hello all. i am a little confused and wondered if someone can point me in the right direction.

1) firstly, is the following statement true.... image a turbulent jet flow. if we over predict the dissipation of turbulent kinetic energy, then there will be less enegy and motion in the eddies to mix the jet. therefore if we DO over predict the dissipation of turbulent kinetic energy the modelled jet will mix and spread less. are these statements true???

2) secondly, based on the first points, if in solving the navier stokes equations we use say an upwind scheme, i.e. a dissiaptive scheme, what effects does this have on the same jet simulation. i would have thought the dissipation would again damp fluctuations and therefore cause less mixing? but also the upwind scheme will 'smear' the solution on the mesh and therefore 'apparently show more mixing'. what are the effects of the upwind scheme on a spreading jet and how do these effects differ from the ones discussed in the first point i made.

any comments would be great as i am (as you may tell) a little confused.

Hi again Paul,

Why don't you try simulating a number of cases with different assumptions, & then testing the sensitivity of the solutions to your assumptions. I would imagine that you could then benchmark some of these results against experimental data. I'm sure that you would then be in reasonable position to judge which way to go & what 'exactly' is happening.

diaw...

Hi there,

In order to shed some light , you might be interested in ready the following paper:

R. Mittal & P. Moin (1997). Suitability of Upwind Biased Schemes for Large-Eddy Simulation of Turbulent Flows. AIAA J. Vol. 35, No. 8, 1415-1417

It is available to download on R. Mittal's website: http://project.seas.gwu.edu/~fsagmae...A_Apr_1997.pdf

Hope this helps. Sincerely,

Frederic Felten, Ph.D

Hello,

the link http://project.seas.gwu.edu/~fsagmae...A_Apr_1997.pdf and http://project.seas.gwu.edu or http://www.project.seas.gwu.edu don't work.

1) You are right here. If you predict more dissipation the jet will mix and spread less. The dissipation in most of the models is strictly solenoidal dissipation i.e. dissipation due to cascading of eddies. The good news for you is most of the models will predict turbulent dissipation less, so in most probability your jet spread rate will be predicted higher. (less dissipation more mixing). As you jet speed (Mach no. to be exact) increases you may need to add more and more dissipation in the models to come near to physical reality. Zeman & Sarkar are the two well known models to add such dissipation to epsilon equation.

2) Not able to comment with my present level of knowledge

Did I add to the confusion

Jitendra