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Initial condition for time dependent study

Posted 12 dic 2012, 07:00 GMT-5 5 Replies

Andrea Morbio
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Hi,

I'm trying to model free vibration of a small PZT cantilever (MEMS). I need to apply some initial conditions on displacement in a time dependent study. How can i do it?

Thanks

Andrea
5 Replies Last Post 3 gen 2013, 08:56 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 12 dic 2012, 07:21 GMT-5
Hi

you can for example apply a boundary load of the type f0*sin(2*pi*freq*t) or better if you have a continuous sinus type laod, use the harmonic development with the frequency domain solver and apply a load of amplitude F0, and define the frequency freq in the frequency domain solver node. This gives ou amplitudes as responses

--
Good luck
Ivar
Hi you can for example apply a boundary load of the type f0*sin(2*pi*freq*t) or better if you have a continuous sinus type laod, use the harmonic development with the frequency domain solver and apply a load of amplitude F0, and define the frequency freq in the frequency domain solver node. This gives ou amplitudes as responses -- Good luck Ivar
Andrea Morbio
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Posted: 1 decade ago 12 dic 2012, 08:18 GMT-5
Hi Ivan,

thanks for the answer but I need only to apply an initial condition like w(t0)=1 on the end face of my cantilever and not a "sin boundary load" ( I've just create it with success :-) )..
Hi Ivan, thanks for the answer but I need only to apply an initial condition like w(t0)=1 on the end face of my cantilever and not a "sin boundary load" ( I've just create it with success :-) )..
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 12 dic 2012, 14:28 GMT-5
Hi

if you impose a displacement at launch (t=0) and let it go, i.e. wit a step() from 1 to 0 times your value, you will have someproblems, as its not only the tip of the canteliever that should be displaced, but the full model.

Then I believe it's better to start with a stationary solver case, with your load or displacement BC, get the solution, store it and then do a times series from the stationary solution, using it as the seady state initial conditions, and in your time series solver you disable the boundary displacement / load.

--
Good luck
Ivar
Hi if you impose a displacement at launch (t=0) and let it go, i.e. wit a step() from 1 to 0 times your value, you will have someproblems, as its not only the tip of the canteliever that should be displaced, but the full model. Then I believe it's better to start with a stationary solver case, with your load or displacement BC, get the solution, store it and then do a times series from the stationary solution, using it as the seady state initial conditions, and in your time series solver you disable the boundary displacement / load. -- Good luck Ivar
Andrea Morbio
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Posted: 1 decade ago 13 dic 2012, 09:30 GMT-5
Hi ivan

thanks for the answer but I have another problem: if I put a buondary condition like w(t0)=0 in the step1(stationary) of my time dipendent analysis how can i disable that condition in the step2(time dipendent) ?

Andrea


Hi

if you impose a displacement at launch (t=0) and let it go, i.e. wit a step() from 1 to 0 times your value, you will have someproblems, as its not only the tip of the canteliever that should be displaced, but the full model.

Then I believe it's better to start with a stationary solver case, with your load or displacement BC, get the solution, store it and then do a times series from the stationary solution, using it as the seady state initial conditions, and in your time series solver you disable the boundary displacement / load.

--
Good luck
Ivar


Hi ivan thanks for the answer but I have another problem: if I put a buondary condition like w(t0)=0 in the step1(stationary) of my time dipendent analysis how can i disable that condition in the step2(time dipendent) ? Andrea [QUOTE] Hi if you impose a displacement at launch (t=0) and let it go, i.e. wit a step() from 1 to 0 times your value, you will have someproblems, as its not only the tip of the canteliever that should be displaced, but the full model. Then I believe it's better to start with a stationary solver case, with your load or displacement BC, get the solution, store it and then do a times series from the stationary solution, using it as the seady state initial conditions, and in your time series solver you disable the boundary displacement / load. -- Good luck Ivar [/QUOTE]
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 3 gen 2013, 08:56 GMT-5
Hi

if you solve for a stationary solver first, you can
a) enable and disable different BC from the solver node (latest versions)
b) you can also use step(t[1/s]) functions (and adding in a global Parameter t=0[s] for the stationary case) to set a given BC to "0" (i.e. heat flux etc)

But fixed BC might have to be "disabled/enabled" per solver study node and cannot always be easily "switched" with a Boolean (t==0)

--
Good luck
Ivar
Hi if you solve for a stationary solver first, you can a) enable and disable different BC from the solver node (latest versions) b) you can also use step(t[1/s]) functions (and adding in a global Parameter t=0[s] for the stationary case) to set a given BC to "0" (i.e. heat flux etc) But fixed BC might have to be "disabled/enabled" per solver study node and cannot always be easily "switched" with a Boolean (t==0) -- Good luck Ivar

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