The turbulent boundary layer over transverse square cavities

Laser-induced fluorescence (LIF) and laser Doppler velocimetry (LDV) are us ed to explore the structure of a turbulent boundary layer over a wall made up of two-dimensional square cavities placed transversely to the flow direc tion. There is strong evidence of occurrence of outflows of fluid from the cavities as well as inflows into the cavities. These events occur in a pseu do-random manner and are closely associated with the passage of near-wall q uasi-streamwise vortices. These vortices and the associated low-speed strea ks are similar to those found in a turbulent boundary layer over a smooth w all. It is conjectured that outflows play an important role in maintaining the level of turbulent energy in the layer and enhancing the approach towar ds' self-preservation. Relative to a smooth wall layer, there is a discerni ble increase in the magnitudes of all the Reynolds stresses and a smaller s treamwise variation of the local skin friction coefficient. A local maximum in the Reynolds shear stress is observed in the shear layers over the cavi ties.