====================================================================== CFD Boundary Condition Setup Checklist ====================================================================== DEFINITION ---------------------------------------- A CFD Boundary Condition Setup Checklist is a systematic, step-by-step verification guide used to ensure physically consistent, numerically stable, and solver-compatible boundary conditions are correctly defined in computational fluid dynamics simulations. It bridges theoretical fluid mechanics with practical numerical implementation by validating the type, location, magnitude, and coupling of boundary specifications. Proper use prevents convergence failure, unphysical results, or misinterpretation of transport phenomena such as momentum, heat, and mass transfer. OVERVIEW ---------------------------------------- Boundary conditions (BCs) are essential inputs that define how a fluid domain interacts with its surroundings—such as walls, inlets, outlets, and symmetry planes—and directly govern the uniqueness and well-posedness of the governing partial differential equations (e.g., Navier–Stokes, energy, and species transport equations). Incorrect or inconsistent BCs are among the most common sources of simulation divergence, non-physical oscillations, or erroneous predictions of pressure drop, separation, heat flux, or mixing efficiency. The checklist enforces discipline in verifying not only the mathematical type (Dirichlet, Neumann, Robin, or mixed) but also physical plausibility—e.g., enforcing no-slip at solid walls, ensuring mass conservation across inlets/outlets, and applying appropriate turbulence intensity and length scale at velocity inlets. Furthermore, it addresses solver-specific requirements: for instance, pressure-based solvers often require careful outlet pressure specification to avoid backflow instability, while density-based solvers may mandate total pressure/temperature at inlets. Advanced considerations include transient BCs (time-varying profiles), coupled BCs (conjugate heat transfer interfaces), and multiphase or reactive flow constraints (e.g., species mass fractions, wall catalysis), all of which must satisfy thermodynamic consistency and numerical compatibility with discretization schemes and mesh resolution near boundaries. KEY COMPONENTS ---------------------------------------- 1. Boundary Type Identification 2. Physical Consistency Validation 3. Solver-Specific Compatibility Check APPLICATIONS ---------------------------------------- - Aerodynamic design of aircraft wings and vehicle underbodies - Thermal management of electronics and battery packs - Chemical reactor flow distribution and mixing optimization KEY FORMULAS ---------------------------------------- No-slip condition: u = v = w = 0 at solid wall -> Enforces zero relative velocity between fluid and stationary wall surface Turbulent kinetic energy inlet: k = (3/2)(U_{turb}^2) -> Estimates turbulent kinetic energy from turbulent intensity U_turb = I * U_mean Hydraulic diameter: D_h = 4A_c / P_w -> Used to compute Reynolds number and turbulence parameters for non-circular inlets RELATED CONCEPTS ---------------------------------------- - Navier-Stokes Equations - Well-Posed Problem Theory - Mesh Boundary Layer Resolution REFERENCES ---------------------------------------- ANSYS Fluent User's Guide: Boundary Conditions (https://ansyshelp.ansys.com/viewer/2023/R2/flu_ug/flu_ug_sec_bc_overview.html) Versteeg & Malalasekera — An Introduction to Computational Fluid Dynamics, 2nd ed., Ch. 7 (https://www.pearson.com/us/higher-education/program/Versteeg-Introduction-to-Computational-Fluid-Dynamics-2nd-Edition/PGM349165.html) CFD Online Boundary Condition Best Practices (https://www.cfd-online.com/Wiki/Best_practice_guidelines_-_Boundary_conditions) TAGS ---------------------------------------- CFD, boundary_conditions, fluid_dynamics