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March 3, 2013, 10:04 |
Boundary Type Help
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#1 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
Hi there! My name is Zack. This is my first post on the CFD Online forums. I am pretty new to CFD and have been using Star CCM 7 for an independent project I am working on. As I am new to CFD, I have a question that might seem somewhat newbie-ish. I ran through a bunch of tutorials that came with Star CCM at first, and now I am trying to figure out everything all at once. I am trying to simulate water running down a funnel (with gravity) and observing the resulting vortex effects. I am using the following physics models: eulerian multiphase, gravity, laminar, multiphase interaction, segregated flow, steady, three dimensional, and volume of fluid. At first the funnel is filled with air and I have an inlet at the top which feeds a constant velocity of water which is supposed to cause a vortex effect. I have a flow split outlet at the bottom.
When I run the simulation, the solution diverges. Velocity at the outlet goes way up. I am guessing this is because all of the air in the device has to exit out of the hole in the bottom. This causes a lot of crazy effects, the water rushes around unpredictably at very high speed. I believe a possible solution to this would be to make it so that the air could escape through the top, but not the water that I am sending through the inlet. However I am not sure what type of boundary at the top I would need to do this. I tried making the top a porous boundary to the air, but I have no clue what to set the alpha and beta coefficent as. I would like this to simulate the top surface of the cone to be open to the environment where air could escape. I would appreciate any type of help I could get with this. If you need any details or pictures, let me know. Thank you! |
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March 3, 2013, 16:40 |
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#2 |
Member
Ryan Coe
Join Date: Jun 2010
Location: Albuquerque, NM
Posts: 98
Rep Power: 15 |
Hard to say what's wrong for sure without more details, but here are some things to look at:
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Ryan |
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March 4, 2013, 00:05 |
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#3 |
New Member
Andrew L. Bloxom
Join Date: Dec 2010
Location: Blacksburg, VA
Posts: 7
Rep Power: 15 |
Are you using a separate inlet to pour water in than just the top where you want atmospheric conditions?
Will a pressure outlet at the bottom at patm not work for the free jet? |
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March 6, 2013, 21:13 |
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#4 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
I figured out what the problem was. I can define the top wall as phase permeable for the air. Now I have a new problem, haha.
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March 11, 2013, 19:16 |
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#5 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
It turns out I still have an issue. I set the top and bottom to be pressure outlets at patm. I am using the following physics models:
Eulerian Multiphases(water, air) Gravity Gradients Implicity Unsteady Volume of FLuid Laminar Multiphase Equation of State Multiphase interaction Segregated flow (with source term conservation option) Three Dimensional I have two meshes. one is the background mesh, and the second is the mesh of a turbine at the bottom of the funnel. I am using a rotating overset mesh for the turbine. I am using DFBI rotation for the turbine. Something strange appears to be happening at the bottom of of the funnel in the turbine region. Once as the flow gets to the bottom, the water shoots up at high speed in the +z direction. After running the simulation for a while I have a thin layer of water along the inner surface of the funnel(as is desired), but the air occupying the middle is always rushing around at high speed. What might be the issue here? |
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March 14, 2013, 21:45 |
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#6 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
Any ideas?
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March 15, 2013, 06:42 |
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#7 |
Member
Ryan Coe
Join Date: Jun 2010
Location: Albuquerque, NM
Posts: 98
Rep Power: 15 |
I didn't realize a turbine was involved here. Have you tried a simulation where it is omitted (to see if everything else is working as desired)?
Also, it seems like you could be using an embedded mesh approach to achieve the rotation of the turbine blades.
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Ryan |
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March 25, 2013, 17:59 |
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#8 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
The simulation does work well without the turbine. With the turbine I get high pressures and speeds near the bottom.
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April 2, 2013, 19:27 |
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#9 |
New Member
Zack Hoyle
Join Date: Mar 2013
Posts: 18
Rep Power: 13 |
Any ideas?
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