Fluent-Adv Turbulence 15.0 L04 Les and Utmm

1 © 2014 ANSYS, Inc. April 23, 2014 ANSYS Confidential

15.0 Release

Lecture 4: Large Eddy Simulation (LES) and

Other Scale Resolving Simulation (SRS)

Models

Turbulence Modeling Using ANSYS Fluent


Topics to be Discussed • Unsteady Turbulent Flow Calculations

– Motivation

– Challenges

• Large Eddy Simulation (LES)

– Filtering

– Subgrid models

– Grid size and time step size

– Boundary Conditions

– Wall issues

• Hybrid RANS/LES Approaches

– Detached Eddy Simulation (DES)

– Scale Adaptive Simulation (SAS)

– Embedded LES (ELES)

– Wall Modeled LES (WMLES)

Modeling approaches that can capture the unsteady motion of a range of different turbulent scales (turbulent content) are referred to as Scale Resolving Simulations (SRS)


Motivation for Scale Resolving Simulations • Many flows of interest require the simulation to be able to resolve the unsteady

motion of the most important turbulent scales – Large scale separation – Acoustics – Small scale processes like micro-mixing or chemical reactions

• This motion is inherently unsteady and three-dimensional

• Unsteady RANS (URANS) simulations might be able to capture some mean flow unsteadiness, such as vortex shedding, but are generally incapable of capturing unsteadiness arising from turbulence

SST k-w model LES


Overview of SRS modeling approaches • Large Eddy Simulation (LES)

• Detached Eddy Simulation (DES)

• Scale Adaptive Simulation (SAS)

• Additional Hybrid RANS/LES approaches – Embedded LES (ELES)

– Wall modeled LES (WMLES)


Large Eddy Simulation (LES) • Recall: Two methods can be used to eliminate the need to resolve small scales

– Reynolds Averaging Approach: Temporal averaging

• All scales are modeled

• Periodic and quasi-periodic unsteady flows

– Filtering (LES): Spatial averaging

• Transport equations are filtered such that only larger eddies need be resolved

– Difficult to model large eddies since they are

• anisotropic

• subject to history effects

• dependent upon flow configuration, boundary conditions, etc.

• Only smaller eddies are modeled

– Typically isotropic and so more amenable to modeling

• Deterministic unsteadiness of large eddy motions can be resolved


Energy Cascade (Richardson, 1922)

LES and the Energy Cascade


LES: Spatial Filtering

2

E

Energy spectrum against the length scale

t,xu

f2

t,xu

scalesubgrid scaleresolved

ttt ,,, xuxuxu


LES: Spatial Filtering • A random variable, f (x), is filtered using a space- filter function, G

• With the top-hat filter (among others)

• The filtered variable becomes

• Most LES codes use implicit filters – Filter width determined by grid resolution

D

dG xxxxx ),()()( ff

otherwise

for

0

/1

,

xxx

VG )(

V dV

xxxx ,)(1

)(

ff


jijiij uuuu

Filtering the Navier-Stokes Equations

• Filtering the original Navier-Stokes equations gives filtered Navier-Stokes equations that are the governing equations in LES

N-S

equation

Filtered N-S

equation

Needs modeling Sub-grid scale (SGS) stress

Filter

j

ij

j

i

jij

jii

xx

u

xx

p

x

uu

t

u

1

j

i

jij

jii

x

u

xx

p

x

uu

t

u

1


DNSLES i iDNS

j j

u u

x x

ˆ ˆLES i iLES t

j j

u u

x x

• Role of LES model: – Eddies cannot be resolved down to

the molecular dissipation limit

– Dissipation of turbulent kinetic energy at grid-resolution limit requires eddy viscosity

– An important role of the SGS model is to dissipate the energy of the small scales

– Everything of importance should be resolved

Effect of SGS Model

Eddy viscosity calculated by the SGS model should ensure energy at the grid cutoff is dissipated at the appropriate rate

Log E

Log k

Viscous dissipation

LES dissipation

Grid Cut-off

Generation of eddies

Energy transfer


Decaying Isotropic Turbulence

– Without SGS model energy is accumulated at small scales (large wave number k)

– With SGS models, energy is dissipated at grid resolution limit

Without SGS Model With SGS Model

Grid size cutoff 1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00 1.00E+01 1.00E+02

