Article

A Review of Rotor Wake Physics and Modeling

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  • komerath.space
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Abstract

This paper presents an overview of developments in rotorcraft wake modeling, particularly in the past decade, from the experimental, theoretical, and computational viewpoints. Current understanding of wake physics and limitations in wake modeling are summarized and juxtaposed with trends experimental and numerical research. Advances in flow imaging and velocimetry have provided evidence that supports simplified models of rotor wake behavior. Persistence of tip vortices to long ages, deterministic rollup phenomena leading to the transition to the far wake, and entrainment of a part of the nascent tip vortex (TV) vorticity into the counterrotating inboard wake have been shown through experiments. Advances in computational fluid dynamics have improved near- and mid-wake modeling capabilities. Hybrid methods that employ vortex element, vorticity transport, or vorticity confinement methods will be necessary for long-age (far-wake) modeling in the near future. A new hover experiment with high-accuracy measurements that tie the blade loading, rotor performance, and wake characteristics is one suggestion to facilitate numerical model correlation and development.

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... Nonetheless, wind-tunnel testing is required as the last step in the manufacturing process to validate numerical simulations [5][6][7][8]. A valuable list of empirical research done to study rotor flow fields is provided in the references [9][10][11]. ...
... Therefore, vortex methods are still being implemented to model the rotor wake flow field [115] since they can offer accurate solutions with affordable computational burden and model the wake geometry without numerical dissipation error. References [9,34,116] provide exhaustive information on the history of wake models, as well as their development and application in rotorcraft studies. In this section, two wake models based on vortex theory, the freewake method (FWM) and the viscous vortex particle method (VPM), are introduced. ...
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Electric vertical take-off and landing (eVTOL) aircraft with multiple lifting rotors or prop-rotors have received significant attention in recent years due to their great potential for next-generation urban air mobility (UAM). Numerical models have been developed and validated as predictive tools to analyze rotor aerodynamics and wake dynamics. Among various numerical approaches, the vortex method is one of the most suitable because it can provide accurate solutions with an affordable computational cost and can represent vorticity fields downstream without numerical dissipation error. This paper presents a brief review of the progress of vortex methods, along with their principles, advantages, and shortcomings. Applications of the vortex methods for modeling the rotor aerodynamics and wake dynamics are also described. However, the vortex methods suffer from the problem that it cannot deal with the nonlinear aerodynamic characteristics associated with the viscous effects and the flow behaviors in the post-stall regime. To overcome the intrinsic drawbacks of the vortex methods, recent progress in a numerical method proposed by the authors is introduced, and model validation against experimental data is discussed in detail. The validation works show that nonlinear vortex lattice method (NVLM) coupled with vortex particle method (VPM) can predict the unsteady aerodynamic forces and complex evolution of the rotor wake.
... Far wake numerical investigations are also almost nonexistent, at least for helicopter rotors. By essence, conventional free wake methods are inappropriate for far wake predictions due to their inviscid assumption [112,102], and further advanced computational methods must be used. Among very accurate approaches, detached eddy simulation with adaptive mesh resolution effectively captures the near wake of 129 hovering and advancing rotors [30,88], at the cost of tremendously high resolution simulations with fine body-fitted meshes. ...
... Despite the overall increase in computational power over the last decades, the simulation of hovering rotors still represents a challenge today [59]. Affordable design tools offer often limited accuracy [102], while realistic predictions are only accessible through very large CFD simulations requiring a huge amount of resources (among recent ones, see e.g. [96]). ...
Thesis
When an object generates lift as a result of its motion through the air, it leaves a typical signature in its wake invariably featuring two parallel vortices. These structures are of great interest, both fundamentally considering their prevalence in fluid dynamics, and industrially for the critical impact they have on air traffic. This thesis is devoted to the study of the formation of such vortices in the wake of wings and helicopter rotors, and to the development of the numerical methods upon which the related analyses depend. Wake vortices generally result from the roll-up of vortex sheets. We aim to simulate this entire process over space and time, starting with the shedding of vorticity from the aerodynamic devices (wings and blades). To this end, we develop and validate a novel immersed lifting-dragging line model, implemented in a vortex particle-mesh method which provides the necessary large eddy simulation capabilities. Despite their inherently different generation processes, the far wakes of wings and rotors appear similar. With our simulations, we investigate their universal character, unveiling some intriguing specificities which depend on the device operating conditions. Focusing on the three-dimensional effects affecting the wake development, we emphasize the important role of vortex interactions and instabilities in the transition to turbulence. We also identify the airfoil parasitic drag as the leading cause of vortex wandering, one of such instabilities. Finally, taking advantage of the present numerical framework, we design multiple wake sensing strategies for airplane formation flight, a technique that could one day help commercial airliners save energy.
... While much research has been performed to understand the aerodynamic loading on isolated wind turbines, much less has been accomplished in efficiently capturing and quantifying the long-age aerodynamic wakes. Determining the near-and far-field wake structure, strength, and position are of critical importance to the calculation of performance and aeroelasticity, as inaccurate wake models lead to poor predictions of blade loads [4]. ...
... For rotating systems, such as rotorcraft and HAWT configurations, the helical/epicycloidal nature of the rotor wake results in regions of strong vorticity that remain close to the rotor so that they cannot be neglected per the Biot-Savart law. Unless a costly grid refinement is applied in this region, numerical diffusion over even a fraction of the rotor revolution can artificially smear the vorticity leading to poor predictions [4]. The ability to preserve these discrete vortices and other localized flow features in rotating flow field calculations remains a major challenge in current CFD development. ...
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An approach to rapidly assess complex unsteady aerodynamic and aeroe- lastic configurations using a novel hybrid overset computational uid dynamics (CFD) approach has been developed. This methodology can also be coupled with computational structural dynamics (CSD) solvers to perform aeroelastic simulations that require CFD/CSD coupling. This robust hybrid approach is designed to utilize state-of-the-art codes from government and commercial developers to provide accurate, "carefree" compu- tations. This hybrid strategy has been successfully applied to rotorcraft and wind energy configurations. Computational evaluations of rotorcraft and wind turbine configurations indicate that this hybrid methodology provides comparable, and in some cases improved predictions of these rotating systems compared with traditional CFD predictions at a fractional cost of the original CFD methodology. The cost of the new hybrid methodology approaches an order of magnitude less than the traditional CFD methodology as convergence is dramatically improved, while the background grid can be coarsened.
... Development of many aerospace technologies, not limited to helicopter rotor-fuselage applications, requires accurate resolution of both near-and far-field flow phenomena. Numerical prediction of wakes involve a tradeoff between accuracy, turnaround time, and computational expense [2]. Current grid-based computational fluid dynamics (CFD) codes can theoretically model the entire flowfield, but resolution and preservation of wake features become difficult since typical grid sizes used in industrial simulations are susceptible to numerical dissipation. ...
... Current grid-based computational fluid dynamics (CFD) codes can theoretically model the entire flowfield, but resolution and preservation of wake features become difficult since typical grid sizes used in industrial simulations are susceptible to numerical dissipation. The artificial diffusion of vorticity that results can be mitigated using grid adaptation techniques and higherorder methods [2][3][4], but this may not be practical for all applications since computational cost increases significantly. For this reason, computationally efficient hybrid methods may be more attractive, especially during design and for flight-test support. ...
Article
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The ability of two approaches to hybridize a Reynolds-averaged Navier Stokes (RANS) CFD solver and a free-wake method for the rotor fuselage interaction problem is investigated. Continuum Dynamics, Inc.'s, comprehensive rotorcraft code, CHARM, has been used successfully to model rotorcraft airloads and blade vortex interaction noise. FUN3D, a fully unstructured Navier Stokes RANS solver developed primarily by researchers at NASA, includes overset and adaptive mesh capabilities to enable accurate resolution of multiple frames of motion, making it suitable for rotorcraft analysis. FUN3D and the CHARM wake panel module were coupled together in a conventional hybrid arrangement where the FUN3D airloads and CHARM wake solutions influenced each other in a closed loop. it is observed that, in general, all predictions show good reproduction of the magnitude and phase of the peaks in unsteady pressure. The modified flow predicted by the FUN3D/CHARM computations can be readily observed at this moderate advance ratio.
... Unfortunately, the requirements of computational resources and time may become computationally prohibitive. 10 Another way to capture the rotor wake is the hybrid CFD approach where the near-body flow-field is solved using the CFD, and the off-body wake is modeled by the external wake model. Recently, some hybrid CFD solvers have been developed, and the results indicate that this method can significantly reduce the computational costs while retaining the similar prediction accuracy. ...
... The effect of compressibility is little in the vast region in rotor vortical flow except for a small area around the blade surface. 10 Therefore, the flow field where the rotor wake rolls up and evolutes can be considered to be incompressible. In this paper, the VWM is used to model the rotor-wake dynamics. ...
Article
Full-text available
A coupling fluid-structure method with a combination of viscous wake model (VWM), computational fluid dynamics (CFD) and comprehensive structural dynamics (CSD) modules is developed in this paper for rotor unsteady airload prediction. The hybrid VWM/CFD solver is employed to model the nonlinear aerodynamic phenomena and complicated rotor wake dynamics; the moderate deflection beam theory is implemented to predict the blade structural deformation; the loose coupling strategy based on the 'delt method' is used to couple the fluid and structure solvers. Several cases of Helishape 7A rotor are performed first to investigate the effect of elastic deformation on airloads. Then, two challenging forward flight conditions of UH-60A helicopter rotor are investigated, and the simulated results of wake geometry, chordwise pressure distribution and sectional normal force show excellent agreement with available test data; a comparison with traditional CFD/CSD method is also presented to illustrate the efficiency of the developed method.
... Some notable investigations into coupling methods have been reported in the literature, and their results are in good agreement with the measured data, obtained while dramatically reducing the required computation time (Bhagwat, Moulton, & Caradonna, 2007;Shi, Zhao, Fan, & Xu, 2011;Sitaraman & Bader, 2006;Wie, Im, Kwon, & Lee, 2010;Yang, Sankar, & Smith, 2002). However, limitations still exist regarding the singularity-based vortex module used in these coupling methodologies (Komerath, Smith, & Tung, 2011). Because of the assumption of inviscid flow, the wake solutions have to rely on empirical formulations that are used to account for the roll-up rule, initial vortex core size, and vortex decay factor. ...
Article
Full-text available
A coupled Eulerian–Lagrangian methodology was developed in this paper in order to provide an efficient and accurate tool for rotor wake and flow prediction. A Eulerian-based Reynolds-averaged Navier–Stokes (RANS) solver was employed to simulate the grid-covered near-body zone, and a grid-free Lagrangian-based viscous wake method (VWM) was implemented to model the complicated rotor-wake dynamics in the off-body wake zone. A carefully designed coupling strategy was developed to pass the flow variables between two solvers. A sample case of a forward flying rotor was performed first in order to show the capabilities of the VWM for wake simulations. Next, the coupled method was applied to rotors in several representative flight conditions. Excellent agreement regarding wake geometry, chordwise pressure distribution and sectional normal force with available experimental data demonstrated the validity of the method. In addition, a comparison with the full computational fluid dynamics (CFD) method is presented to illustrate the efficiency and accuracy of the proposed coupled method.
