Mesh displacement failed with User CEL Functions

motorbean · February 9, 2021, 04:28

Hello, everyone !
I'm trying to use a User CEL function to prescribed mesh displacement, but have a problem.

First, the settings are:
1) I interpolated the known vibration mode shape ("mode1.norm meshdisptot") into CFX onto the blade (rotor67), so that the User CEL function is used to prescribed the magnitude of the vibration mode at every physical time step. The User CEL Function is used to compute the mode shape amplitude at each time step.
2) The blade boundary mesh motion option is set as "Specified Displacement", with X,Y,Z components specified. For example, the X-component is expressed as:

mode1.norm meshdisptot x(Initial X, Initial Y, Initial Z)*mode1amplitude(ctstep,citern). (in which, mode1amplitude(ctstep,citern) is the User CEL Function)

The Problem is:
Whenever a User CEL Function is used in the mesh displacement expression, the numerical computation of mesh displacement at the start of the time step will diverge and fail, and it produces a negative volume on the order as huge as 1.0e+10 ! This happens, even when, I deliberately sets the return value of the User CEL Function to 0 (as a test) inside the user fortran source code.

This problem does not a occur if I use a number or a CEL expression for the modeshape amplitude (i.e. the mesh deformation computation converges, no negative volume). So it seems the problem is related to the function calling. But I'm still very confused and don't know how to fix this.

Does anyone have an idea about the reason as well as how to solve the problem, please ? Thank you !

ghorrocks · February 9, 2021, 06:26

I assume you are not trying to do FSI here, in other words you are simply trying to move the CFX boundaries by a pre-defined function. If the CFX fluids simulation affects the deflection then you should just use the built-in FSI capability.

Debugging mesh motion can be tricky. The FAQ on negative volumes describes the best way to debug this sort of problem: https://www.cfd-online.com/Wiki/Ansy..._went_wrong.3F

LDD_TURBO · July 11, 2022, 06:33

Hi! I encountered the same problem, have you solved this problem?

February 9, 2021, 04:28	Mesh displacement failed with User CEL Functions	#1
motorbean New Member motorbean Join Date: Nov 2011 Posts: 17 Rep Power: 15	Hello, everyone ! I'm trying to use a User CEL function to prescribed mesh displacement, but have a problem. First, the settings are: 1) I interpolated the known vibration mode shape ("mode1.norm meshdisptot") into CFX onto the blade (rotor67), so that the User CEL function is used to prescribed the magnitude of the vibration mode at every physical time step. The User CEL Function is used to compute the mode shape amplitude at each time step. 2) The blade boundary mesh motion option is set as "Specified Displacement", with X,Y,Z components specified. For example, the X-component is expressed as: mode1.norm meshdisptot x(Initial X, Initial Y, Initial Z)*mode1amplitude(ctstep,citern). (in which, mode1amplitude(ctstep,citern) is the User CEL Function) The Problem is: Whenever a User CEL Function is used in the mesh displacement expression, the numerical computation of mesh displacement at the start of the time step will diverge and fail, and it produces a negative volume on the order as huge as 1.0e+10 ! This happens, even when, I deliberately sets the return value of the User CEL Function to 0 (as a test) inside the user fortran source code. This problem does not a occur if I use a number or a CEL expression for the modeshape amplitude (i.e. the mesh deformation computation converges, no negative volume). So it seems the problem is related to the function calling. But I'm still very confused and don't know how to fix this. Does anyone have an idea about the reason as well as how to solve the problem, please ? Thank you !

February 9, 2021, 06:26		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	I assume you are not trying to do FSI here, in other words you are simply trying to move the CFX boundaries by a pre-defined function. If the CFX fluids simulation affects the deflection then you should just use the built-in FSI capability. Debugging mesh motion can be tricky. The FAQ on negative volumes describes the best way to debug this sort of problem: https://www.cfd-online.com/Wiki/Ansy..._went_wrong.3F __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 11, 2022, 06:33		#3
LDD_TURBO New Member Sheldon Zheng Join Date: Dec 2021 Posts: 1 Rep Power: 0	Hi! I encountered the same problem, have you solved this problem?