Browsing by Author "Volkov, Alexey N."
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Item 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 TuscaloosaA 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.Item 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 TuscaloosaThe 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.Item Commissioning of high speed imaging system for rainbow schlieren measurements of vaporizing liquid fuel sprays(University of Alabama Libraries, 2016) Mirynowski, Eileen Marie; Bittle, Joshua A.; University of Alabama TuscaloosaThe fuel injection process has been studied since the internal combustion engine was developed. Direct injection has been an integral part to the success of diesel engines, where there is minimal time for the fuel to mix with the compressed air. The benefits of fuel injection center on: fuel efficiency and lower toxic emissions. As the world depletes more fossil fuels each year it is imperative to concentrate research on techniques to lower fuel consumption. Past research on fuel sprays using laser techniques were limited by cross sensitivity in regards to the regions with both liquid and vapor phases present. Quantitative schlieren techniques have been proposed and investigated since the first half of the 20th century, but only recently with the rapid development of digital imaging techniques and computers have they have been used for quantitative analysis. This thesis presents the results for a new hardware installation for a rainbow schlieren diagnostic method. Experiments were performed using a constant pressure flow vessel (CPFV) and a modern common rail diesel injector to obtain high-speed images of the vaporizing fuel sprays. The CPFV ran under steady ambient thermodynamic conditions where the pressure and temperatures were controlled variables. Two cameras were used, Mie scatter liquid phase data and the rainbow schlieren vapor phase data were captured simultaneously. Quantitative results indicate that the axial and radial variation in the fuel sprays seem to match the well-validated variable profile model.Item Development of a high-fidelity engine modeling framework in Simulink with automated combustion parameter tuning(University of Alabama Libraries, 2017) Thompson, Bradley Adam; Yoon, Hwan-Sik; University of Alabama TuscaloosaThe automotive industry continually seeks to improve performance and fuel efficiency due to increasing fuel costs, consumer demands, and greenhouse gas regulations. With advancements in computer-aided design, engine simulation has become a vital tool for product development and design innovation, and as computation power improves, the ability to optimize designs improves as well. Among the simulation software packages currently available, Matlab/Simulink is widely used for automotive system simulations but does not contain a detailed engine modeling toolbox. To leverage Matlab/Simulink’s capabilities, a Simulink-based 1D flow engine modeling architecture is proposed. The architecture allows engine component blocks to be connected in a physically representative manner in the Simulink environment, therefore reducing model build time. Each component model, derived from physical laws, interacts with other models according to block connection. The presented engine simulation platform includes a semi-predictive spark ignition combustion model that correlates the burn rate to combustion chamber geometry, laminar flame speed, and turbulence. Combustion is represented by a spherical flame propagating from the spark plug. To accurately predict the burn rate, the quasi-dimensional model requires tuning. A method is proposed for fitting turbulence and burn rate parameters across an engine’s operating space. The method reduces optimization time by eliminating the intake and exhaust flow models when evaluating the fitness function. Using the proposed method, 12 combustion model parameters were optimized to match cylinder pressure. Optimization and validation results are given for a 2.0 L Mazda Skyactiv-G engine.Item 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 TuscaloosaThe 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]Item 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 TuscaloosaA 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.Item 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 TuscaloosaComputational 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]Item Kinetic and Hydrodynamic Simulations of Laser Ablation and Plasma Plume Expansion Induced by Bursts of Short Laser Pulses(University of Alabama Libraries, 2020) Ranjbar, Omid A.; Volkov, Alexey N.; University of Alabama TuscaloosaAblation of materials by nanosecond laser pulses involves expansion of a laser-induced vapor plume into a background gas. The absorption of the incident laser radiation by the plume can substantially decrease the amount of laser energy absorbed directly by the target, and, correspondingly, the amount of the ablated material. This plasma shielding effect limits