Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Mechanical Engineering.

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3D Computational Model for an Electrochemical Gas Separation and Inerting System
(Journal of The Electrochemical Society, 2022-04-25) Aryal, Utsav Raj; Aziz, Majid; Prasad, Ajay K.
Aircraft fuel tank inerting is employed to reduce the flammability of the fuel vapor in the ullage (air volume above the fuel) by restricting its oxygen concentration to a safe value—12% for commercial aircraft and 9% for military aircraft. Inerting is typically accomplished by displacing oxygen in the ullage with an inert gas like nitrogen. Electrochemical gas separation and inerting system (EGSIS) is an on-board method to generate and supply nitrogen-enriched air (NEA) to the fuel tank. EGSIS combines a polymer electrolyte membrane (PEM) electrolyzer anode which dissociates water to evolve oxygen, and a PEM fuel cell cathode which reduces oxygen from atmospheric air to produce NEA at its outlet. This paper represents the first attempt to model and simulate EGSIS using a three-dimensional, steady state, isothermal model. Various EGSIS performance indicators such as current density, reactant concentration distribution, and polarization curves are studied as a function of operating conditions and design parameters. The results from the computational model are validated against our previous experimental results for various operating conditions. The simulation results reveal the effects of temperature, reactant flowrates, and material property optimization on EGSIS performance. Different operating strategies are explored with the goal of improving system performance.
A traveler-centric mobility game: Efficiency and stability under rationality and prospect theory
(PLOS ONE, 2023-05-05) Chremos, Ioannis Vasileios; Malikopoulos, Andreas A.
In this paper, we study a routing and travel-mode choice problem for mobility systems with a multimodal transportation network as a “mobility game” with coupled action sets. We formulate an atomic routing game to focus on the travelers’ preferences and study the impact on the efficiency of the travelers’ behavioral decision-making under rationality and prospect theory. To control the innate inefficiencies, we introduce a mobility “pricing mechanism,” in which we model traffic congestion using linear cost functions while also considering the waiting times at different transport hubs. We show that the travelers’ selfish actions lead to a pure-strategy Nash equilibrium. We then perform a Price of Anarchy and Price of Stability analysis to establish that the mobility system’s inefficiencies remain relatively low and the social welfare at a NE remains close to the social optimum as the number of travelers increases. We deviate from the standard game-theoretic analysis of decision-making by extending our mobility game to capture the subjective behavior of travelers using prospect theory. Finally, we provide a detailed discussion of implementing our proposed mobility game.
Analytical Representation and Efficient Computation of the Effective Conductivity of Two-Phase Composite Materials
(International Journal for Numerical Methods in Engineering, 2022-04-04) Roy, R. Valéry
Many engineered materials display ordered or disordered microstructures. Such materials exhibit transport properties which are unmatched by their single-phase homogeneous counterparts. These properties are obtained by the mixture of two or more phases typically characterized by a large contrast in their properties. For the development of these materials, it is critical to develop a robust computational framework in order to provide a fundamental understanding of how microstructure affects performance. This hinges on predicting their macroscopic properties, given the constitutive laws and spatial distribution of their constituents. To this end, this work presents a computational framework based on formulating periodic conduction transport problems in terms of boundary integral equations whose kernel is expressed in terms of Weierstrass zeta-function. The components of the effective conductivity tensor are then sought in the form of power series expansions of a conductivity contrast parameter. To accelerate their convergence, these expansions are transformed into Padé approximants. Presently restricted to the case of two-dimensional, two-phase microstructures, this framework is shown to yield accurate results over the entire range of the contrast parameter. Representation of the kernel as a lattice sum allows the use the fast multipole method, thereby making computations significantly more efficient.
