Numerical calculation of the three-dimensional motion of wind-borne debris

Abstract

Most previous studies of wind-borne debris have only considered two-dimensional (2D) motion. While this can demonstrate many basic principles, it is a special case that is unlikely to occur in a real storm situation. In these 2D cases, the normal force coefficient and the position of the centre of pressure are only functions of the angle of attack. In addition, the centre of pressure is always located on the plane of symmetry. In the present study, rectangular plates with side length ratios of 1, 2 and 4 and long rods with sectional side length ratios of 1, 2 and 3 were wind tunnel tested at a range of angles of attack and tilt angles. The results clearly show that the force coefficients depend on both of these angles. Further it is shown that the centre of pressure moves both towards and across the wind. A model for this movement is presented. A 6-degree-of-freedom (6-DoF) trajectory model is presented, which makes use of the measured forces and the model for the centre of pressure position. This programme also incorporates damping terms and hysteresis effects due to dynamic stall and apparent camber. It is shown that the computed trajectories reasonably match those observed with model plates and rods in free flight. The trajectory model is used to predict the three-dimensional motion of full-scale plates. It is shown that this motion includes significant lateral movement and that horizontal speeds very close to, or even exceeding, the wind speed are predicted.