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June 11, 2017, 14:28 
wall, pressure outlet or outflow ??

#1 
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dilaw meda
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hello every one , i am modeling an air jet in a room, and i get confused how to set the boundary conditions because i have convergence problems, well the room is 2.5 widh, 3m lenght, and 2m heigh , the blower of the air jet is placed in ceiling. what are your suggestions for the ceiling , the floor and the walls of the room? initial velocity of the jet is 8m/s ; velociti near to the walls and the floor is about 0.1 m/s or less


June 12, 2017, 03:28 

#2 
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For walls, one uses the wall BC. For an outlet one uses usually pressure Outlet BC.
"Convergence issues" is not specific enough problem. Also it can depend on different things like mesh, time step, BC's... So a more detailed description of your setup is needed to help. 

June 12, 2017, 08:28 

#3  
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June 12, 2017, 08:46 

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In numerical fluid dynamics you need to have a timestep because the continously coupled partial differential equations need to be disrcetized in order to be solved iteratively. If you look at the NS equations you will have a variable t (=time). This needs to be defined as discrete time intervals > the time step. What you see now in your simulations is that when you have a too large time step it can be a) that you simply need more subiterations per time step to reach the defined convergence level or be b) your solution diverges. For a) i suggest you to increase the number of subiterations in the solution window. You will reach the same level of convergence as with a lower time step, it simply takes longer. For b) it is necessary that you decrease your time step, because the Courant Criterium is not satisfied. But this seems not be the case here for you. So for a), ideally you want to find the transient time step, that you reach desired level of convergence in each time step in about 510 subiterations or so.


June 12, 2017, 08:53 

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June 12, 2017, 09:04 

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As long as you think it needs to be representative to your problem.
I can speak for my simulations: I let them run at least 2 theoretical residence times. Meaning the domain volume needs to be changed 2 times by inlet flow rate. BTW: Are you doing single or multiphase? Have you tried steady state solver already? 

June 12, 2017, 09:11 

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June 12, 2017, 09:20 

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Depends on your case. If residuals ocillate around, it usually indicates steady state solver has problems finding the solution and something transient happens in your model, so it's better modeled as transient.
But i would not rely on what Fluent says if its converged or not. You decide if it's converged or not. Also, please don't open several threads for the same problem. I'm assuming that you have different simulations all the time.. 

June 12, 2017, 10:24 

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June 12, 2017, 10:50 

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Everything is OK, when you reached convergence for the values of interest!!!
The Residuals are not the values of interest! Values of interest are velocities, pressure etc. If these values do not change within a certain tolerance, your solution is converged! Not if the residuals are 10e4, 10e8...or whatever. Residuals are at best an indicator that your simulation is iterating stable. So if you reached true convergence (see above what that means), then you compare to experiment. Then you do a mesh independence study. Then you do a time step independence study. And then you are finished and can make certain statements about the quality of your CFD model. Everything before that is just playing around... Sorry. 

June 12, 2017, 10:59 

#11  
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