arXiv subjects

James Chen

Publications and source records attributed to James Chen.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear potential field in contact electrification

The cause of electron transfer in contact electrification is one of the most hotly debated physical problems today. In this study, the electron transfer is hypothesized to be partly driven by the surface dipole induced potential during contact. This phenomena is demonstrated by a combination of atomistic field theory (AFT) and molecular dynamics (MD) simulation. A representative contact system of carbon and silicon dioxide was chosen for its excellent tribo-tunneling power output performance. The results reveal the existence of a nonlinear potential field as well as the existence of a separation dependent potential barrier at the contact interface. Possible scenarios of triboelectric charge transfer are discussed in light of these results. These results are critical to the fundamental understanding of contact electrification.

physics.class-ph↗

arXiv2026

Boltzmann-Curtiss Description for Flows under Translational Nonequilibrium

Continuum-based theories, such as Navier-Stokes equations, have been considered inappropriate for flows under nonequilibrium conditions. In part, it is due to the lack of rotational degrees of freedom in the Maxwell-Boltzmann distribution. The Boltzmann-Curtiss formulation describes gases allowing both rotational and translational degrees of freedom and forms morphing continuum theory (MCT). The first order solution to Boltzmann-Curtiss equation yield a stress tensor that contains a coupling coefficient that is dependent on the particles number density, the temperature and the total relaxation time. A new bulk viscosity model derived from the Boltzmann-Curtiss distribution is employed for shock structure and temperature profile under translational and rotational nonequilibrium. Numerical simulations of argon and nitrogen shock profiles are performed in the Mach number range of 1.2 to 9. The current study, when comparing with experimental measurements and Direct Simulation Monte Carlo (DSMC) simulation, show a significant improvement in the density profile, normal stresses and shock thickness at nonequilibrium conditions than Navier-Stokes equations. The results indicate that equations derived from the Boltzmann-Curtiss distribution are valid for a wide range of nonequilibrium conditions than those from the Maxwell-Boltzmann distribution.

physics.flu-dyn↗

arXiv2026

Damage Prediction of Sintered {\alpha}-SiC Using Thermo-mechanical Coupled Fracture Model

A three-way coupled thermo-mechanical fracture model is presented to predict the damage of brittle ceramics, in particular {\alpha}-SiC, over a wide range of temperatures (20-1400 C). Predicting damage over such a range of temperatures is crucial for thermal protection systems for many systems such as spacecraft. The model, which has been implemented in MOOSE, is divided into three modules: elasticity, damage phase field, and heat conduction. Analytical approaches for determining crack length scales are presented for both simple tension and simple shear. Validation tests are conducted for both flexural strength and fracture toughness over the specified range of temperatures. Flexural strength simulation results fall within the uncertainty region of the experimental data, and mode I fracture toughness simulation results are also in agreement with the experimental data. Mode II and mixed mode fracture toughness simulations results are presented with the modified G-criterion. Finally, the parallel computing capabilities of the model is considered in various scalability tests.

cond-mat.mtrl-sci↗

arXiv2026

The effect of chemical vapor infiltration process parameters on flexural strength of porous {\alpha}-SiC: A numerical model

The flexural strength variability of {\alpha}-SiC based ceramics at elevated temperatures creates the need for an Integrated Computational Materials Engineering (ICME) framework that relates the strength of a specimen directly to its manufacturing process. To create this ICME framework a model must first be developed which establishes a relationship between the chemical vapor infiltration (CVI) process and parameters, the resulting mesoscale pores, and the overall macroscale flexural strength. Here a nonlinear single pore model of CVI is developed used in conjunction with a four-way coupled themo-mechanical damage model. The individual components of the model are tested and a sample system under a four-point bending test is explored. Results indicate that specimens with an initial porosity greater than 30% require temperatures below 1273 K to maintain structural integrity, while those with initial porosities less than 30% are temperature-independent, allowing for optimization of the CVI processing time without compromising strength.