... In literature, only a limited number of experimental efforts exists [3][4][5] that study wing-vortex interaction. Komerath et al. [6], in their review paper, emphasized further fundamental experimental studies to understand rotor wake. Recently, experimental studies by Bhagwat et al. [7] and Ramasamy et al. [8] attempted to understand the fundamental nature of the interaction. ...
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The interaction of trailing vortices with lifting surfaces is investigated using two levels of modeling fidelity. An overset mesh-based Reynolds-averaged Navier–Stokes solver is considered as the high-fidelity computational model. A lower-fidelity model is developed by combining a vortex panel method with a propeller aerodynamic model and slipstream theory. The high-fidelity model is first validated against available experimental data obtained from the interaction of a trailing vortex generated by an upstream wing with a downstream wing. The ability of both models to represent the development of the vortex wake and integrated loads is assessed for a number of parametric configurations, including a case in which the vortex core directly impinges on the wing surface. Following this, isolated propeller and wing-mounted propeller configurations are studied. In all of these cases, the high-fidelity model is effective in predicting the details of the flow and integrated airloads. The low-fidelity model, although less accurate, is shown to accurately predict interactional air loads and performance at orders of magnitude less cost than the high-fidelity model, justifying its role as a viable tool in design and trajectory-planning applications.
... The helicopter forward airspeed was 65 kt, and so the measurement position was about 20D, where D is the rotor diameter, downstream from the rotor center. Far-wake or long-age wake CFD simulation is a significant challenge because it requires high-density grids and needs to overcome numerical dissipation [13]. A CFD actuator disk model and the Beddoes model were applied to the flight condition of Kopp's test. ...
Article
Different methods of modeling helicopter wakes are presented and compared with available wind-tunnel and flight test data. A free-wake model was then used to generate the wake vortices of a helicopter hover-taxiing over an airport runway. A hybrid wake model, with a wake decay law, was also used to generate the far wake of a helicopter in level flight. The wake-induced velocity fields were integrated into an aircraft flight dynamics model, and piloted flight simulations were carried out to study a light aircraft encountering a helicopter wake during landing and level flight. It was found that, for the current landing wake-encounter scenario, the existing wake-encounter criteria and severity metrics for the determination of the hazardous distance might not be appropriate if the wake encounter occurs close to the ground. The landing simulation results suggest that, for a helicopter in low-speed hover-taxiing (less than 40 kt airspeed), the wake-encounter detectable horizontal distance is about three times the diameter of the rotor, which coincides with the current safety guidelines of the Civil Aviation Authority of the United Kingdom. The level-flight simulations revealed the effects of the vertical separation distance and of the wake decay on the encounter severity.
... Hybrid approaches where near-body URANS and less refined wake methodologies are combined may provide near-term alternatives for complex wake analyses (Ref. 11). An inviscid hybrid approach with an octree-Cartesian Euler solver and free-wake model provided the aerodynamic behavior in a real-time rotorcraft simulation code (Ref. ...
Conference Paper
The simple frigate shape geometry (SFS2) has been studied in free-air and wind tunnel environments for both headwind and yawed flow configurations. The ability of an unsteady Reynolds-Averaged Navier-Stokes (URANS) solver and a URANS/Vorticity Transport hybrid computational methodology to capture the bluff body wake characteristics has been assessed. Comparisons have been made with two different SFS2 experimental data sets, including new data from the Naval Surface Warfare Center Carderock Division. Overall good agreement between the hybrid and traditional URANS approaches was observed. Complex wake interactions were captured by the computational methodologies, and they have been assessed to provide insight into differences between the computational and experiment results. The influence of the unsteady vortex shedding from the bluff hangar has been characterized with respect to the changes in the flow field at hangar height and along the landing deck. Copyright © 2014 by the American Helicopter Society International, Inc. All rights reserved.
... Applications of these techniques are given in (Potsdam and Strawn 2005, Saunier et al. 2008, Wissink et al. 2010, Potsdam et al. 2011). A good review of the problem can also be found in Komerath et al. (2011). ...
Article
This paper gives an overview of CFD techniques developed and used at ONERA for rotorcraft applications. First, the complex multidisciplinary environment around helicopters, in which aerodynamics, flight dynamics, aeroelasticity and aeroacoustics strongly interact, is highlighted. Rotorcraft simulations are thus performed by comprehensive codes capable of dealing with the whole system efficiently, using integrated simplified models for each discipline, e.g. the aerodynamics. However, fast aerodynamic models cannot accurately represent the full complexity of rotorcraft aerodynamics, in particular as far as nonlinear phenomena are concerned, contrary to CFD. Nevertheless, helicopter problems are particularly demanding for numerical methods, requiring efficient simulation of unsteady flows with shock waves, massive flow separation, concentrated vortex structures and deforming bodies with large amplitude relative motion, while allowing fine description and analysis of local flow phenomena impacting the vehicle behaviour. Helicopter trim in the CFD solution is obtained by iterative coupling with comprehensive analysis, so that the global multidisciplinary simulation can be achieved with an advanced aerodynamic model. The approaches taken by ONERA for the comprehensive code and the CFD solvers are outlined in the paper. Examples of applications typical of rotorcraft problems are given to illustrate current possibilities and difficulties. They include an isolated rotor in hover, the dynamic stall of an oscillating wing, an isolated rotor in descent flight with Blade-Vortex Interactions, the dynamic-aerodynamic coupling of a rotor in high-speed forward flight and the simulation of a complete helicopter in forward flight. Finally, expected and needed developments are reviewed in order to make CFD a more efficient tool in the design office of helicopter manufacturers.
... Applications of these techniques are given in (Potsdam and Strawn 2005, Saunier et al. 2008, Wissink et al. 2010, Potsdam et al. 2011). A good review of the problem can also be found in Komerath et al. (2011). ...
... Experimental methods and computational fluid dynamics (CFD) simulation are the main approaches to study the aerodynamic characteristics of helicopters. The experimental approach is more convincing, and sometimes more effective, for classic problems such as airframe drag reduction [1,2]. However, the experimental method has the disadvantages of long period and high investment. ...
Article
Full-text available
In this paper, a computational fluid dynamics (CFD) simulation method based on the polyhedral nested grid is developed. By comparing the simulation and test results of the hovering flow field of the Caradonna–Tung rotor, the forward flight flow field of the AH-1G rotor, the interference flow field of the Robin rotor/fuselage, and the hovering and forward flight flow field of a coaxial rotor, it is proven that the method proposed in this paper can achieve high calculation accuracy under various working conditions. The dual time-stepping method is used for the transient simulation, and the Spalart–Allmaras (S-A) turbulence model, which is widely used in aviation, is adopted in the simulation.
... Further dynamic stall research is needed to build a stronger physical foundation for the development of more effective and efficient flow control strategies. Navier-Stokes simulations of entire rotor blades, rotors, or even rotorcraft are challenging (e.g., Komerath et al., 13 Costes et al. 14 ). Because of the great computational expense, simulations of entire rotors are typically only performed for a limited number of data points. ...
... In literature, only a limited number of experimental efforts exists [3][4][5] that study wing-vortex interaction. Komerath et al. [6], in their review paper, emphasized further fundamental experimental studies to understand rotor wake. Recently, experimental studies by Bhagwat et al. [7] and Ramasamy et al. [8] attempted to understand the fundamental nature of the interaction. ...
... Unfortunately, the available studies, mostly numerical, are limited to the near wake, and far wake experimental investigations are almost nonexistent, at least for helicopter rotors. By essence, conventional free wake methods are inappropriate for far wake predictions due to their inviscid assumption [4,5], and further advanced computational methods must be used. Detached Eddy Simulation was employed on hovering and advancing rotors and captured the near wake at high resolutions [6]. ...
Conference Paper
Full-text available
The present work focuses on the Large Eddy Simulation of the wake flow behind an advancing rotor, to support the investigation of rotorcraft wake characteristics and decay mechanisms. A mixed Lagrangian-Eulerian Vortex Particle-Mesh (VPM) method was employed to simulate the near to far wake development of a four-bladed rotor. The rotor configuration was chosen to mimic the setup of the high advance ratio experiment on an articulated rotor conducted at the University of Maryland in 2014. For an advance ratio of 0.41, results on trim control inputs are compared between the present simulations and the experiment. Then, the wake generation and development mechanisms are described (i) qualitatively in terms of vortex dynamics using rotor polar plots and 3D visualizations; (ii) quantitatively using classical integral diagnostics. A preliminary analysis shows that the strength of the emerging wake vortices increases over the first 30 diameter distance behind the rotor. This behavior is traced back to a longitudinal alignment of vorticity, driven by vortex stretching.
... In literature, only a limited number of experimental efforts exists [3][4][5] that study wing-vortex interaction. Komerath et al. [6], in their review paper, emphasized further fundamental experimental studies to understand rotor wake. Recently, experimental studies by Bhagwat et al. [7] and Ramasamy et al. [8] attempted to understand the fundamental nature of the interaction. ...
Article
The interaction of trailing vortices with lifting surfaces is investigated using two levels of modeling fidelity. An overset mesh-based Reynolds-averaged Navier–Stokes solver is considered as the high-fidelity computational model. A lower-fidelity model is developed by combining a vortex panel method with a propeller aerodynamic model and slipstream theory. The high-fidelity model is first validated against available experimental data obtained from the interaction of a trailing vortex generated by an upstream wing with a downstream wing. The ability of both models to represent the development of the vortex wake and integrated loads is assessed for a number of parametric configurations, including a case in which the vortex core directly impinges on the wing surface. Following this, isolated propeller and wing-mounted propeller configurations are studied. In all of these cases, the high-fidelity model is effective in predicting the details of the flow and integrated airloads. The low-fidelity model, although less accurate, is shown to accurately predict interactional air loads and performance at orders of magnitude less cost than the high-fidelity model, justifying its role as a viable tool in design and trajectory-planning applications. Read More: https://arc.aiaa.org/doi/abs/10.2514/1.C034709
... These ways include designing CFD grids appropriately to reduce the numerical dissipation, using high-resolution schemes, and correctly fitting initial and boundary conditions (Ref. 8). ...
Conference Paper
In this paper, the wake optimized CFD method is presented to accurately predict the complicated wake geometry of the coaxial rotor. The truncated vortex tube model is used to assign initial and boundary conditions, and the resolution is improved by applying the Wavenumber Extended Finite Volume (WEFV) scheme, a CAA numerical technique with low numerical dispersion and dissipation. Using these methods, aerodynamic and aeroacoustics characteristics of the coaxial rotor are confirmed. The thrust of a coaxial rotor is periodically varied according to the relative position of the upper and lower rotor. The lower rotor is continuously influenced by the upper rotor wake and showed the BVI phenomenon and unsteadiness of wake geometry. In addition, the vortex pairing is observed near the lower rotor. The unsteadiness of the wake geometry and the BVI phenomenon in the lower rotor result in a unique time signal of the loading noise. Depending on the position of the observer point, loading noise of the lower rotor is larger than that of the upper rotor due to the large time derivative of the pressure. In conclusion, the complicated wake geometry of the coaxial rotor is precisely simulated by the wake optimized CFD method, and this method can be applied to other compound helicopter and rotorcraft analysis.
