Hybrid RANS/LES investigation of the interaction of a longitudinal vortex with an inclined airfoil

Abstract

Engine nacelles of modern commercial aircraft are often equipped with small vortex generators (strakes) to avoid early flow separation of the adverse-pressure-gradient boundary layer on the main wing in low-speed flight. In numerical flow simulations, conventional RANS models show a tendency to overly dissipate these free vortices. Therefore, we study an application scenario for hybrid RANS-LES methods to account for longitudinal vortices and their interaction with airfoils, in order to clarify some open questions: the grey-area problem for free vortices and the extension of synthetic turbulence to such large-scale turbulent structures, the influence of mesh design and resolution, as well as the modelling of the RANS and LES regions over the airfoil. Three approaches are compared: Firstly, pure IDDES is employed with its inherent switching mechanism to wall-modelled LES. Secondly, the switching in the IDDES is controlled by algebraic sensors which activate the WM-LES and the injection of synthetic turbulence upstream of the interaction region of the free vortex with the attached boundary layer. Thirdly, the entire boundary layer is treated in RANS mode.