arXiv subjects

Shengkai Wang

Publications and source records attributed to Shengkai Wang.

13 recordsLinked to original sources

arXiv2026

Reconfiguring Geovisualization in the Age of Generative AI: Insights from Domain Experts

GenAI is increasingly integrated into geovisualization, yet its broader implications for professional practice are insufficiently understood. To examine these implications, we conducted semi-structured interviews with 20 geovisualization experts. The interviews were structured around four broad analytical domains: Data, Ideation, Prototyping, and Iteration, while also encouraging participants to reflect on issues that extend beyond these activities. Our findings show that GenAI expands the capabilities of geovisualization, particularly in terms of data handling, creative exploration, and rapid prototyping, but does not simply remove existing constraints. Instead, key bottlenecks are shifting from production to judgment and verification. As routine technical tasks become more automated, professional value increasingly depends on spatial reasoning, contextual interpretation, aesthetic and ethical judgment, and the ability to assess whether AI-generated outputs are appropriate for use. At the same time, GenAI introduces new challenges regarding provenance, interpretability, and accountability, raising questions about how responsibility should be distributed across models, developers, practitioners, institutions, and users. These shifts are particularly significant in geovisualization because spatial representations are constrained by geographic reality and must balance scientific validity, visual expression, and technical implementation. We therefore argue that responsible GenAI in geovisualization requires domain-specific approaches to spatial validation, provenance, uncertainty communication, human oversight, and accountable use. This study provides an expert-grounded perspective on how GenAI is reconfiguring geovisualization as a practice of spatial knowledge production. It also identifies implications for future professional practice, education, system design, and governance.

cs.CY↗

arXiv2026

The Sword, Shield, and Achilles' Heel: Characterizing the Linguistic Inductive Bias of Large Language Models for Spatial Reasoning in Navigation Planning

Large Language Model (LLM)-based navigation systems commonly construct explicit spatial representations (e.g., topological graphs, semantic raster maps) and translate them into textual descriptions as LLMs' inputs. However, the linguistic structures of such text-based spatial representations and the choices of contextual features (e.g., topology, geometry) they contain are often treated as neutral engineering decisions rather than key factors that shape LLMs' behavior. To fill the gap, we propose a dual-interventional framework that disentangles linguistic structures from different contextual cues to evaluate the linguistic inductive bias of LLMs for navigation planning. In the framework, representation intervention varies the linguistic format and the degree of linguistic compression, clarifying when linguistic representations support or inhibit navigation planning. Context intervention, combined with contextual feature combination and conflict probing, explicitly clarifies the preferences and weaknesses of LLMs when processing different contextual cues. Experiments across diverse spatial reasoning tasks and multiple model scales reveal a consistent pattern: topological information is a sturdy shield and the backbone of robust planning; linguistic format is a double-edged sword whose effect depends on model size, task demands, and the compression level; and semantic information is a fatal Achilles' heel -- incorrect semantic cues can systematically derail the planning process. Overall, our study shows that effective text-based spatial representations in LLM-based navigation should preserve topological integrity, calibrate representational compression to model capacity, and ensure semantic correctness, rather than simply adopting a single representation. Our code is publicly available at https://github.com/jonesdong150/LLM-Navigation-Inductive-Bias.

cs.CL↗

arXiv2026

Effects of Nozzle Roughness on the Streamwise Streaks in Underexpanded Jets -- An Experimental Study

