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Simcenter STAR-CCM+Turbulence ModelingVersion 2019.3
Where today meets tomorrow.Unrestricted © Siemens 2019
Spotlight
On…
Unrestricted © Siemens 20192019-10-30Page 2 Siemens Digital Industries Software
Table of Contents
OverviewWhy is turbulence modeling necessary today?
Turbulence modeling in Simcenter STAR-CCM+
Turbulence Deep Dive:Deep Dive: Turbulence modeling
RANS
Non-Linear Constitutive Relations
Transition Modeling
Wall Treatment
Large Eddy Simulation
Detached Eddy Simulation
Overview:Turbulence Modeling
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Why is Turbulence Modeling Necessary Today?
Virtually all engineering flows are turbulent
Simulating turbulent flows requires turbulence models
• External aerodynamics
• Combustion
• Hydrodynamics
• Conjugate heat transfer
• Mixing
• Fluid-structure interaction
• Aeroacoustics
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Challenges and Implications for Turbulence Modeling
Fully resolving the full turbulent flow is still out of reach due to computational cost
• Turbulent flows can be simulated with a wide range of different methods
• The challenge for engineers is to find the best model for their particular application.
• Finding a balance between accuracy and computational efficiency can be difficult
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Simcenter STAR-CCM+
Key Requirements
• Design improvements require accurate predictions
• Engineers should be able to draw on decades of experience by using validated models
Speed and Performance
Multiphysics
Range of Models
Accuracy
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Simcenter STAR-CCM+
Key Requirements
• No single model is suitable for all applications
• Select between a range of application-specialized models
• From fast, basic models to highest fidelity results
• From steady to unsteady applications
Speed and Performance
Multiphysics
Range of Models
Accuracy
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Simcenter STAR-CCM+
Key Requirements
Speed and Performance
Multiphysics
Range of Models
AccuracySimulate the physics which is affected by tubulence:
• Heat Transfer
• Mixing
• Combustion
• Aeroacoustics
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Simcenter STAR-CCM+
Key Requirements
• Range of methods to balance compute time and accuracy
• Fast solvers with excellent scalability
Speed and Performance
Multiphysics
Range of Models
Accuracy
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Modeling Turbulence in Simcenter STAR-CCM+
Spotlight on Turbulence
Simcenter STAR-CCM+ includes a range of industrially relevant models for turbulence, allowing an appropriate balance between run time, robustness and accuracy to be achieved:
• RANS models – fastest and most robust models• LES – the highest fidelity and most computationally
expensive approach• DES – a hybrid of RANS and LES, providing a powerful
method for industrial high resolution CFD
Accuracy
Range of Models
Multiphysics
Speed and Performance
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Modeling Turbulence in Simcenter STAR-CCM+
Reynolds-Averaged Navier-Stokes (RANS)
• Most commonly used and robust method for turbulence modeling
• Suitable for studying the mean flow including large scale unsteadiness
• Simcenter STAR-CCM+ contains a comprehensive suite of industry-relevant models:• Published, validated model refinements regularly
implemented to ensure best-in-class offering• Range of options to balance accuracy and run time• From basic single equation models to state-of-the-art
Elliptic Blending and Reynolds Stress Models• Hybrid wall treatments greatly simplify meshing of
industrial geometries
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Modeling Turbulence in Simcenter STAR-CCM+
Large Eddy Simulation (LES)
• Most accurate approach but high computational cost• Capture flow unsteadiness in high fidelity• Resolves the large, energy-carrying flow structures while
filtering out small, less important structures• Uses a sub-grid scale model to represent turbulent
scales not resolved by the mesh• Because more of the flow structures are resolved by the
mesh, the mesh resolution must be high, particularly close to walls
• Typical Applications:• Aeroacoustics• Combustion
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Modeling Turbulence in Simcenter STAR-CCM+
Detached Eddy Simulation (DES)
• Hybrid method - blending between RANS and LES• LES where mesh is fine and turbulent structures can
be resolved• RANS where flow can be modeled
• A powerful method for detailed, unsteady, industrial CFD• More accurate than RANS• Faster than LES
• Captures large-scale vortex motion• Typical Applications:
• External aerodynamics• Fluid-structure interaction• Vortex shedding
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Modeling Turbulence in Simcenter STAR-CCM+
Spotlight on Turbulence
Simcenter STAR-CCM+ includes a wide range of methods for accurate simulation of turbulence for both steady and unsteady applications:
• Suite of RANS models• Large Eddy Simulation• Detached Eddy Simulation
• Choice of methods to balance robustness and accuracy• Fast solvers with excellent scalability
Accuracy
Range of Models
Multiphysics
Speed and Performance
Deep Dive:Turbulence Modeling
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Table of Contents
OverviewWhy is turbulence necessary today?
