Unsteady natural convection induced by constant surface cooling in a reservoir

Abstract

This article concerns numerical simulations of unsteady natural convection induced by constant cooling at the water surface of a reservoir. Numerical computations reveal the occurrence of sinking cold water plumes soon after the initiation of cooling. These sinking plumes are responsible for an initial mixing over the local water depth, resulting later in a distinct horizontal temperature gradient with the water temperature decreasing toward the shallow region due to the presence of a sloping boundary. Simulations also show that the relatively higher cooling rate in the shallow water causes a cold water current flowing downwards along the sloping bottom and penetrating into the deeper regions. After a sufficient time a quasi-steady state is attained and at the end two main fluid layers are developed: a relatively stable undercurrent, and a very unstable return flow just below the water surface. Numerical results for different Grashof numbers are discussed. Understanding of the flow mechanisms pertinent to this flow is important for predicting the transport of nutrients and pollutants across the reservoir.