Article

The physics of modeling unsteady flaps with gaps

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Abstract

Research indicates that active control concepts have promise in mitigating numerous adverse phenomena associated with the aeromechanics of lifting surfaces. These techniques are being applied to delay stall of fixed wing aircraft, as well as to eliminate or mitigate vibratory loads, blade-vortex interaction, and dynamic stall of the flow about rotorcraft and wind turbine blades. These phenomena are nonlinear and unsteady for dynamic systems, which add yet another layer of complexity on the physics of the flow. While a plethora of different active control techniques is being explored, the use of trailing edge flaps appears to be one of the more viable and cost-effective concepts. Static multi-element airfoils and wings have been analyzed computationally, but little exists on the ability to model these when the airfoil and flap are dynamic. The costs associated with modeling the gap between the airfoil and flap have led to approximations where the flap is modeled only as a morphed tip of the airfoil (no gap). Using a hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy-Simulation turbulence technique, an oscillating flapped airfoil has been studied to determine the influence of modeling the gap on the performance and acoustic signature of the airfoil. Results are compared with the experimental data to confirm the validity of the computational approach. Both attached and separated (dynamic stall) oscillating flows are examined. The physics within the gap are found to be important for the airfoil performance when stall is encountered, as well as when acoustic signatures are required.

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... Using the Chimera technique and a rigid motion to rotate a flap requires considering narrow gaps between the meshes in both flow and span directions, that is, the control surface is "discrete". Although the presence of a gap in flow direction helps to delay stall, the large performance losses at low angles of attack, which usually occur in cruise flight, discourage using discrete flaps [6]. In reality, the gap is usually sealed. ...
... Computations have been performed for Ma ∞ = 0.3, Re ∞ = 30•10 6 and δ = 0°. All simulations have been carried out with the negative version of the Spalart-Allmaras model [16], which is the standard one-equation turbulence model in TAU. ...
... The computational mesh is unstructured and has 62.4•10 6 nodes. As shown in figure 18, it consists in a main mesh block for the wing (drawn in yellow), a mesh block around the tip area (in green) and a mesh block for the split flap which is inside the former one (in pink). ...
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The simulation of movable control surfaces is of interest for many applications in aerospace engineering, but it is challenging to perform high-fidelity computations considering them. Generating a new mesh for each deflection angle is computationally expensive, so alternative approaches that allow using only one mesh have been developed in the recent years. This work gives an overview of the available methods for modelling of control surfaces, with special focus on the ones that consider the spanwise gaps between the wing and the control surface. This includes the usage of the Chimera technique combined with mesh deformation as well as a sliding interfaces boundary condition. Their performance has been tested with the DLR MULDICON configuration.
... The configuration of a typical trailing-edge flap is shown in Fig. 17. The use of a fixed flap with a slot for flow control was initially proposed by Page and Glauert in the 1920s, and the primary aim of this technique is to achieve a large lift for an aircraft at a relatively low flying speed [121]. As shown in Fig. 18, a jet-like flow emanating from the gap injects additional turbulence and vortices into the downstream flow over the main airfoil, resulting in the delay or even elimination of the trailingedge vortex. ...
... The airfoil-flap gap for NACA 0012[121].Z.Zhao et al. ...
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... As a result, the use of aerodynamic control devices distributed along a wind turbine blade is of great importance in modern wind turbines to control and alleviate the non-uniform loads on the rotor blades 2 . Active control techniques are being applied to delay stall occurrence in aircraft wings, as well as to mitigate vibratory loads and control dynamic stall phenomenon on wind turbine blades 3,4 . 19 (the former for experimental validation and the latter for numerical verification) is considered as the benchmark for pure-pitching motion of an S809 airfoil, so that the airfoil oscillation in this case represents the unsteady motion of the airfoil in the near-stall condition encountering light dynamic stall. ...
... In rotorcraft engineering, the classic plain TEF is implemented which rigidly oscillates around its hinge point. Several experimental and numerical studies have been carried out to investigate the effect of TEFs in rotor blades, helicopter blades in particular4,14,15 . In contrast to the traditional rigid TEFs in rotorcraft engineering, the modern wind turbine blades are equipped with deformable flaps, called Deformable Trailing-Edge Flap (DTEF), attached to the airfoil which have smooth and continuous deformation. ...