Turbulence in Simcenter STAR-CCM+
Turbulence Deep Dive:Reynolds-Averaged Navier-Stokes models
Non-Linear Constitutive Relations
Transition modeling
Wall treatment
Large Eddy Simulation
Detached Eddy Simulation
RANS
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Key Information
RANS Turbulence Models
• Eddy Viscosity Assumption
• Six Reynolds Stresses needed to close the Navier-Stokes Equations (in 3D)
• Simplification reduces computational expense
• Boussinesq Hypothesis used in RANS models
• Defines Reynolds Stresses as function of ‘turbulent viscosity’
• Model turbulent viscosity to close the NS equations
• To model turbulent viscosity, solve for one or more transported quantities, for example:
• Modified turbulent viscosity
• Turbulent kinetic energy
• Turbulent Dissipation
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Spalart-Allmaras
Model Description
When to Use It?
Typical Applications
• A one-equation model• Widely used in aerospace industry
• Generally simple, economical, robust on good meshes• Valid in the near-wall region• Good predictions for attached flow
• Airfoil performance
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Standard K-Omega
Model Description
When to Use It?
Typical Applications
• Two equation model• Transport equations for turbulent kinetic energy and specific
dissipation rate
• Performs well for swirling flows• Does not require sublayer damping functions• Performs well for adverse pressure gradients
• Aerospace• Formula 1• Turbomachinery
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SST K-Omega
Model Description
When to Use It?
Typical Applications
• Two equation model• Blends k-ω near the wall with a transformed k-ε model
• Performs well for swirling flows• Does not require sublayer damping functions• Performs well for adverse pressure gradients• Less sensitive to freestream turbulence than the standard model
• Aerospace• Formula 1• Turbomachinery
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Standard K-Epsilon
Model Description
When to Use It?
Typical Applications
• Two equation model• The original general purpose “complete” model for industry
• Robust industry standard model• Insensitive to inflow conditions
• Ground transportation
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Realizable K-Epsilon
Model Description
When to Use It?
Typical Applications
• Two equation model• Ensures normal stresses are positive• Implemented by varying Cµ spatially
• More physical and accurate than standard k-epsilon• Performs better than std. k-ε for separated flows, swirling and
rotating flows, and flows with large streamline curvature
• Ground transportation• Aerodynamics
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K-Epsilon: Elliptic Blending Model
Model Description
When to Use It?
Typical Applications
• Four equation model extension of k-epsilon• Handles non-local effect of walls by an elliptic blending approach
• Near wall turbulence• Heat transfer• Skin friction• Separation
• Aerospace• Automotive• Aerodynamics
Non-Linear Constitutive Relations
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Key Information
Non-Linear Constitutive Relations
• The Boussinesq approximation assumes a linear relationship between the Reynolds-stress and the strain rate tensors
• Non-linear constitutive relations instead use a higher order expansion to give a quadratic or cubic relationship
• Advantages
• Anisotropy effects accounted for via algebraic formulation
• No further transport equations
• At low strain rates recovers standard model
• Correctly predicts secondary flows in square ducts
• Unlike Boussinesq models
Base SST
SST+QCRRSM + Quad. Press.-Strain
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Non-Linear Constitutive Relations
Model Description
When to Use It?
