Natural Convection Induced by Diurnal Heating and Cooling in a Reservoir with Slowly Varying Topography

Abstract

This study is concerned with natural convection in a reservoir with slowly varying topography in response to diurnal heating and cooling due to heat transfer through the water surface. In the daytime phase, heat is transferred into the water body through absorption of solar radiation; and in the night-time phase, heat is transferred out of the water body through heat loss from the water surface. An unsteady model is formed and solved numerically in order to investigate the transient flow response in the reservoir. Two different scenarios with shallow and deep waters respectively, based on the comparison between the maximum water depth and the penetration depth of the solar radiation, are considered. The numerical results reveal that there is a distinct time lag in the response of the overall flow to the switches of the thermal forcing, and the lag time depends on the Grashof number. It is also found that thermal instabilities play an important role in breaking the residual circulation and reversing the flow in deep waters.