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Transient Laminar Flow with Diffusion/Convection

Posted Jan 12, 2012, 4:05 p.m. EST RF & Microwave Engineering, Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization Version 4.2a 5 Replies

Melanie Pigeon
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I want to create a model that consists of the following 3 parts:
1) laminar flow
2) inject a substance into the inlet flow (assuming well-mixed at inlet)
3) see how the substance moves and the flow rate changes with time

My question to you is what is the best way to proceed. I do not want to do it all at once as I am sure there will be mistakes. What steps should be done first?My second question is related to the solver. The velocity computed with the laminar flow equation is independent of the concentration but the concentration depends upon the velocity. How should the problem be set up? Should I compute the transient flow rate and then compute the transient concentration or do it simultaneous? If simultaneous, what is the best approach to ensure minimal computation time and convergence.

Note that this problem requires an extremely fine mesh with 6 boundary layers.This was determine with a small side analysis.
5 Replies Last Post Jan 23, 2012, 3:45 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 13, 2012, 2:29 a.m. EST
Hi

it's not my main field (CFD and chemistry) but what I have learned so far: mesh you should use boundary layers for no-slip walls (comsol proposes that by default) but I also use boundary layers on diffusion boundaries (when solving in the time domain) with high concentration steps (this applies also for HT by the way, it's general for any diffusion equation, due to the erfc() solution shape for t around 0).

Then when you mix many physics, it's wise to start stepwise (when possible or at least block some when multiple are there by default NITF ...). This is the general question to for segregation which variables to solve first in which order, how to separate. Even if COMSOL proposed good choices, it's not always optimum, and I often set back to direct mode, as I have quite some RAM and that often solves quiker (less iterations). This requires tsts and tries.

But I would aso start flow, then concentration, in that order. But there must be other than me out here that could come with less "guessed" suggestions ;)

--
Good luck
Ivar
Hi it's not my main field (CFD and chemistry) but what I have learned so far: mesh you should use boundary layers for no-slip walls (comsol proposes that by default) but I also use boundary layers on diffusion boundaries (when solving in the time domain) with high concentration steps (this applies also for HT by the way, it's general for any diffusion equation, due to the erfc() solution shape for t around 0). Then when you mix many physics, it's wise to start stepwise (when possible or at least block some when multiple are there by default NITF ...). This is the general question to for segregation which variables to solve first in which order, how to separate. Even if COMSOL proposed good choices, it's not always optimum, and I often set back to direct mode, as I have quite some RAM and that often solves quiker (less iterations). This requires tsts and tries. But I would aso start flow, then concentration, in that order. But there must be other than me out here that could come with less "guessed" suggestions ;) -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Jan 13, 2012, 7:52 a.m. EST
You don’t need a simultaneous solution since the injected substance does not affect the fluid flow. Solve the fluid flow model first. Then setup a second Transport of Diluted Species (mass transfer) model with the velocity taken from the first solution. You make that connection in Model Inputs section in the “Convection and Diffusion” node. I assumed that the “substance” spreads by both convection (from the fluid flow) and diffusion. If there is no diffusive component you can use particle tracing for the second model.

Nagi Elabbasi
Veryst Engineering
You don’t need a simultaneous solution since the injected substance does not affect the fluid flow. Solve the fluid flow model first. Then setup a second Transport of Diluted Species (mass transfer) model with the velocity taken from the first solution. You make that connection in Model Inputs section in the “Convection and Diffusion” node. I assumed that the “substance” spreads by both convection (from the fluid flow) and diffusion. If there is no diffusive component you can use particle tracing for the second model. Nagi Elabbasi Veryst Engineering
MRW
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Posted: 1 decade ago Jan 20, 2012, 9:39 a.m. EST
Hi Nagi,

How would using the mass transfer module differ from using the particle tracing module?


Thanks!
Marjorie
Hi Nagi, How would using the mass transfer module differ from using the particle tracing module? Thanks! Marjorie
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Jan 20, 2012, 12:37 p.m. EST
Mass transfer accounts for convection and diffusion (spreading). It cannot be non-diffusive even if you use near zero diffusivity. There is always a small amount of diffusion depending on the settings and the mesh size. Particle tracing uses a different formulation that does not involve diffusion.You should select between the two methods based on the physics of your specific problem.

Nagi Elabbasi
Veryst Engineering
Mass transfer accounts for convection and diffusion (spreading). It cannot be non-diffusive even if you use near zero diffusivity. There is always a small amount of diffusion depending on the settings and the mesh size. Particle tracing uses a different formulation that does not involve diffusion.You should select between the two methods based on the physics of your specific problem. Nagi Elabbasi Veryst Engineering
MRW
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Posted: 1 decade ago Jan 23, 2012, 3:45 p.m. EST
Hi Nagi,

Can you please expand on the differences?

I am currently working with a simple setup; 3D laminar flow around a regular object. I am trying to incrementally add levels of complexity so that I can fix any convergences issues that arise.

Thanks!
Hi Nagi, Can you please expand on the differences? I am currently working with a simple setup; 3D laminar flow around a regular object. I am trying to incrementally add levels of complexity so that I can fix any convergences issues that arise. Thanks!

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