Mechanical Engineering

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Acoustic Black Hole with Functionally Graded Perforated Rings
(2024) Petrover, Kayla; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis investigates a novel class of acoustic black hole waveguides (ABH) that harnesses the functionality of an array of optimally designed Functionally Graded Perforated Rings (FGPR). Through this approach, the developed ABH exhibits inherent energy dissipation characteristics derived from the flow through the perforations, which enhances its acoustic absorption behavior, resulting in rapid attenuation of the propagating waves as it traverses the length of the waveguide. Furthermore, the proposed ABH structure facilitates the incorporation of additional porous absorbing layers sandwiching the rings to further enhance its absorption characteristics. Consequently, the operational mechanism of this new class of ABH waveguides diverges significantly from that of the conventional ABH waveguide, which generates the black hole effect by employing sequential solid-flat rings of decreasing inner radius to create the necessary virtual power law taper. Instead, the new class of ABH generates the black hole effect through reactive means rather than the effective dissipative means of the conventional ABH. Therefore, this thesis develops a transfer matrix modeling (TMM) approach and a finite element method (FEM) approach to model the absorption and reflection characteristics of the novel class of ABH, aiming to predict its behavior and, more importantly, demonstrate its merits as effective means for controlling sound propagation. The interior-point method for optimization was employed to select optimal geometric design parameters for the FGPR inside the proposed ABH. Accordingly, the ABH with FGPR is manufacturable, unlike the conventional, and its acoustic properties are tuned to minimize the reflection of incoming acoustic waves across the frequency range 0-5 kHz. This optimization process is then repeated for the ABH with FGPR sandwiched by absorbing layers. From the pool of optimal designs generated, those that offer manufacturing advantages are chosen for further testing and evaluation. Numerical simulations are conducted to showcase the advantages and behavior of the proposed ABH configurations. The predictions of the TMM and FEM are compared and validated against experimental results which are collected with the ACUPRO impedance tube. Furthermore, comparisons between the ABH with FGPR and conventional ABH are made to elucidate and distinguish their respective behaviors and underlying principles of operation.
The Natural Response of Uniform and Nonuniform Plates in Air and Partially Submerged in a Quiescent Water Body
(2024) Fishman, Edwin Barry; Duncan, James; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The free vibration of three aluminum plates (.4 m wide, 1.08 m long) oriented horizontally is studied experimentally under two fluid conditions, one with the plate surrounded by air, called the Air case, and the other with the bottom plate surface in contact with a large undisturbed pool of water, called the Half-Wet case. Measurements of the out-of-plane deflection of the upper surfaces of the plates are made using cinematic Digital Image Correlation (DIC) over the center portion of the surface and optical tracking of the center point. Three plate geometries and boundary conditions are studied: A uniform plate with 6.35 mm thickness pinned at the two opposite narrow ends (designated UP), a uniform plate with 4.83 mm thickness simply supported at one narrow end and clamped at the opposite end (UC), and a stepped plate with thickness varying from 12.7 mm to 6.35 mm along its 1.08 m length pinned at two opposite narrow ends (SP). The plate's free response is induced using an impact hammer at three locations along the center-line of the plate. Video frames of the motion of the upper surface of the plate are collected from stereoscopic cameras and processed using DaVis-Strainmaster and MATLAB to extract full-field displacements as a function of time. Two-degree-of-freedom displacements of the plate center are also collected by tracking a target attached to the center of the plate's lower surface. Time and frequency response plots are presented for comparison between the Half-Wet and Air cases and analysis of their dynamics. It is found that the added mass of the water results in lower measured natural frequencies and modified mode shapes. In the Air case, these results are compared to mode shapes/frequencies produced in Creo Simulate and found to agree. Further experiments are discussed.
