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Transient+ Turbulent model always gives error

Posted 16.07.2012, 15:12 GMT-4 Version 4.2 4 Replies

Naser Imran Hossain
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Hi,

Initially I was trying to do a modeling of a high velocity flame flow around a solid surface, but was facing trouble converting in the transient+turbulent mode. But then, I tried doing just a simple transient flow around a solid object in comsol 4.2, but received the same "Failed to find consistent initial values.Last time step is not converged." error. I don't know what's happening. Why is COMSOL not being able to solve a very basic problem of fluid mechanics?

Some help will be highly appreciated.

Thanks.

PS: I've attached my model (just a simple 2d flow around a rectangular object) here.
4 Replies Last Post 30.08.2012, 15:06 GMT-4
Amir Fadel
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Posted: 1 decade ago 17.07.2012, 05:25 GMT-4
Hi,

You need to define a step function and use it to multiply the inlet velocity so that the velocity goes from 0 to some value during the computation otherwise it will never manage to find correct initial values.

Cheers
Hi, You need to define a step function and use it to multiply the inlet velocity so that the velocity goes from 0 to some value during the computation otherwise it will never manage to find correct initial values. Cheers
Naser Imran Hossain
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Posted: 1 decade ago 20.07.2012, 21:15 GMT-4
Hi,

Thank you very much. I tried it out and it worked. Seems the inlet condition was indeed too steep for the software to handle. I smoothened it out with a step function as you suggested and now the simulation runs on transient mode.

One more thing, since the simulations are taking a lot of time to execute, is there any way I can limit the number of iterations or run-time for which the simulation will give me a set of results? I know I can limit the number of iterations or tolerance at the 'segregated step' in the solver node, but is there any other way to do that?

Thanks again.
Hi, Thank you very much. I tried it out and it worked. Seems the inlet condition was indeed too steep for the software to handle. I smoothened it out with a step function as you suggested and now the simulation runs on transient mode. One more thing, since the simulations are taking a lot of time to execute, is there any way I can limit the number of iterations or run-time for which the simulation will give me a set of results? I know I can limit the number of iterations or tolerance at the 'segregated step' in the solver node, but is there any other way to do that? Thanks again.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 21.07.2012, 04:14 GMT-4
Hi

have you tried to run first a stationary solver case and then a transient. In this way you get really a good initial condition and your transent analysis will run smoother. Sometimes its better to start from a stationary high speed case and lower the inlet flow, than starting from "0", but you can also start from 10% or 50% of nominal inlet speed, in stationary, and then ramp up with a smooth step to 100% or whatever oscillatory conditions you want.

One thing if you start with a too slow flow, you are not really in a turbulent case, that might be an issue for your convergence

But, turbulent transient flow is something one mostly do on a cluster ;)

--
Good luck
Ivar
Hi have you tried to run first a stationary solver case and then a transient. In this way you get really a good initial condition and your transent analysis will run smoother. Sometimes its better to start from a stationary high speed case and lower the inlet flow, than starting from "0", but you can also start from 10% or 50% of nominal inlet speed, in stationary, and then ramp up with a smooth step to 100% or whatever oscillatory conditions you want. One thing if you start with a too slow flow, you are not really in a turbulent case, that might be an issue for your convergence But, turbulent transient flow is something one mostly do on a cluster ;) -- Good luck Ivar
Dereje Tefera
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Posted: 1 decade ago 30.08.2012, 15:06 GMT-4
Hi
I have the same problem here while i am simulating flow through fixed bed reactor,porous media, my problem is i tried
multiplying the inlet velocity with "step1" but it says "unknown variable step1".
what should i do in defining the etep funcation? i just used the default
location 0
from 0
To 1

then i simply multiply it with inlet velocity profile but it displays the above error



regards
Hi I have the same problem here while i am simulating flow through fixed bed reactor,porous media, my problem is i tried multiplying the inlet velocity with "step1" but it says "unknown variable step1". what should i do in defining the etep funcation? i just used the default location 0 from 0 To 1 then i simply multiply it with inlet velocity profile but it displays the above error regards

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