Research and Publications - Department of Mechanical Engineering

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    Characterization Of Multi-layered Fish Scales (Atractosteus spatula) Using Nanoindentation, X-ray CT, FTIR, and SEM
    (MyJove Corporation, 2014) Allison, Paul G.; Rodriguez, Rogie I.; Moser, Robert D.; Williams, Brett A.; Poda, Aimee R.; Seiter, Jennifer M.; Lafferty, Brandon J.; Kennedy, Alan J.; Chandler, Mei Q.; United States Department of Defense; United States Army; U.S. Army Corps of Engineers; U.S. Army Engineer Research & Development Center (ERDC); University of Alabama Tuscaloosa
    The hierarchical architecture of protective biological materials such as mineralized fish scales, gastropod shells, ram's horn, antlers, and turtle shells provides unique design principles with potentials for guiding the design of protective materials and systems in the future. Understanding the structure-property relationships for these material systems at the microscale and nanoscale where failure initiates is essential. Currently, experimental techniques such as nanoindentation, X-ray CT, and SEM provide researchers with a way to correlate the mechanical behavior with hierarchical microstructures of these material systems. However, a well-defined standard procedure for specimen preparation of mineralized biomaterials is not currently available. In this study, the methods for probing spatially correlated chemical, structural, and mechanical properties of the multilayered scale of A. spatula using nanoindentation, FTIR, SEM, with energy-dispersive X-ray (EDX) microanalysis, and X-ray CT are presented.
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    Fatigue Testing of Pipeline Welds and Heat-Affected Zones in Pressurized Hydrogen Gas
    (National Institute of Standards & Technology, 2019) Drexler, Elizabeth S.; Slifka, Andrew J.; Amaro, Robert L.; Sowards, Jeffrey W.; Connolly, Matthew J.; Martin, May L.; Lauria, Damian S.; National Institute of Standards & Technology (NIST) - USA; University of Alabama Tuscaloosa; National Aeronautics & Space Administration (NASA); University of Colorado Boulder
    Several welds and associated heat-affected zones (HAZs) on two API X70 and two API X52 pipes were tested to determine the fatigue crack growth rate (FCGR) in pressurized hydrogen gas and assess the area of the pipe that was most susceptible to fatigue when subjected to hydrogen gas. The relationship between FCGRs for welds and HAZs compared to base metal is discussed relative to local residual stresses, differences in the actual path of the crack, and hydrogen pressure effects.
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    Age-related differences in gait adaptations during overground walking with and without visual perturbations using a virtual reality headset
    (Nature Portfolio, 2020) Osaba, Muyinat Y.; Martelli, Dario; Prado, Antonio; Agrawal, Sunil K.; Lalwani, Anil K.; Columbia University; University of Alabama Tuscaloosa; NewYork-Presbyterian Hospital
    Older adults have difficulty adapting to new visual information, posing a challenge to maintain balance during walking. Virtual reality can be used to study gait adaptability in response to discordant sensorimotor stimulations. This study aimed to investigate age-related modifications and propensity for visuomotor adaptations due to continuous visual perturbations during overground walking in a virtual reality headset. Twenty old and twelve young subjects walked on an instrumented walkway in real and virtual environments while reacting to antero-posterior and medio-lateral oscillations of the visual field. Mean and variability of spatiotemporal gait parameters were calculated during the first and fifth minutes of walking. A 3-way mixed-design ANOVA was performed to determine the main and interaction effects of group, condition and time. Both groups modified gait similarly, but older adults walked with shorter and slower strides and did not reduce stride velocity or increase stride width variability during medio-lateral perturbations. This may be related to a more conservative and anticipatory strategy as well as a reduced perception of the optic flow. Over time, participants adapted similarly to the perturbations but only younger participants reduced their stride velocity variability. Results provide novel evidence of age- and context-dependent visuomotor adaptations in response to visual perturbations during overground walking and may help to establish new methods for early identification and remediation of gait deficits.
