cfx-fsi 14.5 lect-03 two way overview

8/13/2019 CFX-FSI 14.5 Lect-03 Two Way Overview

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2011 ANSYS, Inc. July 26, 20131 Release 14.5

14. 5 Release

Solving FSI Applications Using

ANSYS Mechanical and ANSYS CFX

Lecture 3

Two-way FSI Overview, Geometryand Meshing


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Outline

Coupling OverviewThis lecture starts by describing how the FSI coupling process works.

The main features and capabilities are then discussed along with thecurrent limitations.

Workflow Overview

This section provides a high level view of the FSI workflow for asimple 2-way FSI analysis

Geometry & Meshing

Here well cover what you need to consider when creating the

geometry and mesh for a co-simulation analysis


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Coupling Overview

Simulations involving multiple physics requires solution

of multiple fields

Fields are coupled: solution data from one field is

required by one (or more) other field

Solid Mechanics

Structural

Thermal

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer


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Coupling Overview

CFX solves Mass and Momentum in a single matrix (fully coupled)

Other fields (Turbulence, Heat Transfer, ) are solved in a

segregated manner

Iterations are required to convergence sequentially (segregated)solved fields

Solid Mechanics

Structural

Thermal

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer


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Solid Mechanics

Structural

Thermal

Coupling Overview

MAPDL solver usually solves Structural OR Thermal fieldsMultifield (MFS) solver can couple fields in a segregated

manner in MAPDL

MFS

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer


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Coupling Overview

The MAPDL solver can also couple fields using a fullycoupled approach

Different elements are used to couple different fields

E.g. SOLID 226 can couple thermal and structural fields

Solid Mechanics

Structural

Thermal

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer


7/31 2011 ANSYS, Inc. July 26, 20137 Release 14.5

FEA and CFD Element Background

Mechanical/MAPDL use elements that combine mesh andphysics, e.g.

SOLID185 is a structural element

SOLID278 is a thermal element

SOLID226 is a coupled field element

CFX uses mesh elements as a computational stencil

Physics is not associated with elements

Different element types only for different shapes (tet, hex, etc)

Use of multiple elements (for multiphysics) requires: Data transfer between different element types

Sequencing of element solutions

The Multifield solver (MFS) provides an infrastructure for this



The MFX Solver

The MFXsolver is the external variety of the MFS solver

It couples the MAPDL solver and the CFX solver together

MFX and MFS cannot be combined

Notice the Structural Thermal coupling has been removed below

Solid Mechanics

Structural

Thermal

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer

MFX

MFX

OR



The MFX Solver

To couple CFX with a Structural Thermal simulation use

Coupled Field Elements MFX can only couple to 1 element type on the MAPDL solver side

Only one SOLVEcommand is allowed in the MAPDL solver

Solid Mechanics

Structural

Thermal

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer

MFX



Iterative Coupling

Solid Mechanics

Structural

Fluid Dynamics

Mass

Momentum

Turbulence

Heat Transfer

MFX

Iterations are required to converge the quantities

transferred between the MAPDL and CFX solvers

Just like iterations are required to converge segregated fields

within the CFD or FEA solvers

Force/displacement or Temperature/Heat Flow are the

transferred quantities


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Iterative MFX Coupling A transient 2-way FSI simulation has three levels of iterations:

The transient loop each loop/stepmoves forward in time, as in a standard

CFD or FEA transient simulation.

Loads / displacements are

updated between the FEA

and CFD solvers.

The usual inner loop, used to

converge the field(s) within a solver

named Coefficient Loops in CFX and

Equilibrium Iterations in ANSYS.

Time Loop

End Time Loop

End Coupling / Stagger LoopEnd Field Loop

Coupling / Stagger Loop

Field Loop


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Iterative Coupling

Field loop iterations stop when the field

reaches its convergence target (or MaxIterations/Time Step in Fluent)

