Doctoral Dissertations

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Adaptive Predictor-Based Output Feedback Control of Unknown Multi-Input Multi-Output Systems: Theory and Application to Biomedical Inspired Problems
Nguyen, Chuong Hoang (Virginia Tech, 2016-06-03)
Functional Electrical Stimulation (FES) is a technique that applies electrical currents to nervous tissue in order to actively induce muscle contraction. Recent research has shown that FES provides a promising treatment to restore functional tasks due to paralysis caused by spinal cord injury, head injury, and stroke, to mention a few. Therefore, the overarching goal of this research work is to develop FES controllers to enable patients with movement-disorder to control their limbs in a desired manner and, in particular, to aid Parkinson's patients to suppress hand tremor. In our effort to develop strategies for muscle stimulation control, we first implement a model-based control technique assuming that all the states are measurable. The Hill-type muscle model coupled with a simplified 2DoF model of the arm is used to study the performance of our proposed adaptive sliding mode controller for simulation purpose. However, in the more practical situations, human limb dynamics are extremely complicate and it is inadequate to use model based controllers, especially considering there are still technical limitations that allow in vivo measurements of muscle activity. To tackle these challenges, we have developed output feedback adaptive control approaches for a class of unknown multi-input multi-output systems. Such control strategies are first developed for linear systems, and then extended to the nonlinear case. The proposed controllers, supported by experimental results, require minimum knowledge of the system dynamics and avoid many restrictive assumptions typically found in the literature. Therefore, we expect that the results introduced in this dissertation can provide a solution for a wide class of nonlinear uncertain systems, with focus on practical issues such as partial state measurement and the presence of mismatched uncertainties.
Analysis of Bat Biosonar Beampatterns: Biodiversity and Dynamics
Caspers, Philip Bryan (Virginia Tech, 2017-01-24)
Across species, bats exhibit wildly disparate differences in their noseleaf and pinnae shapes. Within Rhinolophid and Hipposiderid families, bats actively deform their pinnae and noseleaf during biosonar operation. Both the pinnae and noseleaf act as acoustic baffles which interact with the outgoing and incoming sound; thus, they form an important interface between the bat and its environment. Beampatterns describe this interface as joint time-frequency transfer functions which vary across spatial direction. This dissertation considers bat biosonar shape diversity and shape dynamics manifest as beampatterns. In the first part, the seemingly disparate set of functional properties resulting from diverse pinnae and noseleaf shape adaptations are considered. The question posed in this part is as follows: (i) what are the common properties between species beampatterns? and (ii) how are beampatterns aligned to a common direction for meaningful analysis? Hence, a quantitative interspecific analysis of the beampattern biodiversity was taken wherein: (i) unit[267]{} different pinnae and noseleaf beampatterns were rotationally aligned to a common direction and (ii) decomposed using principal component analysis, PCA. The first three principal components termed eigenbeams affect beamwidth around the single lobe, symmetric mean beampattern. Dynamic shape adaptations to the pinnae and noseleaf of the greater horseshoe bat (textit{Rhinolophus ferrumequinum}) are also considered. However, the underlying dynamic sensing principles in use are not clear. Hence, this work developed a biomimetic substrate to explore the emission and reception dynamics of the horseshoe bat as a sonar device. The question posed in this part was as follows: how do local features on the noseleaf and pinnae interact individually and when combined together to generate peak dynamic change to the incoming sonar information? Flexible noseleaf and pinnae baffles with different combinations of local shape features were developed. These baffles were then mounted to platforms to biomimetically actuate the noseleaf and pinnae during pulse emission and reception. Motions of the baffle surfaces were synchronized to the incoming and outgoing sonar waveform, and the time-frequency properties of the emission and reception baffles were characterized across spatial direction. Different feature combinations of the noseleaf and pinnae local shape features were ranked for overall dynamic effect.
Autonomous Vehicle Perception Quality Assessment
Zhang, Ce (Virginia Tech, 2023-06-29)
In recent years, the rapid development of autonomous vehicles (AVs) has necessitated the need for high-quality perception systems. Perception is a fundamental requirement for AVs, with cameras and LiDARs being commonly used sensors for environmental understanding and localization. However, there is a research gap in assessing the quality of AVs perception systems. To address this gap, this dissertation proposes a novel paradigm for evaluating AVs perception quality by studying the perception quality of cameras and LiDARs sensors. Our proposed paradigm aims to provide a comprehensive assessment of the quality of perception systems used in AVs.To achieve our research goals, we first validate the concept of surrounding environmental complexity through subjective experiments that rate complexity scores. In this study, we propose a neural network to classify complexity. Subsequently, we study image-based perception quality assessment by using image saliency and 2D object detection algorithms to create an image-based quality index. We then develop a neural network model to regress the proposed quality index score. Furthermore, we extend our research to LiDAR-based point cloud quality assessment by using the image-based saliency map as guidance to generate a point cloud quality index score. We then develop a neural network model to regress the score. Finally, we validate the proposed perception quality index with a novel designed AVs perception algorithm. In conclusion, this dissertation makes a significant contribution to the field of AVs perception by proposing a new paradigm for assessing perception quality. Our research findings can be used to improve the overall performance and safety of AVs, which has significant implications for the transportation industry and society as a whole.
