More Cells = Better Results? Biggest CFD Myth

More cells does not mean better results - and the reasons are more interesting than the slogan. This video builds the honest error budget of a CFD solution. Truncation error falls as you refine; that is the part everyone knows. But error terms can also cancel on a specific mesh, handing you an artificially accurate answer that gets WORSE when you refine and the cancellation breaks - non-monotonic convergence, drawn here from a computed dataset rather than asserted. And modelling error - turbulence model, boundary conditions, geometry simplifications - sets a floor that no amount of cells lowers. Then the method that separates engineering data from unverified claims: the three-grid Grid Convergence Index, worked end to end on screen with every number derived - refinement ratio 1.5, observed order computed from the three solutions, Richardson extrapolation, GCI on the fine grid, and the asymptotic-range checks that the bare GCI number cannot give you. Plus the caption the method deserves: GCI bounds discretisation error. It is verification, not validation - it cannot see a wrong turbulence model, and a shared mesh-quality bias can converge cleanly to a wrong value. Also: the real cost of uniform refinement in 3D (8x cells, 16x with the explicit time step, 256x for two levels); the y+ trap, with the wall-function band drawn from the actual log-law; why compensating for first-order schemes with more cells is a bill you cannot pay (first-order smearing shrinks only linearly with cell size - ten times less smearing costs a thousand times the cells in 3D, while second order is a scheme switch, not a mesh bill); and targeted, adjoint-guided refinement - cells where the flow demands them. Software-agnostic. The numerics, not the buttons. Timestamps: 0:00 The 50-million-cell disaster 0:37 The two enemies: truncation vs round-off 1:26 Error cancellation: right answer, wrong reason 2:02 The camera resolution analogy 2:40 The 8x cost trap in 3D 3:21 What mesh quality actually means 4:01 The y+ trap 4:43 First-order schemes: the silent killer 5:23 The three-grid method: GCI 6:11 Targeted refinement 6:58 The checklist 7:50 Think like a surgeon Key References: Roache, P.J., Verification and Validation in Computational Science and Engineering ASME V&V 20 (Standard for Verification and Validation in CFD and Heat Transfer) Ferziger & Peric, Computational Methods for Fluid Dynamics (error decomposition) Eca & Hoekstra, J. Comput. Phys. 262 (2014) (discretisation uncertainty estimation) Fidkowski & Darmofal, AIAA J. 49 (2011) (output-based / adjoint mesh adaptation review) ANSYS Fluent and OpenFOAM user guides (quality metrics, wall treatment) #CFD #ComputationalFluidDynamics #Meshing --- Disclaimer: This video is for educational and informational purposes only and does not constitute professional engineering advice. Always consult a qualified professional for project-specific requirements. Codes and standards vary by jurisdiction.