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

Boundary Cond. in Supersonic flow

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 9, 2004, 12:51
Default Boundary Cond. in Supersonic flow
  #1
Doctor Blade
Guest
 
Posts: n/a
Hello!

I'm using the conservative formulation programming my own code in Matlab, in order to solve a supersonic flow over a body. I'm using unsteady Euler equations and an aproximation time-dependent. But my question is How can I implement the bundary condition v*n=0 at the body surface?. Remember I'm using conservative formulation.

Thanks.
  Reply With Quote

Old   August 9, 2004, 13:42
Default Re: Boundary Cond. in Supersonic flow
  #2
Takachi
Guest
 
Posts: n/a
Hi!

For supersonic formulation the Euler Equation is not good. Try the N-S formulation with the any turbulence model. This is necessary to a good convergence process. With Euler, in the surface, the velocity is zero, ok! You must attribute this value to all control volume locate in the wall.

Takachi
  Reply With Quote

Old   August 9, 2004, 15:18
Default Re: Boundary Cond. in Supersonic flow
  #3
JF
Guest
 
Posts: n/a
If you haven't unknows on the body surface (like cell-centered formulation), one choice is to impose that the flux through your surface segment reduce to the pressure contribution, i.e.

Flux_normal_wall = [0 p*n_x p*n_y 0]^T

where n_x and n_y are components of the normal to the surface. p is the pressure extrapolated on the surface (with the method of your choice). It works rather well.

Hope this help

JF

  Reply With Quote

Old   August 9, 2004, 19:49
Default Re: Boundary Cond. in Supersonic flow
  #4
Seb
Guest
 
Posts: n/a
Hi "doctor" Blade, before I can answer your question, I need to know the kind of spatial discretization that is being used (finite element with strong or weak formulation, cell centered FVM, vertex based FVM,...).

  Reply With Quote

Old   August 10, 2004, 10:29
Default Re: Boundary Cond. in Supersonic flow
  #5
Doctor Blade
Guest
 
Posts: n/a
Thanks for your answers. JF, I do not understand why flux on the surface must be balanced with pressure.

Hi, Takachi, but I am going to keep the Euler formulation. I'm not an expert in CFD, so I think complete N-S formulation would be more difficult.

Hi, Seb, I'm seeking for solving with a finite difference method. (I think that Mac'Cormack steps in time would be sufficient). I'm using an structured elliptic grid, which external boundary is a circle of 10 times the characteristic body lenght. I'm going to try to explain all of you my method, in order to know your opinion.

i) I would start with some flow solution, rather arbitrarily. I would yield the freestream conditions on the external boundary in all instants of time.

ii) I advance with time steps of Mac'Cormack's algorithm, in order to be able to solve the transonic, subsonic and supersonic field that will be originated. But the problem I have is in the internal boundary condition.

What do you think?, Do you believe I'm going to be alive when I reach the convergence?.
  Reply With Quote

Old   August 10, 2004, 12:57
Default Re: Boundary Cond. in Supersonic flow
  #6
ag
Guest
 
Posts: n/a
If you are using a finite-difference approach then the standard method is to compute the tangential velocity at the point adjacent to the boundary and project that to the boundary. Set the normal component to zero, and then transform those components to Cartesian or whatever coordinate system you are using.
  Reply With Quote

Old   August 10, 2004, 15:28
Default Re: Boundary Cond. in Supersonic flow
  #7
Jarmo Monttinen
Guest
 
Posts: n/a
You should be able to get convergence rather quickly. Depends on how many bugs you will have in the code though...

I would use a "ghost-point" approach on the surface, ie. create an additional point inside the body. Then set the normal velocity (call it V) to zero by setting V(J-1) = -V(J+1) where J is the surface. Works well as long as your grid is sufficiently fine so that the ghost point is as far from the surface as the point inside the flow is. Use a similar approach for other variables, and you can actually solve the equations on the surface as well.

-- Jarmo
  Reply With Quote

Old   August 11, 2004, 20:33
Default Re: Boundary Cond. in Supersonic flow
  #8
Seb
Guest
 
Posts: n/a
Hi Blade, Jarmo and Ag have given good enough info on the good approach on the implementation of the BC.

