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

Transonic Far field - any advice

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 16, 2008, 21:34
Default Transonic Far field - any advice
  #1
Iain
Guest
 
Posts: n/a
Hi Guys, I'm trying to incorporate non-reflecting far-field boundary conditions into my problem. I have very limited experience in this matter. I was hoping someone could shed some light on the issue?

I'm solving the low frequency transonic small disturbance equation and need to use non-reflecting bc's at the top and bottom boundaries of my computational domain. I have a reference that introduces some simple conditions; the far-field conditions at the top and bottom of the domain are stated as

phi_y +/- sqrt(B)*phi_x =0

phi is the disturbance potential, subscripts denote differentiation and B is a constant.

My problem is that I'm not sure where I should use the + or - sign at the top or bottom boundary. If anyone has some insight into this problem I'd greatly appreciate some advice. The reference I'm using is very clear on most of the details but seems to miss this key point of which sign to use at top & bottom boundaries.

many thanks, Iain
  Reply With Quote

Old   June 17, 2008, 11:14
Default Solution
  #2
Iain
Guest
 
Posts: n/a
Hi Guys, I found a second reference that clears this matter up.

For those who are interested - the top BC should be phi_y + sqrt(B)*phi_x =0

and the bottom BC is phi_y - sqrt(B)*phi_x =0.

Figured I'd let you guys know in case anyone has similar problems in future. Iain
  Reply With Quote

Old   June 17, 2008, 16:43
Default Re: Solution
  #3
Praveen. C
Guest
 
Posts: n/a
But why ?
  Reply With Quote

Old   June 17, 2008, 18:45
Default Re: Solution
  #4
Iain
Guest
 
Posts: n/a
For the transonic small disturbance equation (low frequency) an oscillating airfoil generates disturbances that reach the far-field boundary.

Non-reflecting conditions stop some portion of these disturbances from being reflected back into the interior of the computational domain where they can degrade the quality of the result. Much of the literature features "perfecly reflecting" conditions - these send all the waves back into the domain.

The equations were originally derived by Engquist for the general case and specifically for the low frequency case by Kwak. Sorry I don't have the actual reference details with me at the moment. The details of the derivations are in the refs.
  Reply With Quote

Old   June 18, 2008, 15:42
Default Re: Solution
  #5
Praveen. C
Guest
 
Posts: n/a
That still does not explain your boundary conditions choice.

If you read some book on gas dynamics, you will find that the small disturbance equation has two characteristics, each with constant slope. One of them goes out of the computational domain at top and the other one goes out from the bottom. Thats how the +/- sign in your boundary conditions is determined.
  Reply With Quote

Old   June 18, 2008, 18:32
Default Re: Solution
  #6
Iain
Guest
 
Posts: n/a
I'm dealing with transonic, low frequency, small disturbance flow. The characteristics in this case are parabolic.

The derivation of the boundary conditions, in terms of characteristics, is given in Kwak's paper. The parabolic characteristics are also derived also in Cook & Cole's "Transonic Aerodynamics"; at the end of chapter 3.

The straight line characteristics would surely be useful for steady flows but the flow I'm dealing with is unsteady.

  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
Moving mesh Niklas Wikstrom (Wikstrom) OpenFOAM Running, Solving & CFD 122 June 15, 2014 06:20
problems after decomposing for running alessio.nz OpenFOAM 5 April 20, 2011 08:44
Turbulence dampening due to magnetic field in LES and RAS eelcovv OpenFOAM 0 June 8, 2010 11:35
Zero size field taranov OpenFOAM Bugs 2 April 20, 2010 04:51
Problem with rhoSimpleFoam matteo_gautero OpenFOAM Running, Solving & CFD 0 February 28, 2008 07:51


All times are GMT -4. The time now is 06:45.