CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Choked Noozle Simulation

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   February 21, 2006, 11:17
Default Choked Noozle Simulation
  #1
CFX Guy
Guest
 
Posts: n/a
I'm trying to solve a simple nozzle flow using CFX 10 and am having trouble with convergence!!. I'm sure its all due to the boundary conditions I'm using. Please could someone one give me some advice on the most robust boundary conditions for this problem. I've tried inlet velocity specified with an outlet static pressure specified, also inlet velocity specified and a supersonic outlet. The flow is definitely choked. The speeds are high so the steady state time step is very small 1e-07, hence a transient run would probably take weeks do solve. I've tried solving a subsonic case with specified inlet velocity and outlet static pressure and this solves fine for steady state. Any tips guys?
  Reply With Quote

Old   February 21, 2006, 19:21
Default Re: Choked Noozle Simulation
  #2
Name
Guest
 
Posts: n/a
If the flow coming up to the throat is accelerating from subsonic to supersonic after the throat then Total pressure inlet/static pressure outlet should work fine.

You might have to play around with under-relaxation factors to attain convergence. Ask your support rep about how to relax gradients.

  Reply With Quote

Old   February 22, 2006, 04:47
Default Re: Choked Noozle Simulation
  #3
CFX Guy
Guest
 
Posts: n/a
Cheers for the advice. I was playing last night with total inlet pressure and static outlet and having a bit of success. I think to get these residuals down I'll definitely need to adjust under-relaxation factors.
  Reply With Quote

Old   February 22, 2006, 15:00
Default Re: Choked Noozle Simulation
  #4
Robin
Guest
 
Posts: n/a
Hi CFX Guy,

If it is choked and you know the total conditions at the inlet, apply total pressure and total temperature at the inlet. At the outlet a static pressure lower than the critical pressure will give you choked flow, but you might have some funny behavior right at the outlet if the flow accelerates above Mach 1 through the throat. If that is the case, you can change the outlet to supersonic.

To "Name". There are no relaxation factors to meddle with in CFX, playing with the timestep is the most you would have to do. For this case, it should behave quite well, so setting your timestep to the residence time will suffice. A good check for the timestep is to plot streamlines in Post on a backup file and look at the average or maximum time on the streamlines.

Regards, Robin
  Reply With Quote

Old   February 24, 2006, 10:01
Default Re: Choked Noozle Simulation
  #5
Bart Prast
Guest
 
Posts: n/a
In case of supersonic flow we always use

EXPERT PARAMETERS:

max continuity loops = 2 END

works every time!

  Reply With Quote

Old   February 24, 2006, 12:24
Default Re: Choked Noozle Simulation
  #6
Name
Guest
 
Posts: n/a
Robin, generally I agree with you. With Ptotal inlet/Pstatic outlet you should only need to tweak the timestep a bit to get a converged answer.

However, there are under relaxation factors which can be tinkered with if needed to stablise things (not that this is always needed, only occasionally on very specific cases). For example, I've used these to help get through tough spots, especially on something like a converging/diverging nozzle with a shock, in the past:

ADVECTION SCHEME:
Option = High Resolution
Gradient Relaxation = 0.1
Blend Factor Relaxation = 0.1
END
  Reply With Quote

Old   February 27, 2006, 16:56
Default Re: Choked Noozle Simulation
  #7
CFX Guy
Guest
 
Posts: n/a
Cheers guys i've managed to solve the nozzle case and capture the shocks. I didn't actually change the max cont loops to 2. Does changing this speed up to solution?
  Reply With Quote

Old   February 28, 2006, 12:23
Default Re: Choked Noozle Simulation
  #8
Pete
Guest
 
Posts: n/a
Am i right is saying that setting fixed inlet velocity for a choked flow case is not advisable? Since setting total pressure allows both the density, velocity and static pressure to change at the inlet as the solution develops.

  Reply With Quote

Old   March 1, 2006, 06:41
Default Re: Choked Noozle Simulation
  #9
Bart Prast
Guest
 
Posts: n/a
Just try it and compare convergence plots (and CFPU time). Shouldn't be a very long simulation.
  Reply With Quote

Reply

Thread Tools
Display Modes

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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Why my simulation not agree with the wind tunnel experiment zhaowei CFX 4 July 11, 2015 03:36
Simulation of a complex wing in solidworks flow simulation niels1900 FloEFD, FloWorks & FloTHERM 6 April 20, 2011 10:44
GUI crash and simulation engine still running RPJones FLOW-3D 2 November 9, 2010 09:18
velocity profile export from a simulation onto another sudhirlv STAR-CCM+ 1 September 12, 2010 18:57
FSI TWO-WAY SIMULATION Smagmon CFX 1 March 6, 2009 14:24


All times are GMT -4. The time now is 03:42.