Negative torque value

  • Last Post 26 July 2019
abbas posted this 23 July 2019

Hello every one,


I tried to simulate the turbine and in a post CFX I noticed that the torque value is negative (-0.2 [Nm]). However, I looked at CFX tutorial and in "Chapter 17: multiphase flow in a mixing vessel" the torque value is -67.4 [N.m] and multiplying by -4 to find the torque required by all of the impeller blades. Why it is multiplying by -4 instead of 4?


Therefore, can I say that my simulation is correct and because I have 4 blades, total torque would be (-4*-0.2=0.8Nm) and  P=T*w so P=(0.8*3000=240 W) because otherwise P=-0.8*3000=-240W that means the turbine instead of producing power, it needs power.


Best regards


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abenhadj posted this 24 July 2019

In the tutorial power is required to turn the impeller. The action of the fluids on the impeller bladed needs to have a reaction and this reaction requires that we add power to the system.


Action and Reaction principle.

Best regards,


abbas posted this 26 July 2019

I am simulating the turbine and I think that the torque value should be positive for turbine however, I have a negative value. At first, I simulated the turbine with air and the torque value was positive. However, when I change it to the CO2, the torque value is negative. 

How is that possible?


best regards


abenhadj posted this 26 July 2019

Force and Moments calculation are actions of fluids on bodies. From Body perspective as for an impeller one requires torque to be added.

Regarding your question: might be due to convergence, not consistent setup, (Axis of rotation,..). For turbine moment will provided by the Fluid and expelled to the outside.

Best regards,