Aerodynamics — CAE Glossary

That's correct. They are "dimensionless forces"—a metric showing how efficiently a shape and angle of attack generate drag or lift. For example, at the same velocity, air density, and wing area, an object with a drag coefficient

It depends on the application. The most common approach is to solve the time-averaged RANS (Reynolds-Averaged Navier-Stokes) equations, which model turbulence fluctuations and efficiently solve for the mean flow field. The equations are written as:

It is a dimensionless indicator of pressure at each point on an object surface. It is defined as

The critical factor is resolving the boundary layer. The region immediately adjacent to the object surface—the "boundary layer"—has extremely steep velocity gradients. To capture this accurately, we typically use thin prism or polyhedral cells stacked perpendicular to the surface. Specifically, we set the first cell height such that the dimensionless wall distance

Historically, pressure-based solvers were preferred for incompressible flow (Mach number Ma < 0.3) and density-based for compressible flow (Ma > 0.3). However, modern software (Fluent, STAR-CCM+, etc.) has blurred this distinction. Practical guidance is as follows: Pressure-based solvers are robust for a wide range of problems including low-speed automotive and building flows, and natural convection. Density-based solvers excel at supersonic flows and combustion analyses where shock resolution is critical. However, Ansys Fluent's density-based solver has been enhanced to work across all velocity regimes. If uncertain, start with the pressure-based "Coupled" algorithm.

It depends on whether the flow field has "inherently time-varying structures." For example, the vortex trail behind a car can be captured reasonably well in steady-state analysis. However, bridge "galloping," aircraft "flutter," wind turbine "stall," and Karman vortex streets (as in cylinder wakes) require transient analysis. A practical indicator: if you run a steady-state analysis and the residuals oscillate continuously or force coefficients fail to converge, transient phenomena are likely present. In transient analysis, set the time step size based on the characteristic frequency (e.g., estimated from Strouhal number) such that there are at least 20 time steps per period.

Good question. Industry standards and best practices provide guidance. The automotive industry references SAE J1252 and J2966. General guidelines based on the object's characteristic length L (vehicle length, for instance) are:
• Inlet: 3L to 5L upstream of the front
• Outlet: 7L to 10L downstream of the rear
• Sides/top/bottom: 3L to 5L from the centerline
For a 4.5 m sedan, the domain should be roughly 20 m wide and tall, and 50 m long. The AIJ (Architectural Institute of Japan) guidelines on building loads are useful for wind loading on structures. A domain that is too small causes boundary effects to distort the flow, particularly degrading the pressure distribution in the wake.

"Velocity inlet" is used when the inflow velocity is known (as in wind tunnel experiments). "Pressure inlet" is used when inflow velocity results from the solution or to model open atmosphere conditions. In practice, automotive driving simulations use "velocity inlet," while natural wind analysis on buildings uses "pressure inlet" with specified turbulence intensity and length scale.
The most critical aspect of "wall" conditions is ensuring consistency between the boundary layer mesh and wall function choice. For sufficiently fine meshes with

Decreasing residuals are necessary but not sufficient. The most important criterion is whether the quantities you need (drag coefficient

The biggest strength is its "all-in-one" integrated environment and powerful automatic meshing. STAR-CCM+ consolidates preprocessing (CAD processing, meshing), solving, and post-processing in a single GUI, with very strong workflow automation via Java macros. For automotive external flow, its "Surface Remesher" absorbs minor CAD defects, and the combination of "Polyhedral Mesher" and "Prism Layer Mesher" can generate high-quality meshes around complex car geometries with just a few clicks. It also excels at coupled vehicle dynamics (pitch, yaw) and multi-physics coupling with battery thermal-fluid analysis. While Fluent offers similar features, STAR-CCM+ is preferred for workflow integration and automation in many automotive settings.

Fluent benefits from an extensive range of physics models, a long history, and tight integration with the Ansys ecosystem. It particularly excels at:
• Complex combustion with chemical reactions (e.g., aero engines)
• Multiphase flow (sprays, bubbles)
• Advanced turbulence models (DES, LES variants)
• Strong coupling with Ansys Mechanical (fluid-structure interaction, FSI)
It has deep expertise and track record in these areas. Custom development via User-Defined Functions (UDFs) is also highly flexible. Aerospace and research institutions continue to favor Fluent.

The core solver is very high quality and is used widely in research and certain industries (especially wind energy and marine). It implements physics models as comprehensive as commercial software. However, the largest hurdle is preprocessing and postprocessing. The GUI is minimal (or requires commercial add-ons), requiring mesh generation and configuration via command-line or dictionary files—a steep learning curve. For practical industrial deployment, you need internal CFD expertise or must purchase commercial support from distributors (ESI-OpenCFD, foam-extend). OpenFOAM is ideal for R&D projects with limited budget but deep customization needs and in-house expertise.

This indicates extremely poor mesh quality. Specifically:
1. High skew angles in elements: Cells, especially hexahedral ones, are distorted.
2. Excessive aspect ratios: Elements are too elongated (boundary layer meshes are an exception, but interior cells should generally stay below 1000).
3. Abrupt mesh size transitions: Volume ratios between adjacent cells change too rapidly, breaking gradient calculations.
Solutions: Use the mesher's quality report to ensure skew angles are below 0.9 (ideally < 0.8) and aspect ratios in the interior stay below 100. Remeshing or mesh smoothing features help. For coarse boundary layer meshes, remesh with adjusted inflation parameters.

Very likely. Bluff body vortex shedding (e.g., behind a truck cab) is inherently unsteady. The steady solver tries to find a "time-averaged solution" but fails because the physics is transient, causing oscillations. First, estimate the oscillation frequency (via FFT) and decide if it matters for your design. If critical, switch to transient analysis. If only the mean value matters, try strengthening relaxation factors (reduce default 0.2 to 0.1) or switching from SST k-ω to the more diffusive k-ε Realizable model—but these are workarounds, not solutions.

At high angles of attack, separation is severe and its prediction is critical. Main culprits:
1. Turbulence model limitations: Standard RANS models struggle with large separated regions. Consider DES or LES.
2. Insufficient mesh resolution: Separation and reattachment zones need local mesh refinement. Coarsen meshes at edge regions where separation is expected.
3. Inflow turbulence mismatch: Wind tunnels have characteristic turbulence intensity and scale. Match your CFD inflow conditions to the tunnel's measured values.
4. Model fidelity: Does your CFD model ignore support struts or floor boundary layer that the wind tunnel includes? Replicate tunnel interference effects in CFD.
Best practice: Validate at low angles of attack where separation is minimal, then progressively build confidence toward high angles.