VTechWorks Repository :: Browsing by Author "Bayandor, Javid"

Browsing by Author "Bayandor, Javid"

Now showing 1 - 20 of 25

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.
CFD analysis of airflow patterns and heat transfer in small, medium, and large structures
Detaranto, Michael Francis (Virginia Tech, 2014-11-05)
Designing buildings to use energy more efficiently can lead to lower energy costs, while maintaining comfort for occupants. Computational fluid dynamics (CFD) can be utilized to visualize and simulate expected flows in buildings and structures. CFD gives architects and designers the ability to calculate the velocity, pressure, and heat transfer within a building. Previous research has not modeled natural ventilation situations that challenge common design rules of thumb used for cross-ventilation and single-sided ventilation. The current study uses a commercial code (FLUENT) to simulate cross-ventilation in simple structures and analyzes the flow patterns and heat transfer in the rooms. In the Casa Giuliana apartment and the Affleck house, this study simulates passive cooling in spaces well-designed for natural ventilation. Heat loads, human models, and electronics are included in the apartment to expand on prior research into natural ventilation in a full-scale building. Two different cases were simulated. The first had a volume flow rate similar to the ambient conditions, while the second had a much lower flow rate that had an ACH of 5, near the minimum recommended value Passive cooling in the Affleck house is simulated and has an unorthodox ventilation method; a window in the floor that opens to an exterior basement is opened along with windows and doors of the main floor to create a pressure difference. In the Affleck house, two different combinations of window and door openings are simulated to model different scenarios. Temperature contours, flow patterns, and the air changes per hour (ACH) are explored to analyze the ventilation of these structures.
A Comprehensive Entry, Descent, Landing, and Locomotion (EDLL) Vehicle for Planetary Exploration
Schroeder, Kevin Kent (Virginia Tech, 2017-08-26)
The 2012 Decadal Survey has stated that there is a critical role for a Venus In-situ Explore (VISE) missions to a variety of important sites, specifically the Tessera terrain. This work aims to answer the Decadal Survey's call by developing a new comprehensive Entry, Descent, Landing, and Locomotion (EDLL) vehicle for in-situ exploration of Venus, especially in the Tessera regions. TANDEM, the Tension Adjustable Network for Deploying Entry Membrane, is a new planetary probe concept in which all of EDLL is achieved by a single multifunctional tensegrity structure. The concept uses same fundamental concept as the ADEPT (Adaptable Deployable Entry and Placement Technology) deployable heat shield but replaces the standard internal structure with the structure from the tensegrity-actuated rover to provide a combined aeroshell and rover design. The tensegrity system implemented by TANDEM reduces the mass of the overall system while enabling surface locomotion and mitigating risk associated with landing in the rough terrain of Venus's Tessera regions, which is otherwise nearly inaccessible to surface missions. TANDEM was compared to other state-of-the-art lander designs for an in-situ mission to Venus. It was shown that TANDEM provides the same scientific experimentation capabilities that were proposed for the VITaL mission, with a combined mass reduction for the aeroshell and lander of 52% (1445 kg), while eliminating the identified risks associated with entry loads and very rough terrain. Additionally, TANDEM provides locomotion when on the surface as well as a host of other maneuvers during entry and descent, which was not present in the VITaL design. Based on its unique multifunctional infrastructure and excellent crashworthiness for impact on rough surfaces, TANDEM presents a robust system to address some of the Decadal Survey's most pressing questions about Venus.
Comprehensive Multi-Scale Progressive Failure Analysis for Damage Arresting Advanced Aerospace Hybrid Structures
Horton, Brandon Alexander (Virginia Tech, 2017-08-31)
In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development.
