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open boundary

Posted Mar 7, 2012, 3:28 a.m. EST Plasma Physics, RF & Microwave Engineering, Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Microfluidics, Porous Media Flow, Chemical Reaction Engineering, Geometry, Materials, Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization, Studies & Solvers, Structural Mechanics Version 3.5a, Version 4.0, Version 4.0a, Version 4.1, Version 4.2, Version 4.2a 2 Replies

Bhageshvar Mohan
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Hi,
I am trying to model evacuation of air between two plates which means i shud have the right and left boundaries open to atmosphere. The air is evacuated using a narrow outlet at the centre of the bottom plate.

I noticed something called 'open boundary' in the laminar flow physics. does this help in creating the right and left boundaries open to the atmosphere?
Or does anyone have a suggestion to make the right and left boundaries open to atmosphere?
Please help


Thanks in advance.
2 Replies Last Post Mar 9, 2012, 4:41 a.m. EST
Robert Koslover Certified Consultant
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Posted: 1 decade ago Mar 8, 2012, 7:30 p.m. EST
I'm not an expert on fluid models, but I don't think you can build the model properly in the way you described. Comsol Multiphysics needs to solve for the fluid pressure and velocity as a function of position. The left and right "openings" between the plates are not simple uniform boundary conditions, since the fluid motion there depends on the fluid motions beyond the space between the plates. (In that sense, if I am correct, this is analogous to a parallel plate capacitor in an electrostatic problem, where the fringing fields extend beyond the plates.) So again, if I am correct, you need to make your model-space larger, so that it extends well beyond the volume between your plates and includes some volume to the left, right, above, and beneath (since it seems that you are talking about a 2D model). Then you can specify conventional-type boundary conditions (as offered in the drop-down lists) on the more-removed boundaries of your problem, as appropriate.
I'm not an expert on fluid models, but I don't think you can build the model properly in the way you described. Comsol Multiphysics needs to solve for the fluid pressure and velocity as a function of position. The left and right "openings" between the plates are not simple uniform boundary conditions, since the fluid motion there depends on the fluid motions beyond the space between the plates. (In that sense, if I am correct, this is analogous to a parallel plate capacitor in an electrostatic problem, where the fringing fields extend beyond the plates.) So again, if I am correct, you need to make your model-space larger, so that it extends well beyond the volume between your plates and includes some volume to the left, right, above, and beneath (since it seems that you are talking about a 2D model). Then you can specify conventional-type boundary conditions (as offered in the drop-down lists) on the more-removed boundaries of your problem, as appropriate.
Jaap Verheggen
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Posted: 1 decade ago Mar 9, 2012, 4:41 a.m. EST
I agree, if you only model the plates you essentially are modelling evacuation between two infinetely long planes. If you want to take the edges into account, you need the model the edges of the plates.
I agree, if you only model the plates you essentially are modelling evacuation between two infinetely long planes. If you want to take the edges into account, you need the model the edges of the plates.

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