The concept of drift and its application to multiphase and multibody problems.

The concept of drift is built around understanding how a rigid body moving in a straight line distorts a material sheet in an unbounded perfect fluid. As the body moves from infinity through a material sheet, which is initially perpendicular to the direction of translation of the body, the sheet is permanently distorted. Darwin showed that the 'drift' volume, D(f), formed between the distorted and undistorted sheet is equal to C(m)V, where the added-mass coefficient, C(m), characterizes the shape of the body whose volume is V. Darwin's result is important for two reasons: first, it provides a means of quantifying how dyed fluid is transported from one place to another and dispersed; second, it provides a fundamental Lagrangian coordinate system to study inhomogeneous inviscid problems. The aim of this article is to review Darwin's contribution to fluid mechanics. By drawing on recent experimental measurements of drift and the drift volume, we aim to demonstrate how Darwin's drift concept has developed and to describe its broader significance for multiphase and multibody problems.