Development of a hybrid RANS-LES model for transitional boundary layers

Abstract

High-fidelity hybrid RANS/LES methods are unable to accurately predict the transition from laminar to turbulent boundary layer flows. Usually resulting in an incorrect prediction of boundary layer characteristics. In this study, a hybrid RANS/LES model was developed for transitional boundary layer flows. The transitional kkL-omega turbulence model was combined with the RANS/LES blending function from the framework of the Improved Delayed Detached Eddy Simulation (IDDES). Initial simulations predicted a transition location that agreed with experimental data. However, development of turbulent structures were delayed. Therefore, velocity fluctuations were added to the flow at the location of transition. The intensity of the velocity fluctuations were based on the modelled kinetic energy that is present in the pretransitional boundary layer. The addition of the velocity fluctuations, accelerated the development of turbulence and reduced the classical grey area problem. Flat-plate simulations with different free-stream turbulent intensities all predicted a transition location that agrees with experimental data. The predicted skin-friction in the transitional boundary layer improved with decreasing free-stream turbulence levels. Simulation of a flow over a NACA 0012 airfoil predicted a skin-friction distribution and trailing edge displacement thickness that was superior compared to an existing non-transitional hybrid RANS/LES model. Predicted skin-friction values of a DU91W250 wind turbine airfoil agreed with the kkL-omega RANS results. Right after transition, the kinetic energy of the boundary layer was underpredicted in both simulations. This problem might be solved when the introduced velocity fluctuations more accurately represent the turbulent structures of the pretransitional boundary layer.