multiphysics-cfd

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Ramji Kamakoti, Ph.D.Technical Specialist

August 8, 2013

Introduction to Abaqus/CFDand Native FSI

Overview

• Abaqus/CFD• CFD Simulation Workflow• Setting up CFD Analysis: Case Study 1• Modeling Heat Transfer• Modeling Turbulence

• Native FSI• FSI Simulation Workflow• Setting up FSI Analysis: Case Study 2• FSI Analyses with Shells/Membranes• Conjugate Heat Transfer Analysis

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Abaqus/CFD

4

Abaqus/CFD

Abaqus/CFD is the computational fluid dynamics (CFD) analysis capability offered in the Abaqus product suite to perform flow analysis

Scalable CFD solution in an integrated FEA-CFD multiphysics frameworkBased on hybrid finite-volume and finite-element method

Incompressible, pressure-based flow solver:Laminar & turbulent flows

Pressure contours

Aortic Aneurysm

Pressure contours on submarine skin

Submarine

5

Abaqus/CFD

Incompressible, pressure-based flow solver:Transient (time-accurate) method

2nd-order accurate projection methodSteady-state using pseudo-time marching and backward-Euler method 2nd-order accurate least squares gradient estimationImplicit and explicit advection schemesUnsteady RANS approach (URANS) for turbulent flows

Steady-state methodSIMPLE algorithm adopted to hybrid finite-volume and finite-element projection method

Energy equation for thermal analysisBuoyancy driven flows (natural convection)

Uses the Boussinesq approximationIsotropic porous media flow modeling (transient only)

Includes isothermal and non-isothermal flow modeling

Flow Around Obstacles(Vortex Shedding)

Electronics Cooling

Velocity contours

Buoyancy driven flow due to heated chips

Velocity vectors

Inlet

Outlet

Substrate

Pressure

6

Abaqus/CFD

Turbulence modelsSpalart-AllmarasRNG k-SST k-Hybrid wall functions

Boundary conditionsInlet, outlet and wall boundary conditionsUser subroutines for velocity and pressure boundary conditions

Iterative solvers for momentum, pressure and transport equations

Krylov solvers for transport equationsMomentum, turbulence, energy, etc.

Algebraic Multigrid (AMG) preconditioned Krylov solvers for pressure-Poisson equations

Fully scalable and parallel

88 % efficiency (fixed work per processor at 64 cores)

Helicity isosurfaces

Prototype Car Body(Ahmed’s body)

7

Abaqus/CFD

Other featuresFluid material properties

Newtonian and non-Newtonian fluids I. A variety of shear-rate dependent viscosity models are available

Temperature dependence of material propertiesCFD-specific diagnostics and output quantitiesArbitrary Lagrangian-Eulerian (ALE) capability for moving deforming mesh problems

Prescribed boundary motion, Fluid-structure interaction“hyper-foam” model, total Lagrangian formulation

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CFD Simulation Workflow

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CFD Simulation Workflow

CFD Simulation Workflow in Abaqus/CAE

Part module:Create a part

representing the flow domain

Mesh module:Create CFD mesh

Property module:Define fluid

properties; create and assign fluid

section

Assembly module:Instance and

position the parts

Step module:Define fluid analysis step, solver controls, turbulence models,

etc.

Interaction module:Define interactions for FSI problems

Load module:Apply boundary conditions, body

forces, fluid reference pressure

etc.

Job module:Create and submit

CFD job

Visualization module:

Postprocess results

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Setting up CFD Analysis

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Case Study 1: Flow around a Rigid Circular Cylinder

Flow around a rigid circular cylinder is often used as a CFD benchmark caseCharacteristic length scale: D (cylinder diameter)Model the flow as 3-dimensional but with one element through the thickness and symmetry boundary conditions on the front and back faces to enforce 2-dimensional conditions

Steady and symmetric flow; no separation

Steady and symmetric flow; symmetric vortices

Laminar vortex street (von Karman vortex street,

i.e., Hopf bifurcation at Re ~ 60)

Transition to turbulence in the wake and boundary layers

Re < 5 5 < Re < 40 40 < Re < 200 Re > 200

InflowRe VD

D

Vinlet =0.1 m/secD = 0.1 m

Reveals interesting flow features depending on the Reynolds number

Problemdescription

Flow around a rigid circularcylinder

Flow domain Around the cylinder

How do I model it? 1) Model fluid flow2) Mesh is fixed

Cylinder motion None

12

Case Study 1: Defining the CFD Model

1. Create a “CFD” model in Abaqus/CAE

CFD-specific model attributes are only accessible for Abaqus/CAE models of type “CFD”The model type, once set, cannot be changed