PASSIVE AND ACTIVE GRADED-INDEX ACOUSTIC METAMATERIALS: SPATIAL AND FREQUENCY DOMAIN MULTIPLEXING
(2022) Yazdkhasti, Amirhossein; Yu, Miao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Acoustic metamaterials, similar to their electromagnetic counterparts, are artificial subwavelength materials designed to manipulate sound waves. By tailoring the material's effective properties such as bulk modulus, mass density, and reflective index, these materials can be designed to achieve unprecedented acoustic waves control and realize functional devices of novel properties. Specifically, high-refractive-index acoustic metamaterials have an effective refractive index much larger than air, enabling wave compression in space and a strong concentration of wave energy. Another type of acoustic metamaterials closely related to high-index acoustic metamaterials is graded-index metamaterials, which can be obtained by gradually varying material compositions or geometry over a volume of high-index acoustic metamaterials.The overall goal of this dissertation is to achieve a fundamental understanding of passive and active graded-index acoustic metamaterials for spatial and frequency domain multiplexing and explore their applications in far-field acoustic imaging and sonar systems. Three research thrusts have been pursued. In the first thrust, the spatial domain multiplexing of passive graded-index acoustic metamaterials has been investigated for enhancing far-field acoustic imaging. An array of passive graded-index acoustic metamaterials has been designed and developed to achieve a far-field acoustic imaging system. Parametric studies have been carried out to facilitate the performance optimization of the imaging system. The performance of the metamaterial-based imaging system has been investigated and compared to the scenario without the metamaterials. In the second thrust, frequency-domain multiplexing with active graded-index acoustic metamaterials has been investigated. An active graded-index metamaterial system with a number of active unit cells has been designed and fabricated. A fundamental understanding of the frequency multiplexing properties of the metamaterials has been developed through numerical and experimental studies. In the third thrust, the capabilities of an acoustic sensing system with active graded-index metamaterials as an emitter for shape, size, and surface classification have been explored.
ACTIVE NON-RECIPROCAL ACOUSTIC METAMATERIALS
(2022) Zhou, Han; Baz, Amr AM; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation presents different configurations of active Acoustic MetaMaterials (AMM) which are proposed in order to control the flow of vibration and acoustic wave propagations in various applications. Distinct among these configurations is a 1-dimensional (1D) periodic array which consists of an assembly of active acoustic unit cells which are provided with programmable piezoelectric elements. By tuning the structural properties of these cells, the 1D array can impede the wave propagation over specific frequency ranges. In order to achieve non-reciprocal acoustic wave transmission of the AMMs, three different methodologies are introduced including active control of the piezoelectric elements using virtual gyroscopic control actions, eigenstructure shaping controller, and finally spatial-temporal modulation algorithm.Theoretical models are developed to investigate the fundamentals and the underlying physical phenomena associated with all the considered three AMM configurations. Experimental prototypes of all these AMM configurations are built and tested to demonstrate their effectiveness in controlling the propagation of vibration and noise through these materials. Furthermore, the experimental results are used to validate the developed theoretical models. The developed theoretical and experimental approaches are envisioned to be valuable tools in the design of arrays of AMM for various applications which are only limited by our imagination.
HIGH WAVE VECTOR ACOUSTIC METAMATERIALS: FUNDAMENTAL STUDIES AND APPLICATIONS
(2020) Ganye, Randy Tah; Yu, Miao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Acoustic metamaterials are artificially engineered structures with subwavelength unit cells that hold extraordinary acoustic properties. Their ability to manipulate acoustic waves in ways that are not readily possible in naturally occurring materials have garnered much attention by researchers in recent years. In this dissertation work, acoustic metamaterials that enable wave propagation with high wave vector values are studied. These materials render several key properties, including energy confinement and transport, wave control enhancement, and enhancement of acoustic radiation, which are exploited for enhancing acoustic wave emission and reception. The dissertation work is summarized as follows. First, to enable experimental studies of the deep subwavelength cavities in these metamaterials, a low dimensional fiber optic probe was developed, which allows direct characterization of the intrinsic properties of the metamaterials without seriously disrupting the acoustic fields. Second, low dimensional acoustic metamaterials for enhancing acoustic reception were realized and studied. These metamaterials were demonstrated to achieve both passive and active functionalities, including passive signal amplification and frequency filtering, as well as active tuning for switching and pulse retardation control. Third, a metamaterial emitter was realized and studied, which is capable of enhancing the radiative properties of an embedded emitter. Parametric studies enhanced the understanding of the effects of different geometric parameters on the radiation performance of the structure. Finally, the metamaterial emitter and receiver were combined to form a metamaterial-based sonar system. For the first time, the superior performance of the metamaterial enhanced sonar system over conventional sonar systems was analytically and experimentally demonstrated. As a proof of concept, a robotic sonar platform equipped with the metamaterial system was shown to possess remarkably better tracking performance compared to the conventional system. Through this dissertation work, an enhanced understanding of high-k acoustic metamaterials has been achieved, and their applications in acoustic sensing, emission enhancement, and sonar systems have been demonstrated.
