Predictive model of perceived driving stability at high speeds under aerodynamic excitations
Doctoral thesis, 2023
The automotive industry is continuously advancing towards more energy-efficient vehicle designs. Streamlined vehicles have low aerodynamic drag but have the potential to be unstable when exposed to external excitations such as unsteady aerodynamic forces created by the flow of air around them. Before signing off a new vehicle for production, several on-road test scenarios are conducted by professional drivers to evaluate the performance. Finding vehicle instabilities and proposing solutions to problems during late phases of development is challenging and costly.
The objective of this thesis is to correlate and predict the driver's subjective evaluation of high speed straight-line driving stability with measurable quantities in early design phases. In this work, substandard straight-line drivability was investigated on-road using different aerodynamic devices for generating high rear lift and asymmetric aerodynamic forces. These aerodynamic devices were then paired with stabilizers, called side-kicks, which helped to define the flow separation and improved the drivability of the tested vehicle. Vector plots of the mean and standard deviation of lateral acceleration, yaw velocity, steering angle, and steering torque were used to understand vehicle behavior for the paired configurations and relate to the difference of subjective evaluation of drivability within each pair. The ride diagram was used to separate the presence of transient behavior and study its impact on subjective evaluation. The qualitative assessment of the resulting trends agrees well with the subjective evaluation of the driver.
Following this, experimental trials were conducted in driving simulators and on-road, in order to have an in-depth understanding of drivers' subjective evaluation and responses to external excitations. Both common and professional test drivers were involved in the study. The results provided insight into the excitation frequencies and amplitudes of interest. From the test data, mathematical models were generated that can predict the drivers' subjective evaluation after experiencing induced external excitations. The outcome showed the impact of drivers' steering on their subjective evaluations towards these excitations. The on-road study revealed that higher roll and longitudinal noises reduce the drivers' sensitivity to external excitations. Headwind magnitude and lateral motion in a certain frequency range experienced by the human upper body contribute to drivers' identification of excitations. The resulting predictive model can be used to pinpoint the time of occurrence of observable aerodynamic excitations and provides their characteristics in early development phases. Since the models represent measurements from the cabin, they should be valid for different vehicles.
The objective of this thesis is to correlate and predict the driver's subjective evaluation of high speed straight-line driving stability with measurable quantities in early design phases. In this work, substandard straight-line drivability was investigated on-road using different aerodynamic devices for generating high rear lift and asymmetric aerodynamic forces. These aerodynamic devices were then paired with stabilizers, called side-kicks, which helped to define the flow separation and improved the drivability of the tested vehicle. Vector plots of the mean and standard deviation of lateral acceleration, yaw velocity, steering angle, and steering torque were used to understand vehicle behavior for the paired configurations and relate to the difference of subjective evaluation of drivability within each pair. The ride diagram was used to separate the presence of transient behavior and study its impact on subjective evaluation. The qualitative assessment of the resulting trends agrees well with the subjective evaluation of the driver.
Following this, experimental trials were conducted in driving simulators and on-road, in order to have an in-depth understanding of drivers' subjective evaluation and responses to external excitations. Both common and professional test drivers were involved in the study. The results provided insight into the excitation frequencies and amplitudes of interest. From the test data, mathematical models were generated that can predict the drivers' subjective evaluation after experiencing induced external excitations. The outcome showed the impact of drivers' steering on their subjective evaluations towards these excitations. The on-road study revealed that higher roll and longitudinal noises reduce the drivers' sensitivity to external excitations. Headwind magnitude and lateral motion in a certain frequency range experienced by the human upper body contribute to drivers' identification of excitations. The resulting predictive model can be used to pinpoint the time of occurrence of observable aerodynamic excitations and provides their characteristics in early development phases. Since the models represent measurements from the cabin, they should be valid for different vehicles.
driver-vehicle interaction
unsteady aerodynamics
Driving simulator
on-road tests
predictive model
vehicle stability
subjective evaluation
Author
Arun Kumar
Chalmers, Mechanics and Maritime Sciences (M2), Vehicle Engineering and Autonomous Systems
The automotive industry is continuously advancing towards more energy-efficient vehicle designs. Aerodynamic drag is a significant contributor to vehicle energy consumption. Vehicles with low aerodynamic drag are susceptibility to instabilities especially at high speeds. Depending on the type of instability, the driver’s experience can range from the sense of discomfort to poor safety.
Before signing off a new vehicle for production, several on-road test scenarios such as high-speed driving are conducted by professional drivers to subjectively evaluate the vehicle performance. Finding vehicle instabilities and implementing solutions that would solve the issue during late phases of vehicle development is challenging and costly. Furthermore, subjective evaluation of vehicle instability is a result of vehicle-driver interaction. Hence identifying and solving vehicle instability at early stage of development is not an easy task.
This thesis investigates methodologies to correlate and predict the driver's subjective evaluation of vehicle stability with measurable vehicle-driver motions early in the development. Several sensors are used to measure these motions. Experimental trials were conducted on road and in driving simulators. Aerodynamic devices were used to create vehicle instabilities on road. Difference in driving characteristics between common and professional drivers were investigated. This work investigated the characteristics of these measured vehicle-driver motions and their influence on the drivers' ability to identify excitations. Mathematical models were developed to predict the time of occurrence of observable excitations and provided their characteristics. The proposed methodology provides a platform for simulation tools to assess aerodynamic stability in early phase of vehicle development.
Before signing off a new vehicle for production, several on-road test scenarios such as high-speed driving are conducted by professional drivers to subjectively evaluate the vehicle performance. Finding vehicle instabilities and implementing solutions that would solve the issue during late phases of vehicle development is challenging and costly. Furthermore, subjective evaluation of vehicle instability is a result of vehicle-driver interaction. Hence identifying and solving vehicle instability at early stage of development is not an easy task.
This thesis investigates methodologies to correlate and predict the driver's subjective evaluation of vehicle stability with measurable vehicle-driver motions early in the development. Several sensors are used to measure these motions. Experimental trials were conducted on road and in driving simulators. Aerodynamic devices were used to create vehicle instabilities on road. Difference in driving characteristics between common and professional drivers were investigated. This work investigated the characteristics of these measured vehicle-driver motions and their influence on the drivers' ability to identify excitations. Mathematical models were developed to predict the time of occurrence of observable excitations and provided their characteristics. The proposed methodology provides a platform for simulation tools to assess aerodynamic stability in early phase of vehicle development.
Driving Forces
Sustainable development
Areas of Advance
Transport
Subject Categories
Vehicle Engineering
Fluid Mechanics and Acoustics
ISBN
978-91-7905-870-8
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5336
Publisher
Chalmers