An oscillatory turbulent boundary layer in an adverse pressure gradiant.

Abstract

A turbulent boundary layer experiencing a time mean adverse pressure gradient and a controllable travelling wave periodic oscillation, was examined experimentally. An open return low speed wind-tunnel with a semi-open working section was used for this purpose, with oscillating flaps at its exit inducing the oscillations. The boundary layer on a specially designed "S" shaped model of chord 2m and thickness/ chord ratio of3.6%was investigated, for a range of frequencies from 1 to 6Hz, and amplitudes of the order of 10% of the time mean freestream velocity. The turbulent boundary layer evolved naturally around x/c= . 23, and measurements were taken for a Reynolds number Rec=3401. The effect of flap amplitude was examined for a range of amplitudes, from 2 to 4 inches. Unsteady velocity and pressure quantities were measured using Hot-wire techniques and pressure transducers, with the aid of a digital sampling system. Boundary layer mean values, were found to be invýriant with both frequency and amplitude of oscillation, while unsteady components were predominantly affected by frequency and downstream position but not amplitude. Unsteady velocities in the boundary layer lagged the freestream oscillations by as much as 1500 in some cases, while amplitudes exceeded freestream values by as much as 70%