[adilsyyed]

My setup consist of a CD nozzle going into a tank. The flow at the tip of the CD nozzle is supersonic; my question is

The flow at the inlet (inlet of CD nozzle too) is subsonic but it reaches supersonic after passing through the CD nozzle, Should I use subsonic boundary condition at inlet or supersonic?

[Antanas]

Originally Posted by adilsyyed

My setup consist of a CD nozzle going into a tank. The flow at the tip of the CD nozzle is supersonic; my question is

The flow at the inlet (inlet of CD nozzle too) is subsonic but it reaches supersonic after passing through the CD nozzle, Should I use subsonic boundary condition at inlet or supersonic?

Strange question. If inlet is subsonic then it's subsonic.

[adilsyyed]

Originally Posted by Antanas

Strange question. If inlet is subsonic then it's subsonic.

Thank you for reassurance. I have been using it as subsonic but I read somewhere that you have to enable compressible flow in CFX and that got me worried.

[ghorrocks]

Even stranger question. You will need to enable the compressible flow to get any of these effects as they are compressible effects. Your question was about the inlet boundary condition - the flow is subsonic there so use a subsonic inlet boundary.

[adilsyyed]

Originally Posted by ghorrocks

Even stranger question. You will need to enable the compressible flow to get any of these effects as they are compressible effects. Your question was about the inlet boundary condition - the flow is subsonic there so use a subsonic inlet boundary.

I am using "H2Ol as water, and H2O as steam" from inter-mass transfer material list.

I am using "Total energy" as heat transfer model.

Do I have to worry about anything else to enable compressible flow??

[ghorrocks]

CFX will do a compressible flow simulation if a fluid material properties model with a variable density is selected and if Total energy is selected as the heat transfer model.

So make sure you have not selected the constant properties water materials. At least the vapour phase needs to have a variable density.

[beyonder1]

Originally Posted by adilsyyed

My setup consist of a CD nozzle going into a tank. The flow at the tip of the CD nozzle is supersonic; my question is

The flow at the inlet (inlet of CD nozzle too) is subsonic but it reaches supersonic after passing through the CD nozzle, Should I use subsonic boundary condition at inlet or supersonic?

I also have a similar issue. My geometry includes a convergent nozzle with surrounding after the exit. The nozzle inlet is subsonic and final ambient outlet is also subsonic but at the nozzle exit, it reached supersonic flow. This region is not part of boundary.
My question is : will my solution get affected if I have a supersonic flow even though BCs are subsonic.

[ghorrocks]

If the supersonic region is entirely inside the simulation domain then your use of subsonic boundaries is OK. The supersonic boundary options are only required when the flow is supersonic as it crosses the boundary.

[beyonder1]

The boundary of ambient inlet aligns with the exit of nozzle. Should I stretch back the ambient domain(before nozzle exit) or its fine.
The problem is that when i use the ambient inlet as 'inlet'(v=1m/s), solution seems ok. But for an opening BC(relative pressure=0 pa), the solution is wrong. I am not able to figure out the problem why opening BC is not working.
I have uploaded domain for your reference.

[ghorrocks]

Please show where the inlet and ambient domains are, and where you propose stretching.

Also please show what other boundary conditions are present.

[beyonder1]

I was talking about stretching Ambient Inlet towards Nozzle Inlet.

[beyonder1]

I need advice on one thing. The objective of my simulation is to get the radiation intensity variation at different locations normal to nozzle exit on a plane. I am modelling components of air(multi-species).
I am currently using temperature and species concentration at a location to get radiation intensity. My question is: do i need to use a radiation model for this simulation. I am not experienced with this feature.

[ghorrocks]

Yes, you should move the ambient boundary in line with the nozzle further back. The flow will be complex near the nozzle and you don't want to put a boundary through that. Move it back to where the flow is simpler (this is a general rule for placement of boundary conditions at all times, by the way).

Can you explain why you are doing this a multi-species model? Also explain why this is a radiation model - what is hot and why the radiation at the planes is important.

[beyonder1]

To answer your questions:
1. Multispecies: To calculate radiation intensity of separate species(eg. CO2,N2,O2)
2. Flow has high temperature
3. I am not sure how using radiation model and calculating radiation intensity is related. That is my doubt. When do we use a radiation model?

To my knowledge, we need only temperature, species concentration and some coefficients to get radiation intensity. Correct me if I am wrong.

[ghorrocks]

Can you explain what you are trying to model? What results you intend to get from the simulation and why you are doing this. What do you wish to learn from the model?

I think I need to know some basic information about your case before commenting further.