Transient turbulence structure in the unstable boundary layer under the condition of step cooling from below

Abstract

Density-stratification effects on the turbulence transport and its structure were investigated when an unstable boundary layer was cooled stepwise from below, i.e. a stably-stratified internal boundary layer (SIBL) was developed beneath the unstable boundary layer. Measurements of velocity and temperature were made by means of anemometers with hot and cold wires, using an atmospheric diffusion wind tunnel. Even upstream the SIBL, i.e. in the unstable boundary layer, there were significant effects of the SIBL upon the time-averaged velocity and temperature fields and also turbulent transport mechanism. Upward fluid motion together with upward heat transfer was induced near the wall along by the SIBL because the SIBL blocked the development of upstream unstable boundary layer. Above the upward motion, the downward motion occurred simultaneously. Hence, in this region, the intensity of the fluctuation of vertical velocity component was enhanced due to the interaction of the upward and downward motions. As a result the vertical turbulent transport of heat increased significantly. Inside the developing region of the SIBL the streamwise turbulent motion was enhanced so that the turbulent heat transfer in the streamwise direction increased violently. Thus, the mean motion and turbulent transport processes were modified significantly by the abrupt change of the condition at the wall and the convective turbulence eddies, i.e. the change of boundary condition from no-slip condition to nearly free slip one. It is also pointed out that those abrupt changes caused the counter-gradient diffusion of momentum and heat in the wide region of the unstable layer just above the SIBL.