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Old   February 15, 2007, 13:54
Default VOF model
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Winston Gregorio
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Hi, I'm a third year student currently doing my 3rd year project using FLUENT. I am having trouble with the way I should approach this problem. To be honest, I don't really have a clue how to approach it. What results am I expected to find, what flow features will be present in the wake. I have been advise to approach it with a VOF model, however there is very little information on it, and so far have only come across one example which basically has no relevance to my report. I would be very grateful if anyone could help me, maybe provide me with some answers or maybe any references I can use.

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Old   February 16, 2007, 03:21
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JM
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What is your problem? You have not mentioned anything about the problem itself.
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Old   February 16, 2007, 12:57
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Winston Gregorio
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Sorry, i dont think i made it very clear. the whole point of my project is to simulate a flow around a submarine fin. Its a 2D analysis with a free surface representing the phase change between water and air. My supervisor wasnt very clear on what i shud expect from my results, also i am very inexperienced with FLUENT. However from what i've learnt from tutorials i need to get my residuals to converge. is that right? I've started the analysis already and these are the inputs i've chosen. Segregated Implicit k-epsilon model Time unsteady Gravity -9.81 My boundary conditions are inlet - velocity 4m/s submarine fin- wall top surface - free surface right side wall - outlet. I tried different different numbers of iterations, however when i iterate, always near the end of the iterations, i get an error message 'floating error' and i'm not sure what that means. does it mean my geometry's wrong or could it be something to do with my meshing? That was my first attempt. Now i'm trying to approach it with a different model, ie. vof model, however i dont know how to mark where the water level is. Also i was advised to create two inlets..one for water with a velocity of 4m/s and velocity of 0m/s for air. i defined one outlet. this is because as its a free surface, the water level may fluctuate. And also was advised to define the top surface which would be air to be a pressure outlet with a gauge pressure of 0. But the thing is, a message will appear sayin that you cant have both outlet and pressure outlet in the same analysis. So if anyone out there may be able to give me some hints, no matter how small, i'd be very grateful Thanks
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Old   February 16, 2007, 20:01
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Tom
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Is the submarine surfaced or submerged?

I don't use Fluent, but in most CFD codes you can not have an outlet boundary and a pressure boundary.

Tom
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Old   February 16, 2007, 21:39
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Winston Gregorio
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Yes the submarine fin is submerged. In that case, what do you think i should do in regards to those two boundaries. Do you think i should extend the domain and specify the far right boundary as a wall? I'm thinking that if i do this i can stil have a pressure outlet, but stil have an accurate simulation of the wake and other flow features that maybe present.

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Old   February 20, 2007, 18:50
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Tom
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The reason I asked the question was to try to figure out why one would use a VOF method. I am only familiar with US submarines (spent 8 years aboard one). The fins and planes look somewhat like airplane wings. Because of noise considerations they are designed not to cavitate, so VOF should not be necessary if the submarine is submerged. I would start with a standard single phase analysis (probably with simple inlet and outlet conditions) and do a particle track on the solution. If I had any question of cavitation, then I would repeat the analysis using VOF and an inlet and pressure boundary for an exit. Using the info from the first analysis, you can determine what the outlet should be.

Again, I have not run this analyss and have never used Fluent.

Tom
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Old   February 22, 2007, 06:40
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JM
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In my experience, I have used VOF only for two immissicible phases. I didnt use FLUENT for my application. StarCD was used by someone else with pressure inlet and pressure outlet BCs. I used CFD-ACE with the same conditions. The result was very good. Now, I agree with TOMs opinion, that is when the submarine is submerged why do you take VOF for that. You can easily simulate it as a single phase problem. Coming to cavitation. I have no experience with it. But from the little know how I have. Almost all best CFD programs give you a cavitation possibility including Fluent. But since my problem was not just cavitations in my company's pumps but also bubbles in our liquid metering systems. For which, I am using Fluents ASM approach. ASM - Algebraic Slip Model just because the bubbles due to cavitation in my pumps are good captured and it is much more time efficient.

My advice, as TOM mentioned it already. Start with the simplest case and go towards the difficult. Sometimes you spend more time (in the hope you can save time) by starting directly with the complex case when you dont have the basics clear in your table.
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Old   February 23, 2007, 19:05
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Winston Gregorio
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hi, thanks both to tom and JM, i've taken your suggestions on board, and have finally managed to get some decent results. one question i am going to ask, and it is going to seem really stupid, is what is the convergence criterion? when i start iterating i get a plotted graph of residuals against no. of iterations and i'm having trouble understanding what they mean. you may not reply due to the sheer 'stupidity' of this question. But i have tried lookin it up on the internet and in books. but found nothing. so far from my results, i can talk about areas i found wih high and low pressure, the eddy's and circulation that form. But i'm stil having trouble understanding what the residuals and convergence mean. I'd be really grateful if you give me an answer for this
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Old   February 25, 2007, 12:35
Default Re: VOF model
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Tom
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Winston, again I am not a Fluent user. In STAR-CD I set my redisduals to 1.0e-05 for fluids such as water. Sometimes it converges to that in about 5000 iterations and sometims not. I have the option to select a monitoring cell. I choose that in the flow field and then plot velocities, pressures, and temperatures (if being solved for). I assure that these values have reached a stedy state solution. I then check the mass flow rate in several different areas and assure that they agree.

If you are new to CFD, I would suggest getting the MARNET-CFD Best Practices Guide at:

https://pronet.wsatkins.co.uk/marnet...nes/guide.html

Although written for the Marine industry it is base on the EROFTAC Best Practices, but is free.

Good Luck, Tom
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