cond-mat.mtrl-sci↗

arXiv2026

The largest 5th pivot may be the root of a 61st degree polynomial

This paper introduces a number of new techniques in the study of the famous question from numerical linear algebra: what is the largest possible growth factor when performing Gaussian elimination with complete pivoting? This question is highly complex, due to a complicated set of polynomial inequalities that need to be simultaneously satisfied. This paper introduces the JuMP + Groebner basis + discriminant polynomial approach as well as the use of interval arithmetic computations. Thus, we are introducing a marriage of numerical and exact mathematical computations. In 1988, Day and Peterson performed numerical optimization on $n=5$ with NPSOL and obtained a largest seen value of $4.1325...$. This same best value was reproduced by Gould with LANCELOT in 1991. We ran extensive comparable experiments with the modern software tool JuMP and also saw the same value $4.1325...$. While the combinatorial explosion of possibilities prevents us from knowing whether there may not be a larger maximum, we succeed in obtaining the exact mathematical value: the number $4.1325...$ is exactly the root of a 61st degree polynomial provided in this work, and is a maximum given the equality constraints seen by JuMP. In light of the numerics, we pose the conjecture that this lower bound is indeed the maximum. We also apply this technique to $n = 6$, $7$, and $8$. Furthermore, in 1969, an upper bound of $4\frac{17}{18}\approx 4.94$ was produced for the maximum possible growth for $n = 5$. We slightly lower this upper bound to $4.84$.

math.NA↗

arXiv2025

Independent Test Generation for RESTful APIs

Modern REST API testing relies on brittle sequences of calls to build system state. These multi-step tests suffer from non-determinism, poor scalability, and a "reachability tax" where a single failed setup step invalidates the entire test. We introduce TECTON, which breaks this cycle by replacing implicit state construction with explicit state synthesis of both the request payload and mock data it depends on. TECTON achieves this through two complementary mechanisms: it generates diverse, valid payloads directly, and it augments existing test mocks with realistic data so those payloads have valid system state to reference. Both mechanisms apply combinatorial testing to a new domain: the nested property space of Abstract Data Types (ADTs). TECTON decomposes complex API requests into primitive components to unleash LLMs on the more tractable subtasks of identifying equivalence classes of these primitives and generating representative values for them. It then uses LLMs to extract and inject state values via test mocks, enabling payloads to reference valid state. It recomposes these values into covering combinations to directly produce high-coverage test payloads. On standard RESTful benchmarks, TECTON achieves 70% average line coverage - a 20% absolute increase over sequence-based generators. It exposes 2x more runtime errors than any prior tool, including assertions and data constraint failures. TECTON's shift from sequencing API calls to synthesized payloads advances the state of the art in automated API validation.

cs.SE↗

arXiv2025

LadyBug: A GitHub Bot for UI-Enhanced Bug Localization in Mobile Apps

This paper introduces LadyBug, a GitHub bot that automatically localizes bugs for Android apps by combining UI interaction information with text retrieval. LadyBug connects to an Android app's GitHub repository, and is triggered when a bug is reported in the corresponding issue tracker. Developers can then record a reproduction trace for the bug on a device or emulator and upload the trace to LadyBug via the GitHub issue tracker. This enables LadyBug to utilize both the text from the original bug description, and UI information from the reproduction trace to accurately retrieve a ranked list of files from the project that most likely contain the reported bug. We empirically evaluated LadyBug using an automated testing pipeline and benchmark called RedWing that contains 80 fully-localized and reproducible bug reports from 39 Android apps. Our results illustrate that LadyBug outperforms text-retrieval-based baselines and that the utilization of UI information leads to a substantial increase in localization accuracy. LadyBug is an open-source tool, available at https://github.com/LadyBugML/ladybug. A video showing the capabilities of Ladybug can be viewed here: https://youtu.be/hI3tzbRK0Cw

cs.SE↗

arXiv2024

Computing Inverses of Stieltjes Transforms of Probability Measures

The Stieltjes (or sometimes called the Cauchy) transform is a fundamental object associated with probability measures, corresponding to the generating function of the moments. In certain applications such as free probability it is essential to compute the inverses of the Stieltjes transform, which might be multivalued. This paper establishes conditions bounding the number of inverses based on properties of the measure which can be combined with contour integral-based root finding algorithms to rigorously compute all inverses.

