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Old   March 14, 2002, 22:32
Default Wall boundary condition
Enda Bigarelli
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I'am having a problem when simulating laminar viscous flows about rocket-like configurations. This is about the boundary condition, I guess...

For a 3-D application, the mesh has 160x80x21 points in the longitudinal, normal and azimuthal directions. Wall BC's are extrapolation of pressure and internal energy. Velocity components are zero, of course. The grid is stretched near the wall.

What happens is that with about 5000 iterations (that is just a few iterations for my code) the pressure field is already converged. However, the boundary layer presents a very oscillatory profile and take too many iterations to straight up. The same happens with the temperature. What could be wrong? The BC?

I'have seen the same problem happening with other comercial codes, like Fluent and CFD++. I have talked to Chakravarthy once and he told me about this problem of contact surface that builds up near walls at inviscid simulations due to wrong BC's. However, to correct it, very sofisticated BC's should be used but these are prohibiive when regarding computational costs. Could the same goes to laminar viscous simulations using extrapolation???

Thanks in advance for your help.

Best regards, Enda Bigarelli
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Old   March 15, 2002, 00:13
Default contravariant velocities
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Contravariant velocities are normally used in the pressure based finite volume methods (staggered and colocated grids). This looks more appropriate especially while applying boundary conditions like inlet, exit, wall, symmetry and periodic for irregular geometries.

Generally contravariant velocities are not (mentioned) used in density based finite volume methods. Why?. For irregular geometries, is it not more meaningful to use contravariant velocities while applying the boundary conditions. Or is it done but never mentioned. What is the physical meaning of contravariant velocity I will be extremely happy if these can be clarified
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