Browsing by Subject "Mechanical engineering"

Now showing 1 - 5 of 5
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    A model of tension-induced fiber growth predicts white matter organization during brain folding
    (Springer Nature, 2021-11-18) Garcia, Kara E.; Wang, Xiaojie; Kroenke, Christopher D.; Radiology and Imaging Sciences, School of Medicine
    The past decade has experienced renewed interest in the physical processes that fold the developing cerebral cortex. Biomechanical models and experiments suggest that growth of the cortex, outpacing growth of underlying subcortical tissue (prospective white matter), is sufficient to induce folding. However, current models do not explain the well-established links between white matter organization and fold morphology, nor do they consider subcortical remodeling that occurs during the period of folding. Here we propose a framework by which cortical folding may induce subcortical fiber growth and organization. Simulations incorporating stress-induced fiber elongation indicate that subcortical stresses resulting from folding are sufficient to induce stereotyped fiber organization beneath gyri and sulci. Model predictions are supported by high-resolution ex vivo diffusion tensor imaging of the developing rhesus macaque brain. Together, results provide support for the theory of cortical growth-induced folding and indicate that mechanical feedback plays a significant role in brain connectivity.
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    Effect of geometric, material and operational parameters on the steady-state belt response for flat belt-drives
    (2015-05) Yildiz, Cagkan; Wasfy, Tamer M.; Tovar, Andres; El-Mounayri, Hazim A.
    This thesis presents a comprehensive study of the effects of material, geometric and operational parameters on flat belt-drives steady-state belt stresses, belt slip, and belt-drive efficiency. The belt stresses include: belt rubber shear, normal, axial and lateral stresses; reinforcements tension force; and tangential and normal belt-pulley contact stresses. Belt slip is measured using the driven over driver pulleys’ angular velocity ratio. Each parameter was varied over a range to understand its impact on the steady-state belt-drive response. The material parameters studied are belt axial stiffness and damping, belt bending stiffness and damping, and belt-pulley friction coefficient. The geometric parameters studied are pulley center distance, pulleys diameter ratio, and belt thickness. The operational parameters studied are the driver pulley angular velocity and the driven pulley opposing torque (load). A high-fidelity flexible multibody dynamics parametric model of a two-pulley belt-drive system was created using a commercial multibody dynamics code. In the model the belt’s rubber matrix is represented using three-dimensional brick elements and the belt’s reinforcements are represented using one dimensional beam elements at the top surface of the belt. An asperity-based Coulomb friction model is used for the friction forces between the pulley and belt. The pulleys are modeled as rigid bodies with a cylindrical contact surface. The equations of motion are integrated using an explicit solution procedure. Unlike prior models which use one-dimensional truss or beam elements for the belt, the present model uses a three-dimensional belt model which introduces the effect of the thickness of the belt rubber matrix (modeled using brick elements). This enables a more accurate prediction of the belt stresses and slip than prior models. This thesis resolves in more details the complex stick-slip friction behavior of an axially flexible belt coupled with the shear effects of a flexible rubber cushion and at the same time shows the effect of the main system parameters on this stick-slip behavior. Some of the important conclusions of the thesis include: (1) the driver pulley has two distinct contact zones - a negative traction zone and a positive traction zone - while only one traction zone is present over the driven pulley; (2) the width of the negative traction zone on the driver pulley increases with the belt-pulley coefficient of friction and decreases with the belt axial stiffness; (3) the maximum belt tension and normal contact stress occur on the driver pulley and increase with the belt thickness, belt axial stiffness, and coefficient of friction; (4) belt-drive energy efficiency increases with the belt axial stiffness, and decreases with belt thickness, belt bending damping, belt operating speed, and operating torque load. The belt-drive modeling methodology presented in this thesis which enables accurate prediction of the belt stresses and slip can in turn be used to more accurately predict the fatigue life, wear life, and energy efficiency of belt-drives.