The present study investigated the formation of streamwise streaks in underexpanded jets from round sonic nozzles, based on direct experimental observation using high-speed schlieren imaging and PLIF methods. The effect of geometric perturbations of the nozzle exit on the streamwise flow structure was examined through a series of comparative experiments. Underexpanded jets were generated in a vacuum chamber using phi-4 mm nozzles of two different configurations: (a) a "smooth" nozzle, shaped and polished by a high-quality commercial lathe machine; and (b) nozzles with artificially introduced sinusoidal perturbation of various wavenumbers on the circular contour of the exit. In the case of the "smooth" nozzle, experiments were repeated following a 60-degree rotation of the nozzle along its axis, and a similar rotation in the streak patterns was observed. This suggests that the streamwise streaks most likely originated from geometric perturbations caused by the minute roughness at the nozzle exit. In the latter case, the effects of modal distribution of geometric perturbation on the streaks were further investigated. The results showed that the low-wavenumber (k < 5) perturbations exhibited much smaller growth rates of streamwise streaks - likely dominated by residual roughness similar to the "smooth" case - compared to higher-wavenumber (k = 6 and 7) perturbations, where the streak patterns were observed to correlate geometrically with the perturbed nozzle exit contour. Results from the present study should prove useful in enhancing the current understanding of noise patterns in supersonic wind tunnel tests, where nozzles are critical components.

physics.flu-dyn↗

arXiv2026

Spatiotemporally Resolved Multi-Scalar Measurements of Methane Tulip Flames in a Square Channel

Understanding the propagation dynamics of premixed flames in confined spaces is important for fire safety in gas pipelines and for optimizing modern internal combustion engines. In sufficiently long channels, premixed flames routinely develop tulip flame structures, yet the dominant mechanism remains elusive, and quantitative data on the evolution of flame morphology and key scalar fields are critically needed to improve the explanation, characterization, and modeling of tulip flame dynamics. In this study, premixed flames of a stoichiometric methane/air mixture were investigated in a square channel at a reduced pressure of approximately 0.3 atm. Time-synchronized, multi-plane, dual-color PLIF measurements yielded a spatiotemporally resolved 3-D dataset of key scalar fields, including temperature and OH concentration, throughout the formation and evolution of the tulip structure. Significant heat loss across the walls counteracted the heat released by combustion, producing a near-constant-pressure environment throughout the experiment. A super-equilibrium distribution of OH concentration was observed in the thermal boundary layers, suggesting that thermal cooling dominated over chemical relaxation in those regions. Additionally, the flame-front morphology at five representative times was determined using a 3-D reconstruction algorithm, from which the flame surface area was extracted. The results of this study should aid theoretical modeling and numerical simulations of premixed flame propagation dynamics in confined spaces under realistic boundary conditions.

physics.flu-dyn↗

arXiv2026

Cavity-Stabilized Rotating Flames in a Circular Hele-Shaw Burner

We report direct experimental observations of self-organized rotating flames of premixed CH4 and air in an open circular Hele-Shaw burner equipped with an annulus cavity flame holder. These flames formed spontaneously at sufficiently low flow rates, where flame flashback was counteracted by thermal quenching, resulting in a dynamic balance between the local flame speed and flow velocity. Unlike flames propagating in closed micro-channels, these flames exhibited stable traveling-wave patterns with heads gliding along the leading edge of the cavity, where rapid expansion created a low-speed zone that facilitated flame stabilization. At low flow rates, the rotating flames were single-headed, with their rotation frequencies roughly proportional to the laminar flame speeds, suggesting that the flame fronts traveled in a nearly constant-shape fashion. As the flow rate increased, the rotating flames split into multiple heads at approximately equal spacing, and the number of heads and rotation frequency increased with the flow rate, until these rotating flames transitioned into steady ring-shaped flames anchored at the cavity leading edge. Blow-off or extinction occurred at sufficiently high flow rates, where the flame front was pushed out of the rear side of the cavity. Parametric measurements were conducted over a wide range of equivalence ratios and flow rates, from which a regime diagram of different flame modes and their transition boundaries was obtained. Additional experiments were conducted on C3H8 and DME. It was found that the critical total mass flow rate at the rotating-steady flame transition boundary is insensitive to equivalence ratio, gap distance, and fuel type. These results should be useful not only for the fundamental understanding of flame dynamics in micro-channels but also for the practical design of micro-combustors and the application of micro-combustion technologies.

physics.flu-dyn↗

arXiv2026

Effects of Soret diffusion on the intrinsic instability of premixed hydrogen/air flames

