Open Access Publications
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Open access publications by faculty, postdocs, and graduate students in the Department of Mechanical Engineering.
Now showing 1 - 5 of 30
- ItemOn the inviscid energetics of Mack’s first mode instability(Theoretical and Computational Fluid Dynamics, 2022-12-22) Liang, Tony; Kafle, Sulav; Khan, Arham Amin; Paredes, Pedro; Kuehl, JosephHigh-speed boundary layer transition is dominated by the modal, exponential amplification of the oblique Mack’s first mode waves in two-dimensional boundary layers from Mach 1 up to freestream Mach numbers of 4.5 to 6.5 depending on the wall-to-adiabatic temperature ratio. At higher Mach numbers, the acoustic, planar Mack’s second mode waves become dominant. Although many theoretical, computational and experimental studies have focused on the supersonic boundary layer transition due to the oblique Mack’s first mode, several fundamental questions about the source of this instability and the reasons for its obliqueness remain unsolved. Here, we perform an inviscid energetics investigation and classify disturbances based on their energetics signature on a Blasius boundary layer for a range of Mach numbers. This approach builds insight into the fundamental mechanisms governing various types of instability. It is shown that first mode instability is distinct from Tollmien–Schlichting instability, being driven by a phase shifting between streamwise velocity and pressure perturbations in the vicinity of the generalized inflection point and insensitive to the viscous no-slip condition. Further, it is suggested that the obliqueness of the first mode is associated with an inviscid flow invariant.
- ItemNon-Monotonic Capacitance Change of Layered Ti3C2Tx MXene Film Structures under Increasing Compressive Stress(Advanced Functional Materials, 2022-12-09) Zhang, Qing; Ning, Ran; Cao, Jinxin; Song, Qingrui; Ye, Jiaxin; Wei, BingqingThe progress in advanced electronic devices has imposed a great demand for developing flexible electrochemical power devices, which requires a comprehensive understanding of the mechanical–electrochemical coupling behavior of various energy storage materials. Unlike a monotonic capacitance increase of carbon-based double-layer supercapacitors, MXene-based flexible supercapacitors demonstrate a non-monotonic, i.e., “increase-then-decrease” capacitance behavior under the pressure range of 8488 kPa. This non-monotonic capacitance response to pressure is intrinsic to the MXene film as its charge storage is primarily determined by the surface activity, which can be readily affected by pressure-induced dissociation of functionalities, as well as the charge transporting kinetics as limited by the inherent layered structure. The findings described in this study not only expand the knowledge of mechanical–electrochemical coupling to layered MXenes under pressure, but also give a vital design guideline for flexible/stretchable MXene-based energy storage devices or other electronics.
- ItemResilient Ground Vehicle Autonomous Navigation in GPS-denied Environments(Guidance, Navigation and Control, 2022-11-23) Baxevani, Kleio; Yadav, Indrajeet; Yang, Yulin; Sebok, Michael; Tanner, Herbert G.; Huang, GuoquanCo-design and integration of vehicle navigation and control and state estimation is key for enabling field deployment of mobile robots in GPS-denied cluttered environments, and sensor calibration is critical for successful operation of both subsystems. This paper demonstrates the potential of this co-design approach with field tests of the integration of a reactive receding horizon-based motion planner and controller with an inertial aided multi-sensor calibration scheme. The reported method provides accurate calibration parameters that improve the performance of the state estimator, and enable the motion controller to generate smooth and continuous minimal-jerk trajectories based on local LiDAR data. Numerical simulations in Unity, and real-world experimental results from the field corroborate the claims of efficacy for the reported autonomous navigation computational pipeline.
- ItemDevelopment 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.
- ItemComparison 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.