Exp T1

WALE C=0.5

DSM

SMAG C=0.18


LES: SGS Stress Modeling • Fluent offers several eddy viscosity

sub-grid scale models – Smagorinsky-Lilly model

– Wall-Adapting Local Eddy-Viscosity (WALE) model

– Dynamic Smagorinsky-Lilly model

– Dynamic Kinetic Energy Transport model

– Wall-Modeled LES (WMLES)

– WMLES S-Omega


LES: Smagorinsky-Lilly SGS Model • Simple algebraic (zero - equation) model

• Smagorinsky constant Cs= 0.1 ~ 0.2

• Model relies on a local equilibrium of the sub-grid scales (i.e. local production-dissipation of sub-grid scales, no transport)

• The major shortcoming is that there is no Cs universally applicable to different types of flow

• Difficulty with transitional (laminar) flows

• An ad hoc damping is needed in the near-wall region

with ijijSSS 2,3/1 V ijsijkkij SSC 23

1 2


LES: WALE SGS Model • Wall-Adapting Local Eddy-Viscosity model

• Algebraic (zero - equation) model – retains the simplicity of Smagorinsky’s model

• The WALE SGS model adapts to local near-wall flow structure

– Wall damping effects are accounted for without using the damping function explicitly

– Correct asymptotic behavior of eddy viscosity near wall

• Does not allow for non-equilibrium or transport effects for turbulence in sub-grid scales

onmodificati wall-near

4525

23

2

//

/

d

ij

d

ijijij

d

ij

d

ij

sSGS

SSSS

SSC


LES: Dynamic Smagorinsky-Lilly SGS Model • Based on the similarity concept and Germano’s identity (Germano et al., 1991;

Lilly, 1992) – Assumes local equilibrium of sub-grid scales, scale similarity between the smallest

resolved scales and the sub-grid scales

– The model parameter Cs is automatically adjusted using the resolved velocity field

• Fluent’s implementation – Locally dynamic model – Implemented for unstructured meshes (test-filter) – Constant Cs by default clipped between 0 and 0.23

– Dynamic SGS models also available for energy and species

• Overcomes the shortcomings of the Smagorinsky’s model – Can handle transitional flows

– The near-wall (damping) effects are accounted for


j

sgs

k

sgs

j

sgs

j

iij

j

sgsjsgs

x

k

x

kC

x

u

x

ku

t

k

2/3

ijsgskijkkij SkC 2/123

1

LES: Dynamic Kinetic Energy Transport SGS Model • 1- equation (for SGS kinetic energy) model. Kim & Menon (1997)

• Transport equation for sub-grid scale kinetic energy allows for history and non-equilibrium effects

• Like the dynamic Sgamorinsky’s model, the model constants ( Ck, C ) are automatically adjusted on-the-fly using the resolved velocity field


LES: Grid and Time-Step Size • LES requires mesh and the time-step sizes sufficiently fine to resolve the

energy-containing eddies

– The cost of resolving near-wall region in high-Re wall-bounded flows is very high

– The mesh resolution determines the fraction of turbulent kinetic energy directly resolved

2ln

Eln

Energy

spectrum

against the

length scale

f

2ln


0

k

k

0.0

1.0

0.1

6.1

0.8

0.42

LES: Grid Size • Suppose we want to resolve

80% of the turbulent kinetic energy

• Then, we need to resolve the eddies whose sizes are larger than roughly half the size of the integral length scale l0.

Cumulative TKE against length-scale of eddies based on the Kolmogorov’s energy spectrum

6.10

1.6

0.42

0.16

0

kk 1.0

kk 5.0

kk 8.0

kk 9.0


LES: Grid Size • Integral length scale l0

– Turbulent kinetic energy peaks at integral length scale. This scale must be sufficiently resolved

– Crude estimation for l0

– Based on size of bluff body

– Estimate from correlations

– Perform RANS calculation and compute l0=k1.5/

– Generally require 5-10 grid cells per integral length scale to obtain proper LES content (see next slide)

• At least 10 would be preferable


LES: Estimating Grid Size from RANS Calculation

Contour plot showing ratio of integral length scale to cell size Higher values imply more grid points per integral length scale, or higher spatial resolution Lower limit of contour scale in the picture is 10. Areas without contours may require higher grid resolution during mesh generation for LES


• The temporal resolution should match or exceed the spatial resolution in LES

– Let’s say we have a cell with largest edge length of ∆ x and local average velocity, U

– It takes a time interval ∆ t for the flow to travel across the cell

– The time step should be small enough to provide an adequate temporal resolution of the flow as it passes through the cell

