Assessment of the X-LES method and a decomposed turbulence model by application to turbulent channel flow

Master thesis (2020)

Authors

M. Henger Aerospace Engineering

Contributors

S. Hickel Aerodynamics - Aerospace Engineering (mentor)
J.C. Kok Royal Netherlands Aerospace Centre NLR (graduation committee member)
Faculty
Aerospace Engineering, Aerospace Engineering
Copyright: © 2020 Max Henger
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Published Date

20-07-2020

Language

English

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Faculty

Aerospace Engineering

Abstract

Reynolds-Averaged Navier-Stokes (RANS) turbulence models are in widespread use in the commercial application of Computational Fluid Dynamics (CFD), but lack the capability to accurately compute unsteady flows and aeroacoustics due to its implied averaging of turbulent effects. In order to compute these kinds of flows, Large Eddy Simulation (LES) may be applied. However, this comes with a large computational cost. Hybrid RANS/LES aims to combine the turbulence-resolving aspects of LES, while attempting to approach the computational efficiency of RANS by applying a RANS turbulence model near walls. The X-LES turbulence model developed at NLR has been successfully applied to massively separated flows. A future application of the method is to obtain aeroacoustic predictions of turbulent noise near sharp edges and wakes. In anticipation of this goal, this work aims to assess the accuracy of the X-LES model when it resolves part of the near-wall turbulent spectrum of Turbulent Channel Flow (TCF).

Multiple TCF simulations have been performed at a variety of friction Reynolds numbers. It was found that the presence of resolved turbulent stress in the RANS-domain is not detrimental to the velocity profile in this domain, as it still conforms to the laws governing the viscous sublayer and the log-layer. The presence of so-called superstreaks causes a flow system where streamwise ribbons of high eddy viscosity, originating from within the RANS-domain, are transported away from the wall into the LES-domain. Here they contribute to the lack of development of turbulent stress, leading to an undesirable increase in velocity in the log-layer known as the Log-Layer Mismatch (LLM). Applying stochastic forcing led to a reduction in the LLM and broke up the large streamwise regions of eddy viscosity. It remains unclear in which proportion the disappearance of the high eddy-viscosity regions, and the stochastic forcing itself, contributed to the reduction in LLM.

An attempt has been made in formulating a consistent hybrid RANS/LES framework in which the effects of turbulence on the mean and fluctuating flow are governed by separate turbulence models throughout the entire domain. The initial results are presented in this report, which may be used to further develop the model in the future.