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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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Time dependent load

Posted 2014年11月2日 GMT-5 23:20 MEMS & Nanotechnology, MEMS & Piezoelectric Devices, Structural Mechanics Version 4.4 3 Replies

Bhargav Mistry
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Hi,

I am working on a structural mechanics module and I wish to apply a pressure load which is time dependent. I applied a load varying sinusoidally(sine,cos) and a load comprising of square wave form by directly typing in the equation of the wave in the boundary load node. I wish to apply a load which increases linearly (say from time(t) = 0 to t = 1) and then decreases exponentially(say from t = 1 to t = 2) and is periodic. How do I include such piecewise functions as my load? Or is there any method to graphically plot such a distribution in comsol and use this variation as my load? Or since it's a wave, how do I include fourier series approximation and use it as a load?

Thank you.
3 Replies Last Post 2014年11月3日 GMT-5 07:35
Mohamed Zanaty
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Posted: 10 years ago 2014年11月3日 GMT-5 03:24
Hi,

There are different functions under Global Definitions => Functions. These functions include piecewise and interpolation. Even, you can define functions in MATLAB. Just, define a function load(t) using the appropriate function for your case and apply this function as the pressure load in your model.

Also, after defining your function, you can plot the function in COMSOL to check its variation with time.

Hope that this will be useful.

BR
Mohamed
Hi, There are different functions under Global Definitions => Functions. These functions include piecewise and interpolation. Even, you can define functions in MATLAB. Just, define a function load(t) using the appropriate function for your case and apply this function as the pressure load in your model. Also, after defining your function, you can plot the function in COMSOL to check its variation with time. Hope that this will be useful. BR Mohamed
Bhargav Mistry
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Posted: 10 years ago 2014年11月3日 GMT-5 07:03
Hi,
Thank You for your assistance. I tried defining a load having a response of a square wave using the piecewise function. However, it gave the following error,

Failed to evaluate variable jacobian
-variable: load
-geometry:1
-feature: Time-Dependent solver 1(sol2/t1)

Also, many a time I am facing the following error as well.

Failed to find consistent initial values
Last time step did not converge.
-Feature: Time-Dependent Solver 1 (sol2/t1)

What might be the cause of the following errors?

Thank you
Hi, Thank You for your assistance. I tried defining a load having a response of a square wave using the piecewise function. However, it gave the following error, Failed to evaluate variable jacobian -variable: load -geometry:1 -feature: Time-Dependent solver 1(sol2/t1) Also, many a time I am facing the following error as well. Failed to find consistent initial values Last time step did not converge. -Feature: Time-Dependent Solver 1 (sol2/t1) What might be the cause of the following errors? Thank you
Lasse Murtomäki
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Posted: 10 years ago 2014年11月3日 GMT-5 07:35
In my experience, a square-wave function (or a step function) returns error messages because the time derivative of that function (Jacobian) at the "corner" points is ill-defined. Therefore, I tend to use the "flc2hs" function which is a smoothed Heviside step function. I cannot guarantee that this is your problem, but I suggest you to try it instead of a sharp-corner square wave.

Lasse
In my experience, a square-wave function (or a step function) returns error messages because the time derivative of that function (Jacobian) at the "corner" points is ill-defined. Therefore, I tend to use the "flc2hs" function which is a smoothed Heviside step function. I cannot guarantee that this is your problem, but I suggest you to try it instead of a sharp-corner square wave. Lasse

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