C. Mockett, M. Fuchs & F. Thiele

Transcription

1 C. Mockett, M. Fuchs & F. Thiele

2 Overview Background & motivation DES strong points and Grey Area problem The Go4Hybrid project Improved DES with accelerated RANS to LES transition Basic test case: free shear layer Round jet at M = 0.9 Conclusions & outlook 2

3 DES strong points and the Grey Area problem The Go4Hybrid project 3

4 Motivation of hybrid RANS-LES Strategies for turbulent flows in CFD Hybrid RANS-LES: Combine strengths of RANS and LES Target current and near-future computing capacity 4

5 Strengths of hybrid RANS-LES Massively-separated flows: Dramatic accuracy increase w.r.t. (U)RANS Example of DDES application to helicopter fuselage flow PIV 0.5 URANS DDES 5

6 Best-practice output from ATAAC (2012) Method suitability for different classes of flow Weakly separated flows are poorly handled by hybrid RANS-LES methods: The Grey Area problem 6

7 The Grey Area problem Essentially the detrimental influence of the RANS to LES transition (RLT) region in the early shear layer Example for shallow recirculating flow from ATAAC (NTS) 7

8 The Go4Hybrid project Grey Area Mitigation for Hybrid RANS-LES Methods Funded by EU in final call of FP7 2 years duration (Oct 2013 Sept 2015) 7 Partners: CFD Software E+F GmbH (coord), DLR, FOI, ONERA, NLR, NTS, University of Manchester 10 Observers: Ansys Inc., Bombardier Transportation, Electricité de France (EDF), Exa Corporation, GE Global Research, NUMECA International, PSA Peugot-Citroen, Rolls-Royce Deutschland (Aero engines), SAAB AB (Aerospace), Volkswagen AG 2 Associate Partners: EADS Cassidian, Airbus Helicopters Developments in two main areas: Non-zonal approaches Embedded approaches 7 test cases from academic to complex OpenFOAM to be used as common assessment platform Direct comparison of different models with identical underlying numerics 8

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10 Our approach For non-zonal approaches the RLT issue is inherent, even for fine grids and when the switch between RANS and LES modelling is immediate The LES content is missing The problem is however exacerbated in DES by excessive eddy viscosity in the early shear layer This damps the development of resolved turbulence Our approaches aim to improve RLT behaviour by reducing eddy viscosity in the early shear layer Development priorities: DES performance should not be degraded in other respects Retain non-zonal nature of approach Robust and applicable to complex cases Generally-applicable method 10

11 General form of model in LES mode DES behaves like the Smagorinksy model in LES mode Assuming equilibrium of the production and destruction terms: Approach 1 aims to responsibly reduce the grid filter scale, Compares anisotropic grid cell orientation to the local vorticity vector Reduces to max x, y when vorticity axis is aligned with z Approach 2 adopts alternative definitions for the differential operator on the velocity field, WALE and σ models of Nicoud et al. discern between laminar and turbulent flow fields DES production term switched off in initial, 2D shear layer Normal model functionality recovered for fullydeveloped, 3D turbulence Publication of approach in progress: C. Mockett, M. Fuchs, A. Garbaruk, M. Shur, P. Spalart, M. Strelets, F. Thiele, A. Travin: Two non-zonal approaches to accelerate RANS to LES transition of free shear layers in DES. In: Progress in Hybrid RANS-LES Modelling, Springer (2014 hopefully!) 11

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13 Shear layer: WALE/σ + Δ ω The WALE/σ approach can be combined with the ω grid filter scale: SA-DDES SA-WALE-DDES SA-σ-DDES max ω 13

14 Eddy viscosity: WALE/σ + Δ ω SA-DDES SA-WALE-DDES SA-σ-DDES max ω 14

15 Mean vorticity thickness 15

16 Grid refinement Grid refinement further improves agreement with experiment SA-σ-DDES + Δ ω coarse grid refined Δ x at splitter plate trailing edge 16

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18 Effect of model improvements (coarse grid) Turbulent unheated static jet at M = 0.9, Re D = 1.1x10 6 Results on coarsest grid G1 (~1.4M cells) Δ max SA-DDES SA-WALE-DDES SA-σ-DDES Δ ω 18

19 Effect of model improvements (coarse grid) Instantaneous eddy viscosity ratio at zoom of early shear layer: SA-DDES + Δ max SA-σ-DDES + Δ max Convected υ t from RANS BL Strong reduction of new eddy viscosity production due to shear 19

20 Grid refinement (SA-σ-DDES + Δ ω ) G1 G2 x/d = 4 G3 20

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22 Conclusions & outlook Developed approaches appear promising Dramatic improvement in RANS to LES transition Approaches remain generally-applicable Future work: More complex flows e.g. delta wing, helicopter fuselage How do new methods perform for WMLES? e.g. channel flow, in comparison with IDDES Direct comparison with developments of other partners within Go4Hybrid OpenFOAM used as common assessment platform Technology transfer: Implementation of approaches in industrial in-house code Prediction of jet/wing interaction noise 22

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