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

Abstract

A turbulent combined-convection boundary layer, created by imposing an aiding freestream on a turbulent natural-convection boundary layer along a vertical heated plate, was examined with normal hot- and cold-wires and a particle image velocimetry (PIV), with attention to the laminarization process of the boundary layer due to the freestream effects. As the freestream velocity increases, the wall shear stress monotonously increases, whereas the local heat transfer rate suddenly decreases. The reduction in heat transfer rates and the decays in velocity and temperature fluctuations, showing the transition from turbulent to laminar, arise at Grx/ Rex≃3×10 6. With the laminarization of the boundary layer, a similar change in 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 near-wall region. The increasing freestream then restricts large-scale fluid motions in the outer layer, and consequently the generation of turbulence is suppressed and the boundary layer becomes laminar.