... Despite the overall increase in computational power over the last decades, the simulation of hovering rotors still represents a challenge today [1]. Affordable design tools (including free-wake methods) offer often limited accuracy [2], while detailed predictions are only accessible through very large CFD simulations requiring a huge amount of resources (see e.g. [3] for a recent one). ...
Conference Paper
Midway between a vortex method and a grid-based CFD, the Vortex Particle-Mesh method with Immersed Lifting Lines is intended to provide medium-fidelity results on rotor loadings, together with a realistic representation of the 3-D vortical wakes and their dynamics over long distances. We assess the potential of this hybrid approach for the Large-Eddy Simulation of helicopter rotors in hover. A novel Poisson solver with mixed unbounded-outflow boundary conditions here further enables the computation of turbulent hovering scenarios in tight domains. Considering the Knight and Hefner experiment and the S-76 test case as references, we present and compare blade integrated and distributed loads, induction velocities, and wake characteristics. While the quality of the performance predictions highly depends on external polar data, the obtained wakes exhibit similar characteristics to those recently identified in other CFD analyses, here at a moderate computational cost. Based on these results, we further investigate the secondary vortex structures forming between the main tip vortices, and we bring additional clues on their relation to the phenomenon of wake breakdown. We finally discuss the strengths and difficulties of hybrid vortex methods for the challenging analysis of hovering rotors.
... When classical problems such as airframe drag reduction are considered, experimental methods using wind tunnels or water tunnels are sometimes more effective than CFD. 3,4 However, rotor flight experiments are very expensive and time-consuming and field visualization methods (e.g. particle image velocimetry) often have insufficient spatial resolution to enable detailed observation of the rotor flow field characteristics. ...
Article
Full-text available
A computational fluid dynamics (CFD) trimming method based on wind tunnel and flight test data is proposed. Aerodynamic coefficients obtained for a helicopter rotor using this method were compared with both experimental data from a test report and CFD results based on the control parameters that were reported in the same document. The method applies small disturbances to the collective pitch angle, the lateral cyclic pitch angle and the longitudinal cyclic pitch angle of the helicopter’s main rotor during forward flight to analyze the effects of each disturbance on the thrust coefficient, the pitching moment coefficient and the rolling moment coefficient of the rotor. Then, by solving a system of linear equations, the collective pitch angle, the lateral cyclic pitch angle and the longitudinal cyclic pitch angle of the main rotor in the CFD trim state are obtained. The AH-1G rotor is used in this paper. NASA has conducted a comprehensive flight test program on this model and has published detailed test reports. Using this method, the pitch moment and the roll moment can be corrected to almost zero and the calculated thrust coefficient is more consistent with the test data when compared with results from direct CFD simulations.
... Several approaches can be adopted to improve the resolution of CFD. These include designing appropriate grids to reduce the aforementioned numerical dissipation, using high-resolution schemes, and correctly fitting initial and boundary conditions [35]. Many methods can be used to design grids appropriately, for example, adaptive mesh refinement [36], vortex-following meshes [37], and unstructured dynamic overset grids [38]. ...
Article
In this paper, the wake dynamics of coaxial rotors are studied using the high-wake-resolution method. The high-wake-resolution method is a novel combination of the truncated vortex tube model for initial and boundary conditions and the wavenumber-extended finite-volume interpolation scheme. The wake-resolution of this method is verified by comparison with other general CFD techniques. By using this method, we analyze the wake dynamics of coaxial rotors with different inter-rotor spacing (IRS). The aerodynamic characteristics of coaxial rotors are compared with those of single-rotor systems to identify the effect of wake interaction on coaxial rotor blades. Variations of thrust and sectional thrust coefficients indicate that the lower rotor is continuously influenced by the wake of the upper rotor. In addition, the blade-vortex interaction (BVI) and the wake instability phenomena are observed near the lower rotor. Aerodynamics and wake dynamics are strongly correlated with IRS. Details of important features such as wake trajectory, miss distance of each rotor, BVI phenomenon, and wake instability at different IRS are discussed.
... Far wake numerical investigations are almost nonexistent, at least for helicopter rotors. By essence, conventional free wake methods are inappropriate for far wake predictions due to their inviscid assumption, 7,8 and further advanced high-fidelity computational methods must be used. Among very accurate approaches, detached eddy simulation with adaptive mesh resolution effectively captures the near wake of hovering and advancing rotors 9,10 at the cost of tremendously high resolution simulations with fine body-fitted meshes. ...
Article
We perform the large-eddy simulation of the flow past a helicopter rotor to support the investigation of rotorcraft wake characteristics and decay mechanisms. A hybrid Lagrangian–Eulerian vortex particle–mesh method is employed to simulate the wake development with the blades modeled using immersed lifting lines. The validity of the numerical approach is first evaluated through a comparison of the rotor trim parameters with experimental results. Then, the rotor wake at low, medium, and high advance ratios is simulated up to 30 rotor diameters. The wake generation and roll-up are described (i) qualitatively using rotor polar plots and three-dimensional (3D) vortex dynamics visualizations and (ii) quantitatively using classical integral diagnostics in cross sections. The highly 3D unsteady near wake transitions to a system dominated by two parallel vortices over a distance that depends on the advance ratio. This process is accelerated by the multiple interactions between successive tip vortices, supporting the generation of self-induced turbulence and uncovering a mechanism of vorticity alignment along the streamwise axis. The vortices in the far wake are compared to typical aircraft ones and exhibit less compact cores and faster decaying energy. Finally, we illustrate the loss of time periodicity in the far wake using the power spectral density of the kinetic energy, and the backscattering of energy from high rotor harmonics to lower frequencies, as complementary evidence of the intense vortex interaction activity.
... Far wake numerical investigations are almost nonexistent, at least for helicopter rotors. By essence, conventional free wake methods are inappropriate for far wake predictions due to their inviscid assumption, 7,8 and further advanced high-fidelity computational methods must be used. Among very accurate approaches, detached eddy simulation with adaptive mesh resolution effectively captures the near wake of hovering and advancing rotors 9,10 at the cost of tremendously high resolution simulations with fine body-fitted meshes. ...
Article
We perform the large-eddy simulation of the flow past a helicopter rotor to support the investigation of rotorcraft wake characteristics and decay mechanisms. A hybrid Lagrangian–Eulerian vortex particle–mesh method is employed to simulate the wake development with the blades modeled using immersed lifting lines. The validity of the numerical approach is first evaluated through a comparison of the rotor trim parameters with experimental results. Then, the rotor wake at low, medium, and high advance ratios is simulated up to 30 rotor diameters. The wake generation and roll-up are described (i) qualitatively using rotor polar plots and three-dimensional (3D) vortex dynamics visualizations and (ii) quantitatively using classical integral diagnostics in cross sections. The highly 3D unsteady near wake transitions to a system dominated by two parallel vortices over a distance that depends on the advance ratio. This process is accelerated by the multiple interactions between successive tip vortices, supporting the generation of self-induced turbulence and uncovering a mechanism of vorticity alignment along the streamwise axis. The vortices in the far wake are compared to typical aircraft ones and exhibit less compact cores and faster decaying energy. Finally, we illustrate the loss of time periodicity in the far wake using the power spectral density of the kinetic energy, and the backscattering of energy from high rotor harmonics to lower frequencies, as complementary evidence of the intense vortex interaction activity.
... This section presents an overview of the different modelling methodologies used through the years to represent rotor aerodynamics. The general ideas and arrangement are largely based on the books of Johnson [3] and Seddon [19] and the excellent review articles of Conlisk [1], Strawn et al. [20], Komerath et al. [21] and Hariharan et al. [22,23] which the reader is invited to consult for further details. Although the subject of this research focuses more on high fidelity simulations, it is essential to review some elementary concepts coming from momentum and vortex theory as these notions are used throughout the thesis. ...
Thesis
Helicopters distinguish themselves from other aircraft by their unparalleled versatility in their capacity of efficient vertical flight allowing them to access remote areas easily. Although vertical flight capabilities and their performance are of primordial importance, today’s helicopter must still operate in all flight regimes efficiently such as forward flight, descent and in close interaction with surrounding obstacles over challenging terrain. These challenges, in particular the latter, also extend to a widening field of more general vertical take off/landing vehicles such as drones in the form of Micro Air Vehicles and the advent of urban air mobility. However, due to the wide versatility and applicability of the aircraft, it would be impossible for designers to consider all flight cases and off-design configurations that third parties use or operate the aircraft in. For some of these specific applications, there remains a need to evaluate the aerodynamic performances of helicopters in possibly challenging conditions. The main objective of this thesis is to study the high fidelity aerodynamic computations of helicopter rotors in various flight conditions. Conditions such as hover, forward flight, ground effect and in interaction with a low-rise building are assessed. Traditional high fidelity simulations based on Unsteady Reynolds Averaged Navier Stokes (U-RANS) methods can take up to several weeks on hundreds of CPU cores to complete a computation for one configuration, making them time and computationally intensive. In an attempt to curtail this issue, a model, still based on a high fidelity U-RANS framework, is explored. The developed method needs to accurately simulate and predict the main rotor performance metrics such as thrust, torque, figure of merit and general rotor wake. The Actuator Line Method is the selected model. This method, which is widely used for wind turbine simulations, replaces the rotor blades by momentum source terms in the U-RANS equations. The removal of the blade mesh significantly reduces the computational mesh size thus lowering the computational cost. First, in order to adapt the ALM for helicopter rotor flows, a parametric study is performed in a simplified framework. In a two-dimensional setting, different velocity sampling methodologies are tested with the integral velocity sampling proving superior. Other model sensibilities related to the modelling and the flowfield are assessed and characterized. Then, a simple extension in 3D for a fixed wing application shows the appropriateness of the method in handling 3D flows. Second, the ALM is applied to a rotor in hover. A properly tuned truncated-normalized Gaussian kernel showed superior results in terms of integrated coefficient prediction capabilities. The ALM also produces a similar tip vortex wake when compared to an equivalent blade resolved simulation. Third, the method is applied to an axial flight case where the predictive capabilities of the ALM model are near identical when compared to the fully resolved reference. The ALM is also capable of predicting the linear decrease in figure of merit for a rotor in climb with great accuracy. In descent, less agreement is found due to the mispredicted absence of vortex ring state present in the experimental results. Finally, the ALM is tested in increasingly difficult flow conditions including hovering in ground effect, forward flight in ground effect, in ground effect in interaction with a boxshaped obstacle both with and without incoming wind. In all cases, the ALM is capable of predicting global flow trends with a small misprediction of thrust in the most extreme ground effect states. The method is also capable of predicting the relative magnitude and orientation change of the moment coefficient. The ALM otherwise agrees well with the reference blade resolved simulations. The overall agreement of the ALM with both the fully blade resolved simulations and the experimental data demonstrates its capabilities in accurately predicting rotor flows in challenging conditions and its appropriateness to replace more costly fully blade resolved simulations.
... The rotor wake vortex is a dominant feature of the helicopter flow field and it has been identified as one of the primary aspects for predicting helicopter aerodynamics and flight performance [1]. The rotor wake exhibits an overall periodicity state, while it is highly unsteady and aperiodic during transient maneuvers. ...
