Estudo da transição de escoamento causada pela estratificação estável em um escoamento de couette plano utilizando simulação de grandes turbilhões

Abstract

In fluid mechanics three types of flow can be highlighted, laminar flow, transitional flow and turbulent flow, they occur in a region called boundary layer, being this region directly influenced by a boundary surface. The existence of such a region occurs when there is fluid motion, from flows in ducts, flow in a riverbed and also in the atmospheric flow on earth’s surface. The flow in the atmospheric boundary layer (ABL) is predominantly turbulent, however, after sunset, the incidence of solar radiation on the surface ceases and then the surface start to lose heat through the radiation of long waves, giving place to a stable boundary layer (SBL), where the turbulence scales can be decreased in practically all the scales being the production of turbulence predominantly mechanical. In clear sky night with low minds, strong stratification of the layers adjacent to the Earth’s surface can take place, which causes the turbulence scales to be reduced by several orders of magnitude and may suddenly and unpredictably arise. This phenomenon recives the name of global intermittence and is similar to a laminar-turbulent transition and occurs in the SBL. So, we will use a Large Eddy Simulation method where the sub-grid scales will be parametrized through a one equation model. The results show that for a set of parameters it is possible to reproduce an intermittent flow, similar to those observed in a wind tunnel experiments and data sampled from natual flows. In addition, evidence of the existence of a chaotic phenomena for intermittent flow has been found, but this fact is only taken as a hypothesis because this idea go beyond the scope of this work.