Glossary¶

Key terms and concepts used throughout the Gradient Dynamics documentation.

A¶

Adaptive Mesh: A mesh that varies cell size across the domain, using smaller cells where detail is needed and larger cells elsewhere.
Angle of Attack (AoA): The angle between the freestream velocity direction and the reference line of a body (e.g., a wing chord). Measured in degrees.
Aspect Ratio: The ratio of a cell’s longest dimension to its shortest. Boundary layer cells have intentionally high aspect ratios; volume cells should have low aspect ratios.

Boundary Condition: A mathematical specification of what happens at the edge of the computational domain (inlet velocity, outlet pressure, wall no-slip, etc.).
Boundary Layer: The thin region near a solid surface where the fluid velocity changes from zero (at the wall) to the freestream value. Also refers to the mesh layers (prisms) grown on walls to resolve this region.
Bluff Body: A non-streamlined body that creates significant flow separation and a large wake. Examples: trucks, buildings, flat plates.

CAD (Computer-Aided Design): Software and file formats for creating 3D geometry. STEP and IGES are common CAD exchange formats.
Cd (Drag Coefficient): A dimensionless number representing aerodynamic drag force, normalized by dynamic pressure and reference area: Cd = Fd / (0.5 × ρ × V² × A).
Cell: The fundamental volume element of a computational mesh. The flow equations are solved for each cell.
Coupled Solver: A solver family used when multiple physical fields or regions interact strongly.
CFD (Computational Fluid Dynamics): The use of numerical methods to solve fluid flow equations on a computer.
CHT (Conjugate Heat Transfer): A simulation type that couples fluid flow (convection) with solid heat conduction, sharing temperature and heat flux at their interface.
Cl (Lift Coefficient): A dimensionless number representing aerodynamic lift force, normalized by dynamic pressure and reference area.
Convergence: The process by which iterative solver residuals decrease to acceptable levels, indicating the solution has stabilized.
CGNS (CFD General Notation System): A standard file format for storing CFD data, supported by many commercial and open-source solvers.

DES (Detached Eddy Simulation): A hybrid RANS-LES approach that uses RANS near walls and LES in the bulk flow. Good for massively separated flows.
Divergence: When a simulation fails to converge — residuals increase without bound. Usually indicates a numerical or setup problem.
Domain: The region of space being simulated. For external flow, this is a wind-tunnel-like box around the geometry.

Face: A polygon shared between two mesh cells (internal face) or between a cell and the domain boundary (boundary face).
Far-field: The outer boundaries of the domain that are far from the geometry of interest.
FVM (Finite Volume Method): A numerical technique that discretizes the governing equations over control volumes (cells). The standard method in CFD.

Gauge Pressure: Pressure relative to a reference value (usually atmospheric). In incompressible CFD, only pressure differences matter, so outlet is typically set to 0 Pa gauge.
Ground Effect: The aerodynamic influence of the ground on a nearby body (e.g., a car). Requires a moving ground boundary condition for accurate simulation.

Hexahedron (Hex): A six-faced volume cell. Hexahedral meshes generally provide better numerical accuracy and convergence than tetrahedral meshes.

Isosurface: A 3D surface connecting all points where a field has the same value (e.g., a surface of constant pressure).

k (Turbulent Kinetic Energy): The kinetic energy contained in turbulent fluctuations. Half the sum of the velocity variance in all three directions. Units: m²/s².

LES (Large Eddy Simulation): A turbulence modeling approach that resolves large-scale eddies directly and models only the smallest scales. More accurate but much more expensive than RANS.

Manifold: A surface where every edge is shared by exactly two faces. Non-manifold surfaces have topology errors.
MRF (Multiple Reference Frame): A steady-state method for simulating rotating machinery. The flow equations in the rotating zone include Coriolis and centrifugal source terms.

No-Slip: The wall boundary condition where fluid velocity equals zero at the surface. This is the physical reality for viscous flows.
Non-Orthogonality: The angle between a face normal vector and the vector connecting adjacent cell centers. High values indicate poor mesh quality.

Octree: A hierarchical spatial decomposition where each cube is recursively divided into 8 sub-cubes. Used for adaptive mesh generation.

Patch: A named group of boundary faces. Used to apply boundary conditions (e.g., “inlet” patch, “car_body” patch).
Prism: A wedge-shaped cell typically used in boundary layers, with triangular faces on the wall and outer surfaces.

RANS (Reynolds-Averaged Navier-Stokes): A CFD approach that solves time-averaged flow equations with turbulence models. The industry standard for engineering CFD.
Relaxation Factor: A number between 0 and 1 that controls how much the solution is updated each iteration. Lower values are more stable; higher values converge faster.
Residual: A measure of how well the discretized equations are satisfied at the current iteration. Lower is better.
Reynolds Number (Re): A dimensionless number representing the ratio of inertial to viscous forces: Re = ρVL/μ. Determines whether flow is laminar or turbulent.

Skewness: A mesh quality metric measuring how much a cell deviates from its ideal shape (0 = perfect, 1 = degenerate).
Slip Wall: A boundary condition with zero shear stress — the fluid slides freely along the surface.
Streamline: A curve that is everywhere tangent to the velocity field. Shows the path a fluid particle would follow.

Turbulence Intensity (TI): The ratio of velocity fluctuations to the mean velocity, expressed as a percentage. Defines the level of incoming turbulence.

VTU (VTK Unstructured): A file format for unstructured meshes and field data, used by ParaView and other visualization tools.

Watertight: A surface mesh that is completely closed with no gaps or holes. Required for volume mesh generation.

y+: A dimensionless wall distance: y+ = (u_τ × y) / ν. Determines the first cell height needed for proper turbulence model behavior.
ω (Specific Dissipation Rate): The rate of dissipation per unit turbulent kinetic energy. Used in k-ω turbulence models. Units: 1/s.
ε (Dissipation Rate): The rate at which turbulent kinetic energy is dissipated into heat. Used in k-ε turbulence models. Units: m²/s³.