lecture 3 two-way fsi overview, geometry and meshing 14. 5 ... · pdf file1 © 2011 ansys,...

© 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, Geometry and Meshing


Outline

Coupling Overview

This lecture starts by describing how the FSI coupling process works. The main features and capabilities are then discussed along with the current limitations.

Workflow Overview

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

Geometry & Meshing

Here we’ll cover what you need to consider when creating the geometry and mesh for a co-simulation analysis


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

MassMomentum

Turbulence

Heat Transfer


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

MassMomentum

Turbulence

Heat Transfer


Solid Mechanics

Structural

Thermal

Coupling Overview

MAPDL solver usually solves Structural OR Thermal fields

Multifield (MFS) solver can couple fields in a segregated manner in MAPDL

MFS

Fluid Dynamics

MassMomentum

Turbulence

Heat Transfer


Coupling Overview

The MAPDL solver can also couple fields using a fully coupled approach

• Different elements are used to couple different fields

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

Solid Mechanics

StructuralThermal

Fluid Dynamics

MassMomentum

Turbulence

Heat Transfer


FEA and CFD Element Background

Mechanical/MAPDL use elements that combine mesh and physics, 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 MFX solver 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

MassMomentum

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 SOLVE command is allowed in the MAPDL solver

Solid Mechanics

StructuralThermal

Fluid Dynamics

MassMomentum

Turbulence

Heat Transfer

MFX


Iterative Coupling

Solid Mechanics

Structural

Fluid Dynamics

MassMomentum

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


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

The transient loop – each loop/step

moves 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 Loop

End Field Loop

Coupling / Stagger Loop

Field Loop


Iterative Coupling

Field loop iterations stop when the field reaches its convergence target (or Max Iterations/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 Loop

End Field Loop

Coupling Loop

Field Loop


MFX Key Features

• Steady or Transient 1-way and 2-way co-simulation with surface force/displacement coupling and/or surface Temperature/Heat Flow coupling between CFX 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


MFX Key Features

• Load transfer mapping is fully conservative for conserved 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


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)


MFX – What’s 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 contact elements (CONTA17*) are not supported• Work-arounds discussed later


Comparing MFX/CFX with System Coupling/Fluent

• MFX allows 2-way thermal and structural coupling, including coupled field element solutions. 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 MFX only 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 higher license 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.


Outline

Coupling Overview


Workflow Overview


Geometry & Meshing



Workbench Workflow

Standard WB workflow for 2-way FSI• Drop a CFX system 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


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 interfaceregion where fluid forceswill be received


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


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 both codes tracked in the CFX Solver Manager

Both solutions can be post-processed in CFD-Post

• Some limitations for Mechanical results in CFD-Post


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 Solid

Geometry

Solid Mesh: suppress

fluid bodies in here

Structural Setup: Write

Input File when complete

(Tools > Write Input File)

Export the fluid mesh, then run CFX

outside of WB, providing the fluid

mesh and the Mechanical Input File.

Alternatively, connect to a CFX system

then pull the .cfx file outside of WB

Fluid Mesh: suppress

solid bodies in here


Outline

Coupling Overview


Workflow Overview


Geometry & Meshing



Geometry Considerations

Helpful to use a single CAD file containing both the fluid and 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 mesh

motion or if subdomains are needed



To use shell elements in Mechanical, make a Surface body 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 doesn’t matter, but...

Outward pointing normal Inward pointing normal



Shell element assume the nodes are at the 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 top

or bottom of the thickness

• In the pipe example shown, assuming internal flow with outward pointing normals, use Offset Type = Bottom

• Locates the nodes at the true Fluid Solid Interface

Outward pointing normal


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 produce twisting 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


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 > Export to 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 Solid

Geometry in a

single Part

Mesh both fluid and

solid, export mesh,

then suppress fluid

Import fluid mesh


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 if necessary 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


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 in Mechanical 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.

lecture 3 two-way fsi overview, geometry and meshing 14. 5 ... · pdf file1 © 2011 ansys,...

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