Article
Advanced CFD tools are nowadays used routinely for analysis and design of rotorcraft with the research community shifting towards simulations of rotorcraft during maneuvering flight. One of the impediments of this effort is the lack of data for validation, evaluation and thorough assessment of CFD methods when it comes to rotors with time-varying control inputs. This paper presents a first effort to validate CFD tools for step-inputs in rotor control angles against previously un-published experimental data. The experimental study was carried out at the Nanjing University of Aeronautics and Astronautics in China and consists of time varying control inputs for a hovering rotor. The agreement between the simulation and experimental study is good and quantify the overshoot in loads for this dynamic case. The results examine the lag in loads response to and the dynamic response of the wake with recommendations on spatial and temporal resolution to capture these effects.
... Although a number of metrics may be used for CFD validation, the most important quantities are those that may be used as inputs to rotorcraft dynamic models. Despite advances in computing, fully coupled, real-time, predictive Navier-Stokes-based CFD simulations of the ship/helicopter dynamic interface will not be available in the near term [31]. Airwake simulations that now or in the future incorporate helicopter flight may model the airwake-rotorcraft interaction using a number of different techniques [29]. ...
Article
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Velocity measurements in a ship airwake are obtained in situ aboard a 108 ft naval training vessel. Three-component sonic anemometers are placed at the bow of the ship, for reference wind measurement, and at numerous locations above a flight deck at the stern of the ship. The mean flow structure resembles that of a 3D backward-facing step, with a recirculation region covering much of the forward flight deck, and significant downwash over the remainder of the surface. Reynolds stresses and two-point velocity correlations are presented, and placed in the context of shipborne helicopter operations. The influence of an atmospheric boundary layer, often unavailable in ship airwake measurements from a wind tunnel, is discussed.
Article
A highly efficient unsteady panel time-marching free wake is established to achieve fast and accurate prediction of the unsteady aerodynamics of a helicopter. The unsteady panel method is used to calculate the airload of rotor blades, and the free-wake method is applied to simulate the dynamics of rotor wake. The two methods are tightly coupled by convecting panels to vortex filaments, and the rotor wake is divided into three parts to reduce the computational cost. The first is shed wake indicated by doublet panels. The second is near-wake filaments, which are described by a series of vortex filaments attaching to the shed wake. The third is far-wake filaments connecting to the near-wake filaments. A velocity-field integration technique is adopted to overcome the singularity problem during the interaction of the rotor wake with blades. Helicopter rotors including the Caradonna-Tung and the AH-1G rotors are simulated in hover and forward flight to validate the accuracy and efficiency of the present approach. The predicted blade pressure and load distribution agree well with the measured data and computational-fluid-dynamics results in hover and low-speed forward flight. Compared to computational fluid dynamics and the traditional panel/vortex-rings free-wake method, the present method is more efficient.
Conference Paper
A novel computational fluid dynamics (CFD) methodology has been developed as a new approach to wind resource assessment. An unsteady Reynolds-Averaged Navier-Stokes (URANS) CFD solver with advanced Large Eddy Simulation (LES)-based turbulence models has been tightly coupled with a vorticity-velocity CFD solver. This coupling creates a consistent hybrid overset methodology based solely on the resolution of the Navier-Stokes equations that includes both computational efficiency and improved accuracy. Because the hybrid solver is based on government research and commercial codes that are supported and are regularly updated with new state-of-the-art algorithms and methodologies, this approach also provides a robust, "carefree" platform on which to base computations. This hybrid model has been applied to wind-turbine components used in Phase VI of the National Renewable Energy Laboratory (NREL) Unsteady Aerodynamics Experiment. The accuracy and feasibility of this approach are assessed with correlations to experimental data and standalone URANS simulations. The new hybrid approach is shown to provide comparable (or better) wake characteristics and blade loading as the CFD solver using a fraction of the grid and computational time.
Article
Schemes for anisotropic grid adaptation for dynamic overset simulations are presented. These approaches permit adaptation over a periodic time window in a dynamic flowfield so that an accurate evolution of the unsteady wake may be obtained, as demonstrated on an unstructured flow solver. Unlike prior adaptive schemes, this approach permits grid adaptation to occur seamlessly across any number of grids that are overset, excluding only the boundary layer to avoid surface manipulations. A demonstration on a rotor/fuselage-interaction configuration includes correlations with time-averaged and instantaneous fuselage pressures, and wake trajectories. Additionally, the effects of modeling the flow as inviscid and turbulent are reported. The ability of the methodology to improve these predictions is confirmed, including a vortex/fuselage-impingement phenomenon that has before now not been captured by computational simulations. The adapted solutions exhibit dependency based on the choice of the feature to form the adaptation indicator, indicating that there is no single best practice for feature-based adaptation across the spectrum of rotorcraft applications.
Conference Paper
A coupling framework has been developed for unstructured computational fluid dynamics (CFD) solvers to allow for simple activation of a variety of wake solvers. In addition, the interface has been parallelized and extended to support dynamic, overset meshes in rotating frames. Wake capture and performance calculations demonstrate the validity of the framework. Demonstration cases include an oscillating wing, the hovering Caradonna-Tung rotor and ROBIN rotor-fuselage interaction for multiple coupling variations between CHARM, VorTran-M, and FUN3D. These results illustrate that the hybrid methods can provide more accurate results with reduced grid sizes for various applications. The modification of only one solver at each incremental level of analysis permits the user to identify the source of changes in solution results. Copyright © 2011 by the American Helicopter Society International, Inc. All rights reserved.
Conference Paper
In this paper we study dynamic stall phenomenon of a pitching NACA0015 airfoil us-ing feature based grid adaption technique. The fluid solver is based on FUN3D which solves the three-dimensional, compressible, unsteady Reynolds-Averaged Naiver-Stokes equations. The one equation Spalart-Allmaras is used a the turbulence closure. The governing equations are discretized spatially using second-order finite volume methods and temporally using an optimized second order backward difference scheme. The grid adap-tion is based the anisotropic tetrahedral adaptation approach in which grid is adapted to match a desired quality via an anisotropic metric calculated throughout the simulation. Particularly we track vorticity throughout the pitching cycle and adapt the grid in areas where vorticity is damped. Comparisons are also made with results using uniformly refined grids. Results suggest feature-based adaptation has potential in refining the mesh in the wake of the airfoil, allowing vorticity to be carried out several chords behind the airfoil without excessive dissipation. Our study also shows that great care must be placed in allowing the grid to be adapted in the vicinity of the airfoil as grid resolution can be lost.
Article
An efficient comprehensive vibration analysis method for a helicopter rotor–fuselage coupling system is presented. This loose computational fluid dynamics (CFD)/computational structural dynamics (CSD) coupling approach with a free wake–panel coupled model is used for system vibration response analysis. The CSD model of the helicopter consists of a fuselage model using a refined three dimensional (3 D) finite element model (FEM) and a rotor model consisting of nonlinear moderate deflection beam elements with 15 degrees of freedom. The unsteady Euler CFD solver is used for the flow field analysis of the entire vehicle. The induced inflow of the quasi-steady aerodynamic force is calculated with the free wake–panel coupled model, which is used to simulate rotor–fuselage aerodynamic interference. Using a full-scale helicopter as an example, the vibration responses of the typical fuselage position in hovering and level flights are analysed. When compared with the literature results and flight test data, the predictions of the proposed method are closer to the test data than those of the traditional method in hovering and low forward ratio flights, and the difference between the two methods is minimal in high forward ratio flight.
Article
This paper focuses on one of the most prominent flow features of the hovering rotor wake, the close interaction of the tip vortex with a following blade. Such vortex interactions are fundamental determinants of rotor performance, loads, and noise. Yet, they are not completely understood, largely due to the lack of sufficiently comprehensive experimental data. The present study aims to perform such comprehensive measurements, not on hovering helicopter rotors (which hugely magnifies test complexity) but using fixed-wing models in controlled wind tunnel tests. The experiments were designed to measure, in considerable detail, the aerodynamic loading resulting from a vortex interacting with a semi-span wing, as well as the wake resulting from that interaction. The goal of the present study is to answer fundamental questions such as (a) the influence of a vortex passing below a wing on the lift, drag, tip vortex, and the wake of that wing and (b) the strength of the forming tip vortex and its relation to the wing loading and/or the tip loading. This paper presents detailed wing surface pressure measurements that result from the interaction of the wing with an interacting vortex trailing from an upstream wing. The data show large lift distribution changes for a range of wing-vortex interactions including the effects of close encounter with the vortex core. Significant asymmetry in the vortex-induced lift loading was observed, with the increase in wing sectional lift outboard of the interacting vortex (closer to the tip) being much smaller than the corresponding decrease inboard of the vortex.
Article
The aeroelastic problems of helicopter are different from those of fixed wing aircraft. Not only the single blade is considered and analyzed, but also the rotor is considered as an integrated aeroelastic system, for which the dynamic inflow, wake effects and interactions between rotor and fuselage are all considered in the analysis process. For a single blade, the eccentric field and geometrical nonlinearities, as well as nonlinear flap-lag-twist interactions caused by motion involvement should be considered in structural dynamics modeling; while dynamic inflow and wing tip stall effect should be considered in aerodynamic analysis. Therefore, those problems essentially belong to the category of nonlinear aeroelasticity. Furthermore, the airloads of rotor are transferred to fuselage via rotor shaft in a periodic way and cause vibrations and motions of fuselage; while the motions of fuselage change the root conditions of blades and affect rotor aeroelastic characteristics. Such rotor/fuselage interaction problem becomes one of the important research directions and hot spots in helicopter aeroelasticity in recent years. The numerical methods of rotor flow field simulation are becoming mature gradually, among which, the overset grid and sliding mesh techniques are used to simulate the rigid motion of blades, and the dynamic mesh technique is used to simulate the elastic deformation. Thus, the flow field simulation of elastic rotor can be implemented with enough accuracy and efficiency. These methods are showing thriving vitality and becoming another important research direction of helicopter aeroelasticity. New concepts and configurations, such as tiltrotor, advancing blade concept (ABC) and compound helicopter, bring new aeroelastic problems. Discovering and solving problems constantly to promote the discipline development are the lifelong objectives of aeroelastician forever. ©, 2015, AAAS Press of Chinese Society of Aeronautics and Astronautics. All right reserved.
Article
A new adaptation strategy is presented that permits time-dependent anisotropic adaptation for dynamic overset simulations. The current development permits adaptation to be executed over a periodic time window in a dynamic flow field so that an accurate evolution of the unsteady wake may be obtained within a single unstructured methodology. Unlike prior adaptive schemes, this approach permits grid adaptation to occur seamlessly across any number of grids that are overset, excluding only the boundary layer to avoid surface manipulations. Demonstrations on rotor-fuselage interactions, including flow field physics, time-averaged and instantaneous fuselage pressures, and wake trajectories are included. The ability of the methodology to improve these predictions without user intervention is confirmed, including simulations that confirm physics that have before now, not been captured by computational simulations. The adapted solutions exhibit dependency based on the choice of the flow field feature-based metric and the number of adaptation cycles, indicating that there is no single best practice for feature-based adaptation across the spectrum of rotorcraft applications. Copyright © 2011 by the American Helicopter Society International, Inc. All rights reserved.