math.NA↗

arXiv2024

Controllable and Reliable Knowledge-Intensive Task-Oriented Conversational Agents with Declarative Genie Worksheets

Large Language Models can carry out human-like conversations in diverse settings, responding to user requests for tasks and knowledge. However, existing conversational agents implemented with LLMs often struggle with hallucination, following instructions with conditional logic, and integrating knowledge from different sources. These shortcomings compromise the agents' effectiveness, rendering them unsuitable for deployment. To address these challenges, we introduce Genie, a programmable framework for creating knowledge-intensive task-oriented conversational agents. Genie can handle involved interactions and answer complex queries. Unlike LLMs, it delivers reliable, grounded responses through advanced dialogue state management and supports controllable agent policies via its declarative specification -- Genie Worksheet. This is achieved through an algorithmic runtime system that implements the developer-supplied policy, limiting LLMs to (1) parse user input using a succinct conversational history, and (2) generate responses according to supplied context. Agents built with Genie outperform SOTA methods on complex logic dialogue datasets. We conducted a user study with 62 participants on three real-life applications: restaurant reservations with Yelp, as well as ticket submission and course enrollment for university students. Genie agents with GPT-4 Turbo outperformed the GPT-4 Turbo agents with function calling, improving goal completion rates from 21.8% to 82.8% across three real-world tasks.

cs.AI↗

arXiv2024

Compact Ion Beam System for Fusion Demonstration

We demonstrate a compact ion beam device capable of accelerating H$^+$ and D$^+$ ions up to 75keV energy, on to a solid target, with sufficient beam current to study fusion reactions. The ion beam system uses a microwave driven plasma source to generate ions that are accelerated to high energy with a direct current (DC) acceleration structure. The plasma source is driven by pulsed microwaves from a solid-state radiofrequency (RF) amplifier, which is impedance matched to the plasma source chamber at the ISM band frequency (2.4-2.5GHz). The plasma chamber is held at high positive DC potential and is isolated from the impedance matching structure (at ground potential) by a dielectric-filled gap. To facilitate the use of high-energy-particle detectors near the target, the plasma chamber is biased to a high positive voltage, while the target remains grounded. A target loaded with deuterium is used to study D-D fusion and a B$_4$C or LaB$_6$ target is used to study p-$^{11}$B fusion. Detectors include solid-state charged particle detector and a scintillation fast neutron detector. The complete ion beam system can fit on a laboratory table and is a useful tool for teaching undergraduate and graduate students about the physics of fusion.

physics.ins-det↗

arXiv2023

EMPOT: partial alignment of density maps and rigid body fitting using unbalanced Gromov-Wasserstein divergence

Aligning EM density maps and fitting atomic models are essential steps in single particle cryogenic electron microscopy (cryo-EM), with recent methods leveraging various algorithms and machine learning tools. As aligning maps remains challenging in the presence of a map that only partially fits the other (e.g. one subunit), we here propose a new procedure, EMPOT (EM Partial alignment with Optimal Transport), for partial alignment of 3D maps. EMPOT first finds a coupling between 3D point-cloud representations, which is associated with their so-called unbalanced Gromov Wasserstein divergence, and second, uses this coupling to find an optimal rigid body transformation. Upon running and benchmarking our method with experimental maps and structures, we show that EMPOT outperforms standard methods for aligning subunits of a protein complex and fitting atomic models to a density map, suggesting potential applications of Partial Optimal Transport for improving Cryo-EM pipelines.

q-bio.BM↗

arXiv2023

A Search for Technosignatures Around 11,680 Stars with the Green Bank Telescope at 1.15-1.73 GHz

