The flow environment or properties of the flow, such as flow speed, compressibility, and viscosity, are used to classify aerodynamic problems. The study of flow around solid objects of various shapes is known as external aerodynamics. 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. The study of flow through passages in solid objects is known as internal aerodynamics. Internal aerodynamics, for example, is concerned with the flow of air through a jet engine or an air conditioning pipe.
Aerodynamic issues can also be characterized based on whether the flow speed is below, near, or above the sound speed. When all of the speeds in a problem are less than the speed of sound, it is called subsonic. it is called 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. When the flow speed is much greater than the speed of sound, it is called hypersonic. Aerodynamics has concentrated on difficulties such as compressible flow.