using sliding mesh ansys

Tutorial 11. Using Sliding Meshes

Introduction

The analysis of turbomachinery often involves the examination of the unsteady effectsdue to flow interaction between the stationary components and the rotating blades. Inthis tutorial, the sliding mesh capability of FLUENT is used to to analyze the unsteadyflow in an axial compressor stage. The rotor-stator interaction is modeled by allowingthe mesh associated with the rotor blade row to rotate relative to the stationary meshassociated with the stator blade row.

This tutorial demonstrates how to do the following:

• Create periodic zones.

• Set up the unsteady solver and boundary conditions for a sliding mesh simulation.

• Set up the grid interfaces for a periodic sliding mesh model.

• Sample the time-dependent data and view the mean value.

Prerequisites

This tutorial assumes that you are familiar with the menu structure in FLUENT and thatyou have completed Tutorial 1. Some steps in the setup and solution procedure will notbe shown explicitly.

Problem Description

The model represents a single-stage axial compressor comprised of two blade rows. Thefirst row is the rotor with 16 blades, which is operating at a rotational speed of 37,500rpm. The second row is the stator with 32 blades. The blade counts are such that thedomain is rotationally periodic, with a periodic angle of 22.5 degrees. This allows youto model only a portion of the geometry, namely, one rotor blade and two stator blades.Due to the high Reynolds number of the flow and the relative coarseness of the mesh(both blade rows are comprised of only 13,856 cells total), the analysis will employ theinviscid model, so that FLUENT is solving the Euler equations.

c© Fluent Inc. January 31, 2007 11-1

Using Sliding Meshes

inlet

outlet

ω

rotor / statorinterface

rotor blades stator blades

= 37500 rpmX

ZY

Figure 11.1: Rotor-Stator Problem Description

11-2 c© Fluent Inc. January 31, 2007


Setup and Solution

Preparation

1. Download sliding_mesh.zip from the Fluent Inc. User Services Center or copyit from the FLUENT documentation CD to your working folder (as described inTutorial 1).

2. Unzip sliding_mesh.zip.

axial comp.msh can be found in the sliding mesh folder created after unzippingthe file.

3. Start the 3D (3d) version of FLUENT.

Step 1: Grid

1. Read in the mesh file axial comp.msh.

File −→ Read −→Case...

2. Check the grid.

Grid −→Check

FLUENT will perform various checks on the mesh and will report the progress inthe console. Pay particular attention to the minimum volume, and make sure thisis a positive number.

Warnings will be displayed regarding unassigned interface zones, resulting in thefailure of the grid check. You do not need to take any action at this point, as thisissue will be rectified when you define the grid interfaces in a later step.



3. Define the units for the grid.

Define −→Units...

(a) Select angular-velocity from the Quantities selection list.

(b) Select rpm from the Units selection list.

(c) Select pressure from the Quantities selection list.

Scroll down the Quantities list to find pressure.

(d) Select atm from the Units selection list.

(e) Close the Set Units panel.

4. Display the grid (Figure 11.2).

Display −→Grid...



(a) Retain the default selections from the Surfaces selection list.

(b) Click Display and close the Grid Display panel.

(c) Rotate the view to get the display shown in Figure 11.2.

ZY

X

GridFLUENT 6.3 (3d, pbns, lam)

Figure 11.2: Rotor-Stator Outline Display

The inlet to the rotor mesh is colored blue, the interface between the rotor and statormeshes is colored yellow, and the outlet of the stator mesh is colored red.

5. Use the text user interface to change zones rotor-per-1 and rotor-per-3 fromwall zones to periodic zones.

(a) Press <Enter> in the console to get the command prompt (>).

