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Basic doubt: PDE mode initial time change

Posted Apr 25, 2010, 12:31 p.m. EDT 5 Replies

Sriram Pakkam
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Hi...I have a problem where I need to study the interaction of two diffusing waves. I release the first wave at t=0 with the initial conditions that I have. I get a diffusion pattern based on all the time steps I specify.

I need to now release another wave at t=6400(with the same initial condition as at t=0) and see how it interacts with the wave released at t=0. I am clueless as to how to go about this. I think I need to change the initial time to 6400 for this second wave.

I've put second the wave in a separate multiphysics module(to be solved simultaneously with the first) but I am not able to change the initial time here from 0 to 6400.
5 Replies Last Post Oct 5, 2011, 8:51 a.m. EDT
Danial Tz
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Posted: 1 decade ago Apr 25, 2010, 2:11 p.m. EDT
You can have different release times using logical expressions like this:

U_in*(t==0)+U_in_2*(t==1000)

it may not work as your steps may not pass the exact time, so you may use intervals like

U_in*(t<1e-6)+U_in_2*(t>100)*(t<101)

hope this is what you're looking for.
You can have different release times using logical expressions like this: U_in*(t==0)+U_in_2*(t==1000) it may not work as your steps may not pass the exact time, so you may use intervals like U_in*(t100)*(t
Sriram Pakkam
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Posted: 1 decade ago Apr 25, 2010, 2:57 p.m. EDT
Hi Daniel,
I am really sorry but I didn't follow what you explained. Do i put these conditions in the initial value columns for the sub domain settings? If so, how can I put a time dependent equation in the initial condition space? Its assuming everything to be at t=0 in that box.
Thanks in advance.
Hi Daniel, I am really sorry but I didn't follow what you explained. Do i put these conditions in the initial value columns for the sub domain settings? If so, how can I put a time dependent equation in the initial condition space? Its assuming everything to be at t=0 in that box. Thanks in advance.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 26, 2010, 2:49 p.m. EDT
Hi

I agree too that in the initial value (t0=0 mostly) there is no reason to put time conditional equations, but from your description I too understand you were in a transient analysis and driving your volatage by a time function of the type Danial is proposing but I would not put it in the initial condition but in the main variable definition

Have fun Comsoling
Ivar
Hi I agree too that in the initial value (t0=0 mostly) there is no reason to put time conditional equations, but from your description I too understand you were in a transient analysis and driving your volatage by a time function of the type Danial is proposing but I would not put it in the initial condition but in the main variable definition Have fun Comsoling Ivar
Sriram Pakkam
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Posted: 1 decade ago Apr 27, 2010, 12:26 a.m. EDT
Hey Ivar,
How can I define the equation for my variable? There is no such provision in the 1D PDE mode. I tried used a tanh function in the intial time box...but that doesnt seem to make any difference at all. How do I set the time for a second diffusion wave release? Here is what I entered in the u(t0) column in the 'init' section of coefficient form PDE:

u(t0)=A*exp((-(x-x0)^2)/M)*(tanh(10*(t-6000))+1)/2

the part from A to M is my initial condition. and 6000 is the time at which I want release a second wave with the same i.c as at t=0.

Thanks in advance.
Hey Ivar, How can I define the equation for my variable? There is no such provision in the 1D PDE mode. I tried used a tanh function in the intial time box...but that doesnt seem to make any difference at all. How do I set the time for a second diffusion wave release? Here is what I entered in the u(t0) column in the 'init' section of coefficient form PDE: u(t0)=A*exp((-(x-x0)^2)/M)*(tanh(10*(t-6000))+1)/2 the part from A to M is my initial condition. and 6000 is the time at which I want release a second wave with the same i.c as at t=0. Thanks in advance.
Marilena Boccuto
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Posted: 1 decade ago Oct 5, 2011, 8:51 a.m. EDT
I guess that the solution could be to change the source term F..
I guess that the solution could be to change the source term F..

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