CFD Online Discussion Forums

CFD Online Discussion Forums (
-   FLUENT (
-   -   Calculating torque (

Andrew August 21, 2007 11:17

Calculating torque
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.

Joe August 21, 2007 13:05

Re: Calculating torque
Simply transform the moment from the default coord sys to one that you are interested in.

andrew August 21, 2007 15:04

Re: Calculating torque
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?


carno August 22, 2007 02:12

Re: Calculating torque
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.

andrew August 22, 2007 06:27

Re: Calculating torque
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.

nayan.mathur September 4, 2007 11:10

techinical information
sir , hi i wanated to know each detailing formulas conserned with torque & what is it

All times are GMT -4. The time now is 07:53.