Papers by James D.B. Wang
In this thesis, the theoretical and the experimental approaches were performed to study the trans... more In this thesis, the theoretical and the experimental approaches were performed to study the transitional characteristics of aerodynamic coefficients and flow field of the NACA
633-018 airfoil at Re ranging from 1.32×10E4 ~ 1.09×10E5. For theoretical approach, the catastrophe theory was applied to model the aerodynamic transition problem and to predict
some aerodynamic characteristics undiscovered before. The so-called catastrophe theory describes the discontinuous phenomena with continuous functions based on the study of
singularities of nonlinear functions. Results of aerodynamic loading measurement well verified the validity of the catastrophe theory on the aerodynamically transitional problems and flow visualization provided some physical essences of the flow field about the theory employed here. Finally, the aerodynamic transition of the airfoil was redefined and reexamined through the cusp catastrophe model.
Combining the mathematical model and the experimental results, it can be deduced that the transition and sudden change of aerodynamic coefficients, in essence, is a
phase-changing process of the topological structure of the flow field. The predicted sudden-changing points of the aerodynamic coefficients match very well with the experimental results within the error margin of α less than 0.5 degree and Re less than two thousand. It indicates that the lift or drag and Re as well as α actually constitutes a cubic hypersurface relationship during the transitional process, which is believed to be a new direction of related aerodynamic researches.
In order to prevent the spurious wave reflections and to improve the computational
efficiency in ... more In order to prevent the spurious wave reflections and to improve the computational
efficiency in nanomechanical simulation, this dissertation performs a series of
theoretical/numerical studies on the crystalline solid material, including nanomechanics of
monatomic lattice, isothermally non-reflecting boundary condition, fast updating of neighbor
list, and the application/simulation in laser-assisted nano-imprinting.
Firstly, the nanomechanical and statistical behavior of monatomic crystal lattice are
introduced, which is the foundation for the development of non-reflecting boundary condition
and the simulation of corresponding nano materials. Based on the assumptions and the
harmonic approximation for the utilized inter-atomic potential, the equation of motion as well
as dispersive relation will be given. Furthermore, from the statistical properties of phonon
and lattice energy, the thermal amplitude can be calculated and lattice temperature can be
defined, which covers the result previously derived by other researchers at high temperatures.
Method of Time-History-Kernel (THK) and method of absorbing boundary layer (ABL)
are two main philosophies to derive the generalized Langevin equation, which is a general
form of non-reflecting boundary conditions. The general concepts of THK method, including
the formulation, kernel generation, time convolution, application assessments and numerical
verification, are given. Based on the Crump’s method to express the THK function in the
exponential form, a recursive algorithm for THK time-convolution has been proposed to reduce the computational cost from O(N 2 ) down to O(N). By applying the THK method
in lattice relaxation and the system under external forcing or heating, it is demonstrated that
the THK method indeed provides an isothermally non-reflecting boundary condition in
nanomechanical computation.
In ABL methods, two major mapping formulations are proposed and the corresponding
performances are discussed analytically as well as verified numerically. Starting from the
ω -mapping formulation, a series of extensive ABL methods are developed and investigated
to improve the performance of wave absorption, and one ABL method is recommended due
to its lower reflection at low frequency. Finally, the general comparison for all non-reflecting
boundary condition is addressed.
Based on a rigorous definition of Verlet radius with respect to temperature and
list-updating interval, this study gives an estimation formula of computation time, with which
the best algorithm can be chosen according to different total number of atoms, system
average density and system average temperature in the nanomechanical system. It has been
shown that the Verlet Cell-linked List (VCL) algorithm is better than other algorithms for a
system with a large number of atoms. Furthermore, a generalized VCL (GVCL) algorithm
optimized with a list-updating interval and cell-dividing number is analyzed and shows the
reduction of the computation time by 30% ~ 60%, which is verified by the
molecular-dynamics simulation for a two-dimensional system.
Finally, the molecular dynamics simulation accompanied with the isothermally
non-reflecting boundary condition is performed to analyze the related material physics in
laser-assisted nano-imprinting. Results show that the implemented boundary condition relax
the lattice well, and eventually absorb the wave propagation of momentum/energy during
heating and imprinting process. Besides, the temperature/force evolution in substrate, effect
of the molding-demolding interval, and surface situation of mold are discussed
This article experimentally investigated the evolution of the aerodynamic performance parameters ... more This article experimentally investigated the evolution of the aerodynamic performance parameters and flow field of a two-dimensional airfoil called NACA 63 3 -018 with Reynolds numbers ranging from 1.32×10 4 to 1.09×10 5 . Measurement results show that both maximum lift coefficient and minimum drag coefficient rise while the maximum lift-to-drag ratio deteriorates with decreasing Reynolds number with two different slopes. Additionally, flow visualization pointed out that these two different regions correspond to two different topological structures of streamlines around the airfoil.
