Analytical and solver results not matching

Akash C · January 7, 2013, 10:48

Hi,
I modeled flow through a straight pipe and compared results for pressure drop for various Reynold's numbers with analytical ones(obtained from Darcy Weisbach equation using suitable formula for friction factor as per Re number). Results for the turbulent flow matched for various Re and wall roughness values. But for the laminar flow the solver shows double the pressure drop obtained by analytical results for various Re.

Could anyone possibly suggest a reason for the same.
Thanks!

msaeedsadeghi · January 8, 2013, 00:04

You should remesh the geometry for laminar flow

Akash C · January 8, 2013, 08:58

By remeshing how fine do you suggest the mesh be? Can you suggest some quality parameter e.g skewness, warp, etc.

Because I tried remeshing it, but the pipe being 10 cm diameter and 1 m long, it resulted in a very large cell count leading to each iteration needing around 30-40 sec and i thought such a simple case shouldn't take so much time hence discontinued it.

Marion · January 8, 2013, 09:04

Hi Akash,

For a simple case like this, maybe 2D-axisymmetric could be enough?

Marion.

Akash C · January 8, 2013, 10:08

Hello Marion,
You are right no doubt, but I am trying to find out why it's not happening for the 3D model. Even for the 3D model I have pipe cut in half with symmetry as one of the boundary condition.

What's puzzling me is that turbulent results matched to a tee while the results for laminar are varying by constant multiple.

Marion · January 9, 2013, 01:52

Hi,

With 2D you can use more elements and have a much better mesh quality.
If you make a detailed 2D model first and still get wrong results then there is an issue in your modelling. If the results are good, then you can worry about 3D.

Marion.

January 7, 2013, 10:48	Analytical and solver results not matching	#1
Akash C Member Akash Chaudhari Join Date: Dec 2012 Location: Pune, India Posts: 31 Rep Power: 13	Hi, I modeled flow through a straight pipe and compared results for pressure drop for various Reynold's numbers with analytical ones(obtained from Darcy Weisbach equation using suitable formula for friction factor as per Re number). Results for the turbulent flow matched for various Re and wall roughness values. But for the laminar flow the solver shows double the pressure drop obtained by analytical results for various Re. Could anyone possibly suggest a reason for the same. Thanks!

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January 8, 2013, 00:04		#2
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	You should remesh the geometry for laminar flow

January 8, 2013, 08:58		#3
Akash C Member Akash Chaudhari Join Date: Dec 2012 Location: Pune, India Posts: 31 Rep Power: 13	By remeshing how fine do you suggest the mesh be? Can you suggest some quality parameter e.g skewness, warp, etc. Because I tried remeshing it, but the pipe being 10 cm diameter and 1 m long, it resulted in a very large cell count leading to each iteration needing around 30-40 sec and i thought such a simple case shouldn't take so much time hence discontinued it.

January 8, 2013, 09:04		#4
Marion Senior Member Marion Join Date: Jul 2012 Location: France Posts: 122 Rep Power: 14	Hi Akash, For a simple case like this, maybe 2D-axisymmetric could be enough? Marion.

January 8, 2013, 10:08		#5
Akash C Member Akash Chaudhari Join Date: Dec 2012 Location: Pune, India Posts: 31 Rep Power: 13	Hello Marion, You are right no doubt, but I am trying to find out why it's not happening for the 3D model. Even for the 3D model I have pipe cut in half with symmetry as one of the boundary condition. What's puzzling me is that turbulent results matched to a tee while the results for laminar are varying by constant multiple.

January 9, 2013, 01:52		#6
Marion Senior Member Marion Join Date: Jul 2012 Location: France Posts: 122 Rep Power: 14	Hi, With 2D you can use more elements and have a much better mesh quality. If you make a detailed 2D model first and still get wrong results then there is an issue in your modelling. If the results are good, then you can worry about 3D. Marion.