Unsteady natural convection induced by diurnal temperature changes in a reservoir with slowly varying bottom topography

The present numerical investigation is concerned with the transient flow response in a reservoir model to periodic heating and cooling at the water surface. The numerical modelling reveals a stable stratification of the water body during the heating phase and an unsteady mixing flow in the reservoir during the cooling phase. It is shown that thermal instabilities play an important role in breaking up the residual circulation and initiating a reverse flow circulation in deep waters when the thermal conditions switch from heating to cooling. Further, if cooling is sufficiently strong, a clear undercurrent is formed, bringing cold water to the deep region of the reservoir. Moreover, heating from the water surface results in a stable large-scale convective roll which is clearly observed in the simulations. Understanding of the flow mechanisms pertinent to this problem is important for predicting the transport of nutrients and pollutants across reservoirs.