Parts, instances, material properties etc. can be copied between “Standard & Explicit” and “CFD” model types

13


Only 3-dimensional parts can be modeled.Use a 3D sector for axisymmetric models Use a 3D part with one element through the thickness for 2-dimensional models

Orphan CFD meshes can be importedTetrahedral and hexahedral elements only

All Abaqus/CAE features for geometry creation are accessible

“2-dimensional”D4D

4D

12D

Far field boundaries are chosen such that flow is unaffected by the presence of the cylinder at these boundaries

2. Define a part representing the flow domain

14


Hexahedral (FC3D8), tetrahedral (FC3D4) and wedge (FC3D6) element types are available

Mixed meshes are allowedTetrahedral + WedgeHexahedral + Wedge

Proper modeling of the boundary layer requires a fine mesh near the cylinder surface to resolve the velocity gradient

Fluid velocity is zero at the cylinder surface (no-slip, no-penetration condition)Typically, the CFD mesh is refined at no-slip walls and coarsens as we move away from walls

3. Generate the mesh

15


Newtonian fluid Density: 1000 kg/m3

Viscosity: 0.1 kg/m/sProperties are chosen to set the flow Reynolds number at 100 based on cylinder diameter

4. Define fluid material properties

16


Section category “Fluid” is the only choice available when the CFD model type is chosenElements without a section assignment will cause a processing error

Instance the part (or orphan mesh) and position it in the desired geometric location

5. Create and assign a fluid section and instance the part

17


Transient analysis attributesTime periodEnergy equation (off by default)Time incrementation (automatic or fixed)Solver controlsLaminar or turbulent flows

Enable heat transferAnalysis time period

6. Define an incompressible flow analysis

18


CFL limit: Courant-Fredrichs-Levy (CFL) conditionArises due to the explicit treatment of advective terms in the Navier-Stokes equations

Time integration accuracy• 2nd order accuracy for trapezoid scheme

Automatic time integration parameters• Initial time increment• CFL limit

Time increment growth

Time incrementation

Automatic and fixed time incrementation

Controls pressure update

6. Define an incompressible flow analysis (cont’d)

19


Primary solution quantitiesVelocity componentsPressure

Convergence and diagnostic output for each of the equations is not written by default, but they can be toggled onMany solver choices are available for the Pressure Poisson’s Equation

Use preset levels (default 2)

Momentum Equation Pressure Equation

Solver controls


20


Laminar or turbulent flowLaminar flow (default)Choice of turbulence models


21


Output frequency

Flow field output variables are available at nodes

7. Request output variables

22


One element in the thickness direction Symmetry boundary conditions on faces to enforce 2-dimensional conditions

Reynolds number = 100

Flow outletp = 0

Flow inletVx = 0.1Vy = 0 Vz = 0

Wall (no-slip, no-penetration)Vx = 0 Vy = 0 Vz = 0

Far fieldVx = 0.1 Vy = 0 Vz = 0

Symmetry Vz = 0

Fluid boundary conditions are applied on surfaces

8. Define boundary conditions

23


Flow inlet

Flow outlet

8. Define boundary conditions (cont’d)

24


Wall condition Far field velocities Symmetry at front and back faces


25

Case Study 1: Analysis Execution

Job execution optionsInput file writingAnalysis data checkJob submissionJob monitoringAccessing output database (ODB)Job termination

CFD analysis job

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Case Study 1: Postprocessing Results

Turn off mesh display

Use continuous contour intervals

3. Fast view manipulation

(for large CFD models)

Abaqus/Viewer – Tips for postprocessing CFD output databases

3

2

2

11

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Case Study 1: Postprocessing Results

Velocitycontour

Pressurecontour

Vorticityanimation

Contour plots & animations

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Modeling Heat Transfer

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Modeling Heat Transfer: Introduction

Flow around an oscillating rigid circular cylinderCylinder surface is now at an elevated temperature while the incoming flow is cooler

What do I need to model heat transfer?