the overall efficiency of industrial laser systems designed for material removal applications. The goal of the present work is to numerically study the expansion process of plumes induced by irradiation of a metal target by bursts or groups of nanosecond laser pulses and to reveal the implications of the interaction between plumes induced by individual pulses for the efficiency of material removal. The plume expansion induced by irradiation of a copper target in argon background gas is studied based on one- and two-dimensional hybrid computational models that include a hydrodynamic or kinetic model of plasma plumes. The hydrodynamic model is based on finite-difference solution of gas dynamics equations. The kinetic model is implemented in the form of the direct simulation Monte Carlo (DSMC) method. In this work, the generalization of the DSMC method for plasma flows is developed. The effects of laser fluence, spot size, inter-pulse separation, and background gas pressure are thoroughly studied. The numerical simulations of plume expansion induced by a burst of pulses indicate the formation of complicated flow structures with cascades of the primary and secondary shock waves and strong interaction between plumes induced by individual pulses. The simulations reveal the plume accumulation effect when the plumes induced by preceding pulses in a burst change conditions of propagation of plumes generated by subsequent pulses. The degree of plasma shielding increases with increasing number of laser pulses due to the plume accumulation effect. It results in reduction of the effectiveness of material removal by the subsequent pulses. The degrees of the plasma shielding and plume accumulation effects strongly depend on the inter-pulse separation and laser spot size. The trade-off between the plume accumulation and thermal accumulation effects maximizes the ablation depth per pulse at a certain value of the time delay between pulses.Item 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 TuscaloosaUsing 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.Item 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 TuscaloosaThe 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]Item Laser Pulse Duration Is Critical For the Generation of Plasmonic Nanobubbles(American Chemical Society, 2014) Lukianova-Hleb, Ekaterina Y.; Volkov, Alexey N.; Lapotko, Dmitri O.; Rice University; University of Alabama TuscaloosaPlasmonic nanobubbles (PNBs) are transient vapor nanobubbles generated in liquid around laser-overheated plasmonic nanoparticles. Unlike plasmonic nanoparticles, PNBs' properties are still largely unknown due to their highly nonstationary nature. Here we show the influence of the duration of the optical excitation on the energy efficacy and threshold of PNB generation. The combination of picosecond pulsed excitation with the nanoparticle clustering provides the highest energy efficacy and the lowest threshold fluence, around 5 mJ cm(-2), of PNB generation. In contrast, long excitation pulses reduce the energy efficacy of PNB generation by several orders of magnitude. Ultimately, the continuous excitation has the minimal energy efficacy, nine orders of magnitude lower than that for the picosecond excitation. Thus, the duration of the optical excitation of plasmonic nanoparticles can have a stronger effect on the PNB generation than the excitation wavelength, nanoparticle size, shape, or other "stationary" properties of plasmonic nanoparticles.Item Mesoscale Structural and Mechanical Simulations of Cross-Linked Carbon Nanotube Materials(University of Alabama Libraries, 2021) Banna, MD Abu Horaira; Volkov, Alexey N.; University of Alabama TuscaloosaRelatively poor mechanical properties of carbon nanotube (CNT) bulk materials can be improved by formation of bonds or covalent cross-links (CLs) between nanotubes. In this work, an “effective bond model” of covalent CLs between carbon nanotubes is developed for mesoscopic simulations of cross-linked CNT materials. A general approach for fitting the CL model parameters based on results of atomistic simulations is developed. The best-fit parameters of the CL model are found. The developed effective bond model of CLs is included into a dynamic mesoscopic model of CNT materials, where each nanotube is represented in the form of a chain of stretchable cylindrical segments. The mesoscopic force field in this model accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube CLs. The model is applied to generate and equilibrate in silico pristine and cross-linked CNT fiber and film samples with structural characteristics close to observed in experiments. The structural parameters of CNT fibers and films, including the average bundle size, Herman orientation factor, and tortuosity, are calculated. The quasi-static simulations of large-scale cross-linked CNT films are performed to reveal the load transfer mechanism, as well as effects of CNT length, CL density, material density, and network morphology on mechanical properties