The wake is the region of disturbed flow downstream of a solid body moving through a fluid, caused by the flow of the fluid around the body. Parachutes deployed into wakes suffer dynamic pressure deficits which reduce their expected drag forces, example applications include rocket stage separation and aircraft store separation. In incompressible fluids such as water, a bow wake is created when a watercraft moves through the medium, as the medium cannot be compressed, it must be displaced instead, resulting in a wave. As with all forms, it spreads outward from the source until its energy is overcome or lost. The non-dimensional parameter of interest is the Froude number, waterfowl and boats moving across the surface of water produce a wake pattern, first explained mathematically by Lord Kelvin and known today as the Kelvin wake pattern. This pattern consists of two lines that form the arms of a chevron, V, with the source of the wake at the vertex of the V. For sufficiently slow motion, each line is offset from the path of the wake source by around arcsin =19. 47° and is made up of feathery wavelets angled at roughly 53° to the path. The inside of the V is filled with transverse curved waves and this pattern is independent of the speed and size of the wake source over a significant range of values. However, the changes at high speeds, viz. above a hull Froude number of approximately 0.5. The angles in this pattern are not intrinsic properties of water, Any isentropic. Furthermore, this phenomenon has nothing to do with turbulence, everything discussed here is based on the linear theory of an ideal fluid, cf. Airy wave theory. Parts of the pattern may be obscured by the effects of propeller wash, and tail eddies behind the boats stern, and by the boat being a large object and not a point source. The water need not be stationary, but may be moving as in a river. This formula implies that the velocity of a deep water wave is half of its phase velocity. Two velocity parameters of importance for the pattern are, v is the relative velocity of the water. C is the velocity of a wave, varying with wave frequency. As the surface moves, it continuously generates small disturbances which are the sum of sinusoidal waves with a wide spectrum of wavelengths. Those waves with the longest wavelengths have phase speeds above v, other waves with phase speeds at or below v, however, are amplified through constructive interference and form visible shock waves, stationary in position w. r. t
Kelvin wake pattern generated by a small boat.
Visualisation of the Kármán vortex street in the wake behind a circular cylinder in air; the flow is made visible through release of oil vapour in the air near the cylinder.