Development of a lumped capacitance model for heat transfer from a nanoparticle suspended in a fluids medium

The purpose of this study is to examine the feasibility of using a lumped capacitance approach to model the transient heat transfer behavior from a nanoparticle suspended in a fluid. Since the Cattaneo-Vernotte (CV) constitutive model involves solving using complex numerical procedures a new model is developed in this study that can give a simple closed form solution to heat transfer problems when nanoscale regimes are considered. A nanoscale lumped capacitance model is developed by modifying Cattaneo-Vernotte (CV) constitutive relation. The CV relation, which is a hyperbolic heat equation, is based on the correction made to the assumption of infinite speed of heat propagation proposed in Fourier law. An electrical analogy of the proposed nanoscale lumped capacitance model has been developed and solved using the electrical design software. It is observed that the Vernotte number plays an important role in nanoscale heat transfer problems. Since Vernotte number is very low for common macroscale structure it is neglected in classical lumped capacitance model but it should be included when heat transfer at nanoscale levels is considered because of higher Vernotte numbers. The results of nanoscale model differs significantly with that of classical lumped capacitance model for higher values of Vernotte number and as the Vernotte number approaches zero the nanoscale model returns to the classical lumped capacitance model as expected. The accuracy of this model is still in question and needs to be validated with experimental data and/or by comparison with a computational model employing the CV constitutive relation before drawing any further conclusions on the correctness of the model developed in this study. (Published By University of Alabama Libraries

Investigation of supersonic flow over an axisymmetric protuberance

The behavior of a supersonic fluid flow field over an axisymmetric cylindrical body has been simulated using ANSYS Fluent computational fluid dynamics software. The simulated body was altered for successive simulations to incorporate a protuberance interrupting the one dimensional flow along the surface. The effect of introducing this protuberance was investigated for Mach numbers ranging from 1.5 to 5.5 in increments of 0.5. To account for flow characteristics not associated with an infinite wall simulation, the body was modeled as a 2.5 caliber Tangent Ogive nose cone profile leading a cylindrical wall in free stream conditions. Once the initial shock is formed, the flow downstream is investigated for the behavior it exhibits in the presence of a surface protuberance. The surface protuberance will cause the flow to wrap around the curvature of the surface. This abrupt change in direction and redirection results in a separation in the flow structure from the wall, which creates a region known as a recirculation zone. Flow becomes discontinuous due to the turning angle of the surface and therefore detaches and must resume parallel flow farther past the protuberance. Flow reattachment downstream of such an obstacle is determined in this research in order to develop a relationship between flow speed, protuberance geometry and recirculation region length. Pressure distributions along the surface beyond the separation point caused by the protuberance angle were used to find the reattachment length. Each pressure distribution contained a point where the rate of change leveled off from the initial chaotic region following the protuberance apex. These points were considered the beginning of the reattachment region and thus the end of the separation region. Correlations for such behavior as a function of the protuberance length scale and Mach number are developed and discussed. The dominance of the Mach number influence in the reattachment length of the separation region is apparent in the final data analysis, as well as a negative correlation of increasing protuberance height. It is apparent that an increased Mach number results in a longer reattachment length. However, as the aspect ratio of the protuberance increases, the reattachment length decreases. (Published By University of Alabama Libraries

Variation in mechanical behavior due to different build directions of Ti6Al4V fabricated by electron beam additive manufacturing technology

Titanium has always been a metal of great interest since its discovery especially for critical applications because of its excellent mechanical properties such as light weight (almost half of that of the steel), low density (4.4 gm/cc) and high strength (almost similar to steel). It creates a stable and adherent oxide layer on its surface upon exposure to air or water which gives it a great resistance to corrosion and has made it a great choice for structures in severe corrosive environment and sea water. Its non-allergic property has made it suitable for biomedical application for manufacturing implants. Having a very high melting temperature, it has a very good potential for high temperature applications. But high production and processing cost has limited its application. Ti6Al4V is the most used titanium alloy for which it has acquired the title as `workhouse' of the Ti family. Additive layer Manufacturing (ALM) has brought revolution in manufacturing industries. Today, this additive manufacturing has developed into several methods and formed a family. This method fabricates a product by adding layer after layer as per the geometry given as input into the system. Though the conception was developed to fabricate prototypes and making tools initially, but its highly economic aspect i.e., very little waste material for less machining and comparatively lower production lead time, obviation of machine tools have drawn attention for its further development towards mass production. Electron Beam Melting (EBM) is the latest addition to ALM family developed by Arcam, AB® located in Sweden. The electron beam that is used as heat source melts metal powder to form layers. For this thesis work, three different types of specimens have been fabricated using EBM system. These specimens differ in regard of direction of layer addition. Mechanical properties such as ultimate tensile strength, elastic modulus and yield strength, have been measured and compared with standard data available. Besides, these values have been compared among themselves in order to understand the effect of anisotropy due to the production nature. Nano-hardness at local points on the specimens' bodies has also been tested for comparison. (Published By University of Alabama Libraries

