Discussion Forum

Hi

There are a few caveat with library materials. One is that they are often derived with internal variables such as temperature T and pressure p, and often you do not use these variables by default in your model.

Then you MUST define a Constant T=20[degC] or whatever fits you, and p=1[atm]. I believe you do not have any T defined, or did I miss the point ? (that error is at least a common one).

Next, is that when you define a material from the library, COMSOL 3.5a defines variables "mat1.something" "mat2.something" etc. and set the different (sub)domains material properties equal to the relevant "mat1.something" etc.

So either you use the domain definition through COMSOL or you access the "mat1.something" directly (but this is depending on the order you used to call them into the model.

in anycase you have a rho(T) required so T must be defined, if not through the application mode, than as a Constant

Good luck
Ivar

Hi

I'm using COMSOL 4.0 :)

I did again as your advice.

could you check the attached file

I just want to figure out working or not(Is setting path right expression?)

I set the value pA, T, and path "mat1.rho" in the PDE at "f"

rho has "pA*0.02897/8.314/T"

so I think rho is constant now.

but error comes out

Failed to evaluate expression.
- Expression: d((mat1.rho*test(mod1.u)-mod1.ux*test(mod1.ux)-mod1.uy*test(mod1.uy))*(dvol),{test@0})
Failed to evaluate variable.
- Variable: mat1.rho
- Geometry: 1
- Domain: 1

I think the problem is "path"



and then I put the rho equation into variable

rho1 = pA*0.02897/8.314/T, f = rho1 (in PDE)

this second one is working



I do want to know is "how to set the path for using the material property"

please give me your answer again :)

Hi,

You are right about the path; one step is missing in the expression. You need to use the following syntax:

mat1.def.rho(pA,T)

where "def" points to the Basic property group, which is the source for the density rho. "def" is the tag for that node, which you can view and possibly edit by right-clicking the Basic node and choose Properties.

Best regards,
Magnus Ringh, COMSOL

Thank you all :)

I have more or less exactly the same problem. I have "Oil" as a material, it has property tag "mat2", then it has Basic which has tag "def" and then thermal expansion coefficient "alpha" . There are no model inputs.

So I type in mat2.def.alpha as part of expression for a source term and nope, cannot evaluate expression.

I also tried mat2.def.alpha()

If you look at the attached you can see it documented under "Error" ad/or you can find the source of the error in mod2->PorousHeatTransfer->JTSourceInDarcy.

Maybe the problem is that thermal coefficients are defined as anisotropic alpha11, etc in the Basic, so I've also tried alpha11, etc.

Regards, John

Hm, seems to be a difference between calling a function versus a variable.

If you have, say, rho(T) defined in "def" defined in "mat3", then you can write mat3.def.rho(T) and all is good.

But if you have, say, rho defined in "def" defined in "mat3", then you cannot write mat3.def.rho

Tricky!!

Am having the same problem.

Reported error is ... failed to evaluate Jacobian : Varibale mat.rho.

Please help at the earliest.

>> Shailendra

Hi, can you be a little more clear on the error message. Failure to evaluate Jacobian could be lots of things....

Thanks for the prompt response.

I need help on resolving an error :

Have enabled MATERIALS to use Nitrogen properties.

Even though have "user specified" all the thermal properties of Nitrogen and tried a combination of enable and dissable the above MATERIALS LIBRARY, the error is getting repeated.

Have also entered /added the temperature T in MATERIALS >> BASIC and USER DEFINED, as 248 deg K.

The error does not go away.

Have spent the whole day and now night on this issue.

Regards,

>> Shailendra

Hei Magnus

If I understand well all materials are

matN.def.name

where N=1,2,3 ... for the order as the material appear in the list

.def. is there for definition (not really documented so far)

and "name" stands for whatever material property name we are interested in.

Then comes the optional (pA,T), or (T) or just noting more, if the material is defined as a function of (pA,T) or of (T) or noting if a constant.

Is that so ?

to find out if there is any need for the (T) or whatever, we can check the "Materials - My_material - Basic" entry to see what the calling sequence is.

But really, this should be better documented, I cannot see where the .def. comes from or is mentioned. It is neither not appearing in the equation lists, there I only see "mat.rho" no "mat1.def.rho" even for a constant rho, so obviously you have another intermediate level (that cannot be used at postprocessing calling level)

Ivar

Hi Ivar,

May please help on these error messages.

Thanks and Regards,

>> Shailendra (15/7)