Effect of acoustic excitation on stalled flows over an airfoil

Abstract

The effect of acoustic excitation on poststalled flows over an airfoil, i.e., flows that are fully separated from near the leading edge, is investigated. The excitation results in a tendency towards reattachment, which is accompanied by an increased lift and reduced drag, although the flow may still remain fully separated. It is found that with increasing excitation amplitude, the effect becomes more pronounced but shifts to a Strouhal number which is much lower than that expected from linear, inviscid instability of the separated shear layer.

Interaction Between Vortex Rings and a Separated Shear Layer: Towards Active Control of Separation Zones

“…The drag is also expected to attain a minimum value at the same frequency because the height of the separated zone becomes minimum. This frequency when normalized in the form F N c/; is in a range of 3}4 (Zaman & McKinzie 1991;Zaman 1992), 1}3 (Hsiao et al, 1989) and 2 (Bar-Sever 1989), being of the same order as the abovementioned frequency Fc/; "4. At high Reynolds numbers the frequency F N is much lower than the fundamental frequency of Kelvin}Helmholtz (K}H) instability in the separated shear layer F )& (Zaman 1992).…”

“…This frequency when normalized in the form F N c/; is in a range of 3}4 (Zaman & McKinzie 1991;Zaman 1992), 1}3 (Hsiao et al, 1989) and 2 (Bar-Sever 1989), being of the same order as the abovementioned frequency Fc/; "4. At high Reynolds numbers the frequency F N is much lower than the fundamental frequency of Kelvin}Helmholtz (K}H) instability in the separated shear layer F )& (Zaman 1992). Nishioka et al (1990) found that the forcing by acoustic waves of low amplitude ampli"es the fundamental mode of the linear instability, whereas high amplitude forcing ampli"es the mode of much lower frequency.…”

“…Periodic forcing of stalled #ow around two-dimensional airfoils yields a particular forcing frequency, F N at which the lift attains a maximum (Hsiao et al, 1989;Bar-Sever 1989;Zaman & McKinzie 1991;Zaman 1992). The drag is also expected to attain a minimum value at the same frequency because the height of the separated zone becomes minimum.…”

Dynamic-Active Flow Control - Phase I

“…They noted that the excitation was most effective at frequencies that resulted in large transverse velocity fluctuations, rather than large amplitude pressure fluctuations. Zaman (1992) conducted further research in the post-stall region of the airfoil and found that, contrary to his initial postulation, the optimum forcing frequency did not occur at the Kelvin-Helmholtz instability frequency for the separated shear layer. Rather, the effective frequencies were approximately an order of magnitude lower.…”

Low Reynolds Number Flows and Transition

“…Experimental results showed that lift coefficient had a significant improvement. Zaman (1992) also investigated the effects of acoustic excitation on post-stalled flows over an aerofoil. They used a two dimensional aerofoil LRN (1)-1007 with a chord length of 12.7 cm and employed a crossed hot-film probe for the experiments.…”