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    Experimental Investigation of Pressure Development and Flame Characteristics in a Pre-Combustion Chamber
    (2024-08) Miller, Jared; Nalim, Mohamed Razi; Larriba-Andaluz, Carlos; Yu, Huidan (Whitney)
    This study contributes to research involving wave rotor combustors by studying the development of a hot jet issuing from a cylindrical pre-combustion chamber. The pre-chamber was developed to provide a hot fuel-air mixture as an ignition source to a rectangular combustion chamber, which models the properties of a wave rotor channel. The pre-combustion chamber in this study was rebuilt for study and placed in a new housing so that buoyancy effects could be studied in tandem with other characteristics. The effectiveness of this hot jet is estimated by using devices and instrumentation to measure properties inside the pre-chamber under many different conditions. The properties tracked in this study include maximum pressure, the pressure and time at which an aluminum diaphragm ruptures, and the moment a developed flame reaches a precise location within the chamber. The pressure is tracked through use of a high-frequency pressure transducer, the diaphragm rupture moment is captured with a high-speed video camera, and the flame within the pre-chamber is detected by a custom-built ionization probe. The experimental apparatus was used in three configurations to study any potential buoyancy effects and utilized three different gaseous fuels, including a 50%-50% methane-hydrogen blend, pure methane, and pure hydrogen. Additionally, the equivalence ratio within the pre-chamber was varied from values of 0.9 to 1.2, and the initial pressure was set to either 1.0, 1.5, or 1.75 atm. In all cases, combustion was initiated from a spark plug, causing a flame to develop until the diaphragm breaks, releasing a hot jet of fuel and air from the nozzle inserted into the pre-chamber. In the pressure transducer tests, it was found that hydrogen produced the highest pressures and fastest rupture times, and methane produced the lowest pressures and slowest rupture times. The methane-hydrogen blend provided a middle ground between the two pure fuels. An equivalence ratio of 1.1 consistently provided the highest pressure values and fastest rupture out of all tested values. It was also found that the orientation has a noticeable impact on both the pressure development and rupture moment as higher maximum pressures were achieved when the chamber was laid flat in the “vertical jet” orientation as compared to when it was stood upright in the “horizontal jet” orientation. Additionally, increasing the initial pressure strongly increased the maximum developed pressure but had minimal impact on the rupture moment. The tests done with the ion probe demonstrated that an equivalence ratio of 1.1 produces a flame that reaches the ion probe faster than an equivalence ratio of 1.0 for the methane-hydrogen blend. In its current form, the ion probe setup has significant limitations and should continue to be developed for future studies. The properties analyzed in this study deepen the understanding of the processes that occur within the pre-chamber and aid in understanding the conditions that may exist in the hot jet produced by it as the nozzle ruptures. The knowledge gained in the study can also be applied to develop models that can predict other parameters that are difficult to physically measure.
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    Thermoplastic polyurethane flexible capacitive proximity sensor reinforced by CNTs for applications in the creative industries
    (Springer Nature, 2021-01-13) Moheimani, Reza; Aliahmad, Nojan; Aliheidari, Nahal; Agarwal, Mangilal; Dalir, Hamid; Mechanical and Energy Engineering, School of Engineering and Technology
    Wearable sensing platforms have been rapidly advanced over recent years, thanks to numerous achievements in a variety of sensor fabrication techniques. However, the development of a flexible proximity sensor that can perform in a large range of object mobility remains a challenge. Here, a polymer-based sensor that utilizes a nanostructure composite as the sensing element has been presented for forthcoming usage in healthcare and automotive applications. Thermoplastic Polyurethane (TPU)/Carbon Nanotubes (CNTs) composites are capable of detecting presence of an external object in a wide range of distance. The proximity sensor exhibits an unprecedented detection distance of 120 mm with a resolution of 0.3%/mm. The architecture and manufacturing procedures of TPU/CNTs sensor are straightforward and performance of the proximity sensor shows robustness to reproducibility as well as excellent electrical and mechanical flexibility under different bending radii and over hundreds of bending cycles with variation of 4.7% and 4.2%, respectively. Tunneling and fringing effects are addressed as the sensing mechanism to explain significant capacitance changes. Percolation threshold analysis of different TPU/CNT contents indicated that nanocomposites having 2 wt% carbon nanotubes are exhibiting excellent sensing capabilities to achieve maximum detection accuracy and least noise among others. Fringing capacitance effect of the structure has been systematically analyzed by ANSYS Maxwell (Ansoft) simulation, as the experiments precisely supports the sensitivity trend in simulation. Our results introduce a new mainstream platform to realize an ultrasensitive perception of objects, presenting a promising prototype for application in wearable proximity sensors for motion analysis and artificial electronic skin.
