Steady or Transient CFD? The 4-Step Decision Framework

Steady vs transient CFD explained — when steady-state is the right tool, when it silently lies to you, and a four-step framework for picking the correct solver every time. Most CFD engineers default to steady-state without checking whether their physics actually allows time to be frozen. Get it wrong and you waste days on a residual plot that will never converge. This video walks through what steady-state actually assumes, where it is genuinely valid, the three categories of physics where it fundamentally fails, the Strouhal number as a warning sign, turbomachinery approaches from frozen rotor to sliding mesh, the pseudo-transient and URANS hybrids, and the real cost spectrum from RANS through DNS. What you will learn: The single equation that defines steady-state and what it really means Where RANS gives you fast, correct answers — pipe flow, cruise, heat exchangers Three categories of physics where steady-state cannot work The Strouhal number and how to predict unsteady flow before any simulation runs Frozen rotor vs mixing plane vs sliding mesh in turbomachinery Pseudo-transient and URANS as practical hybrids The 4-step decision framework you can apply to every new simulation The 10-100x cost difference between steady and transient — and how to manage it Timestamps: 0:00 The Convergence That Never Comes 0:45 What Steady-State Assumes 1:42 Where Steady-State Is the Right Tool 2:35 Where Steady-State Fundamentally Fails 3:37 Strouhal Number — Your Unsteady Warning Sign 4:41 Turbomachinery — Three Approaches 5:48 Pseudo-Transient and URANS 6:56 The 4-Step Decision Framework 8:12 The Cost of Going Transient 9:17 The Rule Key References: Ferziger & Peric — Computational Methods for Fluid Dynamics Versteeg & Malalasekera — An Introduction to CFD: The Finite Volume Method Pope — Turbulent Flows (RANS / LES / DNS hierarchy) Strouhal, V. (1878) — On a special kind of tone generation (vortex shedding) #CFD #ComputationalFluidDynamics #TransientSimulation --- 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.