“…Another important aspect is that the HNS schemes are known to produce high-order and highquality gradients (e.g., viscous stresses, heat fluxes, and vorticity) on irregular unstructured grids [23,24,26] while the first approach can produce only first-order accurate gradients (unless the inviscid terms dominate) polluted by numerical noise on irregular grids. Moreover, the HNS solvers are known to eliminate numerical stiffness arising from second derivatives in the governing equations and converge faster on fine grids than conventional solvers [23,24,25,26]. Efficiency comparison is, thus, not a simple matter, and will be even more complicated if other aspects, such as solver constructions, code optimizations, other discretization alternatives, etc., are brought into discussion.…”