A Holistic Approach to Aerodynamics of Intercity and Interurban Buses 2014-01-0581

The aerodynamic drag of cars, trucks and buses have been closely examined over the years. Many of them focus on the front end and to some extent on rear end of the vehicles [1]. Of course these are the two surfaces that contribute to more than 85 % of the total drag. This is because these surfaces are almost normal to the direction of air flow and hence create enormous pressure differences and hence drag. A lot of optimization has also gone into these, by way of reducing the sharp corners at ‘A’ pillars, introducing aerodynamic dome and even ‘boat tail flap plates’ [2-3] for some trailers.

However, part of the vehicle that has not received sufficient attention in aerodynamic drag considerations is the ‘transverse outer profile’ of vehicle. This transverse outer profile is nothing but the cross sectional profile formed by the vehicle's sides, roof and their integration. This has a considerable impact on aerodynamic drag, especially in buses travelling on highways at considerably high speeds. In buses the roof and sides constitute 75 to 87% (considered for bus lengths of 7 to 12 meters) of the overall outer surface area of the vehicle.

This paper discusses the effect of this ‘transverse outer profile’ on the overall aerodynamic drag of the vehicle. CFD (Computational Fluid Dynamics) is used as a tool to understand the skin friction, vortices and hence drag, created by the transverse outer profile. Consequently ways of reducing this drag component, by optimizing the transverse outer profile is evaluated. The variables in this evaluation include - roof curvature, side panel profile and the cant rail cross-section. On the other hand the constraints include various regulatory and ergonomic requirements that define the boundaries of interior saloon space that is enveloped by the outer profile. Within this narrow space, various options of outer profile are tested in CFD, to arrive at the profile that has the lowest drag coefficient, popularly known as Cd.