Investigations on the Roll Stability of a Semitrailer Vehicle Subjected to Gusty Crosswind Aerodynamic Forces

Threats of high crosswind gusts on running safety of modern road and rail vehicles have been reported around the world. Under high transient crosswind conditions, sudden changes in vehicle aerodynamic forces can lead to adverse effects on vehicle dynamics and stability. Moreover, due to increase in maximum speed limits and body dimensions of commercial vehicles as well as reduction in their weights, large class vehicles, in particular, are more prone to rollover accidents in strong crosswind situations, especially at cruising speeds or at exposed sites. Such crosswind accidents have been observed even at low vehicle speed of 15 m/s in adverse windy weather. It is therefore essential to conduct detailed investigations on the aerodynamic performance of commercial vehicles under crosswind conditions in order to improve their crosswind stability.

In this study, estimation of unsteady aerodynamic forces acting on a high-sided tractor-trailer vehicle have been carried out based on experiential and numerical simulations. Although natural crosswind gusts are high-turbulent phenomena, and have a large variability in types and origins, this study suggests employing two gust scenarios based on two different methods:

1. Transient wind gust scenario developed in wind-tunnel to represents a high-sided tractor semitrailer vehicle moving on a road in moderate wind condition and immediately being hit by wind gust.

2. Deterministic crosswind scenario with gusts in exponential shapes has been considered to predict crosswind aerodynamic forces of a high-sided tractor semitrailer vehicle moving through wind exposed area. This scenario is specified in the Technical Specification for Interoperability (TSI) standard, but it has been employed in this study in combination with Computational Fluid Dynamics (CFD).

A series of time-dependent crosswind aerodynamic forces acting on the tractor-semitrailer vehicle have been predicted. Moreover, to illustrate the potential influence of crosswind gusts on a high-sided tractor semitrailer vehicle, instantaneous gust flow structures for proposed wind scenarios and wind pressure fields were presented. The results show that both wind gust scenarios have significant unsteady effects on the side aerodynamic force and the roll moment of the vehicle. Furthermore, there are significant variations in aerodynamic loads, and the flow field becomes more complicated, consistent with the gust’s strength. These conclusions strongly suggested the importance of considering the unsteady aerodynamic forces in the analysis of heavy vehicle roll dynamics.

Lateral load transfer ratio (LTR) is a criterion that is often used for designing ground vehicle rollover warning technologies to indicate the vehicles rollover status. Generally, LTR index depends on road geometry and vehicle dynamic characteristics. However, as mentioned above, crosswind loads have the potential to influence the roll stability and therefore the safety of large commercial vehicles. Therefore, this thesis presents the research carried out to improve the traditional LTR for a high-sided tractor semitrailer vehicle to be more efficient in crosswind environment. For this purpose, since experimental investigations on vehicle rollover dynamics are difficult to carry out, a coupled simulation of crosswind aerodynamic forces and multi-body vehicle dynamics has been proposed. In this method, the predicted aerodynamic forces result due to the proposed wind scenarios were input into multi-body dynamic simulations of the tractor semi-trailer vehicle that were performed through Adams/Car software. Based on this coupled analysis, dynamic responses of the vehicle to fluctuating crosswind conditions have been predicted. Moreover, all parameters of the LTR index such as body roll angle and lateral acceleration were estimated through a critical turning manoeuvre with crosswind actions. The investigation results show that, in the same manoeuvre, in comparison with the traditional LTR index (i.e., in which crosswind aerodynamic forces are ignored), the improved LTR rollover (crosswind) indicator, has successfully detected wheel lift–up conditions when crosswind aerodynamic loads are considered. Also, average values of the LTR measured under crosswind effects are about 22% higher than those of corresponding traditional LTR index. Therefore, the rollover indicator that has been improved by the proposed methodology can provide more reliable information to the warning or control system in the presence of wind conditions.