Impact of a large sphere into a bed of cohesionless particles

granular chains of force;computer simulations;granular dynamics;penetrometers;XBP;expendable bottom probe;CBL;deceleration curves;sediment properties;impact physics;seabed penetration

Goodnight, Dana Jon ; Dr. Elana Leithold, Committee Member,Dr. Michael Kimberley, Committee Member,Dr. David Genereux, Committee Chair,Goodnight, Dana Jon ; Dr. Elana Leithold ; Committee Member ; Dr. Michael Kimberley ; Committee Member ; Dr. David Genereux ; Committee Chair

University:North Carolina State University

关键词: granular chains of force;    computer simulations;    granular dynamics;    penetrometers;    XBP;    expendable bottom probe;    CBL;    deceleration curves;    sediment properties;    impact physics;    seabed penetration;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/876/etd.pdf?sequence=1&isAllowed=y

美国|英语

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【 摘 要 】

Impact and penetration of a large sphere into both dry and water-saturated granular beds of smaller particles were studied using physical and numerical experiments. In both sets of experiments the impacting sphere displays characteristic deceleration curves that may be used to infer the geotechnical and sedimentological character of the granular bed. Quantitative comparison of a subset of the physical experiments with numerical ones confirms the utility and potential of the numerical simulation model for geotechnical studies of a variety of impact phenomena. In particular, characteristics of the impact of a large object with the seafloor can be exploited to remotely infer seafloor properties without the need for expensive and time-consuming sampling and laboratory measurements. Penetration phenomena were studied in both dry, non-cohesive sands, pebbles and plastic spheres and also in water-saturated sediment beds composed of bentonite clays, sands, pebbles, and plastic spheres.An accelerometer attached to the impacting sphere or impactor is used to measure a time series of acceleration during freefall, impact, and cessation of motion. The maximum deceleration and the period of damped oscillation of the impactor are excellent quantitative descriptors of the impact process.A 121-g impactor dropped from 400 mm into dry granular particles in a 100-mm-diameter container experiences maximum decelerations ranging up to 29 times the nominal gravitational acceleration on Earth, g, of 9.8 meters per second squared. For bed particles of a given material, the maximum deceleration generally increases with bed particle diameter and with decreasing container diameter.Maximum deceleration in water-saturated particles is generally less than in dry particles, as expected, although impact into a water-saturated mixture of coarse and fine sandgenerated greater maximum deceleration values than impact into similar dry sediment despite a lower initial impact velocity. Numerical experiments using a discrete-particle computer model were used for detailed intercomparison and interpretation of physical experiments. The discrete-particle model solves the equations of motion for the impactor and each bed particle in a granular assemblage for impacts into a bed of identical 6-mm-diameter cellulose acetate spheres having known material properties and closely matching assumptions used in the simulations. Excellent quantitative agreement between maximum decelerations measured in the simulations and physical experiments demonstrates the considerable potential for use of simulations to study impacts under conditions that are not easily amenable to physical experiments, for example, on planets or moons having gravitational accelerations different from Earth's. Differences between physical experiments and simulations can be attributed to uncertainties in the initial conditions used in the numerical simulations, slight differences in the design of the physical apparatus and the simulation impact sphere, the physical properties of the containers used to hold the sediment bed, and, in the case of the water-saturated beds, incomplete description of the fluid dynamical phenomena in the numerical model.