1. Avancées en Modélisation de la Turbulence
SFEN ST 6
CFD pour conception & sureté des réacteurs, Chatou, le 29 Avril 2009
D. Laurence & F. Archambeau, MFEE
16 avril 2017
Groupe EDF

2. What is new in Turbulent Modelling. …
What is new in Turbulent Modelling? …. Computers work more so modellers can sit back
•  Osborne Reynolds (1895): RANS “Re Averaged Navier Stokes” => exact expression for the production of stresses (&fluxes),
•  Joseph Valentin Boussinesq (1897) Eddy Viscosity
•  Ludwig Prandtl (1925): mixing length
•  Andrei Komogorov (1942): energy cascade production – dissipation scales separation
LES Joseph Smagorinsky (1964): Stanford CTR, Kim, Moin, Moser, Ferziger, Piomelli …
k-eps : Jones & Launder (1972), Launder & Sharma (1974),
Re Stress Transport: Launder, Reece, Rodi (1975), Speziale Sarkar Gatsky(1991)
2000 – 2009 , lower cost HPC => LES for everyone and everything ?

3. LES preamble: Only Large Scales Matter (most of the time)
Large Scales & Human activity
Drag, mixing, heat transfer, chem. reactions …
Large Scales dictate physics
Generated by/scale with obstacle
Impose dissipation rate
Exceptions: noise, combustion, 2-phase flows
…and NEAR WALLs !

4. LES => great, colourful results … IF
LES => great, colourful results … IF .. the mesh is such that large scales are well captured (which means ... solution is already known ?)
16 avril 2017
Groupe EDF

5. Coarse-mesh LES is more dangerous than coarse RANS !
Under-Resolved Channel LES
Coarse-mesh LES is more dangerous than coarse RANS !
Channel Flow LES on structured grid at Re*=395 (Re*=y+ at centre )
From N. Jarrin
% error on friction
16 avril 2017
Groupe EDF

6. Detached Eddy Simulation (DES) Scale Adaptive Simulation (SAS)
- LES limitation: too strongly dependent on mesh, unreliable for true “predictions”
- Hybrid RANS-LES methods developed in EU AEROSPACE project DESIDER
Validation limited to aerodynamics, seems premature for Reactor thermal hydraulics !
“SAS-SST model formulation dos not involve the grid spacing, it avoids the undefined model regimes of DES. In case of overly coarse grids, the model reverts back to the underlying RANS formulation. Fig. 5.2: The SST-SAS model can represent the true nature of the flow, by allowing a break-down of the turbulent structures into a spectrum (down to the grid limit)”
From “Best Practice Guidelines for the use of CFD in Nuclear Reactor Safety Applications” OECD NEA workgroup on CFD in reactor studies

7. Flow ventilation of room with release of passive scalar
DES error
SAS error
RANS error
From Dr S. Gant. 'Challenges in validation of unsteady modeling approaches (DES/SAS/URANS)‘ presented at: To appear and J Flow, Turbulence and Combustion

8. Better RANS models to predict Length scales
Kolmogorov lengthscale
Taylor microscale
Turbulent energy scale:
First 3 are only “models” but can be obtained from “good” RANS models(*)
“True” Taylor and Integral scale are only given by 2 point correlations, i.e. curvature and integral, e.g.:
More work needed to extract “true” scales from DNS or expt. and test predictions from recent advanced RANS models
(*) e.g. SST model in a simple channel flow will dramatically underestimate k near wall by a factor 2-3, and over-estimate lenghscale at centre by a factor 2!

9. Channel flow LES with “Taylor scale” mesh, N total = 0
Channel flow LES with “Taylor scale” mesh, N total = 0.44 Million , Re = 395 U U
y/h y+ y+

10. Unstructured meshing strategy for LES
Possible FV near-wall refinements: a) dichotomy, b) non-conforming, c) & d) polyhedral & zoom.
Lenghtscale predicted by PRECURSOR RANS, then L= (k**3/2)/eps

11. Conclusions
Can we design next gen. nuclear power plants using LES/DES ?
Errors can be much larger than in RANS Beware vast majority of LES are “post-dictions”, or “explanations” how many failed LES were never published ?
Mesh influence is tremendous LES quality criteria progressing (see Q-LES workshops B. Guerts et al.) but trial and error most expensive with LES
Use “good” RANS model as precursor for LES feasibility study and mesh generation (but more user practice and control and over meshing software is needed)
Progress in RANS-LES coupling (google ANR STURM4)
RANS models are progressing quietly but surely (elliptic blending, algebraic structure based models… ), but no time to present…
Beyond “cold / aerodynamic” flows “good” RANS models will still be major engineering tool for complex physics (2 phases flows, near wall effects, heat transfer, combustion, natural convection) which need more than just eddy viscosity predictions (time-scales, lenghtscales, anisotropy…)
16 avril 2017
Groupe EDF

12. Zonal modelling / RANS-LES coupling
LES community very active
on this topic
First idea (DES) reduce viscosity when turbulent scale > cells
Bad transition region (kink)
where RANS viscosity
is reduced but resolved
structures are still
too weak
Current solution is to OVERLAP
RANS and LES
16 avril 2017
Groupe EDF

13. Zonal RANS-LES coupling, Uribe model
Uribe, J., Jarrin, N., Prosser, R., Laurence, D "Two velocities hybrid RANS-LES of a trailing edge flow". J. Flow, Turbulence and Combustion.
16 avril 2017
Groupe EDF

14. Integral length scales in channel flow
Turbulent energy scale is easy RANS “model”
but does not represent true (2 point correlation) integral scale for channel flow
1- x : streamwise 2- y : wall normal 3 – z : spanwise
streaks
Solid: stream-wise separation
Dashed: span-wise separation
Addad, Y.,Gaitonde, U., Laurence, D., Rolfo, S. Optimal unstructured meshing for large eddy simulations, ERCOFTAC Series, Springer, Quality and Reliability of Large-Eddy Simulations. Meyers, Geurts & Sagaut, (Eds.), Vol. 12, pp
16 avril 2017
Groupe EDF

15. Near wall RANS model developments – heated pipe
buoyancy aiding