ACTIVE CONTROL OF NON-RECIPROCAL ACOUSTIC METAMATERIAL WITH A DYNAMIC CONTROLLER
(2019) Raval, Suraj; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Reciprocity is one of the fundamental properties in the field of wave propagation. In acoustics, this property helps in various practical applications. But, breaking this reciprocity also has useful applications. As a result of which, many researchers have tried to break the reciprocity in acoustics, which is comparatively difficult, unlike in fields such as electro-magnetics. Majority of these proposed methods to break the reciprocity are hard-wired systems, which work for a very limited frequency range. Thus, we have introduced a non-reciprocal metamaterial having boundary control with the help of piezoelectric sensors and actuators. A theoretical model is introduced to induce the nonreciprocal behavior, and it is backed up by providing experimental evidence. Our setup consists of a cylindrical cell made up of acrylic, filled with water, having four piezo sensors/actuators, two on each end. The idea is to excite the piezo cell through an actuator on one side, collect the resulting signal from the piezo sensor on the other side, and perform appropriate mathematical operations on this signal to produce a feedback/control signal via a specially designed dynamic control action. This control signal affects the propagation of pressure waves through the water medium inside the cell as it introduces a virtual gyroscopic effect of controlled magnitude and direction. Thus, this is how non-reciprocity is introduced and controlled into the metamaterial cell. The obtained theoretical and experimental results demonstrate the effectiveness of the dynamic controller in breaking the acoustic reciprocity. Extension of this work to multi-cell metamaterial configuration is natural extension to be pursued.
NONLINEARITY BASED ACOUSTIC WAVE REDIRECTION
(2019) Telly, Saliou Binet; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Advances in metamaterials technology have revealed novel opportunities for achieving peculiar material properties amenable to the control of wave propagation paths for various applications not realizable with conventional materials. Some prominent examples are schemes for electromagnetic and acoustic cloaking and focusing devices. In the classical approach to the formulations of these devices, one exploits a change of physical coordinates to achieve a desired wave behavior within a finite space. Such a change can be interpreted as a transformation of material properties when the field equations of interest are invariant to coordinate trans- formations. To date, this intuitive approach has led to the formulation of various two-dimensional devices for wave redirection, including infinite circular and square cloaks for both electromagnetic and acoustic fields. For acoustic fields, however, the transformation approach is constrained to fluid-like metamaterials amenable to the propagation of longitudinal waves only. Complications arise with solid materials because of their inherent ability to sustain the propagation of both longitudinal and transverse waves, which refract differently in linear materials due to dissimilar propagation speeds. In this dissertation, the author first seeks a sequence of transformations that may be used for cloaking two-dimensional airfoil sections through the classical transformation method. For the sought sequence, the author takes advantage of the mapping properties of the Joukowsky Transformation, and the closely related Kaman-Trefftz Transformation, which are two well-known complex mappings that have classically been used in the study of the aerodynamics of airfoil sections. Next, the author explores wave redirection mechanisms that may take advantage of nonlinear wave phenomena in elastic solid materials for acoustic wave redirection. Starting from the classical nonlinear Murnaghan model, a hyper-elastic material is formulated to realize couplings between shear and compressional modes that could lead to a more suitable refractive behavior for acoustic wave redirection in a solid metamaterial. The formulated model is studied by using perturbation and approximation techniques. The findings of this dissertation can be useful for redirecting and manipulating acoustic waves for practical applications.
