Aerodinamica

Research on Aerodynamic Drag Reduction by Vortex Generators
Masaru KOIKE* Tsunehisa NAGAYOSHI* Naoki HAMAMOTO*

Abstract
One of the main causes of aerodynamic drag for sedan vehicles is the separation of flow near the vehicle’s rear end. To delay flow separation, bump-shaped vortex generators are tested for application to the roof end of a sedan. Commonly used on aircraft to prevent flowseparation, vortex generators themselves create drag, but they also reduce drag by preventing flow separation at downstream. The overall effect of vortex generators can be calculated by totaling the positive and negative effects. Since this effect depends on the shape and size of vortex generators, those on the vehicle roof are optimized. This paper presents the optimization result, the effect ofvortex generators in the flow field and the mechanism by which these effects take place. Key words: Body, Aerodynamics, Aerodynamic Devices, Flow Visualization, Computational Fluid Dynamics (CFD)

1. Introduction
To save energy and to protect the global environment, fuel consumption reduction is primary concern of automotive development. In vehicle body development, reduction of drag is essentialfor improving fuel consumption and driving performance, and if an aerodynamically refined body is also aesthetically attractive, it will contribute much to increase the vehicle’s appeal to potential customers. However, as the passenger car must have enough capacity to accommodate passengers and baggage in addition to minimum necessary space for its engine and other components, it is extremelydifficult to realize an aerodynamically ideal body shape. The car is, therefore, obliged to have a body shape that is rather aerodynamically bluff, not an ideal streamline shape as seen on fish and birds. Such a body shape is inevitably accompanied by flow separation at the rear end. The passenger car body’s aerodynamic bluffness, when expressed by the drag coefficient (CD), is generally between 0.2and 0.5, while that of more bluff cubic objects is greater than 1.0 and that of the least bluff bullets is less than 0.1. Two elements that have major influence on the drag coefficient of a bluff object are the roundness of its front corners and the degree of taper at its rear end. The importance of the influence of the rear taper in passenger cars can be described as follows: Fig. 1 schematicallyshows the flow around a sedan. Because of the presence of a trunk at the rear, the flow separates at the roof end and then spreads downward. As a result, the flow around the car is similar to that around a streamline-shaped object with a taper at the rear. For this reason, a sedan with a trunk tends to have smaller drag coefficient value than a wagon-type car.
* Studio Package EngineeringDepartment, Research & Development Office

Fig. 1

Flow around a sedan

In other words, taper at the rear has the effect of delaying flow separation (or shifting the flow separation point downstream). A well-known example for intensifying the flow separation delaying effect is utilizing a dimple (like the ones on golf balls)(1). Adding dimple-shaped pieces can lower the CD to a fraction of itsoriginal value. This is because dimples cause a change in the critical Reynolds number (the Reynolds number at which a transition from laminar to turbulent flow begins in the boundary layer). There are reported examples of aircraft wings controlling the boundary layer, in which vortex generators (hereinafter referred to as VG(s)) successfully delayed flow separation even when the critical Reynoldsnumber is exceeded(2). Although the purpose of using VGs is to control flow separation at the roof end of a sedan, it is so similar to the purpose of using VGs on aircraft. To determine the shape of sedan VGs, the data on aircraft VGs are referred to(2).

2. Mechanism of flow separation and objectives of adding vortex generators
Fig. 2 shows a schematic of flow velocity profile on the vehicle’s...