A numerical investigation of debris initialisation in tornado-like wind fields

The process of debris initialisation is examined for two tornado-like vortices with swirl ratios (S) of 0.3 and 0.7. The vortices were modelled using Large-eddy Simulation and the motion of spherical debris were calculated using Lagrangian-particle tracking. A total of 2700 individual debris particles, corresponding to three different groups (A, B and C) with different Tachikawa numbers (K=2.5, 1.2 and 0.6 respectively) were released. The vortex S=0.7 has greater magnitude of tangential velocity (∼12.7 m/s) compared to vortex S=0.3 (∼11.7 m/s). The corresponding maximum vertical velocities in the flow when S=0.3 were approximately twice those corresponding to S=0.7; this resulted in longer flight durations for debris group A, B and C, i.e., 65%, 73% and 58% longer respectively. An analysis of the spatial displacement of the vortex centre relative to the centre of the simulator shows that both vortices have a maximum displacement of less than 0.02 m. This suggests that the low wandering motions of the vortices have minimum effect on the debris initialisation. The vertical velocity component is shown to be key for the generation and sustaining of debris flight. For example, for S=0.3, approximately 22% (group A), 5% (group B) and 10% (group C) more particles were being initialised compared to S=0.7. The ensemble averaged flow fields associated with flight initiation show the greatest difference at low elevation (less than 0.11 m) compared to non-initialising conditions. It was also found that the tangential and radial velocity components have relatively little impact on flight initialisation. The findings presented in this research provide a fundamental insight into the physics which govern debris initialisation in tornado-like flow.