Effects of freestream on turbulent combined-convection boundary layer along a vertical heated plate

A turbulent combined-convection boundary layer, created by imposing an aidi ng freestream on a turbulent natural-convection boundary layer along a vert ical heated plate, was examined with normal hot- and cold-wires and a parti cle image velocimetry (PIV), with attention to the laminarization process o f the boundary layer due to the freestream effects. As the freestream veloc ity increases, the wall shear stress monotonously increases, whereas the lo cal heat transfer rate suddenly decreases. The reduction in heat transfer r ates and the decays in velocity and temperature fluctuations, showing the t ransition from turbulent to laminar, arise at Grx/Rex similar or equal to 3 x 10(6). With the laminarization of the boundary layer, a similar change i n the turbulent quantities appears independently of Grx. For instantaneous velocity vectors obtained with PIV, large-scale fluid motions, which play a predominant role in the generation of turbulence, are frequently observed in the outer layer, while quasi-coherent structures do not exist in the nea r-wall region. The increasing freestream then restricts large-scale fluid m otions in the outer layer, and consequently the generation of turbulence is suppressed and the boundary layer becomes laminar. (C) 2001 Published by E lsevier Science Inc.