# Compute pressure coefficient on the cylinder surface

Hello everyone, I want to compute the pressure coefficient on the cylinder wall (the inner cylinder). As can be seen below. The wall contains density, pressure, velocity, I think.

Could someone tell me how to get the pressure coefficient over the cylinder surface in some details?

Best regards

This sounds like a job for the `Calculator` filter. With it, you can type a math expression using the fields that you have to create a new field.

Yes, I know the Calculator filter, but it computer value all over the computational domain. How can I export the coefficient on the cylinder wall?

So, this question is not really about the pressure coefficient but how to extract the cylinder wall.

That probably depends on the structure of your data. It would help if you would share your data.

Ok, this is file.
Thanks.

I assume the “cylinder wall” you want is the circular boundary of the hole in the middle of your dataset. For your particular dataset, you can do that with the following sequence of filters. I’ll give an explanation of what each one does and why you need it for your data. (Don’t forget to hit `Apply` after each one.)

1. `Clean to Grid`: Your dataset is not completely connected. The data seem to be broken into blocks and then stitched together. The rendering makes everything look contiguous because the repeated points are coincident, but in fact the pieces are not connected. This filter will find coincident points (that is, points at the same place in space) and merge them together.
2. `Extract Surface`: This filter converts your data from an unstructured grid to a polygon dataset. For volume data, this finds the polygons on the boundary. However, your data is already 2D, so the data does not change. However, the conversion from unstructured grid to polydata is necessary for the next step.
3. `Feature Edges`: This will find “special” edges in polydata. In particular, you are interested in `Boundary Edges` (one of the options of the filter). After you run this filter, you will get two rings: one for the inner boundary and one for the outer boundary. If you want both of those boundaries, you are done. I am assuming you just want the inner one, so the next step extracts that.
4. `Extract Cells By Region`: Your data has a nice separation between the two boundaries, and that makes it easy to separate them. You can use this filter to define a simple spatial region to extract data from. After you create the filter, change `Intersect With` to `Box`. Set the box to be around the inner circle but not touching the outer circle. A `Position` of -5, -5, -5 and a `Length` of 10, 10, 10 does the trick for your data. If you actually want the outside circle instead of the inside circle, then change `Extraction Side` from `inside` to `outside`. If you want both, then skip this step.

Here is a state file that does all of these things:
extract-cylinder-wall.pvsm (652.0 KB)

Thanks a lot for your replay.

1. In plot data filter, can I take Mach function into series parameters as shown below?

2. I use these steps to calculate the pressure coefficient on the airfoil. but at step 3, But I get the mesh and edge, as the image below, do you know how to solve it?

Also the vtu file is attached.

Last, thanks again for your kindness help.

Best regards

1. I don’t understand the question.

2. In your second file, some of the coincident points are not exactly coincident. There are slight differences in the position of the points. In the `Clean to Grid` filter, change the `Tolerance` to `1e-06`. Also, the `Feature Edges` filter is picking up some edges a features for reasons I don’t understand. For that filter, uncheck all boxes except `Boundary Edges`.