Boundry condition for free flow
 

when fluid comes out of nozzle, how we may modell exit to nozzle when it is in open free atmosphere. what boundry conditions can be applied if fluid is air. within the nozzle itself it is well defined, but when it comes out of nozzle how it may governed? request to help all

You need to model the region after the nozzle. It should be large enough to capture all jet effects You are interested in.

What are you trying to achieve with your model? There are many different approaches depending on what you are looking for.

plaese give me proper details

please provide Your problem description

problem description
 

i want to analyse the folw of air coming out of rectangular nozzle. within the nozzle we may model the model but exit to nozzle how it can be model and boundry conditions can be apply?

You still have not described what you are trying to do. Are you trying to model the jet in the atmosphere? Or get the right flow rate in the duct? Or model mixing? You would take a different approach for each of these models - that is why we are asking for a description of what you are trying to do.

yes, exactly i am tryinhg to analyse air jet in the atmosphere when it comes out from a rectangular nozzle.

Why are you modelling this jet - For what reason? Mixing? Jet penetration? Flow rates?

dear , actually i am interetsed to make the study of velocity profile at certain distance from the exit of nozzle. so its case of jet penetration. your help may be to useful to me.

OK, now I have some idea of what you are trying to do I can make useful suggestions. In future please describe what you are trying to do up-front.

For accurate jet penetration models you will need to have the flow profile in the jet at the exit accurately modelled. So, assuming the jet is fully developed flow, go upstream from the jet far enough so that the simulation can establish fully developed flow. Likewise in the expansion region go downstream far enough so that the entire jet length and good amount more is modelled. How far perpendicular to the jet you need to go will depend on the size of the structures you expect to see in the jet. Some jet breakups generate very large structures so you will need a lot of width to capture them.

All of these dimensions can be established either by a bit of theory (eg boundary layer theory) or a sensitivity analysis.

LES turbulent model and stability of flow
 

Any one can answer LES Model can be used for stability analysis of the flow

LES can model this type of flow, providing the Re number is not too high or too low.

But what do you mean by stability analysis? Stability of the jet? Why is that important to you?