under conditions of quasi-static deformation. It is found that stretching of CNT segments is the dominant mode of load transfer in cross-linked CNT film during their stretching, while bending and buckling is the dominant mode of load-transfer during compression. Both tensile modulus and strength of CNT films increase strongly with increasing CNT length. The effect of the nanotube length on mechanical properties, however, is altered by the density of CLs. The mutual effect of the nanotube length and CL density on modulus and strength is described by power scaling laws, where the modulus and strength are functions of the average number of CLs per nanotube, i.e., the product of the CNT length and CL linear density. The exponents in the scaling laws for the modulus and strength are strongly different from each other. The material density of the film samples weakly affects the specific mechanical properties. The dispersion of nanotubes in the films without formation of thick bundles results in the few-fold increase of the modulus and strength. In qualitative agreement with experimental observations, the in-plane compression of a large thin CNT film results in collective bending of nanotubes and folding of the whole film with minor irreversible structural changes. Depending on the CNT length, the reliefs of the compressed films vary from quasi-one-dimensional wavy surface to complex two-dimensional landscape.Item Microstructural and mechanical characterizations of metallic parts made by powder-bed fusion additive manufacturing technologies(University of Alabama Libraries, 2017) Wang, Xiaoqing; Chou, Y. Kevin; Volkov, Alexey N.; University of Alabama TuscaloosaTwo typical powder-bed additive manufacturing (AM) technologies are selective laser melting (SLM) and electron beam additive manufacturing (EBAM). Due to the complex thermal history and the interactions among the thermal, mechanical, and metallurgical phenomena during the manufacturing process, further study is needed to comprehensively understand the manufacturing process and achieve finishing parts with desired properties for application in the industries. The primary objectives of this research are: (1) investigate the effects of the beam scanning speed and support structure in the EBAM; (2) determine the influences of build height and thermal cycles in the SLM; (3) estimate the distribution of the induced residual stress; and (4) model the microstructural evolution of Inconel 718 in the SLM. To achieve the research objectives, microstructure characterization technologies including optical microscopy, scanning electron microscopy, Energy-dispersive X-ray spectroscopy, and electron backscattered diffraction have been utilized, and nanoindentation and Vickers indentations were used to evaluate their mechanical properties. In addition, Vickers indentation method was adopted to estimate the residual stress, and the phase field method was developed to model the microstructural evolution. Based on the results obtained, it is found that (a) a typical columnar and equiaxed microstructure were observed in the X-Plane (side surface) and Z- Plane (Scanning Surface) of the AM parts, respectively. The γ phase of Inconel 718 presented a distinct {0 0 1} texture in the Z-plane and a strong {1 0 1} texture in the Y-plane. The α phase in Ti-6Al-4V had relatively weak textures of 〈0 0 0 1〉 and 〈1 1 2 ̅ 0〉 parallel to the z-axis. (2) Fine colonies of cellular dendrites with a cell spacing of 0.511 ~ 0.845 μm were observed in the Inconel 718, which implied a cooling rate of 1.74 ~ 3.88×〖10〗^7 K∙s^(-1) (°C ∙s^(-1)). (3) With increase of the build height, the width of the columnar structure increased from 75 μm till a stable state around 150 μm, and then slightly decreased to 112 μm when closing to the ending process, which results from the variation of the thermal gradient along the build height . (4) Under continue effects of thermal cycles, the morphology of Laves phase changed from coarse and interconnected irregular particles to discrete particles, and the maximum intensity of the texture also increased. (5) Equiaxed grains were formed at the bottom of the overhang region and then translated into wider columnar structures. The solid-gas support structure acted as a heat sink to enhance heat transfer and provided support for the overhang to avoid the occurrence of sink phenomena. (6) The phase field method is a powerful tool for simulation of microstructure evolution in SLM process. The manufacturing parameters significantly affected the thermal gradient which plays an important role in the dendrite growth and a larger temperature gradient resulted in a higher growth speed. Most of the columnar cellular dendrites have a preferred growth direction of 45-72° to the scanning plane. (7) The residual stress is unevenly distributed in the parts with no notable difference in the X-plane and Y-plane. The beam scanning speed and the build height did not show significant effects on the residual stress, while the right angle interface of