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Due to the unsteady nature of the flow around horizontal-axis wind turbines, the blades are subjected to severe unsteady and fatigue loads. This necessitates an in-depth aerodynamic analysis of flow control techniques to enhance the performance of a wind turbine as well as the lifetime of its components. Using OpenFOAM package in this study, a series of two-dimensional incompressible simulations are performed to present a deeper insight into the aerodynamic characteristics of an oscillating deformable trailing-edge flap, as a promising flow control device, in a sinusoidal pitching motion of an S809 airfoil. Herein, it is of particular interest to investigate the effects of deformable trailing-edge flap size, oscillation frequency, and the phase shift with reference to airfoil motion on lift and drag hysteresis loops. For this purpose, a pure-pitching motion of an S809 airfoil without flap deflection is considered as the benchmark problem in which the airfoil oscillates in the near-stall region at Re=106. After validation and verification of our simulations through comparison against the corresponding experimental and numerical work, a comprehensive investigation is conducted to study the effects of the aforementioned parameters on the aerodynamic loads. Our results reveal the fact that an out-of-phase deflection of the deformable trailing-edge flap with a frequency equal to the airfoil frequency can significantly mitigate the fatigue load. Under these circumstances, an increase in the deformable trailing-edge flap size can also help the airfoil experience less-severe loads in a cycle of motion. Furthermore, higher values of deformable trailing-edge flap frequency or other values of phase shift except the out-of-phase oscillation cannot alleviate fatigue loads. An airfoil under these conditions can, however, enhance the resultant load required for a blade rotation in the case of low wind periods.
... Much of the recent work focuses either on aerial application of a conventional plain TEF [21][22][23] or on aeroelastic behavior and control strategy of a DTEF. From aerodynamic point of view, Troldborg et al. [24] performed numerical simulations to study the influence of a DTEF on aerodynamic performance of an oscillating RisØ-B1-18 airfoil. ...
Article
Unsteady operating environment of a horizontal axis wind turbine can induce excessive loads on the blades, originating from rapid variations in angle of attack and consequently dynamic stall (DS) occurrence. Therefore, it is of utmost importance to control the flow around a blade by which fatigue damage is likely to happen. Using two-dimensional incompressible unsteady Reynolds-averaged Navier–Stokes equations in OpenFOAM package, a series of simulations are carried out to assess the viability of an oscillating deformable trailing-edge flap (DTEF) in load and DS control on a pitching wind turbine airfoil which experiences deep DS at Re = 420,000. Results reveal whether or not the airfoil is equipped with an oscillating DTEF, DS vortex forms at high angles of attack. The size, strength and traveling of the DS vortex, however, can be influenced by out-of-phase deflection of the DTEF. More effectively, the change in the airfoil camber line during flap oscillation can remarkably affect the pressure distribution around the airfoil, and hence, significant load alleviation and mean lift enhancement are achievable, all of which help the wind turbine performance and enhance the life span of the components. Moreover, a parametric study on flap size and amplitude of deflection together with a comparison between a discrete flap and a DTEF suggests an out-of-phase oscillation of a large gently curved DTEF, up to 30% of the total chord, with similar amplitude and frequency with respect to the airfoil is the best condition under which fatigue load control as well as enhancement in resultant load for a blade rotation can take place.
... The flap is hinged at the lower surface of the carbon fiber wing and can move in upward and downward directions over a range of at least 7251. Gap flow between the wing and flap as reported by Liggett and Smith (2013) is prevented by seals covering the gaps on both the suction and the pressure side. The manufacturing tolerance is about 1 mm, meaning that the actual shape differs from the DU96-W-180. ...
... A NACA 0012 wing section with a harmonically deflecting TEF was tested in a subsonic wind tunnel by Krzysiak et al. [17] who demonstrated an increase in Cl, max when both the angle of attack of the airfoil and flap deflection angle increase simultaneously. Liggett et al. [22] investigated the impact of an oscillating flap with and without flap gap using a hybrid RANS/LES turbulence model. It was found that the presence of the gaps caused a decrease in aerodynamic performance due to flow recirculation and further confirmed some earlier findings that the oscillating movement drives the unsteadiness in the flow. ...
Article
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... The use of high-lift devices extends to the motorsport industry as well, in which they are optimized in-ground proximity for better handling qualities of modern racing cars [1]. More recently, Liggett and Smith [2,3] showed the importance of understanding the flow physics of multi-element airfoils in active flow control applications using unsteady flaps. ...
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noise has proven to be a useful guide for elucidation of the physics of flow-induced noise generation over the last five years. This process, relying on a close interplay between experiment and computation, is described and demonstrated here on the archetypal problem of flap-edge noise. Some detailed results from both experiment and computation are shown to illustrate the process, and a description of the multi-source physics seen in this problem is conjectured.
