ABSTRACT

Although the readers of this text may intuitionally understand what the term “turbulence” means, and everybody can see turbulent £ows every day by observing the smoke from a chimney, the water in a river, or a milk drop falling into a glass of water, it is very difŸcult to deŸne turbulence precisely and concisely. For instance, Hinze (1975) used the following deŸnition:

Bradshaw (1971) suggested a more sophisticated deŸnition to emphasize the structure of turbulence:

Many other formulations can be found in the literature, but the two aforementioned deŸnitions re£ect the features of turbulence on which the mainstream models of premixed combustion are based. These are the randomness and unsteadiness of the £ow, the existence of eddies of widely varying sizes, and the important role played by vortex stretching. It is worth stressing, however, that the randomness of a turbulent £ow should not be overestimated. Turbulence differs from a totally random process, for example, a turbulent velocity Ÿeld should satisfy the continuity and Navier-Stokes equations.