hi there, i want to calculate streamlines from a given velocity field. where can i get information on the boundary conditions of the laplace equation thank you ulrich

At the wall you can set the value of stream function equal zero.

ok, this is evident (thank ypo anyway).

if i have symetry or von neumann boundary conditions, i impose the gradient of the stream function; unfortunately my conjugate gradient algorithm does not converge in these cases.

You don't have to solve the Laplace eq. if you already knew the velocity field (assume 2-D incompressible). You can simply integrete the velocity along the grid line from the solid boundary to get the stream function(sf = int[udy-vdx])at each grid point. This may not be very accurate, but certainly very simple.