Conference Paper
Effects of trailing-edge flap gaps on rotor performance are investigated using a high fidelity, coupled computational fluid dynamics (CFD) - computational structural dynamics (CSD) analysis. Both integral flap (the flap is an integral part of the blade such that there are no physical gaps at the flap ends) and discrete flap (the flap is a separate entity with physical gaps in the spanwise and chordwise directions) are examined on an UH-60A rotor at high speed forward flight condition. A novel grid deformation scheme based on the Delaunay graph mapping is developed and implemented to allow the CFD modeling of the gaps with minimal distortion of mesh around the flap gap regions. This method offers an alternative to the traditional approach of modeling such configurations using overset meshes. The simulation results show that the effectiveness of the flap is minimally lost with spanwise gaps - the penalty on rotor performance is of the order of 1% compared to the integral flap. On the other hand the chordwise gaps significantly degrade the benefits of active flap on rotor performance due to the flow penetration between the upper and lower surfaces of the flap.
Conference Paper
Stereoscopic PIV technique is applied to a low-speed flow around a simple wing-flap configuration in a large-scale wind tunnel to demonstrate the applicability and capability of the technique to flow fields around high-lift systems. Also, CFD analyses by a Reynolds-averaged Navier-Stokes code are conducted to evaluate usefulness of PIV in the CFD code validation. Test results suggest usefulness of the stereoscopic PIV for investigation of complex, three-dimensional flow fields around a high-lift device configuration, including tip vortices while two-dimensional PIV might be more suitable for detailed measurements of near-wall flow such as slot and cove flows. Comparisons between the PIV and CFD results show that the stereoscopic PIV is a useful tool for validation of CFD code applied to high-lift device design while improvements in terms of measurement accuracy and capability of measuring near-wall velocity are required. Especially, focusing on flows featuring slot and wake flows of the flap, which are considered sensitive to CFD solver and turbulence model, is appropriate in critical CFD validations. Based on the velocity field measurements for different model configurations, it is inferred that the flow field around a flap is sensitive to both flap slot gap and flap deflection angle in the parameter ranges tested in this study, and therefore, large gap height or deflection angle easily results in a large-scale separation with a highly turbulent recirculation zone on the flap upper surface.
Article
The ability to predict SMART active trailing edge flap rotor loads is explored in this study. Full-scale wind tunnel data recently acquired in the NASA Ames 40-by 80-Foot Wind Tunnel are compared with analytical results from CAMRAD II. For the 5-bladed rotor, two high-speed forward flight cases are considered, namely, a 0 deg flap deflection case and a 5P, 2 deg flap deflection case. Overall, the correlation is reasonable, with the following exceptions: the torsion moment frequency and the chordwise bending moment are underpredicted. In general, the effect of the 5P, 2 deg flap motion is captured by the analysis, though there is overprediction in the neighborhood of the 105 deg and 120 deg azimuthal locations. Changes to the flexbeam torsion stiffness are also briefly considered in this study, as this stiffness will be updated in the future. Finally, the indication is that compressibility effects are important, and this suggests that computational fluid dynamics might improve the current correlation.
Article
To address the complex multidisciplinary nature of rotorcraft analysis, high-fidelity computational fluid and structural dynamics models have been developed and coupled for an advanced technology active rotor. Significant advancements have been made in both modeling disciplines to allow for complex bearingless flapped rotors. Comparisons are made between CFD/CSD and comprehensive (lifting-line, free-wake) analyses and experimental data for the Boeing SMART rotor. Flap phase sweeps for 0, 2, 3, 4, and 5/rev flap inputs are investigated in relation to the zero flap deflection baseline at a nominal cruise condition. Changes in performance, aerodynamic and structural loads, control power, and noise are studied. Details of the high-fidelity flowfield solution, including flap gap effects and wake visualizations, are also presented.
Conference Paper
The vortex shedding caused by compressible subsonic flow along a wall cavity has been investigated using a Large-Eddy Simulation (LES)-based turbulence modeling technique that is embedded within a legacy Reynolds-Averaged Navier-Stokes (RANS) solver to assess the improvement in the prediction of the flow field and acoustic of cavity flows beyond the application of classic RANS turbulence models. Numerical simulations applying two-equation Kinetic-Energy Simulation (KES), sub-grid scale hybrid-RANS LES (HRLES-sgs), and Menter k - w shear-stress transport (SST) turbulence methods have been carried out and compared with experiment and LES results. Important frequencies of the flow are determined, illustrating the abilities of advanced turbulence modeling to improve these predictions when compared to RANS models. Evaluation of the influence of the grid, time step and simulation period shows the sensitivity of the predictions to these parameters.
