A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item TACKLING PERFORMANCE AND SECURITY ISSUES FOR CLOUD STORAGE SYSTEMS(2022) Kang, Luyi; Jacob, Burce; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Building data-intensive applications and emerging computing paradigm (e.g., Machine Learning (ML), Artificial Intelligence (AI), Internet of Things (IoT) in cloud computing environments is becoming a norm, given the many advantages in scalability, reliability, security and performance. However, under rapid changes in applications, system middleware and underlying storage device, service providers are facing new challenges to deliver performance and security isolation in the context of shared resources among multiple tenants. The gap between the decades-old storage abstraction and modern storage device keeps widening, calling for software/hardware co-designs to approach more effective performance and security protocols. This dissertation rethinks the storage subsystem from device-level to system-level and proposes new designs at different levels to tackle performance and security issues for cloud storage systems. In the first part, we present an event-based SSD (Solid State Drive) simulator that models modern protocols, firmware and storage backend in detail. The proposed simulator can capture the nuances of SSD internal states under various I/O workloads, which help researchers understand the impact of various SSD designs and workload characteristics on end-to-end performance. In the second part, we study the security challenges of shared in-storage computing infrastructures. Many cloud providers offer isolation at multiple levels to secure data and instance, however, security measures in emerging in-storage computing infrastructures are not studied. We first investigate the attacks that could be conducted by offloaded in-storage programs in a multi-tenancy cloud environment. To defend against these attacks, we build a lightweight Trusted Execution Environment, IceClave to enable security isolation between in-storage programs and internal flash management functions. We show that while enforcing security isolation in the SSD controller with minimal hardware cost, IceClave still keeps the performance benefit of in-storage computing by delivering up to 2.4x better performance than the conventional host-based trusted computing approach. In the third part, we investigate the performance interference problem caused by other tenants' I/O flows. We demonstrate that I/O resource sharing can often lead to performance degradation and instability. The block device abstraction fails to expose SSD parallelism and pass application requirements. To this end, we propose a software/hardware co-design to enforce performance isolation by bridging the semantic gap. Our design can significantly improve QoS (Quality of Service) by reducing throughput penalties and tail latency spikes. Lastly, we explore more effective I/O control to address contention in the storage software stack. We illustrate that the state-of-the-art resource control mechanism, Linux cgroups is insufficient for controlling I/O resources. Inappropriate cgroup configurations may even hurt the performance of co-located workloads under memory intensive scenarios. We add kernel support for limiting page cache usage per cgroup and achieving I/O proportionality.Item EXPERIMENTAL CHARACTERIZATION OF ATMOSPHERIC TURBULENCE SUPPORTED BY ADVANCED PHASE SCREEN SIMULATIONS(2020) PAULSON, DANIEL A; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Characterization of optical propagation through the low turbulent atmosphere has been a topic of scientific investigation for decades, and has important engineering applications in the fields of free space optical communications, remote sensing, and directed energy. Traditional theories, starting with early radio science, have flowed down from the assumption of three dimensional statistical symmetry of so-called fully developed, isotropic turbulence. More recent experimental results have demonstrated that anisotropy and irregular frequency domain characteristics are regularly observed near boundaries of the atmosphere, and similar findings have been reported in computational fluid dynamics literature. We have used a multi-aperture transmissometer in field testing to characterize atmospheric transparency, refractive index structure functions, and turbulence anisotropy near atmospheric boundaries. Additionally, we have fielded arrays of resistive temperature detector probes alongside optical propagation paths to provide direct measurements of temperature and refractive index statistics supporting optical turbulence observations. We are backing up these experimental observations with a modified algorithm for modeling optical propagation through atmospheric turbulence. Our new phase screen approach utilizes a randomized spectral sampling algorithm to emulate the turbulence energy spectrum and improve modeling of low frequency fluctuations and improve convergence with theory. We have used the new algorithm to investigate open theoretical topics, such as the behavior of beam statistics in the strong fluctuation regime as functions of anisotropy parameters, and energy spectrum power law behavior. These results have to be leveraged in order to develop new approaches for characterization of atmospheric optical turbulence.Item Davinder K. Anand - Research and Related Activities 1965-2020(2020-04-04) Anand, Davinder KOver the past fifty-five years, I have worked on problems related to dye dilution for the analysis of cardiac output, heat pipes as a high conductivity device, satellite attitude control, solar energy usage and design of systems for building heating and cooling applications, design and control of magnetically suspended flywheels for energy storage, design of magnetic bearings, magnetic spindles for high speed machining, manufacturing systems, system simulation and virtual environments as a design tool for a group of various mechanical components, and a number of unique problems of interest to me such as STEM education and R&D funding policy of the Navy.Item EXPERIMENTAL EVALUATION AND SIMULATION RESEARCH ON NOVEL VARIABLE REFRIGERANT FLOW