Vinlet =0.1 m/secD = 0.1 mAo = 0.05 mTp = 2 sec initial = 283 K

Inflow

D

= 283 K

= 313 K

Fluid thermal properties

Specific heat

Thermal conductivity

Enable heat transfer

Linear solver for transport equation

Define thermal boundary conditions and initial conditions

Temperature at boundaries

Initial temperature

Relevant output quantities

Temperature

30

Modeling Heat Transfer: Defining the CFD Model

1. Define thermal properties

Thermal Conductivity Specific heat at constant pressurecp is required for incompressible flow analyses

31


Activating energy equation enables heat transfer in the flow Requires transport equation solver for temperatureSolver options are controlled through transport equation solver controls

2. Activate heat transfer

32


Additional boundary conditions for temperatureFlow inlet Cylinder surface


Define inlet temperature Define cylinder surface temperature

33


Solution of a transient flow problem requires initial conditionsZero initial velocity is assumed unless non-zero initial velocities are specifiedIncluding thermal effects, however, requires specification of initial temperature

4. Define initial conditions

initial = 283 K

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5. Output requests

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Modeling Heat Transfer: Results

Temperature contours

t = Tp t = 2Tp

t = 3Tp t = 4Tp Temperature path plot (Nodes behind the cylinder)

Evolution of Temperature iso-surfaces

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Modeling Turbulence

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Modeling Turbulence: Introduction

Many CFD problems are laminar, and/or can be adequately resolved with reasonable mesh sizes, and do not require the use of a turbulence model

For problems that are truly laminar, use of a turbulence model may yield incorrect results that are too dissipative

Before activating a turbulence model, check the Reynolds number for the flowA very large Reynolds number typically indicates the need for a turbulence model

The transition Reynolds number depends on the flow itself, for example: I. For pipe flows: transition Re ~2300II. For flow around a rigid circular cylinder: transition Re ~200

If you are unsure, try this test:1. Run the simulation without a turbulence

model activated2. Plot kinetic energy and/or the time-history of

several flow variables

3. If there are random oscillations in the results, rerun the simulation with a turbulence model activated to achieve desiredsolution

Time

Velo

city

Time

Velo

city

Steady laminar flow Unsteady laminar flow

Time

Velo

city

Time

Velo

city

Steady turbulent flow Unsteady turbulent flow

38

Modeling Turbulence: Introduction

Consider the case of flow around an oscillating rigid circular cylinder that is maintained at an elevated temperature

But the Reynolds number is now 1000 for the stationary cylinder Turbulent flow regime

What do I need to model turbulent flow?

Vinlet = 0.1 m/sec= 1.0 m/sec

D = 0.1 mAo = 0.05 mTp = 2 secinitial = 283 K

Inflow

D

= 283 K

= 313 K

Select turbulence model

Linear solvers for transport equations for turbulence quantities

Define boundary conditions and initial conditions for

turbulence quantities

Dependent on turbulence model

Relevant output quantities

Dependent on turbulence model

Re 1000

39

Modeling Turbulence: Defining the CFD Model

Turbulence model parameters are characteristic of turbulence modelsTurbulence variables that are solved for depend on particular turbulence model chosen

Requires transport equation solver for turbulence variablesSolver options are controlled through transport equation solver controls

1. Activate turbulence model

40


Boundary conditions for turbulence modelsSpalart-Allmaras turbulence model requires specification of a modified turbulent kinematic viscosityAdditionally, the distance from walls needs to be evaluated

2. Boundary conditions

Automatically set by Abaqus/CAE at surfaces where no-slip or shear wall condition is applied

Walls Inlet

Inlet turbulence specification

/

3 5

...kinematic viscosity

At walls :At inlet :Inlet turbulence ( )

00d

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Initial conditions are required for turbulence variablesThe Spalart-Allmaras turbulence model requires specification of the initial eddy viscosityIt is typically specified using the same rules for the inlet/free-stream values:

3. Initial conditions

3 5t

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4. Output requests

Distance function output – Distance from walls

Turbulent kinematic viscosity t

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Modeling Turbulence: Results

Turbulent eddy viscosity

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Native FSI

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Native FSI using Abaqus

Fluid-structure interactionButterfly valve

Conjugate heat transferHeat exchanger

Coupling Abaqus/Standard + Abaqus/CFD

Abaqus/Explicit + Abaqus/CFD

Fluid structure interaction

Conjugate heat-transfer

46

Native FSI using Abaqus

Abaqus/CFD couples with both Abaqus/Standard and Abaqus/Explicit through the co-simulation engine

The co-simulation engine operates in the background (no user intervention required)

Physics-based conservative mapping on the FSI interface

Significantly expands the set of FSI applications that SIMULIA can address

Fluid-structure interaction

Also supports conjugate heat-transfer applications

Abaqus/Standard

Co-Simulation

Abaqus/Explicit

Abaqus/CFD

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FSI Simulation Workflow

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Setting up FSI Analyses

Case study summary1. Flow around a rigid circular cylinder2. Flow around a spring-loaded rigid circular cylinder

Problemdescription

Flow around a rigid circularcylinder

Flow around a spring-loaded rigid circular cylinder

Flow domain Around the cylinder Around the cylinder but domain changes due to cylinder’s oscillation

How do I model it?

