A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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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.
Efficient learning-based sound propagation for virtual and real-world audio processing applications
(2024) Ratnarajah, Anton Jeran; Manocha, Dinesh; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Sound propagation is the process by which sound energy travels through a medium, such as air, to the surrounding environment as sound waves. The room impulse response (RIR) describes this process and is influenced by the positions of the source and listener, the room's geometry, and its materials. Physics-based acoustic simulators have been used for decades to compute accurate RIRs for specific acoustic environments. However, we have encountered limitations with existing acoustic simulators. For example, they require a 3D representation and detailed material knowledge of the environment. To address these limitations, we propose three novel solutions. First, we introduce a learning-based RIR generator that is two orders of magnitude faster than an interactive ray-tracing simulator. Our approach can be trained to input both statistical and traditional parameters directly, and it can generate both monaural and binaural RIRs for both reconstructed and synthetic 3D scenes. Our generated RIRs outperform interactive ray-tracing simulators in speech-processing applications, including Automatic Speech Recognition (ASR), Speech Enhancement, and Speech Separation, by 2.5%, 12%, and 48%, respectively. Secondly, we propose estimating RIRs from reverberant speech signals and visual cues in the absence of a 3D representation of the environment. By estimating RIRs from reverberant speech, we can augment training data to match test data, improving the word error rate of the ASR system. Our estimated RIRs achieve a 6.9% improvement over previous learning-based RIR estimators in real-world far-field ASR tasks. We demonstrate that our audio-visual RIR estimator aids tasks like visual acoustic matching, novel-view acoustic synthesis, and voice dubbing, validated through perceptual evaluation. Finally, we introduce IR-GAN to augment accurate RIRs using real RIRs. IR-GAN parametrically controls acoustic parameters learned from real RIRs to generate new RIRs that imitate different acoustic environments, outperforming Ray-tracing simulators on the Kaldi far-field ASR benchmark by 8.95%.
Learning Autonomous Underwater Navigation with Bearing-Only Data
(2024) Robertson, James; Duraiswami, Ramani; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Recent applications of deep reinforcement learning in controlling maritime autonomoussurface vessels have shown promise for integration into maritime transportation. These could have the potential to reduce at-sea incidents such as collisions and groundings which are majorly attributed to human error. With this in mind the goal of this work is to evaluate how well a similar deep reinforcement learning agent could perform the same task in submarines but using passive SONAR rather than the ranging data provided by active RADAR aboard surface vessels. A simulated submarine outfitted with a passive spherical, hull-mounted SONAR sensor is placed into contact scenarios under the control of a reinforcement learning agent and directed to make its way to a navigational waypoint while avoiding interfering surface vessels. In order to see how this best translates to lower power autonomous vessels (vice warship submarines), no estimation for the range of the surface vessels is maintained in order to cut down on computing requirements. Inspired by my time aboard U.S. Navy submarines, the agent is provided with simply the simulated passive SONAR data. I show that this agent is capable of navigating to a waypoint while avoiding crossing, overtaking, and head-on surface vessels and thus could provide a recommended course to a submarine contact management team in ample time since the maneuvers made by the agent are not instantaneous in contrast to the assumptions of traditional target tracking with bearing-only data. Additionally, an in-progress plugin for Epic Games’ Unreal Engine is presented with the ability to simulate underwater acoustics inside the 3D development software. Unreal Engine is a powerful 3D game engine that is incredibly flexible and capable of being integrated into many different forms of scientific research. This plugin could provide researchers with the ability to conduct useful simulations in intuitively designed 3D environments.
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.
Aeroacoustic Implications of Installed Propeller Interactional Aerodynamics and Transient Propeller Motions
(2023) Jayasundara, Dilhara; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The emergence of advanced air mobility and sustainable aviation concepts have revived the interest in propeller-driven aircraft. A number of electric vertical take-off and landing (eVTOL) aircraft have been developed to cater to the demands of urban air mobility (UAM) and significant advancements have been made in unmanned aerial vehicles (UAV) equipped with vertical take-off and landing capabilities. However, the community acceptance of these new aircraft configurations highly depends on having a low noise footprint as they will operate in dense urban environments. Propeller noise is considered the major source of noise in these aircraft with the introduction of electric propulsion and it can significantly increase with the effects of installation and transient propeller motions. This study aims to comprehend the complex aerodynamic interactions within such aircraft that result from propeller installation and contribute to the generation of high noise levels. To understand the physics of propeller installation, a wingtip-mounted propeller was analyzed at several angles of attack using computational fluid dynamics (CFD) based on Reynolds-averaged Navier-Stokes (RANS) equations and computational aeroacoustics based on the Ffowcs Williams - Hawkings equation. The aeroacoustic implications of the propeller axis inclination and the propeller-wing aerodynamic interaction were studied in-depth. The propeller-wing interaction leads to a significant increase in propeller noise (~20 to 30 dB increase along the rotational axis) and causes the wing to generate a loading noise in the same order of magnitude as the propeller noise. To extrapolate the understanding of installation effects to a full aircraft, the aeroacoustic characteristics of a quadrotor biplane tailsitter were analyzed in both hover and forward flight focusing on the rotor-rotor and rotor-airframe aerodynamic interaction. The rotor-rotor interaction was found to be a significant source of loading noise in hover but having the fuselage as a physical barrier between the rotors largely reduces its effect. The airframe loading noise and rotor broadband noise are equally dominant as the rotor tonal noise when the aircraft is in forward flight. Moreover, the study evaluated the effectiveness of rotor synchrophasing in reducing the aircraft noise footprint and it showed promising results in hover, causing a reduction of aircraft noise by more than 10 dB. Furthermore, an efficient computational aeroacoustics framework was developed to facilitate the computations, ensuring optimal utilization of the computational resources. The CPU and GPU parallelization and other optimization techniques were able to achieve a 98% reduction in computation time for an isolated propeller case. This enabled the rapid aeroacoustic computations of periodic and non-periodic problems. This was used to analyze the aeroacoustics of an isolated propeller undergoing a transition from hover to forward flight. The aerodynamic and acoustic results of the unsteady case were compared with quasi-steady cases performed at intermediate tilt angles. The quasi-steady CFD simulations predicted the unsteady transition aeroacoustics with reasonable accuracy. A tilting quasi-steady approach was proposed to better capture the aerodynamics and acoustics of the unsteady transition.
