Wake (physics)
In fluid dynamics, a wake may either be:
- the region of recirculating flow immediately behind a moving or stationary blunt body, caused by viscosity, which may be accompanied by flow separation and turbulence, or
- the wave pattern on the water surface downstream of an object in a flow, or produced by a moving object (e.g. a ship), caused by density differences of the fluids above and below the free surface and gravity (or surface tension).
Viscosity
The wake is the region of disturbed flow (often
For a blunt body in
Density differences
In incompressible fluids (liquids) such as water, a
The non-dimensional parameter of interest is the Froude number.
Kelvin wake pattern
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.[1]
This pattern consists of two wake 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 wake line is offset from the path of the wake source by around arcsin(1/3) = 19.47° and is made up of feathery wavelets angled at roughly 53° to the path.Other effects
The above describes an ideal wake, where the body's means of propulsion has no other effect on the water. In practice the wave pattern between the V-shaped wavefronts is usually mixed with the effects of propeller backwash and eddying behind the boat's (usually square-ended) stern.
The Kelvin angle is also derived for the case of deep water in which the fluid is not flowing in different speed or directions as a function of depth ("shear"). In cases where the water (or fluid) has shear, the results may be more complicated.[2] Also, the deep water model neglects surface tension, which implies that the wave source is large compared to capillary length.
Recreation
"No wake zones" may prohibit wakes in
Wakes are occasionally used recreationally. Swimmers, people riding personal watercraft, and aquatic mammals such as dolphins can ride the leading edge of a wake. In the sport of
See also
- Bow shock (aerodynamics)
- Slipstream
- Wake turbulence
- Karman vortex street
References
- ^ William Thomson (1887) "On ship waves," Institution of Mechanical Engineers, Proceedings, 38 : 409–34; illustrations, pp. 641–49.
- ^ Norwegian University of Science and Technology, "A 127-year-old physics riddle solved", Phys.org, Aug 21, 2019. Retrieved 22 August 2019
- ^ BoatWakes.org, Table of distances