Large-eddy simulations of complex turbulent flows

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Turbulence occurs in most fluid flows. Its complexity is due to the fact that turbulent transport is mostly due to coherent, whorl-like motions (“eddies”), which are deterministic in character, but appear at random times and locations. Turbulent transport plays the primary role in determining, for instance, the drag of an aircraft or the heat transfer in a heat exchanger. Solution methods in which the eddies are explicitly resolved are gaining popularity because of the higher accuracy and completeness with which the flow can be analyzed; among them is the large-eddy simulation (LES). This proposal will focus, first, on the development of improved models and methodologies for LES, to decrease its computational demands and accelerate its transition to industry; second, on the application of LES to complex flows. My group has recently developed a new model to parameterize the small eddies [Piomelli et al., J. Fluid Mech., 766:499-527, 2015], which, in initial tests, has shown itself to be accurate and computationally cheaper than existing models. We propose to modify it to respond better to the local state of the turbulence, and extend it to more realistic flows. Furthermore, we will work towards the development of an "optimal LES" in which the computational grid adapts to the flow so that the ratio of grid size to the characteristic length-scale of turbulence is uniform. This improvement has the potential of decreasing the cost of a simulation by orders of magnitude, since present applications use grids that are overly refined in some regions and marginal in others. The second theme is the application of LES to problems of technological relevance, in which the more detailed understanding of the flow physics that LES affords in comparison to other techniques can result in significant benefits. First, we propose to study the effect of roughness on heat transfer, and explore the unknown mechanisms that determine the transport of energy, why the analogy with the transport of momentum does not hold when roughness is present, and how to model it. Second, we will investigate the flow over river dunes, extending our previous research to real-life configurations, including the sand motion and the dynamical evolution of the dune. Finally, we will apply LES to study how leading-edge protuberances can affect the performance of wings, and which shapes can improve the aerodynamic behaviour and sound emission of the wing. In summary, this proposal will take a two-pronged approach that coordinates methodological improvements, fundamental studies and applications. The model development is designed to decrease significantly the cost of the calculations, and the fundamental studies and applications will use the increased capabilities of LES to understand the physics of complex, previously unapproachable, problems. This proposal will also result in the training of at least four HQP. *****