Acoustic measurement of boundary layer flow parameters

The principle of an ultrasonic method for measuring the parameters of a boundary layer flow is investigated using a mathematical model of the sound field created by a compact piston in a solid boundary, radiating into a parallel shear flow. The model, based on a wavenumber decomposition of the problem, is used to demonstrate that dispersive effects in the streamwise direction are a function of the free-stream flow velocity and the boundary layer displacement thickness, and that a multifrequency measurement of phase velocity between two wall-mounted transducers can in principle be used to determine these parameters. Results from a preliminary experiment to demonstrate the method are presented.