CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Trying to achieve supersonic flow in a pipe

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 28, 2010, 17:29
Default Trying to achieve supersonic flow in a pipe
  #1
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
Hello

I am trying to create supersonic flow in a pipe. I have mass-flow-inlet and pressure outlet at exit. Pressure outlet setting should be ignored by the solver and extrapolated from the interior if the flow is supersonic. I don't have a nozzle at inlet. My Supersonic/Initial pressure setting at inlet (the static inlet pressure) is low enough, and mass flux is high enough, so velocity is supersonic at initialization.

Still, the flow stays subsonic. What is wrong?

Last edited by jpo; May 28, 2010 at 18:02.
jpo is offline   Reply With Quote

Old   May 30, 2010, 12:50
Default
  #2
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
According to Fluent manual, "the flow must be initialized to a supersonic flow or it will simply choke and remain subsonic"

I tried setting my inlet to mass-flow-inlet, pressure-inlet and pressure-far-field. Always did FMG initialization and plotted the velocity field. I could never initialize a supersonic flow, where speed is supposed to be highest at inlet and decreases toward outlet.

Then I tried patching the velocity to supersonic values. Once iterations start, it returns back to subsonic.

I am running out of ideas...
jpo is offline   Reply With Quote

Old   May 30, 2010, 17:44
Default
  #3
tez
New Member
 
Tez
Join Date: Oct 2009
Posts: 12
Rep Power: 17
tez is on a distinguished road
You can keep your exit pressure as much low as your solver allows. That value may not be physically possible, but as you stated, pressure outlet is ignored by solver when it is supersonic flow. You can keep it 1 Pa (abs) or negative gauge pressure at the outlet. If your pipe is uniform, that alone could drive the solution, on the other hand you can patch supersonic values to prevent chocking.
Hope that helps.
tez is offline   Reply With Quote

Old   May 31, 2010, 04:09
Default
  #4
Member
 
ali
Join Date: May 2009
Posts: 58
Rep Power: 17
alikami is on a distinguished road
as earlier suggested , try zero exit pressure.
Regards
Ali
alikami is offline   Reply With Quote

Old   June 1, 2010, 21:09
Default
  #5
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
Thank you; I tried negative/low outlet pressure, but it didn't work.

"A real-life supersonic wind tunnel, for example, will not "start'' if the back pressure is simply lowered to its operating value; the flow will choke at the tunnel throat and will not transition to supersonic. The same holds true for a numerical simulation: the flow must be initialized to a supersonic flow or it will simply choke and remain subsonic." (Fluent manual, Section 26.15)

Negative/low pressure at outlet may produce supersonic flow initially; Yet after sufficient number of iterations the flow converts to subsonic again.
jpo is offline   Reply With Quote

Old   June 2, 2010, 02:54
Default
  #6
Member
 
ali
Join Date: May 2009
Posts: 58
Rep Power: 17
alikami is on a distinguished road
try 2 things

1. give zero ambient to see flow behavior
2. increase mass flow rate at inlet incrementally , and check flow behavior

Regards
Ali
alikami is offline   Reply With Quote

Old   June 2, 2010, 11:57
Default
  #7
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
Dear Ali,

many thanks for your suggestions...

Set ambient to zero, do FMG initialization, run. Increment inlet mass flux, run again.
This is what happens:
Initially, outlet flow is supersonic; as soon as run starts to converge, it drops down to subsonic; because I increase the mass flux gradually, density increases everywhere. This is choking.

Supersonic flow should be very different: highest velocity is at inlet, gradually decreasing towards outlet and possibly becoming sonic (M=1) there, if the pipe is long enough (Fanno flow).
jpo is offline   Reply With Quote

Old   June 2, 2010, 13:48
Default
  #8
Member
 
ali
Join Date: May 2009
Posts: 58
Rep Power: 17
alikami is on a distinguished road
for fanno flow in ur case , input conditions are super sonic....now as flow move along the constt area duct , normal shock will appear at certain length beyond that flow will be subsonic... right ! if your duct length is less than the distance to normal shock flow, M>=1 exist else not possible.check this link alsohttp://en.wikipedia.org/wiki/Fanno_flow#Applicationshope this solvesRegardsAli
alikami is offline   Reply With Quote

Old   June 2, 2010, 13:50
Default
  #9
Member
 
ali
Join Date: May 2009
Posts: 58
Rep Power: 17
alikami is on a distinguished road
to add , if you increase inlet pressure till the location of normal shock is further the duct length you will have flow with M>=1...
try increase inlet pressure
alikami is offline   Reply With Quote

Old   June 2, 2010, 15:50
Default
  #10
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
The position of the normal shock should depend on inlet conditions and pipe length. For fixed inlet conditions there exist L* such that if pipe length L > L*, normal shock will occur around the pipe exit. Keeping everything the same, if L increases, the position of the shock will move towards inlet and eventually, supersonic conditions become impossible.

