Compresibble Flow

STAR-CCM+ User Guide 3844

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Compressible

Flow Tutorials

The tutorials in this set demonstrate how compressible flow problems canbe simulated in STAR-CCM+. The code is well equipped to handle suchproblems because it features a coupled solver, which allows accurateresolution of flow features such as shockwaves.

The two tutorials presented here model:

• Subsonic compressible flow in an axisymmetric intake pipe.

• Two-dimensional transonic compressible flow over an airfoil.

Both tutorials include verification of the results through comparison withexternal reference data.


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Intake Tutorial

This tutorial illustrates how to solve a three-dimensional compressible flowproblem using STAR-CCM+. The geometry in question is a NACA-typeaxisymmetric inlet in a free-stream air flow of Mach 0.21. The flow capturedby the inlet is forced into an S-bend before reaching the engine face.

It should be noted that, in order to keep the cell count low for this case,certain compromises have been made. Even though the case is run turbulentfor this exercise, the mesh is really more suited for inviscid flow. Because noattempt is made to resolve the boundary layers, the results are thusessentially what one would obtain from an inviscid simulation. This is nota recommended practice for most simulations.

In addition, the mesh has been created in three separate non-conformalsections. This enables the tutorial to illustrate a useful feature ofSTAR-CCM+, the ability to “fuse” separate mesh sections into a contiguousmesh.

In spite of the fact that the mesh is too coarse to produce amesh-independent solution, the simulation results are shown to comparefavorably with experimental data for pressure on the S-duct walls.

The mesh used for this intake example is based on an S-duct inlet mesh fromNASA.

STAR-CCM+ User Guide Intake Tutorial 3846

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Importing the Mesh and Naming the Simulation

Start up STAR-CCM+ and select the New Simulation option from the menubar.

Continue by importing the mesh and naming the simulation. The mesh tobe imported has been created in three separate blocks, and stored inseparate ccm files. These blocks will later be combined into a single,contiguous mesh.

• Select the File > Import... menu item

• In the Open dialog, navigate to the doc/tutorials/intake subdirectory ofyour STAR-CCM+ installation directory.

• Use the <Ctrl><Click> approach to select files sduct-inlet.ccm,sduct-core.ccm and sduct-outlet.ccm. Click Open to start the import.

• In the Import Mesh Options dialog, select:

• Run mesh diagnostics after import

• Open geometry scene after import

• Ensure that Don’t show this dialog during import is not selected and thenclick OK.

• Next select File > Save to create a file for the new simulation. Navigate


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to the directory where you want the file located.

• In the Save dialog type intake into the File Name text box.

• Click Save.

Visualizing the Mesh

The Geometry Scene 1 display will already be present in the Graphicswindow. Initially all parts of the mesh are shown as solid, colored surfaces.

• Click on one of the parts.


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• Click Close.

To display the surface mesh on the selected parts:

• Select the Geometry 1 node

• In the Properties window, tick the checkbox of the Mesh property.

This will show the surface mesh.

Manipulating Regions and Boundaries

In this section, you will combine the separate mesh blocks into a singlecontiguous mesh, then rename the mesh entities prior to continuing withthe simulation.

• In the intake window, open the Regions node.


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are done.

• Save the simulation by clicking the (Save) button.

Setting up the Models

Models define the primary variables of the simulation, including pressure,temperature and velocity, and what mathematical formulation will be usedto generate the solution. In this example, the flow is turbulent andcompressible. The Coupled Flow Model will be used together with thedefault k-epsilon turbulence model.

• Open the Continua node, right-click the Physics 1 node and select itemSelect models...

• In the Model Selection dialog, clear the Auto-select recommended modelscheckbox; you are customizing the models for this exercise.


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selected models now appear within that node.

• Save the simulation.

Setting Fluid Properties

Since the ideal gas model has been chosen, the viscosity, specific heat andthermal conductivity fluid properties must be set.

• Select the Continua > Physics 1 > Models > Gas > Air > Material Properties >Dynamic Viscosity > Constant node.


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Shear Stress Specification node.

• Set the Method property to Slip.

• Select the Boundaries > Extension_Outer > Physics Conditions >Shear Stress Specification node and set the Method property to Slip.

Setting Initial Conditions

In compressible fluid problems it is generally advisable to initialize the flowto a non-zero initial velocity and a meaningful pressure. Here, we use thefree-stream velocity and the duct outlet pressure.


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• Set the Value to -7903.35.


Adding Field Functions

Field functions provide a mechanism by which fields (raw data from thesimulation stored in cells and/or on boundaries) may be viewed anddefined. User field functions are field functions that you create to accessfield data. This section illustrates how to create a new field function forpost-processing purposes that represents the ratio of absolute pressure toatmospheric pressure.