Bat swarming as an inspiration for multi-agent systems: predation success, active sensing, and collision avoidance
Lin, Yuan (Virginia Tech, 2016-02-22)
Many species of bats primarily use echolocation, a type of active sensing wherein bats emit ultrasonic pulses and listen to echoes, for guidance and navigation. Swarms of such bats are a unique type of multi-agent systems that feature bats's echolocation and flight behaviors. In the work of this dissertation, we used bat swarming as an inspiration for multi-agent systems to study various topics which include predation success, active sensing, and collision avoidance. To investigate the predation success, we modeled a group of bats hunting a number of collectively behaving prey. The modeling results demonstrated the benefit of localized grouping of prey in avoiding predation by bats. In the topics regarding active sensing and collision avoidance, we studied individual behavior in swarms as bats could potentially benefit from information sharing while suffering from frequency jamming, i.e., bats having difficulty in distinguishing between self and peers's information. We conducted field experiments in a cave and found that individual bat increased biosonar output as swarm size increased. The experimental finding indicated that individual bat acquired more sensory information in larger swarms even though there could be frequency jamming risk. In a simulation wherein we modeled bats flying through a tunnel, we showed the increasing collision risk in larger swarms for bats either sharing information or flying independently. Thus, we hypothesized that individual bat increased pulse emissions for more sensory information for collision avoidance while possibly taking advantage of information sharing and coping with frequency jamming during swarming.
Closed Loop Control of Muscle Contraction using Functional Electrical Stimulation
Jaramillo Cienfuegos, Paola (Virginia Tech, 2016-02-05)
A promising approach to treat patients with vocal fold paralysis using electrical stimulation is investigated throughout this research work. Functional Electrical Stimulation works by stimulating the atrophied muscle or group of muscles directly by current when the transmission lines between the central nervous system are disrupted. This technique helps maintain muscle mass and promote blood flow in the absence of a functioning nervous system. The goal of this work is two-fold: develop control techniques for muscle contraction to optimize muscle stimulation and develop a small-scale electromagnetic system to provide stimulation to the laryngeal muscles for patients with vocal fold paralysis. These studies; therefore, focus on assessing a linear Proportional-Integral (PI) controller and two nonlinear controllers: Model Reference Adaptive Controller (MRAC) and an Adaptive Augmented PI (ADP-PI) system to identify the most appropriate controller providing effective stimulation of the muscle. Direct stimulation is applied to mouse skeletal muscle in vitro to test the controllers along with numerical simulations for validation of these experimental tests. The experiments included muscle contractions following four distinct trajectories: a step, sine, ramp, and square wave. Overall, the closed-loop controllers followed the stimulation trajectories set for all the simulated and tested muscles. When comparing the experimental outcomes of each controller, we concluded that the ADP-PI algorithm provided the best closed-loop performance for speed of convergence and disturbance rejection. Next, the focus of the research work was on the implementation of an electromagnetic system to generate appropriate currents of stimulation using the aforementioned controllers. For this study, Nickel-Titanium shape memory alloys were used to assess activation (contraction) through a two-coil system guided by the controllers. The application of the two-coil system demonstrated the effectiveness of the approach and a main effect was observed between the PI, MRAC, and ADP-PI controllers when following the trajectories. Lastly, a small scale two-coil system is developed for animal testing in the muscle-mass-spring setup. Experiments were successful in generating the appropriate stimulation controlled by the output-based algorithms for muscle contraction. Trials conducted for this study were compared to the muscle contractions observed in the first study. The controllers were able to provide appropriate stimulation to the muscle system to follow the set trajectories: a step, ramp, and sinusoidal input. More trials are required to draw statistical conclusions about the performance of each controller. Regardless, the small-scale two-coil system along with the applied controllers can be reconfigured to be an implantable system and tested for appropriate stimulation of the laryngeal muscles.
