Bounding Box


To run a simulation for a model in Star CCM+, a bounding box for the object is required. There are a number of different ways to do this. It can either be done on CATIA before importing the model to Star CCM+ or on Star CCM+ itself. Three different ways are shown below, the first of which being on CATIA and the other two being on Star CCM+. It does not really matter which one you use, just choose one that you are most confident with.

 

Method 1 – CATIA

To create a bounding box in CATIA, you must first start by changing the workbench from the part design workbench to the generative shape design workbench. You can do this by going to Start > Shape > Generative Shape Design.

CATIA 1

Next, select the “Plane” tool from the tools along the right-hand side of the screen.

CATIA_2

A dialogue box should appear requesting plane definition. Select a plane from the axis system from which you would like the first wall of the bounding box to be created. In this case, the x-y plane will be chosen.

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Next, an appropriate offset should be chosen. Remember that the bounding box acts as a fluid domain. Therefore, the box should not be too close to the object it is bounding otherwise the simulation would be inaccurate due to deflection at the boundaries. This would subsequently affect the flow around the object. Conversely, it should not be too big otherwise more computational power will be used than necessary because of the increased number of cells. In this case, an offset of 70mm is chosen.

Choose the offset and click OK. A plane should now be created.

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Now create a sketch on the plane using the “Sketch” tool. The sketch should be a rectangle or square, created with the “Rectangle” tool, which will be a defining wall of the bounding box. Make sure to dimension the sketch to the desired size as has been done below:

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Once this has been done, exit the sketch and select the “Fill” tool.

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A dialogue box should appear for defining the fill surface. Select the sketch just made, if it has not already been selected and click OK.

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The first wall of the bounding box should now be created, as seen below:

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Next, the remaining walls must be created. To do this, select the “Extrude” tool.

CATIA_9

A dialogue box should appear for surface definition. Select the wall previously created as the profile and choose an axis for the direction. In this case the z-axis was chosen. If the profile is extruding away from the part, select the “Reverse Direction” button to correct the direction.

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Next define the dimension. The value of this will depend on the desired depth of the box.

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Once these parameters have been defined, click OK. The bounding box has now been created and the part is ready to be tested in Star CCM+.

CATIA 12

 

Method 2A – Star CCM+

This is the first method that will show you how to create a bounding box for your model on Star CCM+. This could be useful if you’ve already imported your model from CATIA but forgotten to create the bounding box there (you may even prefer this way).

The first step is to save your CAT part/product as a .stp file and import it to STAR CCM+.

Star CCM 1

Star CCM 2

After having imported the file, your tree should look something like the image shown below.

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Expand the geometry node by clicking on the plus beside it and your tree should look like the one below.

Star CCM 4

After that right click on Parts > New Shape Part > Block as shown below.

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Your screen should look something like this.

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Adjust the X, Y and Z coordinates so that it fits your model appropriately. Remember that the bounding box acts as a fluid domain. Therefore, the box should not be too close to the object it is bounding otherwise the simulation would be inaccurate due to deflection at the boundaries. This would subsequently affect the flow around the object. Conversely, it should not be too big otherwise more computational power will be used than necessary because of the increased number of cells.

Star CCM 7

The values shown above should be suitable for the plane model.

When you are satisfied with the size of you bounding box, click create and then close.

Star CCM 8

You should return to the original screen with just your model. To view your bounding box, either just drag the block from parts on to the right window as shown below.

Star CCM 9

Or you can expand the scenes node on the tree, then Geometry Scene 1, then displayers, Geometry 1 and parts. Right click on parts then click on edit (or just double click on parts) and click on block on the window that pops up. Click ok and you should end up with the bounding box on the screen.

Star CCM_10

 

Method 2B – Star CCM+

This is another method for creating a bounding box for your model in Star CCM+.

Once your model is loaded into Star CCM+, under Geometry, right click on ‘3D-CAD Models’ and click ‘New’.

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Then when the window opens up for editing the 3D Model, on the left, choose the plane that you want the box to be drawn on. In this example, the bounding box needs to be on the YZ plane.

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Right click on the chosen plane and click ‘create new sketch’.

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Choose the square drawing tool and draw a square to the size required and click ok.

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Right click on ‘Sketch 1’ and click ‘Create Extrude’.

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Define the length of the extrude in the ‘distance’ section. Then click OK.

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Now press ‘Update 3D-CAD’.

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Now click ‘Close 3D-CAD’.

Right click on the newly created 3D part and click ‘New Geometry part’. You should now have a bounding box.

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