Download the vbm_helicopter_tutorial.zip file here .

Unzip vbm_helicopter_tutorial.zip to your working directory and ensure the following files are available:

naca0015.dat

VBM_helicopter_tutorial.msh.gz

xnr_phi0.xy

xnr_phi090.xy

xnr_phi180.xy

xnr_phi270.xy

Use the Fluent Launcher to start Ansys Fluent.

Select Solution in the top-left selection list to start Fluent in solution mode.

Select 3D for Dimension, Double Precision for Solver Options and a suitable number of Processes.

Select the appropriate Working Directory in the General Options panel.

Click Start.

To read the mesh file, go to:

 File → Read → Mesh…

Select the VBM_helicopter_tutorial.msh.gz file and click OK.

To display the mesh, click the following button in the ribbon.

 Domain → Mesh → Display…

Deselect inlet, outlet and tunnel in the Surfaces list, select int_rotor and fuselage and click Display.

Since this case uses an embedded disk (EDM approach) in the first part of this tutorial, the computational domain must be divided into two separate but connected fluid domains (cell zones) (Physics → Zones → Cell Zones). The VBM only acts on the cells that form the live1 cell zone attached to one side of the actuator disk.

As shown in Figure 34.2: From Left to Right, the Three Components of the live1 Cell Zone: Int_Rotor, Int_live1 and Int_Live., Fluent automatically subdivides the live1 cells into three separate entities.

To display the first two surfaces, select each in the list of Surfaces, and click Display.

For the third surface, however, click Adjacency in the Mesh Display dialog box to open the Adjacency dialog box. Select live1 from the list of Cell Zone(s) and int_live1 from the list of the Adjacent Face Zones and then click on the Display Face Zones.

Fluent VBM can be enabled through the VBM Rotor Inputs dialog box or through the Text User Interface (TUI) by using the following text user interface command in the Fluent Console:

define/models/virtual-blade-model/enable?

Answer yes when asked to enable the Virtual Blade Model.

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Note:  The VBM is only available with Dimension set to 3D.

The VBM can only be enabled when a valid Ansys Fluent case or mesh file has been set or read.

The current compatibility of the VBM is limited as it cannot operate with multiphase or inviscid flow. If any of these are active, Fluent will prevent the VBM from being enabled and display a warning message via either the Console or a pop-up message.

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Click the si tab.

Click angle in the Quantities list and choose deg in the Units list.

Close the Set Units dialog box.

Set a value of 101263.15 Pa in the Operating Pressure box.

Set the value of the X component of the Reference Pressure Location to -1 m.

Click OK.

Configure the Fluent solver settings.

 Physics → Solver → General...

Select the following options in Task Page:

Type → Pressure-Based

Velocity Formulation → Absolute

Time → Steady

Enable the energy equation.

 Physics → Models → Energy

Enable the Energy box.

Select the turbulence model.

 Physics → Models → Viscous...

For vorticity-dominated flows, the default Vorticity-Based-production option overpredicts the production of eddy viscosity in the vortex cores. Adding the strain tensor to the vorticity reduces the production of turbulent viscosity in regions where the measure of vorticity exceeds that of strain rate.

Select air as the working fluid in the Fluent Fluid Materials pull-down menu

Select ideal-gas in the Density pull-down menu.

Click the Change/Create button.

Close the Create/Edit Materials dialog box.

Set the boundary condition at the inlet.

Set the boundary conditions at the outlet.

Set wall boundary conditions.

Select inlet from the Compute from pull-down menu.

Set the Area value to 0.65612 m² (disk area).

Set the Length Value to 0.086 m (blade chord).

Select live from the Reference Zone pull-down menu.

Use the pull-down menus to set the following options:

Retain the default solver parameters:

 Solution → Controls → Controls...

Select Standard and click Options... to open the Solution Initialization task page which provides access to further settings.

Select inlet under the Compute from drop-down.

Set Reference Frame to Absolute.

Click the Initialize button.

Select Virtual Blade Model... to open the VBM Rotor Inputs dialog box.

Toggle Enable VBM.

Select Embedded Disk under VBM Mode.

Press Add. A new rotor with a default name disk-1 with default properties will be created. The settings appearing to the right will be updated in the next steps.

Enter main-rotor in the Active Rotor Name box.

