Formation of dry granular fronts and watery tails in debris flows

Debris flows are particle-fluid mixtures that pose a significant hazard to many communities throughout the world. Bouldery debris flows are often characterized by a deep dry granular flow front, which is followed by a progressively thinner and increasingly watery tail. The formation of highly destructive bouldery wave fronts is usually attributed to particle-size segregation. However, the moving-bed flume experiments of Davies (N. Z. J. Hydrol., vol. 29, 1990, pp. 18-46) show that discrete surges with dry fronts and watery tails also form in monodisperse particle-fluid mixtures. These observations motivate the development of a new depth-averaged mixture theory for debris flows, which explicitly takes account of the differing granular and phreatic surfaces, velocity shear, and relative motion between grains and fluid to explain these phenomena. This poster presents the theory of Meng et al. (J. Fluid Mech., vol. 943, 2022, A19), which consists of four coupled conservation laws that describe the spatial and temporal evolution of the grain and water thicknesses and depth-averaged velocities. This system enables travelling wave solutions to be constructed that consist of (i) a large amplitude dry flow front that smoothly transitions to (ii) an under saturated body, (iii) an oversaturated region and then (iv) a pure water tail. It is shown that these solutions are in good quantitative agreement with Davies’ experiments at high bed speeds and slope inclinations. At lower bed speeds and inclinations, the theory produces travelling wave solutions that connect to a steady-uniform upstream flow, and may or may not have a bulbous flow front, consistent with Davies’ observations

miR-541 Contributes to Microcystin-LR-Induced Reproductive Toxicity through Regulating the Expression of p15 in Mice

Microcystin-leucine arginine (MC-LR) is a harmful cyanotoxin produced by cyanobacteria. MC-LR can exert endocrine-disrupting activities in many organisms. We have previously demonstrated that MC-LR exerts both acute and chronic reproductive toxicity in male mice, resulting in a decline in sperm quality and damage to testicular structure. Moreover, we also observed extensive alterations in a panel of microRNAs in spermatogonial cells after exposure to MC-LR. In this study, we have confirmed that miR-541 was significantly increased both in GC-1 cells (in vitro) and in mouse testes (in vivo) after exposure to MC-LR. Our data support that p15 was the target gene of miR-541. Increase in miR-541 led to a reduction of p15 and murine double minute2 (MDM2), promoting the activation of p53 signaling and MC-LR-mediated cell apoptosis. Moreover, cells responded to MC-LR with reduced viability and increased apoptosis. Consistently, inhibiting miR-541 could upregulate the expression of p15 and MDM2, resulting in the downregulation of phospho-p53. Downregulation of miR-541 promoted cell viability by reducing MC-LR-induced cell apoptosis. In conclusion, we demonstrate here a crucial role for miR-541 in MC-LR-induced toxic effects on the reproductive system, in an attempt to provide a rational strategy for the diagnosis and treatment of MC-LR-induced impairment in the reproductive system

Dynamical modelling and numerical simulation of grain-fluid mixture flows

Flows of grain-fluid mixtures are commonly observed in nature and in industry. However, comprehensive understanding of the physics behind them is to date out of reach. This thesis aims to investigate the mechanism underlying flowing grain-fluid mixtures by both analytical and numerical methods. The work of this thesis starts with introducing standard mixture theory to describe the balance equations of mass and momentum for the fluid and the granular phases of grain-fluid mixtures. As the first step, the flowing mixtures are idealized to be saturated media, indicating that the fluid phase fills all the voids between the particles. Accordingly, the granular phase is treated as a frictional Coulomb-like media, while the fluid phase is modelled as a Newtonian fluid. The interaction forces between the two phases include buoyancy force and drag force. Taking into account the flow characteristics that the flow depth is much smaller than the flow length, the thin-layer approximation and the depth-averaged technique are employed to eliminate the dependency of the governing equations on the vertical coordinate, so that a set of depth-averaged equations are derived. The depth-averaged equations are analyzed in terms of steady flows down an inclined plane. It is found that the present model equations can interpret the classical cross-stream profiles of the downslope velocity, the blunt shape of the flowing front, and roll waves. Additionally, the depth-averaged equations are numerically resolved by using a high-resolution scheme with respect to a large-scale unsteady flow, and the numerical results are compared with the experimental data. The comparison demonstrates that this model is capable to describe dynamics of a grain-fluid mixture flow, such as the evolutions of the mixture height and volume fractions. Moreover, unsaturated grain-fluid mixtures are considered, in which the fluid phase cannot fill all interstices of the granular medium. To investigate their dynamic process, it is assumed that the fluid percolates easily down through the interstices of the granular medium and as a result the air is extruded. To describe such a kind of unsaturated mixtures, a two-layer approach is proposed, in which the fluid-saturated granular layer is overlaid by the pure granular material. The upper granular mass is treated as a frictional Coulomb-like medium, and the lower layer is described by the standard mixture theory. The lower and upper layers interact at an interface which is a material surface for the fluid phase, but across which the mass exchange for the granular phase may take place. The proposed model equations are numerically resolved, and the numerical solutions demonstrate that the proposed two-layer model can provide reasonable predictions with respect to dynamic process of unsaturated mixture flows. The last part of this thesis focuses on the improvement of the saturated depth-averaged model, presented in the first part of the thesis, by taking the granular dilatancy into account. The granular dilatancy is described by the critical-state theory. By coupling critical-state theory and mixture theory, we uncover the coupling between the granular dilatancy and the pore fluid pressure, i.e., the granular dilatancy yields the deviation of the pore fluid pressure from the hydrostatic value that, in turn, affects the motion of the granular phase. The formulated model equations describe the coupling of flow thickness, depth-averaged volume fractions and depth-averaged velocities, and the pore fluid pressure. Moreover, a numerical simulation is performed, and quantitative comparison with experimental data is reported. The comparison demonstrates that the proposed depth-averaged equations can provide reasonable predictions on the evolutions of dynamic quantities for a grain-fluid mixture flow. It is noted that this thesis is based on the accepted publications (see Meng & Wang (2015a) and Meng & Wang (2015b) and manuscripts in Meng et al. (2016) and Meng & Wang (2016)

