ABSTRACT

In hydraulics, as in other areas of fluid mechanics, the flows of practical relevance are almost always turbulent; this means that the fluid motion is highly random, unsteady and three-dimensional. Due to these complexities, the turbulent motion and the heat and mass-transfer phenomena associated with it are extremely difficult to describe and thus to predict theoretically. Turbulence models can only give an approximate description, and, with a particular set of empirical constants, they are valid only for a certain flow or at the most a range of flows. Turbulent transport processes are strongly problem-dependent; for example, they depend on geometrical conditions of large and small scale, on viscous and swirl effects, and on buoyancy. A good turbulence model should however allow extrapolation from the empirical data which entered the model. The chapter also presents an overview of the key concepts discussed in this book.