W7 Flow Over Flexible Cylinder

Copyright © 2012 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Workshop 7 – Flexible Ring


Flexible Ring

• Purpose of the Workshop

• Learn the different types of Fluid Structure Interaction (FSI) that AcuSolve supports

• Set up a Practical FSI simulation (P-FSI)

• Set up a Direct Coupled FSI simulation (DC-FSI)

• Gain experience using ALE mesh motion

• Use Surface Manager to assign boundary condition types

• Use the “Propagate” feature to copy settings from one group to another

• Run AcuSolve

• Monitor solution with AcuProbe

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Flexible Ring

• Fluid Structure Interaction Background

• ACUSIM defines FSI as follows:

• FSI is the simulation of the bi-directional interaction (coupling) between fluid flow and a deforming solid/structural model.

• This definition of FSI does not include:

• Fluid and thermal solid/structure analysis (ie., Conjugate heat transfer)

• Fluid coupling with rigid body dynamics

• These simulation types are supported by AcuSolve, but we classify them differently

• AcuSolve supports two different types of FSI

• Practical FSI (P-FSI)

• Direct Coupled FSI (DC-FSI)

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Flexible Ring

• Practical Fluid Structure Interaction

• The fluid and solid codes are run independently and separately

• The solid code is used for a modal or frequency analysis

• No run time coupling is required

• Each may be run with a different time increment and duration

• No fluid mesh size limitation imposed by FSI

• Significantly more stable than alternative approaches

• Eliminates high wave number modes, yields smooth solution

• Very efficient

• Problem setup

• CPU time

• Applicable only for linear structural problems

4


Flexible Ring

• Direct Coupled Fluid Structure Interaction

• The fluid and solid codes are run in tandem

• Data is passed between the codes on wetted surfaces

• AcuSolve provides forces and/or heat fluxes to structural code

• Structural code returns displacements and/or temperatures to AcuSolve

• No third party software involved

• AcuSolve performs all interpolation between dissimilar meshes

• Applicable for linear and nonlinear deformations

• Current application of this technique is for direct coupling with Radioss, Abaqus and

MD Nastran

5


Flexible Ring

• Flexible Ring Problem Description

• In this workshop, we begin by performing a P-FSI simulation of a thin flexible ring

suspended in cross flow. Note that this is a contrived configuration used for training

purposes only.

• The following diagram illustrates the problem set up and the constraints that are placed

on the ring.

This point is constrained in all

directions to have zero displacement

and zero rotation This point is constrained in the

vertical direction, but is free to move

in the stream wise direction

Flow Direction

OD= .01 m

ID= .009 m Young’s Modulus = 24000 Pa

Density = 500 kg/m3

Poisson’s Ratio = 0.3

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Flexible Ring

• Steps in Performing P-FSI

• Background on P-FSI

• AcuSolve requires the mass, stiffness, and damping array for each mode of the flexible body as input for P-FSI analyses. It also requires an array describing the eigenvectors of each mode.

• This information can be obtained from a modal analysis in a structural solver, or derived analytically for simple problems.

• For this analysis, a structural model of the ring was built in RADIOSS and ABAQUS. The resulting model from RADIOSS is exported into an .op2 file and the result from ABAQUS is exported into an .odb file. Any of these files can be used for the CFD simulation.

• We will read the .op2 or .odb file using AcuConsole then project the modes onto the fluids model for the P-FSI simulation.

• With this background info, we can begin constructing the model

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Flexible Ring

• Open the database

• File Open

• Browse to Workshop6 directory

• Choose the file named flexibleRing.acs

• Select ‘Open’

• Model should appear similar to what is

shown to the right

• Same geometry and mesh as Workshop 5,

but we will modify the problem setup

significantly.

