Large-eddy simulations (LES) over a NACA0018 airfoil at a low Reynolds number (Re = 50, 000) fluid flow are performed to investigate the effect of active flow control at different angles of attack (AOA = 10 to 20 degrees) using low amplitude surface morphing backward (opposite to the airfoil’s forward motion) traveling wave actuation on the suction (upper) side of the airfoil. The curvilinear immersed boundary (CURVIB) method is used to handle the moving surface of the airfoil. While our previous simulations indicated the effectiveness of traveling waves at near stall angle of attack (AOA = 15 degrees), the effectiveness of these waves at post-stall AOA such as AOA = 20 degrees is not understood. The actuation amplitude of the surface morphing traveling waves is a* = 0.001 (a* = a/L, a: amplitude, L: chord length of the airfoil), and the range of the reduced frequency (f* = fL/U, f: frequency, U: free stream velocity) is from f* = 4 to 16. The results of the simulations at the post-stall angle of attack (AOA = 20 degrees) show that the lift coefficient, CL, increases by about 23%, and the drag coefficient, CD, decreases by about 54% within the frequency range from f* = 8 to f* = 10.
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Flow Control and Separation Delay in Morphing Wing Aircraft Using Traveling Wave Actuation
This study examines the biomimicry of wave propagation, a mode of locomotion in aquatic life for the use-case of morphing aircraft surfaces for boundary layer control. Such motion is theorized to inject momentum into the flow on the upper surface of airfoils, and as a consequence, creates a forcible pressure gradient thereby increasing lift. It is thought that this method can be used to control flow separation and reduce likelihood of stall at high angles of attack. The motivation for such a mechanism is especially relevant for aircraft requiring abrupt maneuvers, and especially at high angles of attack as a safety measure against stalling. The actuation mechanism consists of lightweight piezoelectric ceramic transducers placed beneath the upper surface of an airfoil. An open-loop system controls surface morphing. A two-dimensional Fourier Transform technique is used to estimate traveling to standing wave ratio, which is verified analytically using Euler Bernoulli beam theory, and experimentally using a prototype wing. Propagating wave control is tuned and verified using a series of scanning laser vibrometry tests. A custom two-dimensional NACA 0018 airfoil tests the concept in a low-speed wind tunnel with approximate Reynolds Number of 50,000. Both traveling waves and the changes in lift and drag will be experimentally characterized.
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- Award ID(s):
- 1905252
- NSF-PAR ID:
- 10214067
- Date Published:
- Journal Name:
- ASME 2020 Conference on Smart Materials, Adaptive Structures and Intelligent Systems
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Large-eddy simulations (LES) of the fluid flow over a NACA0018 airfoil at AOA =20 degrees angle of attack are performed to investigate the effect of surface morphing oscillations on the aerodynamic performance of the airfoil over a wide range of Reynolds numbers (Re = 5,000 to 500,000). These oscillations are in the form of low amplitude backward (opposite to the airfoil's forward motion) traveling wave actuations on the upper surface of the airfoil. The sharp interface curvilinear immersed boundary (CURVIB) method is used to handle the moving surface of the airfoil. The nondimensional amplitude is a*=0.001 (a*=a/L; a: amplitude, L: chord length of the airfoil) and reduced frequency (f*= fL/U; f is the frequency and U is the freestream velocity) is chosen to match the leading edge vortex shedding frequency. The results of the simulations at the post-stall angle of attack (AOA =20 degrees) show that the lift coefficient increases more than 20% and the drag coefficient decreases more than 40% within the Reynolds number range of Re = 50,000-500,000 for traveling wave actuation of amplitude, a*=0.001, and frequency, f*=8. However, the lift and drag coefficients of the actuated airfoil were similar to the baseline airfoil for Re = 5,000.more » « less
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The main two mechanisms of morphing wall flow control are direct injection of momentum in the streamwise direction and indirect momentum transfer via triggering instabilities. Traveling waves have been shown to perform better than standing waves, probably because they can use both mechanisms. However, the relative importance of the two mechanisms is not known. To differentiate between the mechanisms, a range of parameters (frequency, amplitude, and starting location) at stall (15 deg angle of attack) and poststall (20 deg angle of attack) is tested using wall-resolved large-eddy simulations with a sharp-interface curvilinear immersed boundary method at a low Reynolds number of [Formula: see text] over a NACA0018 airfoil. The results of the simulations demonstrate that the flow is reattached within a range of nondimensional frequencies, actuation amplitudes, and starting locations of oscillation at the stall and poststall angles of attack. Significant lift enhancement and drag reduction are also observed within these ranges. The nondimensional frequency range at which the flow is reattached is found to be similar to the dominant nondimensional frequencies of leading-edge vortex shedding of the unactuated airfoil. These indicate that the indirect transfer of momentum is the dominant mechanism because direct injection of momentum increases with the increase of amplitude and frequency; that is, separation should reduce as they increase. Nevertheless, direct injection of momentum improves the performance relative to pure excitations of standing waves when instabilities are triggered.
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Covert feathers are a set of self-actuating, passively deployable feathers located on the upper surfaces of wings that augment lift at post-stall angles of attack. Due to these benefits, the study of covert-inspired passive flow control devices is becoming an increasingly active area of research. In this work, we numerically investigate the aerodynamic benefits of torsionally mounting five covert-inspired flaps on the upper surface of a NACA0012 airfoil. Two-dimensional high-fidelity simulations of the flow past the airfoil–flap system at low Re=1000 and a high angle of attack of 20∘ were performed. A parametric study was conducted by varying the flap moment of inertia and torsional hinge stiffness to characterize the aerodynamic performance of this system. Lift improvements as high as 25% were attained. Two regimes of flap dynamics were identified that provided considerable aerodynamic benefits. A detailed investigation of the flow physics of both these regimes was conducted to understand the physical mechanisms by which the passively deployed flaps augmented the lift of the airfoil. In both regimes, the flap was found to act as a barrier in preventing the upstream propagation of reverse flow due to flow separation and trailing edge vortex. The torsional spring and flap inertia yielded additional flap dynamics that further modulated the surrounding flow and associated performance metrics. We discuss some of these fluid–structure interaction effects in this article.more » « less
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Abstract Conformal mapping techniques have been used in many applications in the two-dimensional environments of engineering and physics, especially in the two-dimensional incompressible flow field that was introduced by Prandtl and Tietjens. These methods show reasonable results in the case of comprehensive analysis of the local coefficients of complex airfoils. The mathematical form of conformal mapping always locally preserves angles of the complex functions but it may change the length of the complex model. This research is based on the design of turbine blades as hydrofoils divided into different individual hydrofoils with decreasing thickness from root to tip. The geometric shapes of these hydrofoils come from the original FX77W121 airfoil shape and from interpolating between the FX77W121, FX77W153, and FX77W258 airfoil shapes. The last three digits of this airfoil family approximate the thickness ratio times 1000 (FX77153 => 15.3 % thickness ratio). Of the different airfoil shapes specified for the optimal rotor, there are 23 unique shapes.[15, 16, 17, 18, 19, 20, 21, 22, 24, 25, 28] This study describes the advantage of using at least one complex variable technique of transformation conformal mapping in two dimensions.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1115/SMASIS2020-2355