(b) Type the commands shown in boxes as follows:



> grid

/grid> modify-zones

/grid/modify-zones> list-zonesid name type material kind

---- ------------------------- ------------------ -------------------- ----13 fluid-rotor fluid air cell28 fluid-stator fluid air cell2 default-interior:0 interior face15 default-interior interior face3 rotor-hub wall aluminum face4 rotor-shroud wall aluminum face7 rotor-blade-1 wall aluminum face8 rotor-blade-2 wall aluminum face16 stator-hub wall aluminum face17 stator-shroud wall aluminum face20 stator-blade-1 wall aluminum face21 stator-blade-2 wall aluminum face22 stator-blade-3 wall aluminum face23 stator-blade-4 wall aluminum face5 rotor-inlet pressure-inlet face19 stator-outlet pressure-outlet face10 rotor-per-1 wall aluminum face12 rotor-per-2 wall aluminum face24 stator-per-2 wall aluminum face26 stator-per-1 wall aluminum face6 rotor-interface interface face18 stator-interface interface face11 rotor-per-4 wall aluminum face9 rotor-per-3 wall aluminum face25 stator-per-4 wall aluminum face27 stator-per-3 wall aluminum face

/grid/modify-zones> make-periodic

Periodic zone [()] 10Shadow zone [()] 9Rotational periodic? (if no, translational) [yes] yes

Create periodic zones? [yes] yes

all 176 faces matched for zones 10 and 9.

zone 9 deleted

created periodic zones.



6. Similarly, change the following wall zone pairs to periodic zones:

Zone Pairs Respective Zone IDsrotor-per-2 and rotor-per-4 12 and 11

stator-per-1 and stator-per-3 26 and 27

stator-per-2 and stator-per-4 24 and 25

Step 2: Models

1. Define the solver settings.

Define −→ Models −→Solver...

(a) Select Density Based from the Solver group box.

(b) Retain the selection of Green-Gauss Cell Based from the Gradient Option groupbox.

(c) Retain the selection of Implicit from the Formulation list.

(d) Select Unsteady from the Time list.

(e) Select 2nd-Order Implicit from the Unsteady Formulation list.

(f) Click OK to close the Solver panel.



2. Enable the inviscid model.

Define −→ Models −→Viscous...

(a) Select Inviscid from the Model list.

(b) Click OK.



Step 3: Materials

1. Specify air (the default material) as the fluid material, using the ideal gas law tocompute density.

Define −→Materials...

(a) Retain the default entry of air in the Name text entry field.

(b) Select ideal-gas from the Density drop-down list in the Properties group box.

(c) Retain the default values for all other properties.

(d) Click Change/Create and close the Materials panel.

As reported in the console, FLUENT will automatically enable the energy equation,since this is required when using the ideal gas law to compute the density of amaterial.



Step 4: Operating Conditions

1. Set the operating pressure.

Define −→Operating Conditions...

(a) Enter 0 atm for the Operating Pressure.

(b) Click OK to close the Operating Conditions panel.

Since you have set the operating pressure to zero, you will specify the boundarycondition inputs for pressure in terms of absolute pressures when you define themin a later step. Boundary condition inputs for pressure should always be relative tothe value used for operating pressure.



Step 5: Boundary Conditions

Define −→Boundary Conditions...

1. Set the boundary conditions for the fluid in the rotor (fluid-rotor).



(a) Make sure that (0, 0, 1) is entered for X, Y, and Z under Rotation-Axis Direction.

(b) Select Moving Mesh from the Motion Type drop-down list.

(c) Enter 37500 rpm for Speed in the Rotational Velocity group box.

Scroll down to find the Speed number-entry box.

(d) Click OK to close the Fluid panel.

2. Set the boundary conditions for the fluid in the stator (fluid-stator).

(a) Make sure that (0, 0, 1) is entered for X, Y, and Z under Rotation-Axis Direction.

(b) Make sure that Stationary is selected from the Motion Type drop-down list.

(c) Click OK to close the Fluid panel.



3. Set the boundary conditions for the inlet (rotor-inlet).

(a) Enter 1.0 atm for the Gauge Total Pressure.

(b) Enter 0.9 atm for the Supersonic/Initial Gauge Pressure.

(c) Click the Thermal tab and enter 288 K for Total Temperature.

(d) Click OK to close the Pressure Inlet panel.