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Flexible Ring

• Set the data tree to “Basic”

• Ensure that the “BAS” button is selected in the Data

tree Manager

• Set the surface display settings

• Expand ‘Model’ in the tree

• Right-click on ‘Surfaces’

• Select ‘Display type’

• Select ‘solid & wire’

• Set the volume display settings

• Right-click on ‘Volumes’

• Select ‘Display off’

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Flexible Ring

• Set up global parameters

• Expand ‘Global’ branch

• Double-click ‘Problem Description’

• Enter the problem title and subtitle

• Set ‘Analysis Type' to ‘Transient’

• Set ‘Mesh Type' to ‘Arbitrary mesh movement

(ALE)’

• Verify turbulence is set to laminar

• Set solution strategy

• Double-click ‘Auto Solution Strategy’

• Set ‘Max time steps’ to 1000

• Set ‘Initial time increment’ to .002 sec.

• Verify that ‘Flow’ and ‘Mesh’ are set to ‘On’

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Flexible Ring

• Create a material model

• Right-click on ‘Material Model’ branch and select ‘New’

• Right click on ‘Material Model 1’ and select ‘Rename’ and

rename it to ‘Fluid’

• Set the material properties

• Double click ‘Fluid’

• Set the material properties as follows:

• Density = 1 kg/m3

• Viscosity = .0001 kg/m-sec

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Flexible Ring

• Create some monitor points

• Expand the ‘Output branch’

• Right click on ‘Time History Output’ and select ‘New’

• Rename ‘Time History Output 1’ to ‘Monitor Points’

• Double-click ‘Monitor Points’

• Change ‘Type’ to Coordinates

• Click ‘Open Array’ to define ‘Coordinates’

• Click ‘Add Row’ to yield 2 rows total

• Enter values as shown:

• Min X and Max Y of cylinder

1

2

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Flexible Ring

• Set the nodal output frequency

• Double-click on ‘Nodal Output’

• Set ‘Time step frequency’ to 2

• Set ‘Output initial condition’ to ‘On’

• Double-click ‘Nodal Initial Condition’

• Set ‘X velocity’ to 1.0

• Leave all other values at 0.0

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Flexible Ring

• Set the data tree to “FSI” mode

• Ensure that the “FSI” button is selected in the

Data tree Manager

• This only shows settings associated with set-up

of FSI models

• Create a Flexible Body

• Right-click on ‘Flexible Body’ and select ‘New’

• Right-click on ‘Flexible Body 1’ and select

‘Rename’ rename to ‘Flexible Walls’

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Flexible Ring

• Double-click on “Flexible Walls” to open it

• In the panel, click on the ‘Open Refs’ button

next to ‘Surface outputs’

• This opens the list editor to specify the name

of the surface outputs that AcuSolve will use

to determine the forces on the flexible body:

• Select ‘Add Row’, then select ‘BODY’ from the pull-down

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Flexible Ring

• Set the data tree back to “Basic”

• Click on the “BAS” button in the Data tree Manager

• Set the element set properties

• Collapse the ‘Global’ branch

• Expand the ‘Model’ branch

• Expand the ‘Volumes’ and ‘Surfaces’ branches

• Expand the ‘FLUID’ branch under ‘Volumes’

• Double-click ‘Element Set’

• Set ‘Material Model’ to Fluid

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Flexible Ring

• Set the boundary condition types

• Right click on ‘Surfaces’, then ‘Surface Manager’

• Click ‘Columns’ and make sure ‘Simple BC Type’ is

enabled

• Set the boundary conditions using the ‘Simple BC Type’

column according to the following:

• BODY Wall

• BOTTOM Slip

• INFLOW Inflow

• OUTFLOW Outflow

• SYMM1 Symmetry

• SYMM2 Symmetry

• TOP Slip

• Close surface manager

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Flexible Ring

• Set the boundary condition details

• Expand the surface named ‘BODY’

• Ensure that the ‘Surface Output’ box is toggled

on

• Double click on ‘Simple Boundary Condition’

• Ensure that ‘Wall Velocity Type’ is set to ‘Match Mesh Velocity’

• Set the ‘Mesh displacement BC Type’ to ‘Flexible Body’

• Set the ‘Flexible Body’ to ‘Flexible Walls’

• These settings tell the mesh on the BODY walls to

move based on the Flexible Body parameters

that we will define later

18


Flexible Ring

• Expand the surface named ‘SYMM1’


• Set ‘Mesh displacement BC Type’ to ‘Slip’

• These settings allow the mesh on the SYMM1

surface to slip tangentially along the surface.

