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Power calculations in Ansys CFX

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Old   April 14, 2023, 13:33
Unhappy Power calculations in Ansys CFX
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Gabriel de Morais Campos
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Hello! I'm using CFX Post and I'm trying to calculate the power required in an agitation system, with 4 blades. In order to obtain my result in Watt, I used the following expression: POWER= Angular Velocity*60*(1/(2*pi[rad]))*torque_y()@Pas Default (Pas Default is my rotating domain)
I am working on a transient analysis, and my problem is that "toque_y" is returning negative values. I checked every part of my rotating system, which were both sides of the 4 blades. The negative values are obtained on the back side of the blades, while on the front side (the one that attacks the water) the values remain positive. Obviously, when the blades are out of the water layer, the value turns zero.
My study is about a raceway tank, thus, the water is impulsed by the blades and circulates the tank, in a closed system.
My questions are: what is causing these negative values? How can I fix it? Is it a referential problem? Because if my toque is negative (and consequently my power), It means that the water is coming back and making my agitation system rotate, creating energy, which is absurd. I'm truly lost, could someone help me?
Thanks a lot!
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Old   April 14, 2023, 17:47
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Opaque will become famous soon enough
The sign of torque is meaningless until every detail of the setup is in context.

Coordinate Frame

Rotation axis respect to the coordinate frame, how was setup? two points, or global/local axis from the frame

Angular velocity sign with respect to the frame/axis

I have seen models with rotation axis set as (0,0,1) to (0,0,0) and positive angular velocity. What do you think that will do? is it Z, or -Z?
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
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Old   April 27, 2023, 12:43
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Gabriel de Morais Campos
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Hi, thanks for the answer! I understand what you're saying. I believe that it's just a matter of referential. My setup is all set taking it into account.
Take a look at the sheet I posted. I calculated the toque at the front and back sides of one blade during its passage on the water layer. The values got inverted after the middle of the water layer. I think I have to take the absolute value of those results. So the total torque would be the sum between the front and back of each blade.
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File Type: xlsx torque.xlsx (78.8 KB, 1 views)
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cfx, post process, torque impeller

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