Study On Internal Shear Behavior And Its Dynamical Mechanisms Response On Granular Flows

In the emplacement processes,rock avalanches gradually disintegrate into clasts of different shapes and grain sizes due to fragmentation and propagating in the form of granular flow.At present,research on the internal deformation of granular flows is still at an early stage and a systematic and in-depth research on the impacting factors,the evolution of the internal deformation and its effects should be further addressed.In this paper,physical modeling experiments of rock avalanches and the corresponding numerical simulations,as well as the numerical simulation of the granular flow through periodic boundaries are carried out to study shear behavior in granular flows and its dynamical mechanisms response.First,based on the velocity profiles of the granular flows obtained by particle image velocimetry method,the internal shear behavior of the experimental granular flow and its effect on depositional features are studied.On this basis,numerical simulations of the experiments were carried out to further research the evolution of shear behavior and its dependence on various factors.Then,the shear behavior of thick-pile granular flows is studied by numerical simulations of granular flows with periodic boundaries,and the size segregation process and its mechanisms are analyzed.The main conclusions of this research are as follows:(1)Through the analysis of the velocity profiles of the granular flows in the experiments and numerical simulations,it is found that the internal shear of the granular flow increases gradually with the increase of flow depth,which indicates that the shear tends to be localized in the bottom of the granular flow.The numerical simulation of the experiments shows that the bulk shear rate first decreases and then increases from leading edge to the trailing edge.With the increase of grain size,the mean depth-averaged shear rate of the granular flow decreases.With the increase of inclination angle,the mean depth-averaged shear rate of the granular flow increases.As the bottom friction coefficient increases,the mean depth-averaged shear rate of the granular flow also increases.(2)In the monodispersed granular flow experiments,the equivalent friction coefficient decreases with the increase of grain size for granular flow with different grain sizes.For granular flow with the same grain sizes and volumes,the higher the bottom friction coefficient is,the greater the equivalent friction coefficient is,and the granular flow at a higher basal friction boundary shows lower flow mobility.The variations in equivalent friction coefficient with grain sizes are consistent with the variations in the mean depth-averaged shear rate when grain size changes.With the decrease of mean depth-averaged shear rate,the equivalent friction coefficient decreases,which indicates that the internal shear deformation plays a key role in the energy dissipation of the granular flow.On the other hand,the Savage numbers of the simulations of the monodispersed granular flow experiments are all greater than 0.1,which indicates that simulated experimental granular flows presented here are mainly inertia regime.In the same experiments,the Savage number decreases first and then increases,which is consistent with the phenomenon that the dispersion degree of the leading and trailing edges of the granular flow increases,indicating that the particle collisions of the leading and trailing edges of the granular flows are enhanced.For granular flows with different grain sizes,the Savage number increases with the increase of grain sizes,indicating that the particle collisions are stronger with the increase of grain sizes.For granular flows with different bottom friction coefficient,Savage number increases as the bottom friction increases,indicating that the higher bottom frictions tend to enhance the particle collisions of the granular flow.Meanwhile,with increasing grain sizes,the proportion of the energy dissipation through particle damping increases,which reflects that the energy dissipation of the granular flow through particle collisions increases with the increase of grain sizes.It is consistent with the fact that Savage numbers and thus the particle collisions increase with increasing grain sizes.(3)In the numerical simulations of the granular flow experiments with polydispersed particles,it is observed that there is obvious segregation occurred in the process of particle flow propagation,and a typical inverse grading can be observed in the granular flow deposit.The phenomenon of fine particles migrating downward and coarse particles migrating upward also appears in the periodic boundary numerical simulations.In these simulations,the increase in flow thickness tends to inhibit the segregation process.The increase in inclination angles also tends to weaken the particle segregation,while the increase of the bottom friction coefficient tends to enhance the segregation process.The segregation mechanisms of fine and coarse particles in the granular flows are different,while the two mechanisms interact with each other.Due to the supports of force chains and the shear deformations,the coarse particles climb over the adjacent particles,which enlarges the pores inside the granular flows.Fine particles are more likely to develop no continuous force chain than coarse particles in the particle flows.Fine particles without continuous supports of force chains are more likely to infiltrate along pores due to gravity.After infiltration,the fine particles re-establish the force chains with the surrounding particles,which plays a supporting role in the force chain network.