Unsteady aerodynamic effects on the driving stability of passenger vehicles

Doctoral thesis, 2023

The passenger car is a vital part of modern society, giving people the freedom of flexible travel. As technology advances, customers increase their demand for future products. The automotive industry must therefore adapt to society's requirements for energy-efficient travel, where developing low-drag vehicles is key. However, if not designed with care, streamlined bodies of low drag might impair the driving stability. In addition, raised customer demands of perceived control and stability elevate the research needs on driving stability.

Vehicles travelling on open roads are continuously exposed to changing crosswind conditions. Most road vehicles have the aerodynamic centre of pressure located at the front, making them sensitive to these unsteady crosswinds. Strong winds and sensitive vehicle designs degrade the driving stability perceived by drivers and passengers. Furthermore, as aerodynamic loads increase with flow velocity, the sensitivity becomes greater at high speeds. High speeds affect stability performance even without variations in crosswind. The balance of the time-averaged lift forces between the front and rear axles influences understeering and, consequently, vehicle handling. However, the averaged forces cannot always predict the stability performance, which increases the need to explore the unsteady aerodynamic effects on vehicle handling.

The assessment of driving stability for a vehicle in development is often done on test tracks in late design phases when prototype vehicles are available. However, the current demands of faster development times require robust virtual tools for earlier assessment. This thesis aims to develop virtual tools for assessing straight-line driving stability and to gain insights into the interdisciplinary physics between aerodynamics and vehicle dynamics.

By conducting on-track measurements, it was demonstrated that crosswinds deteriorate driving stability and that the vehicle motions of lateral acceleration and yaw velocity correlate with the drivers' subjective assessment. A driving simulator study confirmed these lateral motions, and the path curvature, as significant measures. To reduce the lateral vehicle response to crosswinds, the centre of gravity should move forward, while the aerodynamic yaw moment should be reduced (moving the centre of pressure rearward). For high speed stability, without varying crosswinds, it was demonstrated that the unsteady base wake also plays an important role. Stability issues on the test track correlated with bi-stable wake dynamics, primarily affecting the fluctuating rear lift force. Configurations that stabilised the wake led to subjective improvements on the test track, highlighting the importance of unsteady wake aerodynamics.