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March 7, 2008, 06:28 |
Outlet BC
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#1 |
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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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March 11, 2008, 09:55 |
Re: Outlet BC
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#2 |
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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. Regards, Ganesh |
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March 11, 2008, 10:23 |
Re: Outlet BC
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#3 |
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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, http://citeseer.ist.psu.edu/622061.html
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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