Hydrogen flames exhibit multiple intrinsic instabilities. The low molar masses of H and H2 lead to significant Soret diffusion near the flame front; however, its influence on hydrogen flame instabilities remains to be quantified. This study investigates the effect of Soret diffusion on instability evolution dynamics via one-dimensional counterflow analysis and two-dimensional, high-fidelity direct numerical simulations covering both the linear growth regime and the fully developed nonlinear regime over a wide range of equivalence ratios (phi). In the linear regime, Soret diffusion increases the perturbation growth rate at phi < 1.7, especially under lean conditions, but reduces the growth rate at phi > 1.7. A similar sensitivity reversal is observed in the Markstein length near the peak equivalence ratio of unstretched laminar flame speed. In the nonlinear regime, Soret diffusion accelerates the formation of small-scale wrinkles in lean hydrogen flames and reduces the characteristic size of large-scale finger structure by one-third. An interesting observation is that, although Soret diffusion promotes preferential diffusion and increases the local flame displacement speed, the global fuel consumption rate decreases due to a reduction in the overall flame surface area. In addition, curvature-based flame segment analysis reveals a synergistic effect between Soret diffusion and Fickian diffusion that enhances/reduces the local equivalence ratio in positively/negatively curved regions of the flame front. The probability distributions of the Karlovitz number and the density-weighted displacement speed are also analyzed; results suggest that, for lean hydrogen flames, Soret diffusion broadens the distributions for both parameters, particularly on the positive side. These findings promise to advance the fundamental understanding of hydrogen flame dynamics under complex differential transport.

physics.flu-dyn↗

arXiv2026

Effects of gravity on lean hydrogen/air flame instability: From linear scaling law to nonlinear morphology evolution

The instability characteristics of lean hydrogen/air flames have attracted considerable research attention, yet the effect of gravity remains insufficiently understood. In this study, time-resolved two-dimensional simulations with detailed chemistry and transport are conducted to investigate the influence of gravity-induced Rayleigh-Taylor (RT) instability on the linear growth rate of disturbances and nonlinear morphology evolution of cellular flame fronts at different length scales. In the linear regime, a parametric study is performed across various equivalence ratios, initial temperatures and pressures; in each case, the dispersion relation is calculated for various gravity levels. The influence of gravity is most pronounced under ultra-lean, low-temperature, and high-pressure conditions, and a universal scaling law between gravity sensitivity and the Froude number is established. In the nonlinear regime, gravity has opposite effects on the large-scale and small-scale structures of lean hydrogen flames. On the one hand, gravity inhibits the splitting of small-scale cellular structures through a baroclinic torque mechanism; on the other hand, it promotes the development of large-scale finger-like structures, thereby increasing the total surface area and the global consumption speed of the flame. The effects of gravity on the probability distributions of cell size, displacement speed, Karlovitz number, and local curvature are also analyzed. The results and findings of the present study should advance the fundamental understanding of hydrogen flame dynamics under varying gravity conditions and provide insight for relevant applications, including fire safety and space propulsion.

physics.flu-dyn↗

arXiv2026

Edge-Stabilized Rotating Flames in a Circular Hele-Shaw Cell

In this study, we report direct experimental observations of self-sustaining CH4-air rotating flames formed spontaneously in an unheated, open, circular Hele-Shaw cell. These flames are observed under fuel-rich conditions and exhibit stable traveling-wave patterns, with edge velocities that can significantly exceed the nominal flame speed of the unburned mixture. PLIF measurements across the central plane reveal that the flame front consists of a bibrachial structure, with a diffusion branch gliding along the side edges of the cell and a premixed branch extending into the interior. Complementary numerical simulations suggest that the formation of rotating flames is driven by a dynamic balance between local flame speed and unburned-gas velocity near the cell edges, where both wall heat loss and flow expansion play critical roles in stabilizing the rotation pattern. A parametric study is conducted for various equivalence ratios, flow rates, and gap distances, from which the regime diagrams of flame modes and rotation frequencies are obtained. At low flow rates, the rotating state is characterized by a single rotating flame wave, whose rotation frequency increases with flow rate. For this type of flames, a semi-empirical model is established to predict their rotation frequencies and shapes as functions of mass flow rate and surface temperature. At elevated flow rates, multiple rotating waves appear with approximately equal azimuthal spacing, and the product of the wave number and rotation frequency increases with flow rate. Mode transition from rotating flames to steady ring-shaped flames anchored at the burner edges occurs at sufficiently high flow rates, while at sufficiently low flow rates, flame extinction occurs due to thermal quenching. These findings can provide useful guidance for the advancement of micro-combustion technologies.

