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derivatives in global equations

Posted 06.04.2010, 01:11 GMT-4 1 Reply

Joel Dugdale
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I'm trying to solve for a variable that depends on the time derivative of an internal variable. When I try and solve it, it gives me the error "system matrix is zero"

The equation is entered in the global equations box as follows, what am I missing? I am using a parametric segregated solver to solve.

Global Equations:

u:
a11

f(u,ut,utt):
2/3*d(epx_smps,t)-a11t
1 Reply Last Post 08.06.2010, 14:10 GMT-4
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Frederick Sachs
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Posted: 1 decade ago 08.06.2010, 14:10 GMT-4

I'm trying to solve for a variable that depends on the time derivative of an internal variable. When I try and solve it, it gives me the error "system matrix is zero"

The equation is entered in the global equations box as follows, what am I missing? I am using a parametric segregated solver to solve.

Global Equations:

u:
a11

f(u,ut,utt):
2/3*d(epx_smps,t)-a11t


I am trying to do a similar problem. In 2D domain I want to add a surface impedance (area counductance S/m^2 and pecific capacitance F/m^2) so that voltages in the domain can drive currents normal to the domain. Thus I want to specify an external current density JN=V*S+C*dV/dt applied in the external current density routine of elelctric currents. How do I put JN in the ODE? I can use the DC term as V*S in the external desnity expression window but I cannot seem to figure out how to introduce dV/dt where V(t), the voltage, is the state variable that COMSOL is solving. I can't write C*dV(t)/dt. It is marked in red in teh expression window although V*S is OK.

So I added ODEs physics and tried to define JN that way but I can't see how to refer to V(t) and the derivative of V(t) in the eqaution specification. I just want JN to be: JN=V*S+C*dV/dt . How do I do that? Is there a simpler routine?
[QUOTE] I'm trying to solve for a variable that depends on the time derivative of an internal variable. When I try and solve it, it gives me the error "system matrix is zero" The equation is entered in the global equations box as follows, what am I missing? I am using a parametric segregated solver to solve. Global Equations: u: a11 f(u,ut,utt): 2/3*d(epx_smps,t)-a11t [/QUOTE] I am trying to do a similar problem. In 2D domain I want to add a surface impedance (area counductance S/m^2 and pecific capacitance F/m^2) so that voltages in the domain can drive currents normal to the domain. Thus I want to specify an external current density JN=V*S+C*dV/dt applied in the external current density routine of elelctric currents. How do I put JN in the ODE? I can use the DC term as V*S in the external desnity expression window but I cannot seem to figure out how to introduce dV/dt where V(t), the voltage, is the state variable that COMSOL is solving. I can't write C*dV(t)/dt. It is marked in red in teh expression window although V*S is OK. So I added ODEs physics and tried to define JN that way but I can't see how to refer to V(t) and the derivative of V(t) in the eqaution specification. I just want JN to be: JN=V*S+C*dV/dt . How do I do that? Is there a simpler routine?

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