TRANSIENT TURBULENCE STRUCTURE IN THE UNSTABLE BOUNDARY-LAYER UNDER THE CONDITION OF STEP COOLING FROM BELOW

Density-stratification effects on the turbulence transport and its str ucture were investigated when an unstable boundary layer was cooled st epwise from below, i.e. a stably-stratified internal boundary layer (S IBL) was developed beneath the unstable boundary layer. Measurements o f velocity and temperature were made by means of anemometers with hot and cold wires, using an atmospheric diffusion wind tunnel. Even upstr eam the SIBL, i.e. in the unstable boundary layer, there were signific ant effects of the SIBL upon the time-averaged velocity and temperatur e fields and also turbulent transport mechanism. Upward fluid motion t ogether with upward heat transfer was induced near the wail along by t he 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 fluctuatio n of vertical velocity component was enhanced due to the interaction o f the upward and downward motions. As a result the vertical turbulent transport of heat increased significantly. Inside the developing regio n of the SIBL the streamwise turbulent motion was enhanced so that the turbulent heat transfer in the streamwise direction increased violent ly. Thus, the mean motion acid turbulent transport processes were modi fied significantly by the abrupt change of the condition at the wall a nd the convective turbulence eddies, i.e. the change of boundary condi tion from no-slip condition to nearly free slip one. It is also pointe d out that those abrupt changes caused the counter-gradient diffusion of momentum and heat in the wide region of the unstable layer just abo ve the SIBL. Copyright (C) 1996 Elsevier Science Ltd