Why could the torque and power calculations be so different when using different method to compute the values?
They should be the same. There are two ways of calculating torque.
-one is the force acting on all the rotating walls multiplied by its lever arm which is the method used to compute the torque in Function Calculator in CFX-post.
-The other is the angular momentum difference between the inlet and outlet. An expression for this can be created in CFX-Post under the Quantitative tab, as follows.
Assuming the axis of rotation is Z, then:
torque = massFlowInt(rvtheta)@rotor inlet+ massFlowInt(rvtheta)@rotor outlet (1)
rvtheta=x*Velocity in Stn Frame v - y*Velocity in Stn Frame u (2)
In equation (1), the inlet and outlet terms are added rather then subtracted because the convention in CFX massFlowInt is positive when fluid is going into the domain and negative when the fluid is leaving the domain.
Comparison of the results for torque produced by both approaches should match well. Power would be torque multiplied by the angular speed of the impeller (rotating equipment in general), Omega.