Numerical Simulations of Flows around Trains and Buses in Cross Winds

Doctoral thesis, 2008

When cruising in strong winds, ground vehicles can experience strong aerodynamic forces and moments that may increase drag, reduce their stability, generate noise and induce accidents in the case of strong side winds. However, prevention of the unwanted influences of side winds requires that the flow structures around vehicles are fully understood in both the instantaneous and time-averaged flow. The main objective of the present work is to acquire an understanding of the side wind flow physics around two types of ground vehicles, trains and buses. In this work, Large-Eddy Simulation (LES) is used to compute the side wind flow around stationary simplified train models. The side wind flow is obtained at three different yaw angles, 30 degree, 35 degree and 90 degree. The LES results show that the flow around high speed trains involves many small structures because of the instabilities of the shear layers that contribute to the time varying values of their aerodynamic forces and moments. The influence of the shape of the train nose on the flow field under side wind conditions is investigated. The LES results show that the shape of the nose determines to a large extent the characteristics of the wake structures and their associated frequencies. During the second part of the work, Detached-Eddy Simulation (DES) is used to investigate the flow structures around a double deck bus subjected to a 30 degree yaw angle. The influence of gusty winds on the aerodynamics of buses is investigated by making simulations using different proposed wind gust profiles. The influence of the shape of the front of a double-deck bus on its aerodynamics is investigated in CFD simulations using the realizable k-Epsilon RANS model at different designs of the front. The bus is subjected to a 30 degree side wind. For the purpose of shape optimization, the response surface method was used and the optimum shape thereby obtained. The influence of a passive flow control on the boundary layer and heat transfer is investigated by attaching small vortex generators on the surface of a heated, mounted cube. A more turbulent boundary layer is obtained behind the vortex generators and hence an enhancement in the heat transfer coefficient is observed.