Compressible Lubrication Theory in Pressurized Gases
Chien, Ssu-Ying (Virginia Tech, 2019-04-08)
Lubrication theory plays a fundamental role in all mechanical design as well as applications to biomechanics. All machinery are composed of moving parts which must be protected against wear and damage. Without effective lubrication, maintenance cycles will be shortened to impractical levels resulting in increased costs and decreased reliability. The focus of the work presented here is on the lubrication of rotating machinery found in advanced power systems and designs involving micro-turbines. One of the earliest studies of lubrication is due to Osborne Reynolds in 1886 who recorded what is now regarded as the canonical equation governing all lubrication problems; this equation and its extensions have become known as the Reynolds equation. In the past century, Reynolds equation has been extended to include three-dimensional effects, unsteadiness, turbulence, variable material properties, non-newtonian fluids, multi-phase flows, wall slip, and thermal effects. The bulk of these studies have focused on highly viscous liquids, e.g., oils. In recent years there has been increasing interest in power systems using new working fluids, micro-turbines and non-fossil fuel heat sources. In many cases, the design of these systems employs the use of gases rather than liquids. The advantage of gases over liquids include the reduction of weight, the reduction of adverse effects due to fouling, and compatibility with power system working fluids. Most treatments of gas lubrication are based on the ideal, i.e., low pressure, gas theory and straightforward retro-fitting of the theory of liquid lubrication. However, the 21st Century has seen interest in gas lubrication at high pressures. At pressures and temperatures corresponding to the dense and supercritical gas regime, there is a strong dependence on gas properties and even singular behavior of fundamental transport properties. Simple extrapolations of the intuition and analyses of the ideal gas or liquid phase theory are no longer possible. The goal of this dissertation is to establish the correct form of the Reynolds equation valid for both low and high pressure gases and to explore the dynamics predicted by this new form of the Reynolds equation. The dissertation addresses five problems involving our new Reynolds equation. In the first, we establish the form appropriate for the simple benchmark problem of two-dimensional journal bearings. It is found that the material response is completely determined by a single thermodynamic parameter referred to as the "effective bulk modulus". The validity of our new Reynolds equation has been established using solutions to the full Navier-Stokes-Fourier equations. We have also provided analytical estimates for the range of validity of this Reynolds equation and provided a systematic derivation of the energy equation valid whenever the Reynolds equation holds. The next three problems considered here derive local and global results of interest in high speed lubrication studies. The results are based on a perturbation analysis of our Reynolds and energy equation resulting in simplified formulas and the explicit dependence of pressure, temperature, friction losses, load capacity, and heat transfer on the thermodynamic state and material properties. Our last problem examines high pressure gas lubrication in thrust bearings. We again derive the appropriate form of the Reynolds and energy equations for these intrinsically three-dimensional flows. A finite difference scheme is employed to solve the resultant (elliptic) Reynolds equation for both moderate and high-speed flows. This Reynolds equation is then solved using perturbation methods for high-speed flows. It is found that the flow structure is comprised of five boundary layer regions in addition to the main ``core'' region. The flow in two of these boundary layer regions is governed by a nonlinear heat equation and the flow in three of these boundary layers is governed by nonlinear relaxation equations. Finite difference schemes are employed to obtain detailed solutions in the boundary layers. A composite solution is developed which provides a single solution describing the flow in all six regions to the same accuracy as the individual solutions in their respective regions of validity. Overall, the key contributions are the establishment of the appropriate forms of the Reynolds equation for dense and supercritical flows, analytical solutions for quantities of practical interest, demonstrations of the roles played by various thermodynamic functions, the first detailed discussions of the physics of lubrication in dense and supercritical flows, and the discovery of boundary layer structures in flows associated with thrust bearings.
Crossing the Air-Water Interface: Inspiration from Nature
Chang, Brian Lida (Virginia Tech, 2018-06-01)
This dissertation aims to contribute toward the understanding of water-entry and -exit behaviors in nature. Since water is nearly a thousand times denser than air, transitioning between the two mediums is often associated with significant changes in force. Three topics with implications in water-entry are discussed, along with a fourth topic on water-exit. For a plunge-diving seabird, the first two stages of water-entry (initial impact and air-cavity formation) create large stresses on the bird's neck. Linear stability analysis of a cone-beam system impacting water shows buckling and non-buckling behaviors on the beam, which is extended to the diving birds. The next topic is related to the third stage of water-entry (air-cavity pinch-off), in which the chest feathers come in contact with the water. Here, the elasticity of Northern Gannet contour feathers is calculated using the nonlinear bending equation. The third topic will explore the formation of ripples along air cavity walls and their resulting force after pinch-off. An acoustic model predicts the observed wavelengths of the ripples. The final topic will delve into the mechanics of how animals leap out of water. A scaling law that balances the power of thrust and drag will predict the height of the jump. Finally, a bio-inspired robot was built to help identify physical conditions required to jump out of water.
