Experimental and analytical trajectories of simplified debris models in tornado winds.

摘要

Wind-borne debris is considered as a major source of damage to civil structures during strong wind storms such as hurricanes and tornadoes. After wind-induced failure, building components can become airborne as missiles and cause significant damage to the surrounding structures. There are many studies to model simplified wind-borne debris in flight to predict its trajectory and maximum speed in straight-line wind. However, there has been limited research modeling wind-borne debris in three-dimensional wind field of a tornado. In the current study, ISU's tornado simulator was used to validate a quasi-steady numerical model used to simulated free-flight trajectories of two types of wind-borne debris. The coordinates of the trajectories in the experiments were captured using two cameras and the principles of stereo-photogrammetry. The experimental trajectories were compared to a numerical simulation model that used the tornado wind parameters based on empirical models of measured velocity profiles and assumed or measured aerodynamic properties of the selected debris shapes. The wind-borne debris models that were used for validation were (a) a sphere that is representative of compact objects, and (b) a circular cylinder with an aspect ratio of 3:1 (length to diameter) that is representative of a slightly elongated object such as a vehicle or a timber/steel beam used in construction. The comparison between the observed- and numerically-simulated trajectories for both the sphere and cylinder in controlled-flight condition was excellent and thereby it validated the equations used to model the forces acting on the objects. In the numerically-simulated free-flight trajectories of the cylinder, the effects of moment and angular accelerations were neglected to simplify the equations of motion. The error between the observed- and numerically-simulated trajectories for both the sphere and cylinder in free-flight was low at the beginning of flight and increased with time. The trajectory predictions for both objects can be further improved by including turbulence in the velocity model used and modeling the second-order force effects. The prediction of the trajectory for the cylinder can also be substantially improved by considering the rotational components of its motion in free flight.