Aerodynamic drag on trains in tunnels includes friction drag and pressure drag, which are respectively the algebraic sums of the longitudinal components of all shear and normal forces on the train surfaces. The first of these is broadly similar to its counterpart in the open. The second is shown to include two effects that are usually negligible in the open. It is shown that the overall drag force must be regarded as the sum of individual components, each of which behaves differently from the others. The components can be represented by non-dimensional coefficients whose numerical values are nearly constant for a wide range of train journeys. In contrast, the overall drag coefficient is shown to vary significantly, even during any particular journey. The principal causes of aerodynamic drag in tunnels are also the principal causes of pressure waves that give rise to potential aural discomfort for passengers. It is argued that a common method of analysis is appropriate for the prediction of both of these effects. Ill-defined train areas are shown to be a potentially serious source of confusion in the estimation and interpretation of drag coefficients. The relevant train area is shown to be its aerodynamic area, the definition of which is explained.
Effect of drag force and translation of bubbles on nonlinear pressure waves with a short wavelength in bubbly flows
