# Help.. Boundary conditions for external flow...

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 April 13, 2011, 14:47 Help.. Boundary conditions for external flow... #1 New Member   Suresh Join Date: Apr 2011 Posts: 3 Rep Power: 8 Hi, I am solving 3D euler equation on aircraft wing, I am using FVM in unstructured grid with cell centred method. I use Lax-Friedrich scheme to discretize the equation. My results are not correct, temperatures, pressure and density are higher than the stag. values at wall boundaries. I think my boundary conditions are wrong. I am using a circular domain of 15 times of root chord as diameter, my BC are as follows, Far field: Free stream values. (since the domain is circular, no seperate BC for inlet and exit) Wall boundary: reflective velocity method is used for velocity, and pressure and temperature are extrapolated from boundary cell to ghost cell. Can any one suggest me for solution?

 April 13, 2011, 21:12 what is reflective velocity? Does that refer to slipping? #2 New Member   goldling Join Date: Apr 2011 Posts: 5 Rep Power: 8 what is reflective velocity? Does that refer to slipping?

 April 14, 2011, 03:00 #3 New Member   Suresh Join Date: Apr 2011 Posts: 3 Rep Power: 8 I am reflecting normal component of velocity from the boundary cell to the ghost cell to make the velocity normal to the surface zero. (i.e) I implemented v.n = 0, by this method. I have used the expression vg=vb-2(vb.n)n, where g refers to ghost cell and b refers to boundary cell. Above expression is from the book Blazek

 April 14, 2011, 10:19 #4 New Member   goldling Join Date: Apr 2011 Posts: 5 Rep Power: 8 This method should work, actually it is exactly what I use. How is your convergence? I use Roe, but should not make much difference.

 April 14, 2011, 11:11 #5 New Member   Suresh Join Date: Apr 2011 Posts: 3 Rep Power: 8 I am calculating norms by taking residual per time step. (i.e) Residual / t, I a using local time step. My norm converges in the order of 10-4, not increasing more than that.

 April 14, 2011, 11:26 #6 New Member   goldling Join Date: Apr 2011 Posts: 5 Rep Power: 8 If residual does not drop to machine zero, 10e-14, there is bug in your code, especially for Euler problem.

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