## Preprocessing

#### Meshing Element size

• rizwan4081
Subscriber

Greetings.
I am new to Ansys Fluent. I am doing an analysis where I am analyzing the flow over a car in a tunnel. I have the Student version of the software with a limitation on the number of mesh elements of a max being 512000 cells. I would like to know how to calculate the minimum element size of the mesh?
The tunnel is 500 meters and by default the mesh element size is 25 meters, But I want to know if there is a way to calculate a minimum size so as to guarantee the convergence of solution?

Ansys Employee

Converging solution depends on lot many factors like inlet and outlet conditions, mdoels used, mesh density etc.

If you have a car in tunnel, you will net prism plus tet mesh. As 512K is the limit, you can consider reducing you problem size by considering symmetry.

The sizes will depend on the features in your geometry you want to retrain.

Regards,

Keyur

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Ansys Employee

You may also want to see at

Regards,

Keyur

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• rizwan4081
Subscriber

I totally understand about the difference in the size of the carbody and tunnel.
I would like to know if there is anyway to find a minimum element size for meshing that depends on the length of the tunnel so that I could modify the tunnel. The reason for selecting the 500m tunnel is due to the presence of JEt fans inside.

Ansys Employee

It will be bottom up approach. So it would mesh faces first and then volume. So it depends on how many faces/edges you have in your model and defeaturing size you are using. Please start with a larger element size and try to mesh. Then you will get some idea on what should be min size. Without looking at the model, it is difficult to predict the size. Please do some iterations.

Regards,

Keyur

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• rizwan4081
Subscriber

This is the Case Study

• Rob
Ansys Employee

The cell sizing will be based on the jet fan diameter etc: then you need to keep a sensible aspect ratio.  Very roughly you need 5-6 cells across a gap, and keep the aspect ratio below about 3-4 where the flow is changing. Inflation and regions with no change in flow along the cell can be much higher.

• rizwan4081
Subscriber

Thank you.

I am performing an analysis where I analyze the ventilation inside a tunnel with a body emitting heat inside. I came across the CFL condition for determining the time step for running the calculation. But I read its applicable to only explicit cases. How can I determine the minimum time step for my case study?

• Rob
Ansys Employee

Very roughly you want to set a time step so the flow takes about 10 steps to cross a cell. What we're actually aiming for is to converge a time step in around 10-15 iterations: the time step size is tweaked from the approximated value to achieve this.