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-   -   Difference between 2D and 2D axisymetric (http://www.cfd-online.com/Forums/fluent/116056-difference-between-2d-2d-axisymetric.html)

vagaikwa April 12, 2013 00:40

Difference between 2D and 2D axisymetric
 
Hello,
Im solving a flow through circular pipe. I am specifying pressure drop. When I use a axisymetric case, I get the velocity as per the Hagen Poiseuille Equation. But when I try to solve it using a 2D case it gives me velocity exactly twice of the axisymmetric case.
Or simply put the avg. velocity which I am getting after running a 2D case is twice as what I should get from the Hagen Poiseuille Equation.
Where can I possibly go wrong while simulating these two cases? Thanks.

Varad

vagaikwa April 12, 2013 00:45

Or simply put the avg. velocity which I am getting after running a 2D case is twice as what I should get from the Hagen Poiseuille Equation.

ghost82 April 12, 2013 02:19

2d axisymmetric simulates a cilindrical pipe, 2d simulation a square/rectangular duct; what are your boundary conditions?under reference values what is the value for "depth"?

Daniele

vagaikwa April 12, 2013 09:30

Reference value for depth is 1..
pressure inlet and pressure outlet with no slip conditions on the walls..

Far April 14, 2013 02:04

Quote:

Originally Posted by ghost82 (Post 420033)
2d axisymmetric simulates a cilindrical pipe, 2d simulation a square/rectangular duct; what are your boundary conditions?under reference values what is the value for "depth"?

Daniele

I would like to add further. For shapes other than cylinder, rectangle and square you have to model it as quarter or half symmetrical at least (3D).

vagaikwa April 14, 2013 11:06

Thank you guys for ur replies..

ghost82 April 14, 2013 11:19

Quote:

Originally Posted by vagaikwa (Post 420140)
Reference value for depth is 1..
pressure inlet and pressure outlet with no slip conditions on the walls..

This means that when you are simulating a 2d model, you have a rectangular duct which has an inlet/outlet area of 'your coordinate' x 1 m (depth under reference values).

Daniele

stuart23 April 14, 2013 20:14

No, this is incorrect. 2D models are infinitely long in the spanwise direction (Z axis, which you are not modelling). Any geometry that has a finite span, or does not have a perfect cylindrical cross section at all stations along its length must be modeled in 3D.

Stu

ghost82 April 15, 2013 13:09

Quote:

Originally Posted by stuart23 (Post 420535)
No, this is incorrect. 2D models are infinitely long in the spanwise direction (Z axis, which you are not modelling). Any geometry that has a finite span, or does not have a perfect cylindrical cross section at all stations along its length must be modeled in 3D.

Stu

yes you are right, that means that since you are not modeling the z coordinate, you are assuming that interactions occur in the x-y directions.
However I meant that if you are assigning for example an inlet velocity of 1 m/s and you have set a depth of 1 m with the inlet edge 0,2 m long your resulting flowrate will be 0,2x1x1 m3/s.
For sure you are not modeling z interactions.

Daniele


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