• Open the Tools node, right-click the Field Functions node and select New.


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Pressure field function, and states that the atmospheric pressure value is101,325.0 Pa.

Preparing Stopping Criteria

Stopping criteria allow you to specify the length of the simulation, in thiscase the number of iterations to be performed. To set a stopping criterion

• Open the Stopping Criteria node and select the Maximum Steps node.

• Set the Maximum Steps property to 100.

The solution will not run for more than 100 iterations, unless this stoppingcriterion is changed or disabled.

Setting Solver Parameters

In this simulation, the Coupled Flow model is controlled by the CoupledImplicit solver. This solver enables the local iteration step to be adjusted interms of the Courant number. The default value of 5 is quite conservativefor a low Mach number problem on a mesh of reasonable quality, so it canbe increased to speed convergence.


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During the run, it is possible to stop the process by clicking the (Stop)button on the toolbar. If you do halt the simulation, it can be continued laterby clicking the (Run) button. If left alone, the simulation will continueuntil the stopping criterion of 100 iterations is satisfied.

• Save the simulation after the run has finished.

Adjusting Solver Parameters and Continuing

Now that the flow field has become established and the solution isconverging well, the Courant number can be further increased.

• Select the Solvers > Coupled Implicit node.

• Set the Courant Number property to 50.


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The contours of the scalar display now appear smooth.


Visualizing the Velocity Vectors

This section describes how to examine velocity vectors.

• Right-click on the Scenes node. Selecting New Scene > Vector causes anew Vector Scene 1 display to appear.

• Open the Regions > Fluid > Boundaries node, and then drag the Symmetrynode into the display.

When your mouse pointer arrives on the display, a pop-up menu willappear letting you decide which of the part displayers should receive thenew part.

Filled Smooth Filled


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The arrows now appear shorter.


Setting up a Line Surface for Plotting

In order to make a comparison with the experimental data, a part needs tobe created on which to plot the duct wall pressure ratio. This is achieved byintersecting a plane with the surface corresponding to the duct wall,resulting in a line surface.


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• Click Create.

In the intake window, a new node representing the implicit surface justcreated is added under the Derived Parts node: plane section.

• Click Close in the dialog.

It is now possible to plot data on the new implicit surface.


Plotting Simulation Data

In this section, the simulation data will be plotted using the user-definedfield function for pressure ratio and the line surface created from the S-ductwall.

• In the intake window, right-click the Plots node and select NewPlot > XY Plot.

This creates a new empty plot in the tree and displays it in the Graphicswindow.


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• Set the Scalar property to PressureRatio. A plot like the one shownbelow will appear.


Plotting Reference Data

An external data set consisting of experimental data will now be displayedtogether with the simulation data. First, however, the external data must beloaded into a table.

• Right-click on the Tools > Tables node and select New Table > File....


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The external data will appear more clearly.

The external data in the preceding illustration is from the following source:Fluid Dynamics Panel Working Group 13. 1991. Agard Advisory Report 270“Air Intakes for High Speed Vehicles”, pp. 139-162.


Closing and Restarting

• Close the file by closing the intake window.

It is easy to re-open the file and restore the displays.

• Select File > Recent Files > .../intake.sim from the top menus.


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the simulation file and then click Open.

Back in the Load Simulation dialog, the name and path of the simulation filewill appear in the Sim File text box.

• Click OK.

A new window will appear for the simulation.

The next step is to restore the displays and plots.

• Expand the Scenes node and double-click on each scene display’s nodeone at a time

• Expand the Plots node and double-click on the Residuals and XY Plot 1nodes

All the views will be as they were previously.

• To conclude, close the file again.

Summary

This tutorial of STAR-CCM+ introduced the following steps:

• Importing the mesh.

• Saving the simulation.

• Visualizing the mesh.


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Transonic Flow over an Airfoil

The tutorial simulates two-dimensional, turbulent, compressible, transonicair flow over an idealized airfoil, as shown below. The free-stream Machnumber is 0.725 and the angle of attack is 2.54o. This corresponds toRAE2822 case 6 in Reference [1].

The free-stream flow is subsonic, becoming supersonic on the suction sideof the airfoil and subsonic again through a shock wave. The lift and dragcoefficients are monitored to help determine whether convergence isreached. The final distribution of the pressure coefficient on the airfoil isthen compared to experimental data.

Importing the Mesh and Naming the Simulation

Start up STAR-CCM+ and select the New Simulation option from the menubar.

