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Fluid flow problem: Velocity profile

Posted 11 giu 2012, 05:26 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD) Version 4.2a 4 Replies

Abbie
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Hi, hope to get ur help for the following fluid flow (laminar) problem in a 2D axisymmetrical cylinder:

The horizontal boundary at the top of cylinder was selected as inlet while bottom horizontal boundary was outlet.

Boundary condition:
inlet: time-varying velocity (flat velocity profile)
outlet: pressure=0

The model cannot converged and failed. (The failed model is in attached.)

However, it works when the inlet velocity was changed into "laminar inflow" (parabolic velocity profile) while identical time-varying velocity was implemented.

Can anyone please tell me why the flat profile velocity cannot work in this model?
and how to overcome this problem?
A flat profile of inlet velocity is actually what I want.

Thanks!
4 Replies Last Post 12 giu 2012, 05:50 GMT-4
Amir Fadel
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Posted: 1 decade ago 11 giu 2012, 05:58 GMT-4
Hi,

You are using an interpolation function as inlet velocity profile and there is quite a discontinuity in your interpolation, I am not sure comsol can deal with that, you may want to refine the itnerpolation data.

Cheers
Hi, You are using an interpolation function as inlet velocity profile and there is quite a discontinuity in your interpolation, I am not sure comsol can deal with that, you may want to refine the itnerpolation data. Cheers
Abbie
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Posted: 1 decade ago 11 giu 2012, 12:43 GMT-4
Thanks Amir! Regarding the interpolation velocity, do you know why it works with "laminar inflow" (parabolic profile) but not "velocity" (flat profile) in COMSOL?

I tried to replace the interpolation function by a constant velocity with step function (e.g. velocity= 0.5*step1(t)) but it faced convergence error too. I suspect velocity profile is somehow affecting the convergence.

Thanks Amir! Regarding the interpolation velocity, do you know why it works with "laminar inflow" (parabolic profile) but not "velocity" (flat profile) in COMSOL? I tried to replace the interpolation function by a constant velocity with step function (e.g. velocity= 0.5*step1(t)) but it faced convergence error too. I suspect velocity profile is somehow affecting the convergence.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 11 giu 2012, 14:43 GMT-4
Hi

if you have "no-slip" conditions on your wall ,you cannot have a perfectly "flat" inlet velocity it will always tend to "0" along the wall, or you should use slip conditions, that is due to the shear stress of the fluid.

So if you force a flat inlet velocity you have a singulariy at the boundary, hence convergence issues

--
Good luck
Ivar
Hi if you have "no-slip" conditions on your wall ,you cannot have a perfectly "flat" inlet velocity it will always tend to "0" along the wall, or you should use slip conditions, that is due to the shear stress of the fluid. So if you force a flat inlet velocity you have a singulariy at the boundary, hence convergence issues -- Good luck Ivar
Amir Fadel
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Posted: 1 decade ago 12 giu 2012, 05:50 GMT-4
Hi,

All right, this is complicated. After you said the solver does not converge for a constant inlet velocity profile I tested your model a bit, I could not get convergence either. That is troubling, even if you use a constant velocity profile you can can get a solution you just may have to discard the portion close to the inlet.

I re-wrote your model entirely, please find it attached. Please note I used just one Bezier polygon and closed it to form a solid, you do not need so many polygons as in your model. Please note that I could get convergence also with you interpolation function after I re-wrote the model. Please find attached a movie of the 2D velocity distribution (the zip archive).

The results include: a steady state solution for a constant velocity profile of 0.4m/s, a time-dependent solution with a 0.4*step1(t)[m/s] inlet velocity and a time-dependent solution with your interpolation function.

Finally please note that you may note use 0.5m/s as inlet velocity because Reynolds would reach over 2000 just int he half channel, turning the flow into a turbulent one.

I am not sure what happened when you built your model, but I would strongly recommend to start from scratch.

Cheers

PS: I used 4.3 but I don't think it may influence the results
Hi, All right, this is complicated. After you said the solver does not converge for a constant inlet velocity profile I tested your model a bit, I could not get convergence either. That is troubling, even if you use a constant velocity profile you can can get a solution you just may have to discard the portion close to the inlet. I re-wrote your model entirely, please find it attached. Please note I used just one Bezier polygon and closed it to form a solid, you do not need so many polygons as in your model. Please note that I could get convergence also with you interpolation function after I re-wrote the model. Please find attached a movie of the 2D velocity distribution (the zip archive). The results include: a steady state solution for a constant velocity profile of 0.4m/s, a time-dependent solution with a 0.4*step1(t)[m/s] inlet velocity and a time-dependent solution with your interpolation function. Finally please note that you may note use 0.5m/s as inlet velocity because Reynolds would reach over 2000 just int he half channel, turning the flow into a turbulent one. I am not sure what happened when you built your model, but I would strongly recommend to start from scratch. Cheers PS: I used 4.3 but I don't think it may influence the results

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