Explain CFD validation and verification. What are sources of errors in CFD?

Verification confirms equations are solved correctly (solving equations right); validation confirms correct equations are solved (solving right equations). Error sources: (1) Modeling errors: turbulence model inadequacy, simplified geometry, boundary condition assumptions. (2) Discretization errors: truncation from finite differencing, addressed by grid independence study using Richardson extrapolation or GCI (Grid Convergence Index). (3) Iteration errors: incomplete convergence of nonlinear solver, monitored by residuals and integral quantities. (4) Round-off errors: finite precision arithmetic, usually negligible. Validation involves comparison with experimental data, quantified using metrics like error norms, drag/lift coefficients. Best practices: systematic grid refinement (factor 1.3-2), multiple turbulence models, sensitivity studies on boundary conditions, uncertainty quantification (UQ). ASME V&V 20 provides guidelines.