Dynamic response analysis of a flexible flag-like wind energy harvester

A flexible, flag-like wind energy harvester made from metallized PVDF material is introduced. A mathematical model for the piezoelectric flag excited by wind flow is developed. The model is validated with experimental results. The flag is modeled as an Euler-Bernoulli beam with a low bending rigidity. The wind excitation is modeled using the slender-wing theory. Among other modeling assumptions, the partial differential equation is considered separable and is subjected to two different sets of boundary conditions that are applied to find the mode shapes of the system. The temporal relationship for the electromechanical system is normalized with the resultant eigenfunction. The experimental investigations are performed with longitudinal flow along a PVDF flag in a wind tunnel. Experimental and analytical results match and represent the periodic system response of voltage, power and displacement over time. The two sets of boundary conditions applied to the model provide an envelope for predicting the outputs of the system. Results show that for both sets of boundary conditions, the frequency of the system has a nearly linear relationship to the wind velocity. (Published By University of Alabama Libraries

Development of robotic lower-limb prosthetic and orthotic devices

Human lower limbs serve important biomechanical functions, e.g., supporting human body weight, providing power for locomotion, etc. A large number of individuals, however, live with the impairment of such functions. Such functional impairment may be associated with limb loss caused by amputation, or functional degeneration associated with aging or musculoskeletal/neural pathologies (e.g. stroke). Consequently, such individuals are more likely to suffer from impaired mobility and live in a sedentary lifestyle, seriously affecting their independence and quality of life. The research in this thesis seeks to address this problem through the development of actively-powered robotic assistive devices that restore or augment the lost or weakened limb functions. Specifically, two assistive devices are presented, including a robotic knee orthosis that assists the wearer’s knee motion in sit-to-stand transfer and other locomotive functions, and a robotic transtibial prosthesis that restores the lost ankle-foot functions for below-knee amputees. In the development of these devices, the design specifications were determined according to the desired locomotive functions and related biomechanical data. Subsequently, the actuation approaches were selected, along with the corresponding actuation mechanisms. The devices have been proven to provide the desired kinematic and kinetic performances through detailed analysis. In the following detailed design phase, 3D solid models were created for the robotic systems and individual components. Finite-Element Analysis was also performed to ensure the strength and rigidity of the structure under load. Finally, prototypes of the devices were fabricated and assembled, and experiments have been devised to measure their kinetic performances in use. (Published By University of Alabama Libraries

Fatigue behavior and structural stress analysis of coach-peel and lap-shear friction stir welded joints of AZ31 magnesium alloy

In this work the fatigue behavior of coach-peel and lap-shear friction stir linear welded joints of AZ31 magnesium alloy sheet were evaluated under different loads and the results were compared using structural stress analysis. Lap-shear coupons of 30 mm in width were obtained from a welded overlap configuration. However, since coach-peel configuration coupons were not available, aluminum L-shaped brackets were adhesed weld to create the coach-peel configured specimen. The experimental fatigue life results showed an inverse relationship between applied load and the number of cycles to failure for both types of coupons. However, due to differences in the configuration of the joint leading to higher applied stress, coach-peel coupons failed at comparatively lower loads than lap-shear coupons. In order to correlate the fatigue data with stresses developed at the weld, finite element analysis, using shell/plate elements, was employed to model the joints under cyclic loading. The modeling effort focused on several post-process techniques including equivalent stress and structural stress methods. While the structural stress approach has merits over conventional nominal equivalent stress approach for welded joints including accuracy of the results and mesh insensitivity, it could not correlate the lap-shear and coach-peel fatigue results into a master design curve. As such, a fatigue damage parameter (FDP) was introduced to correlate the fatigue results of both the joints, thus establishing a basic relation to account for differences in stress at the weld for lap-shear and coach-peel welded configurations. (Published By University of Alabama Libraries

Three level innovation of the powered above elbow prosthetic arm using pneumatic artificial muscles

