Subjective perception and prediction model of vehicle stability under aerodynamic excitations

Licentiate thesis, 2021

The current automotive era is moving towards electrified vehicle propulsion. As a result, an energy efficient vehicle design becomes one of the top priorities. From an aerodynamics point of view, the vehicle should be more streamlined for minimal aerodynamic drag. Such designs have the potential to enhance vehicle sensitivity when exposed to external disturbances such as unsteady aerodynamic forces and moments created by the flow of air around the vehicle.
Before signing off for production, several on-road test scenarios are conducted by professional drivers to evaluate the new vehicle’s performance. Finding vehicle instabilities and proposing solutions to problem’s during such late phases of development is challenging in many aspects.
The objective of this paper is to correlate and predict the driver’s subjective perception on high-speed straight-line driving stability with measurable quantities in the early phase of development.
In this work, different aerodynamic devices were used for generating higher lift and asymmetric aerodynamic forces resulting in substandard straight-line drivability on-road. An inverted wing, an inverted wing with an asymmetric flat plate, and an asymmetric air curtain attached under the bumper were the selected aerodynamic devices paired with and without bumper side-kicks. The side-kicks help define the flow separation, thus improving the drivability of the tested vehicle. Plots of mean and standard deviation and ride diagrams of lateral acceleration, yaw velocity, steering angle, and steering torque are used to understand vehicle behaviour for the paired configurations and relate to the difference of subjective judgment of drivability within each pair. The ride diagram was used to separate the presence of transient behaviour and study its impact on subjective judgement. The qualitative assessment of the resulting trends agrees well with the subjective judgement of the driver.
Clinical tests were conducted using driving simulators, in order to have an in-depth understanding of the subjective perception and responses of drivers towards external disturbances. Both common and experienced test drivers were involved in this test. The results provided an insight towards the disturbance frequencies and amplitudes of interest. From the test data, a model is generated that can predict the drivers’ subjective perception after experiencing induced external disturbances. The outcome also shows the impact of drivers’ steering action on their subjective perceptions towards these disturbances.