Simulação numérica de correntes de densidade hiperpicnais sob referencial móvel

• Autor(a) principal: Farenzena, Bruno Avila
• Data de Publicação: 2020
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da PUC_RS
• Texto Completo: http://tede2.pucrs.br/tede2/handle/tede/9437
• topic: Correntes de Densidade; Escoamento Hiperpicnal; Referencial Móvel; Density Currents; Hyperpycnal Flow; Moving Frame; ENGENHARIAS

Density currents are flows resulting from a pressure gradient generated by variations in buoyant forces between two fluids that come into contact. The propagation of these flows promotes the formation of different internal regions, such as a frontal region known as the head, a region upstream of the head called the body and a posterior region referred to as the tail. The head of a density stream is a region of strong velocity gradients, responsible for much of the fluid entrainment in the current and also has heterogeneities at its most downstream point known as lobe and cleft structures. In this work a mathematical model is presented that allows to perform numerical simulations of the head of a density current in sufficiently long times with a reduced computational cost. Such mathematical model is based on the coupling of a moving frame in the coordinate system that corresponds to changes in the transport terms of the Navier-Stokes equations and scalar transport and in the boundary conditions of the problem. Based on this mathematical model and using the computational code Incompact3d, Direct Numerical Simulations and Large Eddy Simulations were performed first to verify the validity of the proposed model and later to seek more information about the behavior of the head of a density current over long time periods and on the origin of the lobe and cleft structures. As results of the validation basis of the proposed model, numerical simulations were performed in the flow configuration known in the literature as lock-exchange and also considered empirical relationships available in the literature. From the results obtained with the proposed mathematical model, it was observed that in longer periods of time the density current does not present the formation of new Kelvin-Helmholtz vortices and the lobe and cleft structures have a width that, on average, grows as a function of the head Reynolds number, a behavior that it is the exact opposite of what is documented in the literature of experimental approaches. Based on the Linear Stability Theory, it was concluded that the lobe and cleft structures are originated by the development of gravitational instabilities, of the Rayleigh-Taylor type, associated with the region of unstable stratification at the point further down the head of the density current.