the geometry induced a stress concentration. (8) The mechanical properties of the parts are comparable with the count-parts made by traditional methods. (9) The volume fraction of the porosity is below 2%, and no remarkable effects were found from the thermal cycles and the build height.Item Microstructure and mechanical properties of additively manufactured parts with staircase feature(University of Alabama Libraries, 2017) Keya, Tahmina; Chou, Y. Kevin; Volkov, Alexey N.; University of Alabama TuscaloosaThis thesis focuses on a part with staircase feature that is made of Inconel 718 and fabricated by SLM process. The objective of the study was to observe build height effect on the microstructure and mechanical properties of the part. Due to the nature of SLM, there is possibility of different microstructure and mechanical properties in different locations depending on the design of the part. The objective was to compare microstructure and mechanical properties from different location and four comparison groups were considered: 1. Effect of thermal cycle; 2. External and internal surfaces; 3. Build height effect and 4. Bottom surfaces. To achieve the goals of this research, standard metallurgical procedure has been performed to prepare samples. Etching was done to reveal the microstructure of SLM processed Inconel 718 parts. Young’s modulus and hardness were measured using nanoindentation technique. FEM analysis was performed to simulate nanoindentation. The conclusions drawn from this research are: 1. The microstructure of front and side surface of SLM processed Inconel 718 consists of arc shaped cut ends of melt pools with intermetallic phase at the border of the melt pool; 2. On top surface, melted tracks and scanning patterns can be observed and the average width of melted tracks is ~ 100-150 µm; 3. The microstructure looks similar at different build height; 4. Microstructure on the top of a stair is more defined and organized than the internal surface; 5. The mechanical properties are highest at the bottom. OM images revealed slight difference in microstructure in terms of build height for this specific part, but mechanical properties seem to be vary noticeably. This is something to be kept in mind while designing or determining build orientation. External and internal surfaces of a stair at the same height showed difference in both microstructure and mechanical properties. To minimize that effect and to make it more uniform, gradual elevation can be considered when suitable as far as design modification is concerned. Above all, this study reveals important information about the pattern of microstructure, thus heat transfer mechanism inside a part which is useful to understand the SLM process.Item MOLECULAR-KINETIC SIMULATIONS OF ESCAPE FROM THE EX-PLANET AND EXOPLANETS: CRITERION FOR TRANSONIC FLOW(IOP Publishing, 2013-12-06) Johnson, Robert E.; Volkov, Alexey N.; Erwin, Justin T.; University of Virginia; New York University; University of Alabama TuscaloosaThe equations of gas dynamics are extensively used to describe atmospheric loss from solar system bodies and exoplanets even though the boundary conditions at infinity are not uniquely defined. Using molecular-kinetic simulations that correctly treat the transition from the continuum to the rarefied region, we confirm that the energy-limited escape approximation is valid when adiabatic expansion is the dominant cooling process. However, this does not imply that the outflow goes sonic. Rather large escape rates and concomitant adiabatic cooling can produce atmospheres with subsonic flow that are highly extended. Since this affects the heating rate of the upper atmosphere and the interaction with external fields and plasmas, we give a criterion for estimating when the outflow goes transonic in the continuum region. This is applied to early terrestrial atmospheres, exoplanet atmospheres, and the atmosphere of the ex-planet, Pluto, all of which have large escape rates.Item Numerical modeling of laser-induced melting and drilling of bulk and powder metals using combined smoothed particle hydrodynamics and ray tracing method(University of Alabama Libraries, 2020-10) Shah, Deepak; Volkov, Alexey N.; University of Alabama TuscaloosaA smoothed particle hydrodynamics (SPH) numerical method is first developed to solve thermal transport problems in heterogeneous materials with finite thermal contact conductance and discontinuous temperature field at interfaces between individual grain-like or fiber-like particles. It is applied to study thermal transport and calculate effective conductivity in a linear chain of powder grains, two-dimensional, and three-dimensional random powder systems composed of spherical grains and high-aspect-ratio spherocylinders, and in a nanocomposite material with carbon nanotubes and polymer matrix. In all cases, the numerical errors are found to be sufficiently small at relatively large spacing between SPH particles. The SPH method is further developed to account for melting and