Tutorial 12. Using Sliding Meshes Introduction The analysis of turbomachinery often involves the examination of the transient effects due to flow interaction between the stationary components and the rotating blades. In this tutorial, the sliding mesh capability of ANSYS FLUENT is used to analyze the transient flow in an axial compressor stage. The rotor-stator interaction is modeled by allowing the mesh associated with the rotor blade row to rotate relative to the stationary mesh associated with the stator blade row. This tutorial demonstrates how to do the following: • Create periodic zones. • Set up the transient solver and cell zone and boundary conditions for a sliding mesh simulation. • Set up the mesh interfaces for a periodic sliding mesh model. • Sample the time-dependent data and view the mean value. Prerequisites This tutorial is written with the assumption that you have completed Tutorial 1, and that you are familiar with the ANSYS FLUENT navigation pane and menu structure. Some steps in the setup and solution procedure will not be shown explicitly. Problem Description The model represents a single-stage axial compressor comprised of two blade rows. The first row is the rotor with 16 blades, which is operating at a rotational speed of 37,500 rpm. The second row is the stator with 32 blades. The blade counts are such that the domain is rotationally periodic, with a periodic angle of 22.5 degrees. This allows you to 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 the inviscid model, so that ANSYS FLUENT is solving the Euler equations. Release 12.0 c ANSYS, Inc. March 12, 2009 12-1
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Tutorial 12. Using Sliding Meshes
Introduction
The analysis of turbomachinery often involves the examination of the transient effects dueto flow interaction between the stationary components and the rotating blades. In thistutorial, the sliding mesh capability of ANSYS FLUENT is used to analyze the transientflow 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 transient solver and cell zone and boundary conditions for a sliding meshsimulation.
• Set up the mesh interfaces for a periodic sliding mesh model.
• Sample the time-dependent data and view the mean value.
Prerequisites
This tutorial is written with the assumption that you have completed Tutorial 1, andthat you are familiar with the ANSYS FLUENT navigation pane and menu structure.Some steps in the setup and solution procedure will not be 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 ANSYS FLUENT is solving the Euler equations.
ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Ensure that the reported minimum volume is a positive number.
Warnings will be displayed regarding unassigned interface zones, resulting in thefailure of the mesh check. You do not need to take any action at this point, as thisissue will be rectified when you define the mesh interfaces in a later step.
2. Examine the mesh (Figure 12.2).
Orient the view to display the mesh as shown in Figure 12.2. The inlet of the rotormesh is colored blue, the interface between the rotor and stator meshes is coloredyellow, and the outlet of the stator mesh is colored red.
Figure 12.2: Rotor-Stator Display
3. 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 (>).
1. Specify air (the default material) as the fluid material, using the ideal gas law tocompute density.
Materials −→ air −→ Create/Edit...
(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 Create/Edit Materials dialog box.
As reported in the console, ANSYS FLUENT will automatically enable the energyequation, since this is required when using the ideal gas law to compute the densityof the fluid.
(d) Click OK to close the Pressure Outlet dialog box.
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.
3. Retain the default boundary conditions for all wall zones.
Boundary Conditions −→ rotor-blade-1 −→ Edit...
Note: For wall zones, ANSYS FLUENT always imposes zero velocity for the normalvelocity component, which is required whether or not the fluid zone is moving.This condition is all that is required for an inviscid flow, as the tangentialvelocity is computed as part of the solution.
(b) Click OK to close the Operating Conditions dialog box.
Since you have specified the boundary condition inputs for pressure in terms ofabsolute pressures, you have to set the operating pressure to zero. Boundary con-dition inputs for pressure should always be relative to the value used for operatingpressure.
1. Create a periodic mesh interface between the rotor and stator mesh regions.
Mesh Interfaces −→ Create/Edit...
(a) Enter int for Mesh Interface.
(b) Enable Periodic Repeats in the Interface Options group box.
Enabling this option, allows ANSYS FLUENT to treat the interface between thesliding and 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 Create/Edit Mesh Interfaces dialog box.
2. Check the mesh again to verify that the warnings displayed earlier have been re-solved.
8. Run the calculation for one revolution of the rotor.
Run Calculation
(a) Enter 6.6666e-6 s for Time Step Size.
(b) Enter 240 for Number of Time Steps.
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.
(c) Retain the default setting of 20 for Max Iterations/Time Step.
(d) Click Calculate.
The calculation will run for approximately 3,700 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.
