Doctor of Philosophy

dissertationMicroorganism motility supports many features in the microbial world and is closely linked to their abilities to find food, mate, and colonize, which are essential to their survival. While swimming, the organisms interact with their fluid environment in a manner quite different from our common human experience in fluids, making it interesting to understand the physics of such low Reynolds number hydrodynamics. In this dissertation, I discuss four different aspects of locomotion in viscous fluids. We first study the suitable choice of computational method for swimming and pumping with a helical flagellar filament at a low Reynolds number. Comparing the most commonly used approaches to solve Stokes equations numerically, we provide a guideline for the optimal choice of the tuning parameters for a wide helical geometries range balancing the accuracy and computational time expenses. We then study the effect of helical cell bodies on the swimming speed and trajectory by comparing to rod-shaped geometries. We validate our numerical model with experiments from high-frame-rate digital tracking and image processing for both helical and straight mutant (Helicobacter pylori) swimming mucin and broth solution. We find that the helicity of the cell body makes at most a 15% contribution to the additional thrust and change in swimming speed. We also study the single-flagellated bacterial flicks and instabilities due to hook and flagellum flexibilities. We find that dynamic instability initiates bacterial flick and then flexibility of the flagellar filament plays an important role in reorientation of the swimmer. Finally, we consider hydrodynamic interactions of a swimmer with a complex biological environment and other passive particles in viscous fluid. We examine long-range hydrodynamic interactions of a simple swimmer near spherical and filamentous obstacles and find that swimming velocity fluctuation is closely related to the correlations in density and orientations of obstacles. For the hydrodynamic interactions and approach of organisms with passive particles, we find exact solutions for the approach of spherical shapes in the cases that the swimmer is driven by a localized constant force and with distributed propulsion (squirmer model). We study the feasibility of approach and find that an organism can approach any similar size or bigger target compared to the organism's size, but approaching smaller targets depends on the current strength generated by the swimmer

What it takes to design a supply chain resilient to major disruptions and recurrent interruptions

Global supply chains are more than ever under threat of major disruptions caused by devastating natural and man-made disasters as well as recurrent interruptions caused by variations in supply and demand. This paper presents an optimization model for designing a supply chain resilient to (1) supply/demand interruptions and (2) facility disruptions whose probability of occurrence and magnitude of impact can be mitigated through fortification investments. Numerical results and managerial insights obtained from model implementation are presented. Our analysis focuses on how supply chain design decisions are influenced by facility fortification strategies, a decision maker’s conservatism degree, demand fluctuations, supply capacity variations, and budgetary constraints. Finally, examining the performance of the proposed model using a Monte Carlo simulation method provides additional insights and practical implications

A resilient and sustainable supply chain: Is it affordable?

Developing environmentally and socially sustainable supply chains has become an integral part of corporate strategy for virtually every industry. However, little is understood about the broader impacts of sustainability practices on the capacity of the supply chain to tolerate disruptions. This article aims to investigate the sustainability-resilience relationship at the strategic supply chain design level using a multi-objective optimization model and an empirical case study. The proposed model utilizes a sustainability performance scoring method and a novel programming approach to perform a dynamic sustainability tradeoff analysis and design a “resiliently green” supply chain

New insight into air/spray boundary interaction for diesel and biodiesel fuels under different fuel temperatures

The liquid fuel breakup mechanism in spray injection with ambient air for diesel and biodiesel at different fuel temperatures is studied numerically. We find that biodiesel fuel type injection with low fuel temperature induces more air entrainment volume to the boundary of the spray than diesel fuel injection and higher fuel temperature. Meanwhile, the normalized parcel density for biodiesel is 12% larger than that of diesel and peaks at a shorter distance along the spray line from the injection point (42 vs. 46 mm). Biodiesel fuel demonstrates a maximum 0.395 mg/s of air mass flow while diesel max mass flow is 0.279 mg/s. As a result, the air entrainment volume of biodiesel to the moving spray area at 1.4 ms reaches 3723.98 mm3 while for diesel the amount is 3151.27 mm3. However, the absorbed y-direction air velocity into the spray core for diesel fuel is dominant. The results give new insights into air exchange to spray boundary in the near nozzle and spray tip area: towards the tip of spray the air pushout is remarkable. Higher fuel temperature leads to slightly lower air exchange flow and entrainment (5.2%), cone angle reduction from 300 to 325 K fuel temperature, and increased surface area:volume ratio for diesel

The Comparison of the Ownership Structure at Different Level of the Financial Collapse in Listed Firms of Tehran Stock Exchange

The main objective of this study is to compare ownership structure of different levels of collapse in listed companies of Tehran Stock Exchange (TSE). Study variables are the ownership structure that includes governmental ownership, private ownership (corporate ownership and individual ownership) and the different levels of collapse including latency stage, the stage of cash deficits and commercial and financial insolvency and complete collapse. Altman model (Z´- Score) is used to predict the collapse of firm and Chi-Square test is used to test the research hypothesis. The study is the applied research and statistical population of study includes 96 collapse firms that are active in the TSE (subject to Business Law Article 141). Given the availability of financial information of distressed companies in 7 years between 2004-2010 years, a sample of 81 members was selected. The results show that the ownership structure of collapsed firms is independent from their different levels of collapse

Sustainability Analysis under Disruption Risks

Resilience to disruptions and sustainability are both of paramount importance to supply chains. This paper presents a hybrid methodology for the design of a sustainable supply network that performs resiliently in the face of random disruptions. A stochastic bi-objective optimization model is developed that utilizes a fuzzy c-means clustering method to quantify and assess the sustainability performance of the suppliers. The proposed model determines outsourcing decisions and buttressing strategies that minimize the expected total cost and maximize the overall sustainability performance in disruptions. Important managerial insights and practical implications are obtained from the model implementation in a case study of plastic pipe industry