Complex permittivity and scattering characteristics of forest fire ash particles at microwave and millimetre wave frequencies

摘要

The geometric, physical, dynamic and scattering properties of ash particulates resulting from Australian biomass, along with the complex permittivity, have been presented within this dissertation. The rationale behind this work relates to the characterisation of the fundamental scattering properties of ash particulates, with the primary goals being to aid active radar system design and to provide a basic framework for a complex inverse scattering model. The reflectivity coefficient for a volumetrically dispersed medium has been defined by characterising three distinct properties of ash. Firstly, statistical modelling of ash created from various plant and tree species was conducted in order to describe its geometric and material behaviour. Here, similarities between plant and tree species with comparable foliage were noted. Three probability distribution functions (PDF) relating to the projected area, aspect ratio and through thickness dimensions for large ash particles (&0.2mm2) have been mapped. Material investigation has included analysis of the effects of temperature on biomass and the resultant geometric changes this incurs. Furthermore, the effects of natural moisture absorption rates and porosity estimations using measured and micro-computer-tomography (Micro-CT) techniques have been presented. An analysis of the dynamic behaviour of ash particles within a defined volume of space displaying different modes provides the second area of investigation. Particular focus has been given to the ascent and descent phases of the ash particles, with analysis of three dynamic stability modes; namely tumbling, fluttering and chaotic random. Probability distribution functions for orientation and analysis of velocities and Reynolds numbers have been established using video processing techniques. The complex permittivity of ash at both low and high temperatures has been measured. Here, an empirically derived mixing law has been established to theoretically model the complex permittivity of ash. This model also takes into account concentrations of water that may be absorbed by the highly porous material. By applying the knowledge gained from the analysis of ash particles, extensive modelling and measurement work has been carried out to determine their reflectivity. Simulated modelling of the ash has been achieved using a hybrid simulation scheme to accurately implement statistical models over a wide range of frequencies (1-40GHz).