EFFECT OF A SHORT ROUGHNESS STRIP ON A TURBULENT BOUNDARY-LAYER

The response of a turbulent boundary layer to a short roughness strip is investigated using laser Doppler velocimetry (LDV) and laser induce d fluorescence (LIF). Skin friction coefficients are inferred from acc urate near-wall measurements. There is an undershoot in c(f)/c(f0), wh ere c(f0) is the undisturbed smooth wall skin friction coefficient, im mediately after the strip. Downstream of the strip, c(f)/c(f0) oversho ots before relaxing back to unity in an oscillatory manner. The roughn ess strip has a major effect on the turbulent stresses (<(u(2))over ba r>, <(nu(2))over bar>, -<(uv)over bar>); these quantities Increase, re lative to the undisturbed smooth wall, in the region between the two i nternal layers originating at the upstream and downstream edges of the strip. The increase in the ratio <(nu(2))over bar>/<(u(2))over bar> s uggests a decrease in near-wall anisotropy. From the flow visualizatio ns, it is inferred that streamwise vortical structures are weakened im mediately downstream of the strip. Consistently, streamwise length sca les are also reduced; direct support for this is provided by measured two-point velocity correlations.