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Euler-Lagrange Description Click here for Reynolds Transport Theorem

In a Nutshell: There are two general methods to characterize fluid dynamics ( includes

linear momentum, angular momentum, mass, and energy). They are the Lagrangian

method of description and the Eulerian method of description depicted in the top, left and

right figures shown below. Picture an intersection with cars representing “fluid particles”,
passing thru the intersection.

In the Lagrangian method one characterizes the motion of the fluid by following individual fluid particles such as fluid particles “a” and “c” shown above. Fluid particle “a” enters the

intersection form the right and continues straight thru whereas fluid particle “c” enters the

same intersection but exits to the right. In the Eulerian method you observe fluid particles

at a fixed location and view all of them entering and leaving that location. You will use the Eulerian description to study the dynamics of fluid motion in elementary fluid mechanics.

The Eulerian description gives rise to the definition of a “control surface” (CS) and a “control

volume” (CV) used in finite volume analysis of fluid dynamics. Later on you will discover

that the control surface can be moving and also deformable.

Click here to return to discussion of conservation of mass.

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