why this boundary condition works?
 

I have read manual that when we apply mass flow or velocity at inlet and pressure at outlet then it will more robust.but when we apply pressure at inlet and velocity at outlet it will diverge the solution.The later is applicable only to compressible flow.Can anybody tell me why this is happening? the outlet mass flow is applicable to fully developed flow.so what is fully developed flow? and why it is robust for outer mass flow? Thanx

Re: why this boundary condition works?
 

From basic PDE classification compressible flow equations are hyperbolic in nature and requires upwind differencing schemes for solution ie property(velocity etc) at any point in the domain is determined by the points upstream of the flow. Hence when you fix velocity at the exit you will be fixing the property of a downstream point which is to determined from the upstream points to model the physics of the problem correctly. Remember in starcd we use pressure correction schemes hence pressure field is determined from the velocity field. A fully developed flow is one in which flow profile does not change with distance. I hope this might help.

DB

Re: why this boundary condition works?
 

Thax DB.If mass flow remains same then can we say that it is a fully developed flow or not?The flow profile does not change with dist,so what u want to say?Can u tell me?

Re: why this boundary condition works?
 

Please look this site for more detail

http://instruct1.cit.cornell.edu/courses/fluent/pipe1/

For flow inside a pipe Laminar Re<2000 Transition 2000<Re<4000 Turbulent Re>4000

Re: why this boundary condition works?
 

Both conditions should work almost the same, if not, you should ask a new version.