A Stress-equivalent Spalart-Allmaras Wall Model with Local Boundary Conditions for RANS, DES, and LES
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Ursachi-ursachi-phd-aeroastro-2024-thesis.pdf
Description
Thesis PDF
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13.15 MB
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Adobe PDF
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de7c2d8a125e381c9aeabda393a2ffcb
Author(s)
Ursachi, Carmen-Ioana
Advisor(s)
Darmofal, David L.
Date Issued
February 2024
Publisher
Massachusetts Institute of Technology
Abstract
While high-fidelity, scale-resolving methods in Computational Fluid Dynamics (CFD) are increasingly applied, the cost of these methods remains a significant barrier to their effective use. In this thesis, a new wall model is developed based upon a modified version of the Spalart-Allmaras (SA) turbulence model that lessens the near-wall grid requirements. This is achieved by, below the log layer, making the eddy viscosity approach a constant, non-zero value, and the velocity, which has a non-zero slip, varying approximately linearly with distance from the wall while maintaining the same total shear stress. The wall model introduces one parameter which controls the near-wall behavior of the solution. Unlike typical wall models, this method avoids the need to query the interior solution by utilizing a boundary condition which only requires solution information present at the boundary, making it well-suited for unstructured grids and mesh adaptation. The new approach is combined with mesh adaptation and applied to ReynoldsAveraged Navier-Stokes (RANS), demonstrating accurate predictions of quantities of interest such as aerodynamic coefficients, surface pressure and temperature, skin friction, and heat transfer compared with standard RANS-SA, while requiring substantially less near-wall grid to resolve the solution. Additionally, the new wall model and modified turbulence model are applied to Detached Eddy Simulation (DES) in a hybrid RANS/LES framework, where it is demonstrated that the wall model allows for reliable solutions on near-wall grids that are significantly coarser in the wall-normal direction than those used typically for DES. Finally, the wall model boundary condition is applied to wall-stress Wall-Modeled Large Eddy Simulation (WMLES) and shown to produce similar results to the traditional equilibrium model, while avoiding the need to query the interior solution.
MIT Department
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics
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