Numerical study of debris flight in tornado-like vortices

The dangers and damages caused by tornadoes and the associated flying debris have been an issue and long been recognised. Whilst the flow fields of tornadoes and debris flight under different wind conditions have been investigated comprehensively, the study on tornado-induced wind-borne debris were surprisingly sparse. The aim of this research is to characterise tornado flows and evaluate the flying behaviour of debris in tornado flows. Two tornado-like vortices with different swirl ratios were numerically generated using Large-eddy simulation and the trajectories of five groups of compact debris with varying Tachikawa number were computed using Lagrangian particle tracking. An analysis of the simulated flow field revealed that the two tornado-like vortices have different characteristics but similar flow structure; a core with downwards flow and vortex walls with high tangential velocity and updraft flows around the core. The investigation on debris fight behaviour showed that in both of the vortices, low mass debris group with high values of Tachikawa number had the highest tendency to become wind-borne and had the longest flight duration with considerable variability observed in debris trajectories. However, the high mass debris group with low values of Tachikawa number were observed to have greater impact range despite the short flight duration; this was due to the high mass debris being ejected out of the vortex with greater inertia, while debris with a lower mass had a tendency to circulate around the vortex. Finally, the initiation and flight altitudes of all wind-borne debris were found to be directly correlated with the updraft flows of the vortices

A Study of the Effects of Tornado Translation on Wind Loading Using a Potential Flow Model

This paper investigates the effects of tornado translation on pressure and overall force experienced by an airfoil subjected to tornado loading and presents a framework to reproduce the flow conditions and effects of a moving tornado. A thin symmetrical airfoil was used to explore the effects of tornado translation on a body. A panel method was used to compute the flow around an airfoil and an idealised tornado is represented using a moving vortex via unsteady potential flow. Analysis showed that the maximum overall pressure at a point was found to increase by up to 20% when the normalised translating velocity was 10% of the tangential velocity, but increases up to 60% when the normalised translating velocity is 30% of the tangential velocity. Investigation on the impact of varying airfoil thickness (Case 2) revealed that the location of the tornado has significant effect on the overall lift force. However, the overall lift force appeared to be largely insensitive to the tornado translation velocity due gross changes in pressure on either side of the airfoil cancelling each other out. Further comparison with varying airfoil sizes and distance to tornado translating path (Case 3) showed that the relative inflow and outflow angle is the primary factor affecting the lift on the airfoil. Additionally, the maximum forces on a body subjected to a moving tornado can be predicted using uniform flow providing that the appropriate range of inflow angles are known. Based on the analysis on the database of National Oceanic and Atmospheric Administration (NOAA), the normalised translation speed of the recorded tornadoes across the EF scales, appears to vary from 0.25 to 0.37, with an average of 0.32 (∼18.8 m/s). Finally, the framework using uniform flow to reproduce the flow conditions which are comparable to those generated by a translating vortex simulator is proposed and discussed in detail

Investigation on debris initialization 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

Using crop fall patterns to provide an insight into thunderstorm downburst

This paper examines whether crop fall patterns due to thunderstorm downburst-like events can provide an insight into the flow structure of a downburst. To explore this phenomenon, a novel three-dimensional analytical model for the velocity flow field is derived and coupled with a generalised plant model which is capable of modelling crop failure. Through this approach we have established the concept of the lodging front – a dimensionless variable used to quantify the spatial extent of crop failure. Crop failure is shown to result in a diverging pattern and the angles at which the crop falls has been shown to collapse onto a single curve when suitably normalised. Comparison with full-scale data suggests that the model is capable of predicting realistic crop fall patterns and could potentially be used in the future to assess the strength of downbursts

The Flow Around a Lorry Platoon Subject to a Crosswind—a Detached Eddy Simulation

Technological developments in Connected and Autonomous Vehicles (AVs) have created opportunities to allow groups of vehicles to travel in close proximity, through methods known as platooning. There are potential benefits from platooning in terms of fuel consumption, through a reduction in aerodynamic drag for trailing vehicles in the platoon; however, it is still not understood whether these benefits remain when the platoon is subject to crosswind. For the first time, this study examines the flow structure and aerodynamic response of a platoon of eight closely spaced lorry type vehicles subjected to a crosswind with a 30o yaw angle. The numerical study is conducted using a Delayed Detached Eddy Simulation. It is observed that there is an increase in the overall drag when compared to a similar simulation with no crosswinds. Streamline illustrations indicate that a recirculation region is formed on the leeside of the lorries, which with the chosen vehicle spacing does not exhibit any interactions with the consecutive lorry, resulting in a diminished drag reduction. High pressure on the windward side of the lorries and a low pressure region on the top of the lorry boxes results in high lift, side force and rolling moment coefficients, but relatively minor pitching and yawing moments