Dynamical modelling and numerical simulation of grain-fluid mixture flows

Flows of grain-fluid mixtures are commonly observed in nature and in industry. However, comprehensive understanding of the physics behind them is to date out of reach. This thesis aims to investigate the mechanism underlying flowing grain-fluid mixtures by both analytical and numerical methods. The work of this thesis starts with introducing standard mixture theory to describe the balance equations of mass and momentum for the fluid and the granular phases of grain-fluid mixtures. As the first step, the flowing mixtures are idealized to be saturated media, indicating that the fluid phase fills all the voids between the particles. Accordingly, the granular phase is treated as a frictional Coulomb-like media, while the fluid phase is modelled as a Newtonian fluid. The interaction forces between the two phases include buoyancy force and drag force. Taking into account the flow characteristics that the flow depth is much smaller than the flow length, the thin-layer approximation and the depth-averaged technique are employed to eliminate the dependency of the governing equations on the vertical coordinate, so that a set of depth-averaged equations are derived. The depth-averaged equations are analyzed in terms of steady flows down an inclined plane. It is found that the present model equations can interpret the classical cross-stream profiles of the downslope velocity, the blunt shape of the flowing front, and roll waves. Additionally, the depth-averaged equations are numerically resolved by using a high-resolution scheme with respect to a large-scale unsteady flow, and the numerical results are compared with the experimental data. The comparison demonstrates that this model is capable to describe dynamics of a grain-fluid mixture flow, such as the evolutions of the mixture height and volume fractions. Moreover, unsaturated grain-fluid mixtures are considered, in which the fluid phase cannot fill all interstices of the granular medium. To investigate their dynamic process, it is assumed that the fluid percolates easily down through the interstices of the granular medium and as a result the air is extruded. To describe such a kind of unsaturated mixtures, a two-layer approach is proposed, in which the fluid-saturated granular layer is overlaid by the pure granular material. The upper granular mass is treated as a frictional Coulomb-like medium, and the lower layer is described by the standard mixture theory. The lower and upper layers interact at an interface which is a material surface for the fluid phase, but across which the mass exchange for the granular phase may take place. The proposed model equations are numerically resolved, and the numerical solutions demonstrate that the proposed two-layer model can provide reasonable predictions with respect to dynamic process of unsaturated mixture flows. The last part of this thesis focuses on the improvement of the saturated depth-averaged model, presented in the first part of the thesis, by taking the granular dilatancy into account. The granular dilatancy is described by the critical-state theory. By coupling critical-state theory and mixture theory, we uncover the coupling between the granular dilatancy and the pore fluid pressure, i.e., the granular dilatancy yields the deviation of the pore fluid pressure from the hydrostatic value that, in turn, affects the motion of the granular phase. The formulated model equations describe the coupling of flow thickness, depth-averaged volume fractions and depth-averaged velocities, and the pore fluid pressure. Moreover, a numerical simulation is performed, and quantitative comparison with experimental data is reported. The comparison demonstrates that the proposed depth-averaged equations can provide reasonable predictions on the evolutions of dynamic quantities for a grain-fluid mixture flow. It is noted that this thesis is based on the accepted publications (see Meng & Wang (2015a) and Meng & Wang (2015b) and manuscripts in Meng et al. (2016) and Meng & Wang (2016)