Since this is a 2-d problem, it is required.

19


Flexible Ring

• SYMM1 and SYMM2 are both symmetry planes

that will need to have the same boundary

conditions

• Instead of opening the SYMM2 surface and setting

the mesh displacement type to slip, we will simply

copy the settings from SYMM1:

• Right-click on the “Simple Boundary Condition” entry under SYMM1 and select “Propagate”

• In the panel that opens, highlight “SYMM2”, then press “Propagate”.

• This will copy the boundary conditions settings to SYMM2

20


Flexible Ring

• Expand the surface named ‘INFLOW’


• Set ‘X velocity’ to 1.0

• The remainder of the simple boundary

conditions do not need to be changed and the

case can be run with the default values

• However, it is a good idea to review all of the settings to familiarize yourself with them.

• Note the mesh displacement options could be changed from fixed to slip on INFLOW, OUTFLOW, TOP and BOTTOM….experiment with this if you have time.

21


Flexible Ring

• We will now create a set of nodes surrounding the

ring that we will force to move in conjunction with the

body

• Right-click on “Nodes” and select “New”

• Rename the node set to “25 Layers”

• Right-click on “25 Layers” and select “Define”

• When the “Node Define” dialog box opens, set the type

to Surface, then select “BODY” as the surface, and set

Number of Layers to 25.

• Select “OK”

• This creates a node set containing 25 layers of nodes starting from the surface named “BODY”

22


Flexible Ring

• Right-click on the eye-ball icon in front of the node set to

toggle its visibility on

• Right-click on the “25 Layers” node set and set the display

color to black in the color chooser dialog

• Your visualization area should now show the nodes of the

set

23


Flexible Ring

• The next step is to import the structural model and project the eigenvectors onto

the CFD mesh

• We’ll project the eigenvectors onto the surface of the ring as well as the node set that was just created.

• This projection step tells AcuSolve to move the nodes according to the solution of the flexible body

• The “Eigenmode Manager” will be used to perform this projection and update the boundary conditions with the appropriate data.

• Note that this projection step relies on nodal coordinates and id’s

• If the mesh is changed, this step needs to be performed again!

24


Flexible Ring

• Open the Eigenvalue manager by clicking on the

appropriate icon in the main toolbar

• Click on “Add”, then type “Modes” for the name.

• Click on “Open” next to Import, then navigate to the

Radioss or Abaqus directory within Workshop 6 and

select the structural data file (Ring.op2 or Modal

Analysis.odb)

• Make sure the file filter is set according to the type of results file to be loaded

• Click on “Open” to load the file

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Flexible Ring

• Click on the “Show” tab in the Eigenmode

Manager, then toggle the animation button on

to visualize the modes of the structure.

• Experiment with the “Animation mode id” slider

to look at the different modes of the structure.

• You can also change the amplitude, speed, and

visualization properties of the animation using

this panel.

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Flexible Ring

• Click on the “Transfer” tab in the Eigenmode Manager.

• Select “Transfer” next to the Flexible Body label.

• Ensure that “Flexible Walls” is selected, then click on “OK”

• This will transfer the mass, stiffness, and damping arrays from the structural model over to the “Flexible Walls” flexible body that was created earlier.

27


Flexible Ring

• Select “Transfer” next to the Simple BC label.

• Select the simple boundary condition named “BODY” from the Reference Editor, then click on “OK”.

• This will project the eigenvectors of the structure onto the nodes of the surface named BODY.

