[Vimal Chaudhari]

I am working on the transient simulation of Savonius hydrokinetic turbine with CFX. My setup in CFX is as shown in the photo attached here. In CFD-Post I am using equation torque_z()@turbine. . Rotor initialization is set to 3m/s cartesian velocity components in the x-direction and stationary domain initialization is set to 3m/s cartesian velocity components in the x-direction. But I am getting negative torque. Is my setup ok or its need some correction? Eager to hear from experts.

[ghorrocks]

Assuming your simulation is set up correctly (which I cannot tell from just a few screen shots), negative torque means that the rotation speed you set is too fast.

Physically speaking, the negative torque will act to slow the rotor down until the torque increases and equals the torque is acting against. When the rotor torque equals the applied torque then it is in equilibrium and the speed will be constant.

[Vimal Chaudhari]

Thanks to reply. As per your suggestion, i had decrease rotational speed from 4 rad/sec to 1 rar/sec but its increased the negative value of torque. Sir please check the photos i have attached on previous post. Is it correct for basic setting rotor and stationary domain ?

[ghorrocks]

I have checked your images and the torque appears to be in the expected direction. It is negative simply because of the way you have defined the axis to measure torque around. Your rotor is spinning with a negative rotational velocity, and it is generating negative torque. So the torque is in the same direction as the rotation, which means your rotor is generating power.

And now you have simulated two speed points you are starting to plot out the device performance curve - which is torque versus rotational speed.

[Vimal Chaudhari]

Originally Posted by ghorrocks

I have checked your images and the torque appears to be in the expected direction. It is negative simply because of the way you have defined the axis to measure torque around. Your rotor is spinning with a negative rotational velocity, and it is generating negative torque. So the torque is in the same direction as the rotation, which means your rotor is generating power.

And now you have simulated two speed points you are starting to plot out the device performance curve - which is torque versus rotational speed.

Thanks Sir,
I have some questions.

Q-1: What correction should be done on setup for "axis to measure torque around" to get proper result or positive torque?

Q-2: As per your comments, my rotor is generating power, So is it correct to generate power from my rotor with generating negative torque?

Q-3: When my rotor is perpendicular to the direction of flow then it has lower negative value of torque as the rotor move from that position toward the parallel to the direction of flow its negative value of torque also increases. So as per me torque production is correct with rotational angle but i can't understand why my rotor show negative value of torque ? What correction i have to made on my setup ?

[ghorrocks]

Torque is a vector. So a negative torque just means it is a positive torque but in the opposite direction. Have a look at some background reading, like this: https://en.wikipedia.org/wiki/Torque

Once you understand what torque as a vector means then you should be able to answer your questions yourself.