On laminar boundary layers in oscillatory flow

Abstract

This paper describes an experimental and theoretical investigation of laminar boundary layer response to harmonic oscillations in velocity associated with a travelling wave imposed on an otherwise constant freestream velocity and convected in the freestream direction. An oscillatory flow wind tunnel and recording system were set up to produce and measure laminar boundary layer velocity profiles over frequencies of 2–10 H z for freestream amplitudes of up to 14 % of the mean velocity. An analysis on the lines of Lighthill’s theory but applying for any travelling wave convection velocity has been developed for both the low and high frequency cases. The experiments show that for a wide range of amplitude and frequency of oscillation the mean flow characteristics are the same as those of steady flow. This supports a major assumption of the theory which is linear in terms of the oscillating perturbations. Comparisons between theory and experiment show satisfactory agreement although the experimental results are largely for frequencies between the extreme ranges of the theories and they display features which are special to that intermediate frequency range. It is found that the boundary layer response is predominantly affected by the travelling wave convection velocity and frequency. In the experiments the freestream oscillation amplitudes increased with downstream distance but the effects of this increase are shown to be negligible.