Enter the parameters shown in Figure 34.3: General Disk Data Configuration Window.

Select int_rotor from the Interior Surfaces list to select the actuator disk surface.

Click the Geometry tab and enter the parameters shown in Figure 34.3: General Disk Data Configuration Window.

Click Change/Create to save the settings. Click OK if the following warning message appears:

Click Apply to close the VBM Rotor Inputs dialog box.

Figure 34.3: General Disk Data Configuration Window

Figure 34.4: Geometry Configuration Window

Ensure that Plot and Print to Console options are enabled in the Options group box.

Enable Show Advanced Options and select absolute from the Convergence Criterion drop-down list.

Set the Absolute Criteria values to 1e-6 for Energy equation and to 1e-5 for other equations as shown in Figure 34.5: Solution Residuals Configuration.

Click OK to close the Residual Monitors dialog box.

Figure 34.5: Solution Residuals Configuration

Enter rotor-pressure in the Name box.

Enable Report File, Report Plot and Print to Console in the Create section.

Select Pressure… and Static Pressure with the pull-down menus in the Field Variable section.

Select int_rotor in the Surfaces section.

Click OK to close the Surface Report Definition dialog box.

Enter main-rotor-thrust in the Name box.

Enable Report File, Report Plot and Print to Console under Create.

Select Rotor Thrust under Report Type.

Select Quantity under Output Type.

Select main-rotor under Rotor Names.

Click OK to close the VBM Report Definition dialog box.

Launch the Fluent executable in the working directory using the same launching setup shown in Preparation.

Read the case file,  File → Read → Case... and select VBM_helicopter_tutorial.cas. Alternatively, you can read both case & data file when restarting from a previous solution.

Initialize the solution from inlet boundary values, select  Solution → Initialization → Initialize. If you restarted from a previous solution, you can skip this step.

Apply the VBM, select  Physics → Models → More → Virtual Blade Model… to open the VBM Rotor Inputs dialog box and click Apply. This action will prompt the Fluent VBM to read and pre-process rotor setup in the VBM Rotor Inputs dialog box and report details on each rotor zone.

For the first simulation, the helicopter rotor is operating in fixed-pitch mode without trimming using the EDM approach. Go to:

 Solution → Run Calculation → Run Calculation...

Set the No. of Iterations value to 500 and click the Calculate button.

The convergence history of the residuals is shown in Figure 34.11: Convergence History. Figure 34.12: Pressure Monitor Convergence History shows the convergence history of the integral of the static pressure on the int_rotor surface. The convergence history of the main-rotor thrust, which is a VBM output quantity, is demonstrated in Figure 34.13: Rotor Thrust Monitor Convergence History. It can be observed that the main-rotor thrust converged more quickly than the residuals, although this is not always the case.

Figure 34.11: Convergence History

Figure 34.12: Pressure Monitor Convergence History

Figure 34.13: Rotor Thrust Monitor Convergence History

To display contours of pressure on the cutting planes that have been created in Cutting Planes for the Pressure Distributions, go to:

 Results → Graphics → Contours → New …

Figure 34.15: Pressure Distribution with Fixed Blade Pitch, Y=0 Cutting Plane and Figure 34.16: Pressure Distribution with Fixed Blade Pitch, X=0.3 Cutting Plane show that there is a clear pressure load imbalance on the disk, therefore if the helicopter were free to fly, it would not be able to maintain a straight-and-level course, rather it would tilt up and roll to the left. To achieve stability, the rotor must be trimmed so that the pressure integrated over the rotor produces no net moments, only lift and thrust in the forward direction. This can only be achieved by fine-tuning the rotor collective and cyclic pitch (thrust and moment trimming).

To display contours of your results on the disk planes that have been created in Cutting Planes for the VBM Results, go to:

  Results → Graphics → Contours→ New...

Save the Fluent case and data files (VBM_helicopter_tutorial.cas and VBM_helicopter_tutorial.dat):

  File → Write → Case & Data

To enable collective trimming, go to:

 Physics → Models → More

To run the calculation, go to:

Solution → Run Calculation...

Set a value of 200 in the No. of Iterations box and press the Calculate button.

The residuals, pressure and thrust monitor values should not change for the next 200 iterations. This short run is required only to verify that the thrust coefficient value is precise and the collective angle printed to the Console after 200 iterations converges to the original 10° fixed-pitch blade angle.

