[vagaikwa]

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]

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]

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]

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

[Far]

Originally Posted by ghost82

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]

Thank you guys for ur replies..

[ghost82]

Originally Posted by vagaikwa

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]

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]

Originally Posted by stuart23

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