Hi all, I am new to CFX. I am trying to model a subsonic-supersonic nozzle. Could u help me on what the flow regime should be? inlet says subsonic. but I am not able to choose any option for the supersonic outlet? is static pressure is sufficient?

Hi,

There is a discussion in the manual about selection of sub/super sonic inlets and outlets. Have a read of that section as it should help you.

Glenn Horrocks

I'm sure you've already done this, but make sure you are solving the energy equation. When I've done ConDi nozzles in the past, the most stable BC combination was fixed velocity inlet and supersonic outlet (I think, it was a while ago). Hope this helps.

hi Gayathri, I think you should set the boundary condition like this: inlet:total pressure outlet:average static pressure

you needn't concern the "supersonic outlet" regards.

Hi Glenn and Jim, thanks.. I did try out the inlet total pressure and the average static pressure..i need to try out the one Glenn mentions. I never could fix a supersonic outlet..I shall try both out. Thanks!

@Jim, I never did try that,, will do thanks!

Depending on the nozzle pressure ratio there could be flow separation and backflow through outlet. So better give "Opening" bc with "static pressure for opening"

1. If you are solving nozzle problem in CFX. You use the subsonic inlet at inlet condition and assign supersonic outflow condition at outlet (I assume that nozzle is with divergent portion also).

2. You need to use energy equation as it is compressible flow analysis.

3. The most important in this problem is good approximation of the flow field. Otherwise u wont get solution. you should look at the outlet flow conditions as per the isentropic relations and assign those values or nearer to the entire flow field. You will get in less than 200 iterations.

All the best

Supersonic outlet bc is apt only if the flow is underexpanded or adapted. What if the flow is overexpanded?

[sivarama1]

i am also simulating convergent and divergent nozzle,
1) In CFX-11,Two region of flows how should analysis actually,take
convergent and divergent nozzle (in convergent is subsonic and
divergent is supersonic)
1)subsonic
2)supersonic
in two region in single model (that means convergent and divergent
nozzle in a single geometry in that in that i will give boundary
condition inlet of convergent pressure 15 bar steam, throat of
pressure 8 bar outlet of 1 bar pressure)
i know inlet velocity of steam 856 m/s this is saturation steam.
How will analysis in CFX-11,as a hypercritical point of correct
results came,but i should verified in CFX-11 SOFTWARE analysis,please
let me know.
once again thankyou for replay.
,i am sending .rar file,please verify it,
in that CFX,and ICEM files.
Actually i done same process,but solution not converging
Geometry:combined of convergent and divergent nozzle.
Converging and diverging nozzle flow problem
My doubt,this model i was created in UNIGRAPHICS modeling
software,meshing ICEM CFD,
when ever i am giving analysis:at that time some problem is there
step2:Boundary condition
Fluid :Steam
mass flow rate:0.5 kg/s
at inlet of convergent nozzle:subsonic condition a)pressure=10
bar;Temperature:356 m/s
at outlet of Divergent nozzle :a)supersonic condition .
You told this type of settings and simulations given by tutorial 8
from ANSYS CFX release 11.0. in this different : inlet is supersonic
condition outlet also supersonic condition
but my point of view inlet is subsonic and outlet is supersonic condition
1)i was increased mesh size
2)That flow direction changed
3)Fluidry steam taken
4)Same boundary conditions
But does not converging(up to 200 iterations converging),what i am
thinking know up to convergent portions was converged.after that it
is not converging,)
convergent nozzle:solution converging .
Divergent nozzle:solution not converging as per velocity plot.and pressure plot.
Actually pressure should drop and velocity should increases.but in
this condition;
1)convergent to throat :velocity increases and pressure droping
2)Throat to Divergent nozzle:Velocity not increasing and pressure not droping.
What is the problem. i am thinking Two solutions(means subsonic and
supersonic condition will not give in single solution) that is correct
or not,please verify that.
Thank you very much.