Fluid Particles & Control Volumes


Fluid can pass through any face of the infinitesimal control volume.  Control volumes can be very small as in this case, or very large.

By Joseasorrentino (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

Different frames of reference are used in the study of fluid dynamics. In one frame of reference (Lagrangian) individual particles are tracked as they move during the motion of the fluid. In the other frame of reference (Eulerian), the fluid properties at a specific location are observed as fluid particles pass through this location.

Fluid Particle

A fluid particle is a small volume of material that contains a specific quantity of fluid.  The material in a fluid particle doesn’t change even though the fluid particle can move and change shape and size.

One way of describing the motion of a fluid is to track individual fluid particles and observe changes to the particle as it moves with the flow.Equations that track fluid particles are said to be formulated using a Lagrangian approach.  Fluid particles are used in the derivation of Bernoulli’s Equation.

A fluid system is a collection of fluid particles that will move with the flow and interact with its surroundings.

Control Volume

A control volume is a region of space through which fluid particles may flow in or out.  Since fluid can enter or leave the control volume it contains different fluid particles at different instances in time.

The bounding surface(s) of a control volume are called the control surface(s). 

The control volume can be fixed in size and shape or its size or shape can change with time.  A balloon is an example in which the control volume changes shape and size as fluid enters.

The control volume shape is chosen to facilitate the solution of a particular problem.  A control volume is often fixed in space and size.  Equations based on a control volume that is fixed in space are said to be based on an Eulerian formulation.


Learning Objectives:

Describe the difference between fluid particles and control volumes.


Links and References