4. Set the boundary conditions for the outlet (stator-outlet).

(a) Enter 1.08 atm for the Gauge Pressure.

(b) Enable the Radial Equilibrium Pressure Distribution option.

(c) Click the Thermal tab and enter 288 K for Backflow Total Temperature.

(d) Click OK to close the Pressure Outlet panel.

Note: The momentum settings and temperature you input at the pressure outletwill be used only if flow enters the domain through this boundary. It is impor-tant to set reasonable values for these downstream scalar values, in case flowreversal occurs at some point during the calculation.



5. Retain the default boundary conditions for all wall zones.

Note: For wall zones, FLUENT always imposes zero velocity for the normal velocitycomponent, which is required whether or not the fluid zone is moving. Thiscondition is all that is required for an inviscid flow, as the tangential velocityis computed as part of the solution.

6. Close the Boundary Conditions panel.



Step 6: Grid Interfaces

1. Create a periodic grid interface between the rotor and stator mesh regions.

Define −→Grid Interfaces...

(a) Enter int for Grid Interface.

(b) Enable Periodic in the Interface Type group box.

Enabling this option, allows FLUENT to treat the interface between the slidingand non-sliding zones as periodic where the two zones do not overlap.

(c) Select rotor-interface from the Interface Zone 1 selection list.

Note: In general, when one interface zone is smaller than the other, it isrecommended that you choose the smaller zone as Interface Zone 1. Inthis case, since both zones are approximately the same size, the order isnot significant.

(d) Select stator-interface from the Interface Zone 2 selection list.

(e) Click Create and close the Grid Interfaces panel.

2. Check the grid again to verify that the warnings displayed earlier have been re-solved.

Grid −→Check



Step 7: Solution

1. Set the solution parameters.

Solve −→ Controls −→Solution...

(a) Make sure that Second Order Upwind is selected from the Flow drop-down listin the Discretization group box.

(b) Click OK to close the Solution Controls panel.



2. Enable the plotting of residuals during the calculation.

Solve −→ Monitors −→Residual...

(a) Enable Plot in the Options group box.

(b) Select relative from the Convergence Criterion drop-down list.

(c) Enter 0.01 in the fields under Relative Criteria for every Residual (continuity,x-velocity, y-velocity, z-velocity, and energy).

(d) Click OK to close the Residual Monitors panel.



3. Enable the plotting of the data at the inlet (rotor-inlet), outlet (stator-outlet), andthe interface (stator-interface).

Solve −→ Monitors −→Surface...

(a) Set the Surface Monitors to 3.

(b) Enable Plot, Print, and Write for each monitor (monitor-1, monitor-2, andmonitor-3).

(c) Select Time Step from the When drop-down list for each monitor (monitor-1,monitor-2, and monitor-3).

(d) Click the Define... button for monitor-1 to open the Define Surface Monitorpanel.

i. Select Mass Flow Rate from the Report Type drop-down list.

ii. Select Flow Time from the X Axis drop-down list.



iii. Select rotor-inlet from the Surfaces selection list.

iv. Click OK to close the Define Surface Monitor panel.

(e) Define monitor-2 to report the mass flow rate at the outlet (stator-outlet) asshown in the following panel:

! Be sure to deselect rotor-inlet from the Surfaces selection list, beforescrolling down to select stator-outlet.

(f) Define monitor-3 to report the area-weighted average of the static pressure atthe interface (stator-interface) as shown in the following panel:

! Be sure to deselect stator-outlet from the Surfaces selection list.



(g) Click OK to close the Surface Monitors panel.

4. Initialize the solution using the values at the inlet (rotor-inlet).

Solve −→ Initialize −→Initialize...

(a) Select rotor-inlet from the Compute From drop-down list.

(b) Select Absolute from the Reference Frame list.

(c) Click Init and close the Solution Initialization panel.

5. Save the initial case file (axial comp.cas).

File −→ Write −→Case...



6. Run the calculation for one revolution of the rotor.

Solve −→Iterate...

