Power (shaft power) expression

Jiricbeng · September 7, 2018, 07:26

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?

Jiricbeng · September 11, 2018, 04:52

Any ideas? :-)

ghorrocks · September 11, 2018, 18:41

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.

Gert-Jan · September 12, 2018, 03:31

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.

Jiricbeng · September 12, 2018, 08:32

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??

Gert-Jan · September 12, 2018, 08:41

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.

Opaque · September 12, 2018, 12:18

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.

Jiricbeng · September 14, 2018, 03:42

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 !!!

September 7, 2018, 07:26	Power (shaft power) expression	#1
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	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?

September 11, 2018, 18:41		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,816 Rep Power: 144	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. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

September 12, 2018, 12:18		#7
Opaque Senior Member Join Date: Jun 2009 Posts: 1,850 Rep Power: 33	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. jiaji lu and shk09 like this.

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September 11, 2018, 04:52		#2
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	Any ideas? :-)

September 12, 2018, 03:31		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,885 Rep Power: 27	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.

September 12, 2018, 08:32		#5
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	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??

September 12, 2018, 08:41		#6
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,885 Rep Power: 27	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.

September 14, 2018, 03:42		#8
Jiricbeng Senior Member Jiri Join Date: Mar 2014 Posts: 221 Rep Power: 13	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 !!!