fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) a ...

, Darwin drift refers to the phenomenon that a

fluid parcel In fluid dynamics, within the framework of continuum mechanics, a fluid parcel is a very small amount of fluid, identifiable throughout its dynamic history while moving with the fluid flow. As it moves, the mass of a fluid parcel remains constant, ...

is permanently displaced after the passage of a body through a fluid – the fluid being at rest far away from the body. Consider a plane of fluid parcels perpendicular to the direction of the body's constant velocity vector, far before the passage of the body. During the passage of the body the fluid parcels move, according to their Lagrangian motion. Far after the passage of the body, the fluid parcels are permanently displaced. The volume between the initial plane of the fluid parcels and the surface consisting of the parcel positions long after the body's passage is called the Darwin drift volume. The phenomenon is named after Sir

Charles Galton Darwin Sir Charles Galton Darwin (19 December 1887 – 31 December 1962) was an English physicist who served as director of the National Physical Laboratory (NPL) during the Second World War. He was a son of the mathematician George Howard Darwin a ...

, who proved in 1953 that the drift volume multiplied with the fluid

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

equals the

added mass In fluid mechanics, added mass or virtual mass is the inertia added to a system because an accelerating or decelerating body must move (or deflect) some volume of surrounding fluid as it moves through it. Added mass is a common issue because the ...

of the body, – known as Darwin's theorem. As shown by Eames and McIntyre in 1999, Darwin drift (by the passage of a body through a fluid otherwise at rest) and Stokes drift (in the fluid motion associated with

surface wave In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at ...

s) are closely related.

Notes

References

* * * * * * * * * Fluid dynamics {{fluiddynamics-stub