• forensic science;
  • traffic accident;
  • wind-drag coefficient;
  • Reynolds number;
  • flow analysis;
  • CFX

Abstract:  This case report describes a noncontact traffic accident involving a motor scooter and a tractor–trailer with a focus on the wind-drag effect. We used load cells to measure the drag force acting on a substantially similar motor scooter when a substantially similar tractor–trailer passes by it, taking into consideration various speeds of the tractor–trailer and distances between the two vehicles. A three-dimensional steady-state flow analysis was also performed by using the CFX program for computational fluid dynamics to examine the streamlines and the pressure distribution around the tractor–trailer at various speeds. From the experiment, for a separation distance of 1.0 m (3.28 ft) and a speed of 90 km/h (55.9 mph), the maximum resultant drag force is 124.5 N (28 lb); this constitutes a degree of force that could abruptly disrupt the stability in maneuvering by an operator who is unaware of the approaching tractor–trailer. In addition, a single equation that relates the tractor–trailer speed to the drag force that acts on the motor scooter was derived on the basis of the Reynolds number (Re) and the wind-drag coefficient (Cd): Cd = 1.298 × 10−7 Re.