A Computational Study of the Hydrodynamics of Gas-Solid Fluidized Beds
Teaters, Lindsey Claire (Virginia Tech, 2012-05-31)
Computational fluid dynamics (CFD) modeling was used to predict the gas-solid hydrodynamics of fluidized beds. An Eulerian-Eulerian multi-fluid model and granular kinetic theory were used to simulate fluidization and to capture the complex physics associated therewith. The commercial code ANSYS FLUENT was used to study two-dimensional single solids phase glass bead and walnut shell fluidized beds. Current modeling codes only allow for modeling of spherical, uniform-density particles. Owing to the fact that biomass material, such as walnut shell, is abnormally shaped and has non-uniform density, a study was conducted to find the best modeling approach to accurately predict pressure drop, minimum fluidization velocity, and void fraction in the bed. Furthermore, experiments have revealed that all of the bed mass does not completely fluidize due to agglomeration of material between jets in the distributor plate. It was shown that the best modeling approach to capture the physics of the biomass bed was by correcting the amount of mass present in the bed in order to match how much material truly fluidizes experimentally, whereby the initial bed height of the system is altered. The approach was referred to as the SIM approach. A flow regime identification study was also performed on a glass bead fluidized bed to show the distinction between bubbling, slugging, and turbulent flow regimes by examining void fraction contours and bubble dynamics, as well as by comparison of simulated data with an established trend of standard deviation of pressure versus inlet gas velocity. Modeling was carried out with and without turbulence modeling (k-Ïµ), to show the effect of turbulence modeling on two-dimensional simulations.
Computational Study of Turbulent Combustion Systems and Global Reactor Networks
Chen, Lu (Virginia Tech, 2017-09-05)
A numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques, namely computational fluid dynamics (CFD) and chemical reactor network (CRN) methods. Both methods have been studied and analyzed as individual techniques as well as a coupled approach to pursue better understandings of the mechanisms and interactions between turbulent flow and mixing, ignition behavior and pollutant formation. A thorough analysis and comparison of both turbulence models and chemistry representation methods was executed and simulations were compared and validated with experimental works. An extensive study of turbulence modeling methods, and the optimization of modeling techniques including turbulence intensity and computational domain size have been conducted. The final CFD model has demonstrated good predictive performance for different turbulent bluff-body flames. The NOx formation and the effects of fuel mixtures indicated that the addition of hydrogen to the fuel and non-flammable diluents like CO2 and H2O contribute to the reduction of NOx. The second part of the study focused on developing chemical models and methods that include the detailed gaseous reaction mechanism of GRI-Mech 3.0 but cost less computational time. A new chemical reactor network has been created based on the CFD results of combustion characteristics and flow fields. The proposed CRN has been validated with the temperature and species emission for different bluff-body flames and has shown the capability of being applied to general bluff-body systems. Specifically, the rate of production of NOx and the sensitivity analysis based on the CRN results helped to summarize the reduced reaction mechanism, which not only provided a promising method to generate representative reactions from hundreds of species and reactions in gaseous mechanism but also presented valuable information of the combustion mechanisms and NOx formation. Finally, the proposed reduced reaction mechanism from the sensitivity analysis was applied to the CFD simulations, which created a fully coupled process between CFD and CRN, and the results from the reduced reaction mechanism have shown good predictions compared with the probability density function method.
The development and analysis of a mobile explosive containment unit for on-board aircraft protection
Costain, Andrew J. (Virginia Tech, 2014-09-17)
This body of work examines the process involved in researching a mobile explosive containment unit for use on board a commercial aircraft. If a device with unknown origin were discovered on board a commercial aircraft an explosive containment unit could be used to dispose of it thereby preventing the passengers and the hardware from incurring any harm. A methodology was developed to help understand and effectively capture the properties of nominal explosives, the detonation pulse, ensuing shock and pressure waves. This methodology was developed with the purpose of mitigating these explosive effects. The information concerning the material properties, shape and sizes of an explosive containment unit were all analyzed to identify one optimal containment unit. This containment unit was utilized extensively in modeling to determine a range of possible materials and reinforcement methods, for reducing the total weight of the unit. Upon optimizing the containment unit numerical analysis was performed on a fuselage section of a narrow body commercial aircraft with the containment unit. The containment unit was successful in arresting the explosion before it was able to cause harm to its surroundings. The success of these containment units proves that the methodology discussed and developed here is capable of rabidly developing and analyzing explosive containment units to fit a wide variety of needs.
