[Solved] Trouble with a Laminar Outflow Flow Rate and a pressure at the Inlet

Benjamin Astbury

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Hi,

I'm relatively new to COMSOL. Im trying to set a system with boundary conditions where I define my outflow flow rate and ave an ambient pressure at the inlet (Case 1).

I have tried it th other way around with Flow rate at inlet and pressure out outlet (Case 2) and it seems very defined but my system is specifically defined as a pulling of air (Case 1) through the system not pushing air (Case 2) through the system.

For the flow rate at outflow (Case 1), the problem seems to try to converge and then unconverge again. I would be happy with the Flow rate at Inlet (Case 2) but I have a complex geometry which has a internal tank with holes into the air flow channel. In the pulling of air (Case 1), I know that this tank will have a low pressure. And in the pushing the air (Case 2) it has higher pressure.

Is it possible to have the Case 1 converged with those boundary conditions or do a different set of boundary conditions have to be made like a pressure condition at the outlet where is is equal to the pressure drop across the device?

Any advice on this would be great.


2 Replies Last Post Aug 20, 2018, 9:16 AM EDT

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Posted: 1 month ago Aug 17, 2018, 4:44 PM EDT

For case 1, you could set "Inlet" to "Pressure" with p = 0 (to represent ambient pressure), and set "Outlet" to "Velocity" with velocity specified to give the outflow rate that you want. In the attached file I have used parabolic velocity profile at the outlet as the "Normal outflow velocity".

For case 1, you could set "Inlet" to "Pressure" with p = 0 (to represent ambient pressure), and set "Outlet" to "Velocity" with velocity specified to give the outflow rate that you want. In the attached file I have used parabolic velocity profile at the outlet as the "Normal outflow velocity".


Benjamin Astbury

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Posted: 4 weeks ago Aug 20, 2018, 9:16 AM EDT

[Solution]

I was contacted by technical support. They instricted me to use a blog article:

  • uk.comsol.com/blogs/the-optima...​projects-on-a-deadline/

After simplifying my model to a 2D axisymmetric model to reduced computational outlay, I was noted to use the article to entent the outflow boundary away from the area of interesting physics I had. (A high pressure gradient and high velocity region)

My issue was completly sloved when ententing the outflow region using a column or a cone with a slip boundary condition.

Thanks for you reply DW. I did try your response and it was still not great. however this was after technical had already gotten bck to me so a solution had already been found.

[Solution] I was contacted by technical support. They instricted me to use a blog article: - uk.comsol.com/blogs/the-optima...​projects-on-a-deadline/ After simplifying my model to a 2D axisymmetric model to reduced computational outlay, I was noted to use the article to entent the outflow boundary away from the area of interesting physics I had. (A high pressure gradient and high velocity region) My issue was completly sloved when ententing the outflow region using a column or a cone with a slip boundary condition. Thanks for you reply DW. I did try your response and it was still not great. however this was after technical had already gotten bck to me so a solution had already been found.

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