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January 20, 2005, 16:43 
Unit conversion from 3d to 2d

#1 
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Hi I have this experimental tank (38 x 9 x 9 cm) that has a circular inlet of 1.2 cm diameter. In my experiments, i have the flow coming in at the rate of 60 mL/minute. Now the problem is  when i have to simulate this in CFD, i need the velocity of flow coming in. So, from the equation Q = V.A, i can get the inlet velocity, but I am simulating this in 2d (38 x 9 cm), where the inlet is represented by a line 1.2 cm in length. What will be the inlet velocity for this case? Will it be the same as i get from the above equation for 3d simulation?


January 21, 2005, 01:12 
Re: Unit conversion from 3d to 2d

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You are assuming that the flow is 2D, there is no variation in the flow properties in the third direction. The velocity must be still taken to be Q/A.


January 21, 2005, 01:51 
Re: Unit conversion from 3d to 2d

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I agree with Praveen. However now Q will be V.L.1 where L is characteristic length (may be diameter). amol


January 21, 2005, 11:54 
Re: Unit conversion from 3d to 2d

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Praveen and Amol Thanks for your responses. But what Praveen said is that we can use the Area from the 3d calculation which will be pi*d*d/4 and Amol says it will be d*1. So, i am a bit confused again.


January 21, 2005, 12:17 
Re: Unit conversion from 3d to 2d

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For simulations : A = d*1 For experiments : A = d*d/4 I am asssuming that in experiments the hole is in xy plane and water is flowing in the z direction and in CFD you are neglecting either x or y.


January 21, 2005, 12:38 
Re: Unit conversion from 3d to 2d

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That depends on what you are interested in. Going from 3D to 2D is a big simplification in your case. If you would like to investigate the local effects of a jet impinging on a free surface you might want to keep the actual velocity. Another option would be to match the filling time, in which case you get a smaller velocity.


January 24, 2005, 05:56 
Re: Unit conversion from 3d to 2d

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Hi, Raj As I read your message first time I noticed that turning to 2D you are losing 3D influences at the coners of the tank. Besides, and it's more important, the crossection of the plume is much smaller than the tank has. 2D planar plume interacts with smaller volume as it does in the reality. I would have calculated the problem with cylindical geometry (not x,z but r,z), substituting outer diameter from 9 to sqrt(9*9/Pi/4). This way you conserve the ratio of volumes and crosssections for coming fluid and fluid inside the tank.


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