Contents: Introduction to automobile aerodynamics. Some fundamentals of fluid mechanics. Performance of cars and light vans. Aerodynamic drag of passenger cars. Driving stability in side winds. Operation, safety and comfort. High performance vehicles. Commercial vehicles. Engine cooling systems. Heating, ventilation and air conditioning of motor vehicles. Wind tunnels for automobile aerodynamics. Measurement and test techniques. Numerical methods for computation of flow around road vehicles.

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Aerodynamics of Road Vehicles

Recent experience with different methods of drag prediction

In this study, a passive flow control experiment on a 3D bluff-body using vortex generators (VGs) is presented. The bluff-body is a modified Ahmed body (Ahmed in J Fluids Eng 105:429–434 1983) with a curved rear part, instead of a slanted one, so that the location of the flow separation is no longer forced by the geometry. The influence of a line of non-conventional trapezoidal VGs on the aerodynamic forces (drag and lift) induced on the bluff-body is investigated. The high sensitivity to many geometric (angle between the trapezoidal element and the wall, spanwise spacing between the VGs, longitudinal location on the curved surface) and physical (freestream velocity) parameters is clearly demonstrated. The maximum drag reduction is −12%, while the maximum global lift reduction can reach more than −60%, with a strong dependency on the freestream velocity. For some configurations, the lift on the rear axle of the model can be inverted (−104%). It is also shown that the VGs are still efficient even downstream of the natural separation line. Finally, a dynamic parameter is chosen and a new set-up with motorized vortex generators is proposed. Thanks to this active device. The optimal configurations depending on two parameters are found more easily, and a significant drag and lift reduction (up to −14% drag reduction) can be reached for different freestream velocities. These results are then analyzed through wall pressure and velocity measurements in the near-wake of the bluff-body with and without control. It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant. Investigation of the velocity field in a cross-section downstream the model reveals that, in the same time, the intensity of the longitudinal trailing vortices is strongly reduced, suggesting that the drag reduction is due to the breakdown of the balance between the separation bubble and the longitudinal vortices. It demonstrates that for low aspect ratio 3D bluff-bodies, like road vehicles, the flow control strategy is much different from the one used on airfoils: an early separation of the boundary layer can lead to a significant drag reduction if the circulation of the trailing vortices is reduced.

/pdf/drag-and-lift-reduction-of-a-3d-bluff-body-using-active-2jct7e2gbq.pdf

Drag and lift reduction of a 3D bluff-body using active vortex generators

This paper seeks to further elucidate the wake of the Ahmed body by investigating how the time-averaged flow structures vary with frontal aspect ratio. High-resolution particle image velocimetry results are provided for eight different width Ahmed geometries at Re = 3 × 10^4. It is shown that the narrower the body, the greater the downwash over the back slant, meaning the flow remains more attached. At a critical aspect ratio ( AR = 1.9), the flow downstream changes. The separation over the back slant is shown to be affected by the AR, and this in turn has a significant effect on the circulation in the c-pillar vortices.

https://hal.archives-ouvertes.fr/hal-01167075/document

The effect of aspect ratio on the wake of the Ahmed body

Publisher Summary This chapter focuses on vorticity dynamics on boundaries. One of the central problems in vorticity and vortex dynamics is the interaction of vorticity field and boundaries. The boundary can be a rigid wall, a flexible solid wall, or an interface of two different fluids. Boundaries are the most basic source of vorticity (in particular, the unique source for incompressible flow of a homogeneous fluid in a conservative external body-force field), and the whole life of a vortex usually begins at a boundary. Due to the continuous creation of vorticity from boundaries, a bounded vortical flow is much more complicated than an unbounded flow, in particular in the regions near the boundary. This chapter presents a unified general theory of the vorticity dynamics on various boundaries for viscous compressible flows and then reviews its applications to specific problems mainly confined to incompressible flows. Two types of boundaries are considered in the chapter: a solid boundary, either rigid or flexible; and an interface of two immiscible fluids. Both are regarded as sharp material surfaces. Some other boundaries, such as a porous wall, regrettably are omitted, though they are also important in practice. The chapter also briefly reviews the development of boundary vorticity dynamics.

Vorticity Dynamics on Boundaries

Measurements are presented showing the influence of the trunk length of a road vehicle on drag coefficient. The experiments were made in the ''Politecnico di Torino'' wind tunnel on a 1/5 scale model. Data from balance measurements, body-surface pressure and wake flow surveys are reported. A method is shown for correlating to the aerodynamic drag data from wake survey.

Michele Onorato

Papers

Drag Measurement Through Wake Analysis

Studio della scia di autovetture