# Problem of torque prediction in CFX

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 January 11, 2009, 23:20 Problem of torque prediction in CFX #1 Patrick Guest   Posts: n/a Now I'm simulating an outdoor unit fan of HVAC. And I calculated it at three different speed 500RPM, 700RPM and 950RPM. The pressure rise of all three speed is reasonable matching the test data. As to the torque of blade, it is good at 500RPM. But at 700RPM the value of torque is about 15% over predicted. At 950RPM the torque is about 20-30% over predicted. The whole case use the same mesh and the turbulence model is k-epsilon. I checked the Yplus in Post and found it is about 12-27 at 500RPM, 15-38 at 700RPM and 20-50 at 950RPM. Anyone who have ever met this problem and how could I got a more accurate torque? Thanks.

 January 12, 2009, 20:17 Re: Problem of torque prediction in CFX #2 Glenn Horrocks Guest   Posts: n/a Hi, Have you done all the normal checks? I mean Mesh size, convergence, physical models, timestep etc. Glenn Horrocks

 January 13, 2009, 01:06 Re: Problem of torque prediction in CFX #3 Patrick Guest   Posts: n/a Glenn, I've checked all the things you mentioned. And I make sure the convergence is good. I do not take the mesh-independence test and I just gave the mesh size according my experience so I do not know if the case is mesh independent.Is it necessary to make some more mesh to test this? I'm wondering since the low speed case could give the right toruqe why the high speed case could not give the right toruqe? Is that related to the mesh or the Yplus or other things?

 January 13, 2009, 18:49 Re: Problem of torque prediction in CFX #4 Glenn Horrocks Guest   Posts: n/a Hi, It is typical for a low speed case to run OK on a coarse mesh but a higher speed case requires a finer mesh. This is because the gradients get larger, boundary layers smaller etc. Sounds like a mesh sensitivity check on the high speed run is required. Glenn Horrocks

 January 23, 2009, 18:56 Re: Problem of torque prediction in CFX #5 CycLone Guest   Posts: n/a Patrick, How are you determining the "right" torque? -CycLone

September 19, 2013, 05:44
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 Originally Posted by Patrick ;91498 Now I'm simulating an outdoor unit fan of HVAC. And I calculated it at three different speed 500RPM, 700RPM and 950RPM. The pressure rise of all three speed is reasonable matching the test data. As to the torque of blade, it is good at 500RPM. But at 700RPM the value of torque is about 15% over predicted. At 950RPM the torque is about 20-30% over predicted. The whole case use the same mesh and the turbulence model is k-epsilon. I checked the Yplus in Post and found it is about 12-27 at 500RPM, 15-38 at 700RPM and 20-50 at 950RPM. Anyone who have ever met this problem and how could I got a more accurate torque? Thanks.
Hi,
We have the same issue of torque prediction. Did you conclude anything from your study. I am also simulating HVAC outdoor unit fan.

 September 19, 2013, 05:51 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,704 Rep Power: 98 We have now written an FAQ on this: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

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