user-defined functions with strain rate

ems · May 27, 2009, 13:46

I am trying to model a non-newtonian fluid (blood) that depends on shear rate. I used the equation:

0.026314*pow($StrainRate,(0.47234-1)) + 3.117316

but I get an error that $StrainRate is not recognized. Is this not the correct term for strain rate, or am I using it incorrectly? I haven't been able to find the correct usage in the help or anywhere else. Any help or advice would be appreciated.

Thanks,

Eric

vishyaroon · May 28, 2009, 08:40

Go into Tools > Field Functions and check the format for Strain Rate. I do not have experience in non-Newtonian fluids, but if the Strain Rate exists it should be listed in the Field Functions. The Function Name should give you the syntax when you use the function.

ems · May 28, 2009, 15:33

After talking to support, I understand what I was doing wrong. If anyone has the same problem in the future, you have to check the box for "Temporary Storage Retained" under Solvers->"solver_used," then strain rate appears in the field function list. Thanks for your help.

ahmadbakri · May 30, 2010, 08:56

Quote:

Originally Posted by ems

After talking to support, I understand what I was doing wrong. If anyone has the same problem in the future, you have to check the box for "Temporary Storage Retained" under Solvers->"solver_used," then strain rate appears in the field function list. Thanks for your help.

Hi, how can I use the function Strain Rate magnitude

what is the syntax to follow

thanks

ems · June 2, 2010, 10:38

As I had mentioned, you will have to check the box for "Temporary Storage Retained" in the solvers section. On my particular simulation, I had it checked for Lagrangian Multiphase, Segregated Flow, and K-epsilon Turbulence. Then, in your user-defined field function, call strain rate magnitude using the handle $StrainRate

As a note, there is no strain rate for the first iteration. So, you should let your simulation run one or two iterations without calling the $StrainRate function and then stop it and apply the field function. As an example, my program used a strain-rate dependent viscosity. So for the first couple iterations I used a constant viscosity, then paused and changed my viscosity definition to my field function that included $StrainRate. If I had tried to use that field function from the beginning, I would have gotten an error.

Let me know if you are still having trouble.

-Eric

May 27, 2009, 13:46	user-defined functions with strain rate	#1
ems New Member Eric Join Date: May 2009 Posts: 6 Rep Power: 17	I am trying to model a non-newtonian fluid (blood) that depends on shear rate. I used the equation: 0.026314*pow($StrainRate,(0.47234-1)) + 3.117316 but I get an error that $StrainRate is not recognized. Is this not the correct term for strain rate, or am I using it incorrectly? I haven't been able to find the correct usage in the help or anywhere else. Any help or advice would be appreciated. Thanks, Eric

May 28, 2009, 15:33		#3
ems New Member Eric Join Date: May 2009 Posts: 6 Rep Power: 17	After talking to support, I understand what I was doing wrong. If anyone has the same problem in the future, you have to check the box for "Temporary Storage Retained" under Solvers->"solver_used," then strain rate appears in the field function list. Thanks for your help. sadjad.s and jseamon1 like this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
User defined functions???	student	CFX	0	March 26, 2008 07:54
Usage of user defined functions	Gowrish	FLUENT	1	October 26, 2005 08:44
User defined function of mass flow rate	Eric	FLUENT	1	April 22, 2005 19:15
user defined functions	sivakumar	FLUENT	0	July 30, 2001 16:48
User Defined Reaction Rate	John McKelliget	FLUENT	1	December 17, 1999 11:44

May 28, 2009, 08:40		#2
vishyaroon Senior Member Aroon Join Date: Apr 2009 Location: Racine WI Posts: 148 Rep Power: 17	Go into Tools > Field Functions and check the format for Strain Rate. I do not have experience in non-Newtonian fluids, but if the Strain Rate exists it should be listed in the Field Functions. The Function Name should give you the syntax when you use the function.

June 2, 2010, 10:38		#5
ems New Member Eric Join Date: May 2009 Posts: 6 Rep Power: 17	As I had mentioned, you will have to check the box for "Temporary Storage Retained" in the solvers section. On my particular simulation, I had it checked for Lagrangian Multiphase, Segregated Flow, and K-epsilon Turbulence. Then, in your user-defined field function, call strain rate magnitude using the handle $StrainRate As a note, there is no strain rate for the first iteration. So, you should let your simulation run one or two iterations without calling the $StrainRate function and then stop it and apply the field function. As an example, my program used a strain-rate dependent viscosity. So for the first couple iterations I used a constant viscosity, then paused and changed my viscosity definition to my field function that included $StrainRate. If I had tried to use that field function from the beginning, I would have gotten an error. Let me know if you are still having trouble. -Eric