Distributed, Stable Topology Control of Multi-Robot Systems with Asymmetric Interactions
Mukherjee, Pratik (Virginia Tech, 2021-06-17)
Multi-robot systems have recently witnessed a swell in interest in the past few years because of their various applications such as agricultural autonomy, medical robotics, industrial and commercial automation and, search and rescue. In this thesis, we particularly investigate the behavior of multi-robot systems with respect to stable topology control in asymmetric interaction settings. From theoretical perspective, we first classify stable topologies, and identify the conditions under which we can determine whether a topology is stable or not. Then, we design a limited fields-of-view (FOV) controller for robots that use sensors like cameras for coordination which induce asymmetric robot to robot interactions. Finally, we conduct a rigorous theoretical analysis to qualitatively determine which interactions are suitable for stable directed topology control of multi-robot systems with asymmetric interactions. In this regard, we solve an optimal topology selection problem to determine the topology with the best interactions based on a suitable metric that represents the quality of interaction. Further, we solve this optimal problem distributively and validate the distributed optimization formulation with extensive simulations. For experimental purposes, we developed a portable multi-robot testbed which enables us to conduct multi-robot topology control experiments in both indoor and outdoor settings and validate our theoretical findings. Therefore, the contribution of this thesis is two fold: i) We provide rigorous theoretical analysis of stable coordination of multi-robot systems with directed graphs, demonstrating the graph structures that induce stability for a broad class of coordination objectives; ii) We develop a testbed that enables validating multi-robot topology control in both indoor and outdoor settings.
Dynamic Emission Baffle Inspired by Horseshoe Bat Noseleaves
Fu, Yanqing (Virginia Tech, 2016-03-04)
The evolution of bats is characterized by a combination of two key innovations - powered flight and biosonar - that are unique among mammals. Bats still outperform engineered systems in both capabilities by a large margin. Bat biosonar stands out for its ability to encode and extract sensory information using various mechanisms such as adaptive beam width control, dynamic sound emission and reception, as well as cognitive processes. Due to the highly integrated and sophisticated design of their active sonar system, bats can survive in complex and dense environments using just a few simple smart acoustic elements. On the sound emission side, significant features that distinguish bats from the current man-made sonar system are the time-variant shapes of the noseleaves. Noseleaves are baffles that surround the nostrils in bats with nasal pulse emission such as horseshoe bats and can undergo non-rigid deformations large enough to affect their acoustic properties significantly. Behavioral studies have shown that these movements are not random byproducts, but are due to specific muscular action. To understand the underlying physical and engineering principles of the dynamic sensing in horseshoe bats, two experimental prototypes ,i.e. intact noseleaf and simplified noseleaf, have been used. We have integrated techniques of data acquisition, instrument control, additive manufacturing, signal processing, airborne acoustics, 3D modeling and image processing to facilitate this research. 3D models of horseshoe bat noseleaves were obtained by tomographic imaging, reconstructed, and modified in the digital domain to meet the needs of additive manufacturing prototype. Nostrils and anterior leaf were abstracted as an elliptical outlet and a concave baffle in the other prototype. As a reference, a circular outlet and a straight baffle designed. A data acquisition and instrument control system has been developed and integrated with transducers to characterize the dynamic emission system acoustically as well as actuators for recreating the dynamics of the horseshoe bat noseleaf. A conical horn and tube waveguide was designed to couple the loudspeaker to the outlet of bat noseleaf and simplified baffles. A pan-tilt was used to characterize the acoustic properties of the deforming prototypes over direction. By using those techniques, the dynamic effect of the noseleaf was reproduced and characterized. It was suggested that the lancet rotation induced both beam-gain and beamwidth changes. Narrow outlet produced an isotropic beampattern and concave baffle had a significant time-variant and frequency-variant effect with just a small displacement. All those results cast light on the possible functions of the biological morphology and provided new thoughts on the engineering device's design.
Dynamic Gap-Crossing Movements in Jumping and Flying Snakes
Graham, Michelle Rebecca (Virginia Tech, 2022-05-23)
Gap crossing is a regular locomotor activity for arboreal animals. The distance between branches likely plays a role in determining whether an animal is capable of crossing a given gap, and what locomotor behavior it uses to do so. Yet, despite the importance of gap distance as a physical parameter influencing gap crossing behavior, the precise relationships between gap distance and movement kinematics have been explored in only a very small number of species. One particularly interesting group of arboreal inhabitants are the flying snakes (Chrysopelea). This species is able to use a dynamic "J-loop" movement to launch its glides, but it is not known whether it is also capable of using such jumps to cross smaller gaps between tree branches. This dissertation addresses this knowledge gap, and investigates the influence of gap distance on crossing behavior and kinematics in three closely-related species of snake: Chrysopelea paradisi, a species of flying snake, and two species from the sister genus, Dendrelaphis, neither of which can glide. Chapter 2 is a literature review of the biomechanics of gap crossing, specifically focusing on the role played by gap distance, and establishes the context for the rest of the work. Chapter 3 presents a detailed study of how increasing gap size influences the behavior and kinematics of gap crossing in C. paradisi, showing that this species uses increasingly dynamic movements to cross gaps of increasing size. Chapter 4 explores the same relationships between gap size and kinematics in D. punctulatus and D. calligastra, revealing remarkable similarities between the three species, suggesting the possibility that dynamic gap crossing may have evolved prior to gliding in this clade. Finally, chapter 5 addresses the role played by gap distance in limiting the non-dynamic, cantilever movements used by these species to cross small gaps, comparing observed stopping distances to those predicted by various torque-related limitations.