Article
The vortex shedding generated by compressible subsonic flow interacting with a wall cavity has been investigated using large-eddy-simulation-based turbulence techniques embedded within a legacy Reynolds-averaged Navier-Stokes solver. Cavity simulations using hybrid turbulence approaches seek the accuracy of large-eddy simulation by providing filtering and modeling of subgrid-scale turbulence with the cost of traditional Reynolds-averaged Navier-Stokes. Simulations applying differing techniques of hybridization of the Menter k-omega shear stress transport Reynolds-averaged Navier-Stokes approach include detached eddy simulation (DES-SST), blended subgrid-scale turbulence models (GT-HRLES), and a self-adjusting large-eddy-simulation very-large-eddy-simulation technique (KES) provide an understanding of differing hybrid approaches. Cavity flow results from Reynolds-averaged Navier-Stokes and hybrid simulations are compared with experiment and large-eddy simulation predictions. Evaluation of important flow characteristics illustrates the abilities of these advanced turbulence modeling techniques compared with traditional Reynolds-averaged Navier-Stokes models. Examination of the influence of the grid, time step, and simulation period demonstrates the sensitivity of the aerodynamic and aeroacoustic predictions to these parameters. In particular the subgrid-scale blended model, GT-HRLES, shows significant improvement in the ability to capture the acoustic signatures and flowfield features on a Reynolds-averaged Navier-Stokes or very-large-eddy-simulation grid compared with the other models.
Article
Current rotorcraft research to increase flight speed or to alleviate adverse physical phenomena expand the Mach/angle-of-attack envelope in which the rotor blades operate. For example, rotor blades will experience large areas over the rotor disk where reverse-flow effects cannot be neglected during the design and analysis of an efficient rotor at high advance ratios. A cost-effective alternative to extensive experimental analyses is the use of computational fluid dynamics codes to quantify the behavior of airfoils at high and reverse angles of attack, as well as to add to the knowledge of the behavior of airfoils when they are immersed in these flows. Numerical experiments have been performed with correlation to experimental databases that examine the ability of computational fluid dynamics to accurately model airfoil characteristics at these angles of attack. It is observed that the use of recently developed hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation turbulence methods result in a significant improvement in the ability of computational fluid dynamics to predict the characteristics of airfoils in these angle-of-attack regimes. Modeling of the airfoil trailing edge is more sensitive when reverse-flow angles of attack are considered.
Article
The effects of a harmonically deflected trailing-edge flap, actuated at different start times and amplitudes but with frequency different from the airfoil motion, on the aerodynamic loads of an oscillating NACA 0015 airfoil were investigated experimentally at Re=2.51×105. Both in-phase and 180° out-of-phase flap deflections, relative to the airfoil motion, were tested. The results show that there was a large change in the hysteretic behavior of the dynamic load loops, and that the formation and detachment of the leading-edge vortex (LEV) were not affected by the flap motion, while the low pressure signature of the vortex was affected by the flap actuation start time. The later the flap actuation the larger the change in the strength of the LEV. The present flap control scheme was also found to be as effective as that achieved by a pulsed ramp flap motion, but with a reduced number of control parameters.
Article
A preliminary study of the control of the dynamic-load loops of an oscillating NACA 0015 airfoil through an 18%c LEF (leading-edge flap) and a 25%c TEF (trailing-edge flap), deflected dynamically and independently, was conducted. Both upward and downward deflections actuated at 1 st = Oμ were tested to maximize the effects of the flap motion on the behavior of the dynamic-stall vortex (DSV). The downward LEF motion suppressed leading edge separation and eliminated the occurrence of the DSV, leading to a minor reduction in C i, maxbut a considerably improved poststall lift condition, compared with the baseline airfoil. On the other hand, the upward LEF motion caused an earlier formation and shedding of the DSV and, subsequently, an increased C i,max and negative peak C m value. The downward TEF deflection always increased the lift. The formation and detachment of the DSV was, however, largely unaffected.
Article
In this paper an extensive parametric study concerning the effect of flap and slat riggings on the 2-D high-lift flow past a three-element airfoil system is presented. The numerical approach for solving the Reynolds-averaged Navier-Stokes equations uses an implicit finite volume scheme of second order accuracy in space on a patched multizonal grid. The Spalart-Allmaras one-equation turbulence model is employed. Six design parameters have been investigated comprising the deviation, gap, and overhang of the slat and of the flap whose settings are centered about the values in practice for takeoff. Good agreement with experiments is obtained for prestall angles of attack. Computations show that both C-l and C-l/C-d have an optimum with every design parameter. The trends in the high-lift flow observed are in accordance with both experiments and computations reported in the literature. C-l and C-l/C-d are found to be more sensitive to deviations and gaps than to overhangs.