The CFX field loop does not need to be

converged every Coupling Iteration, only by

the end of the last Coupling Iteration

Coupling loop iterations stop when the forces/ displacements reach their convergence

targets or max number of Coupling Iterations

Ensure the individual field solvers AND the

forces / displacements are converged before

starting the next time step

E.g.: CFX set to 10 iterations/time step,

System Coupling set to 5 Coupling Iterations

and 100 time steps are solved. CFX could

perform a total of 10*5*100 = 5,000

iterations in total if convergence is poor

Time Loop

End Time Loop

End Coupling LoopEnd Field Loop

Coupling Loop

Field Loop


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MFX Key Features

Steady or Transient 1-way and 2-way co-simulationwith surface force/displacement coupling and/or

surface Temperature/Heat Flow coupling betweenCFX and ANSYS Mechanical/MAPDL

Full range of CFX capabilities

FSI interfaces on SOLID, SHELL or SOLSH elements

Restarts supported with CFX and/or MAPDL changes

Integrated post-processing with ANSYS CFD-Post

Parameterization, design exploration and

optimization

Coupling Iterations produce an implicit solution


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MFX Key Features

Load transfer mapping is fully conservative forconserved quantities (Heat Flow, Force)

Both globally and locally at the element level

General Grid Interface (GGI) algorithm for force and profile

preserving algorithm for displacement

Non matching meshes supported

Interface load under relaxation controls

Interface data convergence checking

Supports Large Models CFX can use distributed parallel processing on n machines

MAPDL solver can use local (shared memory) parallel

Remote Solver Manager (RSM) supported


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MFX Key Features

No additional licenses Need any CFX/CFD/CFD-Flo license and an ANSYS Mechanical

or above license

Workbench based setup and execution

Windows 32/64-bit, Linux 64-bit

Command line execution outside of Workbench

Can use separate machines and mix Windows, Linux

Can use MAPDL to create the structural model

Solver data transfer across standard sockets (TCP/IP) Third Party Coupling Scheme Not Required

Efficient, no intermediate files

Takes place in memory (RAM)


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MFX Whats Not Supported

Cannot specify multiple load steps in Mechanical Cannot provide a pre-stressed structural model

Alternative: start with a FSI simulation with some data

transfers suppressed, then restart after unsuppressing data

transfers

Cannot use multi-configurations or remeshing in CFX

Many Mechanical features based on newer contactelements (CONTA17*) are not supported

Work-arounds discussed later


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Comparing MFX/CFX with System Coupling/Fluent

MFX allows 2-way thermal and structural

coupling, including coupled field elementsolutions. This is not available in SC until

version 15.0.

MFX allows forces to be transferred from a

pair of wall boundaries to a set of shell

elements (2-to-1 mapping). SC is limited to

1-to-1 mapping for co-simulation.

Mesh smoothing options with CFX are more

robust and have more control than Fluent.

MFX allows mixed steady/transient couplings

and allows restarts from steady-state to

transient, neither of which are supported

with SC (at 14.5)

Cases requiring transient rotor-stator can be

solved with MFX/CFX but cases requiring FSI

regions in sliding mesh zones cannot be

solved with SC/Fluent

SC/Fluent allows re-meshing whereas MFX is

limited to mesh smoothing.

MFX does not support many features based

on CONTA17* elements. These are fully

supported with SC.

SC support local/shared memory and

distributed parallel for MAPDL, whereas MFXonly support local parallel for MAPDL.

SC has an easier to use WB workflow for

restarts.

Creation of backup points is simpler with SC.

SC requires an "ANSYS Structural" or higherlicense plus any Fluent license. MFX requires

an "ANSYS Mechanical" or higher license plus

any CFX/CFD-Flo license, or a single "ANSYS

Multiphysics" license, or a single "ANSYS

Mechanical CFD-Flo" license.


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Outline

Coupling OverviewThis lecture starts by describing how the FSI coupling process works.


Workflow Overview


Geometry & Meshing




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Workbench Workflow

Standard WB workflow for 2-way FSI Drop a CFXsystem onto the Setup cell of a Transient Structural system

Geometry is shared by default

Fluid and structural meshes are created separately

CFX Solution cell controls the FSI simulation

Structural Solution not used for FSI, but can be used to check the

structural model


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Workflow Overview - Mechanical

Import / create geometry: Extract fluid regions in DM if

necessary

Set up Mechanical model: Very similar to a standard

Mechanical model

Create the FSI interface

region where fluid forces

will be received


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Workflow Overview

Mesh fluid region

Setup CFX model

Define Coupling Timestep controls

Create fluid domain as usual,enabling Mesh Motion

FSI Interface will be a Wall Boundary

where the motion is received from

MAPDL solver

In addition to usual CFX solver

controls, set coupling solver controls


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Workflow Overview

Run in the CFX Solver

Manager Both codes started

automatically

Or can launch one at a time

(different machines, clusters)

