CFX Simulation Physics

12366636 · April 9, 2016, 07:30

Hey,

This is my first post, I am just wondering if anyone could give me a hand understanding the physics behind my model.

The album can be seen here:

http://imgur.com/a/9l1Qw

Air moving at 104 m/s at 300K is coming from the right in the mainstream.

Air moving at roughly 0.5 m/s and 150K is moving up through the bottom inlet.

This creates a cooling film to the left after the hole in the mainstream.

Could anyone explain some of the physics such as shear stress, bouyancy, eddys? and how they effect this fluid flow.

I haven't really done any aerodynamics before so I am at a bit of a loss.

Thank you in advance,

Eddie.

ghorrocks · April 9, 2016, 07:54

There is going to be shear on the walls due to the fluid velocity, but also shear between the fast and slow fluid. This shear will help draw the slow/cold fluid out.

Buoyancy is unlikely to be of significance here are the flow velocities are high.

Eddys: There can be large scale eddies, particularly along the interface of the fast and slow gases (Kelvin Helmholtz instability: https://en.wikipedia.org/wiki/Kelvin...tz_instability). You will also get small turbulent eddies as well, but that is probably handled by your turbulence model.

12366636 · April 9, 2016, 07:59

Hey ghorrocks,

Thank you so much for your reply, that is very helpful!

Are there any other flow physics that you think would significantly effect this model?

Thanks,

Eddie.

12366636 · April 9, 2016, 08:00

Hey ghorrocks,

Thank you so much for your reply, that is very helpful!

Are there any other flow physics that you think would significantly effect this model?

Thanks,

Eddie.

ghorrocks · April 9, 2016, 08:07

The velocities are high enough (Mach number = around 0.3) that compressibility effects are likely to be important. So you will need to do a compressible flow for an accurate simulation.

12366636 · April 9, 2016, 08:15

Brilliant,
Is there anything else I should take into account such as the turbulence of the flow? How the cold fluid is seperating from the surface? Or anything else for that matter.

Sorry if I'm asking stupid questions!

Thanks again,

Eddie.

ghorrocks · April 10, 2016, 07:05

You don't have to include any physics to model separations. You just need an accurate model.

12366636 · April 10, 2016, 07:09

Okay, that's good to know.

I guess the sepration is just from the turbulence in the mainstream flow is it?
I am just interested to know what physics would effect this model, even in an experimental situation.

Thank you, you have been very helpful.

Eddie.

ghorrocks · April 10, 2016, 08:10

If there is a separation it could be due to quite a few factors. It could be the momentum of the side jet, it could be a laminar separation bubble from turbulence transition, it could be a buoyancy effect and that is just a few, with a bit of thought I am sure we could come up with more.

12366636 · April 10, 2016, 09:05

These are all things I will have to look into.

Do you know anything about entrainment? Looking through some papers on similar problems it is something I have come across but don't really understand.

Thanks for spending your time helping a beginner!

Eddie.

ghorrocks · April 10, 2016, 19:30

Entrainment is simply the flow of one fluid inducing a flow in another fluid. In simple cases this occurs due to viscous action and/or the pressure field (ie the Navier Stokes equations).

April 9, 2016, 07:30	CFX Simulation Physics	#1
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	Hey, This is my first post, I am just wondering if anyone could give me a hand understanding the physics behind my model. The album can be seen here: http://imgur.com/a/9l1Qw Air moving at 104 m/s at 300K is coming from the right in the mainstream. Air moving at roughly 0.5 m/s and 150K is moving up through the bottom inlet. This creates a cooling film to the left after the hole in the mainstream. Could anyone explain some of the physics such as shear stress, bouyancy, eddys? and how they effect this fluid flow. I haven't really done any aerodynamics before so I am at a bit of a loss. Thank you in advance, Eddie.

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April 9, 2016, 07:54		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	There is going to be shear on the walls due to the fluid velocity, but also shear between the fast and slow fluid. This shear will help draw the slow/cold fluid out. Buoyancy is unlikely to be of significance here are the flow velocities are high. Eddys: There can be large scale eddies, particularly along the interface of the fast and slow gases (Kelvin Helmholtz instability: https://en.wikipedia.org/wiki/Kelvin...tz_instability). You will also get small turbulent eddies as well, but that is probably handled by your turbulence model.

April 9, 2016, 07:59		#3
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	Hey ghorrocks, Thank you so much for your reply, that is very helpful! Are there any other flow physics that you think would significantly effect this model? Thanks, Eddie.

April 9, 2016, 08:00		#4
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	Hey ghorrocks, Thank you so much for your reply, that is very helpful! Are there any other flow physics that you think would significantly effect this model? Thanks, Eddie.

April 9, 2016, 08:07		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	The velocities are high enough (Mach number = around 0.3) that compressibility effects are likely to be important. So you will need to do a compressible flow for an accurate simulation.

April 9, 2016, 08:15		#6
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	Brilliant, Is there anything else I should take into account such as the turbulence of the flow? How the cold fluid is seperating from the surface? Or anything else for that matter. Sorry if I'm asking stupid questions! Thanks again, Eddie.

April 10, 2016, 07:05		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You don't have to include any physics to model separations. You just need an accurate model.

April 10, 2016, 07:09		#8
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	Okay, that's good to know. I guess the sepration is just from the turbulence in the mainstream flow is it? I am just interested to know what physics would effect this model, even in an experimental situation. Thank you, you have been very helpful. Eddie.

April 10, 2016, 08:10		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	If there is a separation it could be due to quite a few factors. It could be the momentum of the side jet, it could be a laminar separation bubble from turbulence transition, it could be a buoyancy effect and that is just a few, with a bit of thought I am sure we could come up with more.

April 10, 2016, 09:05		#10
12366636 New Member Eddie Join Date: Mar 2016 Location: Great Britain Posts: 10 Rep Power: 10	These are all things I will have to look into. Do you know anything about entrainment? Looking through some papers on similar problems it is something I have come across but don't really understand. Thanks for spending your time helping a beginner! Eddie.

April 10, 2016, 19:30		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Entrainment is simply the flow of one fluid inducing a flow in another fluid. In simple cases this occurs due to viscous action and/or the pressure field (ie the Navier Stokes equations).