(a) Enter 6.6666e-6 s for the Time Step Size.

This time step represents the length of time during which the rotor will rotate1.5 degrees. Since the periodic angle of the rotor is 22.5 degrees, the passingperiod of the rotor blade will equal 15 time steps, and a complete revolution ofthe rotor will take 240 time steps.

(b) Enter 240 for the Number of Time Steps.

(c) Retain the default setting of 20 for the Max Iterations per Time Step in theIteration group box.

(d) Click Iterate.

The solution will converge in approximately 3100 iterations.

The residuals jump at the beginning of each time step and then fall at least two tothree orders of magnitude. Also, the relative convergence criteria is achieved beforereaching the maximum iteration limit (20) for each time step, indicating the limitdoes not need to be increased.

7. Examine the monitor histories for the first revolution of the rotor (Figures 11.4,11.5, and 11.6).



ZY

X

Scaled Residuals (Time=1.6000e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Iterations3500300025002000150010005000

1e+01

1e+00

1e-01

1e-02

1e-03

1e-04

energyz-velocityy-velocityx-velocitycontinuity

Residuals

Figure 11.3: Residual History for the First Revolution of the Rotor

Z

Y

X

Convergence history of Mass Flow Rate on rotor-inlet (Time=1.6000e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(kg/s)RateFlow

Mass

0.00160.00140.00120.00100.00080.00060.00040.00020.0000

0.2900

0.2800

0.2700

0.2600

0.2500

0.2400

0.2300

0.2200

0.2100

0.2000

monitor-1

Figure 11.4: Mass Flow Rate at the Inlet During the First Revolution



Z

Y

X

Convergence history of Mass Flow Rate on stator-outlet (Time=1.6000e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(kg/s)RateFlow

Mass

0.00160.00140.00120.00100.00080.00060.00040.00020.0000

-0.0750

-0.1000

-0.1250

-0.1500

-0.1750

-0.2000

-0.2250

-0.2500

-0.2750

-0.3000

-0.3250

monitor-2

Figure 11.5: Mass Flow Rate at the Outlet During the First Revolution

Z

Y

X

Convergence history of Static Pressure on stator-interface (Time=1.6000e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(atm)Average

WeightedArea

0.00160.00140.00120.00100.00080.00060.00040.00020.0000

1.5000

1.4000

1.3000

1.2000

1.1000

1.0000

0.9000

monitor-3

Figure 11.6: Static Pressure at the Interface During the First Revolution

The monitor histories show that the large variations in flow rate and interface pressurethat occur early in the calculation are greatly reduced as time-periodicity is approached.



8. Save the case and data files (axial comp-0240.cas and axial comp-0240.dat).

File −→ Write −→Case & Data...

! When the sliding mesh model is used, you must save a case file whenever adata file is saved. This is because the case file contains the grid information,which is changing with time.

Note: For unsteady-state calculations, you can add the character string %t to thefile name so that the iteration number is automatically appended to the name(e.g., by entering axial comp-%t for the File Name in the Select File dia-log box, FLUENT will save files with the names axial comp-0240.cas andaxial comp-0240.dat).

9. Rename the monitor file names in preparation for further iterations.

By saving the monitor histories under a new file name, the range of the axes willautomatically be set to show only the data generated during the next set of iterations.This will scale the plots so that the fluctuations are more visible.




(a) Click the Define... button for monitor-1 to open the Define Surface Monitorpanel.

i. Enter monitor-1b.out for the File Name.

ii. Click OK to close the Define Surface Monitor panel.

(b) Similarly, change the File Name for monitor-2 and monitor-3 to be monitor-2b.outand monitor-3b.out respectively.

(c) Click OK to close the Surface Monitors panel.

Extra: Instead of creating a new file for each monitor, you could have adjustedthe ranges of the axes to make the fluctuations visible, and then allowed thedata from the next set of iterations to be appended onto the original monitorfiles. To do this, click the Axes... button in the Define Surface Monitor panelof each monitor, disable the Auto Range option, enter in appropriate values inthe Range group box, and click Apply.



