| With the rapid development of China’s marine industry,the exploration and utilization of marine resources have gradually moved from the initial coastal areas to the deep sea and offshore regions,and the problem of marine geological disasters has become increasingly prominent.Submarine landslides,as the most typical marine geological disaster,can cause chain disasters once triggered,which can cause enormous harm for humans.The formation mechanism,movement law,and secondary disasters caused by submarine landslides have become hot and difficult research topics in the field of marine geotechnical engineering.This thesis focuses on the research of chain disasters caused by submarine landslides.Firstly,the mechanical mechanism of the transformation from debris flow to turbidity flow in the process of submarine landslides is studied,and a functional relationship between shear stress and critical shear strength is proposed under mechanical shear action.The coefficient of transformation efficiency from debris flow to turbidity flow is obtained by fitting different cases.Secondly,numerical simulation methods are used to explore the mechanical characteristics and potential hazards of the propagation of landslide tsunamis.The sensitivity factors of landslide tsunamis are quantified by changing geometric and topographic parameters.Finally,the interaction between tsunami waves and submarine pipelines is studied,and the dynamic interaction mechanism between different porous media layer submarine pipelines and tsunami waves is explored.The numerical wave flume of the immersion boundary method is proposed.By embedding porous packaging and pipeline modules in the numerical wave flume,the hydrodynamics on the porous packaging and pipeline are simulated to further explore the influence of porous packaging on the hydrodynamic characteristics around the submarine pipeline.The detailed research work and new achievements are as follows.（1）Theoretical analyses were conducted on the rheological characteristics of debris flows and turbidity currents,and the common mechanisms of transformation from debris flows to turbidity currents were introduced.The interaction between the slurry and ambient water was simulated using the two-phase mixture model in ANSYS Fluent,and the feasibility was verified through dam-break experiments.The VOF and Mixture modules were used in the numerical simulation,where the VOF module mainly reflects the laminar characteristics of the fluid and is used to reflect the shear process at the top of the debris flow,while the Mixture module mainly reflects the mixing between different substances and is used to reflect the mixing process between the head of the debris flow and the ambient water.The rheological behavior of the slurry was represented by the Herschel-Bulkley model in terms of viscosity.By fitting different cases,a functional relationship between the top shear stress of the slurry and the critical shear strength was obtained,namely,the shear stress equals 6.55e-5 times the critical shear strength.In addition,considering the influence of different initial thicknesses,concentrations,and initial slope angles on the transformation rate,two correlation coefficients in the conversion formula from debris flows to turbidity currents were fitted,which were 1.61 and 0.62,respectively.Finally,the transformation efficiency from debris flows to turbidity currents was predicted through a real-scale submarine landslide case,and the correctness of the fitting formula was verified.（2）A brief introduction is provided on the types,basic characteristics,and key triggering factors of tsunamis.A detailed discussion of numerical models for tsunami generation and propagation is presented,elucidating the theoretical framework and computational approach of these models.The GEOWAVE numerical model,based on the Boussinesq equations,is used to simulate the tsunami disaster triggered by the potential submarine landslide in the Pearl River Mouth Basin of the South China Sea.The sensitivity of the tsunami induced by the submarine landslide is analyzed by varying the geometric parameters of the landslide body（length,width,thickness,etc.）and topographic parameters（direction of destruction,center depth of destruction,slope angle,etc.）.By comparing the results of tsunami numerical simulations under different parameters,it is found that the propagation of tsunami waves is significantly affected by the terrain.Tsunami energy is concentrated at underwater highlands and islands,and the amplitude of tsunami waves increases significantly.The numerical results show that tsunami waves propagate faster in the deep waters of the south and east directions,and relatively slower in the submerged areas of the north and west directions,indicating that water depth also has a significant impact on the propagation speed of tsunami waves.In addition,the hazard of tsunami waves in the near and far fields were studied.The severity of coastal impact caused by tsunamis at the landslide near-field is proportional to the volume of landslide damage,while the impact at the far-field is significantly weaker than that in the near-field.Furthermore,the smaller the volume of landslide damage,the more significant the difference in the impact between the near-field and far-field.This indicates that the volume of landslide damage has a greater impact on distant tsunamis than on near-field tsunamis.In terms of slope angle,the maximum wave amplitude of tsunamis induced by landslides increases with the increasing slope angle of the landslide.Moreover,the influence of the landslide is more significant at the near field than at the far field.Studies have also shown that the larger the initial failure length of the landslide,the faster the propagation speed of the tsunami waves.Among the initial failure dimensions,the initial failure length has a more significant impact on the maximum wave amplitude than the initial failure width and thickness,given the same volume of the landslide.（3）In order to accurately simulate the hydrodynamic characteristics of the interaction between tsunami waves and submarine pipelines,based on the computational fluid dynamics numerical simulation software FLOW-3D,the immersed boundary method（IB）is proposed to establish a numerical wave tank（NWT）that fully couples the submarine pipelines and porous packaging materials in the fluid domain,and the interaction relationship between the tsunami waves and permeable layer proposed in the experiment is verified.The hydrodynamic characteristics on the porous packaging materials and pipelines are simulated by embedding a porous outer package and submarine pipeline module in the numerical wave tank.Based on the established bi-directional coupled numerical model,the influence of extreme tsunami waves representing different wave heights on pipelines with different porous packaging configurations was studied.The results show that the protective effect of the packaging layer on the pipeline is significant,and the hydrodynamic force on the pipeline is reduced,while the hydrodynamic force on the packaging layer is increased.These protective effects are often enhanced by the porosity and thickness of the packaging.In addition,the influence of different pipeline structures and marine environments on hydrodynamics was discussed,and a detailed description of the complex flow field around pipelines with packaging configurations was provided... |
PDF Full Text Request