Assembling metal-polyphenol coordination interfaces for longstanding zinc metal anodes
(EcoMat, 2022-01-11) Huyan, Yu; Wang, Jian-Gan; Tian, Shan; Ren, Lingbo; Liu, Huanyan; Wei, Bingqing
Zn metals have gained the immense attention of researchers for their wide employment as the anode of high-performance aqueous batteries. Nonetheless, the Zn anodes suffer from uncontrollable dendrite growth and parasitic side reactions, which substantially shorten the battery lifespan. This study proposes an interfacial assembly of a metal-polyphenol coordination coating on Zn anodes to regulate Zn2+ deposition behavior. Bismush-coordinated polyphenolic ligands (i.e., tannic acid, TA) create a functional interface that could promote Zn's uniform nucleation and plating/striping kinetics. Moreover, the artificial coating acts as a physical barrier to inhibit surface corrosion. As a consequence, the TA-Bi-modified Zn anodes display a small voltage hysteresis of ~38 mV at 1 mA cm−2 over 2600 h and an ultra-long lifespan for 3100 h (~4.3 months) even at a high-current density of 10 mA cm−2. When assembled with a vanadium-based cathode, the full Zn-ion batteries achieve improved electrochemical performance.
Bladder expandable robotic system and UV materials for rapid internal pipeline repair
(SAMPE Conference Proceedings 2023, 2023-04-18) Tierney, John J.; Vanarelli, Alex; Fuessel, Lukas; Abu-Obaid, Ahmad; Sauerbrunn, Steve; Das, Shagata; Deitzel, Joseph; Tatar, Jovan; Heider, Dirk; Shenton, Harry W. III; Kloxin, Christopher J.; Sung, Dae Han; Thostenson, Erik; Gillespie, John W. Jr.
This paper describes a novel composite placement process to fabricate stand-alone structural pipe within existing legacy pipelines—with no disruption in gas service. The process utilizes low-cost, UV-curable, glass fiber reinforced plastics (GFRP) for discrete preforms made from continuous fiber fabrics. These sections are designed to meet 50-year service life by addressing the unique loading conditions of the pipe repair allowing for the design customization of the preforms to accommodate the state of pipe corrosion, access points or other local features that may vary along the length of the pipe. The approach offers maximum design flexibility and customization while minimizing installation time and cost. The preforms are fabricated above ground using rapid automated manufacturing methods for quality control. The preforms are transported by a tethering system to the robot. The robot is comprised of a self-propelled dual inflation expandable bladder system that places, consolidates, and cures standard or custom composite sections along the entire pipe length in a continuous co-cure process. This system is designed to adapt to pipe features that include lateral tees, service connections, joints, gaps, and irregular cross sections. In addition, variable thickness composite sections can be placed along the pipe where exposed to high external loads under railroads, highways, airports or where soil erosion and movement occurs. This paper presents the robot design, assessment of UV curable resins, embedded sensing methods, and fabrication of pipe sections with this system.
Blending poly(2-ethyl-2-oxazoline) with hydrophobic polymers as a hybrid adhesive with enhanced water-resistant properties
(Journal of Applied Polymer Science, 2021-07-16) Zhang, Yuanyuan; Li, Xuanhua; Guo, Shaohui; Wei, Bingqing
The use of poly(2-ethyl-2-oxazoline) (PEOX) in a wet environment is limited because of its high hydrophilicity. In this study, PEOX based blends were prepared via blending PEOX with hydrophobic polymers, such as poly(styrene-co-acrylonitrile) (SAN), poly(4-vinylphenol) (PVPh), and poly(vinylidene fluoride) (PVDF), in order to improve the water-resistance of PEOX. The blends' water resistance properties are evaluated by the contact angle, solubility, moisture absorption, and mechanical strength in a wet environment. The results show that the water resistance and the adhesion strength of PEOX in a wet environment are dramatically enhanced by polymer blending. The blend with 30 wt% PVPh demonstrates excellent performances in transparency and water-resistant abilities. It is found that the stable hydrogen bonding within the blend plays an important role in hydrophobic modification. The PVPh/PEOX blend can be applied as a new type of transparent coating or adhesive with enhanced water-resistant properties in a wet environment.
