Experimental investigation of 3D acoustic receptivity of an airfoil boundary layer due to surface vibrations

Abstract

The three-dimensional acoustic receptivity of laminar boundary layers in presence of microscopic surface vibrations (the vibro-acoustic receptivity) is examined. The flow under investigation is the boundary layer on an airfoil at relatively high Reynolds numbers close to realistic ones for gliders. This flow has favourable and adverse pressure gradients and develops on a curved wall. The goal of the present study is to obtain quantitative information about the receptivity characteristics of this flow for excitation of the 3D Tollmien–Schlichting (TS) waves in cases when the frequencies of surface vibrations ( f v ) have the same order of magnitudes as those of the acoustic waves ( f ac ) . The experiments were performed at controlled disturbance conditions. The 2D acoustic field was produced by loudspeakers, while the localised non-stationary surface non-uniformities were simulated by a controlled circular surface vibrator. The TS-wave generation was investigated for several values of the parameter of non-stationarity of the surface non-uniformity K = f v / f ac . The receptivity was investigated in two general cases: in ‘ plus-regimes’ when the TS-waves were excited at combination frequencies f TS + = f ac + f v and in ‘ minus-regimes’ when the excitation occurred at frequencies f TS − = f ac − f v . The complex (amplitude and phase) receptivity coefficients are obtained experimentally as functions of the spanwise wavenumber (and the wave propagation angle) for three different frequency ratios K and also for several TS-wave frequencies at fixed values of K. The obtained vibro-acoustic receptivity coefficients are compared with the acoustic-roughness receptivity coefficients (i.e. for K = 0 ) found in previous experiments and DNS.