10. Continue the calculation for 720 more time steps to simulate three more revolutionsof the rotor.


! Calculating three more revolutions will require significant CPU re-sources. Instead of calculating the solution, you can read a data file(axial comp-0960.dat.gz) with the precalculated solution for this tu-torial. This data file can be found in the folder where you found the meshfile.

The calculation will run for approximately 9,100 more iterations.

11. Examine the monitor histories for the next three revolutions of the rotor to verifythat the solution is time-periodic (Figures 11.7, 11.8, and 11.9).

Note: If you read the provided data file instead of iterating the solution for threerevolutions, the monitor histories can be displayed by using the Plot/File...menu option. Simply click the Add button in the File XY Plot panel, select oneof the monitor histories in the Select File dialog box, click OK, and then clickPlot.



Z

Y

X

Convergence history of Mass Flow Rate on rotor-inlet (Time=6.3999e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(kg/s)RateFlow

Mass

0.00650.00600.00550.00500.00450.00400.00350.00300.00250.00200.0015

0.2748

0.2748

0.2748

0.2748

0.2748

0.2748

0.2748

0.2748

monitor-1

Figure 11.7: Mass Flow Rate at the Inlet During the Next 3 Revolutions

Z

Y

X

Convergence history of Mass Flow Rate on stator-outlet (Time=6.3999e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(kg/s)RateFlow

Mass

0.00650.00600.00550.00500.00450.00400.00350.00300.00250.00200.0015

-0.2740

-0.2745

-0.2750

-0.2755

-0.2760

-0.2765

-0.2770

-0.2775

-0.2780

monitor-2

Figure 11.8: Mass Flow Rate at the Outlet During the Next 3 Revolutions



Z

Y

X

Convergence history of Static Pressure on stator-interface (Time=6.3999e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(atm)Average

WeightedArea

0.00650.00600.00550.00500.00450.00400.00350.00300.00250.00200.0015

1.1180

1.1175

1.1170

1.1165

1.1160

1.1155

1.1150

1.1145

1.1140

1.1135

1.1130

1.1125

monitor-3

Figure 11.9: Static Pressure at the Interface During the Next 3 Revolutions

Extra: Note that the y-axis for Figure 11.7 does not show enough significant figuresto fully display the values of the mass flow rate. You can display the full valuesby using the Plot/File... menu option. Use the Add... button in the File XY Plotpanel to select monitor-1b.out, and click the Axes... button to open the Axes -File XY Plot panel. There, select Y from the Axis list, enter 6 for Precision,and click Apply. Finally, click Plot in the File XY Plot panel to redisplay themonitor history.



13. Change the File Name for monitor-1, monitor-2, and monitor-3 to be monitor-1c.out,monitor-2c.out, and monitor-3c.out, respectively (as described in a previousstep), in preparation for further iterations.




14. Continue the calculation for one final revolution of the rotor, while saving datasamples for the postprocessing of the time statistics.


(a) Enter 240 for the Number of Time Steps.

(b) Enable Data Sampling for Time Statistics in the Options group box.

(c) Click Iterate.

(d) Close the Iterate panel.

The calculation will run for approximately 3,000 more iterations.





Step 8: Postprocessing

In the next two steps you will examine the time-averaged values for the mass flow ratesat the inlet and the outlet during the final revolution of the rotor. By comparing thesevalues, you will verify the conservation of mass on a time-averaged basis for the systemover the course of one revolution.

1. Examine the time-averaged mass flow rate at the inlet during the final revolutionof the rotor (as calculated from monitor-1c.out).

Plot −→FFT...



(a) Click the Load Input File... button to open the Select File dialog box.

i. Select All Files from the Files of type drop-down list.

ii. Select monitor-1c.out from the list of files.

iii. Click OK to close the Select File dialog box.



(b) Click the Plot/Modify Input Signal... button to open the Plot/Modify InputSignal panel.

i. Examine the values for Min, Max, Mean, and Variance in the Signal Statis-tics group box.

ii. Close the Plot/Modify Input Signal panel.

