Aerodynamic Investigations of Vehicles under High Side Wind Conditions on Bridges

Abstract

Crosswind stability is a critical safety factor that needs to be addressed for all types of road vehicles. However, it is especially crucial for trucks and buses due to their large lateral areas. As a result, this makes the vehicles more vulnerable to high side forces caused by incoming side winds. This is because of the fact that, the greater the lateral area, the greater the sensitivity to side winds. In the absence of any shielding, the magnitude of the resulting side force on the vehicles commuting across the bridge will be exacerbated. In addition to side force, the yaw moment acting on the truck is also an important factor that needs to be addressed. The lateral stability of the vehicle is mainly affected by yaw moments. When side winds blow, the presence of a bridge structure, such as a pylon, causes huge turbulent wake flows due to flow separation. Furthermore, when the vehicle rides across the pylon wake, the sudden shielding effect in the wake flow region causes a steep change in the vehicle’s side force. The pressure distribution on the surface of the vehicles as well have continuous variation and thus result in fluctuating yaw moments. These disturbances have an impact on the drivability and maneuverability of the vehicle. Handling stability is extremely important for road safety. If these side wind sensitive vehicles are traveling at high speeds, it may even cause them to roll over in severe cases. So, it is pivotal to analyze the side wind induced aerodynamic forces and moments on road vehicles in order to understand their impact and to provide guidance to drivers on how to maintain the vehicle’s stability when driving in this scenario. The road vehicle taken for this study is a truck. 3D Computational Fluid Dynamics (CFD) simulations are performed to investigate the aerodynamic forces and moments on the truck at different side-wind conditions. The commercial code used to simulate the model is the StarCCM+. Initially, a steady state case is developed and solved using Reynolds Averaged Navier Stokes(RANS) transport equations and a realizable k−epsilon turbulence model with all necessary boundary conditions. The aerodynamic coefficients of the truck are computed at different positions on the bridge. Individual and combined effects of bridge structures such as deck, windshield, and pylon are investigated. Unsteady simulations are also performed, and the results are compared to the steady-state model. The steady state results are used as an initial solution for the unsteady model. The unsteady model is built to simulate the motion of a truck across the bridge by using overset mesh methodology. The simulations are run for 30, 60 and 90 degree yaw conditions. The CFD investigations showed that the presence of deck and windshield(non-porous and solid) reduces the side force acting on the truck by diverting the flow above with a local re-circulation wake region beside the exposed windward surface, and thus reduces the static pressure on the truck. On the other hand, the porous windshields increase the side force on truck when compared against the windshields with no cut-outs. The study on porous windshields revealed an intriguing scenario in which the presence of windshields with more than 80% porosity increases the side force on the truck when compared to the case with no shields.