Development and Validation of Fluid-Structure Interaction in Aircraft Crashworthiness Studies
Satterwhite, Matthew Ryan (Virginia Tech, 2013-09-04)
Current Federal Aviation Regulations require costly and time consuming crashworthiness testing to certify aircraft. These tests are only capable of a limited assessment of progressive damage and all crash configurations and scenarios cannot be physically evaluated. Advancements in technology have led to accurate and effective developments in numerical modeling that have the possibility of replacing these rigorous physical experiments. Through finite element analysis, an in-depth investigation of an aircraft equipped with a fabricated composite undercarriage was evaluated during water ditching. The severe impact of aircraft ditching is dynamic and nonlinear in nature; the goal of this work to develop a methodology that not only captures the structural response of the aircraft, but also the fluidic behavior of the water. Fundamental studies were first conducted on a well-researched fluid-solid interaction problem, the water entry of a wedge. Typical modeling strategies did not capture the desired detail of the event. An advanced meshing scheme combining meshed and meshless Lagrangian techniques was developed and multiple wedge angles were tested and compared to analytic and qualitative results. The meshing technique proved valid, as the difficult to model phenomena of splashing was captured and the maximum impact force was within five percent of analytical calculations for the 20° and 30° deadrise wedge. Physical small scale aircraft ditching experiments were then performed with an innovative testing platform capable of producing varied aircraft approach configurations. The model was outfitted with an instrumented composite undercarriage to record data throughout the impact while a high-speed camera recorded the event. Numerical simulations of the model aircraft were then compared to experimental results with a strong correlation. This methodology was then ultimately tested on a deformable model of a fuselage section of a full-size aircraft.
Development of Comprehensive Dynamic Damage Assessment Methodology for High-Bypass Air Breathing Propulsion Subject to Foreign Object Ingestion
Song, Yangkun (Virginia Tech, 2016-11-10)
Foreign object ingestion (FOI) into jet engines is a recurring scenario during the operation life of aircraft. Objects can range from as small as a pebble on the tarmac to the size of a large bird. Among the potential ingestion scenarios, damage caused by smaller objects may be considered to be negligible. Alternatively, larger objects can initiate progressive damage, potentially leading to catastrophic failure, compromising the integrity of the structure, and endangering the safety of passengers. Considering the dramatic increase in air traffic, FOI represents a crucial safety hazard, and must be better understood to minimize possible damage and structural failure. The main purpose of this study is to develop a unique methodology to assess the response and dynamic damage progression of an advanced, high-bypass propulsion system in the event of an FOI during operation. Using a finite element framework, a unique modeling methodology has been proposed in order to characterize the FOI response of the system. In order to demonstrate versatility of the computational analysis, the impact characteristics of two most common foreign object materials, bird and ice, were investigated. These materials were then defined in finite element domain, verified computationally, and then validated against the existing physical experiments. In addition to the mechanics of the two FOI materials, other material definitions, used to characterize the structures of the high-bypass propulsion system, were also explored. Both composite materials and rate dependent definitions for metal alloys were investigated to represent the damage mechanics in the event of an FOI. Subsequently, damage sequence of high-bypass propulsion systems subject to FOI was developed and assessed, using a uniquely devised Fluid-Structure Interaction (FSI) technique. Using advanced finite element formulation, this approach enabled the accurate simulation of the comprehensive damage progression of the propulsion systems by including aerodynamic interaction. Through this strategy, fluid mechanics was combined with structural mechanics in order to simulate the mutual interaction between both continua, allowing the interpretation of both the additional damage caused by the fluid flow and disrupted aerodynamics induced by the dynamic deformation of the fan blade. Subsequently, this multidisciplinary-multiphysics computational approach, in the framework of the comprehensive analysis methodology introduced, enabled the effective determination of details on the overall progressive impact damage, not traditionally available to propulsion designers.