When the execution terminates, save the case and data files (VBM_helicopter_tutorial_VP.cas and VBM_helicopter_tutorial_VP.dat).

 File → Write → Case & Data…

To enable cycling trimming, go to:

 Physics→ Models → More

To run the calculation, go to:

Solution→ Run Calculation...

Set a value of 600 in the No. of Iterations box and press the Calculate button.

At convergence the residual history will look like Figure 34.19: Convergence History with Collective and Cyclic Angles Trimming.

Figure 34.19: Convergence History with Collective and Cyclic Angles Trimming

When the execution terminates, save the case and data files (VBM_helicopter_tutorial_Co_Cy.cas and VBM_helicopter_tutorial_Co_Cy.dat):

 File → Write → Case & Data…

During the saving process, trimming results will also be saved in a Fluent .dat file and printed in the Console. Due to cyclic pitch trimming, the collective pitch angle changes to 10.14838˚ and the coefficient of cyclic cosine and sine components of the blade pitch angle change to -1.91224˚ and 2.83469˚, respectively. The resulting converged rotor thrust with both collective and cyclic trimming is still 74. 94954 N, as it was prior to trimming. These values can be found in the third column of the Rotor_1_Loads.csv output file.

Figure 34.20: Pressure Distribution with Collective and Cyclic Trimming, Y=0 Cutting Plane and Figure 34.21: Pressure Distribution with Collective and Cyclic Trimming, X=0.3 Cutting Plane show that the cyclic trimming has balanced the pressure distribution almost equally along, and especially across, the rotor.

Figure 34.20: Pressure Distribution with Collective and Cyclic Trimming, Y=0 Cutting Plane

Figure 34.21: Pressure Distribution with Collective and Cyclic Trimming, X=0.3 Cutting Plane

Exit Fluent by selecting Exit in the File ribbon tab or by clicking the X icon in the top right corner of the application. For the latter, a Warning dialog box will open. Press OK to exit Fluent.

If not already done, exit Fluent. Create a new working directory and copy VBM_helicopter_tutorial.cas and naca0015.dat to this new directory.

Launch the Fluent executable in the new working directory using the same launching setup as Preparation.

To re-read the case file, go to:

 File → Read → Case…

and select VBM_helicopter_tutorial.cas.

Initialize the solution from inlet boundary values. Go to:

 Solution → Initialization

and click the Initialize button.

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Note:  Before creating the floating disk, an initial solution should be loaded or initialized. Otherwise, Fluent will give a warning message to do so.

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To set up the rotor parameter, go to:

 Physics → Models → More

Figure 34.22: General Disk Data Configuration Window, FDM, Fixed Pitch

To run calculation, go to:

 Solution → Run Calculation → Run Calculation...

Set the No. of Iterations to 500 and click Calculate.

The convergence history of the residuals is shown in Figure 34.23: Convergence History, FDM, Fixed Pitch. Figure 34.24: Pressure Monitor Convergence History, FDM, Fixed Pitch also shows the convergence history of the integral of the static pressure on the int_rotor surface. In addition, the convergence history of the main-rotor thrust is demonstrated in Figure 34.25: Rotor Thrust Monitor Convergence History. As shown in the pictures, the converging history of the current simulation using FDM is similar to that of using EDM (see Rotor Simulation with Fixed-Pitch Using EDM).

Figure 34.23: Convergence History, FDM, Fixed Pitch

Figure 34.24: Pressure Monitor Convergence History, FDM, Fixed Pitch

Figure 34.25: Rotor Thrust Monitor Convergence History

In general, VBM simulations using FDM and EDM should yields the same results as long as the meshes and facets on the disks are the same. In the current simulation, the converged global rotor results are almost the same as those of EDM approach, as shown in Table 34.3: Comparison Between Global Rotor Results Obtained Using EDM and FDM. The negligible discrepancies in the results are due to the difference between the embedded and floating disk’s facets.

When the execution terminates, save the case and data files (VBM_helicopter_tutorial_FDM.cas and VBM_helicopter_tutorial_FDM.dat):

 File → Write → Case & Data

Exit Fluent by selecting Exit in the File ribbon tab or by clicking the button in the top right corner of the application. For the latter, a Warning dialog box will open to confirm, press OK to exit Fluent.

