## Fluids

#### calculation of Torque in CFD Post

• marduin
Subscriber

I know that in CFD Post the torque of a surface can be found using a command like torque_x()@location. But what is the actual internal equation or process that CFD Post and Ansys use to calculate the result from this command?

• fkelecy
Ansys Employee
I know that in CFD Post the torque of a surface can be found using a command like torque_x()@location. But what is the actual internal equation or process that CFD Post and Ansys use to calculate the result from this command?

The moment vector on a surface is computed as the surface integral of the cross product of the position vector r=xi + yj + zk with the local force vector Fx i + Fy j + Fz k at point (x,y,z).  Note that the position vector is relative to the origin of the coordinate frame (0,0,0).   The net moment computed in this way is a vector quantity Mx i + My j + Mz k.  We can then take the dot product of the moment vector with any axis unit vector to get the torque about the axis.  In CFD Post, you can choose the x y or z axis, thus is will give you Mx, My, or Mz.

hope this helps,

Frank

• louis860813
Subscriber

Sir,if my blade is a 3D geometry and I set torque_z()@blade,the position vector will be calculated from the shroud or the hub for the "r"?

Best regards,

Louis