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how to specify the scalar at the outlet?

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Old   May 11, 2010, 11:12
Default how to specify the scalar at the outlet?
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the velocity at the inlet and outlet is known in advance. so the velocity inlet bc is selected at the out, but the other scalar ,such as temperature or concentration, can not be determined ?
how to treat this problem? please give some suggestions.
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Old   May 11, 2010, 15:04
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I think you would be over-defining the problem if you set the velocity at both the inflow and the outflow. How do you know the velocity at the outflow? From an experiment?

If you flow is incompressible, you could use velocity-inlet/outflow boundary conditions, and let FLUENT calculate the conditions at the outflow. Otherwise, use pressure-inlet/pressure-outlet boundary conditions, and specify the pressure difference between the inflow and the outflow boundaries.
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Old   May 12, 2010, 12:31
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from another case, i know the velocity at the outlet. so the velocity inlet is used. but the temperature and concentration at the outlet can not be specified in advance. the outflow and pressure outlet can not be used because these affect the flow field. can you understand? please give me some suggestion. thanks a lot.
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Old   May 12, 2010, 14:52
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If you know the velocity from another simulation, then why can't you use the temperature and concentration from that simulation?
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Old   May 13, 2010, 10:12
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vof model is used. the heat and mass transfer in multiphase is very hard. so i take the liquid phase, and continue to it. thus it is simple. but concentration and temperature at the outlet can not be specified. how can i solve it?
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Old   May 14, 2010, 10:28
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It sounds like you've split your problem up into two smaller problems and are trying to patch them together. Is this correct? I think that if you're going to set the velocity at the outflow, then you need to set the temperature and concentration, which means that you need to solve for them in the other calculation. By setting setting a velocity inlet boundary condition, you're saying that you know what the flow is at that point. If you don't know what the flow is, then you can't set it.

Also, I still don't think you can set two velocity inlet boundary conditions. Could you explain why you are trying to do that?
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Old   May 14, 2010, 11:14
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i solve the multiphase flow, just the flow field. then i take some portion of liquid phase from that case. so i know the inlet and outlet velocity. based on it , i want to continue to calculate the temperature and concentration. but the problem is coming. what is the temperarure and concentration at the outlet? what i need to do is solving it.
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Old   May 14, 2010, 13:31
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What I think is that you need to come up with a new strategy, because I don't think you can do what you are trying to do. If you want to set the velocity at a boundary, then you also need to set the temperature and concentration. You know the velocity, but, since you don't know the concentration and temperature, then you can't set them, meaning that you can't use that boundary condition.

Here's an idea: take your converged single-phase flow, use that as an initial condition for the multiphase problem, and patch the initial condition for the initial volume fraction onto it. See section 23.14.1 under "Solution Strategies for Multiphase Flows".
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Old   May 15, 2010, 11:14
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first i want to simulate the free surface, the heat and mass transfer in liquid-gas flow. so i use the vof model to solve the free surface problem. but the heat and mass transfer problem is very hard. for the liquid phase, the concentration at the interface is greater than the bulk, so the mass transfer happened. and the relative latent heat is released at the interface, so the heat transfer happened too. i spend a lot of time to do it. but is not achieved. so i used another method. take the liquid phase solely to simulate the heat and mass transfer process. the other conditions are solved successfully. but the only unsolved conditon is the concentration and temperature at the outlet.
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