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    Compact Shape Morphing Tensegrity Robots Capable of Locomotion
    (Frontiers, 2019) Rhodes, Tyler; Gotberg, Clayton; Vikas, Vishesh; University of Alabama Tuscaloosa
    Robustness, compactness, and portability of tensegrity robots make them suitable candidates for locomotion on unknown terrains. Despite these advantages, challenges remain relating to ease of fabrication, shape morphing (packing-unpacking), and locomotion capabilities. The paper introduces a design methodology for fabricating tensegrity robots of varying morphologies with modular components. The design methodology utilizes perforated links, coplanar (2D) alignment of components and individual cable tensioning to achieve a 3D tensegrity structure. These techniques are utilized to fabricate prism (three-link) tensegrity structures, followed by tensegrity robots in icosahedron (six-link), and shpericon (curved two-link) formation. The methodology is used to explore different robot morphologies that attempt to minimize structural complexity (number of elements) while facilitating smooth locomotion (impact between robot and surface). Locomotion strategies for such robots involve altering the position of center-of-mass (referred to as internal mass shifting) to induce "tip-over." As an example, a sphericon formation comprising of two orthogonally placed circular arcs with conincident center illustrates smooth locomotion along a line (one degree of freedom). The design of curved links of tensegrity mechanisms facilitates continuous change of the point of contact (along the curve) that results from the tip-over. This contrasts to the sudden and piece-wise continuous change for the case of robots with traditional straight links which generate impulse reaction forces during locomotion. The two resulting robots-the Icosahedron and the Sphericon Tensegrity Robots-display shape morphing (packing-unpacking) capabilities and achieve locomotion through internal mass-shifting. The presented static equilibrium analysis of sphericon with mass is the first step in the direction of dynamic locomotion control of these curved link robots.
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    A Lightweight Exoskeleton-Based Portable Gait Data Collection System
    (MDPI, 2021) Haque, Md Rejwanul; Imtiaz, Masudul H.; Kwak, Samuel T.; Sazonov, Edward; Chang, Young-Hui; Shen, Xiangrong; University of Alabama Tuscaloosa; Clarkson University; University System of Georgia; Georgia Institute of Technology
    For the controller of wearable lower-limb assistive devices, quantitative understanding of human locomotion serves as the basis for human motion intent recognition and joint-level motion control. Traditionally, the required gait data are obtained in gait research laboratories, utilizing marker-based optical motion capture systems. Despite the high accuracy of measurement, marker-based systems are largely limited to laboratory environments, making it nearly impossible to collect the desired gait data in real-world daily-living scenarios. To address this problem, the authors propose a novel exoskeleton-based gait data collection system, which provides the capability of conducting independent measurement of lower limb movement without the need for stationary instrumentation. The basis of the system is a lightweight exoskeleton with articulated knee and ankle joints. To minimize the interference to a wearer's natural lower-limb movement, a unique two-degrees-of-freedom joint design is incorporated, integrating a primary degree of freedom for joint motion measurement with a passive degree of freedom to allow natural joint movement and improve the comfort of use. In addition to the joint-embedded goniometers, the exoskeleton also features multiple positions for the mounting of inertia measurement units (IMUs) as well as foot-plate-embedded force sensing resistors to measure the foot plantar pressure. All sensor signals are routed to a microcontroller for data logging and storage. To validate the exoskeleton-provided joint angle measurement, a comparison study on three healthy participants was conducted, which involves locomotion experiments in various modes, including overground walking, treadmill walking, and sit-to-stand and stand-to-sit transitions. Joint angle trajectories measured with an eight-camera motion capture system served as the benchmark for comparison. Experimental results indicate that the exoskeleton-measured joint angle trajectories closely match those obtained through the optical motion capture system in all modes of locomotion (correlation coefficients of 0.97 and 0.96 for knee and ankle measurements, respectively), clearly demonstrating the accuracy and reliability of the proposed gait measurement system.
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    Modelling and experimental study of fatigue of powder metal steel (FC-0205)
    (Taylor & Francis, 2013) Allison, P. G.; Hammi, Y.; Jordon, J. B.; Horstemeyer, M. F.; United States Department of Defense; United States Army; U.S. Army Corps of Engineers; U.S. Army Engineer Research & Development Center (ERDC); Mississippi State University; University of Alabama Tuscaloosa
    The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets of high and low porosity specimens calibrated the model. Strain-life behaviour depicted that above the plastic strain limit of 0.002 mm mm(-1) in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain-life curves.
