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This thesis investigates the relationship between discrete event and system dynamics simulation modelling. Each modelling technique has its own strengths and limitations. The choice of using one particular modelling technique often depends on the preference and the knowledge of the modeller rather than on the nature of the problem. The basis of this research has been to address the problem from a different perspective. This has been to look at the nature of the problem first, and then determine the most appropriate modelling technique to apply. The basic method adopted was to compare, contrast and experiment with these two modelling techniques in order to determine a number of common and unrelated concepts between them. This investigation discovered that a system dynamics flow diagram could be used to represent an activity cycle diagram of a discrete event model. The converted flow diagram can provide a different viewpoint from the discrete event model due to the feedback characteristic of system dynamics. This research went on to develop a set of guidelines to convert an activity cycle diagram into a system dynamics flow diagram. Experimentation with many examples demonstrates that these conversion guidelines provide a consistent and systematic method for obtaining a system dynamics flow diagram. The final stage of this research was to develop a prototype computer system (SMCP) to demonstrate these guidelines. SMCP consists of two modules, the ACD module which allows the user to specify a discrete event model through textual descriptions, and the SD module which allows the user to build a system dynamics model by drawing symbols and using text inputs. In addition, SMCP allows users to convert an ACD to a system dynamics flow diagram quickly and easy, and also demonstrates the concept of data sharing.

MSc, Faculty of Engineering, University of the Witwatersrand, 1978

by Pun Kong-Pang. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references (leaves 94-[96]). / Acknowledgement --- p.i / Abstract --- p.iii / List of Tables --- p.vii / List of Figures --- p.viii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Role of Device Simulation --- p.2 / Chapter 1.2 --- Classification of Device Models --- p.3 / Chapter 1.3 --- Sections of a Typical Simulator --- p.6 / Chapter 1.4 --- Arrangement of This Thesis --- p.7 / Chapter 2 --- Classical Physical Model --- p.9 / Chapter 2.1 --- Carrier Densities --- p.12 / Chapter 2.2 --- Space Charge --- p.14 / Chapter 2.3 --- Carrier Mobilities --- p.15 / Chapter 2.4 --- Generation and Recombination --- p.17 / Chapter 2.5 --- Modeling of Device Boundaries --- p.20 / Chapter 2.6 --- Limits of Classical Device Modeling --- p.22 / Chapter 3 --- Computational Aspects --- p.23 / Chapter 3.1 --- Normalization --- p.24 / Chapter 3.2 --- Discretization --- p.26 / Chapter 3.2.1 --- Finite Difference Method --- p.26 / Chapter 3.2.2 --- Finite Element Method --- p.27 / Chapter 3.3 --- Nonlinear Systems --- p.28 / Chapter 3.3.1 --- Newton's Method --- p.28 / Chapter 3.3.2 --- Gummel's Method and its modification --- p.29 / Chapter 3.3.3 --- Comparison and discussion --- p.30 / Chapter 3.4 --- Linear System and Sparse Matrix --- p.32 / Chapter 4 --- Cubic Spline Wavelet Collocation Method for PDEs --- p.34 / Chapter 4.1 --- Cubic spline scaling functions and wavelets --- p.35 / Chapter 4.1.1 --- Approximation for a function in H2(I) --- p.43 / Chapter 4.2 --- Wavelet interpolation --- p.45 / Chapter 4.2.1 --- Interpolant operator Ivo in Vo --- p.45 / Chapter 4.2.2 --- Interpolation operator IWjf in Wj --- p.47 / Chapter 4.3 --- Derivative Matrices --- p.51 / Chapter 4.3.1 --- First derivative matrix --- p.51 / Chapter 4.3.2 --- Second derivative matrix --- p.53 / Chapter 4.4 --- Wavelet Collocation Method for Solving Device Equations --- p.55 / Chapter 4.4.1 --- Steady state solution --- p.57 / Chapter 4.4.2 --- Transient solution --- p.58 / Chapter 4.5 --- Reducing Collocation Points --- p.59 / Chapter 4.5.1 --- Error evaluation --- p.59 / Chapter 4.5.2 --- Deleting collocation points --- p.61 / Chapter 5 --- Numerical Results --- p.64 / Chapter 5.1 --- P-N Junction Diode --- p.64 / Chapter 5.1.1 --- Steady state solution --- p.69 / Chapter 5.1.2 --- Transient solution --- p.76 / Chapter 5.1.3 --- Convergence --- p.79 / Chapter 5.2 --- Bipolar Transistor --- p.81 / Chapter 5.2.1 --- Boundary Model --- p.82 / Chapter 5.2.2 --- DC Solution --- p.83 / Chapter 5.2.3 --- Transient Solution --- p.89 / Chapter 6 --- Conclusions --- p.92 / Bibliography --- p.94