9. Examine the monitor histories for the first revolution of the rotor (Figures 12.4,12.5, and 12.6).
The monitor histories show that the large variations in flow rate and interfacepressure that occur early in the calculation are greatly reduced as time-periodicityis approached.
10. Save the case and data files (axial comp-0240.cas.gz and axial comp-0240.dat.gz).
File −→ Write −→Case & Data...
! It is a good practice to save the case file whenever you are saving the datafile especially for sliding mesh model. This is because the case file containsthe mesh information, which is changing with time.
Note: For transient-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 dialogbox, ANSYS FLUENT will save files with the names axial comp-0240.cas
and axial comp-0240.dat).
11. Rename the monitor files in preparation for further iterations.
Monitors −→ surf-mon-1 −→ Edit...
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) Enter surf-mon-1b.out for File Name.
(b) Click OK to close the Surface Monitor dialog box.
12. Similarly, rename surf-mon-2.out and surf-mon-3.out to surf-mon-2b.out andsurf-mon-3b.out, respectively.
13. Continue the calculation for 720 more time steps to simulate three more revolutionsof the rotor.
Run Calculation
! 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 sliding mesh folder.
The calculation will run for approximately 10,600 more iterations.
14. Examine the monitor histories for the next three revolutions of the rotor to verifythat the solution is time-periodic (Figures 12.7, 12.8, and 12.9).
Note: If you read the provided data file instead of iterating the solution for threerevolutions, the monitor histories can be displayed by using the File XY Plotdialog box.
Plots −→ File −→ Set Up...
Click the Add button in the File XY Plot dialog box to select one of the monitorhistories from the Select File dialog box, click OK, and then click Plot.
Figure 12.9: Static Pressure at the Interface During the Next 3 Revolutions
Extra: Note that the Y -axis for Figure 12.7 does not show enough significant fig-ures to fully display the values of the mass flow rate.
15. (Optional) Display the full values by using the File XY Plot dialog box.
Plots −→ File −→ Set Up...
(a) Click the Add... button to open the Select File dialog box.
i. Select surf-mon-1b.out and click OK to close the Select File dialog box.
(b) Click the Axes... button to open the Axes - File XY Plot dialog box.
i. Select Y in the Axis list.
ii. Set Precision to 6.
iii. Click Apply and close the Axes - File XY Plot dialog box.
(c) Click Plot and close the File XY Plot dialog box.
16. Save the case and data files (axial comp-0960.cas.gz and axial comp-0960.dat.gz).
File −→ Write −→Case & Data...
17. Change the file names for surf-mon-1b.out, surf-mon-2b.out, and surf-mon-3b.outto surf-mon-1c.out, surf-mon-2c.out, and surf-mon-3c.out, respectively (asdescribed in a previous step), in preparation for further iterations.
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 surf-mon-1c.out).
Plots −→ FFT −→ Set Up...
(a) Click the Load Input File... button to open the Select File dialog box.
i. Examine the values for Min, Max, Mean, and Variance in the Signal Statis-tics group box.
ii. Close the Plot/Modify Input Signal dialog box.
(c) Select the folder path ending in surf-mon-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 surf-mon-2c.out), and plot the data.
Plots −→ FFT −→ Set Up...
(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 surf-mon-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 dialog box.
i. Examine the values for Min, Max, Mean, and Variance in the Signal Statis-tics group box.
The outlet mass flow rate values correspond very closely with those fromthe inlet, with the mean having approximately the same absolute value butwith opposite signs. Thus, you can conclude that mass is conserved on atime-averaged basis during the final revolution of the rotor.
(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 dialog box.
(e) Rotate the view to get the display as shown in Figure 12.11.
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.
Figure 12.11: Mean Static Pressure on the Outer Shroud of the Axial Compressor
This tutorial has demonstrated the use of the sliding mesh model for analyzing transientrotor-stator interaction in an axial compressor stage. The model utilized the density-based solver in conjunction with the transient, 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 Fourier Transform (FFT) utility in ANSYS FLUENT was employed to determinethe time averages from stored monitor data. Although not described in this tutorial, youcan further use the FFT utility to examine the frequency content of the transient monitordata (in this case, you would observe peaks corresponding to the passing frequency andhigher harmonics of the passing frequency).
Further Improvements
This tutorial guides you through the steps to reach a second-order solution. You may beable to obtain a more accurate solution by adapting the mesh. Adapting the mesh canalso ensure that your solution is independent of the mesh. These steps are demonstratedin Tutorial 1.