Discovering the Complex Aerodynamics of Flapping Flight with Bio-kinematics Using Boltzmann and Eulerian Methods
Feaster, Jeffrey Oden (Virginia Tech, 2017-08-31)
The cross-sectional geometry of an insect wing has historically been simplified to a rectangular, elliptic, or having a streamlined airfoil shape. Up until this point, no analysis has utilized a morphologically accurate insect wing. As such, there remains significant questions as to whether or not there are aerodynamic benefits to the wing vein structure accompanying the already known structural improvements. The present study uses a bumblebee specimen (Bombus pensylvanicus) acquired by the author, scanned using a skyscan microCT scanner, and post-processed for computational analysis. The resulting geometry captures the naturally occurring vein structures present in the bee wing and is used to better understand aerodynamic effects of biological corrugation. The aerodynamics associated with a morphologically accurate bee wing geometry are explored in two and three dimensions for the first time. Multiple methodologies are validated with experimental results presented in the literature to capture the fluid dynamics in two dimensions including the Lattice-Boltzmann method and unstructured dynamic remeshing using a Navier-Stokes approach. The effects of wing cross-section are compared first with common geometries used in the literature in two dimensions and then between cross-sections extracted at different locations along the wing span. A three-dimensional methodology is validated and used to compare the true bee wing with one using a rectangular cross-section in symmetric hovering. The influence of spanwise cross-section is revisited in three dimensions and compared to the results found in two-dimensions for the same kinematics in forward flight. The final focus of the dissertation is the first simulation of a morphologically accurate wing using kinematics described in the literature.
Effects of fish caudal fin sweep angle and kinematics on thrust production during low-speed thunniform swimming
Matta, Alexander; Bayandor, Javid; Battaglia, Francine; Pendar, Hodjat (The Company of Biologists, 2019-06-12)
Scombrid fish lunate caudal fins are characterized by a wide range of sweep angles. Scombrid that have small sweep-angle caudal fins move at higher swimming speeds, suggesting that smaller angles produce more thrust. Furthermore, scombrids occasionally use high angles of attack (AoA) suggesting this also has some thrust benefit. This work examined the hypothesis that a smaller sweep angle and higher AoA improved thrust in swimmers by experimentally analyzing a robophysical model. The robophysical model was tested in a water tunnel at speeds between 0.35 and 0.7 body lengths per second. Three swept caudal fins were analyzed at three different AoA, three different freestream velocities, and four different Strouhal numbers, for a total of 108 cases. Results demonstrated that the fin with the largest sweep angle of 50° resulted in lower thrust production than the 40° and 30° fins, especially at higher Strouhal numbers. Larger AoA up to 25° increased thrust production at the higher Strouhal numbers, but at lower Strouhal numbers, produced less thrust. Differences in thrust production due to fin sweep angle and AoAwere attributed to the variation in spanwise flowand leading edge vortex dynamics.
Hydrodynamic and gasification behavior of coal and biomass fluidized beds and their mixtures
Estejab, Bahareh (Virginia Tech, 2016-03-29)
In this study, efforts ensued to increase our knowledge of fluidization and gasification behavior of Geldart A particles using CFD. An extensive Eulerian-Eulerian numerical study was executed and simulations were compared and validated with experiments conducted at Utah State University. In order to improve numerical predictions using an Eulerian-Eulerian model, drag models were assessed to determine if they were suitable for fine particles classified as Geldart A. The results proved that if static regions of mass in fluidized beds are neglected, most drag models work well with Geldart A particles. The most reliable drag model for both single and binary mixtures was proved to be the Gidaspow-blend model. In order to capture the overshoot of pressure in homogeneous fluidization regions, a new modeling technique was proposed that modified the definition of the critical velocity in the Ergun correlation. The new modeling technique showed promising results for predicting fluidization behavior of fine particles. The fluidization behavior of three different mixtures of coal and poplar wood were studied. Although results indicated good mixing characteristics for all mixtures, there was a tendency for better mixing with higher percentages of poplar wood. In this study, efforts continued to model co-gasification of coal and biomass. Comparing the coal gasification of large (Geldart B) and fine (Geldart A) particles showed that using finer particles had a pronounced effect on gas yields where CO mass fraction increased, although H2 and CH4 mass fraction slightly decreased. The gas yields of coal gasification with fine particles were also compared using three different gasification agents. Modeling the co-gasification of coal-switchgrass of both fine particles of Geldart A and larger particles of Geldart B showed that there is not a synergetic effect in terms of gas yields of H2 and CH4. The gas yields of CO, however, showed a significant increase during co-gasification. The effects of gasification temperature on gas yields were also investigated.
