2-D prototype, round 3: Johansen-Colella / Schwartz-Barad-Colella-Ligocki (2006) style cut-cell FV
(the AMReX EB approach), with Robin eliminated at the wall. Disc gates as in hybrid.py.
values : cell-centred, every kappa>0 cell is an unknown, mass kappa
face flux : aperture * gradient at the open-face CENTROID, linearly interpolated between the
face's own two-point gradient and the parallel neighbouring face's (if open)
[toggle fc=False -> plain aperture two-point flux]
wall flux : quadratic in the wall-normal distance through u_Gamma, u(p1), u(p2); p1,p2 where
the normal ray from the wall centroid crosses the next two cell-centre lines of the
dominant normal axis; u(p) by 3-point quadratic interpolation along that line
[toggle wall='lin' -> 2-point (u(p1)-u_G)/d1]
Robin : D du/ds = k u_G (s into the fluid) -> u_G eliminated per wall:
du/ds = S - beta u_G, u_G = D S / (k + D beta); wall flux into cell = -k A_w u_G
1.9.8