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In mathematics and fluid dynamics, the substantive derivative (sometimes the Lagrangian derivative, material derivative or advective derivative), written D / Dt, is the rate of change of some property of a small parcel of fluid.
Note that if the fluid is moving, the substantive derivative is the rate of change of fluid within the small parcel, hence the other names advective derivative and fluid following derivative.
It is defined as follows
where is the fluid velocity and is the differential operator del.
Consider water undergoing steady flow through a hosepipe that has a gradually decreasing cross section. Because water is incompressible in practice, conservation of mass requires that the flow is faster at the end of the pipe than at the start. Because the flow is steady, the Eulerian derivative of velocity is everywhere zero, but the substantive derivative is nonzero because any individual parcel of fluid accelerates as it moves down the hose.
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