Article
Unsteady aerodynamic loads on NACA 0012 airfoil with a trailing-edge flap were measured in wind tunnel and calculated from a simple theoretical model. The airfoil model of 0.18 m chord length used in wind-tunnel test was oscillating in pitch about an axis located at 35% chord length from the airfoil leading edge. The length of trailing-edge flap was 22.6% of airfoil chord. The Hap was also deflecting harmonically, but with frequency different from airfoil pitching motion. The influence of phase delay between airfoil angle of incidence and flap deflection at the beginning of the motion was considered. The theoretical method used for calculation of unsteady airfoil loads is based on two-dimensional, inviscid, incompressible flow model at subsonic Mach numbers. The expressions for unsteady aerodynamic loads calculations on the airfoil and on the flap were obtained in a closed form using distribution of flow velocity potential along the airfoil chord and along the flap length. Lift and aerodynamic moment measured in the wind tunnel were compared with results of calculations. The correlation between experimental and theoretical results is adequate.
Article
A multielement airfoil designed for helicopter application has been tested for compressible dynamic stall behavior and has been proven to he a robust dynamic stall-free concept. This slotted airfoil has operated into poststall areas without the dynamic stall vortex that is normally present whenever airfoils are tested beyond their static stall boundary. Point diffraction interferogram images of the dynamic flow over the airfoil are presented, showing details of the flow development during the oscillation cycle, and instantaneous pressure distributions on the airfoil and slat during dynamic airfoil motion are included.
Article
Vortex shedding and the associated noise radiation from a trailing edge were experimentally investigated for a leading-edge slat of a multi-element airfoil at stowed chord Reynolds number Re < 5:9 × 105. A particular focus was on the competition between the instability of the slat boundary layer excited by acoustic feedback and the absolute instability of the wake. Periodic vortex shedding was observed to occur from the slat trailing edge at the Reynolds numbers examined. For Re < 1:9 × 105, the vortex shedding is governed by the absolute instability of the laminar wake of the slat without any distinct tonal noise radiation. For Re > 2:1 × 10 5, however, acoustic feedback becomes pronounced between the trailing-edge noise and the boundary-layer instability waves on the suction surface, so that multiple spectral peaks appear both in the velocity fluctuations and sound pressure. At and around the Reynolds number for the first appearance of tonal noise, Re = 1:9 × 105, both of the instability modes coexist. Beyond Re = 2:1 × 105, the boundary-layer instability waves excited by the acoustic feedback evolve into high-intensity vortices before reaching the trailing edge and suppress the absolute instability of the wake through diminishing the reversed-flow region in the wake. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.
Article
A zonal hybrid Reynolds-averaged Navier-Stokes large-eddy simulation (RANS/LES) approach, called zonal-DES, used to handle a two-dimensional high-lift configuration with deployed slat and flap is presented. This method allows to reduce significantly the cost of an accurate numerical prediction of the unsteady flow around wings compared to a complete LES. Some issues concerning grid generation as well as the use of zonal-detached-eddy simulation for a multi-element airfoil are discussed. The basic planar grid has 250,000 points and the finest spanwise grid has 31 points with Delta z/c = 0.002. The effort is geared toward detailed comparison of the numerical results with the Europiv2 experimental particle image velocimetry data including both mean and fluctuating properties of the velocity field (Arnott, A., Neitzke, K. P., Agocs, J., Sammer, G., Schneider, G., and Schroeder, A., "Detailed Characterisation Using PIV of the Flow Around an Aerofoil in High Lift Configuration," EUROPIV2 Workshop on Particle Image Velocimetry, Springer, Berlin, 2003). The results also provide an insight into the real unsteady nature of the flow around a three-element airfoil that cannot be reproduced by classical RANS models. The current calculation displays extremely complex flow dynamics in the slat and flap coves like the ejection process through the gaps oiseveral vortices issued from the impingement of the free shear layers on the lower walls of the different elements.
Article
The control of cavity flows has been investigated by the means of Large Eddy Simulations. The computations have been carried out on unstructured meshes to assess the efficiency of two passive acoustic oscillation suppression devices: the rod-in-crossflow and the flat-top spoiler. Despite a sustained interest and many experiments, a clear explanation for observed reduction in the flow-induced structure load is still missing. This work explores different hypotheses: the modification of the mean field and its linear stability properties, a pure deflection effect of the separated shear layer, or scale coupling between the rod wake and the turbulent mixing layer over the cavity. The aim here is to enhance the experimental database and provide leads towards a better understanding of the phenomena. The selected test-case is a cavity of length/depth ratio equal to 5, at Mach and Reynolds number of M∞=0.85 and ReL=7.106, respectively.
