deviation in calculated force between CFX solver and CFD post

murx · April 9, 2014, 13:45

Hi,

in my simulations, I adjust certain boundary conditions every n-th timestep based on the torque acting on a body. Therefore I need to evulate this torque during my simulation. If I compare the torque shown in the solver manager for the last timestep to the torque shown in CFD post, there are significant deviations. My latest example:

torque_y_CF1@BODY (solver): 1.8 E-4 [Nm]
torque_y_CF1@BODY (post): -2.3 E-2 [Nm] (128 777 % deviation

)

Can anybody explain this deviation?

I know that the torques are pretty low, so the absolute error is not very high. However, I need this accuracy for my investigation. I don't know if it's possible to say which one is closer to the physical reality, but at least having the _same_ "wrong" value in post and the solver would already help me.

Thanks
Max

brunoc · April 9, 2014, 13:56

Trust the values from the solver. They come directly from the momentum equations.

The values shown by Post are calculated from the available variables (pressure, for instance), and I think viscous forces aren't included.

ghorrocks · April 9, 2014, 21:03

I understand the values calculated during the solver are from the control volume integration points, but the CFD-Post values are from the nodal values only. So the solver is using a higher resolution field to evaluate it and is more accurate.

April 9, 2014, 13:45	deviation in calculated force between CFX solver and CFD post	#1
murx Member Max Join Date: May 2011 Location: old europe Posts: 88 Rep Power: 15	Hi, in my simulations, I adjust certain boundary conditions every n-th timestep based on the torque acting on a body. Therefore I need to evulate this torque during my simulation. If I compare the torque shown in the solver manager for the last timestep to the torque shown in CFD post, there are significant deviations. My latest example: torque_y_CF1@BODY (solver): 1.8 E-4 [Nm] torque_y_CF1@BODY (post): -2.3 E-2 [Nm] (128 777 % deviation ) Can anybody explain this deviation? I know that the torques are pretty low, so the absolute error is not very high. However, I need this accuracy for my investigation. I don't know if it's possible to say which one is closer to the physical reality, but at least having the _same_ "wrong" value in post and the solver would already help me. Thanks Max

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April 9, 2014, 13:56		#2
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	Trust the values from the solver. They come directly from the momentum equations. The values shown by Post are calculated from the available variables (pressure, for instance), and I think viscous forces aren't included.

April 9, 2014, 21:03		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I understand the values calculated during the solver are from the control volume integration points, but the CFD-Post values are from the nodal values only. So the solver is using a higher resolution field to evaluate it and is more accurate.