Surface Friction Effects on Thermal Convection in a Rotating Fluid: A Laboratory Simulation

Abstract

Adiabatic thermals rising through an environment which has formed a friction layer by rotating in contact with a roughness plate are studied in the laboratory. The effects of frictionally induced motions on the growth of thermals and vortexes are assessed by comparing the friction situations with identical ones in which the roughness plate has been removed. The analysis shows that convergence created by the friction layer gives rise to additional upward motion at the center of the cloud tank, and this tends to offset the rotational suppression of the thermal's growth. Entrainment of the environmental fluid by the thermal appears to be increased, and the enhancement of volumetric growth may be due in part to entrainment of turbulent fluid from the friction layer. An attempt is made numerically to evaluate this effect. Vortexes formed in the friction layer are found to be fatter and less intense than in the low-friction environment, and they also break contact with the ground. The results of the laboratory experiments are found to agree very well with those of a numerical simulation in which the initial conditions were similar.