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Wall-modeled large-eddy simulation: an application to atmospheric boundary layer across a natural roughness transition

Over the past decade, significant progress has been made in the development of near-wall models for large-eddy simulation (LES) and their application to complex wall-bounded flows. In this talk, I will first summarize the research on wall-modelled LES (WMLES) conducted by my group for engineering flows and test cases at moderate Reynolds number, and then discuss an application of WMLES to large-scale atmospheric flow over heterogeneous terrain featuring smooth-to-rough transition in the wall topology.

A body-fitted WMLES is run over high-resolution topographic data and validated against field observations, to examine the structure of the atmospheric boundary layer (ABL) across a natural roughness transition: the emergent sand dunes at White Sands National Park, NM. We observe that development of the internal boundary layer (IBL) is triggered by the abrupt transition from smooth playa surface to dunes; however, continuous changes in the size and spacing of dunes over several kilometers influence the downwind patterns of boundary stress and near-bed turbulence. Coherent flow structures grow and merge over the entire 10 km distance of the dune field and modulate the influence of large-scale atmospheric turbulence on the bed.

We analyze turbulence producing motions and how they change as the IBL grows over the dune field. Frequency spectrum and Reynolds shear stress profiles show that IBL thickness determines the largest scales of turbulence. More, the developing IBL enhances the frequency, magnitude and duration of sweep and ejection events which are the primary turbulence-producing events. We also show that IBL height and the edge velocity set the scales of the mean and defect velocity profiles within the IBL for our simulation data as well as laboratory-scale measurement of boundary layers at lower Reynolds number. Improved similarity with the suggested scaling will be discussed.

George Ilhwan Park

George Park is an Assistant Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. He received his Ph.D. and M.S. in Mechanical Engineering (ME) from Stanford University in 2014 and 2011, respectively, and his B.S. in ME from Seoul National University, South Korea in 2009. He worked as a postdoctoral fellow and an engineering research associate at the Center for Turbulence Research (Stanford) prior to joining UPenn as a faculty member. His research interests include high-fidelity numerical simulation of complex wall-bounded turbulent flows, computational methods with unstructured grids, non-equilibrium turbulent boundary layers, and fluid-structure interaction.