Learning Forum

[snr1]

Hi sir, i am trying to simulate the 3D turbulent pipe flow. But i am not getting the values as mentioned in the existing literature/Cornell 2D turbulent flow. Can you help me how to do the complete analysis and post processing.

[Keyur Kanade]

Hi, 

Can you please describe your issue in detail? Insert some images. 

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Regards,

Keyur

[snr1]

 

The images are attached above. As we can see, the center line velocity has to increase from starting point to ending point. But here the trend is opposite. And nature of curve is also not smooth. Its wavy nature. How to get accurate center line profile? i am using 2 equation model with enhanced wall treatment . Geometry dia 0.2m ,length 8m and Re of 10000. Can anyone help me?

[DrAmine]

The curve will depend on your mesh topology. Try to use a line with 1000 samples. Moreover have you heard about max. velocity overshoot? Please look for that.

By the way go through the BPG shared by Keyur and add more detailed screenshot where one can judge the quality of the grid.

[seeta gunti]

Adding to Keyur and Amine, You are modeling turbulent flow in a pipe with two equation models and plotting the center line velocity along x-axis right! If the flow is fully turbulent, your velocity magnitude at the centerline should be more or less uniform since at the center, flow is fully developed. You can check the velocity profiles in a pipe ( search in google) If you are modeling the developing region, then it will increase from starting and reach to certain mean velocity and remains constant. 

 

 

If you are plotting the boundary layer velocity, it will increase from starting to ending. Please check the reference paper what they are referring.

Thanks,

Seeta

[Keyur Kanade]

Please cross check all inputs as given in tutorial. 

Make sure you have a good mesh before proceeding to solver. 

Make sure that solution is converged. 

Regards,

Keyur

[snr1]

Its 3D shape, not 2D. So we can't use line meshing. what max. velocity overshoot? BPG? the grid was attached below. I have used inflation near walls to capture effectively.

[Keyur Kanade]

Looking at mesh, it looks like you have created very fine boundary layer prisms. 

With this you may need to use k-e realizable enhanced wall treatment for turbulence. 

Please check the mesh settings given in tutorial. Then use appropriate turbulence model. 

Regards,

Keyur

[seeta gunti]

By seeing the mesh image, As keyur mentioned, you have used very fine BL but the aspect ratio is looks higher. Can you refine near the center of the pipe . Since it is a simple pipe, you can create a nice hex mesh.

Regards,

Seeta

[mcoderon]

Hi, 

 

As Seeta suggested you can generate a hex mesh. You could use the "sweep method" for that since you have a simple pipe (See the Meshing User Guide). 

Check that your mass balance is verified at the inlet and at the outlet. If not your solution may not be converged, if that so you may want to double check your boundary conditions. (Do you expect a negative velocity?)

Since you have a refinement for the inflation layer follow Keyur's suggestion about using a different turbulence model. The k-omega SST is a valid option too. However make sure that your grid satisfies the condition of having a y+ = 1. 

Best Regards, 

Marco

[snr1]

Dear Marco, how to create hex mesh? and how to use sweep method? how to calculate y+ value? as because i am new to fluent. Please help me. Greatly appreciated for your help

[Keyur Kanade]

Please have look at 3 part video for hex meshing. 

[snr1]

The problem is that, in post processingi have drawn lines at different location for finding the  velocity profile. For example, at outlet cross section, i have drawn line as shown in below.

As we can see, the line is crossing the diameter of circle. Though, its original diameter is 0.2 m. That we can check from the scale, that as shwon in below fig.

Why the line drawn in post processing is crossing the circle, though i have drawn  with actual dimension like diameter 0.2 m. Also i am not getting any plot date on this line. Its showing empty plot. How to resovle this issue?

[DrAmine]

Choose the option cut at first to see it the cut will be detected. Other option will be through intersection of plane with a boundary.

[snr1]

thank you Amine, can you suggest me the some papers regarding turbulent flow through 3D pipe flow?

 Materials regarding turbulent models and some modifications in that models

Let me know the possible meshing options for 3D cylindrical pipe

 

Thank you

[DrAmine]

It would be meaningless if one create a non structures O-grid mesh for a simple geometry like a pipe. Go for that. Regarding papers and books start looking in Pope' s turbulent flow book and BPG from Florian Manger.

[snr1]

Dear amine, i am doing meshing in fluent itself. I am not doing in ICEM.

[DrAmine]

Then native poly with inflation layers: Yplus= 1 / 15-20 Cells in the Boundary layer for Low-Re approach.

[snr1]

Dear amine, what you are trying to say? i didnt understand.

[DrAmine]

1/Yplus =1-5

2/15-20 Cells in the boundary layers

 

in order to have a high quality Low-Re mesh.

[Ayodele]

Hello everyone,

I am studying air flow through a rectangular duct. I wish to confirm if I can use ratio of 1.2 for my mesh independence study to generate the fine and finer meshes from the initial coarse mesh.

Thanks a ton for your responses.

Ay.

[Keyur Kanade]

Please use 1.1 to 1.2 ratio. It would be ok. 

[Ayodele]

Thank you kkanade.

I also observed that my Continuity and epsilon solutions aren't converging below e-03. I just attempted to run the simulation longer (about 1800 iterations) but not dropping to e-04 still. Any suggestions?

Thanks I advance.

Ay.

[DrAmine]

Screenshot of solution methods please.

[snr1]

How to find the heat transfer coefficient in fluent 18.1?

[DrAmine]

Fluent has a good documentation where you might find almost everything within a couple of trials.

Under Wall Fluxes you can find the HTC's: Surface HTC and Wall Func. HTC.

[Ayodele]

Dear Abenhadj,

Many thanks for your response. Below is a screen shot of my solution method.

I await your response.

Thanks a ton!

 

[DrAmine]

Run double precision, coupled with pseudo-transient option and you might change Pressre interpolation to PRESTO! or Body-Force-Weighted (not really relevant)

[Ayodele]

Hello Abenhadj,

Thanks for your response. Kindly confirm I can use the pseudo-transient option for steady flow simulation. Not sure what the pseudo-transient mean?

Secondly, do you mean there is no need to change Pressure interpolation to PRESTO! or Body-Force-Weighted?

Thank you for your time.

Ay. 

 

 

[snr1]

But fluent is giving wrong values. How to find exactly?

[DrAmine]

This is not efficient anymore. Please create your own tgread with your question.

For coupled steaty state solver there is an option in the solution page named pseudo time stepping. The pressure interpolation is not s must at first and please start going through all standard tutorials to get familiar with the software you are using.