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Variable defining/refering

Posted Aug 28, 2011, 10:24 a.m. EDT Parameters, Variables, & Functions Version 5.0 7 Replies

Mike Brown
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I am going through some of available examples of models and and I have some difficulty understanding syntax of certain variables' references such as tpf.G0i, chds.ndflux_c, chns.G0i, nitf.rho or chns.Vf2. The bit before dot is obvious (physics ID), but how do you identify the part after it?
I went through the part of manual on variables (from COMSOL Multi. User Guide) but somehow it did not clarify it. For instance in one place I found what ndflux is (Normal conductive heat flux), but what does it mean with _c at the end? Or where does rho come from (no such variable appears in the model, only symbolic). Same for G0i or Vf2.

I know what some of you may think right now (RTFM), but I kindly ask for a more specific answer. Actually the relevant part of manual would be nice, but one that actually DOES mention such construction and variables above.

Thank you,
Mike
7 Replies Last Post Jan 12, 2015, 4:20 a.m. EST
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Mohammad Hossein Babaei
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Posted: 1 decade ago Dec 19, 2011, 11:13 a.m. EST
Hi, Did you find any answer for your question? I need to know wat tpf.G0i is, too. Thanks...
Hi, Did you find any answer for your question? I need to know wat tpf.G0i is, too. Thanks...
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 20, 2011, 3:21 a.m. EST
Hi

have you tried an indexed search of the doc ? Mostly you get to the values there, then where I search is in the equation view. OK COMSOL does still not allow us to sort them in alphabetic order so it takes time, but everything is in there, including their definition. ("rho" is the generic name for the material density)

--
Good luck
Ivar
Hi have you tried an indexed search of the doc ? Mostly you get to the values there, then where I search is in the equation view. OK COMSOL does still not allow us to sort them in alphabetic order so it takes time, but everything is in there, including their definition. ("rho" is the generic name for the material density) -- Good luck Ivar
Magnus Ringh COMSOL Employee

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Posted: 1 decade ago Dec 20, 2011, 5:27 a.m. EST
Hi,

Activating the Equation View from the Show menu at the top of the Model Builder window gives you access to Equation View subnodes under each node in the physics interface branches. In the Equation View nodes' Settings windows, the Variables table includes all variables that the physics interface defines for that feature, include the expression, unit, and description. In general, the documentation does not list all variables that a physics interface includes.

Specifically,

tpf.G0i (and chns.G0i) is an auxiliary initial interface distance variable for the two-phase flow (tpf) interfaces.

chds.ndflux_c is the normal diffusive flux for species "c" in a Transport of Diluted Specied (chds) interface.

chns.Vf2 is the volume fraction of fluid 2 in a two-phase flow interface.

Best regards,
Magnus Ringh, COMSOL

Hi, Activating the Equation View from the Show menu at the top of the Model Builder window gives you access to Equation View subnodes under each node in the physics interface branches. In the Equation View nodes' Settings windows, the Variables table includes all variables that the physics interface defines for that feature, include the expression, unit, and description. In general, the documentation does not list all variables that a physics interface includes. Specifically, tpf.G0i (and chns.G0i) is an auxiliary initial interface distance variable for the two-phase flow (tpf) interfaces. chds.ndflux_c is the normal diffusive flux for species "c" in a Transport of Diluted Specied (chds) interface. chns.Vf2 is the volume fraction of fluid 2 in a two-phase flow interface. Best regards, Magnus Ringh, COMSOL
Siamak Nejati
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Posted: 1 decade ago Jan 7, 2012, 1:06 p.m. EST
This is very helpful,

Thanks for your comments.
I have a similar question about calling a variable or domain probe. Let say I want to average T on a boundry in domain A and use it in expression for its adjacent domain(B) in an expression for heat source.

How can I define this average and call it?

Thanks,
Siamak
This is very helpful, Thanks for your comments. I have a similar question about calling a variable or domain probe. Let say I want to average T on a boundry in domain A and use it in expression for its adjacent domain(B) in an expression for heat source. How can I define this average and call it? Thanks, Siamak
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 7, 2012, 6:29 p.m. EST
Hi

The reuse of dependent variables, such as T (defined as a field) requires a projection operator to define it, still as a field, but "projected" onto another entity, by a known projection law.

But for you case, you want an average temperature over an element, i.e. a boundary. That you can define as a Variable (independent of the coordinates x,y,z) by using a Define Variable (global) and equal it to the myT = aveop1(T), where aveop1() is an operator defined in the "Definition coupling operator, average" section, where you specify the entities (boundaries) where you want to average. Then myT can be called everywhere within the model, also to define the temperature along another boundary. Note this makes the model non-linear

--
Good luck
Ivar
Hi The reuse of dependent variables, such as T (defined as a field) requires a projection operator to define it, still as a field, but "projected" onto another entity, by a known projection law. But for you case, you want an average temperature over an element, i.e. a boundary. That you can define as a Variable (independent of the coordinates x,y,z) by using a Define Variable (global) and equal it to the myT = aveop1(T), where aveop1() is an operator defined in the "Definition coupling operator, average" section, where you specify the entities (boundaries) where you want to average. Then myT can be called everywhere within the model, also to define the temperature along another boundary. Note this makes the model non-linear -- Good luck Ivar
Geoff Nunes
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Posted: 10 years ago Jan 9, 2015, 1:48 p.m. EST
Magnus- Thank you for this insight. It is not complete, however. I have worked Model Gallery example 6192 (evaporative cooling of water) in the hope that I would learn what I need for another problem. In this model the COMSOL variable ht.psat is used without explanation. This variable does not appear in the Equation View node. Nor does it appear in any of the PDF documentation that I have searched. How do we learn about these things so that it becomes possible to use COMSOL in a reasonable way?
Magnus- Thank you for this insight. It is not complete, however. I have worked Model Gallery example 6192 (evaporative cooling of water) in the hope that I would learn what I need for another problem. In this model the COMSOL variable ht.psat is used without explanation. This variable does not appear in the Equation View node. Nor does it appear in any of the PDF documentation that I have searched. How do we learn about these things so that it becomes possible to use COMSOL in a reasonable way?
Magnus Ringh COMSOL Employee

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Posted: 10 years ago Jan 12, 2015, 4:20 a.m. EST
Hi,

ht.psat is the saturation pressure, and that variable is defined in the Equation View node under Heat Transfer in Fluids>Heat Transfer in Fluids 2. In the model documentation, the definition of the concentration on page 2 is that "the source term for water vapor at the water surface is given by the ideal gas law at saturation pressure." That expression is what you type in as the species concentration in Concentration 2 as ht.psat/(R_const*T), where T is the temperature, and R_const in a built-in physical constant for the universal gas constant.

Best regards,
Magnus Ringh, COMSOL
Hi, ht.psat is the saturation pressure, and that variable is defined in the Equation View node under Heat Transfer in Fluids>Heat Transfer in Fluids 2. In the model documentation, the definition of the concentration on page 2 is that "the source term for water vapor at the water surface is given by the ideal gas law at saturation pressure." That expression is what you type in as the species concentration in Concentration 2 as ht.psat/(R_const*T), where T is the temperature, and R_const in a built-in physical constant for the universal gas constant. Best regards, Magnus Ringh, COMSOL

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