Article
Stall hysteresis discovered in the wind-tunnel performance of a GA(W)-2 airfoil with a 25% chord slotted flap is examined further by using the data obtained for lift, pitching moment, surface pressure distribution, and the hot-film velocity vector. Test cases include 30- and 40-deg flap deflections, each having an optimum and narrow gap at a chord Reynolds number of 2.2 x 10(6) and a Mach number of 0.13. The flap optimized to produce the highest C-Imax for each flap angle apparently did not have a proper contour and nose location for the slot flow to function effectively at off-design conditions. It is shown that suction pressures over the flap, suppressed by a thickening wing wake at stall, are not reversible to their prestall values within the decreasing alpha side of loop. It is suggested that the flap design include the use of a new flap parameter called the slot flow angle to describe the slot flow orientation and a pressure recovery factor to select a proper contour for the flap upper surface.
Article
A investigation has been made in the N.A.C.A. 7-by-10 foot wind tunnel of a large-chord N.A.C.A. 23012 airfoil with several arrangements of venetian-blind flaps to determine the aerodynamic section characteristics as affected by the over-all flap chord, the chords of the slats used to form the flap, the slat spacing, the number of slats, and the position of the flap with respect to the wing. Complete section data are given in the form of graphs for all the combinations tested. The optimum arrangement of the venetian-blind flap was a combination in which the flap was located near the wing trailing edge. These arrangements of the venetian-blind flap were superior to any flaps previously tested for producing lift and giving low drag coefficients at high lift coefficients. The wing with this flap, however, had very large pitching- moment coefficients. When operated as split flaps, the venetian-blind flaps were inferior to the simple split flap in producing lift.
Article
Results of a study using a passive approach to recover the loss of lift that occurred when a variable droop leading edge (VDLE) airfoil was used to successfully control compressible dynamic stall by attaching a small Gurney flap to its trailing edge are reported. Gurney flaps of different heights were tested. The airfoil performance was evaluated by measuring the unsteady pressures while it executed a sinusoidal pitch-up maneuver over a range of Mach numbers from 0.2 to 0.4, at different reduced frequencies, with both static and dynamic leading edge droops. Not only was the “lost“ lift recovered completely with a 1% chord-height Gurney flap, the drag and moment coefficients were also dramatically reduced and a lift-to-drag ratio greater than 10 was achieved, making it an acceptable choice for this purpose. The improved performance is explained through the basic fluid mechanics of the problem by discussing the various pressure distributions and the surface vorticity fluxes derived from these.
Article
The ability to model accurately and efficiently unsteady aerodynamic effects for 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 computational fluid dynamics (CFD) codes and a CFD-based reduced-order model (ROM). This ROM is based on the rational function approximations approach, which yields a state-space, time-domain aerodynamic model suitable for incorporation into comprehensive rotorcraft simulation codes. The accuracy of this model is demonstrated across a practical range of unsteady flow conditions encountered by active flaps. Two Reynolds-averaged Navier-Stokes solvers (CFD++ and OVERFLOW) are employed in conjunction with various turbulence models, including large eddy simulation based models, so as to examine code independence. Flow physics associated with three-dimensional effects, flap hinge gap, as well as compressibility effects are also examined.
Article
Turbulent flows over lifting surfaces exhibiting trailing-edge vortex shedding often cause adverse and complex phenomena, such as self-induced vibration and noise. In this paper, a numerical study on flow past a blunt-edged two-dimensional NACA 0015 section and the same section with various base cavity shapes and sizes at high Reynolds numbers has been performed using the unsteady Reynolds-averaged Navier–Stokes (URANS) approach with the realisable κ–ε turbulence model. The equations are solved using the control volume method of second-order accuracy in both spatial and time domains. The assessment of the application of URANS for periodic trailing-edge flow has shown that reasonable agreement is achieved for both the time-averaged and fluctuating parameters of interest, although some differences exist in the prediction of the near-wake streamwise velocity fluctuation magnitudes. The predicted Strouhal numbers of flows past the squared-off blunt configuration with varying degrees of bluntness agree well with published experimental measurements. It is found that the intensity of the vortex strengths at the trailing-edge is amplified when the degree of bluntness is increased, leading to an increase in the mean square pressure fluctuations. The numerical prediction shows that the presence of the base cavity at the trailing-edge does not change the inherent Strouhal number of the 2D section examined. However, it does have an apparent effect on the wake structure, local pressure fluctuations and the lift force fluctuations. It is observed that the size of the cavity has more influence on the periodic trailing-edge flow than its shape does.
