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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. 

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. 

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. 

Large-eddy simulations (LESs) of low-Reynolds-number flow (Re=50,000) over a NACA0018 airfoil are performed to investigate flow control at the stall angle of attack (15 deg) by low-amplitude surface waves (actuations) of different types (backward/forward traveling and standing waves) on the airfoil’s suction side. It is found that the backward (toward downstream) traveling waves, inspired from aquatic swimmers, are more effective than forward traveling and standing wave actuations. The results of simulations show that a backward traveling wave with a reduced frequency f∗=4 (f∗=fL/U, where f is frequency; L, chord length; and U, free flow velocity), a nondimensional wavelength λ∗=0.2 (λ∗=λ/L, where λ is dimensional wavelength), and a nondimensional amplitude a∗=0.002 (a∗=a/L, where a is dimensional amplitude) can suppress stall. In contrast, the flow over the airfoil with either standing or forward traveling wave actuations separates from the leading edge similar to the baseline. Consequently, the backward traveling wave creates the highest lift-to-drag ratio. For traveling waves at a higher amplitude (a∗=0.008), however, the shear layer becomes unstable from the actuation point and creates periodic coherent structures. Therefore, the lift coefficient decreases compared with the low-amplitude case. 

Some anguilliform swimmers such as eels and lampreys swim near the ground, which has been hypothesized to have hydrodynamic benefits. To investigate whether swimming near ground has hydrodynamics benefits, two large-eddy simulations of a self-propelled anguilliform swimmer are carried out—one swimming far away from the ground (free swimming) and the other near the ground, that is, midline at 0.07 of fish length (L) from the ground creating a gap of 0.04 L . Simulations are carried out under similar conditions with both fish starting from rest in a quiescent flow and reaching steady swimming (constant average speed). The numerical results show that both swimmers have similar speed, power consumption, efficiency, and wake structure during steady swimming. This indicates that swimming near the ground with a gap larger than 0.04 L does not improve the swimming performance of anguilliform swimmers when there is no incoming flow, that is, the interaction of the wake with the ground does not improve swimming performance. When there is incoming flow, however, swimming near the ground may help because the flow has lower velocities near the ground.