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August 9, 2004, 13:51 
Boundary Cond. in Supersonic flow

#1 
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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 timedependent. 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. 

August 9, 2004, 14:42 
Re: Boundary Cond. in Supersonic flow

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Hi!
For supersonic formulation the Euler Equation is not good. Try the NS 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 

August 9, 2004, 16:18 
Re: Boundary Cond. in Supersonic flow

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If you haven't unknows on the body surface (like cellcentered 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 

August 9, 2004, 20:49 
Re: Boundary Cond. in Supersonic flow

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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,...).


August 10, 2004, 11:29 
Re: Boundary Cond. in Supersonic flow

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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 NS 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?. 

August 10, 2004, 13:57 
Re: Boundary Cond. in Supersonic flow

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If you are using a finitedifference 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.


August 10, 2004, 16:28 
Re: Boundary Cond. in Supersonic flow

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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 "ghostpoint" 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(J1) = 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 

August 11, 2004, 21:33 
Re: Boundary Cond. in Supersonic flow

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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,....). 

August 15, 2004, 16:06 
Re: Boundary Cond. in Supersonic flow

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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). 

August 16, 2004, 03:47 
Re: Boundary Cond. in Supersonic flow

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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. 327356, 1983).


August 16, 2004, 06:10 
Re: Boundary Cond. in Supersonic flow

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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 

August 16, 2004, 07:12 
Re: Boundary Cond. in Supersonic flow

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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... 

August 16, 2004, 09:44 
Re: Boundary Cond. in Supersonic flow

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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).


August 16, 2004, 17:26 
Re: Boundary Cond. in Supersonic flow

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OK!
IŽll send to you the formulae about the wall boundary conditions. My messenger is: takachi_e0028@hotmail.com Wait my email. Regards, Takachi 

August 17, 2004, 11:26 
Re: Boundary Cond. in Supersonic flow

#15 
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I've sent you an email to both directions, telling you to write me to this other email (see above).
thanks. 

September 6, 2004, 08:45 
Boundary Cond. in Supersonic flow

#16 
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sir,
i need to know how to compute fluxes for wall boundary conditions,inflow,outflow,farfield boundary. i'm using van leer fvs cellcentred fvm for my scheme. 

September 7, 2004, 14:37 
Re: Boundary Cond. in Supersonic flow

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Sorry, I've not used never FVM. But If I can help you, you must elaborate your question a little bit.


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