Professor Dr. Johan Larsson, Chair Professor Dr. Amir Riaz Professor James Duncan Professor Kenneth Kiger Professor Dr. James D. Baeder,Representative
Title: A multi-fidelity approach to sensitivity estimation in large eddy simulation.
Abstract:
An approach to compute approximate sensitivities in a large eddy simulation (LES) is proposed and assessed. The multi-fidelity sensitivity analysis (MFSA) solves a linearized mean equation, where the mean equation is based on the LES solution. This requires closure modeling which makes the computed sensitivities approximate. The closure modeling is based on inferring the eddy viscosity from the LES data and predicting the change in turbulence (or the perturbed eddy viscosity) using a simple algebraic model. The method is assessed for the flow over a NACA0012 airfoil at a fixed angle of attack, with the Reynolds number as the varying parameter and the lift, drag, skin friction, and pressure coefficients as the quantities-of-interest. The results show the importance of accurate closure modeling, specifically that treating the eddy viscosity as “frozen” is insufficiently accurate. Also, predictions obtained using the algebraic model for closing the perturbed eddy viscosity are closer to the true sensitivity than results obtained using the fully RANS-based method which is the state-of-the-art and most common method used in industry. The proposed method aims to complement, rather than replace, the current state-of-the-art method in situations in which sensitivities with higher fidelity are required.
Professor Miao Yu, Chair/Advisor Professor Amir Baz Professor Balakumar Balachandran Professor Nikhil Chopra Professor Timothy Horiuchi, Dean’s Representative
Title: PASSIVE AND ACTIVE GRADED-INDEX ACOUSTIC METAMATERIALS: SPATIAL AND FREQUENCY DOMAIN MULTIPLEXING
Abstract:
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.
Professor Nikhil Chopra, Chair/Advisor Professor Miao Yu Professor Yancy Diaz-Mercado Professor Shapour Azarm Professor Dinesh Manocha, Dean’s Representative Dr. Brian Sadler
Title: On the Control of Robotic Parasitic Antenna Arrays
Abstract:
Wireless networking is challenging in safety, security, and rescue contexts where network infrastructure may be damaged or compromised. Radio communication between ground robots at the lower end of the Very High Frequency (low-VHF) band is generally more reliable in complex indoor and urban environments when compared to communication systems such as Wi-Fi and cellular which operate at Ultra High Frequencies (UHF) and higher frequencies. Exciting antenna design research in the last 5 to 10 years has approached what is theoretically possible to create compact, moderately high bandwidth antennas at low-VHF. At the beginning of this dissertation research, we discovered that we could distribute these low-VHF antennas across closely positioned ground robots to create a robotic parasitic antenna array. When these robots are optimally positioned, they create a directional signal through the mutual coupling of their antennas. Consequently, these low-VHF arrays have the potential to extend the communication range of a reliable signal in urban and indoor environments with a proportionally small amount of robotic motion.
In this dissertation, we research the control of robotic platforms constituting these arrays from two perspectives. First, we research how robots control their positions to optimize or maintain the gain of a single robotic parasitic array to improve the quality of a communication link. Then, we investigate where these robots should collect to form an array in a network of robotic parasitic arrays to increase a metric of overall network connectivity.
To improve individual network communication links, we consider a two-element parasitic array formed by a static antenna and a ground robot and propose a technique by which this array can optimize its gain in a direction of interest. First, we propose and test an optimization approach for actuating spacing between the two antennas and passive antenna length to increase gain. Next, we propose and test an approach for using robotic motion to rotate the antenna array. In these experiments, we show that the robotic parasitic antenna array can provide a gain of 2 dB which is close to twice the effective transmission power in line-of-sight and non-line-of-sight conditions.
From a network perspective, we research where robots should form arrays to maintain a metric of overall connectivity. However, existing control formulations for maintaining connectivity are not general enough to support this new capability. We first propose a generalized model that we integrate into a Fiedler value maximization approach for maintaining communication. Afterward, we develop approaches for allocating a finite number of robots for forming these robotic parasitic arrays while ensuring that our metric for overall network connectivity between robotic parasitic array forming robots remains lower bounded.
Dr. Keir C. Neuman Professor Peter Chung Professor Paul Paukstelis Professor Don DeVoe Professor Jason Kahn, Dean’s Representative
Title of Dissertation: EFFECT OF MISMATCHED BASE PAIRS ON DNA PLECTONEMES
Abstract: Base pair mismatches in DNA occur during replication and can result in mutations and certain types of cancer. The exact mechanism by which mismatch repair proteins recognize mismatches is still not well understood. Structures of mismatch recognition proteins bound to a mismatch indicate that the process involves introducing a sharp bend in the DNA and flipping out the mismatched base. Under external torsional stress, an elastic rod with a defect would buckle at the defect, provided the defect reduces the local bending stiffness. In vivo, if the same energetic scenario prevails, it could localize (or pin) the mismatch at the plectoneme end loop (plectoneme refers to a structure formed by the DNA when it buckles and its helical axis wraps or writhes around itself in the presence of a critical torsional stress) and make the mismatched base pair more accessible to the mismatch repair protein. In genomic DNA, however, the entropic cost associated with plectoneme localization could make pinning unfavorable. Magnetic-tweezers-based studies of DNA supercoiling, performed at high salt concentrations, have shown that in DNA harboring a single mismatch, the plectoneme will always localize at the mismatch. Theoretical studies have predicted that under physiological salt concentrations, plectoneme localization becomes probabilistic. However, both experimental and theoretical approaches are currently limited to positively supercoiled DNA. In the current dissertation, we aim to study plectoneme localization, in physiologically relevant conditions, using state-of-the-art molecular dynamics (MD) simulations and single molecule magnetics tweezers-based experiments.
