Flow regime: subsonic or supersonic

ingdade · May 1, 2016, 07:11

Hi,

I am trying to model a water jet in air. The velocity of the water in the inlet is about 700 m/s. I'd like to choose supersonic as flow regime in the Boundary Details, but there is only the subsonic as option. I have read in the manual that I have to employ the Total Energy model, but unfortunately I don't find how to employ it.

Could anyone help me to solve this problem?

urosgrivc · May 1, 2016, 09:16

Isnt this a subsonic flow? as speed of sound in water is 1500 m/s at 30°C.
And than for air you just set up air ideal gas, and use total energy model, and probably LES tur. model, an than wery small timesteps

I wonder what opinions experts will have on this topic, I am looking forward to hearing from them.

I do think that rate of shear will be extremly high where water will enter the air domain, and i think that this is quite hard to model accurately.

ghorrocks · May 1, 2016, 09:46

Yes, this is going to be a difficult model. urosrivc's comments are correct - an incompressible model for the water and compressible for the air is probably required.

What are you trying to learn from doing the model?

What do you expect the water jet to do? Does it say coherent or does it break up?

ingdade · May 1, 2016, 10:55

I'd like to see how much the width of the water jet increases along its axis. I've defined as Buoyancy Ref. Density 1.2 kg m^-3. Is it correct?

ghorrocks · May 1, 2016, 11:10

In other words you are trying to model the initial breakup of the jet. Something like this: https://www.youtube.com/watch?v=9XjEX929nK4

This model involves supersonic flow, free surfaces, surface tension and LES just to name a few. Just one of those physical models makes for a tricky simulation. Having all 4 in one simulation means you will need to be a CFD expert to have any hope of completing this model in anything less than a year.

There are experimental results in the literature which show jet width versus distance. Why not just use that data?

May 1, 2016, 07:11	Flow regime: subsonic or supersonic	#1
ingdade New Member Davide Ferrario Join Date: May 2016 Posts: 2 Rep Power: 0	Hi, I am trying to model a water jet in air. The velocity of the water in the inlet is about 700 m/s. I'd like to choose supersonic as flow regime in the Boundary Details, but there is only the subsonic as option. I have read in the manual that I have to employ the Total Energy model, but unfortunately I don't find how to employ it. Could anyone help me to solve this problem?

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May 1, 2016, 09:16		#2
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	Isnt this a subsonic flow? as speed of sound in water is 1500 m/s at 30°C. And than for air you just set up air ideal gas, and use total energy model, and probably LES tur. model, an than wery small timesteps I wonder what opinions experts will have on this topic, I am looking forward to hearing from them. I do think that rate of shear will be extremly high where water will enter the air domain, and i think that this is quite hard to model accurately.

May 1, 2016, 09:46		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	Yes, this is going to be a difficult model. urosrivc's comments are correct - an incompressible model for the water and compressible for the air is probably required. What are you trying to learn from doing the model? What do you expect the water jet to do? Does it say coherent or does it break up?

May 1, 2016, 10:55		#4
ingdade New Member Davide Ferrario Join Date: May 2016 Posts: 2 Rep Power: 0	I'd like to see how much the width of the water jet increases along its axis. I've defined as Buoyancy Ref. Density 1.2 kg m^-3. Is it correct?

May 1, 2016, 11:10		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	In other words you are trying to model the initial breakup of the jet. Something like this: https://www.youtube.com/watch?v=9XjEX929nK4 This model involves supersonic flow, free surfaces, surface tension and LES just to name a few. Just one of those physical models makes for a tricky simulation. Having all 4 in one simulation means you will need to be a CFD expert to have any hope of completing this model in anything less than a year. There are experimental results in the literature which show jet width versus distance. Why not just use that data?