Efectos De La Compresibilidad En El Arrastre (Inglés)

COMPRESSIBILITY EFFECTS ON DRAG AND LIFT
As an object approaches the speed of sound, shock waves are formed. These may occur when the overall Mach number of the object is less than 1, since therewill be regions near the body where there is a local increase in flow velocity (see diagram below). Shocks will form wherever the local Mach number becomes 1. The transonic flow range begins when shocksfirst start to form. The transonic regime ends and the supersonic regime begins when there are no remaining subsonic flow regions (relative to the object). The transonic regime may be as wide as 0.5< M < 1.5 or 2.

Note that for blunt objects, such as a sphere, there may always be a region near the bow where subsonic flow exists.

Thermofluids 313

Compressibility Effects on Lift and DragMB Bush 2001

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The shock waves bring about abrupt changes in pressure and may cause boundary layer separation, leading to increases in the pressure drag. This increase is referred to as wavedrag. In supersonic flow the wave drag dominates skin friction. Streamlining the trailing part of the body, which is very important in subsonic flow, has the reverse effect in supersonic flow; thedrag increases. The compromises required to design an aircraft that is capable of flying at both subsonic and supersonic speeds begin to become obvious! The wave drag is related to the angle of the bowshock (and other shocks that may be present). This angle is reduced if the nose of the body is sharp. Minimum drag in a supersonic flow is therefore achieved by having a sharp, pointed nose ratherthan the smoothly streamlined nose required in subsonic flow. This is illustrated below.

The wave drag on a sharp nosed object is significantly less than that of the blunt object. The figure alsoillustrates that when compressibility effects begin to have an influence (M > 0.5), the drag coefficient becomes a function of Mach number (or Reynolds number). In incompressible flow (M < 0.5) the drag...