We conducted a search for narrowband radio signals over four observing sessions in 2020-2023 with the L-band receiver (1.15-1.73 GHz) of the 100 m diameter Green Bank Telescope. We pointed the telescope in the directions of 62 TESS Objects of Interest, capturing radio emissions from a total of ~11,680 stars and planetary systems in the ~9 arcminute beam of the telescope. All detections were either automatically rejected or visually inspected and confirmed to be of anthropogenic nature. In this work, we also quantified the end-to-end efficiency of radio SETI pipelines with a signal injection and recovery analysis. The UCLA SETI pipeline recovers 94.0% of the injected signals over the usable frequency range of the receiver and 98.7% of the injections when regions of dense RFI are excluded. In another pipeline that uses incoherent sums of 51 consecutive spectra, the recovery rate is ~15 times smaller at ~6%. The pipeline efficiency affects calculations of transmitter prevalence and SETI search volume. Accordingly, we developed an improved Drake Figure of Merit and a formalism to place upper limits on transmitter prevalence that take the pipeline efficiency and transmitter duty cycle into account. Based on our observations, we can state at the 95% confidence level that fewer than 6.6% of stars within 100 pc host a transmitter that is detectable in our search (EIRP > 1e13 W). For stars within 20,000 ly, the fraction of stars with detectable transmitters (EIRP > 5e16 W) is at most 3e-4. Finally, we showed that the UCLA SETI pipeline natively detects the signals detected with AI techniques by Ma et al. (2023).

astro-ph.IM↗

arXiv2018

First-order Approximation to the Boltzmann-Curtiss Equations for Flows with Local Spin

The first-order approximation to the solution of the Boltzmann--Curtiss transport equation is derived. The resulting distribution function treats the rotation or gyration of spherical particles as an independent classical variable, deviating from the quantum mechanical treatment of molecular rotation found in the Wang Chang-Uhlenbeck equation. The Boltzmann-Curtiss equation, therefore, does not treat different rotational motions as separate molecular species. The first-order distribution function yields momentum equations for the translational velocity and gyration that match the form of the governing equations of morphing continuum theory (MCT), a theory derived from the approach of rational continuum thermomechanics. The contribution of the local rotation to the Cauchy stress and the viscous diffusion are found to be proportional to an identical expression based off the relaxation time, number density, and equilibrium temperature of the fluid. When gyration is equated to the macroscopic angular velocity, the kinetic description reduces to the first-order approximation for a classical monatomic gas, and the governing equations match the form of the Navier--Stokes equations. The relaxation time used for this approximation is shown to be more complex due to the additional variable of local rotation. The approach of De Groot and Mazur is invoked to give an initial approximation for the relaxation of the gyration. The incorporation of this relaxation time, and other physical parameters, into the coefficients of the governing equations provides a more in-depth physical treatment of the new terms in the MCT equations, allowing experimenters to test these expressions and get a better understanding of new coefficients in MCT.

physics.flu-dyn↗

arXiv2018

Atomistic Field Theory for contact electrification of dielectrics

The triboelectrification of conducting materials can be explained by electron transfer between different Fermi levels. However, triboelectrification in dielectrics is poorly understood. The surface dipole formations are shown to be caused by the contact-induced surface lattice deformations. An Atomistic Field Theory (AFT) based formulation is utilized to calculate the distribution of the polarization, electric and potential fields. The induced fields are considered as the driving force for charge transfer. The simulation results show that a MgO/BaTiO3 tribopair can generate up to 104 V/cm^2, which is comprable with the data in the published literature.