Boosting photocatalytic hydrogen production from water by photothermally induced biphase systems
(Nature Communications, 2021-02-26) Guo, Shaohui; Li, Xuanhua; Li, Ju; Wei, Bingqing
Solar-driven hydrogen production from water using particulate photocatalysts is considered the most economical and effective approach to produce hydrogen fuel with little environmental concern. However, the efficiency of hydrogen production from water in particulate photocatalysis systems is still low. Here, we propose an efficient biphase photocatalytic system composed of integrated photothermal–photocatalytic materials that use charred wood substrates to convert liquid water to water steam, simultaneously splitting hydrogen under light illumination without additional energy. The photothermal–photocatalytic system exhibits biphase interfaces of photothermally-generated steam/photocatalyst/hydrogen, which significantly reduce the interface barrier and drastically lower the transport resistance of the hydrogen gas by nearly two orders of magnitude. In this work, an impressive hydrogen production rate up to 220.74 μmol h−1 cm−2 in the particulate photocatalytic systems has been achieved based on the wood/CoO system, demonstrating that the photothermal–photocatalytic biphase system is cost-effective and greatly advantageous for practical applications.
Carbon Additive Manufacturing with a Near-Replica “Green-to-Brown” Transformation
(Advanced Materials, 2023-05-30) Zhang, Chunyan; Shi, Baohui; He, Jinlong; Zhou, Lyu; Park, Soyeon; Doshi, Sagar; Shang, Yuanyuan; Deng, Kaiyue; Giordano, Marc; Qi, Xiangjun; Cui, Shuang; Liu, Ling; Ni, Chaoying; Fu, Kun Kelvin
Nanocomposites containing nanoscale materials offer exciting opportunities to encode nanoscale features into macroscale dimensions, which produces unprecedented impact in material design and application. However, conventional methods cannot process nanocomposites with a high particle loading, as well as nanocomposites with the ability to be tailored at multiple scales. A composite architected mesoscale process strategy that brings particle loading nanoscale materials combined with multiscale features including nanoscale manipulation, mesoscale architecture, and macroscale formation to create spatially programmed nanocomposites with high particle loading and multiscale tailorability is reported. The process features a low-shrinking (<10%) “green-to-brown” transformation, making a near-geometric replica of the 3D design to produce a “brown” part with full nanomaterials to allow further matrix infill. This demonstration includes additively manufactured carbon nanocomposites containing carbon nanotubes (CNTs) and thermoset epoxy, leading to multiscale CNTs tailorability, performance improvement, and 3D complex geometry feasibility. The process can produce nanomaterial-assembled architectures with 3D geometry and multiscale features and can incorporate a wide range of matrix materials, such as polymers, metals, and ceramics, to fabricate nanocomposites for new device structures and applications.
Catalytic Boosting Bidirectional Polysulfide Redox using Co0.85Se/C Hollow Structure for High-Performance Lithium-Sulfur Batteries
(ChemElectroChem, 2022-02-17) Zhang, Xingyuan; Gu, Honghui; Shen, Chao; Wei, Bingqing; Wang, Jian-Gan
Achieving effective adsorption and fast conversion of soluble polysulfides confined in the sulfur cathode is critical yet challenging for building high-performance lithium-sulfur batteries. Herein, we construct a unique hollow-structured Co0.85Se/C as a separator modifier (CSPP) to effectively suppress the polysulfide shuttle effect. The Co0.85Se/C demonstrates strong anchoring with polysulfide species and smooth bidirectional electrocatalysis. The unique mesoporous hollow architecture affords sufficient catalytic sites and Li+ diffusion channels for promoting the reaction kinetics. Benefiting from the merits, the CSPP-cell could yield a superior electrochemical utilization of active sulfur, excellent rate capability (679 mAh g−1 at 5 C), and stable cycling performance with an ultralow fading rate of 0.056 % per cycle over 500 cycles. The work highlights great promise of developing cobalt-based materials as kinetic regulators for highly stable lithium-sulfur batteries.
Comparing steady and unsteady rectangular jets issuing into a crossflow
(Journal of Fluid Mechanics, 2022-05-31) Tricouros, Frank A.; Amitay, Michael; Van Buren, Tyler
The foundational differences of steady and unsteady jets issued into a laminar boundary layer crossflow are considered. Jets have been used widely for flow control applications, due to their ability to enhance mixing and mitigate separation, but it is unclear what role jet steadiness plays in flow control effectiveness. Here we compare experimentally unsteady (synthetic) and steady rectangular jets issued into a flat-plate laminar boundary layer with varying orifice pitch and skew. The coherent streamwise vortices produced by unsteady jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns. These differences are rooted in how vorticity is generated in the orifice, through either a Stokes layer (unsteady) or a Blasius boundary layer (steady). Exploring the time- and phase-averaged vorticity transport equation reveals that the time-varying vorticity term is the reason for the enhanced vortical structure. When considering flow control metrics, we find that the unsteady jet produced greater added momentum in the boundary layer and added vorticity when compared to a momentum-matched steady jet. Both the steady and unsteady jets produced similar jet penetration characteristics.