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A fully coupled, effective stress based elasto-plastic model is presented for a rigorous analysis of flow and deformation in variably saturated porous media subjected to monotonic and cyclic loading. The governing equations are derived based on the effective stress concept, equations of equilibrium, and conservation equations of mass and momentum using a systematic macroscopic approach. Both elastic and elasto-plastic constitutive equations are developed. All model coefficients are identified in terms of measurable parameters. The governing equations presented are general in nature, embodying all previously presented formulations in the field. A unified bounding surface plasticity model is developed to describe the stress-strain behaviour of variably saturated soils subjected to monotonic and cyclic loading. The model is formulated incrementally within the critical state framework using the effective stress approach. The model takes into account the effects of both plastic volumetric strain and matric suction on the hardening of the bounding surface. Cyclic behaviour is captured through a new mapping rule in which the point of stress reversal is taken as the centre of projection. The effect of particle crushing at high stresses is considered through a three-segmented critical state and isotropic compression lines. A non-associative flow rule is employed to generalise application of the model to all soils. Solution to the governing equations is obtained numerically using the finite element approach, with the finite difference method employed for the time integration of the rate equations. The elasto-plastic constitutive equations are integrated explicitly using Euler???s forward and the modified Euler integration schemes. Yield surface correction schemes are adopted to improve accuracy of the solution. Essential elements of the proposed model are validated by comparing numerical predictions with experimental data from the literature for fully and partially saturated soils subjected to monotonic and cyclic loadings in drained, undrained, isotropic and deviatoric conditions. The results demonstrate capability of the coupled model to predict essential characteristics of variably saturated soils subjected to monotonic and cyclic loadings in a unified manner.

This thesis reports on research into the causes of local optima when optimization algorithms are applied to aerospace structural design. A thorough understanding of local optima will enable the engineers to select the algorithm for optimization or to guide the optimization to ensure either global optima or near optimal solutions are achieved. Therefore, a comprehensive literature review has been conducted and several illustrative examples have been identified to help fully understand the cause and importance of local optima. The first application involved the design of the internal structure of a simplified wing spoiler. MSC.NASTRAN was used to optimize each discretized location of an additional rib with the aid of a Patran Command Language (PCL) algorithm. The objective function of minimum weight was approximated as a multimodal function in a 2D smooth curve where the local and global optima were identified. The theory of continuous rectangular plates was used to explain the phenomena. The second problem considered buckling of a wing rib. A PCL code was written to obtain the rib buckling factors as the position of the center of a square cutout was varied within a constrained area. The rib linear buckling factor versus the centre position O(X, Y) of the square cutout was plotted in a 3D surface contour plot. Load path theory and relevant plate buckling theories were used to explain the local and global maxima identified. The final example considered the maximization of the buckling load of a simply supported composite laminated plate under in-plane loading. A conventional Genetic Algorithm was used to examine the local and global optima of the critical buckling load factor. Many local and global optima were identified and explained and many near-optimal solutions were found in a single run. A significant understanding of local optima in aerospace structural design with the optimal utilization of available software and the appropriate selection of optimization algorithms has been achieved. Further work could either include implementing the proposed global optimization strategies or include implementing rapid methods for identifying multiple local optima.

