CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Secondary Flow in Square Duct

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   September 4, 2012, 22:55
Default Secondary Flow in Square Duct
  #1
New Member
 
Lee Yin Jen
Join Date: May 2011
Location: Malaysia
Posts: 10
Rep Power: 6
YJ Lee is on a distinguished road
Good day.

I'm running LES in a square duct case to test the effect of various inlet boundary treatment. The first thing I tried is the uniform inlet boundary. However, I can't get the correct secondary flow pattern.

The parameter of the simulation:
Geometry: Square duct, dimension 1.0x0.05x0.05m (aspect ratio 20)
Inlet velocity: 0.2 m/s
Reynolds number: 10,000
Simulation time: 20s (4 through-flow)
Time step size: 1e-4 s
No. of grid cells: ~1 million
Turbulence model: Smagorinsky SGS
Solver: OpenFOAM (pisoFoam)

After 20s simulation time, I tried to view the secondary flow by extracting velocity vectors from the cross section and yields something like this:

MeanVel.png

(actual grid density is higher)

I'm not sure why the velocity vector direction is reverse direction from what I've seen in literature. Does anyone have suggestions? Your help is much appreciated, thank you.
YJ Lee is offline   Reply With Quote

Old   September 5, 2012, 03:58
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,646
Rep Power: 23
FMDenaro will become famous soon enough
first, the inflow you used is not physical and is not congruent to an LES approach...
I suggest to set the case of a periodic inflow/outflow and run until an energy equilibrium is reached. Then you can store a time-sequence of velocity on a plane and use it as inflow for your simulation

second, have you used the dynamic Smagorinsky or the static one? If you use the static one you get probabily too much dissipation

Third, hwo about the clustering of the grid near the wall? are you resolving th BL?
FMDenaro is offline   Reply With Quote

Old   September 5, 2012, 22:16
Default
  #3
New Member
 
Lee Yin Jen
Join Date: May 2011
Location: Malaysia
Posts: 10
Rep Power: 6
YJ Lee is on a distinguished road
Hi Denaro, thanks for taking the time to reply.

I'm using velocity inlet boundary condition because the objective of our study is to determine the performance of several turbulent velocity inlet boundary conditions designed for LES, including uniform velocity, random white noise and other attempts to impose a reasonable amount of fluctuations. If it is the effect of the unphysical inlet as you've explained, perhaps extending the duct length will help?

I'm currently using static Smagorinsky model.

In terms of grid clustering, I did set the grid up such that the ratio of the wall-adjacent grid size to the domain centre grid size is roughly 1:5 ratio. The y+ was reported to be about the value of 4.5 at maximum (for 95% of the duct length, excluding the inlet region). Velocity profile seems reasonable.

Thanks again for your suggestions. I'm planning to re-run the simulation with Dynamic Smagorinsky, extended duct length and slightly increase the grid density and hope it yields something better.
YJ Lee is offline   Reply With Quote

Old   September 6, 2012, 03:43
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,646
Rep Power: 23
FMDenaro will become famous soon enough
Some suggestions:

- use only the dynamic Smagorinsky model;
- the grid must be refined furhter near walls, ensure that 2 - 3 point are within y+=1; 1 milion of cells for Re=10^4 is too coarse. Perhaps you could try at lower Re number..
- Try also to run the simulation without any SGS model (so called LES no-model) to see the effect of the model on the fluctuation
- Control the total kinetic energy in time

As you probably know, many studies were done to specify a suitable way for inlet conditions in LES.
FMDenaro is offline   Reply With Quote

Old   September 6, 2012, 21:39
Default
  #5
New Member
 
Lee Yin Jen
Join Date: May 2011
Location: Malaysia
Posts: 10
Rep Power: 6
YJ Lee is on a distinguished road
Thanks for the pointers.
YJ Lee is offline   Reply With Quote

Old   October 5, 2012, 03:03
Default
  #6
New Member
 
MS Zhang
Join Date: Aug 2011
Posts: 3
Rep Power: 6
Hellen is on a distinguished road
Dear Lee,

Have you got the correct secondary flow pattern?
Recently I am also doing the simulation of the square duct flow based on some anisotropic turbulent models and Reynolds stress turbulent models using OpenFoam. But I got the totally inverse secondary rotating direction to the experimental observation.

If you have found the problems, could you please give me some hint about that.

Thank you in advance.
Hellen is offline   Reply With Quote

Old   October 11, 2012, 01:55
Default
  #7
New Member
 
Lee Yin Jen
Join Date: May 2011
Location: Malaysia
Posts: 10
Rep Power: 6
YJ Lee is on a distinguished road
Hi Hellen,

Sorry to say that I haven't really worked out the problem at this time (I've been putting this matter in the background while I worked on some other things, since I couldn't resolve it). Just to describe some modifications I made and didn't really work, I changed the LES model to dynamic Smagorinsky, almost doubled the number of elements, and tried a periodic boundary condition. So far, still having the same issue.

One thing I suspect is that the flow is still developing and therefore attempts to visualize the secondary flow will be affected by the development of the mean flow. Perhaps the simulation time needs to be longer (unfortunately our workstation is running something else, so I can't really try that).

All the best in your attempts. Do tell me if you succeed!
YJ Lee is offline   Reply With Quote

Old   October 13, 2012, 00:49
Default
  #8
New Member
 
MS Zhang
Join Date: Aug 2011
Posts: 3
Rep Power: 6
Hellen is on a distinguished road
Hi, Lee.

Thank you very much for the reply and the information.
I have not solved this problem either.
These days, I have checked the code of the different turbulent models. It looks no problem on the equations. Besides, I have also tried the longer ducts but met the same problems.
And now I begin to suspect that the processing of the wall boundary conditions at the corners in OpenFoam. But now I do not know how to modify it.

Also DO tell me if you have some progress.

Best regards
Hellen is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
DNS of square duct at Re_tau = 600 sbaffini Main CFD Forum 23 November 19, 2012 18:38
Turbulent Flow in Straight Square Duct - k-ep Validation mcclud STAR-CCM+ 2 August 7, 2012 16:56
Review: Reversed flow CRT FLUENT 0 July 20, 2012 13:03
in flow and out flow in a short duct jane Main CFD Forum 0 March 28, 2004 00:08
Duct flow Omar Mohamud FLUENT 0 December 2, 1999 21:45


All times are GMT -4. The time now is 08:16.