Fractional steps method
Can anybody give information about implementing chorin's fractional steps method? I have problems specially about solving poisson equation

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V* = Vn+1 + Grad (phi) Taking divergence of both sides you get Div (Grad (phi)) = Div V* with the BCs: d phi/dn = n . (V*  Vn+1) The latter condition says that you hav to modify Div V* on the boundary using the normal component of Vn+1 and use homogeneous Neumann BC d phi/dn=0. This way the compatibility equation is verified and you get a unique solution apart a constant value. 
Dear FMDenaro
Could you please explain more simply how to modify Div.V* on the boundary, using the normal components of Vn+1?? 
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Div (Grad (phi)) = Div V* (1) with the BCs: d phi/dn = n . (V*  Vn+1) > n . V* = d phi/dn  n . Vn+1 then substitute in RHS of (1) and simplify the terms 
But how do we know the value of Vn+1
if we have a free stream inlet velocity in the inlet, So, Vn+1 will be known in the inlet. what about the other boundaries i.e walls and outlet. Vn+1 is not known at these boundaries ??? Sorry if my understanding is wrong 
on a wall n.Vn+1 must be prescribed ... some difficulties can appear for an outlet where you have to prescribe some condition derived by physical assumption

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