– The real velocity can be higher than the averaged velocity

– Pre-cursor RANS simulation is used for the assessment of ∆ t

• Good practice to account for differences between instantaneous and averaged velocities as well as for the errors introduced from RANS by

LES: Time Step Size

U

xt

U

xt

2

5.0

x

tU


LES: Inlet Boundary Conditions

• It is often important to specify a realistic turbulent inflow velocity for accurate prediction of the (downstream) flow

• Fluent offers two specification methods for inflow perturbations, available at velocity inlets

– Spectral Synthesizer

– Vortex Method

randomcoherent

i

averagedtime

ii tuUtu

,, xxx



• Spectral Synthesizer – Based on the work of Celik et al.(2001)

– Able to synthesize anisotropic, inhomogeneous turbulence from RANS results (k-, k-w, and RSM fields)

– The velocity-field satisfies the continuity by design, i.e. it is divergence-free



• Vortex Method

– Vorticity transport is modeled by distributing and tracking many point-vortices on a plane (Sergent, Bertoglio)

– Velocity field computed using the Biot-Savart law

ttt k

N

k

k ,,

1

xxx

w

xxx

exxxxu

dt z

22

1,

w


LES: Initial Conditions • Initial condition for velocity field is generally not

important for statistically steady-steady flows

• Patching a realistic turbulent velocity field can however help shorten the simulation time substantially to get to a statistically steady state

• The spectral synthesizer can be used to superimpose turbulence on top of the mean velocity field

• TUI command

/solve/initialize/init-instantaneous-vel

Velocity field generated by

turbulence synthesizer for

homogeneous turbulence


LES: Walls & Near-Wall Resolution (1) • Up to this point walls in LES have not been considered

• LES can be prohibitively expensive for complex medium to high Reynolds number flows involving wall boundary layers

– Meaning most technical flows

• The problem stems from the mesh resolution required to properly resolve wall boundary layers in LES, typically reported as

– y+ = 1 ( NY = 30 ), x+ = 40, z+ = 20

– This is called Near-Wall Resolved LES, sometimes referred to as NWR

• Because the streamwise and spanwise resolution scales with wall coordinates ( x+, z+) there is a strong Reynolds number dependency resulting in excessive mesh requirements for practical flows


LES: Walls & Near-Wall Resolution (2) • The challenge with NWR

– As Reynolds number increases and the boundary layer becomes thinner, the size of important energy bearing eddies decreases

– In LES, the important energy bearing eddies must be resolved, thus the cost of maintaining the grid resolution becomes prohibitive • NWR is only currently feasible for low Reynolds number problems

• Unlike RANS, both the wall normal and wall parallel spacing (both directions) must also decrease to resolve smaller eddies, so the number of grid points increases in all 3 directions

• As size of boundary layer cells decreases, so also does the size of the time step needed to maintain local cell Courant numbers less than 1

• It is estimated that it should be possible to perform LES with NWR on an airplane wing by 2045 (Spalart, 2000)

• Therefore, some form of near-wall modeling (NWM) is required for many cases in which it would be desirable to perform LES


LES Wall Modeling in Fluent • Wall Functions in LES

– The near-wall turbulence is explicitly calculated inside the boundary layer, but not necessarily down to the laminar sublayer

• The first grid point can be located at 20 < y+ < 150

• The log law is used to relate instantaneous wall shear stress to wall parallel instantaneous velocity at the centroid of wall adjacent cells

– Werner-Wengle (cheaper) wall functions also available through TUI

/define/models/viscous/near-wall-treatment/werner-wengle-wall-fn?

• Wall functions, or other wall layer modeling approaches may be satisfactory in simple flows, but in many cases improved approaches involving hybrid RANS/LES models are desirable: – Detached Eddy Simulation (DES)

– Scale Adaptive Simulation (SAS)

– Embedded LES (ELES)

– Wall-Modeled LES (WMLES)


Detached Eddy Simulation (DES) • Goal of DES is to produce a hybrid modeling approach combining the benefits

of RANS and LES while minimizing their disadvantages

– RANS • Can achieve good predictions for attached boundary layers

• Does not capture unsteady motions of large eddies, even if spatial, temporal resolution permits

– LES • Achieves good prediction of separated regions

• Becomes prohibitively expensive in boundary layer with increasing Reynolds number as size of energy-bearing eddies that need to be resolved decreases

LES region RANS region


DES Model Formulation • Example: Spalart-Allmaras model based DES (Spalart et al., 1997)

• One-equation SGS turbulence model with two regions in flow calculation

– Near walls, the flow calculation reduces to unsteady RANS with the S-A model

– In the high-Re turbulent core region, where large turbulence scales play a dominant role, DES recovers LES with a one-equation model for the sub-grid scale viscosity

• DES also available with SST and Realizable k- models

– Appropriate turbulent length scale, i.e. d ~ k1/2/w , k3/2/ , for selected model used to determine RANS/LES boundary.

