Avances en Modlisation de la Turbulence

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

2

• What is new in Turbulent
  Modelling?
•   Osborne Reynolds (1895) RANS Re Averaged
  Navier Stokes gt 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)
  1980-1990 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 gt LES for everyone
  and everything ?

. Computers work more so modellers can retire ?
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
Ex Mixing in T junctions (Watlon test case, T.
Pasutto)

• Mixing FAR from walls OK easier for LES
• Mesh quality still important (nearly cubic hexa
  is best)

Non dimensional Mean temperature
Measurements
Temp.
RMS T.
Non dimensional RMS temperature
3M cell LES wall functions
Hexa mesh better than tetra
5
Under-Resolved Channel LES
Coarse-mesh LES is more dangerous than coarse
RANS !
Channel Flow LES on structured gridat Re395
(Rey at centre ) From N. Jarrin 2006
www.cfdtm.org
error on friction
6
LES gt great, colourful results IF .. the
mesh is such that large scales are well captured
(which means ... solution is already
known ?)
7

• 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 2004-2007.
• 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
www.nea.fr/html/nsd/csni/cfd.htm
8
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 http//cfd.mace.manchester.ac.uk/Ma
in/KnooWorkshop To appear http//www.hse.gov.uk/
and J Flow, Turbulence and Combustion
9
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
  by2 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
• () beware e.g. SST model in a simple channel
  flow will dramatically underestimate k near wall
  by a factor 2-3, and over-estimate lengthscale at
  centre by a factor 2!

10
Channel flow LES with Taylor scale mesh, N
total 0.44 Million , Re 395
Standard 2nd order FV codes can produce DNS like
results on good grids)
U
U
y/h
y
y
11
Unstructured meshing strategy for LES
a)
Possible FV near-wall refinements a) dichotomy,
b) non-conforming, c) d) polyhedral zoom.

• Lenghtscale predicted by PRECURSOR RANS, then L
  (k3/2)/eps

32 hand-hade blocs Very time consuming!
Coaxial swirling jet Taylored grid
12
Many High-Re multiscale complex geom.
challenges

• Need whole range of turbulence models
• RANSWF, Low RANS, LES, LES
  embedded in RANS
• Need to be able to generate quality meshes (in
  volume)

Sodium Fast Reactor (Gen IV) Wire-wrapped fuel
assembly C. Peniguel, S. Rolfo)
Advanced Gas Reactor Effect of fuel pin ribs
13
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
  explanationshow many failed LES were never
  published ?
• Mesh influence is tremendousLES 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)