Turbulent Flows

Abstract

Most flows encountered in engineering practice are turbulent; they are characterized by the following properties:

• Turbulent flows are highly unsteady. A plot of the velocity as a function of time would appear random to an observer unfamiliar with these flows. The word ‘chaotic’ could be used but it has been given another definition in recent years.

• They are three-dimensional. The time-averaged velocity may be a function of only two coordinates, but the instantaneous field appears essentially random.

• They contain a great deal of vorticity. Stretching of vortices is one of the principal mechanisms by which the intensity of the turbulence is increased.

• Turbulence increases the rate at which conserved quantities are stirred. That is, parcels of fluid with differing concentrations of the conserved properties are brought into contact. The actual mixing is accomplished by diffusion. Nonetheless, this behavior is often called diffusive.

• By increasing the mixing of momentum, turbulence brings fluids of differing momentum content into contact. The reduction of the velocity gradients produced by the action of viscosity reduces the kinetic energy of the flow; in other words, it is dissipative. The lost energy is irreversibly converted into internal energy of the fluid.

• It has been shown in recent years that turbulent flows contain coherent structures — repeatable and essentially deterministic events that are responsible for a large part of the mixing. However, the random part of turbulent flows causes these events to differ from each other in size, strength, and time interval between occurrences, making study of them very difficult.