Dynamical Phase-Change Phenomena
Ahmadi, Seyedfarzad (Virginia Tech, 2019-06-28)
Matter on earth exists mostly in three different phases of solid, liquid, and gas. With extreme amounts of energy, temperature, or pressure, a matter can be changed between the phases. Six different types of phase-change phenomena are possible: freezing (the substance changes from a liquid to a solid), melting (solid to liquid), condensation (gas to liquid), vaporization (liquid to gas), sublimation (solid to gas), and desublimation (gas to solid). Another form of phase change which will be discussed here is the wetting or dewetting transitions of a superhydrophobic surface, in which the phase residing within the surface structure switches between vapor and liquid. Phase transition phenomena frequently occur in our daily life; examples include: a ``liquid'' to ``solid'' transition when cars decrease their distance at a traffic light, solidification of liquids droplets during winter months, and the dancing of droplets on a non-sticking pan. In this dissertation we will address seven different phase-change problems occurring in nature. We unveil completely new forms of phase-change phenomena that exhibit rich physical behavior. For example, during traffic flow, drivers keep a large distance from the vehicle in front of them to ensure safe driving. When vehicles come to a stop, for example at a red light, drivers voluntarily induce a ``phase transition'' from this ``liquid phase'' to a close-packed ``solid phase''. This phase transition is motivated by the intuition that traveling as far as possible before stopping will minimize the overall travel time. However, we are going to investigate this phase-change process and show that this long standing intuition is wrong. Phase-change of solidification will be discussed for different problems. Moreover, the complex physics of oil as it wicks up sheets of frost and freezing of bubble unveil completely new forms of multiphase flows that exhibit rich physical behavior. Finally, the ``Cassie'' to ``Wenzel'' transition will be investigated for layered nano-textured surfaces. These phenomena will be modeled using thermodynamics and fluid mechanics equations.
Dynamics of Multi-Agent Systems with Bio-Inspired Active and Passive Sensing
Jahromi Shirazi, Masoud (Virginia Tech, 2020-10-22)
Active sensors, such as radar, lidar and sonar, emit a signal into the environment and gather information from its reflection. In contrast, passive sensors such as cameras and microphones rely on the signals emitted from the environment. In the current application of active sensors in multi-agent autonomous systems, agents only rely on their own active sensing and filter out any information available passively. However, fusing passive and active sensing information may improve the accuracy of the agents. Also, there is evidence that bats who use biosonar eavesdrop on a conspecific's echolocation sound, which shows a successful example of implementing active and passive sonar sensor fusion in nature. We studied the effect of such information fusion in the framework of two problems: the collective behavior in a multi-agent system using the Vicsek model and the canonical robotics problem of Simultaneous Localization And Mapping (SLAM). Collective behavior refers to emergence of a complex behavior in a group of individuals through local interaction. The Vicsek model is a well-established flocking model based on alignment of individuals with their neighbors in the presence of noise. We studied the aligned motion in a group in which the agents employ both active and passive sensing. Our study shows that the group behavior is less sensitive to measurement accuracy compared to modeling precision. Therefore, using measurement values of the noisier passive sonar can be beneficial. In addition, the group alignment is improved when the passive measurements are not dramatically noisier than active measurements. In the SLAM problem, a robot scans an unknown environment building a map and simultaneously localizing itself within that map. We studied a landmark-based SLAM problem in which the robot uses active and passive sensing strategies. The information provided passively can improve the accuracy of the active sensing measurements and compensate for its blind spot. We developed an estimation algorithm using Extended Kalman Filter and employed Monte Carlo simulation to find a parameter region in which fusing passive and active sonar information improves the performance of the robot. Our analysis shows this region is aligned within the common range of active sonar parameters.
The Dynamics of Non-Equilibrium Gliding in Flying Snakes
Yeaton, Isaac J. (Virginia Tech, 2018-03-13)
This dissertation addresses the question, how and why do 'flying' snakes (Chrysopelea) undulate through the air? Instead of deploying paired wings or wing-like surfaces, flying snakes jump, splay their ribs into a bluff-body airfoil, and undulate through the air. Aerial undulation is the dominant feature of snake flight, but its effects on locomotor performance and stability are unknown. Chapter 2 describes a new non-equilibrium framework to analyze gliding animals and how the pitch angle affects their translational motion. Chapter 3 combines flying snake glide experiments and detailed dynamic modeling to address what is aerial undulation and how each kinematic component affects rotational stability and translational performance. Chapter 4 combines the kinematic data of Chapter 3, with elements of the non-equilibrium framework of Chapter 2, to examine the kinematics of snake flight in greater detail. This chapter also tests if our current understanding of flying snake aerodynamics is sufficient to explain the observed center of mass motion.
