An experimental investigation of the formation and development of a wave packet in a laminar boundary layer

Abstract

The formation and development of a wave packet in the laminar boundary layer of a flat plate has been studied experimentally. The packet was arti­ficially generated by a short duration acoustic pulse, which was injected into the boundary layer flow through a small hole in the plate. The flow oscillations created by the passage of the packet were detected by a hot­wire anemometer which was positioned just outside the boundary layer. Repeated excitation of the disturbance enabled the noise content of the data to be significantly reduced by signal averaging which was carried out on­line by a small computer. From records generated at a number of probe locations over the plate it has been possible to show how the packet developed as it propagated downstream. Contours of the signal amplitude from each of the downstream stations showed that the patch of waves was initially roughly elliptic, but this evolved into a distinctly bowed shape as the patch spread out as it progressed downstream. The smoothly contoured wave packet, with peak amplitudes close to the centre, gradu­ally distorted and far downstream two amplitude maxima formed on either side of the centre line. The wave-like character of the fluctuations within the boundaries of the packet was revealed graphically through perspective projections of the signal records on the z, t plane ( z is across the span). These displays showed the gradual development from a small smooth packet close to the source to the final warped and distorted pattern. Spectral decomposition of the fluctuations at each streamwise measuring station provided information relating to the growth of narrow bands of waves. These showed that a particular band of oblique waves, which grew very rapidly at some stage, were linked to the observed distortions.