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    Development of control quality factor for HVAC control loop performance assessment-II: Field testing and results (ASHRAE RP-1587)
    (Taylor & Francis, 2019) Liu, Ran; Li, Yanfei; O'Neill, Zheng D.; Zhou, Xiaohui; University of Alabama Tuscaloosa; United States Department of Energy (DOE); National Renewable Energy Laboratory - USA
    This article is the third paper from the research project RP-1587, focusing on presenting a comprehensive field test of the proposed control quality factors (CQFs; i.e., CQF-Harris and CQF-exponentially weighted moving average [EWMA]) and testing results. Firstly, the simulated control loops and real control loops are evaluated for offline assessment. Then, the field experiment implemented for different HVAC control loops is assessed online using the proposed CQFs. Test results show that the proposed CQFs are capable of adequately and effectively assessing the HVAC control loop performance. The methodology of obtaining those CQFs is provided in the companion paper: Development of Control Quality Factor for HVAC Control Loop Performance Assessment I-Methodology (ASHRAE RP-1587) (Li et al. 2019).
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    Energy savings and ventilation performance from CO2-based demand controlled ventilation: Simulation results from ASHRAE RP-1747 (ASHRAE RP-1747)
    (Taylor & Francis, 2019) O'Neill, Zheng D.; Le, Yanfei; Cheng, Hwakong C.; Zhou, Xiaohui; Taylor, Steven T.; University of Alabama Tuscaloosa
    This is the first journal paper from the ASHRAE research project RP-1747 "Implementation of RP-1547 CO2-based Demand Controlled Ventilation for Multiple Zone HVAC Systems in Direct Digital Control Systems." HVAC designers face challenges in complying with the ventilation requirements in ASHRAE Standard 62.1 due to the complexity of the ventilation rate procedure (VRP) and the lack of direction on how to appropriately apply demand controlled ventilation (DCV) within the context of the VRP. The RP-1747 project aimed to address those issues by developing and testing DCV control sequences that are practical and implementable in typical single-duct variable air volume (VAV) systems with direct digital control (DDC) Systems. These control sequences were also tested for energy and ventilation performance by using a co-simulation of EnergyPlus and CONTAM coupled by a functional mockup unit (FMU). This paper presents the simulation-based study of one office building in four climate zones including Miami (1A), Atlanta (3A), Oakland (3C), and Chicago (5A) for both DCV and non-DCV baselines. The ventilation requirements in non-DCV baselines were set following a simplified ASHRAE 62.1 approach and California Title 24. Simulation results show that RP-1747 DCV control logic could lead to 9% to 33% HVAC energy savings on a source energy basis compared with the non-DCV baseline with the simplified ASHRAE 62.1 approach. The simulated hourly outdoor airflow provided met or exceeded the ASHRAE Standard 62.1 ventilation requirement in the four climate zones for 83% to 97% of the time for the simulated building. Transients and simulation artifacts associated with the discretization of time steps appear to account for a large portion of the time steps when the ventilation provided is less than required by the Standard. After applying a tolerance to account for sensor and control error in real life and time averaging as allowed by Standard 62.1, the DCV strategy met ventilation requirements for 96% to 98% of the time. This indicates the RP-1747 DCV control logic achieves good compliance with the ventilation requirements in Standard 62.1.
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    HVAC control loop performance assessment: A critical review (1587-RP)
    (Taylor & Francis, 2016) O'Neill, Zheng; Li, Yanfei; Williams, Keith; University of Alabama Tuscaloosa
    This article presents a comprehensive review of control loop performance assessments in the context of building HVAC controls. Few studies are available for assessing HVAC control loop performance using a single control quality factor. A control quality factor should be an objective and quantitative metric with simple-to-interpret criteria and should only use data available from the actual control system, such as the control output. The authors systematically reviewed 34 indices and the associated methods of evaluating control loop performance and cataloged the drawbacks and merits of the different indices. Most of these performance assessment indices are currently used in process control industry applications. There were 14 of the 34 indices selected for further review, due to their particular suitability for implementation in HVAC control loop performance assessment. Finally, the selected 14 indices are implemented for assessments of three regulatory control loops with proportional-integral controllers: a heating coil outlet air temperature control loop and variable air volume room air temperature control loop using simulated data from a dynamic Modelica model, and variable air volume room air temperature control loop in a heating mode from real field data. Based on the review and preliminary results, the Normalized Harris Index and exponentially weighted moving averages based index are proposed as potential candidates for control quality factor, and further investigation of the use of them in HVAC control loop performance assessment is recommended.