Localised failure in quasibrittle materials is due mainly to the effects of combined shear and compression. Once the cohesion strength is reached, shear tractions generate slip and aggregate interlocking that cause dilatancy inducing crack opening. Further damage reduces the cohesion and dilatancy so that eventually only a residual friction state remains. The energy dissipated due to friction and interlocking needs to be considered in the constitutive law. Initially, a Mohr-Coulomb yield surface with a tension cut-off will be investigated. A compression cap will be included when the modelled interfaces are not appropriately aligned and compressive failure must be controlled. The evolution of the yield surface and the appropriate flow rules to be used in the interface/particle model, are questions which will be examined. The particle/interface model with plasticity concentrated at the interface nodes, which can produce the correct volumetric expansion, will also be studied. A composite model has been developed to represent the heterogeneity of concrete consisting of coarse aggregates, mortar matrix and the mortar-aggregate interface. The constituents of concrete are modelled using triangular elements with six interface nodes along their sides. Fracture is captured through a constitutive softening-fracture law at the interface nodes, which bound the elastic domain inside each element. The inelastic displacement at an interface node represents the crack opening, which is associated to the conjugate internodal force by a single branch softening law. The path-dependent softening behaviour is derived in irreversible rate formulation within a quasi-prescribed displacement control. At each event in the loading history, all equilibrium solutions for the prescribed mesh can be obtained and the critical equilibrium path with the minimum increment of external work adopted. The crack profile develops restrictively to the interface boundaries of the defined mesh. No re-meshing is carried out. Solutions to the irreversible rate formulation are obtained using a mathematical programming procedure in the form of a linear complementary problem. Other work is aimed at incorporating fibre reinforcement in the model. Fibre particles are modelled by introducing additional linear elements interconnecting distant interface nodes in the matrix media after the generation of matrix-aggregate structure.

Two dimensional experimental and numerical modeling of wave transmission and wave-induced current over detached submerged breakwaters has been carried out in this thesis. Two preliminary 3D and a comprehensive series of 2D laboratory experiments have been conducted in the wave basin and 3 m wide wave flume. The preliminary 3D experimental tests qualitatively investigated the flow behavior behind a submerged breakwater and confirmed the validity of the 2D tests. The 2D laboratory tests examined wave breaking, reflection, transmission as well as wave-induced set-up and currents over submerged breakwater/reef structures. Different approaches to experimental data processing are examined in producing reliable application of the 2D laboratory measurements. Sensitivity of wave transmission coefficient, wave-induced set-up and wave-induced discharge over submerged breakwaters to other dimensional and non-dimensional parameters are comprehensively investigated. Previously published analytical/experimental studies for predicting/calculating wave breaking, wave transmission, wave-induced set-up and current are discussed and compared with the present experimental results. Improved equations/models are presented. Numerical modeling of the hydrodynamic effects of wave breaking and flow over a submerged breakwater is investigated using Delft3D. The capability of the Delft3D numerical model to simulate wave height transformation and wave-induced current over submerged breakwaters is provided. Four different approaches using combinations/options within the two main modules of Delft3D (SWAN and FLOW) are tested in the numerical simulations and the results are compared to the laboratory experimental data. Guidance is provided as to the most appropriate application of WAVE/FLOW/ROLLER modules in Delft3D for the reliable prediction of discharge and wave height over different width submerged breakwaters.

Magnetic and gravity modelling was completed along two E-W transects offshore central Oregon. These models indicate that the backstop-forming western edge of the Siletz terrane has a seaward dip of approximately 40° to 49° at 44°48'N, shallowing to ~28° at 44°1 1 'N. This is a well-determined result, given available a priori information, to a depth of ~7 km. The edge of the Siletz terrane may continue to descend at these dips to the JdF plate, but alternate geometries for the lowermost portion of the backstop are also consistent with the potential field data. The magnetic data also require progressive eastward demagnetization of the subducting JdF crust, which is most likely due to heating of the descending oceanic plate to the Curie temperature. Our southern transect reveals that Heceta Bank is cored by relatively high-density sediments (~2.54 g/cc), consistent with the model proposed by Kulm and Fowler (1974) for submarine bank formation in the Oregon convergent margin. On the basis of magnetic, gravity, and velocity data, we tentatively interpret a deeply buried, linear aseismic ridge to be present beneath the accretionary complex from about 45°N to 42°N. This ridge may collide with the backstop beneath Heceta Bank and may play a role, in concert with imbricate thrust faulting, in the formation of Heceta Bank's high density core. We also speculate that differences in depth to the JdF plate due to juxtaposition of different-aged crust across pseudofaults which intersect the coast at Nehalem and Heceta Banks may be a factor in the construction of these topographic highs. / Graduation date: 1997