Syntethic Inflow Boundary Conditions for the Numerical Simulation of Turbulence

  • Jarrin N
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Abstract

This thesis describes the development and validation of a new method for the generation of synthetic inlet conditions, referred to as the Synthetic Eddy Method (SEM), for Large-Eddy Simulation (LES). The motivation for this work is the growing interest of the engineering community in hybrid methods coupling Reynolds-averaged Navier-Stokes (RANS) approaches in regions of the flow that are at equilib- rium (where RANS can be trusted), with LES approaches elsewhere. The focus of this thesis is on the RANS-to-LES interface inside attached turbulent boundary layers, where an unsteady LES content has to be explicitly generated from a steady RANS solution. The SEM is a stochastic algorithm that gener- ates instantaneous velocity fluctuations from input statistical quantities that are typically available from a RANS solution. The method is based on the classical view of turbulence as a superposition of eddies. The signal is expressed as a sum of synthetic eddies with random position and intensity, and which are subsequently convected through the LES domain inlet. The method generates stochastic signals with prescribed mean velocity, Reynolds stresses, and length and time scale distributions. The SEM is imple- mented into the unstructured finite volume code, Code Saturne, and used to generate inflow data for the LES of plane channel flow. It is shown that the robustness of the SEM depends strongly on the correct specification of its input parameters (i.e. mean velocity, turbulent kinetic energy, and integral length and time scales). Equations to compute all the input parameters of the SEM from simple RANS statistics are then derived, and the SEM is used to couple an upstream RANS simulation with a LES in the case of simple wall-bounded flows (i.e. channel, boundary layer and duct flows). Generally with synthetic tur- bulence, some distance is required downstream of the inlet for realistic fluctuations to develop. With the SEM, this development length is found to be approximately 3,000 wall units for all the cases simulated - much shorter than other comparable methods of generation of synthetic turbulence. Hybrid simulations of more complicated turbulent flows involving separation and reattachment (i.e. the flow over a backward facing step and over an airfoil trailing edge) are then performed. The SEM is compared to (and found to perform better than) other existing methods of generation of synthetic turbulence. It is shown that when the SEM is used, the RANS-to-LES interface can be placed only 1−2 boundary layer thicknesses upstream of the region of interest (where the flow separates) without significantly alterating the results.

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Jarrin, N. (2008). Syntethic Inflow Boundary Conditions for the Numerical Simulation of Turbulence. PhD Thesis, 1–258. Retrieved from papers3://publication/uuid/30C73EC3-BF64-4638-9A13-2FDF556EE3C0

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