A Particle-Based Analysis of the Saltation Process: Models, Numerical Methods and Tests

Windblown sand transport is one of the most important natural erosion mechanism in arid zones. It is responsible for landform dynamics, dust aerosols emission and for damaging human infrastructure. In this thesis we present a particle-based numerical model in order to simulate the saltation phenomenon, that is the sand transport mode involving most of the transported mass under normal wind conditions. The model takes into account the fundamental physical aspects of the saltation process, which are essentially the entrainment of particles by wind, their trajectories and their impact on the ground. Starting from basic principles of single particle behaviour, the aim of the model is to obtain information on global saltation quantities, such as the sand particle concentration with respect to ground distance and the sand mass flux with respect to the wind strenght. In the first part of the thesis the physics of saltation is presented, highlighting its main features and the most critical aspects in the process description. This phenomenon is characterized by random behaviour, due to both the presence of a turbulent flow and the random shape and arrangement of the sand grains. This randomness makes the measurement of the descriptive quantities of saltation and its modelling complex to define. Many aspects involved in the process remain uncertain and debated. The development of numerical models can help shed light on some of these issues. The particle trajectories are studied in detail in chapter 2 in order to understand the role of wind flow as an external source of energy. Then, in chapter 3 a numerical model for the saltation process is proposed, with a focus on the simulation algorithms and their extendibility to future improvements. The grain-bed impact dynamics is described by energy-based models without directly simulating the deterministic interaction between particles. We model the complex impact mechanism by means of stochastic approaches. This allows us to reproduce randomness of grain-bed impact due to random shape of particles and their arrangement on the sand bed. Several impact models are proposed in chapter 4. In chapter 5 we present results obtained from numerical simulations.