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-   -   multiphase flow (https://www.cfd-online.com/Forums/fluent/27834-multiphase-flow.html)

 Shane Gillis August 14, 2000 12:28

multiphase flow

I am trying to model a ships hull in Fluent. I can create the solid model just fine, but when I do a multiphase flow arrangement, I get weird results. It has been suggested that I use a transient solution instead of a steady one. This sounds like it would work, but how do I set things up so that I get more than one second of flow past the hull. I'd like at lease 5 seconds before all the water drains out.

Any suggestions?

Previous question below.

I have asked a similar question before, but now I need a touch more detail. I am modelling a ships hull in Fluent, both 2D & 3D. I can setup the steady state multiphase problem with region adaption and patching and I have seperate inlets & outlets for the initial water volume (a box with an inlet face with an upper & lower half, the upper is air inlet and the bottom is water inlet, the bottom inlet gets a velocity and a fraction of one, gravity is on), but the solution never seems to converge & if I get results when the error gets low, I get nonsensical results (such as all the water behind the vessel seeming to drain out). About the only time I get good results is within the first 20 iterations (then it looks realistic), but I don't know if I can trust those results to far. Is there any way I can improve the accuracy of the model and solution.

 John C. Chien August 14, 2000 14:38

Re: multiphase flow

(1). First of all, you can visit the vendor's website and go through the examples displayed, to see if they have solved a problem similar to yours. (2). If the answer is yes, then e-mail the vendor for a sample input case so that you can learn how to do it. (3). If the answer is no, then you can still e-mail the vendor for their opinion, whether their code is suitable for this type of applications of not. If you don't get the answer, then you can safely assume that they are not ready to attack this type of problem. (4). Try to visit the website of Flow Science, you may be able to find something useful there.

 Shane Gillis August 14, 2000 15:37

Re: multiphase flow

I am not sure if there is any such help from Fluent.

 John C. Chien August 14, 2000 15:56

Re: multiphase flow

(1). At the same time, you may want to search through the previously posted messages, to see if there is anything related to your applications.

 Shane Gillis August 14, 2000 15:58

Re: multiphase flow

Done that. Could not find the answer to my question that way.

 Trac August 14, 2000 18:14

Re: multiphase flow

Hi Shane,

If you have an inlet for both the air and water, the water shouldn't drain out over time. You should patch the air and water to begin with (which I think you've mentioned you've done) and for a 3D simulation you'll probably have to use fairly small timesteps. However, depending on the eventual steady state solution (is there one?) you may be able to use a steady state, implicit VOF scheme. You might also want to consider adapting by volume fraction to get the interface finer. (second order will also make this finer, but work up to this). how big is the domain? i've found you need the boundaries a long way off to stop reflection problems (unless of course you're looking for wall effects, such as a river bank.) have you tried starting with a 2D simulation? You might this very useful in terms of getting all the procedure worked out, and much quicker!

 Shane Gillis August 15, 2000 11:09

Re: multiphase flow

I will explain a 2D case that I have done first. I made a long box (50m long) with a square in the center .5m on a side. The box is 20m tall. The velocity inlet & outlet (pressure outlet & outlet vent, I've tried both) are each divided in half with the top being air & the bottom being water. The bottom half of the inlet & outlet are set to a volume fraction of one for water fraction (water being the second phase). The top & bottom edges are set as symmetry, and the edges of the square are walls. I mesh it with quads. If I do a steady state solution using the implicit VOF model, the solution explodes, no matter how I adjust the relaxation factors or if I include body forces or not. I use PISO and 1st & 2nd order solutions and it still explodes. (I am patching the water to the entire bottom half (-25<x<25 & -10<y<0)).

If I use a transient solution, the system solves just fine & everything works great, but all the water drains out the outlet. Of course, I don't have the volume fractions of the inlet & outlet set to one, so maybe that is the problem. However, I would still like to solve this in steady state if possible.

Any ideas?

Thank you,

 Trac August 15, 2000 17:40

Re: multiphase flow

This might not be a steady state problem - you will have lots happening off the sharp edges of the square and it may only be able to be solved as transient.

You'll need to set the volume fractions for the inlets (1 for the water inlet, 0 for the air inlet) to make sure the air and water keep coming through. Try using one big outflow boundary at the exit, that way you won't have to enforce a volume fraction. Although if its far enough away (which yours sounds like it is) you shouldn't have too many problems from the downstream boundary.

 Shane Gillis August 15, 2000 17:44

Re: multiphase flow

Sounds good, thanx, I'll try that.

 Sung-Eun Kim August 28, 2000 18:57

Re: multiphase flow

Hi Shane,

I wonder if what BC you're using at the exit boundary. You should use "pressure-outlet" boundary. And the pressure you specify on the pressure out let boundary should include the hydrostatic head. Admittedly, the whole procedure to compute the hull-generated wave is not very straightforward mainly due to the BC at the exit boundary through which the waves radiates out. Yet it's certainly possible.