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Old   October 5, 2010, 06:21
Default variable diffusion coefficient
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ali
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Hi
I like the diffusion coefficient in additional variable as a function of shear strain rate can define. When I do this (in cfx expression language)I'm faced with the following message. Please help if possible.
Grateful
kinematic diffusivity:0.18*(radious^2)*shear strain rate
cfx message:
Parameter 'Kinematic Diffusivity' in object '/FLOW:Flow Analysis 1/DOMAINomain 1/FLUID
MODELS/ADDITIONAL VARIABLE:Concentration of LDL' has been assigned an expression that
references the following unavailable variables: Shear Strain Rate
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Old   October 5, 2010, 21:22
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The error message is completely clear to me. The variable name is "Shear Strain Rate" - you need the capital letters.

The names of all variables is listed in the documentation.
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Old   October 19, 2010, 07:34
Default variable diffusion coefficient
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thank you ghorrocks
I did do it . But once again gave the same error message.</SPAN></SPAN></SPAN></SPAN>
Parameter 'Kinematic Diffusivity' in object '/FLOW:Flow Analysis 1/DOMAINomain 1/FLUID
MODELS/ADDITIONAL VARIABLE:Concentration of LDL' has been assigned an expression that
references the following unavailable variables: Shear Strain Rate
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Old   October 19, 2010, 18:50
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Have you tried the variabel short name "sstrnr"
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Old   October 19, 2010, 19:28
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Michael P. Owen
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The variable Shear Strain Rate is unavailable for this purpose. Instead create an algebraic additional variable and set it equal to the Shear Strain Rate and use that.
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Old   October 23, 2010, 03:57
Default kinematic diffusivity
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hi michael_owen
My relationship to the coefficient of diffusion into expression, I've defined and then define an algebraic equation and enter the name expression. How i can impose the algebraic equation in the diffusion coefficient ?

Last edited by alinematollahi; October 24, 2010 at 02:02.
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Old   October 24, 2010, 01:59
Default immediate help-variable kinematic diffusivity
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hi
Please on how to create a relationship for the diffusion coefficient based on the shear strain rate below.I need immediate help.thank you
radious=0.01m
kinematic diffusivity:0.18*(radious^2)*shear strain rate
cfx message:
Parameter 'Kinematic Diffusivity' in object '/FLOW:Flow Analysis 1/DOMAINomain 1/FLUID
MODELS/ADDITIONAL VARIABLE:Concentration of LDL' has been assigned an expression that
references the following unavailable variables: Shear Strain Rate
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Old   October 24, 2010, 07:11
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Michael has suggested how to do it. Have a look in the documentation and tutorials about additional variables for tips on how to set it up. The idea is basically: Set up an additional variable which is defined by a CEL expression. Set the CEL expression to simply be "=Shear Strain Rate". Then you should be able to refer to the additional variable in your kinematic diffusivity CEL expression.

On another matter:
Please do not post duplicates of this post on other threads, it does not help. Also if you need urgent help then talk to CFX support. That is what you pay them for. This forum is not appropriate for urgent work, people only post here when they feel like it.
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Old   October 25, 2010, 09:42
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Quote:
Originally Posted by alinematollahi View Post
hi michael_owen
My relationship to the coefficient of diffusion into expression, I've defined and then define an algebraic equation and enter the name expression. How i can impose the algebraic equation in the diffusion coefficient ?

This is pretty hard to decipher. If I understand you correctly, you've defined an Additional Variable with the same dimensions as the Shear Strain Rate, you've turned on the use of that Additional Variable in you domain, set it to Algebraic, and entered Shear Strain Rate as the expression that defines the Additional Variable, and now you are looking for how to use that in the kinematic diffusivity? Just reference your Additional Variable the same way you are trying to reference the Shear Strain Rate in the expression for the the kinematic diffusivity. If your Additional Variable is called "ShearStrainRate" (no spaces), then make your expression for the Kinematic Diffusivity equal to 0.18*(radious^2)*ShearStrainRate.




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Old   October 26, 2010, 01:57
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hi ghorrocks
I do apologize. I believe I've tried but did not solve my problem. I was hoping someone could solve it .
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Old   October 26, 2010, 02:25
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hi michael,thank you
You think wrong about my problem . I'm working on blood flow as single phase and I need the equation of mass transport(concentration) with the fluid equations(navier-stockes) can solve (additional variable with unit (kg m^-3)).i select "transport equation" in the domain, with define it,i need the diffusion coefficient(kinematic diffusivity with unit(m^2 s^-1)) that i want to define it as a function of shear strain rate. i think that with this work that fluid and mass transfer equations are coupled(is it correct?)how with defining expression or algebric equation i define the"kinematic diffusivity"?this is my problem
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Old   October 26, 2010, 06:55
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If you do as Michael suggests you will not be truly coupling the fluid and mass transfer equations but linking them with an explicit-type function as I suspect the CEL will get calculated on the values from the last iteration. As long as you keep your timestep under control it should work - need to do a sensitivity check to make sure it is OK.
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Old   October 26, 2010, 09:45
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Quote:
Originally Posted by alinematollahi View Post
hi michael,thank you
You think wrong about my problem . I'm working on blood flow as single phase and I need the equation of mass transport(concentration) with the fluid equations(navier-stockes) can solve (additional variable with unit (kg m^-3)).i select "transport equation" in the domain, with define it,i need the diffusion coefficient(kinematic diffusivity with unit(m^2 s^-1)) that i want to define it as a function of shear strain rate. i think that with this work that fluid and mass transfer equations are coupled(is it correct?)how with defining expression or algebric equation i define the"kinematic diffusivity"?this is my problem
I told you exactly how to do this.
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Old   October 28, 2010, 05:54
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thank you very much michael_owen
if you help me,I'm very grateful.
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Old   November 15, 2010, 06:10
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hi
my problem is not solve.Can someone help me?
thank you
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Old   November 16, 2010, 08:47
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It doesn't mattter whether you try to use Shear Strain Rate, sstrnr, or an Additional Variable because the solver knows (directly or indirectly) that you are trying to use a variable which is not allowed to be used to define the parameter Kinematic Difffusivity.

The variables that can be used are defined in the Dependency List for the parameter, and all the parameters are defined in the RULES file. Often, the Dependency List can be extended to include other viariables without error (although sometimes it does not work).

You can include in your CCL file the following lines:

RULES:
PARAMETER: Kinematic Diffusivity
Dependency List = ANY
END
END

This will override the default settings for your current run and remove any need to modify the installation version.

This will get past the error regarding allowing variables, but the solver may still fail if the dependency has not been coded/allowed internally.
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