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Old   August 21, 2007, 11:17
Default Calculating torque
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I am running a simulation of a rotor, with constant cross section in the axial direction (see:, rotating in air. I would like to use fluent to calculate the power required to turn the rotor at a give angular velocity. For this, I would need to know the required torque. Is there a method for predicting this with Fluent? I have been looking at using the Report -> Forces... -> moments option but I cannot find a way of calculating the moment from the axis, rather than a fixed point.

Any help, as always, appreciated.
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Old   August 21, 2007, 13:05
Default Re: Calculating torque
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Simply transform the moment from the default coord sys to one that you are interested in.
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Old   August 21, 2007, 15:04
Default Re: Calculating torque
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Joe, thanks for your help on this matter. Please excuse my ignorance as I am having a little trouble understanding how to implement what you suggest.

When I open the forces on boundaries panel, in order to calculate the pressure moment, I have to supply a force orign ("B" in the manual, section 29.3.1). The origin is specified as a single, fixed point with an x, y, and z value.

I am looking for the torque that would be experienced about the axis of my rotor, in my case the z axis. How do get fluent to ignore the third dimension in the specification of the force centre?

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Old   August 22, 2007, 02:12
Default Re: Calculating torque
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I have not yet understood the problem yet, buttry this command on TUI of Fluent 6.3.26.

First find the zone name for rotor wall of interest. The use below command.

>report forces wm n

!* it will ask for the zones, give the zone name (say rotor)*!


!*It will ask for the centre of rotation. In your case z co-ordinate will be zero.*!

you will get the moment acros the z axis for the supplied wall BC (rotor)

Hope this helps. Same can be done with GUI also.
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Old   August 22, 2007, 06:27
Default Re: Calculating torque
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I suppose the problem that I am trying to resolve it this: Say, for example, the image of the rotor I have previously linked to is axisymmetric around the z axis, running from 0,0,0 to 0,0,100. When fluent calculates the total moment vector around A (in manual sec 29.3.1) it uses the moment vector r_AB. r_AB will be calculated based on the distance between specified moment centre (A), and the face that is being used to calculated the pressure force vector (B). If A is specified as 0,0,0 then the faces at the top of the rotor (with larger z value) will have a larger value of r_AB and hence a larger contribution to moment than the faces lower down (with lower z value).

For example, the force on a face near 0,0,0 may have a force of 10 N on it. Another face near 0,0,100 may have the same x and y values as the first and also a torque of 10 N. However, the moment about A (0,0,0) for the second point will be larger than that of the first, as r_AB will be larger for the second. Similarly, if the moment A were taking about 100,0,0, the first face would have a larger moment than the second. As these two points are equally spaced from the z axis, they should contribute equally to the torque on the rotor, regardless of the position taken as the moment centre.

In summary, I would like to be able to specify a moment centre around the axis, rather than a point.

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Old   September 4, 2007, 11:10
Default techinical information
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sir , hi i wanated to know each detailing formulas conserned with torque & what is it
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