boundary onditions for the level set approach
 

Now I use the level set method to simulate the incomressible two-phase flow. After solving the NS equation one step, i have to solve phi_t + u * gradient(phi) = 0. By the 2nd or 3rd order eno scheme, some ghost cells should be imposed. But since no boundary conditions for phi, how can i extrapolate the ghost cells? Thanks in advance.