cond-mat.mtrl-sci↗

arXiv2018

A Morphing Continuum Simulation of Transonic Flow over an Axisymmetric Hill

Finite volume simulations of turbulent boundary layer flow over an axisymmetric hill are performed for $Re_H$ = 6500 using Morphing Continuum Theory (MCT), and compared with DNS data from Castagna et. al. and experimental data obtained by Simpson. The inlet profile was specified by inputting values from the profile specified by Castagna et. al. Root- mean-square velocity fluctuations are inputted using the new variable of gyration from MCT. Additional terms introduced by MCT lead to a new formulation of the Q-criterion, which allows for the visualization of three-dimensional turbulent structures including a hairpin vortices. Streamline plots of the separation bubble show a more confined bubble than Castagna et. al., but agree well on the separation and reconnection points. The surface pressure coefficient matches Simpson much more closely than Castagna et. al. MCT data was obtained on a $6.72 \times 10^6$ cell mesh containing a smaller number of cells by an order of magnitude than that of the DNS mesh of $5.4 \times 10^7$ by Castagna et. al. The effects of the particle on the larger structures are inferred from the variation in the new variable of gyration.

physics.flu-dyn↗

arXiv2018

A Morphing Continuum Analysis of Energy Transfer in Compressible Turbulence

A shock-preserving finite volume solver with the generalized Lax-Friedrichs splitting flux for Morphing Continuum Theory (MCT) is presented and verified. The numerical MCT solver is showcased in a supersonic turbulent flow with Mach 2.93 over an $8^{\circ}$ compression ramp. The simulation results validated MCT with experiments as an alternative for modeling compressible turbulence. The required size of the smallest mesh cell for the MCT simulation is shown to be almost an order larger than that in a similar DNS study. The comparison shows MCT is a much more computationally friendly theory than the classical NS equations. The dynamics of energy cascade at the length-scale of individual eddies is illuminated through the subscale rotation introduced by MCT. In this regard, MCT provides a statistical averaging procedure for capturing energy transfer in compressible turbulence, not found in classical fluid theories. Analysis of the MCT results show the existence of a statistical coupling of the internal and translational kinetic energy fluctuations with the corresponding eddy rotational energy fluctuations, indicating a multiscale transfer of energy. In conclusion, MCT gives a new characterization of the energy cascade within compressible turbulence without the use of excessive computational resources.

physics.flu-dyn↗

arXiv2017

An Advanced Kinetic Theory For Morphing Continuum With Inner Structures

Advanced kinetic theory with the Boltzmann-Curtiss equation provides a promising tool for polyatomic gas flows, especially for fluid flows containing inner structures, such as turbulence, polyatomic gas flows and others. Although a Hamiltonian-based distribution function was proposed for diatomic gas flow, a general distribution function for the generalized Boltzmann-Curtiss equations and polyatomic gas flow is still out of reach. With assistance from Boltzmann's entropy principle, a generalized Boltzmann-Curtiss distribution for polyatomic gas flow is introduced. The corresponding governing equations at equilibrium state are derived and compared with Eringen's morphing (micropolar) continuum theory derived under the framework of rational continuum thermomechanics. Although rational continuum thermomechanics has the advantages of mathematical rigor and simplicity, the presented statistical kinetic theory approach provides a clear physical picture for what the governing equations represent.

physics.flu-dyn↗

arXiv2017

Morphing Continuum Theory for Turbulence: Theory, Computation and Visualization

A high order morphing continuum theory (MCT) is introduced to model highly compressible turbulence. The theory is formulated under the rigorous framework of rational continuum mechanics. A set of linear constitutive equations and balance laws are deduced and presented from the Coleman-Noll procedure and Onsager's reciprocal relations. The governing equations are then arranged in conservation form and solved through the finite volume method with a second order Lax-Friedrichs scheme for shock preservation. A numerical example of transonic flow over a three-dimensional bump is presented using MCT and the finite volume method. The comparison shows that MCT-based DNS provides a better prediction than NS-based DNS with less than 10% of the mesh number when compared with experiments. A MCT-based and frame-indifferent Q-criterion is also derived to show the coherent eddy structure of the downstream turbulence in the numerical example. It should be emphasized that unlike the NS-based G-criterion, the MCT-based Q-criterion is objective without the limitation of Galilean-invariance.

physics.flu-dyn↗