...

~~1~

~~~

~

2

11

jj

wwbxxd

fCSCDt

D

DESw Cdd ,min


Delayed Detached Eddy Simulation (DDES)

• In the original DES formulation, the LES mode can be activated inside the boundary layer for <<

– This can have undesirable side effects such as Grid Induced Separation (GIS) where the flow separates too far upstream of the actual separation point

• In delayed detached eddy simulation (DDES) the length scale is redefined

• The use of additional physical quantities prevent LES mode activation inside the boundary layer except in very extreme cases

dSffCdfdd ijtddesd ,,,, ; ,0 max~

dd ~

:A Grid Δ~

: BGrid desCd


Delayed Detached Eddy Simulation (DDES)

• Implemented for all DES options in ANSYS CFD codes

– The use of DDES is recommended for all DES simulations with DES-SA or DES-RKE

• With DES-SST the SST blending functions can be used

– The DDES function is recommended in R14 and higher

– In earlier versions, use F2


IDDES

• Improved Delayed Detached Eddy Simulation (IDDES) is a new formulation of the DES model with the following goals – Provide improved shielding against grid induced separation (GIS)

compared to DDES

– Provide a new subgrid length scale definition that will allow the model to run in wall-modeled LES (WMLES) mode when unsteady inlet conditions are available, e.g. using vortex method with accurate flow and turbulence profiles

• The IDDES implementation is based on SST-DES

• The IDDES subgrid length scale definition is also used in the WMLES model


Detached Eddy Simulation (DES) • Useful to relax the strong LES mesh constraints close to the wall

– Mesh constraints relaxed in wall-parallel directions

– The near-wall cell should still have y+ = 1 and a sufficient number of grid points with moderate expansion ratio in the wall normal direction to resolve the boundary layer

• RANS/LES interface depends on the mesh – Need sufficient cells close to wall for resolving RANS layer

– All the boundary layer should be covered by RANS

– Production mechanisms far from wall must be correctly resolved with LES • Grid spacing must be small enough to resolve turbulent eddies with CFL ~ 1

– Need to check the RANS/LES interface a posteriori • See next slide


Checking RANS and LES Regions in DDES • When performing Delayed DES, the variable DES TKE

Dissipation Multiplier can be used to check where the model is operating in RANS mode

– The entire boundary layer should be covered by RANS

• Depending on the RANS model, there are different possibilities

– DDES-SA and DDES-RKE

• The value of the shielding function fd (Slide 31) is displayed and is equal to 0 in the RANS region (boundary layer) and 1 in the LES region

– DDES-SST

• The value of FDES (below) is displayed and is >1 in the LES region and 1 in the RANS region.

DESk FkY w * :equation TKE in termnDissipatio


• RANS models, even in unsteady mode (URANS), are only able to resolve unsteady structures that are comparable with typical characteristic length scale

– Even if calculation is performed with a grid and time step size that are capable of resolving smaller scales

• This is often attributed to the time/ensemble averaging procedure

– Is that really the case?

Unsteady Flow Calculations Without LES/DES

RANS with SST k-ω model LES


Scale Adaptive Simulation (SAS) Model • Transport equations for length scale determining equations (, w) are not

exact scale equations, but simply modeled in a manner analogous to the k-equation

• In the SAS model, an exact transport equation for the integral length scale is used instead

– The exact integral length scale equation is transformed by change of variables into w for implementation within SST k-w framework, resulting in:

2

2

22

22 ~12

vKjjjjj

t

jj

j

L

LS

xx

k

xx

k

xxS

x

U

t

ww

w

w

w

w

w

ww

w

Wilcox Model BSL (SST) Model SAS Model


von Karman Length Scale • The von Karman length scale in the

transformed w −equation allows the model to detect which scales are already resolved structures in the flow field

• This allows the evolution of a turbulent spectrum in unstable flow regions whereas conventional URANS models only predict a single scale

Turbulent structures in the wake of a cylinder in crossflow

22

yU

yULvK


SAS Model • If the grid is too coarse, or the time step size is too large, the model may

revert to steady RANS.

• The model may not go unsteady for all flows

– Can give steady results in attached and mildly separated wall bounded flows

– At worst, if this happens, the end result is still a valid RANS solution, which is not necessarily the case with other hybrid approaches

