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.

Effect of acoustic excitation on flow over a partially grooved circular cylinder

“…At Rea = 7.7 • 104 it was found that maximum attenuation occurred at the excitation frequency rex = 2.5 kHz. This excitation frequency agreed with that on airfoil McKinzie 1991 andZaman 1992). At Rea= 1.22 x 105 fex for maximum attenuation was 2.3 kHz.…”

“…At this Reynolds number the momentum thickness 0is at the laminar separation point was found to be 0.1438 mm, giving the Strouhal number Sto( =fex 0is/Uo) of 0.015. This is very near the Strouhal number of about 0.016, when rex matches the instability frequency of the separated shear layer (Zaman 1992). The excitation level, as measured on the tunnel wall, was 120 dB.…”

“…However, the laterally induced excitation does suppress the separated, locally decelerating layer, resulting in the reattachment, though more disturbed layer (Morkovin 1964). As this separated and locally decelerating layer is strongly sensitive to three-dimensional disturbances and freestream turbulence (Morkovin 1964), the lateral disturbances introduced by the acoustic waves primarily act on the inviscid instability of the separated shear layer (Nishioka et al 1990, Hsiao and Shyu 1991and Zaman 1992. With the 'lock-in' between the forcing and separated shear layer amplification of the instability waves at Sto ~ 0.015 the entrainment of fluids and influx of momentum towards the wall region are enhanced, resulting in the suppression of the separation and reattachment.…”

Weakly non-linear analysis on identification of vortex sound & heat and their interactions in shear flow

“…Combined with the result of [4], we can find the importance of large scale vortex in interaction between sound and vortex. In [5], the phenomena that the effect of the sound on the lower stream flow is small is related with isotropy of the lower stream vortex: which is nearly consistent. In [6] the pressure amplitude is also large at fi= 1220Hz.…”

“…There exists free shear layer over the separated area on the stalled flow of the airfoil in [5]. [6], so the perturbation equations developed above can be applied.…”