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thezack March 3, 2013 11:04

Boundary Type Help
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!

ryancoe March 3, 2013 17:40

Hard to say what's wrong for sure without more details, but here are some things to look at:
  • You say that the funnel is initially filled with air. Are you doing this to match an experimental setup? If not, I think it would be easier to start with the funnel filled with water.
  • What's your time-step (and inner-iteration limit)? Make sure this is sufficiently small in comparison to your cell size. If you're using the semi-implicit solver, you don't have to satisfy Courant number = 1 for stability, but it shouldn't be outrageously large either.
Hope this helps.

AndrewLBloxom March 4, 2013 01:05

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?

thezack March 6, 2013 22:13

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.

thezack March 11, 2013 20:16

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)
Implicity Unsteady
Volume of FLuid
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?

thezack March 14, 2013 22:45

Any ideas?

ryancoe March 15, 2013 07:42

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.

thezack March 25, 2013 18:59

The simulation does work well without the turbine. With the turbine I get high pressures and speeds near the bottom.

thezack April 2, 2013 19:27

Any ideas?

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