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Fluid Dynamics of a High Aspect-Ratio Jet
... All of these points are consistent with the results of Munro and Ahuja. 21 At very slow speeds, where the jet appears to be quasi-planar for the entire visible region, some notable oscillations are observed downstream. These disappear as the flow speed increases, however, it is suspected that this is due to the oscillations increasing in frequency to a rate the camera could no longer detect. ...
... The flow visualisation results ( figure 11) show that the downstream flow-field changes significantly with jet-speed, confirming the results of Munro and Ahuja. 21 The results also highlight an optical phenomena unseen in previous jet-noise studies, although it remains to be seen whether this is useful or merely parasitic. ...
This paper covers the design, analysis, manufacture and testing of an experimental rig for aeroacoustic measurements on high aspect ratio rectangular jets. The design of the rig is detailed with emphasis given to flow control and acoustic characteristics. Results of the far-field noise spectra and directivity are presented from the initial tests. These show that the jet noise is notably directional about both the major and minor jet axes and that the peak frequency component is weakly dependant on both aspect ratio and Mach number. The 'zone of silence' close to the streamwise axis can also be seen clearly. Flow visualization tests were also conducted that showed how the flow develops close to the nozzle.
... Two aspects could yield significant improvements in this respect: (a) the possibility to avoid the use of gaps, thus reducing noise emissions [110], and (b) steep climb-out and approach flight paths, which reduce noise exposure to surrounding communities [40]. Experimental measurements of the noise generated by a high aspect-ratio jet were carried out by Munro et al. [111,113,112]. The experimental set-up was designed to investigate the flow dynamics and the aeroacoustic characteristics of a free jet, with the possibility of varying its width and thickness. ...
Active flow control applied to high-lift systems is a promising solution to improve low-speed flight capabilities and reduce noise emissions of commercial aircraft. However, too high power requirements in relation to the achieved lift gains have prevented active high-lift systems from being largely employed in the aeronautical industry. In this context, this work develops technologies to enhance the aerodynamic efficiency of an active high-lift system by means of RANS numerical simulations. The transonic airfoil DLR-F15 is equipped with an active internally-blown flap, which consists of a thin air jet tangentially blown over the shoulder of a simple-hinged flap deflected by 65°. To improve the lift generated by the airfoil, the effects of a flexible droop-nose device, wall suction and unsteady blowing are investigated. The fundamentals of gap-less droop-nose design are presented, describing the aerodynamic sensitivities of the main geometrical parameters and the physical phenomena that determine the lift performance. The efficiency of the resulting droop-nose configuration is also tested on a wing-body aircraft model. The analysis reveals three-dimensional flow mechanisms that limit the lift performance in operative conditions.
The airfoil efficiency is then further improved by adding a boundary-layer suction device. The effects of shape and location of the suction slot are studied to maximize the lift coefficient and pressure recovery. Finally, the effectiveness of unsteady excitation of the mixing layer by means of dynamic blowing is investigated. As a final result, a target maximum lift coefficient of 5.0 can be achieved with a 43% lower jet-momentum coefficient with respect to the baseline airfoil configuration.
... To further understand the aeroacoustic characteristics and benefits of the Circulation Control Wing, Munro, Ahuja and Englar [16, 17, 18, 19] have recently conducted several acoustic experiments comparing the noise levels of a conventional high-lift system with that of an advanced CC wing at the same lift setting. The present Computational Fluid Dynamics (CFD) study [20] is intended to be a complement to this work, and to numerically investigate the aerodynamic characteristics and benefits associated with the CC airfoil. ...
... But overall, the hinged flap design still maintains most of the advantages of the Circulation Control, while greatly reducing the drag in cruising condition associated with the rounded trailing edge CCW design. To understand and quantify the aeroacoustic characteristics and benefits of the Circulation Control Wing, Munro, Ahuja and Englar [7, 8, 9, 10] have recently conducted several acoustic experiments comparing the noise levels of a conventional high-lift system with that of an advanced CC wing at the same lift setting. The present Computational Fluid Dynamics (CFD) study [11] is intended to complement this work, and numerically investigate the aerodynamic characteristics and benefits associated with the CC airfoil. ...
Circulation Control technology is a very effective way of achieving high lift forces required by aircraft during take-off and landing. This technology can also directly control the flow field over the wing. Compared to a conventional high-lift system, a Circulation Control Wing (CCW) can generate comparable or higher lift forces during take-off/landing with fewer or no moving parts and much less complexity. In this work, an unsteady three-dimensional Navier-Stokes analysis procedure has been developed and applied to Circulation Control Wing configurations. The effects of 2-D steady jets and 2-D pulsed jets on the aerodynamic performance of CCW airfoils have been investigated. It is found that a steady jet can generate very high lift at zero angle of attack without stall, and that a small amount of blowing can eliminate vortex shedding at the trailing edge, a potential noise source. It is also found that a pulsed jet can achieve the same high lift as a steady jet at lower mass flow rates, especially at a high frequency, and that the Strouhal number has a more dominant effect on the pulsed jet performance than just the frequency or the free-stream velocity.