• Unsteady solutions with turbulent content are obtained for flows with large separation and mixing zones

– Many industrial applications are characterized by strong mixing in largely separated regions


Additional Model Combinations for SRS • SAS can be selected together with any w-based

model except the k-kl-w transition model

– Standard k-w, SST, Stress Omega, Transition SST and Intermittency Transition models

• (D)DES can be enabled with the Transition SST and Intermittency Transition models

– First select the transition model in the Viscous Model panel, then DES appears as an option


Embedded LES (ELES) • Models such as DES and SAS are global approaches which use a single model

formulation in the entire domain – There is a RANS region and a LES region, but they are determined by the model itself

based only on the solution and the grid spacing.

– Successful when the flow instability is sufficiently strong to produce unsteadiness in the calculation

• Embedded LES (ELES) is a slightly different concept in that there are still RANS regions and LES regions, but now the user explicitly defines the boundary between them – RANS and LES regions are separately defined and use different models

– Synthetic turbulence is generated at the interface to convert RANS turbulence to unsteady, turbulent resolved scales entering the LES region

• ELES is not a new turbulence model, but the combination of RANS and LES models joined by appropriate interface conditions


Coupled Zonal Modeling for ELES

• Example: zonal definition for backward facing step RANS-LES Interface Acts as B.C. for LES model in Zone 2

• Zonal interfaces can be non-conformal (most common) or conformal •Definition of model interaction

at interfaces is critical

ZONE 1

RANS Model

LES Model

ZONE 2

ZONE 3 RANS Model

LES-RANS Interface Acts as B.C. for RANS model in Zone 3


ELES Model Combinations (1) • RANS Models

– Any RANS model can be selected in the RANS zone except Spalart-Allmaras • Does not have two distinct turbulent scales

– The same RANS model must be used in all RANS zones

– Selected in Viscous Models panel

• LES Models – All algebraic LES models can be selected in the

Embedded LES tab in the Cell Zone conditions panel • Dynamic Smagorinsky model also available

– Bounded central differencing recommended for momentum • Central differencing also available • Discretization used in LES zone is selected in Cell Zone

Conditions panel


ELES Model Combinations (2) • Zonal Interfaces

– RANS-LES Interfaces – modeled turbulence needs to be converted to resolved turbulence

• Vortex method (recommended)

• Spectral synthesizer

– LES-RANS Interfaces

• Select “No Perturbations” in b.c. panel

– There is only a single boundary condition type (RANS/LES Interface) available. The user defines whether it is LES-RANS or RANS-LES through specification of the “Fluctuating Velocity Algorithm”


LES-RANS Interfaces (1) • At LES-RANS interfaces, the RANS transport equations need to obtain

information from the upstream LES zone. The following possibilities are available

– The RANS model from a precursor RANS simulation is frozen in the LES zone

• Recommended for flows where the LES region does not change the overall flow topology and the LES-RANS interface is outside the region of interest

– The SAS model is run passively in the LES zone and then reactivated at the LES-RANS interface

• In the LES zone, the unsteady velocity field is calculated using the LES eddy viscosity

• The SAS transport equations are solved too and are based on the LES velocity field. The SAS µ t is not used in the momentum equation. The SAS µ t is reactivated at the interface and used as the boundary condition for the downstream RANS zone


LES-RANS Interfaces (2) • One additional possibility for running ELES models

– The SAS or DES model is run actively in all zones. Synthetic turbulence is generated at the interface and propagates downstream

• Can be used to trigger SAS into unsteadiness

• It is recommended to locate RANS-LES interfaces where there is no backflow


Non-conformal Interfaces in ELES (1)

• To create a RANS/LES interface boundary type at a non-conformal interface

– Define the interface in Define > Mesh Interfaces

– Left click on the interface in the list on the left of the panel after it has been created

– Note the name of the zone which then appears under “Interface Interior Zone”

(see next slide)


Non-conformal Interfaces in ELES (2)

• (Continued from previous slide)

– Select LES zone in cell zone conditions panel

• Or activate SAS or DES models in the Viscous Models panel

– Select the interface interior zone in the boundary conditions