The Hydrodynamics and Energetics of Bioinspired Swimming with Undulatory Electromechanical Fins
Gater, Brittany L. (Virginia Tech, 2017)
Biological systems offer novel and efficient solutions to many engineering applications, including marine propulsion. It is of interest to determine how fish interact with the water around them, and how best to utilize the potential their methods offer. A stingray-like fin was chosen for analysis due to the maneuverability and versatility of stingrays. The stingray fin was modeled in 2D as a sinusoidal wave with an amplitude increasing from zero at the leading edge to a maximum at the trailing edge. Using this model, a parametric study was performed to examine the effects of the fin on surrounding water in computational fluid dynamics (CFD) simulations. The results were analyzed both qualitatively, in terms of the pressure contours on the fin and vorticity in the trailing wake, and quantitatively, in terms of the resultant forces and the mechanical power requirements to actuate the desired fin motion. The average thrust was shown to depend primarily on the relationship between the swimming speed and the frequency and wavelength (which both are directly proportional to the wavespeed of the fin), although amplitude can be used to augment thrust production as well. However, acceleration was shown to significantly correlate with a large variation in lift and moment, as well as with greater power losses. Using results from the parametric study, the potential for power regeneration was also examined. Relationships between frequency, velocity, drag, and power input were determined using nonlinear regression that explained more than 99.8% of the data. The actuator for a fin was modeled as a single DC motor-shaft system, allowing the combination of the energetic effects of the motor with the fin-fluid system. When combined, even a non-ideal fin model was able to regenerate more power at a given flow speed than was required to swim at the same speed. Even in a more realistic setting, this high proportion of regenerative power suggests that regeneration and energy harvesting could be both feasible and useful in a mission setting.
Impact Characterization of Earth Entry Vehicle for Terminal Landing (on Soil)
Shorts, Daniel Calvert (Virginia Tech, 2017-08-28)
In order to more accurately predict loads subjected to the EEV (Earth Entry Vehicle) upon impact with a variety of materials, finite element simulations of soil/EEV impact were created using the program LS-DYNA. Various modeling techniques were analyzed for accuracy through comparison with physical test data when available. Through variation of numerical methods, mesh density, and material definition, an accurate and numerically efficient representation of physical data has been created. The numerical methods, Lagrangian, arbitrary Lagrangian-Eulerian (ALE), and spherical particle hydrodynamics (SPH) are compared to determine their relative accuracy in modeling soil deformation and EEV acceleration. Experimentally validated soil material parameters and element formulations were then used in parametric studies to gain a perspective on effects of EEV mass and geometry on its maximum acceleration across varying soil moisture content. Additionally, the effects of EEV orientation, velocity, and impact material were explored. Multi-material arbitrary Lagrangian-Eulerian (MMALE) formulation possess the most effective compromise between its ability to: accurately display qualitative soil behavior, accurately recreate empirical test data, be easily utilized in parametric studies, and to maintain simulation stability. EEV acceleration can be minimized through increase of EEV mass (with constant geometry), allowing for maximum penetration depth, and longest deceleration time. A critical orientation was discovered at 30⁰ from normal, such that maximum EEV surface area impacts the soil surface instantaneously, resulting in maximum acceleration. Off-nominal impact with concrete is predicted to increase acceleration by up to 630% from impact with soil.