This paper studies the aerodynamic performance of a finite wing at low Reynolds numbers through w... more This paper studies the aerodynamic performance of a finite wing at low Reynolds numbers through wind tunnel experiments. The 3-component force/moment balance and surface oil-flow technique are respectively used to investigate the aerodynamic performance and flow field characteristics about the wing. Results indicate that three different Reynolds number modes of the aerodynamic performance are found as functions of the Reynolds number and the angle of attack in the operating ranges. That is, the subcritical, critical and supercritical behavior of the aerodynamic performance of the wing can be characterized in terms of the Reynolds number and angle of attack. For the tested wings of lower aspect ratio, the 3-dimensional, tip-vortex effect is more significant in influencing the aerodynamic performance, and this is also witnessed by the oil-flow visualization.
Laser-assisted direct imprinting (LADI) technique has been proposed to utilize an excimer laser t... more Laser-assisted direct imprinting (LADI) technique has been proposed to utilize an excimer laser to irradiate and heat up the substrate surface through a highly-transparent quartz mold preloaded on this substrate for micro-to nano-scaled structure fabrications. While the melting depth and molten duration are key issues to achieve a satisfactory imprinting pattern transfer, many material property issues such as crystalline phase alteration, grain size change and induced film stress variation are strongly affected by transient thermal response. With onedimensional simplification as a model for the LADI technique, the present paper has successfully derived an analytical solution for the arbitrary laser pulse distribution to predict the relevant imprinting parameters during the laser induced melting and solidification processes. The analytical results agree quite well with the experimental data in the literature and hence can be employed to further investigate the effects of LADI technique from laser characteristics (wavelength, fluence and pulse duration) and substrate materials (silicon and copper) on the molten duration, molten depth and temperature distributions. Three kinds of excimer laser sources, ArF (193 nm), KrF (248 nm) and XeCl (308 nm) were investigated in this study. For the silicon substrate, the melting duration and depth were significantly dictated by the wavelength of laser used, indicating that employing the XeCl excimer laser with longer pulse duration (30 ns in the present study) will achieve the longest molten duration and deepest melting depth. As for the copper substrate, the melting duration and depth are mainly affected by the laser pulse duration; however, the wavelength of laser still plays an insignificant role in LADI processing. Meanwhile, the laser fluence should properly be chosen, less than 1.4 J/cm 2 herein, so as to avoid the substrate temperature exceeding the softening point of the quartz mold ($1950 K) and to make sure that the mold can still maintain the original features.
The melting duration and molten depth are key information for laser-assisted direct imprinting (L... more The melting duration and molten depth are key information for laser-assisted direct imprinting (LADI), which raises the issue of the melting and solidification induced by an excimer pulse laser shining through unilaterally transparent binary materials. Based on the matured laser annealing analysis, the thermal contact resistance is taken into account to simulate and predict the melting behavior for this process. The results in this study indicate that the laser annealing case and the perfect-contact case provide the upper-bound as well as the lower-bound values for the physical quantities involved in this process even, without detailed information about the evolution of thermal contact resistance during the heating and cooling process. Sensitivity analysis of the thermal contact resistance is investigated herein and could be employed to optimize fabrication parameters such as pulse duration and the fluence of the laser.
Establishing the neighbor list to efficiently calculate the inter-atomic forces consumes the majo... more Establishing the neighbor list to efficiently calculate the inter-atomic forces consumes the majority of computation time in molecular dynamics (MD) simulation. Several algorithms have been proposed to improve the computation efficiency for short-range interaction in recent years, although an optimized numerical algorithm has not been provided. Based on a rigorous definition of Verlet radius with respect to temperature and list-updating interval in MD simulation, this paper has successfully developed an estimation formula of the computation time for each MD algorithm calculation so as to find an optimized performance for each algorithm. With the formula proposed here, the best algorithm can be chosen based on different total number of atoms, system average density and system average temperature for the MD simulation. It has been shown that the Verlet Cell-linked List (VCL) algorithm is better than other algorithms for a system with a large number of atoms. Furthermore, a generalized VCL algorithm optimized with a list-updating interval and cell-dividing number is analyzed and has been verified to reduce the computation time by 30 ∼ 60% in a MD simulation for a two-dimensional lattice system. Due to similarity, the analysis in this study can be extended to other many-particle systems.
