TURBULENT BOUNDARY-LAYER OVER A SMOOTH WALL WITH WIDELY SEPARATED TRANSVERSE SQUARE CAVITIES

Laser-Doppler velocimetry (LDV) measurements and how visualizations ar e used to study a turbulent boundary layer over a smooth wall with tra nsverse square cavities at two values of the momentum thickness Reynol ds number (R(theta) = 400 and 1300). The cavities are spaced 20 elemen t widths apart in the streamwise direction. Flow visualizations reveal a significant communication between the cavities and the overlying sh ear layer, with frequent inflows and ejections of fluid to and from ca vities. There is evidence to suggest that quasi-streamwise near-wall v ortices are responsible for the ejections of fluid out of the cavities . The wall shear stress, which is measured accurately, increases sharp ly immediately downstream of the cavity. This increase is followed by a sudden decrease and a slower return to the smooth wall value. Integr ation of the wall shear stress in the streamwise direction indicates t hat there is an increase in drag of 3.4% at both R(theta).