Ordered motion of turbulence in a thermally stratified flow under unstable conditions

Abstract

Measurements of the streamwise and vertical velocity fluctuations and temperature fluctuation in a thermally stratified flow in an open channel under unstable conditions were made with two laser Doppler velocimeters and a film probe operated as a resistance thermometer. The result indicates that the contribution to the total turbulent momentum flux from the sweep motion of large amplitude becomes greatest in the free surface region, while that from the ejection motion remains largest in the wall region. The ejection motion makes the largest contribution to the total turbulent heat transport over the whole measurement range from the wall region to the free surface region for both weakly and strongly unstable conditions. The outward interaction motion makes increasing contribution to the total turbulent momentum flux as well as to the total turbulent heat flux and is closely connected with the ejection motion under the strongly unstable conditions. The mean durations and the mean periods of occurrences of the respective motions decrease with increasing instability.