CFX torque calculations single instance vs whole domain

Stanley90 · December 9, 2019, 10:50

Hi all,

I've got probably simple question to answer. I'm modeling simple rotor wheel, but not full 360 deg, just 1/5. I get the results and try to calculate torque for rotating axis X. The problem is, should I calculated torque value multiply by 5 or whether to get torque for whole rotor wheel I need run model 360 deg? My guess is, that torque for on instance should be the same like for whole domain, but it's not agree with my hand calcs. I've checked if can find help in ANSYS manual and here in other topics and FAQ, but I have not seen something like that anywhere. Please help, if you can.

LuckyTran · December 9, 2019, 11:40

Yes you do need to multiply by 5. Torque (forces) are calculated based on the domain size.

Stanley90 · December 9, 2019, 11:46

Quote:

Originally Posted by LuckyTran

Yes you do need to multiply by 5. Torque (forces) are calculated based on the domain size.

Thanks a lot! Have you got any paper explaining it more?

LuckyTran · December 9, 2019, 15:31

No I don't have a paper. Who needs paper for this? And who would even write it? It's trivial.

Torque is based on moments (or force x length).

Force is the surface integral of pressure acting over an area. If you have trouble with this, remember the joke of hide-n-seek where Newton draws a square meter and says that "you found Pascal!"

Area goes like area, which is dependent on the area that you include in your model. That's why it goes like the domain size. If you model 1/5 of the area, you get 1/5th of the force and so on.

December 9, 2019, 10:50	CFX torque calculations single instance vs whole domain	#1
Stanley90 Member Join Date: Sep 2014 Posts: 30 Rep Power: 12	Hi all, I've got probably simple question to answer. I'm modeling simple rotor wheel, but not full 360 deg, just 1/5. I get the results and try to calculate torque for rotating axis X. The problem is, should I calculated torque value multiply by 5 or whether to get torque for whole rotor wheel I need run model 360 deg? My guess is, that torque for on instance should be the same like for whole domain, but it's not agree with my hand calcs. I've checked if can find help in ANSYS manual and here in other topics and FAQ, but I have not seen something like that anywhere. Please help, if you can.

December 9, 2019, 11:40		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Yes you do need to multiply by 5. Torque (forces) are calculated based on the domain size. Stanley90 likes this.

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December 9, 2019, 15:31		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	No I don't have a paper. Who needs paper for this? And who would even write it? It's trivial. Torque is based on moments (or force x length). Force is the surface integral of pressure acting over an area. If you have trouble with this, remember the joke of hide-n-seek where Newton draws a square meter and says that "you found Pascal!" Area goes like area, which is dependent on the area that you include in your model. That's why it goes like the domain size. If you model 1/5 of the area, you get 1/5th of the force and so on.