Comparison of Existing Methods for Characterizing Bi-Linear Natural Ankle Quasi-Stiffness
(Journal of Biomechanical Engineering, 2022-07-01) Nigro, Luke; Arch, Elisa S.
Natural ankle quasi-stiffness (NAS) is a mechanical property of the ankle joint during dynamic motion. NAS has been historically calculated as the average slope (linear regression) of the net ankle moment versus ankle angle during discrete phases of stance. However, recent work has shown that NAS is nonlinear during the stance phase. Specifically, during the loading phase of stance (∼10 to 60% of total stance), plantarflexion moment increases at an accelerating rate compared to dorsiflexion angle. Updated models have been developed to better capture this inherent nonlinearity. One type of model called bi-linear NAS (BL-NAS) divides the loading phase of stance into two subphases, called early loading (EL) and late loading (LL) NAS. Two papers, written by Crenna and Frigo (2011, “Dynamics of the Ankle Joint Analyzed Through Moment-Angle Loops During Human Walking: Gender and Age Effects,” Hum. Mov. Sci., 30(6), pp. 1185–1198) and Shamaei et al. (2013, “Estimation of Quasi-Stiffness and Propulsive Work of the Human Ankle in the Stance Phase of Walking,” PLoS One, 8(3), p. e59935), outline different BL-NAS models. Both models fit measured data better (lower root-mean-squared error (RMSE)) than standard single linear NAS (SL-NAS) models but have not been widely adopted, possibly because of methodological discrepancies and lack of applicability to physical devices at the time. This paper compares and contrasts these existing BL-NAS models and translates those findings to possible orthotic device designs. Results showed that both BL-NAS models had lower RMSE than SL-NAS, EL-NAS was not significantly different across walking speeds, and LL-NAS increased significantly at faster walking speeds. These improved models of NAS much better approximate natural human movement than commonly used SL-NAS models, and thus provide a basis to design ankle-foot devices with multiple stiffness properties to emulate and facilitate natural human motion.
Data-Driven Abstractions for Robots With Stochastic Dynamics
(IEEE Transactions on Robotics, 2021-11-22) Tanner, Herbert G.; Stager, Adam
This article describes the construction of stochastic, data-based discrete abstractions for uncertain random processes continuous in time and space. Motivated by the fact that modeling processes often introduce errors which interfere with the implementation of control strategies, here the abstraction process proceeds in reverse: the methodology does not abstract models; rather it models abstractions. Specifically, it first formalizes a template for a family of stochastic abstractions, and then fits the parameters of that template to match the dynamics of the underlying process and ground the abstraction. The article also shows how the parameter-fitting approach can be implemented based on a probabilistic model validation approach which draws from randomized algorithms, and results in a discrete abstract model which is approximately simulated by the actual process physics, at a desired confidence level. In this way, the models afford the implementation of symbolic control plans with probabilistic guarantees at a desired level of fidelity.
Decoupled Right Invariant Error States for Consistent Visual-Inertial Navigation
(IEEE Robotics and Automation Letters, 2022-01-04) Yang, Yulin; Chen, Chuchu; Lee, Woosik; Huang, Guoquan
The invariant extended Kalman filter (IEKF) is proven to preserve the observability property of visual-inertial navigation systems (VINS) and suitable for consistent estimator design. However, if features are maintained in the state vector, the propagation of IEKF will become more computationally expensive because these features are involved in the covariance propagation. To address this issue, we propose two novel algorithms which preserve the system consistency by leveraging the invariant state representation and ensure efficiency by decoupling features from covariance propagation. The first algorithm combines right invariant error states with first-estimates Jacobian (FEJ) technique, by decoupling the features from the Lie group representation and utilizing FEJ for consistent estimation. The second algorithm is designed specifically for sliding-window filter-based VINS as it associates the features to an active cloned pose, instead of the current IMU state, for Lie group representation. A new pseudo-anchor change algorithm is also proposed to maintain the features in the state vector longer than the window span. Both decoupled right- and left-invariant error based VINS methods are implemented for a complete comparison. Extensive Monte-Carlo simulations on three simulated trajectories and real world evaluations on the TUM-VI datasets are provided to verify our analysis and demonstrate that the proposed algorithms can achieve improved accuracy than a state-of-art filter-based VINS algorithm using FEJ.