Article
Measured, open loop and closed loop data from the SMART rotor test in the NASA Ames 40-by 80-Foot Wind Tunnel are compared with CAMRAD II calculations. One open loop high-speed case and four closed loop cases are considered. The closed loop cases include three high-speed cases and one low-speed case. Two of these high-speed cases include a 2 deg flap deflection at 5P case and a test maximum-airspeed case. This study follows a recent, open loop correlation effort that used a simple correction factor for the airfoil pitching moment Mach number. Compared to the earlier effort, the current open loop study considers more fundamental corrections based on advancing blade aerodynamic conditions. The airfoil tables themselves have been studied. Selected modifications to the HH-06 section flap airfoil pitching moment table are implemented. For the closed loop condition, the effect of the flap actuator is modeled by increased flap hinge stiffness. Overall, the open loop correlation is reasonable, thus confirming the basic correctness of the current semi-empirical modifications; the closed loop correlation is also reasonable considering that the current flap model is a first generation model. Detailed correlation results are given in the paper. Notation c m Pitching moment coefficient C T Helicopter thrust coefficient KTEF Flap hinge stiffness, ft-lb/rad M Mach number NP Integer (N) multiple of rotor speed Per rev Per revolution RmPtn NASA wind tunnel Run "m" Point "n" α Angle of attack α s Rotor shaft angle µ Rotor advance ratio σ Rotor solidity ratio Sign Convention Chordwise moment, + tip toward trailing edge. Flap deflection, + trailing edge down. Flap lift, + up; flap chordwise force, + toward leading edge. Flatwise moment, + tip up. Pitch link load, + in tension. Torsion moment, + leading edge up.
Article
Modern wind turbines are steadily increasing in size, with recent models boasting rotor diameters greater than 120 m. Wind turbines are subjected to significant and rapid fluctuating loads, which arise from a variety of sources including turbulence, tower shadow, wind shear and yawed flow. Reducing the loads experienced by the rotor blades can lower the cost of energy of wind turbines. ‘Smart rotor control’ concepts have emerged as a solution to reduce fatigue loads on wind turbines. In this approach, aerodynamic load control devices are distributed along the span of the blade, and through a combination of sensing, control and actuation, these devices dynamically control the blade loads. This research investigates the load reduction capabilities of smart rotor control devices, namely trailing edge flaps (TEFs), in the operation of a 5 MW wind turbine. A feedback control approach is implemented for load reduction, which utilizes a multiblade coordinate transformation. Single input–single output control techniques are employed to determine the appropriate response of the TEFs based on the blade loads. The use of TEFs and this control approach is shown to effectively reduce the fatigue loads on the blades, relative to a baseline controller. The load reduction potential is also compared to an alternative individual pitch control (IPC) approach, in the time and frequency domain. The effects on the pitch and power systems are briefly evaluated, and the limitations of the analysis are assessed. Finally, a combined approach that uses both TEFs and IPC is evaluated. Copyright
Article
Detailed measurements on mean flow and turbulence around a multielement airfoil model have been made using pressure and hot-wire probes. The results obtained in two test cases at chord Reynolds number of 3 x 10 exponent 6 and freestream Mach number of 0.2 show a number of features of the complex flows that are important for accurately modeling these flows by numerical methods. Many parts of the shear flow deviated vastly from classical flows, and the interaction with the external flow is very strong.
Conference Paper
Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carried out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving laminar separation bubbles. One of the methods computes transition locations across laminar separation bubbles whereas the other assumes transition onset where laminar separations occur. For validation of the numerical methods an extensive measurement campaign has been carried out. It is shown, that the methodology mentioned first can simulate the size of laminar separation bubbles for angles of attack up to where the separation bubble and the turbulent separation at the trailing edge are well behaved and steady in the mean. With trailing edge separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Cases with large 3D flow structures inside the turbulent trailing edge separations in windtunnel experiments for high angles of attack are compared and analysed along with 2D and 3D steady RANS calculations that model the measurement section of the windtunnel.
Article
The self-excitation mechanism of the acoustic diametral modes of an axisymmetric internal cavity–duct system is studied for a Mach number range up to 0.4. The effect of cavity dimensions on the excitation mechanism is investigated experimentally and numerically. Experiments are conducted on three cavity depths and six cavity lengths for each depth. Numerical simulations of the mode shapes are also performed to determine the effect of cavity dimensions on the particle velocity field of the diametral modes. For all the tested configurations, the diametral modes are strongly excited at relatively low Mach numbers (as low as 0.1). The pulsation amplitude at resonance is found to increase as the cavity becomes shorter or deeper, relative to the main pipe diameter. The test results provide new insights into the excitation mechanism of diametral modes, the effect of the cavity length to depth ratio on the Strouhal numbers of acoustic resonances caused by various shear-layer modes of the cavity, and into the effect of the particle velocity field of the acoustic modes on the mode selectivity mechanism which determines the dominant acoustic mode during resonance.
