Conditional Reynolds stress on a V‐grooved surface

Measurements of both mean and turbulent dynamic fields over a ribbed wall show the existence of a minute zone in which the boundary layer characteristics are strongly altered. The turbulent kinetic energy (k+), deduced from the three velocity standard deviations, is strongly increased above the ridge, and reduced over the valley. A similar effect is also visible on the Reynolds stress (−〈uv〉+) profiles, but the relative variations of k+ and −〈uv〉+, with respect to the flat plate case, are not of the same magnitude. Close to the ridge plane, contributions to the Reynolds shear stress from ejection and sweep types of motions are seen to be larger and more frequent above the ridge than over the smooth wall, whereas they are reduced above the valley. Comparison of the respective productions of turbulence above a riblet and above smooth walls, inferred at the distance y+=9 above the ridge plane, indicates a reduction of about 5% in the manipulated flow.