Fluid mechanics concepts

Abstract This chapter sets out some basic fluid mechanics concepts that are required in the study of train aerodynamics and that will be referred to later in this book. Dimensionless analysis and dimensionless variables are described, and this is followed by a discussion of two different frames of reference – the ground and train frames. Qualitative descriptions are then given of the principles of conservation of mass, momentum and energy, and of the concepts of free flows, boundary layers, turbulence, separated flows and wakes. This leads into a more quantitative discussion of the equations of motion in a variety of different forms, which will all be referred to later in this book. This section attempts to show the interrelated nature of many of the practical problems in train aerodynamics that might otherwise appear quite diverse. The nature of flow turbulence is then discussed in more detail, and this is followed by a description of the natural wind close to the ground, including wind speed probabilities, wind speed and turbulence profiles and turbulence spectra.