Continue by importing the mesh and naming the simulation. Aone-cell-thick, three-dimensional, hexahedral mesh has been prepared forthis analysis. The mesh corresponds to an angle of attack of 0o in the defaultLaboratory coordinate system.

• Select File > Import... from the menus.

• In the Open dialog, simply navigate to the doc/tutorials/aerofoilsubdirectory of your STAR-CCM+ installation directory and select file

STAR-CCM+ User Guide Transonic Flow over an Airfoil 3900

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Graphics window. (If the image does not appear immediately, simply clickthe (Reset View) button on the toolbar.)

All the geometry parts will be shown, viewed from the +z-direction. Themouse rotation option is suppressed for two-dimensional scenes.

• Right-click the Physics 1 continuum node and select Delete.

• Click Yes in the confirmation dialog.

Setting up the Models

Models define the spatial and temporal solution methods and the physicalproperties of the flow. In this example, the flow is steady, turbulent andcompressible. The default Spalart-Allmaras turbulence model and the idealgas model will be used. The analysis will also use the coupled solver, whichis recommended for all supersonic and transonic compressible flows.

By default, a continuum called Physics 1 2D is created when the mesh isconverted to two-dimensional. To use a more appropriate name:


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group box.

The Physics Model Selection dialog should look as shown below when youare done.

• Click Close.

Save the simulation by clicking the (Save) button.

Setting Material Properties

• Open the Continua node in the simulation tree.

The color of the Aerofoil node has turned from gray to blue to indicate thatmodels have been activated.

• Open the Aerofoil node and then the Models node.


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Save the simulation.

Setting Boundary Conditions and Values

The geometry used for this tutorial has only two boundaries:

• A wall boundary representing the surface of the airfoil.

• A free-stream boundary at the external edge of the solution domain.

• Open the Regions node, then right-click the Default_Fluid 2D node andselect Rename....

• Enter the name Fluid and click OK.

• Select the Fluid > Boundaries > freestream > Physics Conditions > FlowDirection Specification node.

• In the Properties window, change the Method property to Components.


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• Change the Value property to 0.725.

• Select the Static Temperature > Constant node.

• Change the Value property to 291 K.

All other conditions for the free-stream boundary and the default wallboundary conditions are suitable for this problem.


Setting Solver Parameters

The simplicity of this problem allows a rapidly converging solution to beattained using a large Courant number. In problems involving morecomplex geometries or physics, attempting to shorten the run time in thisway may cause the run to diverge. To increase the Courant number:

• Select the Solvers > Coupled Implicit node.

• In the Properties window, change the Courant Number to 20.0.



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of the scalar scene.

To change the style of the Mach number contours:

• Select the Scalar Scene 1 > Displayers > Scalar 1 node.

• In the Properties window, change the Contour Style property to SmoothFilled.


Plotting Graphs

The lift and drag coefficients will be plotted to help in determining when theanalysis has converged.


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• Select the XY Plot 1 > Axes node.

• Click on the Axis Orientation property and select the option shown below.

The setup is now complete. Save the simulation.

Running the Simulation

• To run the simulation, click the (Run) button in the top toolbar. If youdo not see this button, use the Solution > Run menu item.

The Residuals display will be created automatically and will show theprogress being made by the solver. You may observe the run progress by


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Both monitors have reached constant values so it is reasonable to concludethat the solution has converged.


Visualizing the Results

The Scalar Scene 1 display shows the Mach number profile at the end of therun. The profile shows the transonic flow around the airfoil, including theshock wave produced above it.


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• Right-click the Reports > Lift Coefficient node and then select Run Report.

In the Output window, a tab named Lift Coefficient Report will display therelevant report and show a lift coefficient of 0.732, which is within 2% of theexperimental value. Similarly, the drag coefficient report will give a valueof 0.0137, which also compares well to the experimental value of 0.0127.

Summary

This STAR-CCM+ tutorial introduced the following features:

• Starting the code and creating a new simulation.

• Importing a mesh.

• Converting the three-dimensional mesh to two-dimensional.

• Defining models for compressible flow problems.

• Defining the material properties required for the selected models.

• Setting initial conditions.

• Defining boundary conditions.

• Setting solver parameters for a steady-state run.

• Plotting graphs comparing results with experimental data.

• Initializing and running the solver to a specified stopping criterion.

• Analyzing the results using the built-in visualization facilities.

Reference[1] Cook, P.H., M.A. McDonald, M.C.P. Firmin “Aerofoil RAE 2822 -

Pressure Distributions, and Boundary Layer and Wake Measurements

Documents

Compresibble Flow