All this reasoning confirms your arguments. I read more about it here:
canteach.candu.org/library/20040207.pdf

You are suggesting that increasing the inlet pressure should "move" the cutoff length L* further from inlet. Why is that? Can you elaborate more? Appreciate your thoughts.
jpo is offline   Reply With Quote

Old   June 3, 2010, 04:46
Default
  #11
Member
 
ali
Join Date: May 2009
Posts: 58
Rep Power: 17
alikami is on a distinguished road
http://www.potto.org/gasDynamics/node152.php
link above gives a good description

I hope it works for you.
Regards
Ali
alikami is offline   Reply With Quote

Old   June 3, 2010, 10:01
Default
  #12
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
Ali, thank you very much for the link. It is a very nice and detailed treatment of supersonic flows in a pipe.

So far, I used "no slip" walls for my pipe in Fluent. Now if I set "zero shear stress" on all walls, it means the pipe length factor 4fL/D will become zero, because f = wall_friction_coefficient/rho*v^2

Then I should be able to see supersonic flow no matter how long the pipe is.
I will try this and see how it works.
jpo is offline   Reply With Quote

Old   June 3, 2010, 18:30
Default mixture in T
  #13
New Member
 
constantinos kast
Join Date: May 2010
Posts: 5
Rep Power: 16
kkostas1982 is on a distinguished road
I would like to make a mixture of two identical liquid (water) with different densities from two different entrances, which will lead to a common output.
it could be done in laminar flow? would help too if you receive even a simple instruction. I'm new to the program and try to find a solution to this problem.
kkostas1982 is offline   Reply With Quote

Old   June 3, 2010, 18:56
Default
  #14
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
Ali,

yes, it works now that I have "perfectly smooth" walls. The link you posted is a GNU-licensed free textbook and is really helpful.


Now I can continue and work with "no slip" walls and examine things in detail
jpo is offline   Reply With Quote

Old   June 3, 2010, 18:59
Default
  #15
jpo
Member
 
Join Date: Apr 2009
Posts: 94
Rep Power: 17
jpo is on a distinguished road
kkostas1982,

I guess it depends what is your Reynolds number.

Please read
http://en.wikipedia.org/wiki/Reynolds_number

And also the thread
http://www.cfd-online.com/Forums/flu...-shedding.html
looking specifically at "laminar", "turbulent" and "Reynolds number" there.
jpo is offline   Reply With Quote

Old   October 26, 2015, 09:03
Default Supersonic inlet becomes subsonic
  #16
New Member
 
Jenny Majeed
Join Date: Oct 2015
Posts: 2
Rep Power: 0
Jenny_1994 is on a distinguished road
Hello..!

I'm trying to simulate in FLUENT, a very complex flow through a supersonic inlet, involving multiple oblique shocks and one normal shock near outlet. I had given mass-flow inlet conditions and pressure-outlet conditions. I'm able to get the required Pressure temperature results at the outlet, but after a few iterations, the inlet became subsonic, with a feeble velocity when compared to a high inlet mach number that i had assigned.
I have tried segregated solving, zero shear-stress etc.still the inlet flow is subsonic. please advice me.

Thank you..!
Jenny_1994 is offline   Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
supersonic flow Steffen FLUENT 6 October 26, 2015 14:06
Reverse Flow at Rotating Pipe Outlet vismech STAR-CCM+ 1 August 11, 2009 11:38
About Turbulence Intensity (Pipe flow assimilated) gRomK13 Main CFD Forum 1 July 10, 2009 04:11
LES of highly heated pipe flow - incorrect results MichalH FLUENT 3 June 3, 2009 06:00
stepped pipe flow Tom Cloutier Main CFD Forum 0 April 20, 2003 14:19


All times are GMT -4. The time now is 07:25.