Numerical study of debris flight in a tornado-like vortex

Abstract

This paper presents the numerical study on the flight behaviour of spherical compact debris in a tornado-like wind field. The tornado-like vortex corresponding to a swirl ratio of 0.7 was generated using Large-eddy Simulation and the trajectories of 2250 individual debris particles placed in the flow were computed using Lagrangian-particle tracking. The debris corresponded to five groups (A, B1, B2, B3 and C) based on the value of the Tachikawa number (K) which ranged between 0.6 and 2.5. An analysis of the simulated flow field revealed that the tornado-like vortex consisted of two main features - a core at the centre with low velocity (∼0.25 m/s) which was surrounded by thick vortex wall composed of high velocity magnitudes (∼9.4 m/s). Updraft flows were observed around the core of the vortex and as a result, debris positioned around the core radius region were found to be 24% more likely to become wind-borne than debris positioned at the vortex wall region. Three groups of debris (B1, B2 and B3) with varying mass and density were studied for the aerodynamic similarity by retaining the fixed value of K = 1.2; all three debris groups exhibited the propensity to travel with similar flight characteristics. An analysis of the data pertaining to the fight behaviour of the three debris group (A, B1 and C) with varying K revealed that the low mass debris group A (K = 2.5) had the highest propensity to become wind-borne and was more likely to travel for the longest time with considerable variability observed in individual debris trajectories. However, somewhat counterintuitively, the high mass debris group C (K = 0.6) were found to have the furthest impact range despite their short flight duration; this was due the high mass debris being ejected out of the vortex with greater inertia, while debris with a lower mass had a tendency to be trapped in the flow that circulates around the vortex core.