Effect of a Short Roughness Strip on a Turbulent Boundary Layer

Abstract

The response of a turbulent boundary layer to a short roughness strip is investigated using laser Doppler velocimetry (LDV) and laser induced fluorescence (LIF). Skin friction coefficients are inferred from accurate near-wall measurements. There is an undershoot in $c_f /c_{f_0 }$, where $c_{f_0 }$ is the undisturbed smooth wall skin friction coefficient, immediately after the strip. Downstream of the strip, $c_f /c_{f_0 }$ overshoots before relaxing back to unity in an oscillatory manner. The roughness strip has a major effect on the turbulent stresses $\left( {\overline {u^2 } ,\overline {v^2 } , - \overline {uv} } \right)$; these quantities increase, relative to the undisturbed smooth wall, in the region between the two internal layers originating at the upstream and downstream edges of the strip. The increase in the ratio${\overline {v^2 } /\overline {u^2 } }$ suggests a decrease in near-wall anisotropy. From the flow visualizations, it is inferred that streamwise vortical structures are weakened immediately downstream of the strip. Consistently, streamwise length scales are also reduced; direct support for this is provided by measured two-point velocity correlations.