Article
A hybrid Eulerian-Lagrangian solver is developed by coupling the computational fluid dynamics (CFD) method with viscous vortex particle method for complicated rotor vortex field analysis. In order to capture the detailed features of a flow field such as its three-dimensional blade tip effect, shock wave, etc., the CFD method is adopted to compute the near-body flow, while a high revolution model is established using the viscous vortex particle method to track the wake since the tip vortex has compact structural characteristics in a high Reynolds number rotor flow field. The "integrated vorticity source method" and the "boundary surface method" are used to exchange the information between the two computational domains. Using the proposed method, a standard test (Caradonna-Tung rotor) for CFD method verification is performed as an example, and numerical simulation is carried out for a hover condition in which the wake plays a more obvious role than in other flight conditions. By comparing the flow characteristics in coupled boundary and pressure coefficient distribution of the blade surface, the validity of the method is verified. Furthermore, different methods are comparatively analyzed from the aspects of the accuracy of capturing the rotor wake, vorticity dissipation characteristics and calculation time. The results show that the hybrid method possesses the advantages of both the CFD method and the Viscous Vortex Particle Method, and has a unique advantage in the rotor flow field numerical simulation.
Article
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In this paper, a computational fluid dynamics trimming method is proposed and compared with wind tunnel experiment and the blade element method. The NASA’s generic ROBIN helicopter model is adopted for transient simulations to obtain the final main rotor trimming conditions. Totally three steps were applied to the computational fluid dynamics method. The first step is associated with no cyclic pitch motion, the second is regarding pure longitudinal cyclic pitch motion and the last is concerning with pure lateral cyclic pitch motion. At the same time, a simple linear equation system between the roll and pitching moment was established to get the final longitudinal and lateral cyclic pitch angles for the main rotor through the above three steps. An overset grid approach was used where the volume around each blade was modeled in an individual overset grid region. The rotor rotation was resolved with three degrees per time-step. Turbulence was modeled with the well-known SST K-omega model with second-order convection. The helicopter was in straight forward flight with an advance ratio of . Three sources of stick angles, which are also called rotor trimming angles, were shown and compared with each other. And the corresponding results were also plotted with a type of history plot in the computational fluid dynamics condition. In the simulations, the results became quasi periodic after about 1.5 rotations and four rotor rotations were simulated for each case. The pitch moment coefficient and roll moment coefficient were all trimmed to about zero by the computational fluid dynamics trimming method while moments were not removed thoroughly in the other two source conditions.
Article
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This study presents a hybrid non-linear unsteady vortex lattice method-vortex particle method (NL UVLM-VPM) to investigate the aerodynamics of rotor blades hovering in and out of ground effect. The method is of interest for the fast aerodynamic prediction of helicopter and smaller rotor blades. UVLM models the vorticity along the rotor blades and near field wakes with panels that are then converted into their equivalent vortex particle representations. The standard Vreman subgrid scale model is incorporated in the context of a large eddy simulation for mesh-free VPM to stabilize the wake development via particle strength exchange (PSE). The computation of the pairwise interactions in the VPM are accelerated using the fast-multipole method. Non-linear UVLM is achieved with a low computational cost viscous-inviscid alpha coupling algorithm through a stripwise 2D Reynolds-averaged Navier–Stokes (RANS) or empirical database. The aerodynamics of the scaled S76 rotor blades in and out of ground effect from the hover prediction workshop is investigated with the proposed algorithm. The results are validated with experimental data and various high-fidelity codes.
Article
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We perform the large-eddy simulation of the flow past a helicopter rotor to support the investigation of rotorcraft wake characteristics and decay mechanisms. A hybrid Lagrangian-Eulerian Vortex Particle-Mesh method is employed to simulate the wake development, with the blades modeled using immersed lifting lines. The validity of the numerical approach is first evaluated through a comparison of the rotor trim parameters with experimental results. Then, the rotor wake at low, medium and high advance ratios is simulated over up to 30 rotor diameters. The wake generation and roll-up are described (i) qualitatively using rotor polar plots and three-dimensional (3-D) vortex dynamics visualizations; (ii) quantitatively using classical integral diagnostics in cross-sections. The highly 3-D unsteady near wake transitions to a system dominated by two parallel vortices, over a distance that depends on the advance ratio. This process is accelerated by the multiple interactions between successive tip vortices, supporting the generation of self-induced turbulence, and uncovering a mechanism of vorticity alignment along the streamwise axis. The vortices in the far wake are compared to typical aircraft ones, and exhibit less compact cores and faster decaying energy. Finally, we illustrate the loss of time periodicity in the far wake using the power spectral density of the kinetic energy, and the backscattering of energy from high rotor harmonics to lower frequencies, as complementary evidence of the intense vortex interaction activity.
Article
We perform the large-eddy simulation of the flow past a helicopter rotor to support the investigation of rotorcraft wake characteristics and decay mechanisms. A hybrid Lagrangian–Eulerian vortex particle–mesh method is employed to simulate the wake development with the blades modeled using immersed lifting lines. The validity of the numerical approach is first evaluated through a comparison of the rotor trim parameters with experimental results. Then, the rotor wake at low, medium, and high advance ratios is simulated up to 30 rotor diameters. The wake generation and roll-up are described (i) qualitatively using rotor polar plots and three-dimensional (3D) vortex dynamics visualizations and (ii) quantitatively using classical integral diagnostics in cross sections. The highly 3D unsteady near wake transitions to a system dominated by two parallel vortices over a distance that depends on the advance ratio. This process is accelerated by the multiple interactions between successive tip vortices, supporting the generation of self-induced turbulence and uncovering a mechanism of vorticity alignment along the streamwise axis. The vortices in the far wake are compared to typical aircraft ones and exhibit less compact cores and faster decaying energy. Finally, we illustrate the loss of time periodicity in the far wake using the power spectral density of the kinetic energy, and the backscattering of energy from high rotor harmonics to lower frequencies, as complementary evidence of the intense vortex interaction activity.
Conference Paper
Advanced CFD tools are nowadays used routinely for analysis and design of rotorcraft. Computations for flows around rotors in trim are also common place and slowly the research community is shifting towards simulations of rotorcraft during maneuvering flight. One of the impediments of this effort is the lack of detailed data for validation, evaluation and thorough assessment of CFD methods when it comes to rotors with time-varying inputs. This paper presents a first effort to validate CFD tools for step-inputs in rotor control angles and presents both novel simulations, and unpublished experimental data. The results show that there is always a lag involved between the wake and loads response and the operation of low-thrust rotors with dynamic wakes and collective input is a challenging task for modern CFD. The results used in this work originate from a study carried out at the Nanjing University of Aeronautics and Astronautics in China and represent a unique set of great value to the research community. The agreement with simulation results further contributes to the value of the test data.
Article
The flow field around a helicopter is characterized by its inherent complexity including effects of fluid–structure interference, shock–boundary layer interaction, and dynamic stall. Since the advancement of computational fluid dynamics and computing capabilities has led to an increasing demand for experimental validation data, a comprehensive wind-tunnel test of a fully equipped and motorized generic medium size transport helicopter was conducted in the framework of the GOAHEAD project.In this paper the test campaign results in terms of three-component velocity field and fluid–structure interaction are discussed. The effect of the interaction between the main rotor wake and the fuselage for cruise/tail shake conditions is investigated, analyzing the flow characteristics downstream the rotor hub and the rear hatch for the case of isolated fuselage and full equipped model. The results indicate a sensible increment of the intensity of the vortices shedding form the lower part of the fuselage and a strong influence of the main rotor in the upper region.The rotor/fuselage interaction is further discussed mainly taking into account the static and dynamic loads and the static and dynamic pressure distributions measured on the fuselage model.Furthermore, the pitch-up phenomenon is considered by detecting the blade tip vortices impacting on the horizontal tail plane, and by measuring the effects on the horizontal tail plane in terms of vertical loads, pressure distribution and integral sectional forces and finally the effect on the full fuselage loads. For high-speed forward flight, the shock wave forming on the advancing blade is investigated by measuring its intensity and the location on the blade chord.
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The development of a generalized viscous vortex model is described for application in rotor aeroacoustic analyses. A brief summary is given of previous models for the viscous core growth and induced velocity field of a trailing tip vor- tex. The assumptions made for the vortex core growth are shown to influence the predictions of the rotor wake ge- ometry, as well as the induced velocities, airloads, and performance, thereby emphasizing the need for a more universal and physically representative vortex model. In the present approach, the viscous core growth was mod- eled using an extension of the classic Lamb-Oseen model for the diffusion of laminar vortices. Turbulence in the tip vortex affects the diffusion of vorticity, and these effects were incorporated using an empirically validated correc- tion for the average apparent or "eddy" viscosity. The vortex induced velocity profiles measured in experiments were found to exhibit strong self-similarity when using the vortex core radius as a length-scale, suggesting that a gen- eralized model is possible. A family of algebraic models for the swirl, axial, and radial components of the veloc- ity induced by a viscous trailing tip vortex is proposed. These velocity components were found from a solution to a simplified form of the incompressible Navier-Stokes equations. The velocity profiles are written in terms of a single integer exponent with the viscous core growth be- ing given by a semi-empirical relation for the turbulent viscosity that scales as a function of the vortex Reynolds number. The model is compared with velocity field mea- surements for both fixed-wing and rotating-wing tip vor- tices, with good agreement.
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A study was made involving simultaneous helicopter rotor blade pressure measurements and tip vortex surveys. Measurements were made for a wide range of tip Mach numbers including the transonic flow regime. The measured tip vortex strength and geometry permit effective blade loading predictions when used as input to a prescribed wake lifting surface code. It is also shown that with proper inflow and boundary layer modeling, the supercritical flow regime can be accurately predicted.
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Flow visualization and velocity measurements are used to show that the outer edge of the trailing vortex sheet from an untwisted rotor blade rolls up into a discrete vortical structure which is opposite in sense and approximately one-half the strength of the tip vortex. This occurs both in hover and low-speed forward flight of a two-bladed teetering rotor, and at both upstream and downstream edges of the wake in forward flight. The roll-up process and the trajectories of the tip vortex and the counter-rotating vortex are examined using digitized laser sheet video images. Azimuth-resolved velocity data are used to quantify vortex strengths.
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An adjoint-based methodology for design optimization of unsteady turbulent flows on dynamic unstructured grids is described. The implementation relies on an existing unsteady three-dimensional unstructured grid solver capable of dynamic mesh simulations and discrete adjoint capabilities previously developed for steady flows. The discrete equations for the primal and adjoint systems are presented for the backward-difference family of time-integration schemes on both static and dynamic grids. The consistency of sensitivity derivatives is established via comparisons with complex-variable computations. The current work is believed to be the first verified implementation of an adjoint-based optimization methodology for the true time-dependent formulation of the Navier-Stokes equations in a practical computational code. Large-scale shape optimizations are demonstrated for turbulent flows over a tiltrotor geometry and a simulated aeroelastic motion of a fighter jet.