28


Flexible Ring

• Select “Transfer” next to the Nodal BC label.

• Select the node set named “25 Layers”, then click on “OK”.

• This will project the eigenvectors of the structure onto the nodes of the set named 25 Layers and activate the appropriate boundary conditions.

• This projection step causes the nodes of this set to move directly with the structure

• Note that there is an option to scale the eigenvectors for more complex applications.

• Close the Eigenmode Manager

29


Flexible Ring

• Save the model

• Click on the save icon in the toolbar, or type

Ctrl+S

• Write the AcuSolve input files and launch the

solver:

• Click on the solve icon in the toolbar, or type

Ctrl+Shift+S

• Ensure that ‘Problem name’ is set to flexibleRing

• Ensure ‘Problem directory’ is set to

path\Workshop6

• Ensure ‘Working directory’ is set to

path\Workshop6\ACUSIM.DIR

• Select “OK”.

30


Flexible Ring

• Inspect the solution using acuProbe and AcuFieldView

• Plot the mesh displacements at the time history output points to get an idea of how

much the ring is deforming

• Expand ‘Time History’

• Expand ‘Node 1’ and ‘Node 2’

• Plot the displacements

• Animate the solution using AcuFieldView

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Flexible Ring

• Perform DC-FSI calculation with Abaqus (if

available on your system)

• File > Save As and enter flexDC as the new database

name

• Expand ‘Global’ and double-click ‘Problem

Descripton’

• Modify ‘Title’ and ‘Sub title’ and set ‘External Code’ to ‘On’

• Select “All” in the Data tree Manager

• Right-click ‘Multiplier Function’ and select ‘New’

• Set ‘Type’ to ‘Piecewise linear’ and ‘Curve fit variable’

to ‘Time step’

• Click ‘Open Array’ for ‘Curve fit values’ and enter as

shown

• Forces passed to Abaqus will be ramped over 10 time steps

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Flexible Ring

• Double-click ‘External Code Parameters’

• Set ‘Socket Initiate’ to ‘Off’

• Enter name of machine running Abaqus in ‘Socket

host’

• Select a ‘Socket Port’ to use - 10000 in this example

• Set ‘Multipler function’ to ‘Multiplier Function 1’

that was just created

• Double-click ‘Auto Solution Strategy’

• Set ‘Max stagger iterations’ to 4

• This allows for better convergence of flow and mesh equations

33


Flexible Body

• Expand ‘Model,’ ‘Surfaces,’ and ‘BODY’

• Disable ‘Simple Boundary Condition’ by clicking in

the check-box

• Enable ‘External Code Surface’ by clicking in the

check-box

• Set Gap factor to 0

• This is the surface used for information exchange with Abaqus

• Expand ‘Model’, ‘Nodes’, ‘25 Layers’

• Scroll down to the ‘Mesh X-Displacement’ variable

• Change the type to ‘external code’

• Repeat this process for the ‘Mesh Y-Displacement’

and ‘Mesh Z-Displacement’ variables.

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Flexible Body

• Launch AcuSolve

• Tools -> AcuSolve

• Set ‘Launch AcuSolve’ to ‘On’

• Set ‘Generate Input Files’ to ‘On’

• Hit ‘Ok’ to start the solver

• Launch Abaqus

• Open the Abaqus command prompt and/or

browse to the ‘Abaqus’ directory under

‘Workshop6’

• Issue the abaqus command:

• abaqus -job Ring_FSI -port 10000

• (Use appropriate abaqus command and port number)

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Flexible Body

• Convert AcuSolve results to Abaqus .odb format

• acuOdb -ts a

• Converts all available time steps to the .odb file

• acuOdb -h

• Gives options for the acuOdb command (similar to acuTrans)

• The figure shows contours of fluid pressure from AcuSolve and maximum principal

stress from Abaqus

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Documents

W7 Flow Over Flexible Cylinder