[Publius]

Hello,

Some background:
I am trying to find the 3D point for the center of pressure in Fluent. I have seen several other posts saying that you have to effectively eliminate one moment in order to get a single 3D point, but I think there's another way (since I am comparing CFD results to an analytical result obtained via another method, and I was able to determine a single number from that method with the same logic as I will present below). I am almost certain that one can sum the pressure times area times distance (in one dimension) and divide by the sum of the total force to find the center of pressure coordinate for that dimension. My logic is confirmed with this site:
https://www.grc.nasa.gov/www/k-12/airplane/cp.html


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What I am trying to do:

COPx = sum(Pi*Ai*x)/sum(Pi*Ai) for the x coordinate
COPy = sum(Pi*Ai*y)/sum(Pi*Ai) for the y coordinate
COPz = sum(Pi*Ai*z)/sum(Pi*Ai) for the z coordinate

In this way, no manual iteration is required to find the 3D location of the center of pressure (since discrete data is used to find the center of pressure in the first place).


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Where I am going wrong:
I am not sure how to access these specific variables in Fluent. I know that they must exist since Fluent is capable of calculating forces and moments on certain surfaces. However, I am having issues with the syntax in actually defining the report. For example, I have tried:

Sum(StaticPressure*FaceAreaMagnitude*Position.x)/Sum(StaticPressure*FaceAreaMagnitude)

but there are a few issues with this:
1. I need to limit the pressures/areas/positions/cells to certain surfaces. I read somewhere (maybe in the user manual?) that pressure reports are limited to surfaces, but I am unable to find how to limit the StaticPressure, FaceAreaMagnitude, or Position.x variables to just those that are related to a certain boundary.
2. Typing this yields a "Sum: Empty Location List" error. I am not sure what this means. Perhaps it is related to the above, where the arrays are of different sizes? E.g. the StaticPressure array is for every cell in the 3D mesh whereas the FaceAreaMagnitude array is for only the cells on the boundaries.
3. Using the variables above (with ZFaceArea instead of FaceAreaMagnitude), I can't reproduce the force report in the z direction, which tells me I'm doing something wrong in general. I can't even calculate it, I just get an error.


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Any help would be greatly appreciated. I've been iterating manually for hours for the center of pressure, and I know there has to be an easier way to get an x, y, and z location where all of the resultant moments are 0.

It it matters, I am using Fluent 2020 R2.

Thank you in advance!