Conference Paper
Motivated by the demand for a fast flow simulation tool that takes the interaction between rotor and helicopter fuselage into account an actuator disc boundary condition suited for helicopter rotors in forward flight has been implemented in the unstructured DLR TAU code. The time-averaged effect of the rotor, which accelerates the flow and adds energy to the fluid is imposed using source terms in the Navier-Stokes equations where the actuator disc is located in the grid. The transfer of this approach previously implemented in the structured DLR FLOWer code, involved adapting the strategy to the unstructured framework. It is shown that propeller simulation results are in accordance to FLOWer results and simple 1D theory predictions. Moreover, rotor in forward flight cases prove the robustness of the implementation and resemble FLOWer results. Further development involved testing the implementation in parallel mode and a more sophisticated rotor force distribution is applied instead of a constant pressure jump. Finally, a comparison of the viscous flow field around the EC145 helicopter computed by TAU and FLOWer is performed. It shows that there is good agreement between the two codes in predicting the effect of the actuator disc on the fuselage pressure distribution.
Article
Wind-tunnel experiments were conducted to investigate tail-rotor performance for flight conditions in ground effect where helicopter directional control deficiencies have been experienced. Tail-rotor thrust was measured on a small-scale helicopter model for various wind speeds and azimuths. Measurements were made with the fuselage and fin removed to emphasize the primary interference effects of a main rotor in ground effect. Interpretation of the measured data showed that the ground vortex and trailing vortex systems of the main-rotor flow field produced tail-rotor thrust perturbations that are due to rotational and axial interference with the tail rotor.
Article
This paper studies the concept of extracting hover performance from model rotor climb data by extrapolating to the limit of zero climb speed. A 2-bladed rotor was mounted horizontally and tested in the 30' a 31' settling chamber of the Ames 7' x 10' #1 wind tunnel, The collective pitch and tunnel speed were varied. Video cameras were used to visualize the flowfield illuminated by pulsed white light sheets. Facility recirculation effects were eliminated at all but the lowest rates of climb. With a steady, non-recirculatory flow, the rotor wake was seen to be fully periodic, with little diffusion of the vortices for several rotor cycles. The variation of the computed figure of merit,vith climb rate compared very well with the experiments and it is shown that extrapolation is a good method to get an accurate value for the hover figure of merit. The transition to the far wake was seen to occur through a periodic pairing of the tip vortices, followed by their merging into a single diffuse vortex far each rotor cycle, The number of discrete vortex turns in the near wake before the pairing varied with the thrust coefficient and rate of climb. The climb-extrapolation method appears to be a reliable and practical approach to obtaining performance data which are free of facility recirculation effects.
Article
In planning a research investigation of a hovering model helicopter rotor, a relatively large test chamber was selected because of the desire to minimize room interference effects. However, initial testing revealed large random variations of thrust, torque, and blade flapping. The source of the problem was diagnosed as unsteady recirculation of the test chamber flow. The test chamber was modified by the addition of a flow 'straightener' which successfully reduced the variations to 1. 5% of their mean values. The rotor performance was then measured outside in calm free-air conditions and was compared with the results that were obtained inside the modified chamber. This paper documents the problem, the diagnosis, the solution, and it presents examples of the performance data and wake flow visualizations obtained inside the test chamber (before and after the modification, and in free air).
Article
An error correction and grid adaptive method is presented for improving the accuracy of functional outputs of compressible flow simulations. The procedure is based on an adjoint formulation in which the estimated error in the functional can be directly related to the local residual errors of both the primal and adjoint solutions. This relationship allows local error contributions to be used as indicators in a grid adaptive method designed to produce specially tuned grids for accurately estimating the chosen functional. The method is applied to two-dimensional inviscid and viscous (laminar) flows using standard finite volume discretizations, and to scalar convection-diffusion using a Galerkin finite element discretization. Isotropic h-refinement is used to iteratively improve the grids in a series of subsonic, transonic, and supersonic inviscid test cases. A commonly-used adaptive method that employs a curvature sensor based on measures of the local interpolation error in the solution is implemented to comparatively assess the performance of the proposed output-based procedure. In many cases, the curvature-based method fails to terminate or produces erroneous values for the functional at termination. In all test cases, the proposed output-based method succeeds in terminating once the prescribed accuracy level has been achieved for the chosen functional.
Article
The aerodynamics of a helicopter undergoing steady-state maneuvers has been studied using a free-vortex model of the rotor wake. The wake model is based on a finite-difference approximation to the vorticity transport equation, and is solved using a pseudo-implicit relaxation method. It is shown that, in both hover and forward flight, maneuvers are a source of additional distortion to the vortical rotor wake. This wake distortion can be sensitive to the maneuvering rates. In hover and low speed forward flight, the maneuver induced wake distortion manifests as a counter intuitive change in inflow velocity through the rotor. This suggests that the free wake dynamics during pitching and rolling maneuvers may manifest as a contributor to the 'off-axis' blade flapping response. At higher advance ratios, the effects of maneuver induced wake distortions are still present, but are reduced by about half relative to the hover condition. The results provide guidelines as to the importance of the main rotor wake in predicting the rotor flapping response during maneuvers.
Article
This paper studies the concept of extracting hover performance from model rotor climb data by extrapolating to the limit of zero climb speed, A 2-bladed rotor was mounted horizontally and tested in the 30' x 31' settling chamber of the Ames 7' x 10' #1 wind tunnel. The collective pitch and tunnel speed were varied. Video cameras were used to visualize the flowfield illuminated by pulsed white light sheets. Facility recirculation effects were eliminated at all but the lowest rates of climb. With a steady, non-recirculatory flow, the rotor wake was seen to be fully periodic, with little diffusion of the vortices for several rotor cycles. The variation of the computed figure of merit,vith climb rate compared very well with the experiments and it is shown that extrapolation is a good method to get an accurate value for the hover figure of merit The transition to the far wake was seen to occur through a periodic pairing of the tip vortices, Followed by their merging into a single diffuse vortex for each rotor cycle, The number of discrete vortex turns in the near wake before the pairing varied with the thrust coefficient and rate of climb. The climb-extrapolation method appears to be a reliable and practical approach to obtaining performance data which are free of facility recirculation effects.
Article
Steady-state Reynolds-averaged Navier-Stokes computations are presented for a range of UH-60A model-rotor test cases in hover. The computations are designed to assess grid-related effects on the numerical results and employ 1) structured overset grids with high resolution on the rotor blades, 2) a systematic variation of grid resolution in the rotor wake, and 3) a systematic variation of outer-boundary locations. Computed rotor performance values agree very well with experimental measurements and show little sensitivity to either grid resolution or outer-boundary locations. However, the computations uniformly overpredict the blade sectional thrust near the rotor tip. This overprediction of blade tip thrust is explained by an analysis of the circulation in the computed rotor wake system.
Conference Paper
An analytical method for predicting the distorted geometry of a helicopter rotor wake is described. The development of this method is ultimately directed toward providing a more accurate aerodynamic theory for computing the instantaneous rotor flow field and associated blade airloads. It is shown that: (1) realistic self‐induced distortions of a rotor wake represented by numerous discrete vortex elements can be computed by application of the classical Biot‐Savart law and numerical integration techniques and (2) by dividing the wake into a series of near and far wake regions, the computation requirements which have previously prohibited the use of such an analysis as a cost‐effective engineering tool can be appreciably reduced. Sample comparisons of analytical and experimental wake geometries of a rotor in forward flight are presented and indicate the ability of the analysis to predict the characteristic distortion features of the wake.
Conference Paper
This paper describes a computational infrastructure that supports Chimera-based interfacing of different CFD solvers - a body-fitted unstructured grid solver with a block-structured adaptive cartesian grid solver - to perform time-dependent adaptive moving-body CFD calculations of external aerodynamics. The goal of this infrastructure is to facilitate the use of different solvers in different parts of the computational domain - body fitted unstructured to capture viscous near-wall effects, and cartesian adaptive mesh refinement to capture effects away from the wall. The computational infrastructure, written using Python, orchestrates execution of the different solvers and coordinates data exchanges between them, controlling the overall time integration scheme. Details about the infrastructure used to integrate the codes, the parallel implementation, and results from demonstration calculations are presented. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
Conference Paper
This paper presents a method to improve the tip vortex conservation in a CFD simulation of a helicopter main rotor. Our approach uses vortex-adapted Chimera child grids in order to achieve a local refinement of the grid in the vicinity of the vortex and thus reduce the numerical dissipation of the vortex. The effect of the improvement in tip vortex conservation is demonstrated by comparison with simulations without Chimera refinement and with the experimental results of the HART-II test campaign. In order to allow a meaningful comparison to the experimental data, the rotor is trimmed for thrust as well as longitudinal and lateral mast moment using weak fluid structure coupling with a flight mechanics code.
Conference Paper
A high-order discontinuous Galerkin finite element discretization and output-based adaptation scheme for the compressible Euler equations are presented and applied to an isolated rotor in hover. A simplex cut-cell mesh generation technique is used to support robust and autonomous creation of higher-order meshes. The calculations are performed using a parallel implementation of the DG discretization and the results are compared to experimental data. As accurate simulation of rotorcraft wakes and blade-vortex interactions continues to be a challenge, the output-based adaptation scheme is used with thrust as the output of interest to refine the mesh. The result is a solution with less than three million degrees of freedom that is capable of preserving a rotor tip vortex for three and a half revolutions.
Conference Paper
This paper describes recent work on the application of vortex tracking grids (VTG) in a dynamic manner for a UH-60A rotor in hover to predict the blade airloads and wake. It was found that with appropriate time step size and limiters combined with an automated adaptation process using implicit hole-cutting that allows the nested vortex tracking grids to follow the tip vortices, reasonable computations of the blade section loading and wake geometry characteristics were obtained. While the results are currently inviscid, they indicate the potential of the VTG approach for wake capturing in an affordable manner, but more work is still needed to mature the approach for design application. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Conference Paper
Comprehensive rotorcraft analysis programs are traced from their origins in the 1960s to the present. Many first generation comprehensive analyses began as special-purpose codes, which were subsequently enhanced to perform additional computational tasks. These analysis tools provided significant value to their developers, but all were limited by major deficiencies. One of the principal objectives for developing second generation analysis programs was to eliminate the deficiencies identified in first generation codes. While a great many of these deficiencies were indeed alleviated in second generation codes, shortcomings remain, particularly in the areas of loads prediction, usability, and documentation. For third generation comprehensive analysis programs, that are yet to be developed, four areas of improvement are identified; and potential approaches are suggested.
Article
To advance the understanding of the brownout phe-nomenon, the dual-phase flow environment induced by a rotor hovering above a sediment bed was studied us-ing high-speed flow visualization and particle image ve-locimetry (PIV). The high frame rate of the camera, com-bined with advanced particle recognition software, per-mitted an understanding of the temporal evolution of the rotor wake in ground effect, simultaneously with the pro-cesses of sediment uplift. High-resolution PIV measure-ments in the surface boundary layer showed large ex-cursions in the ground shear produced by the convect-ing wake vortices, these excursions being correlated with localized, intermittent increases in sediment entrainment rates. Once entrained, significant quantities of sediment were trapped and vertically transported by the vortex-induced upwash field. Large sediment particles were often spun out of the vortical flow, and proceeded in a mod-ified saltation trajectory. In particular, the surface and upwash velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the con-centration of entrained sediment, and also the maximum height to which sediment could be transported. Particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground, causing sediment ejection through the process of reinges-tion bombardment. While providing new insight into the time-and length-scales associated with sediment entrain-ment by a rotor wake, the observations made here also bring into question the validity of equilibrium particle flux models currently being used for brownout simulations.
