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-   -   Unit conversion from 3-d to 2-d (http://www.cfd-online.com/Forums/main/8578-unit-conversion-3-d-2-d.html)

 Raj January 20, 2005 16:43

Unit conversion from 3-d to 2-d

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 2-d (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 3-d simulation?

 Praveen C January 21, 2005 01:12

Re: Unit conversion from 3-d to 2-d

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

 amol palekar January 21, 2005 01:51

Re: Unit conversion from 3-d to 2-d

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

 Raj January 21, 2005 11:54

Re: Unit conversion from 3-d to 2-d

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

 amol palekar January 21, 2005 12:17

Re: Unit conversion from 3-d to 2-d

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

 O. January 21, 2005 12:38

Re: Unit conversion from 3-d to 2-d

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.

 Alexander Starostin January 24, 2005 05:56

Re: Unit conversion from 3-d to 2-d

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 cross-sections for coming fluid and fluid inside the tank.

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