Direct numerical simulation of combined forced and natural turbulent convection in a vertical plane channel

The fully developed, turbulent combined forced and natural convection between two vertical parallel plates kept at different temperatures was investigated through a series of direct numerical simulations (DNSs). The pressure gradient drives the mean flow upward, while the buoyant force acts upward (aiding flow) and downward (opposing flow) near the high- and low-temperature walls, respectively. The Reynolds number based on the channel half-width and the friction velocity is assumed to be 150; whereas, the Grashof number based on the channel width and the wall temperature difference varies from 0 to 1.6 x 10(6) The buoyancy effect on the turbulent statistics including the mean velocity and temperature, the Reynolds stress tensor, and the turbulent heat flux vector are examined. In the opposing flow, the turbulent transport is greatly enhanced with both the Reynolds stresses and the turbulent heat fluxes being remarkably increased; whereas, in the aiding flow, the opposite change is observed. The DNS results presented here are compared with those of the channel flow with uniform wall mass injection and suction (Sumitani and Kasagi 1995) and of the liquid metal channel flow with a transverse magnetic field (Ohtsubo and Kasagi 1992). As a result, it is found that the opposing and aiding buoyancy affects not only the turbulent statistics but also the quasi-coherent structures in much the same way as the wall injection/suction and the Lorenz force. This correspondence should result from the near-wall force balance modified similarly by the additional body force or momentum transport. (C) 1997 by Elsevier Science Inc.