This thesis presents a novel above elbow prosthetic arm design, innovating on three different levels: the actuation method, system integration, and the mechanism design. The Chemo-Muscle actuation system expands pneumatic artificial muscles to mobile applications. The integrated design makes use of the internal dead space of the muscle, incorporating external components and reducing fuel consumption. A novel design is presented using smaller muscles to control multiple degrees of freedom, providing 50% more strength than other prosthetic arms and an excellent range of motion for each degree of freedom. A simple control method is proven and the future development is discussed. (Published By University of Alabama Libraries

A comparison of mechanical models for the viscoelastic response of human breast carcinomas

The mechanical response of a living cell is notoriously complicated. The complex, heterogeneous characteristics of cellular structure introduce difficulties that simple linear models of viscoelasticity cannot overcome, particularly at moderate indentation depths. Herein, a nano-scale stress-relaxation analysis performed with an Atomic Force Microscope reveals that isolated human breast cells do not exhibit simple exponential relaxation capable of being modeled by the Standard Linear Solid (SLS) model. Therefore, this work proposes the application of a progression of more sophisticated models that may extract the mechanical parameters from the entire relaxation response, improving upon existing physical techniques to probe isolated cells. The first model under consideration is the Generalized Maxwell (GM) model that distributes the response of the cell across multiple time scales in an attempt to replicate the interaction of subcellular components. The second is a fractional model that operates without a priori assumptions of the cell's internal structure and describes the fractional time-derivative dependence of the response. The results show an exceptional increase in conformance to the experimental data compared to that predicted by the SLS model. Both models excel at mapping the relaxation behavior of the cells that occurs within a few seconds of the initial force. This area is generally ignored with an SLS fit and therefore not included in most cell differentiation studies. The results of the GM model show a significant change in the mechanical properties of the first relaxation mode, which validates the necessity of the early behavior's inclusion. The FZ model preserves the distinctions highlighted in the SLS model, but also incorporates the disparity in the early-relaxation times seen in the GM model as a change in the composite relaxation time. (Published By University of Alabama Libraries

Experimental investigation of a magnetic induction pebble-bed heater with application to nuclear thermal propulsion

NASA explored the idea of nuclear thermal rockets in the 1950's and 60's and has recently shown interest in reviving the nuclear rocket program in an attempt to reach manned mission to Mars by 2035. One problem with nuclear rockets is finding ways to test them inside the atmosphere. NASA's Stennis Space Center has considered using a non-nuclear device to simulate a nuclear reactor during testing. The reactor is responsible for heating the propellant to over 1,922 K (3,000 °F), so the reactor simulator should be capable of heating to this temperature. A pebble-bed heater at Glenn Research Center was used for nuclear rocket testing in the past; however, the device no longer exists. This particular pebble-bed heater used hot gases to heat the pebble bed made of high melting temperature ceramics and was able to reach 2,755 K (4,500 °F) but could only sustain the temperature for 30 seconds at most. If the pebbles were heated by magnetic induction, then heat would consistently be generated within the heater, and tests could run longer. Magnetic induction heats a ferrous metal by inducing a current on its surface and by rapidly reversing a magnetic field surrounding the metal. Unfortunately, it was found that a magnetic induction pebble-bed heater using steel could not reach 1,922 K (3,000 °F) due to the Curie and melting temperatures. However, the device could be used if a higher melting temperature metal was found that was also magnetic. A small-scale pebble-bed heater heated by magnetic induction was designed, built, and tested to analyze its behavior at 27 different combinations of flow rates, pebble sizes, and power levels. The temperature changes were recorded for each test. With this data, a relationship between dimensionless heat transfer, dimensionless power, and Reynolds number was found. (Published By University of Alabama Libraries

A finite element analysis of microcutting mechanics on surface integrity and white layer formation in hard machining

Precision hard milling AISI H13 steel and AISI 52100 steel finds wide applications in mold and die industries. Surface integrity of machined surfaces is critical to component performance of fatigue and tribology. It has shown that micro cutting edge geometry has dominant effects on process efficiency and surface integrity. Therefore, a basic understanding of the cutting edge/workpiece interactions is critical for design of a cutting process for optimal product performance. Many experiments have been performed to understand the cutting edge/workpiece interactions by revealing the unique cutting mechanics and surface integrity due to the size effect of micro cutting edge. However, the cutting edge/workpeice contact zone is too small to study using cutting experiments. In this study the cutting edge/workpiece contact has been modeled with different cutting tool geometry. A 3D finite element simulation of cutting edge/workpiece was performed to study the material flow and frictional behaviors around the cutting edge in hard milling H13 steel. 2D finite element models have been developed to predict the cutting temperature penetration in the subsurface and the white layer thickness in hard machining AISI H13 and 52100 steel. The simulations results have been reasonably correlated with the experimental data. (Published By University of Alabama Libraries