CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

LES of Jet

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

LinkBack Thread Tools Display Modes
Old   February 28, 2002, 10:08
Default LES of Jet
Posts: n/a
Dear all,

I would like to calculate a turbulent jet using LES. I have problems at the outflow. The BCD are as follow:

ENTRAINMENT |---------------------------------| | | | I |O ||N |U ||L |T ||E |L | T |E | |T |---------------------------------|


at the Inlet (nozzle): tanh-velocity profile Entrainment region : v- or w velocity

(for entrainment mass flow) outlet : convective bcd or

extrapolated bcd Model : Smagor.

The domain has a size of 18D x 10D x 10D where D is the size of the nozzle.

The problem: At the outlet the pressure decreases and subsequently the velocity increases and finaly the solution "runs away".

Has anybody calculated such a problem? Where are my mistakes. Any suggestions/remarks are very appreciated.


  Reply With Quote

Old   March 1, 2002, 13:49
Default Re: LES of Jet
Posts: n/a

How do you specify v- and/or w-velocities at the entrainment boundaries. What you really need are some sort of unsteady entrainment BCs which are hard to prescribe. The scaling laws for turbulent jets would only provide steady state estimates of entrainment.

Some have suggested the computation of entrainment velocities by extrapolating from the inside of the computational domain. Such an approach would not be mathematically sound.

Could you settle for a confined jet simulation instead of a free jet simulation. That is perhaps the easiest way of simulating jets. Also the computational domain needs to be extended far more than 18D. Since no convective outflow condition is perfectly non-reflecting (unless you use parabolized NS solvers or fully upwinded schemes), the domain of interest needs to be as far from the outflow condition as possible. It would not hurt to have a sponge region that would hinder (if not fully prevent) the noise created at the outflow boundary from propagating upstream.

Here are a few more observations from my brief experience with LES of jets. I had noticed that the effects of outflow boundary are minimal if I used a 5th order upwind-biased scheme instead of a second order scheme. Though, a recent AIAA paper by Mittal and Moin suggests the use of 2nd order central scheme over a 5th order upwind biased scheme, it was Rai and Moin who suggested earlier that the 5th order upwind-biased scheme was more preferable. Also, much is being made of the so-called kinetic energy preserving schemes in LES which are necessarily central. I have not seen a single one of these extended for variable density flows or for use on general grids. So the answer to the question : which scheme is better depends on the problem and for jets upwind-biasing helps. Since the truncation error is 5th order, I would suspect that the numerical dissipation would be much smaller than the dissipation resulting from the viscous and subgrid stresses (which usually tend to be second order in grid spacing). Hope this helps.
  Reply With Quote

Old   March 6, 2002, 03:09
Default Re: LES of Jet
Posts: n/a
Dear Kalyan

thanks for the answer, currently I am trying to use convectiveinflow BCD.

Of course a confined jet simulation is much more easier to model. But I'd like to simulate this problem.

You mentioned a sponge layer at the outflow, I have read about this in a paper of Freud, but without details. Can you please refer to a paper where it is described in more detail or can you say sth. about the implementation.

Thanks a lot

  Reply With Quote


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
Low Mach number Compressible jet flow using LES ankgupta8um OpenFOAM Running, Solving & CFD 7 January 15, 2011 14:38
LES of a confined impinging jet reactor.. vishwa OpenFOAM Running, Solving & CFD 8 December 7, 2010 19:11
LES of jet injection giving pulsations nsalpeter Main CFD Forum 3 October 2, 2009 19:12
LES of a jet tsjb00 OpenFOAM Running, Solving & CFD 0 March 29, 2006 20:03
LES of jet in cross flow dirk FLUENT 0 April 25, 2005 08:39

All times are GMT -4. The time now is 11:57.