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

Too high mach number...

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 11, 2010, 07:28
Default Too high mach number...
  #1
Senior Member
 
Roland Rakos
Join Date: Mar 2009
Posts: 122
Rep Power: 8
Roland R is on a distinguished road
Hello,

I have a complicated geometry which contains some narrow cross sections. In my tutorial simulation (see below) there is a simple choke. It is a 2D axisymmetrical geometry, the pressure ratio is 3.
The main settings:
- steady state
- total pressure inflow/static pressure outflow
- air ideal gas
- SST turb.modell
- high resolution

The results:


Uploaded with ImageShack.us


Uploaded with ImageShack.us


Uploaded with ImageShack.us


Uploaded with ImageShack.us


Uploaded with ImageShack.us

Based on the convergence history the simulation is converged. I don't understand the cause of the too high Mach number. The geometry doesn't contain Laval tube, this is a simple choke.

I have tried to set the expert parameter/high speed models but the result is unchanged.

Can the material model cause the problem?
...perhabs other ideas?

Thanks in advance;
Roland
Roland R is offline   Reply With Quote

Old   May 11, 2010, 07:58
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,951
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Looks like a standard expansion shock to me. What don't you like about it?

If you want to see an expansion shock in real life have a look at a fighter jet engine exhaust, especially with afterburners on. You can see it clearly then.
ghorrocks is offline   Reply With Quote

Old   May 11, 2010, 10:14
Default
  #3
Senior Member
 
Roland Rakos
Join Date: Mar 2009
Posts: 122
Rep Power: 8
Roland R is on a distinguished road
Hello Glenn,

As far as I know, the Mach number can not be larger than 1. Although a laval nozzle develops from the flow separations after the choke, but Ma=2.5??? In my opinion this is unrealistic...

I have some measurements to the original investigated geometry. The total temperature was measured before and after the choke. The total temp. is in the case of my simulation ~24C (after the choke). This value was ~15C in case of the measurement...

Roland
Roland R is offline   Reply With Quote

Old   May 11, 2010, 18:27
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,951
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Quote:
In my opinion this is unrealistic...
Looks like you need to do some further research into compressible flow then. In the nozzle the flow does not exceed Ma=1. In the expanding region afterwards it can - and in your case it is. Look at expansion shocks in some textbooks.

Quote:
The total temp. is in the case of my simulation ~24C (after the choke). This value was ~15C in case of the measurement...
This is a question about simulation accuracy - have you done the basics and checked mesh sensitivity, convergence tolerance and that the physics is correct (material properties is a key one here, properties vary considerably over this temperature/pressure range)
ghorrocks is offline   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
mesh file for flow over a circular cylinder Ardalan Main CFD Forum 6 April 17, 2010 23:40
mach number and CFX 11.0 solver turbinesv CFX 5 January 5, 2009 06:43
compressible at low Mach number with uniteration ricklee Main CFD Forum 2 October 20, 2005 23:15
How to find Mach Number Tobias FLUENT 2 October 18, 2005 09:59
Difficulties in solving a high Reynolds number Flow? wowakai Main CFD Forum 10 December 29, 1998 14:46


All times are GMT -4. The time now is 15:14.