Investigation of Dynamic Ultrasound Reception in Bat Biosonar Using a Biomimetic Pinna Model
Pannala, Mittu (Virginia Tech, 2013-12-03)
Bats are a paragon of evolutionary success. They rely on parsimonious sensory inputs provided by echolocation, yet are able to master lives in complex environments. The outer ears (pinnae) of bats are intricately shaped receiver baffles that encode sensory information through a diffraction process. In some bat species with particularly sophisticated biosonar systems, such as horseshoe bats (Rhinolophidae), the pinnae are characterized by static as well as dynamic geometrical features. Furthermore, bats from these species can deform their pinnae while the returning ultrasonic waves impinge on them. Hence, these dynamic pinna geometries could be a substrate for novel, dynamic sensory encoding paradigms. In this dissertation, two aspects of this dynamic sensing process were investigated: (i) Do local shape features impact the acoustic effects during dynamic deformation of the bat pinna? and (ii) do these shape deformations provide a substrate for the dynamic encoding of sensory information? For this, a family of simplified biomimetic prototypes has been designed based on obliquely truncated cones manufactured from sheets of isobutyl rubber. These prototypes were augmented with biomimetic local shape features as well as with a parsimonious deformation mechanism based on a single linear actuator. An automated setup for the acoustic characterization of the time-variant prototype shapes has been devised and used to characterize the acoustic responses of the prototypes as a function of direction. It was found that the effects of local shape features did interact with each other and with the deformation of the overall shape. The impact of the local features was larger for bent than for upright shape configurations. Although the tested devices were much simpler than actual bat pinnae, they were able to reproduce numerical beampattern predictions that have been obtained for deforming horseshoe bat pinnae in a qualitative fashion. The dynamically deformable biomimetic pinna shapes were estimated to increase the sensory encoding capacity of the device by unit[80]{%} information when compared to static baffles. To arrive at this estimate, spectral clustering was used to break up the direction- and deformation-depended device transfer function into a discrete signal alphabet. For this alphabet, we could estimate the joint signal entropy across a bending cycle as a measure for sensory coding capacity. The results presented in this thesis suggest that bat biosonar posses unique dynamic sensing abilities which have no equivalent in man-made technologies. Sensing paradigms derived from bat biosonar could hence inspire new deformable wave-diffracting structures for the advancement in sensor technology.
Investigations of Hypervelocity Impact Physics
Thurber, Andrew (Virginia Tech, 2014-09-17)
Spacecraft and satellites in orbit are under an increasing threat of impact from orbital debris and naturally occurring meteoroids. While objects larger than 10 cm are routinely tracked and avoided, collisions inevitably occur with smaller objects at relative velocities exceeding 10 km/s. Such hypervelocity impacts (HVI) create immense shock pressures and can melt or vaporize aerospace materials, even inducing brief plasmas at higher speeds. Sacrificial shields have been developed to protect critical components from damage under these conditions, but the response of many materials in such an extreme event is still poorly understood. This work presents the summary of computational analysis methods to quantify the relevant physical mechanisms at play in a hypervelocity impact. Strain rate-dependent behavior was investigated using several models, and fluid material descriptions were used to draw parallels under high shear rate loading. The production and expansion of impact plasmas were modeled and compared to experimental evidence. Additionally, a parametric study was performed on a multitude of possible material candidates for sacrificial shield design, and new shielding configurations were proposed. A comparison of material models indicated that the Johnson-Cook and Steinberg-Cochran-Guinan-Lund metallic formulations yielded the most consistent results with the lowest deviation from experimental measures in the strain rate regime of interest. Both meshless Lagrangian and quasi-Eulerian meshed schemes approximated the qualitative and quantitative characteristics of HVI debris clouds with average measurable errors under 5%. While the meshless methods showed better resolution of interfaces and small details, the meshed methods were shown to converge faster under several metrics with fewer regions of spurious instability. Additionally, a new technique was introduced using hypothetical viscous fluids to approximate debris cloud behavior, which showed good correlation to experimental results when such models were constructed using the shear rates seen in hypervelocity impacts. Formulations using non-Newtonian fluids showed additional capability in approximating solid behavior, both quantitatively and qualitatively. Such fluid models are significant, in that they reproduced the qualitative and quantitative characteristics of evolving debris clouds with better fidelity than purely hydrodynamic models using inviscid fluids. This indicates that while inertial effects can dominate overdriven shock phenomena, neglecting shear forces invariably introduces errors; such forces can instead be simplistically approximated via viscous models. The viscous approximation also allowed for a successful scaling analysis using dimensionless Pi terms, which was unfeasible using solid constitutive relations. Attempts to model plasma dynamics saw success in the simulation of a laser ablation-driven flyer plate by using a hot gas with solid initial conditions; similar strategies were used to analyze plasma production in hypervelocity impacts with reasonable correlation to experimental measurements. Lastly, the analysis of bumper material candidates showed that metals with a low density such as beryllium and magnesium yield a higher specific energy and momentum reduction of incident projectiles with lower weight requirements than a similarly constructed bumper using aluminum. Investigations of bumpers using a combination of materials and variations in microstructure showed promise in increasing weight-normalized efficacy. Through these computational models, the parameters which influence damage and debris in hypervelocity impacts are more critically understood.
