[joggis]

Hi,

I am currently working on a displacement project. The idea is to displace the oil in a pipe with water. The pipe has a diameter of 0.06 meters and the other specifications can bee seen in the attachment. The largest velocity i will be running is 2.236 m/s.

I was wondering if any of you could offer some expertice when it comes to setting up the mesh parameters.

1) I am using a o-grid in the center of the pipe, which distance do you recommend placing the edges of the o-grid from the center? i am currently using 0.02 meters.

2) How many nodes do you recommend for the pipe wall per meter? i am currently using 100 nodes per meter and 100 nodes for a 90 degree bend.

The problem i am having is that the number of nodes in the mesh i quite large. I would like to minimize it without compromising the accuracy.

Also, i have tried to do some research into recommended y+ value, without finding any good solution. Which value do you recommend?

[shereez234]

Originally Posted by joggis

Hi,

I am currently working on a displacement project. The idea is to displace the oil in a pipe with water. The pipe has a diameter of 0.06 meters and the other specifications can bee seen in the attachment. The largest velocity i will be running is 2.236 m/s.

I was wondering if any of you could offer some expertice when it comes to setting up the mesh parameters.

1) I am using a o-grid in the center of the pipe, which distance do you recommend placing the edges of the o-grid from the center? i am currently using 0.02 meters.

2) How many nodes do you recommend for the pipe wall per meter? i am currently using 100 nodes per meter and 100 nodes for a 90 degree bend.

The problem i am having is that the number of nodes in the mesh i quite large. I would like to minimize it without compromising the accuracy.

Also, i have tried to do some research into recommended y+ value, without finding any good solution. Which value do you recommend?

Hello Joggis;

Welcome to the Forum. I am no expert in flow through pipes. But I can tell you this much:

* Achieve a mesh independent result. Theoretically a mesh independent result is achieved when you increase the number of nodes/elements in the mesh and the results you achieve stop changing. I will give you an example. Lets say in the case of an airfoil mesh independence study. You have

Case 1) A mesh with 20,000 elements for which you obtain result CL = 0.800
Case 2) A mesh with 40,000 elements for which you obtain result CL = 0.850
Case 3) A mesh with 80,000 elements for which you obtain result CL = 0.855
Case 4) A mesh with 160,000 elements and for which you obtain CL = 0.856
So one would say at Case 4) a mesh independent result is obtained.

But in practical CFD since every does not have enough resources or time or for other reasons one could argue that Case 2) presents satisfactory results and case 3) presents good results and both of them can be used as acceptable results or atleast I would argue so. ( Unless you are going to NASA drag workshop or something )

My point is it's a compromise between economizing time, resources and accuracy of the result.

* About your second doubt: Since you are using hexa meshing - there is an option in ICEM CFD called mesh scaling it's in mesh edge parameters. Multiply your dense mesh by 0.9 or something (less than 1.0) so number of nodes on all edges gets reduced consistently/equally giving a more uniform distribution.

* On your doubt about y+; I can't tell you which spacing to use. May be some one else can. But my advice is understand the physics of the flow. If you are going for a y+=1 that means you are resolving the boundary layer and there are formulaes to calculate the wall spacing. you can search google for y+ calculators and inputyour inlet velocity, density .. other factors and get the correct wall spacing.

Apologies for the lengthy post.

Best Regards

Shereez

[joggis]

Originally Posted by shereez234

Hello Joggis;

Welcome to the Forum. I am no expert in flow through pipes. But I can tell you this much:

* Achieve a mesh independent result. Theoretically a mesh independent result is achieved when you increase the number of nodes/elements in the mesh and the results you achieve stop changing. I will give you an example. Lets say in the case of an airfoil mesh independence study. You have

Case 1) A mesh with 20,000 elements for which you obtain result CL = 0.800
Case 2) A mesh with 40,000 elements for which you obtain result CL = 0.850
Case 3) A mesh with 80,000 elements for which you obtain result CL = 0.855
Case 4) A mesh with 160,000 elements and for which you obtain CL = 0.856
So one would say at Case 4) a mesh independent result is obtained.

But in practical CFD since every does not have enough resources or time or for other reasons one could argue that Case 2) presents satisfactory results and case 3) presents good results and both of them can be used as acceptable results or atleast I would argue so. ( Unless you are going to NASA drag workshop or something )

My point is it's a compromise between economizing time, resources and accuracy of the result.

* About your second doubt: Since you are using hexa meshing - there is an option in ICEM CFD called mesh scaling it's in mesh edge parameters. Multiply your dense mesh by 0.9 or something (less than 1.0) so number of nodes on all edges gets reduced consistently/equally giving a more uniform distribution.

* On your doubt about y+; I can't tell you which spacing to use. May be some one else can. But my advice is understand the physics of the flow. If you are going for a y+=1 that means you are resolving the boundary layer and there are formulaes to calculate the wall spacing. you can search google for y+ calculators and inputyour inlet velocity, density .. other factors and get the correct wall spacing.

Apologies for the lengthy post.

Best Regards

Shereez

Thank you for the answer, when you are talking about CL, what do you mean by that?

Any other input on my question would be great.

[shereez234]

Dear Joggis;

Forget about the CL; I was using an aerospace example. CL = Coefficient of Lift force for a wing/airfoil/body.

What i mean is convergence of something you are interested in.

[joggis]

Originally Posted by shereez234

Dear Joggis;

Forget about the CL; I was using an aerospace example. CL = Coefficient of Lift force for a wing/airfoil/body.

What i mean is convergence of something you are interested in.

Ok, that was what I figured, as most of the google results where with aerospace results.

I have done a study, with a 0.5 meter straight pipe, to determine the required number of nodes per meter, from input to output. The results can be seen in the attachment. In my opinion, 250 nodes should be good.

On the subject of y+, should I aim for a value of 1? I am able to achieve that with water, but the value will be much less for water. Any input on this subject would be great, as I am interested in the residual oil volume fraction.

Regards

[shereez234]

Hello Joggis;

It depends on the turbulence model you choose and the decision to use wall treatment or not.

If you use wall treatment I believe, the wall y+ is not = 1 and above ( may be 5 may be 30 to 100).

if you dont use wall treatment you need to ensure y+ = 1.

Also, to save me time I will refer you to:

http://www.cfd-online.com/Forums/mai...lus-value.html

Best regards

Shereez