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-   -   How to calculate pressure forces using custom field functions? (http://www.cfd-online.com/Forums/fluent/65644-how-calculate-pressure-forces-using-custom-field-functions.html)

 tsagaro June 22, 2009 07:57

How to calculate pressure forces using custom field functions?

Dear all,

I am trying to obtain forces and torques on the blades of an axial fan using a transient simulation. I can obtain them by using the Report => Forces panel. However, I would like to obtain them as a function of time. Therefore, I want to create a Custom Field Function and then make a Monitor => Surface of that Custom Field Function.

In order to be confident with what I am doing, I am currently checking if the Custom Field Function I have written gives the same answer as the Report => Forces panel... and I am making one (or several) mistake(s) I am unable to detect.

For the x component of the pressure force on the surface, I define a Custom Field Function as: p * x-surface- area and Sum over the surface on the Report => Surface Integrals. The result is very different from that I obtain using the Report => Force panel (it also disagrees with y and z components).

Any help?

Thanks,

Toni

 f0208secretx February 9, 2011 06:10

Hi, Toni.

I ran to the same problem with you. I'm wondering if we misunderstand the definition of "X Face Area" in the droplist of "Mesh" or something else, and neither do I know how Fluent calculate the integrals/sum in the surface monitor operation. This makes me really uncomfortable... If you already find the answer, could you share it some time? It would be helpful, thanks. :)

JH

 f0208secretx February 10, 2011 02:29

Since that no one reponse to this question, I would answer it myself.

First, I need to admit that "X Face Area" is a strange quantity and it seems no one know what it represents. Even the official manual is written in so abstruse language that I doubt that they cannot figure it out either... I have did some experiments on the quantity yet I still coulnd't get it. After spending hours, I quit on this thing.

I temporarily use the monitor panel to obtain the force coefficient and it sucks because of the restrictions, anyway if there are people who have better advices I would be much appreciated. :D

JH

 sailor April 13, 2011 03:25

Hi， JH. Have you solved this problem now?
I have a similar question similar with you, and want to get some advices from you.
You know, in Force Report, we can get the force on Wall only. However, I need the force on Wall and another kind of boundary. So I used p*x-surface-area as you ever used before, but the result seems not reasonable.
Do you have any idea about my problem?

Quote:
 Originally Posted by f0208secretx (Post 294578) Since that no one reponse to this question, I would answer it myself. First, I need to admit that "X Face Area" is a strange quantity and it seems no one know what it represents. Even the official manual is written in so abstruse language that I doubt that they cannot figure it out either... I have did some experiments on the quantity yet I still coulnd't get it. After spending hours, I quit on this thing. I temporarily use the monitor panel to obtain the force coefficient and it sucks because of the restrictions, anyway if there are people who have better advices I would be much appreciated. :D JH

 f0208secretx April 13, 2011 03:34

Actually we are taking different issues: I was stucked on the transient force report, while you are confused about outputting properties of the non-wall boundaries.

What kind of boundaries are you handling here? Try to identify them in the pre-process procedure as an independent wall. That might help.

JH

 f0208secretx April 13, 2011 03:36

One more thing, the x-surface-area and the x-face-area is tricky as it seems, you shall examine the mechanisms underlying carefully before utilization. Teach me when you figure it out. :-D

 sailor April 13, 2011 04:03

Yeah, I set then as Velocity Inlet because I want to simulate the plasma on the surface. But when it is set as Velocity Inlet, cannot be set as Wall at the same time. I will try to figure out.

Quote:
 Originally Posted by f0208secretx (Post 303379) Actually we are taking different issues: I was stucked on the transient force report, while you are confused about outputting properties of the non-wall boundaries. What kind of boundaries are you handling here? Try to identify them in the pre-process procedure as an independent wall. That might help. JH

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