Aerodynamics

A vortex is created by the passage of an aircraft wing, revealed by smoke. Vortices are one of the many phenomena associated to the study of aerodynamics. The equations of aerodynamics show that the vortex is created by the difference in pressure between the upper and lower surface of the wing. At the end of the wing, the lower surface effectively tries to 'reach over' to the low pressure side, creating rotation and the vortex.
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with the difference being that gas dynamics applies to all gases. Understanding the motion of air (often called a flow field) around an object enables the calculation of forces and moments acting on the object. Typical properties calculated for a flow field include velocity, pressure, density and temperature as a function of position and time. By defining a control volume around the flow field, equations for the conservation of mass, momentum, and energy can be defined and used to solve for the properties. The use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simulations form the scientific basis for heavier-than-air flight.
Aerodynamic problems can be identified in a number of ways. The flow environment defines the first classification criterion. External aerodynamics is the study of flow around solid objects of various shapes. Evaluating the lift and drag on an airplane or the shock waves that form in front of the nose of a rocket are examples of external aerodynamics. Internal aerodynamics is the study of flow through passages in solid objects. For instance, internal aerodynamics encompasses the study of the airflow through a jet engine or through an air conditioning pipe.
The ratio of the problem's characteristic flow speed to the speed of sound comprises a second classification of aerodynamic problems. A problem is called subsonic if all the speeds in the problem are less than the speed of sound, transonic if speeds both below and above the speed of sound are present (normally when the characteristic speed is approximately the speed of sound), supersonic when the characteristic flow speed is greater than the speed of sound, and hypersonic when the flow speed is much greater than the speed of sound. Aerodynamicists disagree over the precise definition of hypersonic flow; minimum Mach numbers for hypersonic flow range from 3 to 12.
The influence of viscosity in the flow dictates a third classification. Some problems may encounter only very small viscous effects on the solution, in which case viscosity can be considered to be negligible. The approximations to these problems are called inviscid flows. Flows for which viscosity cannot be neglected are called viscous flows.

ROAD VEHICLE AERODYNAMICS:
For a vehicle, external and internal flow are closely related. The external flow has great influence on the performance characteristics and the directional stability of the vehicle, whereas the internal flow makes reference to the cooling system of the engine and the ventilation or heating of the passengers cabin. In both cases the flow is viscous, incompressible, turbulent and three-dimensional. Until a few years ago, the simulation of fluid dynamics in vehicles was unapproachable, though the Navier-Stokes equations that govern the fluid flow were perfectly known. The arrival of the digital computer has permitted to model physical phenomenon with less cost and in a faster way than with experimental procedures in wind tunnels. The science that tries to determine a numerical solution to the equations that govern the fluid flow is called CFD (Computational Fluid Dynamics). In the last years it has been a considerable growth in development and application of numerical methods to all the aspects of the fluid dynamics. At present, with the appearance of supercomputers, CFD methods are used as a tool of design analysis within industry and research organizations. The use of these numerical methods is widely recognized together with the experimentation and the measurement. In fact, the advance is leaded toward the theoretical-experimental treatment, so the results obtained in the wind tunnel serve to verify and calibrate the numerical models. Numerical simulation permits furthermore investigation and study of real situations that can not be accomplished in a wind tunnel.