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-   -   how to apply a symmetry boundary? (https://www.cfd-online.com/Forums/main/95233-how-apply-symmetry-boundary.html)

 houkensjtu December 10, 2011 08:49

how to apply a symmetry boundary?

hey guys
i am working on coding a simulation by SIMPLE method, the flow is laminar and axissymmetric, all the momentum and pressure correction equations are written in cylindrical form.
however i found it conquers error especially near the axis.the velocity derivative du/dr is not zero.

here is how i applied the symmetry boundary condition;
for all u-velocity(parallel to the symmetry axis) cells near the axis, the discrete momentum equation is:
ap*u(i,j) = ae*u(i,j+1) + aw*u(i,j-1) + an*u(i+1,j) + as*u(i-1,j) +dp/dz*r*dz*dr
then i set u(i-1,j) = u(i,j) and repeat

i found that the problem maybe caused by that the "as" coefficient is very close to zero near the axis because radius is near to zero.