Solving FSI Applications Using ANSYS Mechanical and ANSYS...

© 2011 ANSYS, Inc. June 26, 2014 1 Release 15.0

15. 0 Release

Solving FSI Applications Using ANSYS Mechanical and ANSYS Fluent

Workshop Coriolis Flow Meter


Workshop Description:

This example considers flow through a vibrating Coriolis flow meter. Fluid flow causes a phase shift between the inlet and outlet arms of the meter which is used to measure mass flow rate.

Learning Aims:

This workshop shows how to prepare a modal analysis and a 2-way coupled FSI analysis in workbench. This includes:

– Set up of the modal analysis for the flow meter

– Set up of the Transient Structural case for the flow meter piping

– Set up of the transient Fluent case including coupling effects

– Set up and solution of the coupled flow case

Learning Objectives:

To understand the key steps in setting up a modal analysis and to use information obtained from the modal analysis for a 2-way coupled FSI simulation within Workbench.

Introduction


In a Coriolis mass flow meter fluid passes though a U-shaped or similar tube. A forcing displacement is applied to the centre of the U-bend. The elbows upstream and downstream of the centre are displaced accordingly, but there is a phase shift between the upstream and downstream sides which is related to the fluid mass flow through the tubes. Typically two counter vibrating tubes are used so that external vibrations do not affect the measuring device. In this workshop, only one tube will be modelled since there will be no external vibrations acting on the flow meter.

The displacements and phase shifts are highly exaggerated in the animations below. Also see http://en.wikipedia.org/wiki/Mass_flow_meter to view the animations.

Simulation to be performed

http://en.wikipedia.org/wiki/Mass_flow_meter

Tube vibrations with no flow Tube vibrations with flow

http://en.wikipedia.org/wiki/Mass_flow_meter


The forcing displacement applied to the tubes should be at the natural frequency required to excite the appropriate mode shape for the system. In the first part of this workshop we will find the frequency of interest using a Modal analysis. We must consider that the tubes are filled with fluid when calculating this frequency.

In the second part of the workshop the forcing displacement is applied at the correct frequency to the tubes in a Transient Structural analysis. The fluid flow is modelled in Fluent. The structural and fluid systems are connected via System Coupling to facilitate a 2-way FSI analysis.

Simulation to be performed


Starting Workbench

1. Start ANSYS Workbench (R15.0) and select File > Restore Archive: a) Select CoriolisFlowMeter.wbpz b) Save to your working directory (save

to a local hard disk, not a USB memory stick)

The restored archive contains a Modal block, a Transient Structural block, and a Fluent block with the geometry and meshes already created.


1. Double-click on the Modal Setup cell (A5)

The mesh has already been generated. This modal analysis will include both the solid and fluid regions. The fluid region will be modelled using FLUID30 elements. This element is often used to model acoustics but also includes acceleration effects and motion at the fluid-solid interface. The element type, material properties and identification of the fluid-solid interface must be completed in a Commands object.

2. In the Model tree, expand Geometry and Part then right-click on fluid and select Insert > Commands

3. In the Units menu select Metric (m, kg, N, s, V, A)

4. Enter the commands as shown (they are not case sensitive)

These commands will first delete the structural material properties (mpdele), then switch the element type to FLUID30 (et). Material properties are then defined for the fluid density (mp,DENS) and the speed of sound (mp,SONC). See “Using Coupled Field Elements.pptx” for general background information on changing element types.

Modal Analysis


5. Right-click on Modal (A5) and select Insert > Fixed Support

6. Using the edge filter, select both outer pipe edges then click Apply in the Geometry field

Modal Analysis


7. Right-click on Modal (A5) in the Outline tree select Insert > Commands

8. Enter the commands as shown

These commands set the FSI interface flag on the FLUID30 elements that touch the tube. The cmsel command selects the nodes associated with the Named Selection “interface”. The esel commands then selects only the FLUID30 elements attached to these nodes. The sf commands then set the FSI interface flag.

9. Click Analysis Settings and under Solver Controls, change Solver Type to Unsymmetric

The documentation for FLUID30 elements notes that when coupled to structural elements an unsymmetric modal analysis is necessary.

10. Right-click Solution (A6) and solve the system

Modal Analysis


11. Click Solution Information and scroll down to the PARTICIPATION FACTOR CALCULATION tables

Mode 3 has a significant participation factor in the Y direction and is likely the mode of interest (this is the direction we want to excite).

12. Right-click on Solution (A6) branch and select Insert > Deformation > Total

13. Select Mode 3, then right-click on Total Deformation and select Evaluate All Results. Play the animation to view the mode shape.

14. Check the other mode shapes in a similar way

Modal Analysis


Mode 3 at 50.4 Hz is the mode of interest. Note how it matches the animations at the start of the workshop.

15. Close the Mechanical window and save the project

Modal Analysis

Mode 3, 50.4 Hz


1. Double click the Setup cell in the Transient Structural system (B5)

2. Check the Metric Units system is still selected

3. Right-click Transient (B5) and select Insert > Fixed Support, using the same two edges as in the Modal analysis

4. Right-click Transient (B5) and select Insert > Displacement

5. Using the face selection filter, and select the face shown and click Apply in the Geometry field

6. In the Y Component field click the arrow to select Function

7. Type the function:

0.000025*sin(50.4*360*time)

A 25 μm amplitude sine wave with a frequency of 50.4 Hz is applied

Transient Structural


8. Click Analysis Settings and change Define By to Substeps and turn Auto Time Stepping to Off

9. Change the Number of Substeps to 1

10. Right-click Transient (B5) and select Insert > Fluid Solid Interface

11. Change Scoping Method to Named Selection and select fsi_solid

12. Right-click Solution Information and select Insert > Deformation

13. Change Scoping Method to Named Selection and select Node1

14. Change the Orientation to Y Axis

15. Duplicate this deformation tracker and change the selection to Node2

These two Results Trackers will provide the y-direction displacement history for the two pipe elbows. A chart can also be added to plot both displacements.