In order to simulate plectoneme localization we first develop a framework using the widely available sequence and salt dependent OxDNA2 model. We verify that the OxDNA2 model can quantitively reproduce a reduction in bending rigidity due to the presence of the mismatch(es), similar to all-atom MD simulations. We then verify that the current framework can reproduce the experimentally observed plectoneme pinning (at the location of the mismatches). Next, we simulate plectoneme pinning under physiologically relevant conditions. We find that the plectoneme pinning (at the location of the mismatches) becomes probabilistic and this probability of plectoneme pinning increases with an increase in the number of mismatches. We also simulate a longer 1010 base pair long DNA to study the influence of entropic effects on plectoneme pinning.
Next, we extend the simulation framework to simulate a negatively supercoiled, i.e., under-wound, DNA molecule. In vivo, DNA is maintained in a negatively supercoiled state. Negative supercoiling can result in local melting at the mismatched base pairs: this local melting would further reduce the local bending rigidity at the mismatched base pairs and could enhance plectoneme pinning. We find that negative supercoiling significantly enhances plectoneme pinning in comparison with equivalent levels of positive supercoiling. We also find that the mismatched base pairs are locally melted and the plectoneme end loop is bent significantly more as compared to the positive supercoiling case. Additionally, we simulate the 1010 base pair long DNA under two different negative super-helical densities, i.e., two different degrees of unwinding. We find that the super helical density does not affect the plectoneme pinning probabilities. We also conduct simulations of DNA under different stretching forces (0.3 pN, 0.4 pN and 0.6 pN). Negatively supercoiled DNA under relatively high stretching force (~0.6 pN) absorbs tortional stress by locally melting instead of supercoiling. Simulations of DNA under different forces allow us to study the effect of mismatches on the competition between supercoiling and local melting in a negatively supercoiled DNA. We find that higher stretching forces, up to a maximum set by the onset of melting, increase plectoneme pinning at the location of mismatch.
Finally, we propose and develop a single molecule assay to validate the simulations results presented in the previous chapters. Previous single-molecule magnetic tweezers measurements of mismatch DNA buckling and pinning were limited to the high force (~2 pN) – high salt (>0.5 M NaCl) regime. We propose to overcome this limitation by attaching a small gold nano-bead via a di-thiol group close to the mismatched base pairs, which permits direct observation of transient DNA buckling at the mismatch. We fabricate a DNA substrate that can be used to directly observe plectoneme pinning at the mismatch. We perform single-molecule magnetic tweezers measurements to verify that the presence of the di-thiol group does not result in anomalous pinning in an intact DNA molecule.
Come stop by for a SAMPE BBQ on April 6 at 5:30 and learn about the composite bridge competition and the additive manufacturing competitions! More details in the attached flyer. If you have any questions, please contact Colleen at cmmurray@umd.edu.
Clemson University is looking to fill multiple faculty positions in Mechanical Engineering. They are looking for faculty at all ranks and candidates with exceptional credentials could be considered for endowed faculty positions as well.
The University of Louisiana at Lafayette, Mechanical Engineering Department invites applications and nominations for the 9-month tenure track position of Assistant Professor (Mechanical Engineering, Cluster Hire in Solar Energy). A Tenure track faculty member for this position is expected to be an excellent teacher in the area of Thermo-Fluid systems, and to develop strong funded research programs in the broad area of solar energy. All faculty members participate in departmental, service, and undergraduate student advising activities.
On the basis of an application to the Foundation in the form of a research proposal, six awards will be made to doctoral candidates enrolled in academic institutions located in the United States and Canada. Each award will consist of a grant of $34,000. There are no citizenship restrictions.
Basis for Award:
An independent panel of experts in the fields of ocean engineering and ocean instrumentation will review the applications. The main evaluation criteria include the degree of innovation, technical merit and relevance to ocean engineering/instrumentation of the proposed research. Additionally, each candidate should demonstrate intellectual ability and achievement, evidence of creativity and initiative and the potential for a career that will impact ocean engineering and ocean instrumentation
Title: Dynamic Bayesian Network Updating Approaches for Enabling Causal Prognostics and Health Management of Complex Engineering Systems
Committee Members:
Associate Professor Katrina Groth, Chair
Assistant Professor Michelle Bensi
Professor Jeffrey Herrmann
Professor Mohammed Modarres
Professor Gregory Baecher, Dean’s Representative
Abstract:
Complex engineering systems (CESes), such as nuclear power plants or manufacturing plants, are critical to a wide range of industries and utilities; as such, it is important to be able to monitor their system health and make informed decisions on maintenance and risk management practices. However, currently available system-level monitoring approaches either ignore complex dependencies in their probabilistic risk assessments (PRA) or are prognostics and health management (PHM) techniques intended for simpler systems. The gap in CES health management needs to be closed through the development of techniques and models built from a systematic integration of PHM and PRA (SIPPRA) approach that considers a system’s causal factors and operational context when generating health assessments.
The following dissertation describes a concentrated study that addresses one of the challenges facing SIPPRA: how to appropriately discretize a CES’s operational timeline derived from multiple data streams to create discrete time-series data for use as model inputs over meaningful time periods. This research studies how different time scales and discretization approaches impact the performance of dynamic Bayesian Networks (DBNs), models that are increasingly used for causal-based inferences and system-level assessments, specifically built for SIPPRA health management. The impact of this research offers new insight into how to construct such DBNs to better support system-level health management for CESes.
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