Experimental and semi-analytical studies on the aerodynamic forces acting on a vehicle passing through the wake of a bridge tower in cross wind

In a strong cross wind, sudden change of wind velocity in the wake of a bridge tower causes rapid change of the aerodynamic forces acting on a passing vehicle, which may result in an accident due to driver’s miss-steering. This paper discusses and clarifies the transient characteristics of the aerodynamic forces acting on a ground vehicle in high cross winds when the vehicle is passing through the wake of a bridge tower using a scale model in a wind tunnel experiment. Various parameters, such as wind speed and direction, type of vehicle and tower, were considered in the study. The side force and yawing moment acting on the vehicle model were measured using strain gauges on the model-supporting system that was designed as two cantilever beams with high stiffness. Characteristics of the aerodynamic forces acting on the vehicle model were described and the prediction methods that considered the wind non-uniformity acting on the vehicle were proposed and their applicability was studied. For the side force, the quasi-steady forces based on the relative wind velocity acting at the c.g. of the vehicle model generally agreed with the measured results. Changes in side force were found to be more or less proportional to the wind speed distribution. For the yawing moment, however, the quasi-steady prediction did not agree well with the measured results due to the wind non-uniformity. Therefore, a modified prediction method was proposed that included the effects of wind non-uniformity in a simple manner. Depending on the case, the agreement of prediction and measured results was still not good quantitatively, but from the qualitative point of view, the proposed prediction method could reproduce the yawing moment peak that was similar to the experimental results.