Investigation On Granular Collapsing Flow Driven By Gravity And Its Interaction With Different Media

The collapse and flow of discrete granular media widely exist in geophysical and engineering environments,such large-scale complex systems composed of a huge number of solid particles with different shapes,which will yield and subsequently flow under the influence and trigger of gravity or other factors,and form a long-distance and large-scale diffuse deposition downstream.Real mass flow events usually exhibit typical characteristics of high mobility and large volume,and its flow process can be affected by various external factors,which may produce drastic impact effects on media and structures in the flow path.Such destructive flow and deposition processes can pose significant threats to the surroundings,infrastructures,and human safety.Therefore,it is of great value for scientific research and engineering application to deeply understand the basic dynamic behavior of complex mass flow and to analyze the interaction process between the mass flow and the surrounding media or structures.In this dissertation,granular matter with typical discrete and energy dissipation characteristics is selected as our main research object,and the dynamic behavior of the granular system in different initial configurations,the flow regime and deposition morphology on the erodible surface,and the interaction process with the baffle of finite width along the flow direction are investigated by building a small-scale experimental setup with controllable initial configuration and state and utilizing a high-speed measurement system.This provides an effective way to directly and quantitatively understand the complex flow processes of large-scale mass flow in geophysics.The main contents of the dissertation are as follows.1.A small-scale inclined granular flow experimental platform with precisely controllable initial configuration and flow state is designed,which includes three parts,namely the external structure,internal PLC control system,and additional force sensors.A high-speed image synchronization system for measuring granular flow is developed,which contains a high-speed camera,an IPC,a LED light source,and professional control software.By adopting a reliable image processing tool and PIV(Particle Image Velocimetry)method,the measurement and analysis of granular flow and deposition processes in diverse regimes along different orientations can be realized.2.The collapse and deposition characteristics of columns with a range of aspect ratios on erodible substrates are systematically investigated based on the experimental runs and the corresponding DEM numerical simulation of horizontal granular column collapse configurations.Three typical erosion-deposition regimes are observed in the experiments,which do not depend on the initial aspect ratio of the column but are determined by the difference in material properties between the particle of column and substrate.The transient dynamic behavior of the particles moving on the erodible bed was analyzed in terms of the evolution characteristics of the instantaneous profile,the distance of the flow front,and the velocity distribution of the collapsed material over time.By analogy with the theory of the rigid ball obliquely impacting granular bed,kinetic equations are developed to characterize the motion of the flow front of the column.The experimental findings are also reproduced and supplemented by the numerical simulation results,the microscopic mechanism that the generation of different erosion-deposition patterns is further revealed,and the critical density ratios at which the flow regimes transition occurs are also given.Finally,based on the results of experiments and simulations with different initial aspect ratios and density ratios of particles,a generalized scaling law of deposition and erosion morphologies of granular columns collapse on the horizontal substrate with erosion effect is established.3.A bilinear experimental configuration of inclined granular flow is constructed by placing a horizontal substrate with different properties at the bottom of the inclined chute,so as to realize the modeling of the acceleration and deceleration processes of the particle spreading on different surfaces.In our experiments,the inclination angle and initial volume of granular material were taken as the main variables,two highspeed cameras were used to record the motions of particles simultaneously along different viewing angles,which help us to track and analyze in detail the dynamic behaviors of particles during the flow process,such as transient profile,migration of the mass center,horizontal propagation,and velocity evolution.On this basis,the flow and deposition morphologies of particles spreading on the horizontal substrate with different properties(rigid or erodible)in diverse regimes are quantitatively studied.Furthermore,a kinetic equation of the flow front is derived,which can simply predict the evolution characteristics of the runout distance versus the time of the particles in different flow regimes when they propagate on the horizontal substrate.4.In the experiments,a series of rigid baffles with different widths and a specific force sensor is installed in the inclined channel to systematically study the interaction between the granular flow on the inclined plane with the rigid baffles along the flow path in different incoming flow regimes.Firstly,some experimental measurements are performed on a simple configuration of inclined granular flow,the parameter space where the steady-state flow regime occurs is identified,and the stability of the experimental setup and the reliability of the flow measurements could therefore be verified by comparing with the available results.Then,for the case of the presence of a baffle,the flow behavior and deposition morphology of the particles disturbed by rigid baffles with limited width were investigated,and three typical flow regimes with the characteristics of reverse propagation and stationary jump are detected.Based on the different flow and jump patterns observed in the experiments,a complete phase diagram that can describe the flow regimes of the granular flow under the blocking effect of the baffle is established.Meanwhile,using the force sensor installed on the baffle,the dynamic impact forces of the granular flow in different regimes on the baffles with limited width are measured.Finally,by effectively correlating the transitions of the different flow regimes of the particles with the evolution of the dynamic impact force,we discussed in detail the complex interaction between the granular flow of different regimes and the rigid baffle with finite width in the flow path.5.Based on the SH model which is derived from the depth average theory and can be used to describe the granular flow downstream along an inclined plane,a theoretical wave model describing the reversed propagation upstream of particles after impacting the limited-width baffles is subsequently developed by introducing a dimensionless parameter that characterizes the relative width of the baffle and considering the source term related to its deposit volume in the control equation.According to the different characteristics on reversed propagation of the shock wave along the slope,this model can accurately describe and effectively predict the evolution of the upstream propagation of particles in diffuse granular bore and stationary jump regimes,respectively.On the other hand,according to the deposition characteristics of particles after being blocked by the baffle,as well as the drag force,passive lateral pressure,and the gravity-and-friction induced force when the flow contacts the baffle,a normal impact force theoretical model suitable for particles impacting the rigid baffles,which is proved to be used to accurately predicted the evolution characteristics for granular flows impacting rigid baffle of limited width down inclines in the diffuse granular bore and stationary jump regimes,is constructed.In his dissertation,we focus on the fundamental problem of the collapse and flow of discrete materials,several small-scale granular flow experimental devices are designed and constructed to systematically study the collapsing characteristics of granular flow and its interactions with the medium in the flow path.The details mainly cover the observation and description of different granular flow and deposition phenomena,quantitative analysis of the dynamic behavior during the flow,various collapse-erosion modes,the revelation of micromechanics in different collapse and flow regimes,and theoretical modeling of granular jump patterns and dynamic impact forces.During the research,some new macroscopic flow phenomena were observed,a few important microscopic mechanisms of granular flow were revealed,and several meaningful quantitative results were also obtained.Of course,these research works can help to deeply understand the complex collapse and flow process of large-scale mass flow which essentially has similar flow characteristics in nature,on the other hand,it can also provide an important theoretical basis and methodology for the optimization and design of protective structures in the engineering environment.