But in the order to solve your problem and get to convergence, you will also need a descent artificial dissipation scheme (Leer, QUICK,....).
  Reply With Quote

Old   August 15, 2004, 15:06
Default Re: Boundary Cond. in Supersonic flow
  #9
Doctor Blade
Guest
 
Posts: n/a
Talking about this...

Does anybody know some website or paper in which I could see an example of procedure of this kind of simulation?.

I'm not searching for an advanced method, but one simple (e.g. Maccormack's).
  Reply With Quote

Old   August 16, 2004, 02:47
Default Re: Boundary Cond. in Supersonic flow
  #10
Rami
Guest
 
Posts: n/a
Why not using Jameson's FVM, developed in the 80's? It is simple, well docummented and addresses the wall BC you asked about originally. You may find it in most textbooks, or go to the original papers (e.g., A. Jameson, Solution of the Euler Equations by a Multigrid Method, Appl. Math. Comput., vol. 13, pp. 327-356, 1983).
  Reply With Quote

Old   August 16, 2004, 05:10
Default Re: Boundary Cond. in Supersonic flow
  #11
Bart Prast
Guest
 
Posts: n/a
It is about NORMAL velocity=0 (not a problem for Euler). Not TANGENTIAL velocity=0.

Depending on the spatial order of your numerical schema you can use 1 or 2 mirror cells. Worked fine for me
  Reply With Quote

Old   August 16, 2004, 06:12
Default Re: Boundary Cond. in Supersonic flow
  #12
Doctor Blade
Guest
 
Posts: n/a
does FVM mean Finite Volume Method?. If so, I have no idea of FVM at all. I only know something about Finite Differences.

You know, the beginning is a hard time...
  Reply With Quote

Old   August 16, 2004, 08:44
Default Re: Boundary Cond. in Supersonic flow
  #13
Rami
Guest
 
Posts: n/a
FVM indeed stands for Finite Volume Method. It is much simpler than you seem to imagine. I still suggest you to read the references I recommended or other introductory textbooks to CFD (e.g. Patankar or Ferziger & Peric).
  Reply With Quote

Old   August 16, 2004, 16:26
Default Re: Boundary Cond. in Supersonic flow
  #14
Takachi
Guest
 
Posts: n/a
OK!

IŽll send to you the formulae about the wall boundary conditions.

My messenger is: takachi_e0028@hotmail.com

Wait my e-mail.

Regards,

Takachi
  Reply With Quote

Old   August 17, 2004, 10:26
Default Re: Boundary Cond. in Supersonic flow
  #15
Doctor Blade
Guest
 
Posts: n/a
I've sent you an e-mail to both directions, telling you to write me to this other e-mail (see above).

thanks.
  Reply With Quote

Old   September 6, 2004, 07:45
Default Boundary Cond. in Supersonic flow
  #16
amit kumar khatri
Guest
 
Posts: n/a
sir,

i need to know how to compute fluxes for wall boundary conditions,inflow,outflow,far-field boundary. i'm using van leer fvs cell-centred fvm for my scheme.
  Reply With Quote

Old   September 7, 2004, 13:37
Default Re: Boundary Cond. in Supersonic flow
  #17
Doctor Blade
Guest
 
Posts: n/a
Sorry, I've not used never FVM. But If I can help you, you must elaborate your question a little bit.

  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
non reflective boundary conditions for incompresible flow Pascal_doran OpenFOAM Programming & Development 16 August 25, 2015 05:35
Flow Direction: normal to boundary!!! Atit CFX 1 August 2, 2015 13:42
Can 'shock waves' occur in viscous fluid flows? diaw Main CFD Forum 104 February 16, 2006 06:44
Boundary condition for not fully-developed flow John FLUENT 0 July 15, 2005 07:07
Boundary Layer Flow Paradox Wen Long Main CFD Forum 3 September 24, 2002 08:47


All times are GMT -4. The time now is 20:16.