Read the case data files.

 File → Read → Case & Data…

and read the VBM_helicopter_tutorial_FDM.cas and .dat files.

Display floating disk surface and srf-fdisk-main-rotor. Go to:

 Domain → Mesh → Display…

and select srf-fdisk-main-rotor in the list of Surfaces and click Display. If this step is neglected, the VBM will not recognize the floating disk, potentially leading to it crashing.

To read and pre-process VBM data.

 Physics → Models → More

Select Virtual Blade Model... to open the VBM Rotor Inputs dialog box. Ensure srf-fdisk-main-rotor is selected from the list of Rotor Surface and press Apply to perform the VBM pre-processing.

Run the calculation.

 Solution → Run Calculation → Run Calculation...

Set the No. of Iterations to 100 and click Calculate.

Based on your preference, answer yes or no when asked to create the rotor-pressure-rfile.out and main-rotor-thrust-rfile.out report file. These report files were defined to monitor convergence history of surface pressure integral on the int_rotor surface and the thrust force on main-rotor. Answer yes if asked to overwrite these files in case you do not want to create new files.

The convergence history of the residuals is shown in Figure 34.23: Convergence History, FDM, Fixed Pitch. As shown in the pictures, residuals continue with the same trends as re- siduals of the previous simulation (Rotor Simulation With Fixed Pitch Using FDM. The converged rotor thrust, 74.97935 N, listed in column 3 of the Rotor_1_Loads.csv log file, is also the same as that of in the previous simulation. Therefore, there is no need to save the case data files since the results obtained with 500 iterations in the previous simulation was sufficient.

Figure 34.26: Convergence History, FDM, Restarting From Previous Saved Data File

Download the vbm_propeller_tutorial.zip file here .

Unzip vbm_propeller_tutorial.zip to your working directory.

Ensure the following files are available in the working directory:

naca16016.dat

VBM_propeller_tutorial.msh.gz

Use the Fluent Launcher to start Ansys Fluent.

Select Solution in the top-left selection list to start Fluent in solution mode.

Select 3D for Dimension, Double Precision for Solver Options and a suitable number of Processes.

Select the appropriate Working Directory in the General Options panel.

Click Start.

To read the mesh file, go to:

 File → Read → Mesh…

Select the VBM_propeller_tutorial.msh.gz file, and click OK.

To display the mesh, click the following button in the ribbon.

 Domain → Display…

Deselect inflow, outer and outflow in the Surfaces list and select int_acdisk and click Display.

Since this case uses an embedded disk, the computational domain must be divided into two separate but connected fluid domains (Physics → Zones → Cell Zones). The VBM only acts on the cells that form the disk cell zone attached to one side of the actuator disk.

As shown in Figure 34.34: From Left to Right, the Three Components of the Disk Cell Zone: Int_Acdisk, Int_Disk and Int_Disk:003, Fluent automatically subdivides the disk cells into three separate entities:

To display the first two surfaces, select each in the list of Surfaces, and click Display.

For the third surface, however, click Adjacency in the Mesh Display dialog box to open the Adjacency dialog box, select disk from the list of Cell Zone(s) and int_disk from the list of the Adjacent Face Zones and then click the Display Face Zones.

Click the si tab.

Click angle in the Quantities list and choose deg in the Units list.

Close the Set Units dialog box.

Set a value of 97013.91 Pascal in the Operating Pressure box.

Set a value of -1 m in the X component of the Reference Pressure Location section.

Click OK.

To configure the Fluent solver settings, go to:

  Physics → General

and select the following options in the General task page:

Type → Pressure-Based

Velocity Formulation → Absolute

Time → Steady

To enable the energy equation, go to:

 Physics → Models

and enable the Energy equation box.

To select the turbulence model, go to:

 Physics → Models → Viscous...

For vorticity-dominated flows, the default Vorticity-Based-production option overpredicts the production of eddy viscosity in the vortex cores. Adding the strain tensor to the vorticity reduces the production of turbulent viscosity in regions where the measure of vorticity exceeds that of strain rate.

Select air as the working fluid in the Fluid Materials pull-down menu.

Select ideal-gas in the Density pull-down menu.

Click the Change/Create button.

Close the Create/Edit Materials dialog box.

Set the boundary condition at the inlet.

Set the boundary condition at the outlet.