Development and Testing of an Aerial Radiation Detection System
(IEEE Sensors Journal, 2021-12-15) Yadav, Indrajeet; Tanner, Herbert G.
This paper reports on the design and implementation of an airborne radiation detection system together with its associated signal processing and decision-making algorithms. This system is envisioned as the building block of an aerial radiation sensor network and it is specifically designed to detect weak radiological signatures in transit. The whole system is developed based on low-cost commercial off the shelf (COTS) components, and through a series of detailed experiments and Monte-Carlo tests, the paper shows how it can be deployed in time-critical application scenarios, where the time allocated to detect the source is limited. Performance metrics for the detection algorithms utilized in the system indicate that the reported technology can offer a significant improvement on the detection speeds compared to alternative techniques utilizing the same hardware resources.
Development and Validation of a Framework for Predictive Simulation of Treadmill Gait
(Journal of Biomechanical Engineering, 2022-07-14) Pariser, Kayla M.; Higginson, Jill S.
Treadmill training is a common intervention to promote healthy walking function for individuals with pathological gait. However, because of the heterogeneity of many patient populations, determining how an individual will respond to new treadmill protocols may require extensive trial and error, causing increased patient fatigue. The purpose of this study was to develop and validate a framework for predictive simulation of treadmill gait, which may be used in the design of treadmill training protocols. This was accomplished through three steps: predict motion of a simple model of a block relative to a treadmill, create a predictive framework to estimate gait with a two-dimensional (2D) lower limb musculoskeletal model on a treadmill, and validate the framework by comparing predicted kinematics, kinetics, and spatiotemporal parameters across three belts speeds and between speed-matched overground and treadmill predictive simulations. Predicted states and ground reaction forces for the block-treadmill model were consistent with rigid body dynamics, and lessons learned regarding ground contact model and treadmill motion definition were applied to the gait model. Treadmill simulations at 0.7, 1.2, and 1.8 m/s belt speeds resulted in predicted sagittal plane joint angles, ground reaction forces, step length, and step time that closely matched experimental data at similar speeds. Predicted speed-matched overground and treadmill simulations resulted in small root-mean-square error (RMSE) values within standard deviations for healthy gait. These results suggest that this predictive simulation framework is valid and can be used to estimate gait adaptations to various treadmill training protocols.
Effect of the Coastline Geometry on the Boundary Currents Intruding through the Gap
(Fluids, 2022-02-08) Kuehl, Joseph; Sheremet, Vitalii A.
The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. However, for more complex geometries, such as angled or offset gap configurations, the question of multiple states and hysteresis is unresolved. In such cases, the inertia of the western boundary current is oriented into the gap, hence the assumption that increased inertia promotes gap penetrating loop current states. Here we address the problem numerically in an idealized setting. It is found that despite the inertia of the current being directed into the gap, for large western boundary current transport values, leaping states will be present. That is, we show here that the presence of multiple states with hysteresis for gap-leaping western boundary current systems is robust to both angled and offset gap geometries.
Electrochemical gas separation and inerting system
(Journal of Power Sources, 2021-05-15) Aryal, Utsav Raj; Chouhan, Ashish; Darling, Robert; Yang, Zhiwei; Perry, Mike L.; Prasad, Ajay K.
Following the TWA 800 flight disaster in 1996 which was attributed to an explosion in the fuel tank, inerting of the ullage (air volume above the fuel in the tank) has gained prominence. Fuel tank inerting is the process of reducing the flammability of the ullage by supplying it with an inert gas like nitrogen. Current inerting techniques are expensive, consume large amounts of energy, and fail prematurely. Here, we propose a novel in-flight electrochemical gas separation and inerting system (EGSIS) to produce and supply nitrogen-enriched air (NEA). EGSIS combines a polymer electrolyte membrane (PEM) fuel cell cathode with a PEM electrolyzer anode to generate humidified NEA as the cathode output which can be dehumidified and supplied directly to the fuel tank. The required rate of NEA varies during a typical flight and a major advantage of EGSIS is that the rate of NEA generation can be conveniently controlled by varying the voltage applied to the system. Here, we report on the performance of a single-cell EGSIS apparatus and evaluate its suitability for aircraft fuel tank inerting.