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    Assessing the validity, reliability, and practicality of ASHRAE's performance measurement protocols (ASHRAE Research Project 1702)
    (Taylor & Francis, 2019) Wang, Liping; Mcmorrow, Gabrielle; Zhou, Xiaohui; O'Neill, Zheng D.; University of Wyoming; University of Alabama Tuscaloosa
    The objective of this study was to provide a basis for future updates to the 2010 ASHRAE Performance Measurement Protocols for Commercial Buildings (PMP) through case studies. The PMP defines a standardized method of measuring and analyzing building performance in six categories: energy, water, thermal comfort, indoor air quality, lighting, and acoustics. We conducted case studies for five buildings following the PMP. Based on experiences following the protocol in this wide range of buildings, we assessed the validity, reliability, and practicality of the PMP and provided comments and recommendations for future revisions. Most of the measurement protocols at the basic level are reliable, practical, and valid. Many tasks at the intermediate and advanced levels, however, can be difficult to perform for some building types. Some of the tasks or measurement procedures were results from past research projects, and the software or tools recommended may not be readily available or fully supported. The measurement protocols at the intermediate level are only somewhat reliable and some of them are impractical. Most measurement protocols at the advanced level are complex and need to be performed by qualified or specially trained personnel and, thus, are impractical as a performance measure, except for specialized applications.
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    Numerical Modelling of the Magnus Force and the Aerodynamic Torque on a Spinning Sphere in Transitional Flow
    (2007) Volkov, Alexey N.; University of Alabama Tuscaloosa
    Three dimensional transitional flow over a spinning sphere is studied numerically by the direct simulation Monte Carlo method. The flow is assumed to be steady-state, gas molecules interact with each other as hard spheres and the speculardiffuse scattering model describes the interaction between molecules and the sphere surface. The translational and rotational velocities of the sphere is assumed to be perpendicular to each other. The drag coefficient, the Magnus force coefficient and the torque coefficient are found as functions of the Mach and Reynolds numbers and the dimensionless rotation parameter for subsonic and supersonic flows. Computational results are compared with the analytical solution for a spinning sphere in free molecular flow and with available semi-empirical data. The "critical" Knudsen number when the Magnus force is equal to zero is found as a function of the Mach number.
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    3D Numerical Modelling of a Rarefied Gas Flow in the Nearby Atmosphere around a Rotating Cometary Nucleus
    (2007) Volkov, Alexey N.; Lukyanov, German A.; University of Alabama Tuscaloosa
    A combined 3D model of a nearby atmosphere around an arbitrary rotating spherical cometary nucleus is developed. The model includes a 3D unsteady model of solar radiation absorption and heating of the nucleus material (water ice), its evaporation and condensation and a 3D quasi-stationary kinetic model of flow inside the near nucleus coma. This model can be used to predict the coma flow in conditions typical for rendezvous projects such as ESA project Rossetta. Calculations are carried out with the help of this model to reveal the influence of nucleus rotation on its temperature field and the flow field in the nearby atmosphere. It was found that the nucleus rotation influences significantly the nucleus temperature field and the coma flow. Vapor flow around a rotating nucleus is essentially three dimensional and differs qualitatively from the coma flow around a non-rotating nucleus.
