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

Turbulent Round Jet

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 13, 2009, 11:07
Default Turbulent Round Jet
  #1
New Member
 
Andrew
Join Date: May 2009
Posts: 5
Rep Power: 8
Obnates is on a distinguished road
I'm fairly new to FLUENT and turbulence modeling, so I need some help on a problem.

I'm trying to model a turbulent jet with Re ~ 2*10^4.

I am having problems getting my solution to properly converge, i.e. the centerline velocity is not linearly decaying like I might expect.

My computational domain is as follows:
North side: pressure outlet, 0 gauge pressure
East side: pressure outlet, 0 gauge pressure
South side: Axis
West side: top region is a pressure outlet, 0 gauge pressure; bottom region is a velocity inlet with V = 10 m/s

There is a jet wall separating the top and bottom regions (viewed from the west).

I'm currently using the realizeable k-e with enhanced wall treatment; my grid is more fine near the wall and coarse out in free space.

I may be approaching this entirely wrong -- what would proper boundary conditions be, and which turbulence model would work best?

Thanks.
Obnates is offline   Reply With Quote

Old   May 13, 2009, 11:48
Default
  #2
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
Hi!

your BC do look right....

Check your yplus value on the wall... For enhanced wall treatment, I think, it should be between 1 and 5.

How do you determine the convergence of your solution? Do you take the residuals with standard convergence check?
Turn off that criteria - the standard values are set not low enough. And lowering the values by yourself might result in to low values (that cannot be reached....

Use an alternative criteria: a surface monitor of the (area averaged) axial (or radial) velocity on your axis (or on a point in your flow field). If that value does not change any more, your solution is (more or less) convergent!

Another point: have you set the problem to 2D axis symmetrical?

What is the distance between the jet and the boundaries? It should be something like 10 jet diameter perpendicular to the jet and more than 20 jet diameter in flow direction of the jet.

Best wishes
Ralf
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 13, 2009, 11:54
Default
  #3
New Member
 
Andrew
Join Date: May 2009
Posts: 5
Rep Power: 8
Obnates is on a distinguished road
The y+ value on the wall is indeed <5.

I check convergence by a) monitoring the residuals with standard convergence check, b) monitoring the mass flow rate out of the free surfaces and c) monitoring the average velocity along the jet centerline.

I have set the problem to 2D axisymmetrical.

The distance between the velocity inlet and the jet face is 20 jet diameters.
The distance between the jet face and the east boundary is 30 jet diameters.
The distance between the jet wall and the north boundary is 5 jet diameters.
Obnates is offline   Reply With Quote

Old   May 13, 2009, 11:59
Default
  #4
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
mhh.. ok.. that looks very well...

I do not really get your geometry... can you upload a picture of it?(including the grid)

And another idea: what are your reference values?
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 13, 2009, 12:06
Default
  #5
New Member
 
Andrew
Join Date: May 2009
Posts: 5
Rep Power: 8
Obnates is on a distinguished road
A picture (not to scale) of my domain is shown.

I don't have a picture of the grid currently handy -- I could try to get one sometime later.

By "reference values" do you mean operating conditions?
Attached Images
File Type: jpg domain.JPG (7.6 KB, 45 views)
Obnates is offline   Reply With Quote

Old   May 13, 2009, 13:35
Default
  #6
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
so... that is what I thought about...
you distance between the jet wall and the north boundary might be a little low...

The reference values are given in -> report-> reference values.
It is a good idea, to compute them from the velocity inlet. They are used to calculate volume flow or heat transfer...

Have you scaled your grid?

can you give a plot of the axial velocity on the axis? What is the problem with these values?

Do you have a temperature change in your domain? What kind of material properties are you using?
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 13, 2009, 13:47
Default
  #7
New Member
 
Andrew
Join Date: May 2009
Posts: 5
Rep Power: 8
Obnates is on a distinguished road
I considering making the distance between the jet and the north boundary 10 diameters, but someone I know suggested starting at 5 diameters and that he had success with that in the past.

What reference value in particular would be of interest in this case?

I have not scaled my grid -- how would I scale it, and what advantage does that have?

A plot of the axial velocity is attached -- from -0.3048, you can see the velocity increase as the flow goes down the pipe. Then, shortly after 0 (the jet exit), you can see the velocity decreases slightly (the potential core). Then it decays rapidly, then decays slowly. I expect to see (from self-similarity) the end of the potential core, and then a slow, constant (linear) decay.

There is no temperature change or heat transfer. The pipe is aluminum and the fluid is air into air.
Attached Images
File Type: jpg centerline_velocity.JPG (17.4 KB, 27 views)
Obnates is offline   Reply With Quote

Old   May 14, 2009, 04:18
Default
  #8
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
Hi!

ok.. the reference values are not really interesting her... I grant... it was just an idea... The operating pressure should be atmosphere pressure unless your jet is in a vacuum...

The grid scaling is very important!! You create your grid in gambit WITHOUT any units. You just have dimensionless distances.

Fluent does import the grid WITHOUT any units as well. So, in Fluent you have to assign units to the grid (-> grid-> scale). The standard in Fluent is meters. So if you have created your model in mm, there will be a factor of 1000 to your geometry.

At the moment, I have no further idea, what is going wrong with your simulation....
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 14, 2009, 09:55
Default
  #9
New Member
 
Andrew
Join Date: May 2009
Posts: 5
Rep Power: 8
Obnates is on a distinguished road
Re: grid scaling -- I created the geometry in meters, so would I still need to scale the grid?

I'm going to try to create a new grid with 10 diameters between the jet wall and the north boundary to see if maybe that makes a difference.

Thanks for all your help.
Obnates 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
CFD simulation of round turbulent jet Sheeko Main CFD Forum 4 June 18, 2013 20:53
Round jet model in FLUENT PurdueME Main CFD Forum 2 April 8, 2009 16:41
Modelling a turbulent jet and k-epsilon constants Ant CD-adapco 3 January 24, 2005 16:56
Turbulent Intensity good or bad for a jet Christian Main CFD Forum 0 November 19, 2003 06:47
Transient behaviour of a round jet Clifford Arnold Main CFD Forum 15 November 10, 1998 17:47


All times are GMT -4. The time now is 15:56.