Methodologies for RANS-LES interfaces in turbulence-resolving simulations

Doctoral thesis, 2017

Hybrid Reynolds-Average Navier-Stokes (RANS)-Large-Eddy Simulations (LES) techniques are now seen by the aeronautical industry as the most promising turbulence-resolving approaches for complex high-Reynolds-number turbulent flows with regard to cost and accuracy. However, connecting RANS and LES simulated flows is a challenging task and needs special attention in order to increase simulation robustness and accuracy. This thesis presents a new low-Reynolds-number k − ω based zonal hybrid RANS-LES turbulence model with which novel interface methodologies for connecting RANS and LES regions are explored. The emphasis of the work reported in the thesis is on RANS-LES interface techniques for grey-area mitigation and reduction of log-layer mismatch since these are the two most important issues to resolve in hybrid RANS-LES modeling in order to meet industry requirements for robustness and accuracy. The proposed hybrid RANS-LES model was applied to Decaying Homogeneous Isotropic Turbulence (DHIT) and channel flow for calibration purposes. It was concluded from the simulations of fully developed channel flow that a wall distance based LES length scale was superior in reducing the log-layer mismatch compared to other LES length scales discussed in the literature. It was shown from simulations of spatially developing boundary layer flow and flow over a wall-mounted hump that a RANS-LES interface technique combining commutation terms, introduced in the k and ω equations to reduce the turbulent viscosity, and synthetic turbulent fluctuations substantially mitigates the grey area as compared to commonly used RANS-LES interface methods and gives results that are in good agreement with experimental data. In simulations of a plane mixing layer flow, commutation terms were introduced in the k, ω and momentum equations in order to represent the transfer of energy between modeled and resolved turbulent scales at the  wall-normal RANS-LES interface, located at the trailing edge of the flat plate. It was found that the commutation terms in the momentum equations are able to trigger the equations to resolve turbulence, thus mitigating the RANS-to-LES transition region and improving the prediction of the resolved turbulent stresses. These were in good agreement with experimental data. Moreover, the proposed hybrid RANS-LES model has been successfully used to simulate a transonic flow in a rectangular duct with shock-induced corner flow separations.