1)Model fluid flow2)Mesh is fixed

1)Model fluid flow2)Allow mesh at cylinder surface to

accommodate displacements (ALE)3)Model the cylinder and the spring in

structural solver (co-simulation)

Cylinder motion

None Determined by structural analysis(two separate models)

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Setting up FSI Analysis

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Case Study 3: Flow around a Spring-loaded Rigid Circular Cylinder

Consider the case of flow around a spring-loaded rigid circular cylinderFlow at Reynolds number = 100

Boundary motion due to structural deformationThe structural deformation and fluid velocities are governed by the coupled physicsBoundary conditions on the mesh displacements and fluid velocities are dictated by the structural deformation

Vinlet = 0.1 m/secD = 0.1 m

Inflow

D

Spring-loaded rigid cylinder

Fluid StructureForce

Displacement, Velocity

51


Wall (no-slip, no-penetration)

Flow outletp = 0

Flow inletVx = 0.1Vy = 0Vz = 0

Far fieldVx = 0.1Vy = 0Vz = 0

Symmetry Vz = 0


FSI kinematic conditions require that the displacements and velocities at the FSI interface match

The co-simulation interface is used to define fluid-structure interaction and kinematic constraints

Suppress the wall BC on the cylinder surface

52



Boundary conditions for mesh motion

SymmetryUz = 0

Fixed *Ux = 0Uy = 0Uz = 0

Cylinder surface still requires mesh displacement boundary condition but this boundary condition is now defined on the co-simulation interface by the FSI coupling

Define fluid-structure interaction instead

Flow domain is still around the cylinder but the domain changes due to cylinder’s oscillation Arbitrary Lagrangian-Eulerian (ALE) capability is required within CFD to model fluid flow when the boundary

moves due to prescribed motion (boundary condition) or interaction (fluid-structure interaction) “Deform” the mesh Mesh motion requires additional boundary conditions

53


Boundary conditions for mesh motion

Fixed mesh Symmetry


54


The interaction surface needs to be definedOnly one surface per co-simulation definition*

Only one FSI co-simulation definition per analysis step

3. Define fluid-structure interaction

Define fluid-interaction surface

* If necessary, multiple surfaces can be merged into a single surface

55


3. Define fluid-structure interaction (cont’d)

The displacements and velocities that are imported into Abaqus/CFD from Abaqus/Standard or Abaqus/Explicit serve as prescribed boundary conditions for Abaqus/CFD at the FSI interface

Fluid StructureForce

DisplacementVelocity

Abaqus/Standard +

Abaqus/CFD

Abaqus/Explicit +

Abaqus/CFD

Option Abaqus/Standard

Abaqus/CFD

Abaqus/Explicit

Abaqus/CFD

Fluid-structure

interaction

Export DisplacementVelocity

Forces DisplacementVelocity

Forces

Import Forces DisplacementVelocity

Forces DisplacementVelocity

56

Case Study 3: Defining the Structural Model

The FSI interface in the structural and CFD models should be co-located

Fixed at ground (U = 0)

Axial Connector definitionDefines a spring with linear stiffness, K = 1 N/m

Rigid cylinderThe cylinder is constrained to move in the axial flow direction

57

Case Study 3: Defining the Structural Model

Define fluid-structure interaction

The interaction surface needs to be definedOnly one surface per co-simulation definition*

Only one FSI co-simulation definition per analysis step can be defined

Define fluid-interaction surface

* If necessary, multiple surfaces can be merged into a single surface

58

Case Study 3: FSI Analysis Execution

Coupled fluid-structure interaction jobs can be set up and run interactively using the co-execution framework in Abaqus/CAE

Creating co-execution jobs

Define structural jobDefine CFD job

Automatically chosenbased on selected models

59

Case Study 3: Postprocessing FSI Analyses

Open the structural and CFD output databases simultaneously Can also overlay viewports from multiple output databases

Cylinder displacementPressure contours

61

FSI Analyses with Shells/Membranes

Fluid-structure interaction analyses can be performed when structures are modeled with shells/membranes