Set the Wall boundary conditions.

Select inflow from the Compute from pull-down menu.

Set the Area value to 0.785398 m² (disk area).

Set the Length Value to 0.05 m (the blade tip chord).

Select live from the Reference Zone pull-down menu.

To set the discretization options, go to:

  Solution → Solution Methods…

In the Solution Methods task page, use the pull-down menus to set the following options:

Retain the default solver parameters in:

  Solution → Controls → Controls...→ Solution Controls.

.

Select Standard and click Options... to open the Solution Initialization task page, which provides access to further settings.

Select inflow from the Compute from drop-down list.

Set the Reference Frame to Absolute.

Click the Initialize button.

Select Virtual Blade Model... to open the VBM Rotor Inputs dialog box.

Ensure Enable VBM is enabled.

Select Embedded Disk as VBM Mode.

Press Add under Rotor Names list to add a new rotor. A new rotor with a default name disk-1 with default properties will be created. These entries will be updated in the next steps.

Enter main-rotor in the Active Rotor Name box to change the rotor name.

Enter the parameters shown in Figure 34.35: General Disk Data Configuration Window.

Click int_acdisk in the list of Surfaces to select the actuator disk surface.

Click the Geometry button of the ribbon and enter the parameters shown in Figure 34.36: Geometry Configuration Window.

Click Change/Create to save the settings.

Click Apply to close the VBM Rotor Inputs dialog box.

To configure the residuals monitors that will appear in the Fluent graphics window and Console, go to:

  Solution → Reports → Residuals…

Additionally, you may want to monitor pressure convergence on the actuator disk int_acdisk. Go to:

 Solution → Reports → Definitions → New… → Surface Report → Integral…

When the calculation begins, the pressure convergence history will be displayed on screen and written in the file rotor-pressure-rfile.out.

Additionally, the convergence histories of the thrust, torque, power, moments and forces are automatically written to the Rotor_1_Loads.dat file. If you want to see the convergence histories of any output quantity (for example, VBM Thrust) for the rotor, Go to:

 Solution → Reports → Definitions → New → VBM Report → Rotor Thrust…

When the calculation begins, the rotor thrust convergence history will be displayed in the Graphics window, printed in the Console and written in main-rotor-thrust-rfile.out.

Example 34.2: The ribbon at the top of the Graphics window can be used to change the view options defined above.

(residual/pressure-integral/rotor-thrust convergence history)

For the first simulation, the propeller is operating in fixed-pitch mode without trimming. Go to:

Solution → Run Calculation → Run Calculation...

Set the No. of Iterations value to 600 and click the Calculate button. The convergence history of the residuals of the governing equations is shown in Figure 34.38: Convergence History with Fixed Pitch. Figure 34.39: Pressure Monitor Convergence History with Fixed Pitch shows the convergence history of the integral of the static pressure on the int_acdisk surface.

Figure 34.38: Convergence History with Fixed Pitch

Figure 34.39: Pressure Monitor Convergence History with Fixed Pitch

Figure 34.40: Rotor Thrust Monitor Convergence History

Save the Fluent case and data files (VBM_propeller_tutorial_FP.cas and .dat):

 File → Write → Case & Data…

To display contours of flow velocity on the cutting plane that has been created in Cutting Plane for the Velocity Distributions, go to:

 Results → Graphics → Contours → New …

Figure 34.42: Velocity Magnitude Distribution Around the Actuator Disk, Z = 0 Cutting Plane

Figure 34.42: Velocity Magnitude Distribution Around the Actuator Disk, Z = 0 Cutting Plane shows the flow contraction by the propeller in the downstream of the propeller. The axial velocity is increased from 84.5 to 97 at some regions.

To display the VBM data like angle of attack on the disk, go to:

 Results → Graphics → Contours → New …

To enable collective trimming, go to:

  Physics → Models → More

To run the calculation, go to:

 Solution → Run Calculation → Run Calculation...

Enter a value of 600 in the No. of Iterations box and press the Calculate button.

This run is intended to verify that the thrust coefficient value is precise and the collective angle printed to the Console after 600 iterations converges towards the original 61.85345-degrees fixed-pitch blade angle.

When the execution terminates, save the case and data files (VBM_propeller_tutorial_VP.cas and .dat):

  File → Write → Case & Data…