Turbulence structure in a stratified boundary layer under stable conditions

Turbulence structure in stably stratified boundary layers is experimentally investigated by using a thermally stratified wind tunnel. A stably stratified flow is created by heating the wind tunnel airflow to a temperature of about 50 degrees C and by cooling the test-section floor to a surface temperature of about 3 degrees C. In order to study the effect of buoyancy on turbulent boundary layers for a wide range of stability, the velocity and temperature fluctuations are measured simultaneously at a downwind position of 23.5 m from the tunnel entrance, where the boundary layer is fully developed. The Reynolds number, Re-delta, ranges from 3.14 x 10(4) to 1.27 x 10(5), and the bulk Richardson number, Ri(delta), ranges from 0 to 1.33. Stable stratification rapidly suppresses the fluctuations of streamwise velocity and temperature as well as the vertical velocity fluctuation. Momentum and heat fluxes are also significantly decreased with increasing stability and become nearly zero in the lowest part of the boundary layer with strong stability. The vertical profiles of turbulence quantities exhibit different behaviour in three distinct stability regimes, the neutral flows, the stratified flows with weak stability (Ri(delta) = 0.12, 0.20) and those with strong stability (Ri(delta) = 0.39, 0.47, 1.33). Of these, the two regimes of stratified flows clearly show different vertical profiles of the local gradient Richardson number Ri, separated by the critical Richardson number Ri(cr), of about 0.25. Moreover, turbulence quantities in stable conditions are well correlated with Ri.