16. Insert a Chart using the toolbar icon shown, and select the two Directional Deformation objects in the Outline Selection field

The locations for the two displacement trackers are the upstream and downstream bends of the meter. We expect a phase shift between these two displacement trackers.

17. Type in X-Axis and Y-Axis labels as shown

18. Close Mechanical and save the project



1. Edit the Fluent Setup cell (C4), select the Double Precision option and click OK

2. Select Models from the tree, edit Viscous – Laminar and change to k-omega (2 eqn) with the SST model and Curvature Correction enabled. Click OK.

3. Select Materials, click Create/Edit followed by Fluent Database...

4. Locate water-liquid and click Copy then Close

5. Change the Density (kg/m3) to 1000 then click Change/Create and Close

6. Select Cell Zone Conditions, click Edit... for the fluid zone and change the Material Name to water-liquid then hit OK

7. Select Boundary Conditions, edit inlet and enter a Velocity Magnitude of 20 m/s

8. Change the Turbulence to Intensity and Length Scale with a Turbulence Intensity (%) of 1 and the Turbulent Length Scale (m) to 0.02. Click OK

Steady State Fluent


9. Edit the outlet, leaving the Gauge Pressure at 0 Pa and changing the turbulence settings to the same as the inlet

10. Select Solution Methods, change the Pressure-Velocity Coupling Scheme to Coupled and enable the Pseudo Transient and High Order Term Relaxation check boxes

11. Select Solution Controls and set the Pressure and Momentum relaxation factors to 1

12. Select Run Calculation and change the Number of Iterations to 200

13. Click Calculate and click yes when prompted to initialize the case

14. Close the Fluent window when the solution has finished and save the project

Steady State Fluent


Project Schematic

1. Right-click on the Fluent Setup cell (C4) and select Duplicate. Rename the new Fluent system to Fluent Coupling.

Performing the Duplicate operation from the Setup cell copies setup information to the new Fluent system

2. Connect C5 to D5 to provide the initial solution to the Fluent Coupling system

3. Drag a System Coupling component system onto the Project Schematic and drop it on the Setup cell (B5) of the Transient Structural system. Connect the Fluid Coupling Setup cell (D4) to the System Coupling Setup cell (E2) as well.


1. Edit the Fluent Coupling Solution cell (D5)

2. Select General from the tree and change the Time option to Transient

3. Select Dynamic Mesh from the tree and enable the Dynamic Mesh check box

4. Under Mesh Methods select Settings... then pick the Diffusion method and set a Diffusion Parameter of 1. Click OK

5. Under Dynamic Mesh Zones click Create/Edit. Select fsi_fluid from the Zone Names drop-down menu, pick the System Coupling option then click Create and close the window

Fluent Coupling


6. Select Solution Methods and set the Transient Formulation to Second Order Implicit

7. Under Run Calculation, change the Number of Time Steps to 1 and the Max Iterations/Time Step to 5

The Number of Time Steps and Time Step Size are controlled by System Coupling

8. Close Fluent and notice the Setup cell has a check mark for Fluent Coupling but not for Transient Structural

9. Right click the Transient Structural Setup cell (B5) and click Update

Fluent Coupling


1. Edit the System Coupling Setup cell (E2) and click Yes at the prompt

2. Under Analysis Settings, set the End Time [s] to 0.1 and the Step Size [s] to 0.0005

3. Multi-select Fluid Solid Interface under Transient Structural and fsi_fluid under Fluent Coupling using the CTRL key, then right-click and select Create Data Transfer

4. Save the project, then right-click Solution and select Update

The solution will take about 1.5 hours to complete

System Coupling


5. When the solution is complete return to the Project page and then open the Transient Structural Results cell (B7)

6. Select the previously created Chart

The outlet arm (green) leads the inlet arm (red) after start up effect have died out. The phase shift is very small and is difficult to see on this chart. It can be calculated by determining the difference between the times that the inlet and outlet arm have a displacement of 0.

Note that the outlet arm has a vertical phase shift so this would have to be accounted for when determining the 0 displacement time. Alternatively, the time difference between peaks can be used to determine the phase shift.

System Coupling

Small phase shift seen in tracked displacements


Simulations can be run at different flow velocities and the phase shift calculated for each flow velocity. The results can be used to create a calibration curve where mass flow rate can be determined given a specific phase shift. Simulation results are shown below for this flow meter

Further Work

0

0.1

0.2

0.3

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0.7

0 1 2 3 4 5

Mas

s fl

ow

rat

e (

kg/s

)

Phase shift (degrees)


• This workshop has demonstrated how to set up a modal analysis and use the results from this analysis in a 2-way coupled FSI simulation

• The displacement applied on the flow meter tube and the observed phase shift between the inlet and outlet arms is small but can still be seen through simulation

• A steady-state fluid simulation is used to initialize the FSI simulation so that a shock is not applied to the system which could cause instability

• Different flow velocities can be used resulting in different phase shifts, which can be used to create a calibration curve

Wrap-up

Solving FSI Applications Using ANSYS Mechanical and ANSYS...

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