SYSTEM(2017) Lin, Xiaojie; Radermacher, Reinhard; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variable refrigerant flow (VRF) system is a popular building air conditioning system which could provide cooling or heating to individual rooms independently. The system is called “variable refrigerant flow” system due to its capability of regulating the refrigerant flow via the precise control of variable speed compressors and electronic expansion valves in each indoor unit. In this dissertation, an advanced VRF system which could provide space cooling, heating and water heating is experimentally evaluated in cooling and heating season for both heat recovery operation and water heating operation. The VRF system is simulated in EnergyPlus and validated with experimental data. Based on the deviation analysis and literature review, it is found that the existing VRF model could not fully reflect the operation characteristic of VRF systems, leading to a high uncertainty in cooling/heating energy and energy consumption. A new VRF model is thereafter proposed, validated in this research and resulted in a model uncertainty less than 5%. Based on the new model, the seasonal performance of an energy saving control strategy and the concept of chilled water storage are investigated. Meanwhile, to solve the mismatch between the building’s thermal load and cooling/heating capability of the VRF system, a new VRF system with phase change material (PCM) based thermal energy storage (TES) is proposed. The new VRF system utilizes single TES device to support both cooling and heating season operation. The performance of new VRF system with PCM based TES is investigated and compared to that of the baseline VRF system. It is found that the new VRF system with PCM based TES could achieve both energy efficiency and demand response goals in cooling and heating season. Based on the comparison, the effect of operation strategies and grid incentive program are discussed. Finally, the economic analysis of the new VRF system with PCM based TES based on annual performance is carried out.Item On agent-based modeling: Multidimensional travel behavioral theory, procedural models and simulation-based applications(2015) Xiong, Chenfeng; Zhang, Lei; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation proposes a theoretical framework to modeling multidimensional travel behavior based on artificially intelligent agents, search theory, procedural (dynamic) models, and bounded rationality. For decades, despite the number of heuristic explanations for different results, the fact that "almost no mathematical theory exists which explains the results of the simulations" remains as one of the large drawbacks of agent-based computational process approach. This is partly the side effect of its special feature that "no analytical functions are required". Among the rapidly growing literature devoted to the departure from rational behavior assumptions, this dissertation makes effort to embed a sound theoretical foundation for computational process approach and agent-based microsimulations for transportation system modeling and analyses. The theoretical contribution is three-fold: (1) It theorizes multidimensional knowledge updating, search start/stopping criteria, and search/decision heuristics. These components are formulated or empirically modeled and integrated in a unified and coherent approach. (2) Procedural and dynamic agent-based decision-making is modeled. Within the model, agents make decisions. They also make decisions on how and when to make those decisions. (3) Replace conventional user equilibrium with a dynamic behavioral user equilibrium (BUE). Search start/stop criteria is defined in the way that the modeling process should eventually lead to a steady state that is structurally different to user equilibrium (UE) or dynamic user equilibrium (DUE). The theory is supported by empirical observations and the derived quantitative models are tested by agent-based simulation on a demonstration network. The model in its current form incorporates short-term behavioral dimensions: travel mode, departure time, pre-trip routing, and en-route diversion. Based on research needs and data availability, other dimensions can be added to the framework. The proposed model is successfully integrated with a dynamic traffic simulator (i.e. DTALite, a light-weight dynamic traffic assignment and simulation engine) and then applied to a mid-size study area in White Flint, Maryland. Results obtained from the integration corroborate the behavioral richness, computational efficiency, and convergence property of the proposed theoretical framework. The model is then applied to a number of applications in transportation planning, operations, and optimization, which highlights the capabilities of the proposed theory in estimating rich behavioral dynamics and the potential of large-scale implementation. Future research should experiment the integration with activity-based models, land-use development, energy consumption estimators, etc. to fully develop the potential of the agent-based model.Item Simulation of Dual-Mode Scramjet Under Thermally Choked vs. Supersonic Combustion Mode(2014) Butcher, Cameron; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Effects of combustion mode and cavity flame-holder on dual-mode scramjet performance were investigated using a two-dimensional computational framework developed from commercial finite element software. The objectives were to simulate the experimental data from a laboratory model scramjet with mixing enhancement device, provide better understanding of the physical processes, and to analyze the quantitative effects on the potential performance. The isolator flow field was modeled separately to match the experimentally obtained pressure rise during the Mach 2.1 isolator entry condition. The combustor heat release distribution was systematically adjusted to reproduce the wall pressure distributions from the experiments. Case studies were conducted with and without the presence of the wall cavity for scramjet operation under both thermally-choked and supersonic-combustion mode. The combustion mode affected potential tradeoffs between thrust increase and higher thermal