Boundary condition, incompressible colocated code
 

Hello,

I have written a simple FDM 1d colocated code with the nodes coinciding with boundaries. I have attached the code (it is C/C++).

My question is about the velocity derivatives that I need from outside the inlet boundary. I try to derive an expression based on the (normal) momentum equation but this does not work. If I just extrapolate the derivatives from the first interior node to the boundary node then everything works perfect. However, I do not understand why this is the case. Perhaps someone can point out what I am doing (or thinking) wrong?

Thank you.