Dynamics of smart materials in high intensity focused ultrasound field
Bhargava, Aarushi (Virginia Tech, 2020-05-06)
Smart materials are intelligent materials that change their structural, chemical, mechanical, or thermal properties in response to an external stimulus such as heat, light, and magnetic and electric fields. With the increase in usage of smart materials in many sensitive applications, the need for a remote, wireless, efficient, and biologically safe stimulus has become crucial. This dissertation addresses this requirement by using high intensity focused ultrasound (HIFU) as the external trigger. HIFU has a unique capability of maintaining both spatial and temporal control and propagating over long distances with reduced losses, to achieve the desired response of the smart material. Two categories of smart materials are investigated in this research; shape memory polymers (SMPs) and piezoelectric materials. SMPs have the ability to store a temporary shape and returning to their permanent or original shape when subjected to an external trigger. On the other hand, piezoelectric materials have the ability to convert mechanical energy to electrical energy and vice versa. Due to these extraordinary properties, these materials are being used in several industries including biomedical, robotic, noise-control, and aerospace. This work introduces two novel concepts: First, HIFU actuation of SMP-based drug delivery capsules as an alternative way of achieving controlled drug delivery. This concept exploits the pre-determined shape changing capabilities of SMPs under localized HIFU exposure to achieve the desired drug delivery rate. Second, solving the existing challenge of low efficiency by focusing the acoustic energy on piezoelectric receivers to transfer power wirelessly. The fundamental physics underlying these two concepts is explored by developing comprehensive mathematical models that provide an in-depth analysis of individual parameters affecting the HIFU-smart material systems, for the first time in literature. Many physical factors such as acoustic, material and dynamical nonlinearities, acoustic standing waves, and mechanical behavior of materials are explored to increase the developed models' accuracy. These mathematical frameworks are designed with the aim of serving as a basic groundwork for building more complex smart material-based systems under HIFU exposure.
EEG-Based Control of Working Memory Maintenance Using Closed-Loop Binaural Stimulation
Beauchene, Christine Elizabeth (Virginia Tech, 2018-05-17)
The brain is a highly complex network of nonlinear systems with internal dynamic states that are not easily quantified. As a result, it is essential to understand the properties of the connectivity network linking disparate parts of the brain used in complex cognitive processes, such as working memory. Working memory is the system in control of temporary retention and online organization of thoughts for successful goal directed behavior. Individuals exhibit a typically small capacity limit on the number of items that can be simultaneously retained in working memory. To modify network connections and thereby augment working memory capacity, researchers have targeted brain areas using a variety of noninvasive stimulation interventions. However, few existing methods take advantage of the brain's own structure to actively generate and entrain internal oscillatory modulations in locations deep within the auditory pathways. One technique is known as binaural beats, which arises from the brain's interpretation of two pure tones, with a small frequency mismatch, delivered independently to each ear. The mismatch between these tones is perceived as a so-called beat frequency which can be used to modulate behavioral performance and cortical connectivity. Currently, all binaural stimulation therapeutic systems are open-loop "one-size-fits-all" approaches. However, these methods can prove not as effective because each person's brain responds slightly differently to exogenous stimuli. Therefore, the driving motivation for developing a closed-loop stimulation system is to help populations with large individual variability. One such example is persons with mild cognitive impairment (MCI), which causes cognitive impairments beyond those expected based on age. Therefore, applying a closed-loop binaural beat control system to increase the cognitive load level to people with MCI could potentially maintain their quality of life. In this dissertation, I will present a comparison of algorithms to determine brain connectivity, results of open-loop based binaural stimulation, the development of a closed-loop brain network simulation platform, and finally an experimental study to determine the effectiveness of closed-loop control to modulate brain networks hence influencing cognitive abilities.
