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Transport through diluted species-Time dependent problem
Posted 13.08.2010, 07:27 GMT-4 Fluid & Heat, Computational Fluid Dynamics (CFD), Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.4 3 Replies
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I am performing a time dependent analysis for transport through diluted species based on the solution I obtain by performing transport through free and porous region in the same geometry. Basically first I solve Naviers stokes and Brinkmann equation in stationary mode and use the velocity profile obtained in the Convection -diffusion equations . I am using Direct Paradiso solver. Using BDF solver, the solution never converges. When I use the generalisd alpha method, it converges for small time interval upto 500 second and gives a good solution.
However when I use a high value for time 10000 seconds, the solution starts rapidly diverging after 1077 seconds. can you please suggest where could be possible error.
Regards
Pranay Ghosh
Hello Pranay Ghosh
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If you have managed to find a solution, please update forum- Any help would be greatly appreciated.
Caroline
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Pranay, I also have this problem. I too am modelling a diluted species transport model with a modified navier stokes for flow through a porous media. The time point at which is starts diverging seems random, however does change when timestep and numerical damping is varied (amplification factor for high frequency) in the generalized alpha time stepping method. Sometimes its at 16sec, others it is at 160 second.
If you have managed to find a solution, please update forum- Any help would be greatly appreciated.
Caroline
Do you have a high Pe and Sc? if so, it seems you have the common problem of divergence in C-D. You can try increasing the mesh where possible, and use artificial diffusion. But yet again, I had to invest a year to get a feeling of my model :(
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Comsol 4.1
Ubuntu 10.04.1
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Since this topic is quite old, I re-open it because we currently have the same king of problem with the diluted species solver. Our case is the following:
- the case: an axi-symetric geometry (a simple cylinder)
- first study with bubbly flow: computation of an airlift system to get the fluid velocity field inside the cylinder
- second study with diluted species: a patch of concentration is added at the top of the cylinder, and a time dependent computation is performed. The boundary conditions are no flux everywhere.
The concentration first correctly convects/diffuses inside the cylinder, but after a while the concentration begins to grow exponentially. Looking at the solutions shows that there is actually several mesh points on the walls (on different boundaries) where the concentration diverge and then diffuse in the whole domain.
If we change the boundary condition from no flux to a constant concentration at these locations, no divergence happens.
If we desactivate convection (ie diffusion only), no divergence too.
Any idea or news about this problem ? Is the mesh refinement the only solution to reduce the divergence ? I could provided our case if necessary. Thank you !