Conference Paper
Under the US-France Memorandum of Agreement on Helicopter Aeromechanics, ONERA, US Army, and Georgia Institute of Technology have performed full configuration, unsteady CFD analyses on a Dauphin helicopter configuration using individual, discrete moving blades. The three different multiblock, overset, and unstructured mesh calculations using compressible, unsteady low Mach preconditioning, and incompressible formulations are described. Spatial and temporal convergence studies are reported. Results are compared against ONERA wind tunnel data, including fuselage forces and unsteady surface pressures, rotor thrust, and wake PIV visualizations. Generally good agreement is seen between the partners' CFD calculations. At the flight conditions investigated, fuselage force comparisons with data are fair. Comparison of the PIV data with reasonably resolved CFD calculations shows clear qualitative agreement for wake strength and location. The CFD visualizations permit determining the origin of the flowfield structures in the PIV planes oriented streamwise and perpendicular to the mean flow. The flowfield and surface pressure visualizations indicate significant rotor-fuselage-empennage interactions which cannot be seen in lower-order actuator disk models.
Conference Paper
Accurate prediction of complex rotor-airframe integration is essential for optimum design and accurate analysis of rotorcraft. Over the years, a broad array of computational methodologies has been employed to predict the rotor-airframe interaction. In the present study three leading computational approaches are developed and employed in the study of rotor-airframe integration problems. These approaches include an overset grid-based, unstructured tetrahedral-based, and unstructured Cartesian grid-based methods. The computational results are compared with experimental data on a generic rotor-airframe configuration tested at Georgia Tech and on the Robin helicopter geometry tested at NASA Langley. The agreement with experimentally measured surface pressure is found to depend strongly on the assumed rotor load distribution. These studies are valuable in helping develop and document the capability of these approaches to provide accurate predictions of rotor-airframe interaction.
Conference Paper
ow eld in an unstructured legacy RANS computational uid dynamics code. The hybrid method consists of a blending of the k ! SST RANS model with a one-equation LES model for the subgrid-scale turbulent kinetic energy (k sgs ). Unstructured grids provide better resolution of complex geometries which is the motivation for extending this method. Correlations include theoretical data, experimental data and computational results with RANS turbulence models.
Conference Paper
An investigation into the physics of utilizing various rotor approximation techniques on a rotor-fuselage interaction aerodynamics problem has been undertaken. A very simple experimental model consisting of a teetering rotor over a cylinder with a semi-hemispheric nose was chosen for comparison to isolate the basic physical mechanisms. A comparison of actuator disk and actuating blade models with an overset rotor blade model is presented. Methods to implement these models within an unstructured computational code are also described.
Conference Paper
The ability to accurately and efficiently model the unsteady aerodynamic effects of actively controlled trailing-edge flaps (ACFs) is crucial for practical application of such systems for vibration and noise reduction as well as performance enhancement. Two-dimensional unsteady airloads due to oscillating flap motion are calculated and compared using various CFD codes and a CFD based reduced order model (ROM). This ROM is based on the Rational Function Approximation (RFA) approach, which results in a state-space, time-domain aerodynamic model suitable for incorporation into comprehensive rotorcraft simulation codes. The accuracy of this model was demonstrated across the practical range of unsteady flow conditions encountered by the active flaps. Two Reynolds-Averaged Navier-Stokes (RANS) solvers (CFD++ and OVERFLOW) were employed in conjunction with various turbulence models, including large eddy simulation based models. Unsteady sectional airloads obtained using the different CFD methods were compared so as to determine code independence. Furthermore, detailed flow physics associated with three-dimensional effects, flap hinge gap, as well as compressibility effects were analyzed using CFD.
Article
This paper presents a method to improve the tip vortex conservation in a computational fluid dynamics simulation of a helicopter main rotor. Our approach uses vortex-adapted Chimera child grids to achieve a local refinement of the grid in the vicinity of the vortex and thus reduce the numerical dissipation of the vortex. The method is applied to the higher-harmonic-control aeroacoustic rotor test case to evaluate its potential with respect to the prediction of blade-vortex interaction airloads. To allow a meaningful comparison with the experimental data, the rotor is trimmed for thrust, as well as for longitudinal and lateral mast moments, using weak fluid-structure coupling with a flight mechanics code. We obtained a good overall agreement with the experimental data for both aerodynamics and blade dynamics. The effect of the improvement in tip vortex conservation is demonstrated by comparison with simulations without Chimera refinement and with the experimental results. It turned out that a very fine grid resolution in the vicinity of the vortex is necessary to capture the blade-vortex interaction airloads. The required grid resolution was provided by a refinement of the vortex-adapted grids, allowing for a very good reproduction of the blade-vortex interaction airloads, especially on the retreating blade side.
Article
Blade surface pressure data obtained during a model helicopter rotor test are examined to determine the locations and strengths of impulsive loading events caused by blade vortex interactions (BVI). Data from 43 descent conditions are used to identify, locate, and characterize the BVI events. The combination of azimuthal, radial, and chordwise resolution has generated a detailed picture of the aerodynamic response of a rotor blade to BVI. The paper examines both global and local features of the interaction. Global features include the dependence of the impulsive airload strength and position on three independent parameters: rotor tip path plane angle, rotor thrust, and advance ratio. Local features are examined by using the unsteady pressure distributions to determine the strength and the chordwise dependence of the amplitude and phase of a representative BVI event.
Article
A computational helicopter rotor wake model, based on the numerical solution of the unsteady fluid-dynamic equations governing the generation and convection of vorticity through a domain enclosing the helicopter, has been developed. The model addresses several issues of specific interest in the context of helicopter flight dynamic modeling. The problem of excessive numerical dissipation of vortical structure, common to most grid-based computational techniques, is overcome using a vorticity conservation approach in conjunction with suitable vorticity-flux limiter functions. Use of a time-factorization-type algorithm allows the wake model to avoid the stiffness that is introduced in flight dynamic applications by the disparity between the rotor and fuselage timescales and to generate rapid solutions to the time-varying vortical structure of the helicopter wake. The model is demonstrated to yield valid solutions to the blade loading and wake structure for isolated and interacting rotors in both hover and forward flight.
Article
Systematic model rotor performance and wake geometry data were acquired to evaluate the influence of wake geometry on rotor hover performance. Analysis of the wake data resulted in (1) the development of a simple generalized representation of the near wake which facilitates the rapid estimation of realistic wake geometries for a wide range of rotor designs and operating conditions and (2) the discovery of a reduction in wake stability with increasing distance from the rotor. The results of a theoretical method for predicting the wake geometry are also presented. Results of a prescribed wake analysis for predicting rotor hovering performance show that the incorporation of a realistic wake geometry provides significantly improved predictions of rotor performance characteristics. Finally, initial results of advanced experimental techniques for determining rotor make characteristics are presented dong with a brief discussian of other current efforts at UARL pertaining to rotor make geometry and performance.
Article
In recent experiments, it was observed that the tip vortices shed from a two-bladed rotor can interact significantly. The interaction consists of a turn of the tip-vortex from one blade rolling around a turn of the tip-vortex from the other blade. Visualized in a planar light sheet, the two vortices spiral around each other prior to merging into a single vortex. One complete cycle of the roll-up process takes about one and one-half rotor cycles. This behavior is analogous to the interaction which takes place between two ring vortices and the numerical calculations shown here demonstrate this similarity. It is observed that the pair of rings, placed initially parallel and of the same strength roll around each other, by alternate contraction and expansion of the ring radius. Numerical calculations show that the ring interaction is complicated when the rings are initially inclined,with respect to one another. The experimental configuration of a two-bladed rotor in axial flight is considered next. A lifting-line theory is used to model each rotor blade and a fully unsteady computation of the motion of the tip-vortices is carried out using a fourth-order accurate Adams-Moulton method to advance the vortices. The modified Biot-Savart law is used to describe the vortex structure. It is shown that the tip-vortex interaction is periodic and deterministic and results for the tip-vortex paths are seen to be in good agreement with experimental results.
Article
Numerical prediction of the geometry of the rotor wake and its effect on the performance of the helicopter, when in ground effect, remains a challenge. This is because certain experimentally observed features of the rotor flow field, such as the formation of the characteristic "ground vortex" and the dynamics of its interaction with the remainder of the rotor flow, require extremely long-term calculations, in computational terms, to capture. The development of the rotor flow field in ground effect is studied using a computational model which, through its vorticity conserving properties, is ideally suited to capturing vortical features in the flow that take a large number of rotor revolutions to develop. Computations confirm experimental observations that the geometry of the rotor wake undergoes a transition through a set of qualitatively different flow states as the helicopter's forward speed is increased. Transition between states is mediated by what appears to be a convective instability of the vortex sheet generated on the ground plane by the rotor. Certain characteristic features in the rotor thrust and power, and in the variation of control angles with forward speed, can be traced back to the dynamics of the vortical structures produced by the growth of this instability.
Article
High-resolution three-dimensional velocity field measurements and flow visualization results were acquired in the flow field near the tip of a rotor blade operating in hover. Using three-component laser Doppler velocimetry (LDV), the measurements documented the trailing vortex formation, the initial core structure, and the viscous evolution of the vortex properties. The test conditions covered a range of wake-ages from as young as one-degree, up to about one rotor revolution. For each wake age, vortex core properties were estimated from the velocity field measurements. The high spatial resolution obtained with LDV has shown that the tip vortex core radius can be less than 3% chord at early wake-ages, but grows asymptotically as it ages. A significant axial velocity deficit (on the order of the peak swirl velocity) existed in the vortex core at early wake-ages, but the deficit rapidly diminished as the vortex aged. Using a Richardson parameter combined with flow visualization, the results suggest that the inner core of the vortex is mostly laminar at the vortex Reynolds numbers tested in this experiment. It was found that bands of turbulent eddies, originating from an adjacent vortex sheet, underwent a re-laminarization process as they were entrained into the vortex core. The evidence suggests that the entire tip vortex structure is neither fully laminar nor fully turbulent, but is instead in a continuous state of dynamic evolution with a region of relatively slow laminar diffusion and a region of accelerated turbulent diffusion. It is suggested that the variation of peak swirl velocity is the result of the competing influences of an inviscid roll-up process, relaminarization inside the vortex core, and viscous diffusion around the core boundary.
