Cell-Quality QC

Skew Has a Limit

Corner-point freedom let the grid bend to follow faults and dip, but that same freedom lets a cell bend too far. The standard flow solver uses a two-point flux approximation: it assumes fluid crosses a cell face roughly perpendicular to that face. A cell whose corner angles stay near ninety degrees honors that assumption; a badly sheared, non-orthogonal cell violates it, and the computed flow across its faces drifts away from the truth. Geometry that is good for honoring structure can be bad for solving flow.

Measuring the Damage

Cell-quality QC puts a number on the problem. A common measure is how far each cell's corner angles stray from ninety degrees, and cells past a chosen tolerance are flagged. In the widget, increasing the structural relief drapes the cells over a fault: the cells on the steep limb skew first and light up red, while the flat-lying cells away from the fault stay within limits. Real grids report the same thing as a map of problem cells, usually clustered along faults and steep flanks where the geometry has to work hardest.

What To Do About It

There is no free fix, only trades. You can resmooth or relayer the grid to cut the worst skew, at the cost of honoring the structure a little less faithfully. You can accept the skew and switch to a flux scheme that tolerates it, such as a multi-point flux approximation, at the cost of more computation per step. Or you can judge that the affected cells carry little flow and the error is acceptable. The point of the QC is to make that an informed decision rather than a silent error: you should know where your grid is non-orthogonal before the simulator does.