Discussion Forum

September 1, 2010 12:19pm UTC in response to Leendert van der Tempel

Re: Subdomain Settings - Equation system

Hi

For me I understand this that they are the same but you are at two different levels of COMSOL interiours:

Normal user entry is your:
1 F8 Via Physics - Subdomain settings - Coefficients
Because her you input your values and COMSOL has strict rules (physics/application mode rules) how to handle them and how to adapt the equations when you change physics settings

One level deeper is your:
2 CTRL+F8 Via Physics - Equation system - Subdomain settings
COMSOl will "lock" the equations you touch because your are "below" its "physics /application mode interpreter and you can do things that COMSOL cannot link to the physics, so it cannot adapt or change it. You are at the raw equation level.
You can always reset the equations to the COMSOL default (this level is also awailable in V4, turn on "lower node level" by "options preferences equations")
I use this level typically in structural to disable some DoFs in the default weak equations for special load cases, but this can apply to any physics/application mode.

Conclusion: use 1) for normal work, use 2) to understand the deeper functioning, to search for internal variable names (unfortunately the sort column does not work, neither the cut/copy names, would be nice ;), or to really tweak the physics equations

Comsol is really applying physics fully out, that's why I'm having so fun Comsoling, hope you too
--
Good luck
Ivar


--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

September 1, 2010 10:03pm UTC in response to Leendert van der Tempel

Re: Subdomain Settings - Equation system

Hi

as I do not have access to COMSOL from where I'm just now, I cannot check, but from memory: 2 and 3 should be the same, the easiest is to fill in your case 2, and "apply" then go to the "physics equation settings ..." and check what COMSOL has defined as h&r

From my earlier reply what you fill in under "F8" (higher normal user level) will analyse and fill in the values under "CNTRL-F8" (lower level) while the opposite is not true

Now imposing relations on boundaries can be done in different ways, and which depends aso on the dependent variables you have and what equation(s) you want. You can also define your own variables and impose combination of these. (i.e. see the example of applying a moment on an axis on the structural v3.5a doc)

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

January 4, 2011 10:04am UTC in response to LE HUY TOAN

Re: Subdomain Settings - Equation system

Hi

in which version, 3.5 ?

in v4.1 I can easily write for the Young modulus

E = (0.1+Param*1*(Len-y[1/m]))*1E9[Pa]

and do a param sweep from 0 to 1 and see that by Len long solid beam deflects differently, so my E is E(y) jsut by writing out he equation

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

January 4, 2011 8:06pm UTC in response to LE HUY TOAN

Re: Subdomain Settings - Equation system

Hi

Indeed I did no switch on the anisotropic elastic material proerties in the linear elastic node, because then I need to define the full tensor.
But there is a line in the GUI node to select "isotropic", "orthotropic" or fully "anisotropic" material, and then if you select "user defined" you can set up the full tensor, with equations at your will. Note it's limited to the default cartesian coordinate, so you must take care of any coordinate transforms yourself

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

January 6, 2011 8:28am UTC in response to Ivar Kjelberg

Re: Subdomain Settings - Equation system

BOUNDARY CONDITION IMPLEMENTATION
Ivar's two suggestions to implement the resistance boundary condition are indeed equivalent; the easiest is to fill in the first:
• Via Physics - Subdomain settings -Coefficients (F8) put
for the voltage h=1 and r=I*Rs
for the current h=1 and r=V/Rs where Rs is the closing spark resistance
• Via Physics - Equation system - Subdomain settings (Ctrl+F8) put
h=[[1 -Rs] [-1/Rs 1]] and r=[[0] [0]]

PROBLEM WITH TELEGRAPHER'S EQUATION AND SOLUTION
However no boundary condition imposes the telegrapher’s equations there, hence the telegrapher equations are violated if the closing spark resistance is time dependent.
Impose similarly the telegrapher’s equation 2pi ri·dV/dr = µl·dI/dt+R#·I at the inner boundary
via Physics - Equation system - Boundary settings (Ctrl+F8) put:
q=[[0 -R#/2pi ri] [0 0]] and g=[[µlIt#/2pi ri] [0]] and h=[[1 -Rs] [-1/Rs 1]] and r=[[0] [0]]

REMAINING PROBLEM
The other telegrapher's equation is not imposed there, hence violated if the closing spark resistance is time dependent.
It may be imposed similarly, but how to impose it while imposing I=V/Rs when only 2 bc are available in ComSol??

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

January 10, 2011 8:42am UTC in response to LE HUY TOAN

Re: Subdomain Settings - Equation system

Hi

you can add in complex equations based on integration etc, so long you are not making circular references. But, youoften make the solution highly non-linear, sometimes you must specify it in the solver settings, and take care of scaling manually, but try first with the COMSOl default values, often it detects things Okby itself ;)

For the integration, use the coupling operatos, or create a global expression

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse

January 10, 2011 9:46am UTC in response to LE HUY TOAN

Re: Subdomain Settings - Equation system

Hi

I'm not sure I understand what you are doing, often its easier to upload a simple model to allow us to better understand

--
Good luck
Ivar

Reply  |  Reply with Quote  |  Send private message  |  Report Abuse