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    Kinetic Model of a Collisional Admixture in Dusty Gas and its Application to Calculating Flow Past Bodies
    (2000) Volkov, Alexey N.; Tsirkunov, Yury M.; University of Alabama Tuscaloosa
    Using the methods of statistical physics, the basic kinetic equation describing the dynamics of a polydisperse admixture of solid particles in a dilute dusty-gas flow is derived. Particle rotation, inelastic collisions, and interaction with the carrier gas are taken into account. The basic kinetic equation is used to obtain a Boltzmann-type equation for the one-particle distribution function, for which the boundary conditions for the problem of dusty-gas flow past a body are formulated. On the basis of the kinetic model developed, using direct statistical modeling, the flow patterns and the fields of the dispersed-phase macroparameters in a uniform crosswise dusty-gas flow past a cylinder are obtained for various free-stream particle sizes and concentrations.
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    Fluid/Kinetic Hybrid Simulation of Atmospheric Escape: Pluto
    (2011) Tucker, Orenthal J.; Erwin, Justin T.; Johnson, Robert E.; Volkov, Alexey N.; Cassidy, Timothy A.; University of Alabama Tuscaloosa
    A hybrid fluid/molecular kinetic model was developed to describe the escape of molecules from the gravitational well of a planet's atmosphere. This model was applied to a one dimensional, radial description of molecular escape from the atmosphere of Pluto and compared to purely fluid dynamic simulations of escape for two solar heating cases. The hybrid simulations show that the atmospheric temperature vs. altitude and the escape rates can differ significantly from those obtained using only a fluid description of the atmosphere.
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    Kinetic simulations of thermal escape from a single component atmosphere
    (American Institute of Physics, 2011-06-06) Volkov, Alexey N.; Tucker, Orenthal J.; Erwin, Justin T.; Johnson, Robert E.; University of Virginia; University of Alabama Tuscaloosa
    The one-dimensional steady-state expansion of a monatomic gas from a spherical source in a gravity field is studied by the direct simulation Monte Carlo method. Collisions between molecules are described by the hard sphere model, the distribution of gas molecules leaving the source surface is assumed to be Maxwellian, and no heat is directly deposited in the simulation region. The flow structure and the escape rate (number flux of molecules escaping the atmosphere) are analyzed for the source Jeans parameter lambda(0) (ratio of the gravitational energy to thermal energy of the molecules) and Knudsen number Kn(0) (ratio of the mean free path to the source radius) ranging from 0 to 15 and from 0.0001 to infinity, respectively. In the collisionless regime, flows are analyzed for lambda(0)=0-100 and analytical equations are obtained for asymptotic values of gas parameters that are found to be non-monotonic functions of lambda(0). For collisional flows, simulations predict the transition in the nature of atmospheric loss from escape on a molecule-by-molecules basis, often referred to as Jeans escape, to an organized outflow, often referred to as hydrodynamic escape. It is found that the structure of the flow and the escape rate exhibit drastic changes when lambda(0) varies over a narrow transition range 2-3. The lower limit of this range approximately corresponds to a critical Jeans parameter equal to 2.06, which is the upper limit for isentropic, supersonic outflow of a monatomic gas from a body in a gravity field. Subcritical, lambda(0)<= 2, flows are qualitatively similar to free outgassing in the absence of gravity, resulting in hypersonic terminal Mach numbers and escape rates that are independent of lambda(0) in the limit of small Knudsen numbers. Supercritical, lambda(0)>= 3, flows are controlled by thermal conduction and demonstrate qualitatively different trends. The ratio of the actual escape rate to the Jeans escape rate at the source surface is found to be a non-monotonic function of Kn(0) spanning the range from similar to 0.01 to similar to 2. At lambda(0)>= 6, the ratio of the actual escape rate to the Jeans escape rate at the exobase is found to be similar to 1.4-1.7. This is unlike the predictions of the slow hydrodynamic escape model, which is based on Parker's model for the solar wind and intended for the description of the atmospheric loss at lambda(0)>similar to 10. At lambda(0) < 6, the actual escape rate can be well approximated by a modified Jeans escape rate, which accounts for non-zero gas velocity. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3592253]
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    Effect of bending buckling of carbon nanotubes on thermal conductivity of carbon nanotube materials