Estimating ground reaction force with novel carbon nanotube-based textile insole pressure sensors
(Wearable Technologies, 2023-03-02) Burch, Kaleb; Doshi, Sagar; Chaudhari, Amit; Thostenson, Erik; Higginson, Jill
This study presents a new wearable insole pressure sensor (IPS), composed of fabric coated in a carbon nanotube-based composite thin film, and validates its use for quantifying ground reaction forces (GRFs) during human walking. Healthy young adults (n = 7) walked on a treadmill at three different speeds while data were recorded simultaneously from the IPS and a force plate (FP). The IPS was compared against the FP by evaluating differences between the two instruments under two different assessments: (1) comparing the two peak forces at weight acceptance and push-off (2PK) and (2) comparing the absolute maximum (MAX) of each gait cycle. Agreement between the two systems was evaluated using the Bland–Altman method. For the 2PK assessment, the group mean of differences (MoD) was −1.3 ± 4.3% body weight (BW) and the distance between the MoD and the limits of agreement (2S) was 25.4 ± 11.1% BW. For the MAX assessment, the average MoD across subjects was 1.9 ± 3.0% BW, and 2S was 15.8 ± 9.3% BW. The results of this study show that this sensor technology can be used to obtain accurate measurements of peak walking forces with a basic calibration and consequently open new opportunities to monitor GRF outside of the laboratory.
Extrusion-Based Additively Manufactured PAEK and PAEK/CF Polymer Composites Performance: Role of Process Parameters on Strength, Toughness and Deflection at Failure
(Journal of Composites Science, 2023-04-11) Sharafi, S.; Santare, M. H.; Gerdes, J.; Advani, S. G.
Poly aryl-ether-ketone (PAEK) belongs to a family of high-performance semicrystalline polymers exhibiting outstanding material properties at high temperatures, making them suitable candidates for metallic part replacement in different industries such as aviation, oil and gas, chemical, and biomedical. Fused filament fabrication is an additive manufacturing (AM) method that can be used to produce intricate PAEK and PAEK composite parts and to tailor their mechanical properties such as stiffness, strength and deflection at failure. In this work, we present a methodology to identify the layer design and process parameters that will have the highest potential to affect the mechanical properties of additively manufactured parts, using our previously developed multiscale modeling framework. Five samples for each of the ten identified process conditions were fabricated using a Roboze-Argo 500 version 2 with heated chamber and dual extruder nozzle. The manufactured PAEK and PAEK/carbon fiber samples were tested until failure in an Instron, using a video extensometer system. Each sample was prepared with a speckle pattern for post analysis using digital image correlation (DIC) to measure the strain and displacement over its entire surface. The raster angle and the presence of fibers had the largest influence on the mechanical properties of the AM manufactured parts, and the resulting properties were comparable to the mechanical properties of injection molded parts.
A Hybrid PAC Reinforcement Learning Algorithm for Human-Robot Interaction
(Frontiers in Robotics and AI, 2022-03-09) Zehfroosh, Ashkan; Tanner, Herbert G.
This paper offers a new hybrid probably approximately correct (PAC) reinforcement learning (RL) algorithm for Markov decision processes (MDPs) that intelligently maintains favorable features of both model-based and model-free methodologies. The designed algorithm, referred to as the Dyna-Delayed Q-learning (DDQ) algorithm, combines model-free Delayed Q-learning and model-based R-max algorithms while outperforming both in most cases. The paper includes a PAC analysis of the DDQ algorithm and a derivation of its sample complexity. Numerical results are provided to support the claim regarding the new algorithm’s sample efficiency compared to its parents as well as the best known PAC model-free and model-based algorithms in application. A real-world experimental implementation of DDQ in the context of pediatric motor rehabilitation facilitated by infant-robot interaction highlights the potential benefits of the reported method.

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