CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

CFL limits for separated flow

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

LinkBack Thread Tools Display Modes
Old   August 7, 2002, 21:54
Default CFL limits for separated flow
Posts: n/a

I'm working on a finite difference method for aerospace applications. This is a 2-D (actually 3-D, but think 2-D for this problem), explicit, centred-difference method, that solves the compressible RANS plus eddy viscosity turbulence models (SA, SST). I use a 5-stage, 2-nd order Runge-Kutta for time stepping, plus implicit residual smoothing and multigrid. The limiting CFL number for this RK is 2*SQRT(2), but the residual smoothing allows for CFL = 4 for some of my aplications.

The flows I'm interested in have Mach numbers of M=0.9 to 3.0 and Reynolds number of about 9 million to 30 million. The rocket geometry I work at is something like this monster:


/ \junction_____ --> air speed ( _____________|<


(obviously not so rough) =D

The problem I'm facing is that, for some conditions, the flow separates in the junction of the payload fairing and the lower-diameter cylinder, and this separation forms a little bubble there. My code, however, blows at it. I would like to know which possible reasons could be generating it? Some options:

1. High CFL number? It is currently CFL = 4. Is it to high for a Runge-Kutta scheme with separated flow? 2. The multigrid? Does it handle well separated flows? 3. Should I throw it all away and go to the beach?

Thanks in advance, Biga
  Reply With Quote

Old   August 8, 2002, 10:57
Default Re: CFL limits for separated flow
Li Yang
Posts: n/a

I have got the impression that the central difference scheme can only be used for a Mach number up to 1.1 or 1.2. Above that it fails.

  Reply With Quote

Old   August 8, 2002, 11:01
Default Re: CFL limits for separated flow
Posts: n/a
Thanks for your reply.

I don't think so! I have very good results for Mach=4, it depends on the quality of your artificial dissipation scheme.
  Reply With Quote

Old   August 11, 2002, 07:22
Default Re: CFL limits for separated flow
Li Yang
Posts: n/a
It is very interesting to know that central difference scheme can perform well at Mach=4. How is the shock position has been predicted in your calculation ? Does the artificial viscosity need to be specially specified for such a high Mach number ? I thought that only upwind schemes such as Roe's scheme can capture the shock position precisely for strong shock problem.

I would suggest you to use a smaller CFL number to start with. To my knowledge, even for an implicit scheme, a solver may not work with a large CFL number for a specific flow.


  Reply With Quote


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
benchmark: flow over a circular cylinder goodegg Main CFD Forum 12 January 22, 2013 12:47
Fully Developed Flow in Star-cd SMM STAR-CD 0 September 5, 2011 22:08
Flow meter Design CD adapco Group Marketing Siemens 3 June 21, 2011 08:33
reversed flow at velocity inlet / mass flow inlet ib FLUENT 1 March 26, 2007 13:11
Can 'shock waves' occur in viscous fluid flows? diaw Main CFD Forum 104 February 16, 2006 06:44

All times are GMT -4. The time now is 01:25.