Solution output from bothcodes tracked in the CFX Solver

Manager

Both solutions can be post-

processed in CFD-Post

Some limitations forMechanical results in CFD-Post


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Workflow Outside Workbench

Can also run the solutions outside of Workbench

Can be easier when restarts are required

Can use WB for Geometry, Mesh and Mechanical, then export the

CFX mesh/setup and ANSYS Input file and continue outside of WB

Fluid and Sol id

Geometry

Sol id Mesh: supp ressf lu id bodies in here

Struc tural Setup: Write

Input Fi le when com plete

(Tool s > Write Inpu t File)

Export the f lu id m esh, then run CFX

outside of WB, provid ing the f lu id

mesh and the Mechanic al Inpu t File.

Alternat ively, conn ect to a CFX system

then pu l l the .c fx f i le outs ide of WB

Fluid Mesh: supp ress

sol id bodies in here


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Outline

Coupling Overview

This lecture starts by describing how the FSI coupling process works.


Workflow Overview


Geometry & Meshing




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Geometry Considerations

Helpful to use a single CAD file containing both the fluidand solid regions

Ensures that FSI interface region lines up

Fluid geometry/mesh may need to include fillets if present in the

solid

Small mismatch OK ~ half the local element edge length

Can import separate CAD files into DesignModeler and

move / transform as necessary

Consider if the fluid bodies should be split to control meshmotion or if subdomains are needed


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To use shell elements in Mechanical, make a Surfacebody in DM

Given a fluid body, use Create > Thin/Surface with zero thickness

When you select a face, the side with the positive

surface normal is highlighted in green

Force passed to Mechanical is a vector, so the surface normal

direction doesnt matter, but...

Outward pointing normal Inward pointing normal


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Shell element assume the nodes are atthe mid-plane of the true geometry

Forces from a Fluid Solid Interface will be

applied at the mid-plane by default

Negligible error for thin geometries

Correct interpretation of the physics is

given by using Offset Type

Offsets the shell element nodes to the topor bottom of the thickness

In the pipe example shown, assuminginternal flow with outward pointing

normals, use Offset Type = Bottom

Locates the nodes at the true Fluid Solid

Interface

Outward pointing normal


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Meshing

When meshing the fluid, suppress the

solid region in Meshing

And visa-versa

Interface mesh does not need to match

Force still locally and globally conservative

Similar mesh length scales will maintain the

load transfer resolution

Difference in element normal

directions across interface can producetwisting on coarse meshes with surface

curvature

Create a matching mesh if necessary...

FluidSolid

Fluid exerts pressure normal

to fluid boundary elements

Force vector is transferred to the solid,

but does not act normal to the solid

elements results in a twisting force

Solid

Fluid


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Meshing

The structural Model cell contains the fluid & structural bodies in one Part

Mesh both fluid and solid regions in the Mechanical Model, then use File

> Exportto write out a Fluent mesh (.msh)

Suppress the fluid region, continue with the Mechanical Model definition

Import the Fluent mesh into CFX-Pre in a CFX Component System

No automatic mesh update in CFX-Pre

Matching interface mesh mesh the fluid and solid parts

together, using a single Part

Fluid and Sol id

Geometry in a

single Part

Mesh both f lu id and

sol id, export m esh,

then suppress f lu id

Import f lu id mesh


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Meshing

It is also possible to create fluid and solid meshes

outside Workbench (e.g. ICEM)

Import the fluid mesh into CFX-Pre manually

Import the solid mesh into Finite Element Modeler, then connect

to a Mechanical system:

Fluid and solid meshes can be created together ifnecessary so that nodes match at the interface

Meshing the fluid volume then extracting the surface mesh to

use as a Mechanical shell mesh is also possible


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Summary

The MFX solver allows fields solved in different solvers to be

coupled together

For 2-way FSI analyses iterations are typically used between

the solvers within each time step so that the

forces/displacements can converge at that time step

The workflow involves identifying boundary regions inMechanical and CFX that will send/receive data. All coupling

settings are defined in CFX-Pre in Workbench.

The fluid and structural geometries should physically match at

the FSI interface

An offset may be required when using shell elements

Poor mesh resolution on curved surfaces can lead to errors in

the force transfer. Use of a matching mesh avoids this.

cfx-fsi 14.5 lect-03 two way overview

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