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Old   March 7, 2008, 06:28
Default Outlet BC
Praveen. C
Posts: n/a
In some flow situations like viscous flow over a cylinder, the outlet BC in FVM is taken as a zero-normal-gradient condition i.e.,

d(rho)/dn = 0

d(vel)/dn = 0

dp/dn = 0

First of all is this correct ? Secondly how do you implement this in practice in a 3d unstructured grid finite volume formulation ? I would be interested to know if there are differences for cell-centered and node-centered schemes.
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Old   March 11, 2008, 09:55
Default Re: Outlet BC
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Dear Praveen,

1. The conditions that you have mentioned are all extrapolation bc's which are not correct simply because they do not look into whether the flow is subsonic/supersonic. A full extrapolation of all variables is correct only when the flow is supersonic, although with a genuinely large farfield radii, the above mentioned BCs could work. However, I would suggest that a Reimann invariant or solver based NRBC would be the more ideal choice. The farfield normally being free of viscous activity, the BC would be the same for Euler/NS cases. This NRBC is capable of handling both subsonic/supersonic flows.

2. If you had to implement the BCs mentioned in a 3D framework, you would have to use a suitable generalised gradient finding strategy. One possibility would be to use LS/GG to find the x-- and y-- gradients and then compute the normal derivatives using these and the unit normals nx and ny as,

d(rho)/dn = d(rho)/dx *nx + d(rho)/dy * ny and so on ...

3. As far as node- and cell- centered schemes are concerned, I do not see any major differences, except in the construction of a dual volume being necessary for the node-centered approach.

Hope this helps.



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Old   March 11, 2008, 10:23
Default Re: Outlet BC
Praveen. C
Posts: n/a
I am currently using a Reimann bc. But it does not allow the flow to pass through the outlet without perturbing it. Well, I have put the outlet bc at 20*diameter which is probably too close. But I would prefer not to enlarge the domain. The flow conditions are M=0.1 and Re=40. Reimann bc is not very appropriate since the flow is strongly parabolic due to the wake. There are some sophisticated methods but I have not tried them, see for example,

I was hoping there was some extrapolation-type bc which could be implemented within the FVM framework, but maybe its not possible.

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