Aircraft noise is a major inhibitor of the growth of air transport. Airports in key locations are operating at full capacity and the noise in the vicinity of airports is so intrusive that local communities object to any further expansion. A functionally-silent aircraft is aimed at reducing airframe and propulsion system noise by as much as 30 dB. Silent in this context means sufficiently quiet that the aircraft noise is less than that of the background noise in a typical well populated environment. There are two technical aspects to be dealt with in reducing aircraft community noise: propulsion system noise and airframe noise. The work presented focuses on both aspects for noise during take-off, approach and landing. The main objectives of this paper are to (1) investigate the technological barriers and requirements for a functionally-silent aircraft, (2) assess the potential noise reductions of selected low-noise concepts for such aircraft, and (3) delineate design implications and airframe/propulsion system configurations for a silent aircraft. The theme of the technical approach is based on a systems view rather than an individual component view of the airframe and interacting propulsion system. Simple analytical modeling and existing semi-empirical noise prediction methods and scaling laws are used to predict the acoustic signature of low-noise airframe and propulsion system concepts envisioned for a silent aircraft. A noise reduction assessment framework is developed to explore the necessary technologies. The design study and acoustic analysis is based on an aerodynamically clean blended-wing-body type airframe configuration. Results from the noise assessment studies are discussed and preliminary design implications for a silent aircraft are given.
The rapid air travel growth during the last three decades, has resulted in runway congestion at major airports. The current airports infrastructure will not be able to support the rapid growth trends expected in the next decade. Changes or upgrades in infrastructure alone would not be able to satisfy the growth requirements, and new airplane concepts such as the NASA proposed Super Short Takeo and Landing and Extremely Short Takeo and Landing (ESTOL) are being vigorously pursued. Aircraft noise pollution during Takeoff and Landing is another serious concern and efforts are aimed to reduce the airframe noise produced by Conventional High Lift Devices during Takeoff and Landing. Circulation control technology has the prospect of being a good alternative to resolve both the aforesaid issues. Circulation control airfoils are not only capable of producing very high values of lift (Cl values in excess of 8.0) at zero degree angle of attack, but also eliminate the noise generated by the conventional high lift devices and their associated weight penalty as well as their complex operation and storage. This will ensure not only satisfying the small takeoff and landing distances, but minimal acoustic signature in accordance with FAA requirements. The Circulation Control relies on the tendency of an emanating wall jet to independently control the circulation and lift on an airfoil. Unlike, conventional airfoil where rear stagnation point is located at the sharp trailing edge, circulation control airfoils possess a round trailing edge, therefore the rear stagnation point is free to move. The location of rear stagnation point is controlled by the blown jet momentum. This provides a secondary control in the form of jet momentum with which the lift generated can be controlled rather the only available control of incidence (angle of attack) in case of conventional airfoils. The use of Circulation control despite its promising potential has been limited only to research applications due to the lack of a simple prediction capability. This research effort was focused on the creation of a rapid prediction capability of Circulation Control Aerodynamic Characteristics which could help designers with rapid performance estimates for design space exploration. A morphological matrix was created with the available set of options which could be chosen to create this prediction capability starting with purely analytical physics based modeling to high fidelity CFD codes. Based on the available constraints, and desired accuracy metamodels has been created around the two dimensional circulation control performance results computed using Navier Stokes Equations (Computational Fluid Dynamics). DSS2, a two dimensional RANS code written by Professor Lakshmi Sankar was utilized for circulation control airfoil characteristics. The CFD code was first applied to the NCCR 1510-7607N airfoil to validate the model with available experimental results. It was then applied to compute the results of a fractional factorial design of experiments array. Metamodels were formulated using the neural networks to the results obtained from the Design of Experiments. Additional validation runs were performed to validate the model predictions. Metamodels are not only capable of rapid performance prediction, but also help generate the relation trends of response matrices with control variables and capture the complex interactions between control variables. Quantitative as well as qualitative assessments of results were performed by computation of aerodynamic forces and moments and flow field visualizations. Wing characteristics in three dimensions were obtained by integration over the whole wing using Prandtl's Wing Theory. The baseline Super STOL configuration was then analyzed with the application of circulation control technology. The desired values of lift and drag to achieve the target values of Takeoff and Landing performance were compared with the optimal configurations obtained by the model. The same optimal configurations were then subjected to Super STOL cruise conditions to perform a tradeoff analysis between Takeoff and Cruise Performance. Supercritical airfoils modified for circulation control were also thoroughly analyzed for Takeoff and Cruise performance and may constitute a viable option for Super STOL and STOL Designs. The prediction capability produced by this research effort can be integrated with the current conceptual aircraft modeling and simulation framework. The prediction tool is applicable within the selected ranges of each variable, but methodology and formulation scheme adopted can be applied to any other design space exploration. Ph.D. Committee Chair: Dimitri N. Mavris; Committee Member: Daniel P. Schrage; Committee Member: Lakshmi N. Sankar; Committee Member: Robert C. Michelson; Committee Member: Robert J. Englar
Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the blowing slot. This jet sheet is essentially a high aspect-ratio rectangular jet. Thus, to fully understand the noise of a circulation control wing, the noise of high aspect-ratio rectangular jets must also be understood. A high aspect-ratio nozzle was fabricated to study the general characteristics of high aspect-ratio jets with aspect ratios from 100 to 3000. The jet noise of this nozzle was proportional to the 8" power of the jet velocity. It was also found that the jet noise was proportional to the slot height to the 312 power and slot width to the 1/2 power.
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