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Need help solving a stationary followed by time-dependent problem

Posted 2 gen 2011, 00:38 GMT-5 Materials Version 4.1 4 Replies

Steve Conrad
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I have a fluid flow problem with the inlet pressure driven by an interpolated function of time that has a large value at time 0. To solve this I set up a stationary problem to use inlet function value at time zero, then solve a time-dependent problem for t values of 0 through 1. Since my inlet pressure is a function of time funct(t), it produces an error when it tries to solve the stationary problem. I tried to get around this by defining my input function as:

func( if( depends(t), t, 0)[1/s] )

My read on this is that it should give func(0) during the stationary problem since t is not defined, and func(t) for the time-dependent problem, but it always gives func(0).

Anyone with any experience in doing what I am attempting?
4 Replies Last Post 4 gen 2011, 01:21 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2 gen 2011, 03:00 GMT-5
Hi

have you tried to use a step function (Heaviside derivative step) to ramp up quickly your pressue value ?

--
Good luck
Ivar
Hi have you tried to use a step function (Heaviside derivative step) to ramp up quickly your pressue value ? -- Good luck Ivar
Steve Conrad
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Posted: 1 decade ago 2 gen 2011, 13:52 GMT-5
Thanks, Ivar. I gave that some thought by changing my initial step from stationary to time-dependent, but then I have to deal with some way to figure out which time-dependent step I'm in in order to determine whether to use a constant or the function.

What I really would like to do is use is define my inlet based on:

func( t[1/s] )

The COMSOL documentation says that for stationary problems, t is defined as 0, which is what I want, However when I try to use this approach, I get a message "Failed to evaluate variable" for t, indicating it is undefined.

Any other thoughts?

Steve
Thanks, Ivar. I gave that some thought by changing my initial step from stationary to time-dependent, but then I have to deal with some way to figure out which time-dependent step I'm in in order to determine whether to use a constant or the function. What I really would like to do is use is define my inlet based on: func( t[1/s] ) The COMSOL documentation says that for stationary problems, t is defined as 0, which is what I want, However when I try to use this approach, I get a message "Failed to evaluate variable" for t, indicating it is undefined. Any other thoughts? Steve
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2 gen 2011, 14:16 GMT-5
Hi

I have noticed that, depending on version/patch too the behaviour with the internal variabloes such as "t" and "lambda" changes somewhat.
Often I define a parameter such as t=0, if accepted it solves the issue. But then when adding a transient solver often you get back the error message.
Another way, is to add a dummy (unused but declared) transient solver study case, this defines t=0 for you.

Now the heaviside (or v4 "step" function, already defined in v4) is typically used in transient, but you can also, for stationary use it to ramp with a local "Param" parameter

--
Good luck
Ivar
Hi I have noticed that, depending on version/patch too the behaviour with the internal variabloes such as "t" and "lambda" changes somewhat. Often I define a parameter such as t=0, if accepted it solves the issue. But then when adding a transient solver often you get back the error message. Another way, is to add a dummy (unused but declared) transient solver study case, this defines t=0 for you. Now the heaviside (or v4 "step" function, already defined in v4) is typically used in transient, but you can also, for stationary use it to ramp with a local "Param" parameter -- Good luck Ivar
Steve Conrad
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Posted: 1 decade ago 4 gen 2011, 01:21 GMT-5
Ivar, with a little help from COMSOL support I was able to solve the problem. By defining the variable t as a global parameter with a value of 0, it was picked up and used by the stationary solver. The time-dependent solver then reused t for its own purposes.

Steve
Ivar, with a little help from COMSOL support I was able to solve the problem. By defining the variable t as a global parameter with a value of 0, it was picked up and used by the stationary solver. The time-dependent solver then reused t for its own purposes. Steve

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