Aerodynamics is the branch of fluid mechanics concerned with the motion of gases around bodies and the forces, moments and flow structures produced by that motion. It is most strongly associated with atmospheric flight and aerospace engineering, but its principles apply to automobiles, buildings, turbines, projectiles, sports equipment, ventilation systems and any object interacting significantly with moving air. Aerodynamics examines quantities such as velocity, pressure, density, temperature, viscosity, vorticity and Mach number, together with the resulting aerodynamic forces.
The two principal force components commonly discussed for a body moving through air are lift and drag, while pitching, rolling and yawing moments are also important. Aerodynamic analysis can be experimental, analytical, computational or a combination of all three. It connects fluid mechanics with mechanics, thermodynamics, materials engineering, control systems and aircraft design.
The forces acting on an aerodynamic body arise from the distribution of pressure and viscous shear over its surface. Lift is the component of the resultant aerodynamic force perpendicular to the reference relative wind direction, while drag is the component parallel to it. For a conventional aircraft wing in suitable conditions, differences in pressure between the upper and lower surfaces contribute substantially to lift, while viscous effects and pressure related phenomena contribute to drag.
The common lift relationship is L equals one half rho V squared S C sub L, where rho is air density, V is reference velocity, S is reference area and C sub L is the lift coefficient. A corresponding expression exists for drag using C sub D. These coefficients are dimensionless and depend on factors such as angle of attack, Reynolds number, Mach number, geometry and surface condition.
Angle of attack is the angle between a body's reference chord or characteristic direction and the relative airflow, with the exact definition depending on the body and application. For a conventional wing, increasing angle of attack generally increases lift over a useful operating range, but beyond a critical angle the flow can separate substantially from the surface and lift can decrease, producing stall. Stall is therefore fundamentally a flow separation phenomenon rather than simply a low speed condition. An aircraft can stall at different speeds depending on weight, configuration, load factor, air density and other factors. Aerodynamic design seeks to manage the pressure distribution and boundary layer behaviour so that the desired lift is produced with acceptable drag and controllable separation characteristics.
Drag has several physical components and its relative importance varies with flight regime and geometry. Skin friction drag arises from viscous shear within the boundary layer, while pressure or form drag is associated with pressure differences created by flow separation and wake formation. Induced drag is associated with the generation of lift by finite wings and the resulting trailing vortices. Wave drag becomes particularly important when compressibility and shock waves develop at high Mach numbers.
Parasite drag is a broad engineering category that includes drag from components and surfaces not directly associated with induced drag. Aerodynamic optimisation therefore requires tradeoffs because a design change that reduces one component can increase another. Streamlining can reduce separation and pressure drag, while surface treatment and boundary layer control can affect skin friction and separation.
Compressibility becomes important when air velocity is sufficiently large relative to the local speed of sound. Mach number is defined as the ratio of flow speed to local speed of sound and is a central parameter in compressible aerodynamics. At low Mach numbers, air can often be treated as approximately incompressible for many engineering calculations, whereas at higher Mach numbers density variations become important.
Supersonic flow can produce shock waves and expansion waves, causing abrupt changes in pressure, density, temperature and velocity. Transonic flow is particularly complex because subsonic and supersonic regions can coexist around the same body. Hypersonic aerodynamics involves still higher Mach numbers and introduces strong aerodynamic heating, chemical nonequilibrium and other effects. These regimes require governing equations and numerical methods appropriate to compressible flow rather than simple incompressible approximations.
Boundary layers are thin regions adjacent to solid surfaces in which viscosity causes the velocity to change from the wall condition toward the external flow velocity. They may be laminar or turbulent and can transition from one state to the other. Boundary layer behaviour strongly influences drag and separation. A favourable pressure gradient can help keep flow attached, while an adverse pressure gradient can slow the near wall flow and cause separation.
Aerodynamic surfaces are therefore often shaped to manage pressure gradients and boundary layer development. Turbulent boundary layers generally contain greater mixing and momentum transfer than laminar ones, which can make them more resistant to separation but often increases skin friction. Devices such as vortex generators can deliberately modify boundary layer behaviour, demonstrating that aerodynamic design frequently involves controlled manipulation of flow rather than simply minimising turbulence everywhere.
Aerodynamics is studied through wind tunnel testing, flight testing, analytical theory and computational fluid dynamics. Wind tunnels allow controlled investigation of pressure distributions, forces, moments and flow structures, although the validity of results depends on similarity parameters and experimental corrections. Computational fluid dynamics solves discretised forms of governing fluid equations and can provide detailed information about flow fields that may be difficult to measure experimentally.
Reynolds number, Mach number and geometric similarity are among the important parameters when relating model results to full scale systems. Computational results are also subject to numerical discretisation, turbulence modelling, boundary conditions and other modelling assumptions. High fidelity simulation can therefore complement rather than completely replace experimental validation.
Aerodynamics has broad practical importance in aircraft efficiency, vehicle fuel consumption, wind turbines, sports equipment, architecture and environmental engineering. Aircraft designers seek to produce required lift while controlling drag, stability, structural loads and noise. Automotive designers use aerodynamic analysis to manage drag and high speed stability, while wind turbine designers study blade aerodynamics to extract energy efficiently from moving air. Buildings can experience wind induced pressures and flows that affect structural loading, ventilation and pedestrian comfort. Aerodynamic design is consequently not simply about making shapes streamlined. It involves understanding fluid forces, stability, flow separation, compressibility, turbulence, noise, thermal effects and interactions among components. Successful aerodynamic design requires these factors to be considered together with structural, manufacturing, operational and safety requirements.