SURROGATE MODELING AND CHARACTERIZATION OF BLADE-WAKE INTERACTION NOISE FOR HOVERING SUAS ROTORS USING ARTIFICIAL NEURAL NETWORKS
(2022) Thurman, Christopher; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This work illustrates the use of artificial neural network modeling to study and characterize broadband blade-wake interaction noise from hovering sUAS rotors subject to varying airfoil geometries, rotor geometries, and operating conditions. Design of Experiments was used to create input feature spaces over 9 input features: the number of rotor blades, rotor size, rotor speed, the amount of blade twist, blade taper ratio, tip chord length, collective pitch, airfoil camber, and airfoil thickness. A high-fidelity strategy was then implemented at the discrete data points defined by the designed input feature spaces to design airfoils and rotor blades, predict the unsteady rotor aerodynamics and aeroacoustics, and isolate the blade-wake interaction noise from the acoustic broadband noise, which was then used for prediction model training and validation. An artificial neural network tool was developed and implemented into NASA's ANOPP2 code and was used to identify an optimal prediction model for the nonlinear functional relationship between the 9 input features and blade-wake interaction noise. This optimal artificial neural network was then validated over test data, and exhibited prediction accuracy over 91% for data previously unseen by the model. First- and second-order sensitivity analyses were then conducted using the developed artificial neural network tool and it was seen that input features which serve to directly modify the thrust coefficient, such as airfoil camber and collective pitch, had a dominant effect over blade-wake interaction noise, followed by second-order interaction effects related to the mean rotor solidity. The optimal prediction model along with aerodynamic simulations were used to further study the effect of varying input features on blade-wake interaction noise and three types of blade-wake interaction noise were identified. Blade-wake interaction noise caused by impingement of the turbulence entrained in a tip vortex on the leading edge of a subsequent rotor blade showed to be the most prominent type of blade-wake interaction noise, exhibiting an acoustic contribution upward of 7 dB. Blade-wake interaction noise caused by a direct impingement of a tip vortex on the leading edge of a subsequent rotor blade had the second largest acoustic significance, exhibiting roughly 6 dB of broadband noise. The third, and least significant type of blade-wake interaction noise was shown to be caused by impingement of a blade-wake sheet on the mid-span of a subsequent rotor. This last type of blade-wake interaction noise was seen to only occur in the turbulent-wake operating state and possibly mild vertical descent conditions, and had approximately a 2.5 dB acoustic contribution.
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.
Aeroacoustic Analysis of Asymmetric Lift-Offset Helicopter in Forward Flight
(2021) Arias, Paulo; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In recent years, the University of Maryland has worked on an asymmetric lift-offset compound helicopter. The configuration consists of a single main rotor helicopter with the addition of two key ways to increase the forward speed: a stubbed wing on the retreating fuselage side, and a slowed down rotor. Experiments and simulations have shown that the novel concept provides improved thrust potential and lift-to-drag ratios in high-speed forward flight. This study aims to determine whether the asymmetric lift-offset configuration also provides aeroacoustic benefits in forward flight in addition to its aerodynamic advantages. The aerodynamic results from previous computational and experimental studies are recreated using the Mercury framework, in-house Computational Fluid Dynamics solver based on Reynolds-Averaged Navier-Stokes (RANS) coupled to a comprehensive rotor analysis for structural deformations and trim. The acoustic analysis is performed using an acoustic code based on the Ffowcs William-Hawkings equation to solve for the tonal noise propagating from the surfaces of the aircraft. The BPM model is used for broadband noise prediction. It was found that for an advance ratio of 0.5 the wing-lift offset configuration can produce 56.8% more thrust at the same collective angle without any penalties in total noise. When the configurations produce equal thrust it was found that the wing-lift offset case has a 4 dB reduction in maximum overall sound pressure level. At an advance ratio of 0.3 with trim for equivalentthrust between configurations, a 3 dB maximum OASPL reduction was obtained with the inclusion of the wing. The rotor of the wing-lift offset case was also slowed down while maintaining equal thrust to find a 6 dB reduction at an advance ratio of 0.55. Blade flap and lag bending moments near the root were also significantly reduced for the wing-lift offset configuration with equal thrust.
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.

A. James Clark School of Engineering

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