solidification of the target material based on the enthalpy formulation and major interfacial effects, including surface tension force, Marangoni stresses, recoil force due to vapor pressure, mass removal, and evaporative cooling and is coupled with the ray tracing (RT) method to simulate the propagation, multiple reflections and absorption of incident laser radiation. The SPH-RT method is used to investigate the melt pool expulsion mechanism during high aspect ratio laser drilling of aluminum, and 316L stainless steel bulk targets. The drilling velocity is found to be non-linear which is dependent on the shape of the heating surface. The simulations reveal very strong effect of multiple reflections of laser radiation inside the keyhole on the drilling velocity. The main driving force of melt expulsion is the repulsive force produced by vapor pressure, while the Marangoni stresses only marginally affect the drilling velocity. The spattering of melt pool occurs at high laser intensities which is well captured by SPH-RT method. For small intensity a steady state is achieved where the incident laser energy is balanced by the thermal conduction and evaporative cooling and hence the drilling stops. The SPH-RT method is also used to study the drilling of copper with bursts of nanosecond laser pulses. The ablated material, surface temperature, and cavity depth after first pulse match with the solution obtained with the finite-difference solution of the heat conduction equation. The rate of ablation increases with number of laser pulses.Item Numerical modeling of laser-induced plumes and jets(University of Alabama Libraries, 2019) Palya, Austin; Volkov, Alexey N.; University of Alabama TuscaloosaThe goal of the current work is to perform numerical modeling and identify important phenomena associated with vapor plume and jet flows induced by irradiation of metal targets with short pulse or continuous wave (CW) lasers, discover and explain the mechanisms responsible for vaporized material motion in applications of lasers for material processing and analysis such as deep laser drilling, laser-induced breakdown spectroscopy (LIBS), and selective laser melting (SLM) of metallic powders. The simulations of laser-induced vapor expansion into a background gas are performed with a combined computational model, including the thermal model of irradiated targeted and a kinetic model of multi-component gas flows. The latter is implemented for simulations in the form of the Direct Simulation Monte Carlo method. Based on this model, two major problems are considered. In the first problem, vapor plume expansion under conditions of spatial confinement when the plume, which is induced by irradiation of a copper target by a short-pulse laser, propagates inside a cavity or trench, is considered. The simulations identify two major effects, the focusing effect, appearing due to transient motion of shock waves inside the cavity, and the confinement effect, induced due to overall deceleration of the plume with increasing background gas pressure, as two major mechanisms affecting removal of vaporized material out of the cavity and formation of high-density and high-temperature regions in the plume core. Due to the trade-off between these effects, an optimum background gas pressure exists, when the efficiency of the vapor removal from the cavity is maximized. At later stages of plume expansion, the simulations also reveal a suction effect, when the vapor flow at the cavity throat can be temporarily directed into the cavity, inducing a decrease of the overall efficiency of vapor removal. The balance between the focusing and confinement effects is studied in a range of the background gas pressure, for various background gas species, and various geometrical parameters of the cavity and laser beam. It is also shown that application of double laser pulses with short inter-pulse separation can be beneficial for both laser drilling and LIBS. In the second problem, a vapor jet induced by irradiation of a stainless steel target by a CW laser is simulated under conditions specific for SLM of metallic powders. The generated vapor jet is then used to predict motion of powder particles that can be efficiently entrapped into the ambient gas flow induced by the vapor jet. It is shown that powder particles in a broad range of their diameters can be efficiently entrained into the gas flow and, thus, removed from the irradiated surface. These results are in agreement with experimental observations of the surface denudation effect in SLM.Item Numerical Modelling of the Magnus Force and the Aerodynamic Torque on a Spinning Sphere in Transitional Flow(2007) Volkov, Alexey N.; University of Alabama TuscaloosaThree 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.Item 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 TuscaloosaThe 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.Item 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 TuscaloosaThermally 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.