Laser-assisted direct imprinting (LADI) is an etching-free and high throughput fabrication proces... more Laser-assisted direct imprinting (LADI) is an etching-free and high throughput fabrication process in a submicron scale, though complete theoretical modeling and physical analysis have not yet been investigated. The present study formulates, analyzes and discusses the laser-induced melting, elastic stretching and low-Reynolds number flow during a LADI fabrication process. With proper assumptions and simplifications for the proposed mathematical modeling, the involved physics has been well tackled and the theoretical prediction as well as engineering optimization can be efficiently obtained. The predicted melting duration and imprinting depth show good agreements with the experimental measurement performed in our group.
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Papers by James D.B. Wang
633-018 airfoil at Re ranging from 1.32×10E4 ~ 1.09×10E5. For theoretical approach, the catastrophe theory was applied to model the aerodynamic transition problem and to predict
some aerodynamic characteristics undiscovered before. The so-called catastrophe theory describes the discontinuous phenomena with continuous functions based on the study of
singularities of nonlinear functions. Results of aerodynamic loading measurement well verified the validity of the catastrophe theory on the aerodynamically transitional problems and flow visualization provided some physical essences of the flow field about the theory employed here. Finally, the aerodynamic transition of the airfoil was redefined and reexamined through the cusp catastrophe model.
Combining the mathematical model and the experimental results, it can be deduced that the transition and sudden change of aerodynamic coefficients, in essence, is a
phase-changing process of the topological structure of the flow field. The predicted sudden-changing points of the aerodynamic coefficients match very well with the experimental results within the error margin of α less than 0.5 degree and Re less than two thousand. It indicates that the lift or drag and Re as well as α actually constitutes a cubic hypersurface relationship during the transitional process, which is believed to be a new direction of related aerodynamic researches.
efficiency in nanomechanical simulation, this dissertation performs a series of
theoretical/numerical studies on the crystalline solid material, including nanomechanics of
monatomic lattice, isothermally non-reflecting boundary condition, fast updating of neighbor
list, and the application/simulation in laser-assisted nano-imprinting.
Firstly, the nanomechanical and statistical behavior of monatomic crystal lattice are
introduced, which is the foundation for the development of non-reflecting boundary condition
and the simulation of corresponding nano materials. Based on the assumptions and the
harmonic approximation for the utilized inter-atomic potential, the equation of motion as well
as dispersive relation will be given. Furthermore, from the statistical properties of phonon
and lattice energy, the thermal amplitude can be calculated and lattice temperature can be
defined, which covers the result previously derived by other researchers at high temperatures.
Method of Time-History-Kernel (THK) and method of absorbing boundary layer (ABL)
are two main philosophies to derive the generalized Langevin equation, which is a general
form of non-reflecting boundary conditions. The general concepts of THK method, including
the formulation, kernel generation, time convolution, application assessments and numerical
verification, are given. Based on the Crump’s method to express the THK function in the
exponential form, a recursive algorithm for THK time-convolution has been proposed to reduce the computational cost from O(N 2 ) down to O(N). By applying the THK method
in lattice relaxation and the system under external forcing or heating, it is demonstrated that
the THK method indeed provides an isothermally non-reflecting boundary condition in
nanomechanical computation.
In ABL methods, two major mapping formulations are proposed and the corresponding
performances are discussed analytically as well as verified numerically. Starting from the
ω -mapping formulation, a series of extensive ABL methods are developed and investigated
to improve the performance of wave absorption, and one ABL method is recommended due
to its lower reflection at low frequency. Finally, the general comparison for all non-reflecting
boundary condition is addressed.
Based on a rigorous definition of Verlet radius with respect to temperature and
list-updating interval, this study gives an estimation formula of computation time, with which
the best algorithm can be chosen according to different total number of atoms, system
average density and system average temperature in the nanomechanical system. It has been
shown that the Verlet Cell-linked List (VCL) algorithm is better than other algorithms for a
system with a large number of atoms. Furthermore, a generalized VCL (GVCL) algorithm
optimized with a list-updating interval and cell-dividing number is analyzed and shows the
reduction of the computation time by 30% ~ 60%, which is verified by the
molecular-dynamics simulation for a two-dimensional system.
