2D vs 3D

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 January 28, 2005, 16:42 2D vs 3D #1 Riham Guest   Posts: n/a Hi all, Please can anyone tell me if velocity value or flow rate value have to be changed in velocity inlet boundary condition when making 2D simulations? I calculated my inlet flow velocity for 3D model, and I do not know if I have to keep the same value for the 2D model because instead of circle opening in 3D model I will have only a straight line. Do I have to consider the same flow rate for both models and then calculate velocity values for 2D model? (incompressible fluid) Or I can just keep same velocity value for both models? Thank you so much in advance I am new in this field

 January 31, 2005, 12:19 Re: 2D vs 3D #2 swarup Guest   Posts: n/a Hi Riham, a 2D cylinder will not look like a rectangle but will look like a circle. a 2D axisymmetric cylinder will be a rectangle and as such you will have an edge instead of a circle for inlet. you need not change your velocity in 2D axisymmetric case and should use the one you used in 3D case. you can check this for yourself by simulating a simple pipe flow in 3D and in 2D axisymmetry. you can check inlet mass flow and should find that it remains same in both the cases provided you use same velocity in 3D and 2D axisymmetric case. also, a pure 2D problem for cylinder will use same velocity as 3D problem. this may solve your problem. Swarup.

 January 31, 2005, 13:30 Re: 2D vs 3D #3 Riham Guest   Posts: n/a Hi Swarup, Thank you so much for your concern. Actually, I am simulating ventilation in rectangular box, it has a circle opening where ventilation flow comes in, this is my 3D model. In a 2D model, I am trying to simulate a plane cutting in the half of the box. So, ventilation opening is an edge as you said. Is your answer still valid? Have a nice day Riham

 February 1, 2005, 11:04 Re: 2D vs 3D #4 swarup Guest   Posts: n/a Yes. i think your geometry somewhat resembles a bottle (a rectangular one!) with small stopper. since the circular opening becomes an edge in your 2D model, you should solve it as a 2D axisymmetric problem where circulation may or may not be present and the solution will be pseudo 2D or 2.5D. if my guess is correct, this is what you are aiming at. i should say however that by introducing the plane, you are simply introducing symmetry in your 3D model and that does not necessarily mean that the flow problem is automatically axisymmetric. as i said, a 3D problem can be solved as 2D axisymmetric with or without swirl motion in FLUENT. Swarup.

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