(c) Select the folder path ending in monitor-1c.out from the Files selection list.

(d) Click the Free File Data button.

2. Examine the time-averaged mass flow rate at the outlet during the final revolutionof the rotor (as calculated from monitor-2c.out), and plot the data.

Plot −→FFT...

(a) Click the Load Input File... button to open the Select File dialog box.

i. Select All Files from the Files of type drop-down list.

ii. Select monitor-2c.out from the list of files.

iii. Click OK to close the Select File dialog box.



(b) Click the Plot/Modify Input Signal... button to open the Plot/Modify InputSignal panel.

i. Examine the values for Min, Max, Mean, and Variance in the Signal Statis-tics group box.

Note that the outlet mass flow rate values correspond very closely withthose from the inlet, with the mean having approximately the same ab-solute value but with opposite signs. Thus, you can conclude that massis conserved on a time-averaged basis during the final revolution of therotor.

ii. Click the Set Defaults button.

iii. Click Apply/Plot to display the mass flow rate at the outlet (Figure 11.10).

iv. Close the Plot/Modify Input Signal panel.

(c) Close the Fourier Transform panel.



Z

Y

X

Convergence history of Mass Flow Rate on stator-outlet (in SI units) (Time=7.9999e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

Flow Time

(kg/s)RateFlow

Mass

0.0080.00780.00760.00740.00720.0070.00680.00660.0064

-2.75e-01

-2.75e-01

-2.75e-01

-2.75e-01

-2.75e-01

-2.75e-01

-2.75e-01

Figure 11.10: Mass Flow Rate at the Outlet During the Final Revolution

3. Display contours of the mean static pressure on the walls of the axial compressor.

Display −→Contours...

(a) Enable Filled in the Options group box.

(b) Select Unsteady Statistics... and Mean Static Pressure from the Contours ofdrop-down lists.



(c) Select wall from the Surface Types selection list.

Scroll down the Surface Types selection list to find wall.

(d) Click Display and close the Contours panel.

(e) Rotate the view to get the display shown in Figure 11.11.

Contours of Mean Static Pressure (atm) (Time=7.9999e-03)FLUENT 6.3 (3d, dbns imp, unsteady)

1.34e+00

1.29e+00

1.23e+00

1.18e+00

1.12e+00

1.07e+00

1.01e+00

9.60e-01

9.06e-01

8.52e-01

7.98e-01

7.43e-01

6.89e-01

6.35e-01

5.81e-01

5.27e-01

4.72e-01

4.18e-01

3.64e-01

3.10e-01

2.55e-01 ZY

X

Figure 11.11: Mean Static Pressure on the Outer Shroud of the Axial Compressor

Shock waves are clearly visible in the flow near the outlets of the rotor and stator,as seen in the areas of rapid pressure change on the outer shroud of the axialcompressor.



Summary

This tutorial has demonstrated the use of the sliding mesh model for analyzing unsteadyrotor-stator interaction in an axial compressor stage. The model utilized the density-based solver in conjunction with the unsteady, dual-time stepping algorithm to computethe inviscid flow through the compressor stage. The solution was calculated over timeuntil the monitored variables displayed time-periodicity (which required several revolu-tions of the rotor), after which time-averaged data was collected while running the casefor the equivalent of one additional rotor revolution (240 time steps). The Fast FourierTransform (FFT) utility in FLUENT was employed to determine the time averages fromstored monitor data. Although not described in this tutorial, you can further use theFFT utility to examine the frequency content of the unsteady monitor data (in this case,you would observe peaks corresponding to the passing frequency and higher harmonicsof the passing frequency).

Further Improvements

This tutorial guides you through the steps to reach a second-order solution. You maybe able to obtain a more accurate solution by adapting the grid. Adapting the grid canalso ensure that your solution is independent of the grid. These steps are demonstratedin Tutorial 1.


using sliding mesh ansys

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