Finally, the molecular dynamics simulation accompanied with the isothermally
non-reflecting boundary condition is performed to analyze the related material physics in
laser-assisted nano-imprinting. Results show that the implemented boundary condition relax
the lattice well, and eventually absorb the wave propagation of momentum/energy during
heating and imprinting process. Besides, the temperature/force evolution in substrate, effect
of the molding-demolding interval, and surface situation of mold are discussed
633-018 airfoil at Re ranging from 1.32×10E4 ~ 1.09×10E5. For theoretical approach, the catastrophe theory was applied to model the aerodynamic transition problem and to predict
some aerodynamic characteristics undiscovered before. The so-called catastrophe theory describes the discontinuous phenomena with continuous functions based on the study of
singularities of nonlinear functions. Results of aerodynamic loading measurement well verified the validity of the catastrophe theory on the aerodynamically transitional problems and flow visualization provided some physical essences of the flow field about the theory employed here. Finally, the aerodynamic transition of the airfoil was redefined and reexamined through the cusp catastrophe model.
Combining the mathematical model and the experimental results, it can be deduced that the transition and sudden change of aerodynamic coefficients, in essence, is a
phase-changing process of the topological structure of the flow field. The predicted sudden-changing points of the aerodynamic coefficients match very well with the experimental results within the error margin of α less than 0.5 degree and Re less than two thousand. It indicates that the lift or drag and Re as well as α actually constitutes a cubic hypersurface relationship during the transitional process, which is believed to be a new direction of related aerodynamic researches.
efficiency in nanomechanical simulation, this dissertation performs a series of
theoretical/numerical studies on the crystalline solid material, including nanomechanics of
monatomic lattice, isothermally non-reflecting boundary condition, fast updating of neighbor
list, and the application/simulation in laser-assisted nano-imprinting.
Firstly, the nanomechanical and statistical behavior of monatomic crystal lattice are
introduced, which is the foundation for the development of non-reflecting boundary condition
and the simulation of corresponding nano materials. Based on the assumptions and the
harmonic approximation for the utilized inter-atomic potential, the equation of motion as well
as dispersive relation will be given. Furthermore, from the statistical properties of phonon
and lattice energy, the thermal amplitude can be calculated and lattice temperature can be
defined, which covers the result previously derived by other researchers at high temperatures.
Method of Time-History-Kernel (THK) and method of absorbing boundary layer (ABL)
are two main philosophies to derive the generalized Langevin equation, which is a general
form of non-reflecting boundary conditions. The general concepts of THK method, including
the formulation, kernel generation, time convolution, application assessments and numerical
verification, are given. Based on the Crump’s method to express the THK function in the
exponential form, a recursive algorithm for THK time-convolution has been proposed to reduce the computational cost from O(N 2 ) down to O(N). By applying the THK method
in lattice relaxation and the system under external forcing or heating, it is demonstrated that
the THK method indeed provides an isothermally non-reflecting boundary condition in
nanomechanical computation.
In ABL methods, two major mapping formulations are proposed and the corresponding
performances are discussed analytically as well as verified numerically. Starting from the
ω -mapping formulation, a series of extensive ABL methods are developed and investigated
to improve the performance of wave absorption, and one ABL method is recommended due
to its lower reflection at low frequency. Finally, the general comparison for all non-reflecting
boundary condition is addressed.
Based on a rigorous definition of Verlet radius with respect to temperature and
list-updating interval, this study gives an estimation formula of computation time, with which
the best algorithm can be chosen according to different total number of atoms, system
average density and system average temperature in the nanomechanical system. It has been
shown that the Verlet Cell-linked List (VCL) algorithm is better than other algorithms for a
system with a large number of atoms. Furthermore, a generalized VCL (GVCL) algorithm
optimized with a list-updating interval and cell-dividing number is analyzed and shows the
reduction of the computation time by 30% ~ 60%, which is verified by the
molecular-dynamics simulation for a two-dimensional system.
Finally, the molecular dynamics simulation accompanied with the isothermally
non-reflecting boundary condition is performed to analyze the related material physics in
laser-assisted nano-imprinting. Results show that the implemented boundary condition relax
the lattice well, and eventually absorb the wave propagation of momentum/energy during
heating and imprinting process. Besides, the temperature/force evolution in substrate, effect
of the molding-demolding interval, and surface situation of mold are discussed