Assessment of LES techniques for mitigating the Grey Area in DDES models

The slow RANS-LES transition is a well-known shortcoming in hybrid turbulence models such as DelayedDetached Eddy Simulation (DDES). The present work assesses the feasibility of 2D sensitive LES models for naturally triggering turbulence, rather than using the specifically designed techniques available in the literature. This research has been carried out with OpenFOAM, which has been used for simulating the flow in a Backward-Facing Step configuration. The results have been compared with a DNS data set, showing the good mitigation capabilities of such LES techniques. Nevertheless, other cases should be studied before extracting any relevant conclusion. Introduction Accurate numerical simulations are essential for understanding the complex flow physics present in many aeronautical applications. RANS models are commonly used in the industry, as they are cost-effective, but their limitations for predicting complex flow behaviours and providing unsteady data are also well-known. Moreover, the routine use of accurate numerical methodologies such as Large Eddy Simulation (LES) require heavy computational cost, so their applications are not yet feasible. In this regard, Delayed-Detached Eddy Simulation (DDES)9 is intended to circumvent the massive costs of pure LES simulations, modelling the boundary layer using RANS and simulating the unsteady flow behaviour with LES at the core. This hybrid turbulence model is widely used due to its user-friendly non-zonal approach and its proved success in several applications. Especially in those situations where RANS applications are unreliable. Apart from that, hybrid turbulence models (in contrast to RANS) can provide high quality transient data, which is completely necessary for simulating complex coupled physics, such as Fluid-Structure Interaction (FSI) and Computational Aeroacoustics (CAA). It is therefore not surprising that during the last decade, these methods have been gaining importance in the aeronautical industry. However, some of their well-known weaknesses are still present. In particular, the slow transition from RANS to LES leads to unphysical results, delaying the flow instabilities in complex zones such as free shear layers. The zone where this issue takes place is named Grey Area (GA). In the literature, there are two main strategies for leading this shortcoming4 . One of them consists on using artificial oscillations in specific areas (zonal approach), whereas the other is based on reducing the subgrid-scale viscosity, νsgs = (Cm∆) Dsgs (ū) , (1) νsgs, in LES 2D flow regions. The second approach is preferable as it is aligned with the initial non-zonal DES philosophy. This reduction could be forced by any of the terms present in Eq. 1. The idea of attributing kinematic sensitivity to the Subgrid-Length Scale (SLS) coefficient, ∆, was initially explored by Mockett et al.4 (∆̃ω) and Shur et al.8 (∆S LA). Later we proposed another approach, which was initially developed for LES12 (∆lsq). Surprisingly, even though ∆lsq was initially designed for LES applications, first studies6 show how its performance mitigating the GA in DDES was Copyright© 2019 by ADD AUTHORS. Published by the EUCASS association with permission. DOI: 10.13009/EUCASS2019-777