simcenter star-ccm+ topic version 2019gerry/class/me448/lecture/pdf/... · 2020. 2. 17. · page 7...

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


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


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


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


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


Multiphysics

Range of Models

Accuracy


Simcenter STAR-CCM+

Key Requirements


Multiphysics

Range of Models

AccuracySimulate the physics which is affected by tubulence:

• Heat Transfer

• Mixing

• Combustion

• Aeroacoustics


Simcenter STAR-CCM+

Key Requirements

• Range of methods to balance compute time and accuracy

• Fast solvers with excellent scalability


Multiphysics

Range of Models

Accuracy


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




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



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



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



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


Deep Dive:Turbulence Modeling


Table of Contents

OverviewWhy is turbulence necessary today?

Turbulence in Simcenter STAR-CCM+

Turbulence Deep Dive:Reynolds-Averaged Navier-Stokes models


Transition modeling

Wall treatment




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


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


Standard K-Omega

Model Description

When to Use It?


• 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


SST K-Omega

Model Description

When to Use It?


• 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


Standard K-Epsilon

Model Description

When to Use It?


• Two equation model• The original general purpose “complete” model for industry

• Robust industry standard model• Insensitive to inflow conditions

• Ground transportation


Realizable K-Epsilon

Model Description

When to Use It?


• 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


K-Epsilon: Elliptic Blending Model

Model Description

When to Use It?


• 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


Key Information


• 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



Model Description

When to Use It?


• 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


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


Cross Flow Modification for γReθ and γ models

Model Description

When to Use It?


• 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


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


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


Defect Layer

log y+

U+

5 30 500-1000


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


Defect Layer

log y+

U+

5 30 500-1000


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


LES Sub-Grid Scale (SGS) Models

Model Description

When to Use It?


• 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


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)


Elliptic Blending DDES

Model Description

When to Use It?


• 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


LES/DES Boundary Conditions & Initialization

Model Description

When to Use It?


• 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


Anisotropic Linear Forcing

Model Description

When to Use It?


• 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

simcenter star-ccm+ topic version 2019gerry/class/me448/lecture/pdf/... · 2020. 2. 17. · page 7...

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