Two Non-zonal Approaches to Accelerate RANS to LES Transition of Free Shear Layers in DES

Abstract

We present two novel approaches to improve the behaviour of DES in the region where an attached boundary layer (handled with RANS) flows into a separated shear layer (to be resolved using LES). The approaches aim to be generally-applicable and retain the non-zonal nature of DES. Furthermore, the formulations are local and can be readily implemented in general-purpose solvers. One approach introduces an adaptive grid scale definition, sensitised to the local vorticity orientation. The second approach, which can be combined with the first, involves the incorporation of alternative SGS model formulations that discern between quasi 2D and developed 3D flow states. Both modifications lead to a strong reduction of eddy viscosity in the early shear layer. Consequently, a significant acceleration of RANS to LES transition is demonstrated for a plane shear layer, a backwards-facing step and a round jet, with results from two different flow solvers shown. The greatest improvement is seen when the approaches are applied in combination.

An Enhanced Version of DES with Rapid Transition from RANS to LES in Separated Flows

“…The first modification, =˜ ω (1) is inspired by a recent proposal of Chauvet et al [7] but, unlike this proposal, does not involve the minimum grid step of the cell in case of anisotropic grids. The second modification, = SLA =˜ ω F KH (3) ensures similar improvements of model's performance as that provided by the WALE-based DES version combined with =˜ ω [8] but is simpler and universal, i. e, it is directly applicable to a pure LES and to any hybrid methods.…”

“…This definition has been already published in Mockett et al [8], where it is used both in isolation and in a combination with a special modification of the SGS version of the SA model. Unlike its original formulation [3,9] which at equilibrium (assuming that generation is equal to dissipation) essentially reduces to the Smagorinsky model, this version reduces to the WALE SGS model of Nicoud et al [10].…”

Assessment of novel DES approach with enhanced SGS modelling for prediction of separated flow over a delta wing

“…This effectively applies an alternative SGS functionality to DDES, where we seek to keep the RANS mode unaltered. In Mockett et al 11 , we presented two different SGS formulations which potentially could improve standard DDES, namely the WALE model of Nicoud et al 12 and the σ model of Nicoud et al 13 . Both models draw on a more complex formulation for the velocity operator D sgs (u) which vanishes in the presence of pure 2D shear.…”

“…As an additional means to accelerate RANS-to-LES transition in separated shear layers, a novel formulation for the LES filter width ∆ is evaluated for the delta wing, which was proposed in Mockett et al 11 and more recently published by Shur et al 15 . It is based on the idea to sensitise the filter width ∆, which for standard DDES is usually a geometric measure of the local grid size, i.e.…”

“…It is based on an alteration of SGS functionality of DES, which generally acts as a Smagorinsky-like LES model, to a more elaborate formulation that aims to improve RANS-to-LES transiton in free shear layers. The approach was first published by Mockett et al 11 and subsequent case studies were presented in Fuchs et al 5 . It could be demonstrated that for cases featuring pure planar shear, e.g.…”