The effect of acoustic excitation on the later stages of transition in a laminar separation bubble

Abstract

The effect of external acoustic excitation on flow development within a separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125 000 and an angle of attack of 4°is investigated experimentally. Two-component, time-resolved particle image velocimetry is used to study both streamwise and spanwise aspects of the flow development. Acoustic excitation is applied at the frequency of the most amplified disturbance in the naturally developing separated shear layer and several excitation amplitudes are considered. The results show that periodic shear layer vortices form in the separation bubble with strong spanwise coherence at roll-up; however, the vortex filaments develop a spanwise unsteadiness which leads to rapid deformation and breakdown to smaller scales. Applying acoustic excitation causes the separation bubble to decrease in size, with the effect becoming more pronounced with increasing excitation amplitude. The changes in the mean bubble topology are directly linked to alterations in the salient characteristics of the shear layer vortices. Specifically, excitation is shown to cause earlier shear layer roll-up while enhancing the spanwise coherence of the dominant structures, with the former resulting in the aft portion of the bubble shifting upstream, and the latter reducing the streamwise distance between the mean roll-up and reattachment locations. Nomenclature c = airfoil chord length C P = surface pressure coefficient; (P − P 0 ) / 1 2 ρU 0 2 f = frequency l = airfoil leading to trailing edge arc length z = spanwise coherence length P = mean local surface pressure P 0 = free-stream static pressure p = fluctuating pressure Re = chord-based Reynolds number; U 0 c/ν St = chord-based Strouhal number; f c/U 0 t, t * = time and non-dimensional time; t * = tU 0 /c U, U = instantaneous and time-averaged x-velocity component u = fluctuating component of U U 0 = free-stream velocity U c = convective velocity of shear layer vortices U e = separation bubble edge velocity v = fluctuating y-velocity component v = root-mean-square of v x, y, z= wall-tangent, wall-normal, and spanwise coordinates x S , x H , x R = mean streamwise locations of separation, maximum bubble height, and reattachment α = angle of attack λ x , λ z = streamwise and spanwise disturbance wavelengths ν, ρ = kinematic viscosity and density of air φ pp = frequency auto-spectral density of p φ uu = wavelength auto-spectral density of u φ vv = frequency auto-spectral density of v ω = spanwise vorticity * Graduate Student

Effects of free-stream turbulence intensity on laminar separation bubbles

“…The authors noted that the decrease in bubble size was a result of a downstream shift in mean separation and an upstream shift in mean reattachment, similar to the results presented here. Similarly to the present results, Kurelek and Yarusevych [166] noted that these shifts were much more significant when estimated using the zero velocity contour line measured using PIV as compared to that estimated from mean surface pressure distributions. In fact, no change in the separation location could be detected in their investigation with the mean surface pressure distributions, despite a downstream shift in mean separation of about 0.03c measured with the PIV estimates.…”

“…A comparison of mean bubble topology using the two methods discussed here is also shown by Kurelek and Yarusevych [166]. In their investigation, acoustic excitation was used to excite a separation bubble formed over the same airfoil used in the present investigation, resulting in a decrease in separation bubble size.…”