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time dependent turbulent flow and step function

Posted Feb 12, 2012, 8:53 p.m. EST Version 4.2, Version 4.2a 3 Replies

Asal Bidarmaghz
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Hi,
I am modeling heat transfer and turbulent fluid flow(RANS) in a pipe.
i could run the model and get reasonable results as long as fluid flow is in laminar regime but converting to RANS model, the process continues for ever and even after one day no progress is made. but still going and not interrupted.

by browsing through the forum i realized i should define a step function so what I did is a step function as follows:
step1
location: 0.025
from:0
to:0.2
smoothing: 0.05
i am not sure if these are correct but my inlet velocity is 0.2m/s. the problem is that when i multiply the inlet velocity with the step function (U0=0.2*step1(t)) comsol does not recognizes it and stays in red. even if i define t=0 as a parameter in advance.

i would appreciate if anyone could help me with this.

ASAL
3 Replies Last Post Feb 19, 2012, 5:29 p.m. EST
Asal Bidarmaghz
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Posted: 1 decade ago Feb 15, 2012, 5:48 p.m. EST
now the model in being run but only 4% progress in 4 days and i think this is going to continue forever.
may this be because of the step function i defined?


ASAL
now the model in being run but only 4% progress in 4 days and i think this is going to continue forever. may this be because of the step function i defined? ASAL
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 16, 2012, 2:04 a.m. EST
Hi

turbulence models are tough and really demanding on RAM and processing time, as you often need a very fine mesh.

Sometimes starting from a laminar flow case can help, by gently increasing the velocity

Anyhow you should monitp what is gping on, by looking at the plots wile solving of both velocity and pressure, then you often identify rapidly that the solver is going the wrong direction , and you can stop before haviong waited ages for the progress to move on its sand bar ;)

--
Good luck
Ivar
Hi turbulence models are tough and really demanding on RAM and processing time, as you often need a very fine mesh. Sometimes starting from a laminar flow case can help, by gently increasing the velocity Anyhow you should monitp what is gping on, by looking at the plots wile solving of both velocity and pressure, then you often identify rapidly that the solver is going the wrong direction , and you can stop before haviong waited ages for the progress to move on its sand bar ;) -- Good luck Ivar
Asal Bidarmaghz
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Posted: 1 decade ago Feb 19, 2012, 5:29 p.m. EST
Hi Ivar,
I have divided it to two studies. stationary for fluid flow and transient for heat transfer and forced the heat transfer to use the stationary results as initial conditions. the model is run but I get very unrealistic results.

i think that step function should work well for me but i could not figure out what the step should be. imagine my velocity is 0.3m/s for a 0.025m diameter pipe.

Regards
ASAL
Hi Ivar, I have divided it to two studies. stationary for fluid flow and transient for heat transfer and forced the heat transfer to use the stationary results as initial conditions. the model is run but I get very unrealistic results. i think that step function should work well for me but i could not figure out what the step should be. imagine my velocity is 0.3m/s for a 0.025m diameter pipe. Regards ASAL

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