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November 15, 2008, 03:27 
how to set the final velcoity at the end of time

#1 
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I have a question concerned with transient analysis. If the value of velocity at the initial & final time is given in fluent the initial velocity can be set easily but setting the final velocity is unavailable ? can someone guide me how I can set it please?
Many thanks Nicolas Hewitt 

November 15, 2008, 06:53 
Re: how to set the final velcoity at the end of ti

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You can't because it will be an ill posed problem. The final velocity is your solution not a b.c.


November 15, 2008, 09:12 
Re: how to set the final velcoity at the end of ti

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thanks to the reply. Actaully I want to solve laplace equation for temperature and stream function so why setting like this bc not available in fluent?


November 15, 2008, 11:35 
Re: how to set the final velcoity at the end of ti

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When you solve unsteady problems the "time direction" always is hyperbolic, that is some kind of information (your initial condition) can only be propagated forward in time. Figure it out...how could the future influence the past (besides of general relativity issues)?
In fact, if you look at the time derivative stencil of all the time integration methods, they never makes use of future informations, only past (explicit methods) and current ones (implicit methods). The classic Laplace equation only has space derivatives and is a steady problem. The problem has an elliptic nature and you must give b.c.s an all your boundaries. This is quite different from trying to set the final solution as a b.c. which is always an illposed problem. Fluent has the "capability" (but it is not a capability, not at all) to let you impose almost every kind of b.c., also the wrong ones. Only one condition is always illposed, fixing your final solution in unsteady problems. Maybe there's some misunderstanding because my english is not so good. What is, exactly, the problem you're trying to solve with fluent? 

November 15, 2008, 13:59 
Re: how to set the final velcoity at the end of ti

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Paolo first thing I want to express my thankful for answering my request. Actually I have two phase (gasliquid) stratified flow within a pipe due to acceleration the water level is reduced I need exactly the water level decreases at a time nearest to the experiments to compare the velocity of gas in this case?
Thanks a gain 

November 15, 2008, 21:05 
Re: how to set the final velcoity at the end of ti

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You're welcome, but you don't have to thank me...also because i don't understand so much of your problem. If i'm not wrong, you have a sort of tank or whatever in which the level of water, starting from an initial value, is progressively reduced, for some reason, until a specific time is reached and you want to compare your gas phase velocity field at this time with some experimental data. Also you need the level of water at this specific time. Is it correct?
I'm pratically without any knowledge of multiphase simulation in fluent so i can't be of so much help in the specific task. But, you just have to integrate in time your equations until the specific time T is reached. This is just n time steps of lenght dt = T/n. The choice of dt comes from accuracy and stability requirements so n follows directly. Once this time is reached you just compare the actual results with the reference ones and the work is done. Everything you need is an initial condition and boundary conditions for all your variables. Hope this helps 

November 16, 2008, 05:29 
Re: how to set the final velcoity at the end of ti

#7 
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thanks a gain but how i can integrate with time my equations are nonlinear i cannot do this so?


November 16, 2008, 06:18 
Re: how to set the final velcoity at the end of ti

#8 
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For "integrate in time" i mean let fluent solve this job for you, that is solving the unsteady equations


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