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using a time dependent function
Posted Oct 31, 2012, 3:12 p.m. EDT Interfacing, LiveLink for MATLAB, Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.3a 4 Replies
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I'm trying to use a time dependent boundary condition with matlab. I need to simulate a boundary condition that change in a fixed sampling time, for example, every 15 seconds the value in the boundary condition changes. These values of boundary conditions are in a data file of matlab. I made the following code:
I initialize the variables c = 0 and n =1 in the workspace of matlab
function out = myfunction(t)
global Vector n c; % global variables
load('Data.mat') % load my Vector
if t == c
c = c + 15;
n = n+1;
end
out = Vector(n,1);
the argument of the function is the time (t) of the comsol simulation, but the comsol simulation does not work.
Anyone can say me if the time (t) in comsol is a 1x1 parameter? because in my matlab function I'm trying to compare the time with a 1x1 value, maybe this is causing the error.
kind regards
Geovani
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there can be several issues, one that strikes me is that depending on your mdel, by applying dirac functions like that (boundary values changes abruptly, your are exciting many "modes" and your solver might go unstable. It's far better to use smoothed steps, as for the rect() function in the COMSOL GUI, these are based on the Heaviside step functions, that provide derivable functions over a steep transition.
Then you must ensure your model is not responding by oscillating for rapid boundary changes, in "solid" this can be improved, by ignoring the inertial terms (2nd time derivative values of the driving equations)
--
Good luck
Ivar
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I made some tests trying to use constant boundary values from matlab, and I conclude that the interface is not working. I declared a vector with the same value in all lines and I made the algorithm to change the line every sampling time. I think that the global variables c, n are not working. I have no idea how I can simulate this conditions.
With a rect() function I can't reproduce the same values that are stored in the data file.
best regards
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I'm using a time dependent boundary condition and it perfectly works.
I suggest you to start programming an easier case, for instance a ramp function. Of course you can define everything in Matlab, and I believe is far better to define a continuous function instead than a discrete number of values. So, try to define an analytical function.
Coming back to your question, I'm not sure if I understood what you're asking.... Mind that the variable time t is a scalar which varies into its domain, then is discretized to be a scalar function which varies into its discretized domain. In Matlab the discretization is really easy since you just have to define a range of values of t. This is done in the model.sol sequence when defining a range for the feature 'tlist'. I didn't get what you mean with "compare the time with a 1x1 value", but all you can do is to play with the syntax range(t1,t2,dt)
Hope it helps!
Mattia
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thanks for the help. I tryed to make the simulation in matlab, but I have a problem with the initial values, if you can see, I post this in the next link:
www.comsol.com/community/forums/general/thread/32751/#p89649
Did you have this problem ?
best regards