Modeling and Approximation of Nonlinear Dynamics of Flapping Flight
Dadashi, Shirin (Virginia Tech, 2017-06-19)
The first and most imperative step when designing a biologically inspired robot is to identify the underlying mechanics of the system or animal of interest. It is most common, perhaps, that this process generates a set of coupled nonlinear ordinary or partial differential equations. For this class of systems, the models derived from morphology of the skeleton are usually very high dimensional, nonlinear, and complex. This is particularly true if joint and link flexibility are included in the model. In addition to complexities that arise from morphology of the animal, some of the external forces that influence the dynamics of animal motion are very hard to model. A very well-established example of these forces is the unsteady aerodynamic forces applied to the wings and the body of insects, birds, and bats. These forces result from the interaction of the flapping motion of the wing and the surround- ing air. These forces generate lift and drag during flapping flight regime. As a result, they play a significant role in the description of the physics that underlies such systems. In this research we focus on dynamic and kinematic models that govern the motion of ground based robots that emulate flapping flight. The restriction to ground based biologically inspired robotic systems is predicated on two observations. First, it has become increasingly popular to design and fabricate bio-inspired robots for wind tunnel studies. Second, by restricting the robotic systems to be anchored in an inertial frame, the robotic equations of motion are well understood, and we can focus attention on flapping wing aerodynamics for such nonlinear systems. We study nonlinear modeling, identification, and control problems that feature the above complexities. This document summarizes research progress and plans that focuses on two key aspects of modeling, identification, and control of nonlinear dynamics associated with flapping flight.
Modeling and Control of Flapping Wing Robots
Murphy, Ian Patrick (Virginia Tech, 2013-03-05)
The study of fixed wing aeronautical engineering has matured to the point where years of research result in small performance improvements. In the past decade, micro air vehicles, or MAVs, have gained attention of the aerospace and robotics communities. Many researchers have begun investigating aircraft schemes such as ones which use rotary or flapping wings for propulsion. While the engineering of rotary wing aircraft has seen significant advancement, the complex physics behind flapping wing aircraft remains to be fully understood. Some studies suggest flapping wing aircraft can be more efficient when the aircraft operates in low Reynolds regimes or requires hovering. Because of this inherent complexity, the derivation of flapping wing control methodologies remains an area with many open research problems. This thesis investigates flapping wing vehicles whose design is inspired by avian flight. The flapping wing system is examined in the cases where the core body is fixed or free in the ground frame. When the core body is fixed, the Denavit Hartenberg representation is used for the kinematic variables. An alternative approach is introduced for a free base body case. The equations of motion are developed using Lagranges equations and a process is developed to derive the aerodynamic contributions using a virtual work principle. The aerodynamics are modeled using a quasi-steady state formulation where the lift and drag coefficients are treated as unknowns. A collection of nonlinear controllers are studied, specifically an ideal dynamic inversion controller and two switching dynamic inversion controllers. A dynamic inversion controller is modified with an adaptive term that learns the aerodynamic effects on the equation of motion. The dissipative controller with adaptation is developed to improve performance. A Lyapunov analysis of the two adaptive controllers guarantees boundedness for all error terms. Asymptotic stability is guaranteed for the derivative error in the dynamic inversion controller and for both the position and derivative error in the dissipative controller. The controllers are simulated using two dynamic models based on flapping wing prototypes designed at Virginia Tech. The numerical experiments validate the Lyapunov analysis and illustrate that unknown parameters can be learned if persistently excited.
