Direct against iterative

Solvers, size and conditioning. Factorise or iterate, what conditioning does to either choice, how turbulence changes the bargain, and what happens when a mesh is split across processes.

• 01Direct against iterativeFactorisation is robust and memory-hungry; iteration is cheap and depends entirely on the preconditioner.
• 02Conditioning, and why it bitesA poorly conditioned system converges slowly or not at all, and mesh quality is usually the reason.
• 03RANS, LES and DNS — three bargainsTurbulence is the open problem, and each approach trades cost against how much of it is modelled rather than resolved.
• 04Parallel, and the partitionSplitting a mesh across processes is its own graph problem, and load balance decides the wall clock.