Article
The rotorcraft industry is pricing itself right out of the commercial transportation marketplace. This is illustrated by helicopter prices that have inflated significantly faster than consumer product prices and by helicopter productivity per dollar that decreases with increased purchase price. Specifically, inflation in helicopter purchase price has significantly exceeded the U. S. consumer price index since 1980. When measured in ton-knots, productivity per 1994 purchase price dollar has diminished with increased size, cruise speed and added features. In sharp contrast, the propeller driven, fixed wing airliner industry has not followed the rotorcraft industry in this unsatisfactory trend. Purchase price analysis of 120 helicopters using linear regression statistical analysis has yielded a price estimating equation. This equation shows helicopter prices are more sensitive to installed power than to weight empty. Inclusion of 126 General Aviation aircraft and 163 airliners in the regression analysis has shown that price is linearly dependent on a size factor. This pseudo, universal size factor contains both weight empty and total engine(s) rated horsepower design parameters. At equal size factor, helicopters are priced about 50 percent higher than airplanes in the commercial marketplace. This appears to be the premium for vertical takeoff and landing capability. Preliminary price and performance data for two emerging tiltrotors show that the helicopter's low cruise speed problem has been solved. The rotorcraft industry can now expand into the airliner marketplace if it can substantially reduce the premium price for VTOL. The traditional minimum weight empty design approach results in excessive installed power due to high disc loading. The design approach has been necessary to meet military requirements such as fitting helicopters inside an Air Force C-130 and operating tiltrotors from a Navy ship. This military oriented design approach is wrong for commercial products. The price estimating relationship developed shows that designing for minimum weight empty does not equate to minimum helicopter purchase price for the commercial operator. Continuing a military oriented design approach for advanced commercial products such as civil tiltrotors is not recommended.
Article
Extensive airload measurements have been obtained during wind tunnel testing of a pressure-instrumented model of the UH-60A Black Hawk helicopter main rotor. The test included level flight, descent, and hover conditions, and was conducted at the Duits Nederlands Windtunnel as part of a joint acoustics and aerodynamics program by the Army Aeroflightdynamics Directorate, NASA, United Technologies Research Center, and Sikorsky Aircraft. The combination of the variety of measurements (blade pressures, acoustics, dynamics, and performance), the density of measurement stations, and the number and range of test conditions produced a unique and comprehensive picture of the helicopter rotor. This paper describes the model and discusses the aerodynamic results. The blade pressures and integrated airloads are examined to identify regions of wake interaction, supersonic flow, negative loading, and flow separation. For descent conditions, the radial and azimuthal locations of impulsive loading events related to blade-vortex interaction noise are determined and compared with predictions. Supersonic flow regions are examined in terms of their potential for generating high-speed impulsive noise.
Article
For the past 10 years, experimental and theoretical studies have been performed at the Aerodynamics Department of the Office National d'Etudes et de Recherches Aerospatiales (ONERA), in order to improve the capability to perform accurate measurements and to predict the characteristics of the flow around a helicopter blade. The different phases of the experimental work, as well as analytical studies concerning blade tip planform effects, are presented, and the main results obtained are analyzed. From a purely aerodynamic point of view, these results demonstrate that it is possible for high-speed rotor blades to define new blade tip shapes that should minimize some of the problems due to transonic flows such as an increase in the power required in the vibration level, or in the impulsive noise generated by the rotor.
Article
A Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM) incorporating fast vortex and fast panel methods for modeling complete rotorcraft has been developed and applied to the problems of rotor/wake/body interaction. The background and technical foundations of this model are reviewed and sample calculations are presented to demonstrate the capabilities of fast methods in these modeling applications. Correlations with test data are described that illustrate the performance of the analysis in capturing measured values of average rotor performance, rotor-induced fuselage loading, and wake geometry associated with interactional aerodynamics phenomena. Results suggest that these fast methods can provide good accuracy for a wide variety of key interactional aerodynamics problems with computation times compatible with routine use in rotorcraft design analysis tasks.
Article
This paper focuses on the validation of a hybrid computational fluid dynamics (CFD) method for the prediction of isolated rotor hover performance of the model UH-60A and tapered-tip variant rotors by comparison with available data sets. The hybrid method combines a local Reynolds-averaged Navier-Stokes solver to resolve the near-blade flow, with a vorticity-embedding potential flow solver for the wake flow field. The comparisons entail wake trajectories, surface pressures, integrated sectional thrust and torque, and rotor performance. The wake and loads comparisons demonstrate that the current method has a predictive accuracy and speed that is unusual to most CFD methods. The resulting performance comparisons are inconclusive owing partially to the fact that two available data sets do not agree to the level of accuracy required for validation. It appears therefore that development of hover CFD methods requires validation and test data in greater detail and variety than currently exists.
Article
Rotary-wing vehicles encounter a wide variety of complex aerodynamic phenomena and these phenomena present substantial challenges for computational fluid dynamics (CFD) models. This paper presents highlights from the past 30-years of CFD modeling and development to address these aerodynamic problems. During this time, a variety of CFD approaches have been used, ranging from early inviscid potential-flow methods from the 1970s and 1980s to more recent high-resolution solutions of the Reynolds-Averaged Navier-Stokes (RANS) equations for combined rotors and airframes. Historically, the choice of CFD methodology and the numerical resolution for the overall problem have been driven mostly by available computer speed and memory at any point in time. This fact is still largely true today. However the combination of ever-larger computers and improved CFD algorithms now allows the solution of a number of important rotorcraft aerodynamics problems. Future improvements in solution algorithms and computer speed will continue to make rotorcraft CFD solutions more robust and useful. However fundamental problems in solving for the rotor wake and in turbulence modeling remain challenges for the future.
Article
WITH THE DEVELOPMENT OF LASER VELOCIMETRY, SPECIALIZED FLOWVISUALIZATION TECHNIQUES, AND ANALYTICAL WAKE METHODS, MEASUREMENT AND PREDICTION OF DETAILED HELICOPTER AIRFLOW AND WAKE CHARACTERISTICS HAVE BECOME FEASIBLE. THIS WAS DEMONSTRATED IN A SERIES OF EXPERIMENTAL AND ANALYTICAL RESEARECH PROGRAMS DIRECTED TOWARD THE INVESTIGATION OF THE AIRFLOW AT SELECTED ROCKET TRAJECTORY, WIND SENSOR, AND ROTOR WAKE LOCATIONS OF A MODEL HELICOPTER SIMULATING HOVER AND LOW-SPEED FLIGHT CONDITIONS. CONSIDERING THE CURRENT INCREASED INTEREST INLOW-SPEED, LOW-LEVEL FLYING, RESULTS OF THESE INVESTIGATIONS ARE PRESENTED TO PROVIDE INSIGHT INTO THE AIRFLOW AND WAKECHARACTERISTICS ASSOCIATED WITH HOVER AND LOW-SPEED FLIGHT NEAR AND AWAY FROM THE GROUND. LASER VELOCIMETER AND FLOW VISUALIZATION DATA ARE PRESENTED AND COMPARED WITH THEORY TO PROVIDE INDICATIONS OF THE INFLUENCE OF AIR-SPEED, FLIGHT DIRECTION, FUSELAGE PRESENCE, THRUST LEVEL, GROUND EFFECT, AND BLADE-VORTEX INTERACTION ON ROTOR AIRFLOW VELOCITIES AND WAKE GEOMETRY AND TO DEMONSTRATE THEORY-TEST CORRELATION.
Article
The theoretical basis and the numerical implementation of free-vortex filament methods are reviewed for application to the prediction and analysis of helicopter rotor wakes. The governing equations for the problem are described, with a discussion of finite difference approximations to these equations and various numerical solution techniques. Both relaxation and time-marching wake solution techniques are reviewed. It is emphasized how the careful consideration of stability and convergence (grid-independent behavior) are important to ensure a physically correct wake solution. The implementation of viscous diffusion and filament straining effects are also discussed. The need for boundary condition corrections to compensate for the inevitable wake truncation are described. Algorithms to accelerate the wake solution using velocity field interpolation are shown to reduce computational costs without a loss of accuracy. Several challenging examples of the application are shown to reduce computational to helicopter rotor problems are shown, including multirotor configurations, flight near the ground, maneuvering flight conditions, and descending flight through the vortex ring state.
Article
Modeling of rotor-induced velocity receives continued attention in the literature as the rotorcraft community addresses limitations in the fidelity of simulations of helicopter stability, control, and handling qualities. A comparison is presented of results obtained using a rigid-blade rotor-fuselage model configured with two induced velocity models: a conventional, first-order, finite state, dynamic inflow model and a wake model that solves a vorticity-transport equation on a computational mesh enclosing the rotorcraft. Differences between the two models are quantified by comparing predictions of trimmed rotor blade Bap, lag and feather angles, airframe pitch and roll attitudes, cross-coupling derivatives, response to control inputs, and airframe vibration. Results are presented in the context of measurements taken on a Puma aircraft in steady flight from hover to high speed. More accurate predictions of the cross-coupling derivatives, response to control, and airframe vibration obtained using the vorticity transport model suggest that incorporation of real flowfield effects is important to extending the bandwidth of applicability of helicopter simulation models. Unexpectedly small differences in some of the trim predictions obtained using the two wake models suggest that an overall improvement in simulation fidelity may not be achieved without equivalent attention to the rotor dynamic model.
Article
Detailed measurements with a laser Doppler velocimeter (LDV) have been performed in the tip region and in the tip vortex core of a single-bladed mode rotor in hover. The testing was conducted at a rotor tip speed of 32 m/s, a Reynolds number of 269,000, and at two values of the rotor thrust coefficient, 0.0022 and 0.0057. Strobed laser sheet flow visualization was used to verify the steadiness of the tip vortex trajectory in the near wake and quantify the vortex trajectory to guide LDV surveys of the vortex core. A remotely aligned off-axis receiving optics system enabled measurement of vortex core velocity profiles at large focal lengths. The core self-induced velocity components extracted from these data are presented.
Article
We recently proved that a dissipative residual-based scheme of second-order accuracy is vorticity-preserving for the compressible Enter equations. In the present paper, this scheme is extended to curvilinear grids and applied to the computation of the interaction between a Scully vortex and a NACA0012 airfoil at a Mach number of 0.5. A grid convergence study and a comparison with a conventional scheme and with experimental measurements are presented. The new scheme shows a faster grid convergence, especially for the vortex trajectories and deformations during the interaction.
Article
Advanced concepts designed to improve the lift, drag, and pitching moment characteristics of rotor blades have been investigated for the purpose of enhancing rotor maneuver capability. The advantages and disadvantages of these concepts have been evaluated using both computational and experimental means. The concepts that were considered in this study included a leading-edge slat, a deformable leading-edge, and upper-surface blowing. The results show the potential of these concepts for substantially improving the performance of a rotor.
Article
Two fifth-order spatial weighted essentially nonoscillatory schemes for the convective terms were added to the OVERFLOW 2 implicit overset Navier-Stokes flow solver. The method used to incorporate the schemes is similar to a monotone upstream-centered scheme for conservation laws and requires no modification of the viscous terms, transport equations, or turbulence models in the code. The new flux calculation schemes were applied to problems involving vortex convection, strong shocks, and large scale unsteady flows. The weighted essentially nonoscillatory schemes were found to have much lower numerical dissipation/dispersion than traditional third-order spatial monotone upstream-centered schemes for conservation laws. Both weighted essentially nonoscillatory schemes were numerically robust over a wide range of Mach numbers when solved using the existing implicit schemes within OVERFLOW 2, The weighted essentially nonoscillatory schemes also provided improved numerical accuracy over traditional third-order spatial monotone upstream-centered schemes for conservation laws on the same computational grid for all the applications examined here. The weighted essentially nonoscillatory schemes are 10-30% more expensive than the third-order spatial monotone upstream-centered schemes for conservation laws depending on choice of implicit solver.