Torque Calculation at Arbitrary Point
Hi everybody I've simulated motion of a rigid body (solid structure) in fluid stream. I've performed this in twoway FSI manner (CFX and Workbench together). I've encountered a problem in post processing in CFXPOST. How can I calculate the torque on arbitrary point? In other word I want to calculate torque on a certain point attached to a certain node which moves? Do I have to do anything in CFXPRE before run my simulation? For example define a local coordinate system? I didn't succeed in define a local coordinate system which its origin attached to the certain node. Thank a lot.

Re: Torque Calculation at Arbitrary Point
Hi,
I did not quite understand your question. I assume you are trying to find the torque of a surface about a specified axis. To do this you get the torque about the x,y and/or z axis, and the forces in x, y and z directions and use some simple mechanics to transpose the torque to the desired axis. Is this what you mean? Glenn Horrocks 
Re: Torque Calculation at Arbitrary Point
You can also define a new system of coordinates in CFXPost (you have an icon for this) and then evaluate your torque relative to this new set of axis. It can be done quite easily using Tools > Function calculator.
Felix 
Re: Torque Calculation at Arbitrary Point
Dear Horrocks, Thank for your attention, I would have explained my problem better, I want to calculate torque of a surface about a Z axis, but the center of torque calculation is varying, for example I want to calculate torque of a surface about a Z axis on the center of surface while the surface and its center are moving. So I think I have to define a point attached to a certain node. How can I do this? Do I have to try in CFXpost or CFXpre? I did successfully this for calculate pressure history for a point on a surface while that point moves. (A surface which falls in water) Apologize for my English sincerely

Re: Torque Calculation at Arbitrary Point
Dear Felix, Thank for your attention, I want to calculate torque of a surface about a Z axis, but the center of torque calculation is varying, for example I want to calculate torque of a surface about a Z axis on the center of surface while the surface and its center are moving. So I think I have to define a point attached to a certain node. I did successfully this for calculate pressure history for a point on a surface while that point moves. (A surface which falls in water). Can I do this for define a point attached to a node for origin of new coordinate system? I will try and announce you.. Best wishes

Re: Torque Calculation at Arbitrary Point
Hi,
I assume you are trying to get this torque as the run progresses rather than as postprocessing. In postprocessing it is easy as Felix has suggested. To do it as the run progresses so you can output the result to a monitor point, get the torque about the X/Y/Z axis using the torque_x/y/z CEL expressions and the force on the surface using force_x/y/z CEL expressions. Then using simple mechanics you can transfer the torque axis to any axis you like. I trust you can do the mechanics required here  if you can't then forget about CFD and go back to school. Glenn Horrocks 
Re: Torque Calculation at Arbitrary Point
thank Glenn

Re: Torque Calculation at Arbitrary Point
Dear Glenn Horrocks Thank for your advice. I was busy for awhile and I could not use your advices. But last night I was trying to do that, I think there is not any icon as Felix said or I can't find it. Maybe I have to define a local coordinate system in CFXPRE, but its origin moves unpredictably. I mean that I can't predict motion of structure and its center which I want to calculate torque about it, because the structure is moved due to water stream effect. You said:" To do it as the run progresses so you can output the result to a monitor point, get the torque about the X/Y/Z axis using the torque_x/y/z CEL expressions and the force on the surface using force_x/y/z CEL expressions. Then using simple mechanics you can transfer the torque axis to any axis you like." I can use your advice to calculate torque on each point. It is possible but it is very timeconsuming. Because I have to calculate pressure distribution on walls in every time steps! CFX can calculate force in any direction, but it is necessary to calculate where the result of pressure distribution acts, so how I can use your advice. I'm not beginner in CFX, and I can calculate pressure on every point on every wall which moves due to fluid stream. I do this by select a node which attached a node. But unfortunately I can't define a moving coordinate system which its origin is a certain node or a point attached a certain node, similarly. I wait for your answer. It would be greatly appreciated. My Email is m.qafari@yahoo.com if it will be needed to send you some picture. Sincerely Mojtaba

Re: Torque Calculation at Arbitrary Point
Dear Felix Thank for your advice. I think there is not any icon as you said or I can't find it. Where is it? Maybe I have to define a local coordinate system in CFXPRE, but its origin moves unpredictably. I mean that I can't predict motion of structure and its center which I want to calculate torque about it, because the structure is moved due to water stream effect. I'm not beginner in CFX, and I can calculate pressure on every point on every wall which moves due to fluid stream. I do this by select a node which attached a node. But unfortunately I can't define a moving coordinate system which its origin is a certain node or a point attached a certain node, similarly. I wait for your answer. It would be greatly appreciated. My Email is m.qafari@yahoo.com if it will be needed to send you some picture. Sincerely Mojtaba

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