Browsing by Author "Eberhard, Peter (Prof. Dr. -Ing.)"

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    Contact investigations of granular mechanical media in a tumbling sorting machine
    (2007) Alkhaldi, Hashem; Eberhard, Peter (Prof. Dr. -Ing.)
    Chapter 1 briefly introduced some contact problems in granular media with some computational procedures used in sequential and parallel computations. In Chapter 2, a general description of the molecular dynamic problems and clarification of the basics of the granular media are presented. Some of the frequently-used algorithms and models, e.g. Discrete Element Method (DEM) and penalty method of the spring-dashpot model are involved in this chapter.Some basic techniques for speeding up simulations of particulate systems by using some proper sorting algorithms and neighbor list computations, e.g. the Verlet approach and the linked linear list method, are used and compared. Different integration approaches was also discussed. It was found that Verlet integrators are efficient, accurate and appropriate to solve the equations of motion of the granular systems. In Chapter 3, the spatial decomposition method is basically used in building the parallel programing codes. This method allows scalability and good results especially when load balancing is done. Needless to say that the important factor which affects the success of the numerical procedure is how much one has access to a computer system which is powerful enough to handle the problem of interest. In this chapter, existing sequential algorithms are extended and modified in such a way that modern high performance computers can be utilized for their parallel evaluation. The library functions of the Parallel Virtual Machine (PVM) are used to handle communication between processors in a distributed memory environment. This chapter also underlined the relation between the speedup, which is the usually used measure of the program scalability, and the size of the system. It was found that the performance improves with increasing the number of particles. The reason is due to the communication and data flow which become more efficient between the different tasks as the number of particles increases and therefore, the communication cost will directly decrease and accordingly, the computational speedup will increase. In some cases, a superlinear behavior is recorded when using different computers with many processors due to the individual cache memory effect of each of the machines used in the network. As a practical industrial application of granular studies, particle screening, which is considered as an essential technology of particle separation in many industrial fields, is selected to be investigated in Chapter 4. This chapter presents a numerical model for studying the particle screening process using the discrete element method that considers the motion of each particle individually. Dynamical quantities like particle positions, velocities and orientations are tracked at each time step of the simulation. The particular problem of interest is the separation of round shaped particles of different sizes using a rotating tumbling vertical cylinder while the particulate material is continuously fed into its interior. This rotating cylinder can be designed as a uniform or stepped multi level oblique vertical vessel and is considered as a big reservoir for the mixture of particulate material. The finer particles usually fall through the sieve openings while the oversized particles are rebounded and ejected through outlets located around the machine body. Particle-particle and particle-boundary collisions will appear under the tumbling motion of the rotating structure. Herein, the penalty method, which employs spring-damper models, is applied to calculate the normal and frictional forces. For specific geometrical and contact parameters particle transportation, sifting rates and machine efficiency are recorded. Particles are simulated in uniform and stepped models of tumbling cylinders. For both continuous screening and batch sieving, it was found that the segregation process is very sensitive to the rotational speed of the machine. Furthermore, the particle feeding rates, inclination angles and shaft eccentricity have a great influence on the machine efficiency. Small angles between 0.5 degree to 1 degree and eccentricities between 25 to 50mm are recommended. The sieve roughness has also an influence on the number of particles that stay or leave the machine. An optimal value of relatively medium friction coefficient is recommended. Moreover, the barrel oscillation has a significant influence on the sorting process. Oscillatory motion of the barrel shows better performance relative to the non-rotating or even continuous-rotating motion. Finally, the thesis ends in Chapter 5 with a general summary of the presented work and a short overview of the proposed work in the future.