Numerical analysis of bat noseleaf dynamics and its impact on the encoding of sensory information
Gupta, Anupam Kumar (Virginia Tech, 2017-02-06)
Horseshoe bats possess a sophisticated biosonar system that helps them to negotiate complex unstructured environments by relying primarily on the sound as the far sense. For this, the bats emit brief ultrasonic pulses and listen to incoming echoes to learn about the environment. The sites of emission and reception in these bats are surrounded by baffle structures called "noseleaves" and "pinnae (outer ears)". These are the the only places in the biosonar system where direction-dependent information gets encoded. These baffle structures in bats unlike the engineering systems like megaphones have complex static geometry and can undergo fast deformations at the time of pulse emission/reception. However, the functional significance of the baffle motions in biosonar system is not known. The current work primarily focuses on: i) the study of the impact of noseleaf dynamics on the outgoing sound waves, ii) the study of the impact of baffle dynamics on encoding of sensory information and localization performance of bats. For this, we take a numerical approach where we use computer-animated digital models of bat noseleaves that mimic noseleaf dynamics as observed in bats. The shapes are acoustically characterized (beampatterns) numerically using a finite element implementation. These beampatterns are then analyzed using an information-theoretic approach. The followings findings were obtained: i) noseleaf dynamics altered the spatial distribution of energy, ii) baffle dynamics results in encoding of new sensory information, and iii) the new sensory information encoded due to baffle dynamics significantly improves the performance of biosonar system on the two target localization tasks evaluated here -- direction resolution and direction estimation accuracy. These results affirm the importance of dynamics in biosonar system of horseshoe bats and point at the possibility of biosonar dynamics as a key factor behind the astounding sensory capabilities of these animals that are not yet matched by engineering systems. Thus, these biosonar dynamic principles can help improve the man-made sensing systems and help close the performance gap between active sensing in biology and in engineering.
Parametric Sensitivities of XFEM Based Prognosis for Quasi-static Tensile Crack Growth
Prasanna Kumar, Siddharth (Virginia Tech, 2017-08-21)
Understanding failure mechanics of mechanical equipment is one of the most important aspects of structural and aerospace engineering. Crack growth being one of the major forms of failure in structural components has been studied for several decades to achieve greater reliability and guarantee higher safety standards. Conventional approaches using the finite element framework provides accurate solutions, yet they require extremely complicated numerical approaches or highly fine mesh densities which is computationally expensive and yet suffers from several numerical instabilities such as element entanglement or overly soften element behavior. The eXtended Finite Element Method (XFEM) is a relatively recent concept developed for modeling geometric discontinuities and singularities by introducing the addition of new terms to the classical shape functions in order to allow the finite element formulation to remain the same. XFEM does not require the necessity of computationally expensive numerical schemes such as active remeshing and allows for easier crack representation. In this work, verifies the validity of this new concept for quasi-static crack growth in tension with Abaqus' XFEM is employed. In the course of the work, the effect of various parameters that are involved in the modelling of the crack are parametrically analyzed. The load-displacement data and crack growth were used as the comparison criterion. It was found that XFEM is unable to accurately represent crack growth in the models in the elastic region without direct manipulation of the material properties. The crack growth in the plastic region is found to be affected by certain parameters allowing us to tailor the model to a small degree. This thesis attempts to provide a greater understanding into the parametric dependencies of XFEM crack growth.

Browsing by Author "Bayandor, Javid"

Results Per Page

Sort Options