    (American Institute of Physics, 2012-03-01) Volkov, Alexey N.; Shiga, Takuma; Nicholson, David; Shiomi, Junichiro; Zhigilei, Leonid V.; University of Tokyo; University of Virginia; University of Alabama Tuscaloosa
    The effect of bending buckling of carbon nanotubes (CNTs) on thermal conductivity of CNT materials is investigated in atomistic and mesoscopic simulations. Nonequilibrium molecular dynamics simulations of the thermal conductance through an individual buckling kink in a (10,10) single-walled CNT reveal a strong dependence (close to inverse proportionality) of the thermal conductance of the buckling kink on the buckling angle. The value of the buckling kink conductance divided by the cross-sectional area of the CNT ranges from 40 to 10 GWm(-2) K-1 as the buckling angle changes from 20 to 110 degrees. The predictions of the atomistic simulations are used for parameterization of a mesoscopic model that enables calculations of thermal conductivity of films composed of thousands of CNTs arranged into continuous networks of bundles. The results of mesoscopic simulations demonstrate that the conductivity of CNT films is sensitive to the angular dependence of the buckling kink conductance and the length of the individual CNTs. For a film composed of 1 mu m-long CNTs, the values of the in-plane film conductivity predicted with a constant conductance of 20 GWm(-2) K-1 and the angular-dependent conductance obtained in atomistic simulations are about 40 and 20% lower than the conductivity predicted for the same film with zero thermal resistance of the buckling kinks, respectively. The weaker impact of the angular-dependent buckling kink conductance on the effective conductivity of the film is explained by the presence of a large fraction of kinks that have small buckling angles and correspondingly large values of conductance. The results of the simulations suggest that the finite conductance of the buckling kinks has a moderate, but non-negligible, effect on thermal conductivity of materials composed of short CNTs with length up to 1 mu m. The contribution of the buckling kink thermal resistance becomes stronger for materials composed of longer CNTs and/or characterized by higher density of buckling kinks. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3687943]
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    Atomistic simulations, mesoscopic modeling, and theoretical analysis of thermal conductivity of bundles composed of carbon nanotubes
    (American Institute of Physics, 2013-09-10) Volkov, Alexey N.; Salaway, Richard N.; Zhigilei, Leonid V.; University of Virginia; University of Alabama Tuscaloosa
    The propensity of carbon nanotubes (CNTs) to self-organize into continuous networks of bundles has direct implications for thermal transport properties of CNT network materials and defines the importance of clear understanding of the mechanisms and scaling laws governing the heat transfer within the primary building blocks of the network structures-close-packed bundles of CNTs. A comprehensive study of the thermal conductivity of CNT bundles is performed with a combination of non-equilibrium molecular dynamics (MD) simulations of heat transfer between adjacent CNTs and the intrinsic conductivity of CNTs in a bundle with a theoretical analysis that reveals the connections between the structure and thermal transport properties of CNT bundles. The results of MD simulations of heat transfer in CNT bundles consisting of up to 7 CNTs suggest that, contrary to the widespread notion of strongly reduced conductivity of CNTs in bundles, van der Waals interactions between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic conductivity of the CNTs. The simulations of inter-tube heat conduction performed for partially overlapping parallel CNTs indicate that the conductance through the overlap region is proportional to the length of the overlap for CNTs and CNT-CNT overlaps longer than several tens of nm. Based on the predictions of the MD simulations, a mesoscopic-level model is developed and applied for theoretical analysis and numerical modeling of heat transfer in bundles consisting of CNTs with infinitely large and finite intrinsic thermal conductivities. The general scaling laws predicting the quadratic dependence of the bundle conductivity on the length of individual CNTs in the case when the thermal transport is controlled by the inter-tube conductance and the independence of the CNT length in another limiting case when the intrinsic conductivity of CNTs plays the dominant role are derived. An application of the scaling laws to bundles of single-walled (10,10) CNTs reveals that the transition from inter-tube-conductance-dominated to intrinsic-conductivity-dominated thermal transport in CNT bundles occurs in a practically important range of CNT length from similar to 20 nm to similar to 4 mu m. (C) 2013 AIP Publishing LLC.
