What is RANS turbulence modeling and name common turbulence models.

Reynolds-Averaged Navier-Stokes (RANS) decomposes flow variables into mean and fluctuating components (Reynolds decomposition), averaging equations over time. This introduces Reynolds stresses that require closure models. Common models: (1) k-ε standard: robust, widely used for fully turbulent flows, struggles with separation. (2) k-ω: better near-wall behavior, sensitive to freestream values. (3) k-ω SST: combines k-ε far-field with k-ω near-wall, excellent for adverse pressure gradients. (4) Spalart-Allmaras: one-equation model for aerospace. Model selection depends on flow physics: separated flows need SST or RSM, jets/mixing need k-ε realizable.