Article
An experimental study was made of the flow regimes associated with the cove regions of a multielement airfoil. These regions occur when the leading edge slats and trailing edge flaps on the wings are extended to generate high lift during takeoff and landing of the aircraft. The results include mean velocity and turbulence intensity data, measured between the slat and wing and between the wing and flap, together with comprehensive pressure distributions around the three airfoil components. This information should be of value in the development of numerical models for predicting the flow around high-lift airfoils.
Article
Experiments have been performed at low speeds documenting certain broad aspects of hysteresis on a mildly swept wing under high-lift conditions. Aerodynamic load measurements were carried out at two values of incidence, 10.5 and 15.5 deg, and flap deflections of 20 and 30 deg, essentially under quasi-steady conditions. Two-dimensional particle image velocimetry (PIV) was utilized to measure the mean velocity vector field in the rear part of the wing and flap. These velocity measurements have revealed the complex nature of separated flows on the wing and flap. Also, in general, the slot flow angle determined by geometric considerations is very different from those actually inferred from PIV measurements.
Article
Several issues relating to the application of Chimera overlapped grids to complex geometries and flowfields are discussed. These include the addition of geometric components with different grid topologies, gridding for intersecting pieces of geometry, and turbulence modeling in grid overlap regions. Sample results are presented for transonic flow about the Space Shuttle launch vehicle. Comparisons with wind tunnel and flight measured pressures are shown.
Analysis of Development of Dynamic Stall based on Oscillating Airfoil Experiments
  • L Carr
  • K Mcalister
  • W Mccroskey
Carr, L., McAlister, K., McCroskey, W., 1977. Analysis of Development of Dynamic Stall based on Oscillating Airfoil Experiments. Technical Report NASA TN D-8382. NASA.
Systematic Investigations of the Effects of Planform and Gap Between the Fixed Surface and Control Surface on Simple Flapped Wings
  • B Gothert
  • C Rober
Gothert, B., Rober, C., 1949. Systematic Investigations of the Effects of Planform and Gap Between the Fixed Surface and Control Surface on Simple Flapped Wings. Technical Report TM-1206. NACA.
Boeing-Smart Test Report for DARPA Helicopter Quieting Program Unsteady airfoil with a harmonically deflected trailing-edge flap
  • B Lau
  • N Obriecht
  • T Gasow
  • B Hagerty
  • K Cheng
  • B Sim
Lau, B., Obriecht, N., Gasow, T., Hagerty, B., Cheng, K., Sim, B., 2010. Boeing-Smart Test Report for DARPA Helicopter Quieting Program. Technical Report TM 2010-216404. NASA. Lee, T., Su, Y., 2011. Unsteady airfoil with a harmonically deflected trailing-edge flap. Journal of Fluids and Structures 27, 1411–1424.
Active Flap Control of the Smart Rotor for Vibration Reduction
  • S R Hall
  • R V Anand
  • F K Straub
  • B H Lau
Hall, S.R., Anand, R.V., Straub, F.K., Lau, B.H., 2011. Active Flap Control of the Smart Rotor for Vibration Reduction. Technical Report ARC-E-DAA-TN590. NASA.
non-contoured gaps are not recommended for design due to large associated performance losses at low angles of attack Flow-excited resonance of trapped modes of ducted shallow cavities
  • K Ziada
Although gap flows can delay stall, non-contoured gaps are not recommended for design due to large associated performance losses at low angles of attack. References Aly, K., Ziada, S., 2010. Flow-excited resonance of trapped modes of ducted shallow cavities. Journal of Fluids and Structures 26, 92–120.
Zonal hybrid RANS-LES method for static and oscillating airfoils and wings Flow regimes in the cove regions between a slat and wing and between a wing and flap of a multi-element airfoil
  • M Sanchez-Rocha
  • M Kirtas
  • S Menon
Sanchez-Rocha, M., Kirtas, M., Menon, S., 2006. Zonal hybrid RANS-LES method for static and oscillating airfoils and wings. In: 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV. Savory, E., Toy, N., Tahouri, B., Dalley, S., 1992. Flow regimes in the cove regions between a slat and wing and between a wing and flap of a multi-element airfoil. Experimental Thermal and Fluid Science 5, 307–316.