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#1 |
Senior Member
Jiri
Join Date: Mar 2014
Posts: 221
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Hello,
I searched this forum and found a similar thread. However, I still do not understand the background of the equation for "shaft power" as shown below: abs(sum(Velocity in Stn Frame u * Force X + Velocity in Stn Frame v * Force Y + Velocity in Stn Frame w * Force Z)@Calc Wall Region) This equation is also used in "Fan template" provided by Ansys. But why is it different than torque_z()@Calc Wall Region * Angular velocity ? Assuming rotation around z axis. What is the physical difference between these two equations? |
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#2 |
Senior Member
Jiri
Join Date: Mar 2014
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Any ideas? :-)
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#3 |
Super Moderator
Glenn Horrocks
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I have no idea why you would use the first equation. It appears to be a clumsy way of doing it, but I may be missing something important. You equation torque_z()@Calc Wall Region * Angular velocity appears to be the obvious way of calculating power.
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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#4 |
Senior Member
Gert-Jan
Join Date: Oct 2012
Location: Europe
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I checked the equation for a mixer in water. The results was about 5% lower than Torque*omega. Did you find the same difference?
The manual states that Force includes Wall shear, so that cannot explain the difference. Then, as Glenn mentioned, I think it is a clumsy and, possible, inaccurate method. |
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#5 |
Senior Member
Jiri
Join Date: Mar 2014
Posts: 221
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Thank you very much to both for the reply.
I checked the deviations between the results of the two equaitons and I can confirm also that "sum" equation gives about 5% - 10% lower values than "torque" equaiton. I absolutely agree with you and I also cant figure out any background for the "sum" equation. So, why is this equation applied in Ansys template expressions?? |
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#6 |
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Gert-Jan
Join Date: Oct 2012
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Maybe because it is more general applicable. So, you can estimate the power in case you don't have an Angular velocity? I don't know. If you want to be sure, ask ANSYS.
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#7 |
Senior Member
Join Date: Jun 2009
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In your comparisons, did you evaluate such expression using the "Hybrid", or "Conservative" setting?
I am not aware of the source of such formula, but I think it is bypassing the axis of rotation definition, i.e. it attempts to compute the "correct value" w/o even knowing the axis of rotation definition. However, I would have done differently. Shaft power = Angular Velocity . (Position x Force) which by vector identities, a.(b x c) = c.(a x b), can be written as Shaft power = Force . (Angular Velocity x Position) = Force . (Rotation Velocity) Adding over all the points in the locator Shaft power = abs ( sum( Force X * Rotation Velocity X + Force Y * Rotation Velocity Y + Force Z * Rotation Velocity Z)@Calc Wall Region ) Have not done the comparison vs "torque_z()@Calc Wall Region * Angular Velocity Magnitude", but it should work. |
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#8 |
Senior Member
Jiri
Join Date: Mar 2014
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Mr. Opaque,
you hit it. I switched the velocity from conservative to hybrid values and it is the same as "torque" equation. Problem solved. Thank you very much !!! |
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