In mathematics and

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...

, especially the study of

mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objects ...

and

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) ...

, the d'Alembert-Euler condition is a requirement that the streaklines of a flow are

irrotational In vector calculus, a conservative vector field is a vector field that is the gradient of some function (mathematics), function. A conservative vector field has the property that its line integral is path independent; the choice of any path betwee ...

. Let x = x(X,''t'') be the coordinates of the point x into which X is carried at time ''t'' by a (fluid) flow. Let

\ddot=\frac

be the second

material derivative In continuum mechanics, the material derivative describes the time rate of change of some physical quantity (like heat or momentum) of a material element that is subjected to a space-and-time-dependent macroscopic velocity field. The material de ...

of x. Then the d'Alembert-Euler condition is: :

\mathrm\  \mathbf=\mathbf. \,

The d'Alembert-Euler condition is named for

Jean le Rond d'Alembert Jean-Baptiste le Rond d'Alembert (; ; 16 November 1717 – 29 October 1783) was a French mathematician, mechanician, physicist, philosopher, and music theorist. Until 1759 he was, together with Denis Diderot, a co-editor of the '' Encyclop� ...

and

Leonhard Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ma ...

who independently first described its use in the mid-18th century. It is not to be confused with the Cauchy–Riemann conditions.

References

* See sections 45–48.
d'Alembert–Euler conditions
on the Springer Encyclopedia of Mathematics Fluid mechanics Mechanical engineering Vector calculus {{math-stub