• The list of zone types will now contain “rans-les-interface”. Change the type and enter the appropriate interface settings

(see previous slide)


Wall Modeled LES (WMLES) Simulation • WMLES is designed to reduce the mesh resolution requirements of LES for medium

to high Reynolds number wall bounded flows

– Boundary layers, channel and pipe flows

• WMLES is based on the concept of modeling the flow close to the wall (inner part of the boundary layer) with RANS and resolving the central part of the boundary layer with LES

• WMLES decides on the wall-parallel RANS/LES interface automatically based on grid resolution and boundary layer properties

max maxmin max , , , , 0.15W W W Wn wC d C h h h C

dW − distance from the wall

hmax − length of the longest edge cell

hWn − wall normal edge length of cell

“A hybrid RANS-LES approach with delayed-DES and wall-modelled LES capabilities”, Mikhail L. Shur , Philippe R. Spalart , Mikhail K. Strelets , Andrey K. Travin , Int. J of Heat and Fluid Flow 29, 2008


WMLES Eddy Viscosity • RANS / LES blending

• RANS Model − Prandtl mixing length model with van Driest damping

• LES Model − Smagorinsky model with van Driest damping

• The WMLES S-Omega option replaces S with abs(S-W) where W is the vorticity magnitude – Allows prediction of transitional effects – Improved predictions of separating shear layers

S − Strain rate magnitude CSMAG − Smagorinsky constant CSMAG = 0.2 − Modified grid spacing (see previous slide)

32 2min , 1 exp / 25t W SMAGd C y S


WMLES Grid Requirements • For a flow with boundary layer thickness, , each boundary layer volume, x

x , in the computational domain, the grid used for WMLES should be as follows

– x = / 10, z = / 20, NY = 40, y+ at first grid point should be less than 1

• In wall resolved LES, as Reynolds number increases, it is required to maintain x+ and z+, such that the total cell count increases (significantly) with Reynolds number

• In WMLES, it is only necessary to maintain x = / 10 and z = / 20 Thus the required number of grid points is much less sensitive to increasing Reynolds number

– The number of grid points in the wall normal direction will increase to maintain

y+ < 1 at the first grid point


WMLES Summary • WMLES can be used with IDDES or as a standalone subgrid model for LES

• WMLES reduces grid resolution requirements (compared to wall-resolved LES) for medium to high Reynolds number flows

• WMLES grid resolution requirements

• The time step size should be defined so that the cell Courant number in the boundary layer is 0.3

• WMLES is still very expensive compared to RANS

– 10 x 40 x 20 grid points per boundary layer volume compared with 1 x 30 x 1 for RANS

– Unsteady calculation with low cell Courant number compared to steady state calculation

• Keep resolved boundary layer regions as small as possible

4030 ,20

,10

yNzx


Summary of SRS Models (1)

• LES – Not suitable if wall boundary layers at high Re • Resolution requirements are excessive for such flows

• SAS – Safest hybrid model • Fallback RANS solution if model does not go unsteady

– No explicit grid sensitivity in RANS zone – Needs flow instability to generate resolved turbulence • Stays in RANS/URANS mode if instability is missing or too weak

• DES – Explicit combination of RANS and LES models – For many flows results are similar to SAS – Allows computation of flows with weaker instabilities than needed for SAS – Grid sensitivity in RANS regions requires shielding – use DDES as default


Summary of SRS Models (2) • ELES

– Useful to limit LES zone to critical area – Allows manual definition of LES zone • Unlike SAS or DES which depend on both the grid and the solution

– Synthetic turbulence generated at RANS-LES interface – Can also be used to trigger SAS/DES into unsteady mode

• WMLES – The inner part of the boundary layer is covered by RANS and the central part by LES,

drastically reducing the LES grid requirements for high Re flows – Boundary layer resolution

• 10 x 40 x 20 grid points for x x boundary layer

• Cell courant number in boundary layer = 0.3 – Potentially 1.0 for free shear flows

• All models are necessary. The question is not “Which model is best?” but “Which model is best for a given type of application?”


Best Practices for Scale Resolving Simulations • A Best Practices document for SRS is available from support.ansys.com

– Detailed monograph covering theory, model selection and numerical settings

– Enter “bpg” in the search field on the main landing page

Fluent-Adv Turbulence 15.0 L04 Les and Utmm

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