protection need. The presence of the cavity dampened the extent of the tradeoffs by reducing the temperature change.Item Flight Dynamics Simulation Modeling and Control of a Large Flexible Tiltrotor Aircraft(2014) Juhasz, Ondrej; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A high order rotorcraft mathematical model is developed and validated against the XV-15 and a Large Civil Tiltrotor (LCTR) concept. The mathematical model is generic and allows for any rotorcraft configuration, from single main rotor helicopters to coaxial and tiltrotor aircraft. Rigid-body and inflow states, as well as flexible wing and blade states are used in the analysis. The separate modeling of each rotorcraft component allows for structural flexibility to be included, which is important when modeling large aircraft where structural modes affect the flight dynamics frequency ranges of interest, generally 1 to 20 rad/sec. Details of the formulation of the mathematical model are given, including derivations of structural, aerodynamic, and inertial loads. The linking of the components of the aircraft is developed using an approach similar to multibody analyses by exploiting a tree topology, but without equations of constraints. Assessments of the effects of wing flexibility are given. Flexibility effects are evaluated by looking at the nature of the couplings between rigid-body modes and wing structural modes and vice versa. The effects of various different forms of structural feedback on aircraft dynamics are analyzed. A proportional-integral feedback on the structural acceleration is deemed to be most effective at both improving the damping and reducing the overall excitation of a structural mode. A model following control architecture is then implemented on full order flexible LCTR models. For this aircraft, the four lowest frequency structural modes are below 20 rad/sec, and are thus needed for control law development and analysis. The impact of structural feedback on both Attitude-Command, Attitude-Hold (ACAH) and Translational Rate Command (TRC) response types are investigated. A rigid aircraft model has optimistic performance characteristics, and a control system designed for a rigid aircraft could potentially destabilize a flexible one. The various control systems are flown in a fixed-base simulator. Pilot inputs and aircraft performance are recorded and analyzed.Item Rotorcraft Brownout Mitigation Through Flight path Optimization Using a High Fidelity Rotorcraft Simulation Model(2012) Alfred, Jillian; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Brownout conditions often occur during approach, landing, and take off in a desert environment and involve the entrainment and mobilization of loose sediment and dust into the rotor flow field. For this research, a high fidelity flight dynamics model is used to perform a study on brownout mitigation through operational means of flight path. In order for the high fidelity simulation to model an approach profile, a method for following specific profiles was developed. An optimization study was then performed using this flight dynamics model in a comprehensive brownout simulation. The optimization found a local shallow optimum approach and a global steep optimum approach minimized the intensity of the resulting brownout clouds. These results were consistent previous mitigation studies and operational methods. The results also demonstrated that the addition of a full rotorcraft model into the brownout simulation changed the characteristics of the velocity flow field, and hence changing the character of the brownout cloud that was produced.Item A discrete-continuous modeling approach with applications to vehicle holding and use(2012) Tremblay, Jean-Michel; Cirillo, Cinzia; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Transportation and automobile use is a major concern today in the United- States. The use of automobile has impacts on congestion, urban dynamics, environ- ment and on the economy in general. Good indicators of transportation demand are the number of vehicles owned by a household and the total number of miles traveled. This thesis aims at building a model that can predict the total vehicle miles traveled and number of cars owned by households, simultaneously. The discrete- continuous model that we present correlates the error terms of a utility-based probit with the error term of an ordinary regression. The objective is to capture the relationship between preferred ownership alternatives and miles traveled. We successfully show that households with high utility for owning a lot of cars also drive more and that households with high utility for owning few cars drive less. The correlation is between utilities and miles traveled. It also correlates the two transportation demand indicators without assuming that one precedes the other and, thus, does not suffer from circular variable inclusions. The thesis ends by incorporating sampling weights into the model before pa- rameters are estimated. We find slight changes in parameters' values calculated with weights. The difference however, is more quantitative than qualitative since the general analysis we make with the weighted coefficients remains the same, only the magnitude of the effects changeItem Design and Analysis of New Gasification Apparatus based on the Standard Cone Calorimeter(2012) Liu, Xuan; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A simple, inexpensive, safe version of pyrolysis apparatus is developed base on the standard cone calorimeter (ASTM E 1354). A controllable oxygen concentration (0% to 21% by volume) environment in the vicinity of 80 mm by 80 mm square sample positioned under the cone radiant heater is achieved by means of "Controlled Atmosphere Pyrolysis Apparatus". Valid gasification mass loss rate measurements have been obtained for both poly(methyl methacrylate) (PMMA) and polypropylene (PP) samples under external heat fluxes of 35kW/m^2 and 50kW/m^2. Reasonable value of thermal conductivity for PMMA is measured. With the thermal conductivity and parameters defined by Differential Scanning Calorimeter (DSC) of PMMA, the gasification mass loss rate is well simulated using Thermo-Kinetic Model of Burning (ThermaKin).