Experimental investigation of instability wave propagation in a three-dimensional boundary-layer flow

Abstract

Wave propagation phenomena in three-dimensi onal boundary-layer flows with crossflow instability were investigated experimentally. A 10-element hot-film sensor was flush-mounted on a rotatable insert in a swept flate plate with imposed favorable pressure gradient. By means of cross-spectra l analysis it was possible to obtain direction and magnitude of the phase velocity and the group velocity of the traveling instability waves, thus filling a gap in the knowledge of crossflow instability characteristics. The waves were found to propagate approximately normal to the potential streamline direction, according to linear theory. Phase velocity and the resulting wavelengths also agree satisfactorily, whereas the measured direction and magnitude of the group velocity shows significant differences. Nomenclature c = chord length, 0.5 m cgr = group velocity cph = phase velocity E = voltage F = dimensionless frequency, / • / = dimensional frequency Ga = power spectrum of Xa(t) Gab(f) = cross spectrum of Xa(t) and Xb(t) k - complex wave number vector, k = (kr, &/), IA>l=27r/X