THE DYNAMIC BEHAVIOR OF POLYUREA COMPOSITES SUBJECTED TO HIGH STRAIN RATE LOADING
(2018) ALI, MOHAMMAD MAQSUD; Baz, Amr M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A comprehensive theoretical and experimental study of the characterization of Polyurea composites subjected to high strain impact loading are conducted. The composites under consideration consist of multi-layers of polyurea/aluminum arranged in one dimension configuration. Finite element models (FEM) are developed by describing the dynamics of the viscoelastic behavior of the polyurea using the Golla-Hughes-Mctavish (GHM) mini-oscillator approach. The model enables the predictions of the structural stress, strain, strain rate, relaxation modulus, loss factor of the polyurea composites for different layering arrangements. The predictions of the developed FEM are validated against the predictions of the commercial finite element package ANSYS. Also, the FEM predictions are validated experimentally using the Split Hopkinson Pressure Bar (SHPB) which is used to monitor the dynamics of the polyurea composites at different levels of strain rates. Close agreements are demonstrated between the theoretical predictions and the obtained experimental results. The properties of periodic structures are used to develop an analytical model to create attenuation band gaps in dynamical response of periodically placed polyurea composites. The influence of various design parameters that controls the width of pass and stop-bands including multi layered periodic structures and different material configurations is compared. The presented theoretical and experimental approaches are envisioned to provide invaluable tools for the design of polyurea composites that can be used in impact mitigation and protection of critical structures subjected to high impact and blast loading. Keywords: Polyurea composites, dynamics under high strain loading, finite element modeling, Golla-Huges-Mctavish (GHM) mini-oscillators, Split Hopkinson Pressure Bar (SHPB), periodic structures, Bloch wave propagation theory, band gaps.
Framework for Small Fatigue Crack Propagation and Detection Joint Modeling Using Gaussian Process Regression
(2017) Smith, Reuel Calvin; Modarres, Mohammad; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Engineers have witnessed much advancement in the study of fatigue crack detection and propagation (CPD) modeling. More recently the use of certain damage precursors such as acoustic emission (AE) signals to assess the integrity of structures has been proposed for application to prognosis and health management of structures. However, due to uncertainties associated with small crack detection of damage precursors as well as crack size measurement errors of the detection technology used, applications of prognosis and health management assessments have been limited. This dissertation defines a new methodology for the assessment of CPD parameters and the minimization of uncertainties including detection and sizing errors associated with a series of known CPD models that use AE as the precursor to fatigue cracking. The first step of the procedure is defining the separate crack propagation and crack detection models that are to be used for the testing of a joint-CPD model. The two propagation models for this study are based on a Gaussian process regression model that correlates crack shaping factors (CSFs) to the propagation of the crack. One of these propagation models includes a particle filtering technique that includes several AE data. The testing of this joint-CPD model is facilitated by the Bayesian inference of the CPD likelihood where the posterior models are extracted and tested for correctness. The CSFs, the CPD data, and the AE signal data used for testing of this methodology come from a series of fatigue tests done on dog-bone Al 7075-T6 specimens. The data is first corrected for measurement error that is present based on the initial crack measurements. Then the data is used to generate the prior CPD models that is needed for the Bayesian inference procedure. With the resulting posterior CPD models, a correlation procedure that estimates the CPD model parameters of validation specimens based on the relationship that exists between the CSFs and the CPD model parameters is performed as well as a model error correction procedure. The result of this correlation provides reasonable estimates for the remaining useful life of a given validation specimen.
PRACTICAL ASPECTS OF ACOUSTIC SOURCE LOCALIZATION USING POLYMER-CARBON BLACK COMPOSITE PHASED ARRAYS
(2015) Cullen, Robert; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
An array of Carbon Black (CB) filled polyurethane sensors is developed to identify the location and intensity of a single acoustic source. The manufacturing procedure of the proposed array is outlined, in details, and several prototypes are manufactured. Characterization of the viscoelastic and piezo-resistive material properties of different samples of the CB filled polyurethane is carried out using Dynamic, Mechanical, Thermal Analyzer (DMTA) and uni-axial compression testing. The structural, dynamical, and sensing performance characteristics of the array sensor are modeled mathematically using the theory of finite elements and utilizing the well-known localization theories of phased arrays. The experimental performance characteristics of the proposed array sensor are evaluated in comparison to an array of conventional condenser microphones. The purpose of such an experimental effort is to demonstrate the capabilities and limitations of the proposed array sensor as compared with conventional condenser microphones. Furthermore, the obtained experimental results are utilized to validate the theoretical predictions of the localization of acoustic sources. It is envisioned that the proposed CB filled polyurethane array sensor presents a cost effective and viable means for identifying the location and intensity of acoustic sources which can vary from stationary to moving sources in air or underwater. Accordingly, the applications of such an array sensor are only limited by our imagination.

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