Recommended in cases when modeling thin structures

Alleviates small element sizes in the surrounding CFD modelImproves time increment sizes

FSI analyses require

Zero-thickness shells in CFD domain“Seam” in the CFD mesh

Define positive & negative surfaces (SPOS & SNEG, respectively) for defining fluid-structure interaction interface

CFD Mesh

Shell structure“Seam”

62


Creating a seam in a CFD meshRequires use of the Crack feature in Abaqus/CAE

Available in Interaction module

Create the seamSelect internal surfaces and “Assign Seam”

Mesh the part

Proceed with the CFD model setup

Partition representing the seam

1

2

3

63


Creating FSI surfaces

Pick the surfaces associated with the seam

Create separate positive and negative (SPOS, SNEG) shell surfacesTwo-sided shell surfaces are not supported for co-simulation

Only one surface per co-simulation definition can be definedMerge individual shell surfaces

3

2

1

65

Conjugate Heat Transfer Analyses: Introduction

Model heat transfer within a solid region that interacts with the surrounding fluidWe will show a simple example to demonstrate the basic concepts involved

Transient conjugate heat transfer between a printed cicuit board (PCB)-mounted electronic component and ambient airSpecified power dissipation within the component

Heat transfer mechanism involvedHeat transfer within the component and the PCB due to conduction – Modeled in Abaqus/StandardHeated surface of the PCB/Component induces a temperature-dependent density differential in the surrounding air

Buoyancy-driven natural convection is set upModeled in Abaqus/CFD

Component

PCB

66

Conjugate Heat Transfer Analyses: Defining the CFD Model

CFD mesh is built around the PCB/ComponentMaterial properties for air

Density: 1.127 kg/m3

Viscosity: 1.983x105 kg/m/sThermal conductivity: 2.71x102 W/m/KSpecific heat (Cp): 1006.4 J/kg/KThermal expansion coefficient: 3.43x103 /KReference temperature: 293 K

Thermal expansion property for air has been specified to enable coupling between momentum and energy equations – Natural convection

Define thermal initial condition Initial temperature of air = 293 K

Define gravity load

g

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Boundary conditions

Bottom surface:No-slip/no-penetration

Top surface:p = 0

PCB/Component interface:No-slip/no-penetration

PCB/Component surfaces require thermal boundary conditions Should not be specified in CFD or Standard model Dictated by thermal coupling with the solid Create co-simulation interaction instead

No boundary conditions are needed on surrounding wallsFree outflow surfaces

Fluid Structure

Heat flux

Temperature

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Abaqus/Standard +

Abaqus/CFD

Abaqus/Explicit +

Abaqus/CFD

Coupling type Option Abaqus/Standard

Abaqus/CFD Abaqus/Explicit Abaqus/CFD

Conjugate heat transfer

Export Temperature Heat flux Temperature Heat flux

Import Heat flux Temperature Heat flux Temperature


Define thermal interaction

The co-simulation definition enables thermal coupling between the fluid and the structure

Fluid Structure

Heat flux

Temperature

69

Conjugate Heat Transfer Analyses: Defining the Structural Model

Printed circuit board (PCB)-mounted electronic componentHeat transfer step in Abaqus/Standard

Density(Kg/m3)

Specific heat(J/Kg/K)

Thermal conductivity

(W/m/K)

Thermal expansion

(/K)

PCB substrate 8950 1300 19.25 1.6 × 105

Encapsulant 1820 882 0.63 1.9 × 105

Die 2330 712 130.1 3.3 × 106

Heat slug 8940 385 398 3.3 × 106

70

Conjugate Heat Transfer Analyses: Defining the Structural Model

Define thermal interaction

71

Conjugate Heat Transfer Analyses: Results

PCB/Component:Temperature contours

Surrounding air:Temperature and velocity contours

72

Abaqus/CFD 6.13 New Features

Steady state SolverHybrid FE/FV SIMPLE

New Pyramid ElementTet-to-brick transition, tet-to-tet transition

Turbulence ModelsNew RANS model - SST variant of k-Hybrid wall function

Heat TransferRadiation and film boundary conditionsConduction heat transfer within CFD

ALE Mesh MotionA new mesh smoother based on an implicit algorithm using a “matrix-free” method

multiphysics-cfd

Documents

isothermal flow modeling

flow domainmesh module

cfd meshproperty module

pressurebased flow solver

fluid sectionassembly

fsi analysis

fluid reference pressure

fsi problemsload module