Electroelastic Modeling and Testing of Direct Contact Ultrasonic Clothes Drying Systems
Dupuis, Eric Donald (Virginia Tech, 2020-07-06)
Energy efficient appliances and devices are becoming increasingly necessary as emissions from electricity production continue to increase the severity of global warming. Many of such appliances have not been substantially redesigned since their creation in the early 1900s. One device in particular which has arguably changed the least and consumes the most energy during use is the electric clothes dryer. The common form of this technology in the United States relies on the generation of thermal energy by passing electrical current through a metal. The resulting heat causes liquid within the clothing to evaporate where humid air is ejected from the control volume. While the conversion of energy from electrical to thermal through a heating element is efficient, the drying characteristics of fabrics in a warm humid environment are not, and much of the heat inside of the dryer does not perform work efficiently. In 2016, researchers at Oak Ridge National Laboratory in Knoxville, Tennessee, proposed an alternative mechanic for the drying of clothes which circumvents the need for thermal energy. This method is called direct-contact ultrasonic clothes drying, utilizing atomization through direct mechanical coupling between mesh piezoelectric transducers and wet fabric. During the atomization process, vertical oscillations of a contained liquid, called Faraday excitations, result in the formation of standing waves on the liquid surface. At increasing amplitudes and frequencies of oscillation, wave peaks become extended and form "necks" connecting small secondary droplets to the bulk liquid. When the oscillation reaches an acceleration threshold, the droplet momentum is sufficient to break the surface tension of the neck and enable the droplets to travel away from the liquid. For smaller drops where surface tension is high, a larger magnitude of acceleration is needed to reach the critical neck lengths necessary for droplet ejection. The various pore sizes within the many fabrics comprising our clothing results in many sizes of droplets retained by the fabric, affecting the rate of atomization due to the differences in surface tension. In this study, we will investigate the physical processes related to the direct contact ultrasonic drying process. Beginning with the electrical actuation of the transducer used in the world's first prototype dryer, we will develop an electromechanical model for predicting the resulting deformation. Various considerations for the material properties and geometry of the transducer will be made for optimizing the output acceleration of the device. Next, the drying rates of fabrics in contact with the transducer will be modeled for identification of parameters which will facilitate timely and energy efficient drying. This task will identify the first ever mechanically coupled drying equation for fabrics in contact with ultrasonic vibrations. The ejection rate of the water atomized by the transducer and passed through microchannels to facilitate drying will then be physically investigated to determine characteristics which may improve mass transport. Finally, future considerations and recommendations for the development of ultrasonic drying will be made as a result of the insight gained by this investigation.
An Exploration of Attunement in Counselor Education
Snead, Katherine F. (Virginia Tech, 2018-05-02)
Experiences of attunement, a deeply felt and embodied state of consciousness that results from tuning in to oneself, others, events, energies, and the environment, are the basis for realization of innate human capacities for connection and growth (Kossak, 2015). In an educational context, the process of 'tuning in' fosters the development of relationally based and embodied knowledge (Blades and Bester, 2013; Lutzker, 2014). Though rarely referenced and never studied comprehensively in counselor education, attunement contributes to the development of relational qualities and creativity necessary to provide effective counseling services (Duffey, Haberstroh, and Trepal, 2009; Kossak, 2015). Based on a synthesis of knowledge from diverse fields, this study was an exploration of attunement in counselor education involving several forms of measurement and the expressive arts as vehicles through which to foster attuned states. The researcher explored attunement as it occurred among master's level counselors-in-training engaging in improvisational group drumming, an intervention that has been shown to promote attunement (e.g., Kossak, 2008a). Perceptual, behavioral, and physiological measures were used to identify an occurrence of attunement. Audio and video data were used to contextualize the overall drumming experience and the process of 'tuning in' that led to attuned states. Findings from this study increase understanding of the phenomenon of attunement in the context of improvisational group drumming. Results shed light on how relational qualities and creativity develop and may promote more relational-responsive pedagogical practices in counselor education. Ultimately, results may contribute to the development of counselors with greater capacities for relating to diverse clients, responding to the complexities of their work, and creating meaningful change within their communities and society at large.
Fluid Dynamics in Liquid Entry and Exit
Kim, Seong Jin (Virginia Tech, 2017-10-05)
Interaction between a solid body and a liquid-air interface plays an important role in multiphase flows, which includes numerous engineering applications such as mineral flotation, dip coating operations, and air-to-sea and sea-to-air projectiles. It is also crucial in animal behaviors like the locomotion of water-walking animals, the plunge-diving of birds, and the jumping out of water of marine creatures. Depending on the moving direction of a solid, such diverse phenomena can be classified into two categories; liquid-entry and liquid-exit. Liquid-entry, or more widely called water-entry, is the behavior of a solid object entering liquid from air. The opposite case is referred to as liquid-exit. Liquid-entry has been extensively studied, especially focusing on cavity formation and the estimation on capillary and hydrodynamic forces on a solid object. However, as the behavior of a triple contact line has not been understood on a sinking object, previous studies were limited to the special case of hydrophobic object to fix the contact line. Moreover, a more recent study pointed out the important role of contact line behavior to characterize the performance of film flotation, which is one of the direct applications of liquid-entry. However, there are no existing previous studies on the dynamics of the contact line on a sinking object. This subject will be first discussed in Chapter 2. In Chapters 3 and 4, the topics related to liquid-exit will be discussed, where a solid sphere exits out of a liquid toward air with constant velocity, acceleration, or deceleration. Chapter 3 will focus on the penetration and bouncing behaviors of a solid sphere while impacting a liquid-air interface. The solid sphere experiences the resistance of surface tension and gravity while impacting the interface. Thus, liquid-exit spheres should have enough momentum to penetrate the interface to overcome these resistances, which indicates that the critical momentum exits. This understanding would give a mechanistic explanation as to why some aquatic species, especially plankton, are able to jump out of water while the others cannot despite their similar size. This study can help to understand the particle-bubble interaction for froth flotation applications, in which the particle tends to attach to the bubble. In the last Chapter, the formation of a liquid column during the liquid-exit will be discussed. It has been observed that the evolution of a liquid column strongly depends on experimental conditions, especially the acceleration of a solid sphere. The pinch-off dynamics of a liquid column is categorized as two branches: upper and lower pinch-off's. The pinch-off location affects the entrained liquid volume adhered to the solid object, which is directly related to the uniform quality of a dip-coating operation. In addition to the pinch-off location and time in relation to the aforementioned experimental conditions will be discussed. In summary, studies in the present dissertation are designed and performed to provide mechanistic insight to the problems in the liquid-entry and liquid-exit, which are all closely related to animal's daily life as well as engineering applications.