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    Heat conduction in carbon nanotube materials: Strong effect of intrinsic thermal conductivity of carbon nanotubes
    (American Institute of Physics, 2012-07-24) Volkov, Alexey N.; Zhigilei, Leonid V.; University of Virginia; University of Alabama Tuscaloosa
    Computational study of thermal conductivity of interconnected networks of bundles in carbon nanotube (CNT) films reveals a strong effect of the finite thermal conductivity k(T) of individual nanotubes on the conductivity k of the CNT materials. The physical origin of this effect is explained in a theoretical analysis of systems composed of straight randomly dispersed CNTs. An analytical equation for quantitative description of the effect of finite k(T) on the value of k is obtained and adopted for continuous networks of bundles characteristic of CNT films and buckypaper. Contrary to the common assumption of the dominant effect of the contact conductance, the contribution of the finite k(T) is found to control the value of k at material densities and CNT lengths typical for real materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737903]
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    THERMALLY DRIVEN ATMOSPHERIC ESCAPE: TRANSITION FROM HYDRODYNAMIC TO JEANS ESCAPE
    (IOP Publishing, 2011-02-16) Volkov, Alexey N.; Johnson, Robert E.; Tucker, Orenthal J.; Erwin, Justin T.; University of Virginia; New York University; University of Alabama Tuscaloosa
    Thermally driven escape from planetary atmospheres changes in nature from an organized outflow (hydrodynamic escape) to escape on a molecule-by-molecule basis (Jeans escape) with increasing Jeans parameter, lambda, the ratio of the gravitational to thermal energy of the atmospheric molecules. This change is described here for the first time using the direct simulation Monte Carlo method. When heating is predominantly below the lower boundary of the simulation region, R-0, and well below the exobase of a single-component atmosphere, the nature of the escape process changes over a surprisingly narrow range of Jeans parameters, lambda(0), evaluated at R-0. For an atomic gas, the transition occurs over lambda(0) similar to 2-3, where the lower bound, lambda(0) similar to 2.1, corresponds to the upper limit for isentropic, supersonic outflow. For lambda(0) > 3 escape occurs on a molecule-by-molecule basis and we show that, contrary to earlier suggestions, for lambda(0) > similar to 6 the escape rate does not deviate significantly from the familiar Jeans rate. In a gas composed of diatomic molecules, the transition shifts to lambda(0) similar to 2.4-3.6 and at lambda(0) > similar to 4 the escape rate increases a few tens of percent over that for the monatomic gas. Scaling by the Jeans parameter and the Knudsen number, these results can be applied to thermally induced escape of the major species from solar and extrasolar planets.
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    THERMAL ESCAPE IN THE HYDRODYNAMIC REGIME: RECONSIDERATION OF PARKER's ISENTROPIC THEORY BASED ON RESULTS OF KINETIC SIMULATIONS
    (IOP Publishing, 2013-03-10) Volkov, Alexey N.; Johnson, Robert E.; University of Virginia; New York University; University of Alabama Tuscaloosa
    The one-dimensional steady-state problem of thermal escape from a single-component atmosphere of mon- and diatomic gases is studied in the hydrodynamic (blow-off) regime using the direct simulation Monte Carlo method for an evaporative-type condition at the lower boundary. The simulations are performed for various depths into an atmosphere, indicated by a Knudsen number, Kn(0), equal to the ratio of the mean free path of molecules to the radial position of the source surface, ranging from 10 to 10(-5), and for the range of the source Jeans parameter, lambda(0), equal to the ratio of gravitational and thermal energies, specific to blow-off. The results of kinetic simulations are compared with the isentropic model (IM) and the Navier-Stokes model. It is shown that the IM can be simplified if formulated in terms of the local Mach number and Jeans parameter. The simulations predict that at Kn(0) < similar to 10(-3) the flow includes a near-surface non-equilibrium Knudsen layer, a zone where the flow can be well approximated by the IM, and a rarefied far field. The corresponding IM solutions, however, only approach Parker's critical solution as lambda(0) approaches the upper limit for blow-off. The IM alone is not capable for predicting the flow and requires boundary conditions at the top of the Knudsen layer. For small Kn(0), the scaled escape rate and energy loss rate are found to be independent of lambda(0). The simulation results can be scaled to any single-component atmosphere exhibiting blow-off if the external heating above the lower boundary is negligible, in particular, to sublimation-driven atmospheres of Kuiper belt objects.