Typical Applications
• Quadratic & Cubic Constitutive Relations• Compatible with:
• Std. K-Epsilon• SST K-Omega
• Where secondary flows are important• Anisotropic flows• Flows with strong swirl• Boundary layer flows
• Ducted flows• Aerodynamics
Transition Modeling
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Key Information
Transition Modeling
• Three models available• Turbulence Suppression Model – User specified
regions of laminar flow• Gamma ReTheta transition model – Predictive four
equation model extension of SST-kω, additional transport equations for intermittency and transition momentum thickness
• Gamma transition model – Predictive three equation model extension of SST-kω – achieves similar levels of accuracy to Gamma ReTheta at reduced computational cost
• Gamma and Gamma ReTheta models have predictive (correlation based) capability for transition
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Cross Flow Modification for γReθ and γ models
Model Description
When to Use It?
Typical Applications
• Option to include cross flow • Takes into account additional effects:
• Secondary flows normal to the streamlines• More prevalent in swept wings• Chordwise pressure gradient has component normal to
streamlines• Significantly modifies onset of transition
• High speed compressible cases
• Aerospace• Transonic swept wings
γReθ
γReθ + Crossflow Modification
Sickle wing testcase – Comparison of skin friction with and without crossflow mod. and experiment (red line)
Wall treatment
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Key Information
Near Wall Modeling
• For most industrial CFD problems the mesh resolution is insufficient near the wall
• A wall function is used to determine the relationship between the first cell center and the wall
• For cases requiring accurate predictions of heat transfer and separation it is necessary to resolve the viscous sublayer with a fine prism layer mesh (y+ ~1) • Not all turbulence models can resolve down to the wall and
require special near wall treatment (some k-epsilon models, RSM)
Viscous Sublayer
Buffer Layer
Fully Turbulent Log-Law Region
Defect Layer
log y+
U+
5 30 500-1000
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Key Information
Wall Treatment
• Three basic approaches:
• 1) High y+ uses standard wall function approaches
• 2) Low y+ uses the low Re approach (no wall function)
• 3) All y+ uses a hybrid approach
• Valid for models that can be solved through sublayer
• Blends turbulence source terms between wall function and low Re
• Uses continuous wall laws Viscous Sublayer
Buffer Layer
Fully Turbulent Log-Law Region
Defect Layer
log y+
U+
5 30 500-1000
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Key Information
Two-Layer Models
• Realizable two-layer with hybrid wall treatment has been the default since STAR-CCM+'s inception
• Key to robustness is the blending of linear equation coefficients
• Discretized equation 𝑎𝑎𝑝𝑝𝜔𝜔∆𝜖𝜖𝑝𝑝 + ∑𝑛𝑛 𝑎𝑎𝑛𝑛 ∆𝜖𝜖𝑛𝑛 = 𝑟𝑟 𝑥𝑥𝛼𝛼
• Algebraic equation ∆𝜖𝜖𝑝𝑝 = 𝜖𝜖𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑟𝑟𝑎𝑎𝑎𝑎𝑎𝑎 − 𝜖𝜖𝑜𝑜𝑎𝑎𝑜𝑜 𝑥𝑥(1 − 𝛼𝛼)
• Key to accuracy is using the minimum possible Re for the crossover location
Viscous Sublayer
Buffer Layer
Fully Turbulent Log-Law Region
Defect Layer
log y+
U+
5 30 500-1000
Large Eddy Simulation
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Key Information
Large Eddy Simulation (LES)
• Largest energy containing eddies are resolved
• The mesh is used as a filter
• Turbulent length scales smaller than the grid are modeled by a sub-grid scale model (see next slide)
• The Taylor Microscale can be used as an indication of an appropriate mesh size
log κ (wavenumber)
log
E(κ
)
Energy Containing Eddies
Dissipating
Eddies
Inertial Sub-range
Taylor Microscale, λ≈(10νk/ε)½
Kolmogorov Lengthscale, η≈(ν3/ε)¼
ResolvedModeled
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LES Sub-Grid Scale (SGS) Models
Model Description
When to Use It?