Full Field Reconstruction Enhanced With Operational Modal Analysis and Compressed Sensing for General Dynamic Loading
Fu, Gen (Virginia Tech, 2021-06-09)
In most applications, the structure components have to be tested under different loading conditions before being placed in operation. A reliable and low cost measuring technique is desirable. However, most currently employed measuring approaches can only provide the structural response at several discrete locations. The accuracy of the measurements varies with the location and orientation of the sensors. Practically, it is not possible to place sensors at all the critical locations for different excitations. Therefore, an approach that derives the full field response using a limited set of measured data is desirable. In contrast to experimental full field measurement techniques, the expansion approach involves analytically expanding the limited measurements to all the degrees of freedom of the structure. Among all the analytical methods, the modal expansion method is computationally efficient and thus more suitable for real time expansion of measured data. In this method, the full-field response is approximated by the linear combination of mode shapes. In previous studies, the modal expansion method is limited by errors from mode aliasing, inaccuracy of the calculated mode shapes and the noise in measurements. In order to overcome these limitations, the modal expansion method is enhanced by mode selection and error compensation in this study. First, the key parameters used in modal expansion method were analyzed using a cantilever beam model and a method for optimal placement of sensors was developed. A mode selection method and error compensation method based on operation modal analysis and adaptive compressed sensing techniques were then developed to reduce the effects of mode aliasing, mode shape inaccuracy and measurement noise. The developed approach was further tested virtually using a numerical model of rotor 67. The numerical model was created using a two-way coupled fluid structure interaction technique. By developing these methods, the enhanced modal expansion approach can provide full field response for structures under different load conditions. Compared to the traditional modal expansion method, it can expand the data with high noise and under general dynamic loading.
Geometrical Investigation on Escape Dynamics in the Presence of Dissipative and Gyroscopic Forces
Zhong, Jun (Virginia Tech, 2020-03-18)
This dissertation presents innovative unified approaches to understand and predict the motion between potential wells. The theoretical-computational framework, based on the tube dynamics, will reveal how the dissipative and gyroscopic forces change the phase space structure that governs the escape (or transition) from potential wells. In higher degree of freedom systems, the motion between potential wells is complicated due to the existence of multiple escape routes usually through an index-1 saddle. Thus, this dissertation firstly studies the local behavior around the index-1 saddle to establish the criteria of escape taking into account the dissipative and gyroscopic forces. In the analysis, an idealized ball rolling on a surface is selected as an example to show the linearized dynamics due to its special interests that the gyroscopic force can be easily introduced by rotating the surface. Based on the linearized dynamics, we find that the boundary of the initial conditions of a given energy for the trajectories that transit from one side of a saddle to the other is a cylinder and ellipsoid in the conservative and dissipative systems, respectively. Compared to the linear systems, it is much more challenging or sometimes impossible to get analytical solutions in the nonlinear systems. Based on the analysis of linearized dynamics, the second goal of this study is developing a bisection method to compute the transition boundary in the nonlinear system using the dynamic snap-through buckling of a buckled beam as an example. Based on the Euler-Bernoulli beam theory, a two degree of freedom Hamiltonian system can be generated via a two mode-shape truncation. The transition boundary on the Poincar'e section at the well can be obtained by the bisection method. The numerical results prove the efficiency of the bisection method and show that the amount of trajectories that escape from the potential well will be smaller if the damping of the system is increasing. Finally, we present an alternative idea to compute the transition boundary of the nonlinear system from the perspective of the invariant manifold. For the conservative systems, the transition boundary of a given energy is the invariant manifold of a periodic orbit. The process of obtaining such invariant manifold compromises two parts, including the computation of the periodic orbit by solving a proper boundary-value problem (BVP) and the globalization of the manifold. For the dissipative systems, however, the transition boundary of a given energy becomes the invariant manifold of an index-1 saddle. We present a BVP approach using the small initial sphere in the stable subspace of the linearized system at one end and the energy at the other end as the boundary conditions. By using these algorithms, we obtain the nonlinear transition tube and transition ellipsoid for the conservative and dissipative systems, respectively, which are topologically the same as the linearized dynamics.

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Browsing Doctoral Dissertations by Author "Abaid, Nicole"

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