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    Three-dimensional transient numerical study of hot-jet ignition of methane-hydrogen blends in a constant-volume combustor
    (2015) Khan, Md Nazmuzzaman; Nalim, Mohamed Razi; Yu, Whitney; Zhu, Likun
    Ignition by a jet of hot combustion product gas injected into a premixed combustible mixture from a separate pre-chamber is a complex phenomenon with jet penetration, vortex generation, flame and shock propagation and interaction. It has been considered a useful approach for lean, low-NOx combustion for automotive engines, pulsed detonation engines and wave rotor combustors. The hot-jet ignition constant-volume combustor (CVC) rig established at the Combustion and Propulsion Research Laboratory (CPRL) of the Purdue School of Engineering and Technology at Indiana University-Purdue University Indianapolis (IUPUI) is considered for numerical study. The CVC chamber contains stoichiometric methane-hydrogen blends, with pre-chamber being operated with slightly rich blends. Five operating and design parameters were investigated with respect to their eff ects on ignition timing. Di fderent pre-chamber pressure (2, 4 and 6 bar), CVC chamber fuel blends (Fuel-A: 30% methane + 70% hydrogen and Fuel-B: 50% methane + 50% hydrogen by volume), active radicals in pre-chamber combusted products (H, OH, O and NO), CVC chamber temperature (298 K and 514 K) and pre-chamber traverse speed (0.983 m/s, 4.917 m/s and 13.112 m/s) are considered which span a range of fluid-dynamic mixing and chemical time scales. Ignition delay of the fuel-air mixture in the CVC chamber is investigated using a detailed mechanism with 21 species and 84 elementary reactions (DRM19). To speed up the kinetic process adaptive mesh refi nement (AMR) based on velocity and temperature and multi-zone reaction technique is used. With 3D numerical simulations, the present work explains the e ffects of pre-chamber pressure, CVC chamber initial temperature and jet traverse speed on ignition for a speci fic set of fuels. An innovative post processing technique is developed to predict and understand the characteristics of ignition in 3D space and time. With the increase of pre-chamber pressure, ignition delay decreases for Fuel-A which is the relatively more reactive fuel blend. For Fuel-B which is relatively less reactive fuel blend, ignition occurs only for 2 bar pre-chamber pressure for centered stationary jet. Inclusion of active radicals in pre-chamber combusted product decreases the ignition delay when compared with only the stable species in pre-chamber combusted product. The eff ects of shock-flame interaction on heat release rate is observed by studying flame surface area and vorticity changes. In general, shock-flame interaction increases heat release rate by increasing mixing (increase the amount of deposited vorticity on flame surface) and flame stretching. The heat release rate is found to be maximum just after fast-slow interaction. For Fuel-A, increasing jet traverse speed decreases the ignition delay for relatively higher pre-chamber pressures (6 and 4 bar). Only 6 bar pre-chamber pressure is considered for Fuel-B with three di fferent pre-chamber traverse speeds. Fuel-B fails to ignite within the simulation time for all the traverse speeds. Higher initial CVC temperature (514 K) decreases the ignition delay for both fuels when compared with relatively lower initial CVC temperature (300 K). For initial temperature of 514 K, the ignition of Fuel-B is successful for all the pre-chamber pressures with lowest ignition delay observed for the intermediate 4 bar pre-chamber pressure. Fuel-A has the lowest ignition delay for 6 bar pre-chamber pressure. A speci fic range of pre-chamber combusted products mass fraction, CVC chamber fuel mass fraction and temperature are found at ignition point for Fuel-A which were liable for ignition initiation. The behavior of less reactive Fuel-B appears to me more complex at room temperature initial condition. No simple conclusions could be made about the range of pre-chamber and CVC chamber mass fractions at ignition point.