Typical Applications
• Three SGS models available in Simcenter STAR-CCM+:• Smagorinsky• Dynamic Smagorinsky• Wall Adapting Local Eddy (WALE)
• When detailed analysis required• High accuracy• Boundary layer instabilities
• Combustion• Aeroacoustics
Detached Eddy Simulation
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Key Information
Detached Eddy Simulation (DES)
• Hybrid method between RANS and LES
• If the grid is fine enough, turbulence model becomes a sub-grid scale model to a locally applied LES model
• If the grid is not fine enough, the turbulence model is the underlying RANS model
• Compatible with RANS models: Spalart-Allmaras, SST k-omega, Elliptic Blending k-epsilon
• The default DES version is:
• Improved Delayed Detached Eddy Simulation (IDDES)
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Elliptic Blending DDES
Model Description
When to Use It?
Typical Applications
• Most recent DES model• Better prediction of the near-wall behavior compared to
other DES models• Model has the property of transitioning to turbulence
almost naturally and predicting better separation
• Massively separated flows
• Ground transportation• Aerodynamics
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LES/DES Boundary Conditions & Initialization
Model Description
When to Use It?
Typical Applications
• Specify turbulent fluctuations at inflow boundaries• Higher level realism • Synthetic Eddy Method (SEM)• Anisotropic Linear Forcing (ALF)
• When accurate inflow turbulence is necessary• To reach fully turbulent flow state faster
• Ducted flows• External aerodynamics
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Anisotropic Linear Forcing
Model Description
When to Use It?
Typical Applications
• Generate synthetic turbulence inside a volume of interest rather than at the inflow boundary
• Applies volume forcing to transition naturally unstable flows into turbulence
• Scale-resolving simulations such as DES/LES• When resolved inflow turbulence is of importance• When region of interest is far downstream of the inflow boundary• Not applicable to (nearly) uniform inflow profiles
• Aerospace airfoils• Automotive aerodynamics• Ducted flows
1. Precursor RANS
3. Generate volumetricsynthetic turbulence
2. Extract Turbulence Statistics
Simcenter STAR-CCM+Turbulence ModelingVersion 2019.3
Where today meets tomorrow.Unrestricted © Siemens 2019
Spotlight
On…
Simcenter STAR-CCM+�Turbulence Modeling�Version 2019.3Table of ContentsOverview:�Turbulence ModelingWhy is Turbulence Modeling Necessary Today?Challenges and Implications for Turbulence ModelingSlide Number 6Slide Number 7Slide Number 8Slide Number 9Modeling Turbulence in Simcenter STAR-CCM+Modeling Turbulence in Simcenter STAR-CCM+Modeling Turbulence in Simcenter STAR-CCM+Modeling Turbulence in Simcenter STAR-CCM+Modeling Turbulence in Simcenter STAR-CCM+Deep Dive:�Turbulence ModelingTable of ContentsRANSRANS Turbulence ModelsSpalart-AllmarasStandard K-OmegaSST K-OmegaStandard K-EpsilonRealizable K-EpsilonK-Epsilon: Elliptic Blending ModelNon-Linear Constitutive RelationsNon-Linear Constitutive RelationsNon-Linear Constitutive RelationsTransition ModelingTransition ModelingCross Flow Modification for Re and modelsWall treatmentNear Wall ModelingWall TreatmentTwo-Layer ModelsLarge Eddy SimulationLarge Eddy Simulation (LES)LES Sub-Grid Scale (SGS) ModelsDetached Eddy SimulationDetached Eddy Simulation (DES)Elliptic Blending DDESLES/DES Boundary Conditions & InitializationAnisotropic Linear ForcingSimcenter STAR-CCM+�Turbulence Modeling�Version 2019.3