physics.flu-dyn↗

arXiv2025

Quantitative 3D Morphology of Cellular H2/O2/N2 Flames on a Porous-Plug Burner: Spatially Resolved Measurements of Temperature and OH Radical

This study presents a systematic characterization of burner-stabilized lean hydrogen flame morphology across a wide range of equivalence ratios, dilution factors, and flow rates. Spatially resolved measurements of three-dimensional temperature and OH distributions were achieved. A comprehensive dataset of over 200 flame cases was obtained, enabling accurate determination of regime diagrams for different flame modes. Linear stability analysis and direct numerical simulations were also performed and compared with the experimental results. The dominant wavenumbers of steady-state cellular flames were found to be consistently lower than the most unstable wavenumbers predicted by the linearized dispersion relation, indicating that nonlinear interactions between finite-amplitude perturbations of different length scales favored the growth of low-frequency components at long times. The cellular structures were found to be critically important in stabilizing the flame, especially at nominal equivalence ratios near the lean flammability limit. The mechanism of cellular flame stabilization was analyzed by complementary numerical simulations using a detailed reaction model. The combined effect of curvature-induced flame acceleration, local flow expansion/compression near the burner surface, and stratification of equivalence ratio caused by Soret diffusion created regions of reduced flow speed and enriched hydrogen concentration that helped anchor flames at nominal conditions where they would have blown off without the flame cells. The results of the present study are useful for understanding the fundamental flame dynamics of lean hydrogen mixtures and for improving the design of practical hydrogen combustors.

physics.flu-dyn↗

arXiv2025

Spatiotemporally Resolved Measurements of CO2 Distribution at the Air-Water Interface Using Tunable Diode Laser Spectroscopy

The transport of CO2 across the air-water interface is central to physical oceanography and carbon sequestration. A comprehensive understanding of this process requires high-resolution diagnostics of diffusion, absorption, and reaction across a wide range of spatial and temporal scales. The current study presents a novel measurement method to quantify the CO2 distribution at the air-water interface. This method combines the advantages of tunable diode laser spectroscopy and rapid spatial beam scanning for in situ, nonintrusive, and spatiotemporally resolved measurement of the CO2 concentration distribution above the interface. The performance of this method was examined in a series of quasi-1D experiments in a miniature gas chamber, where the diffusion and absorption of CO2 into pure water and alkaline solutions of different pH values were continuously monitored. An effective time resolution of 5 ms and an effective spatial resolution of 1 mm were achieved. The observed gas-phase CO2 distribution evolution agreed with the classic one-dimensional diffusion model, which validated the accuracy of the current method. PH-dependent dynamics of interfacial CO2 concentration were also observed. The CO2 depletion rate is highly pH-sensitive at low pH and saturates at pH = 10, revealing complex competition between the gas-phase and the liquid-phase transport processes. The current method's high spatial and temporal resolution holds promise for studying cross-interface gas transport under more complex flow conditions, in both field measurements and laboratory studies.

physics.flu-dyn↗

arXiv2025

Pulsation of Burner-Stabilized CH4-O2 Flames Moderated by CO2 Addition

This study investigated the pulsating instability of burner-stabilized premixed CH4-O2 flames at various levels of CO2 dilution. Experiments were conducted using a water-cooled porous-plug burner of 18 mm diameter over a wide range of mixture compositions and flow rates, during which time-resolved measurements of flame chemiluminescence and gas temperature were obtained. The primary oscillation frequencies of the pulsating flames were determined using fast Fourier transform and harmonic power analysis. Phase-locked analysis of the chemiluminescence images revealed an interesting mode-transition phenomenon of the flame oscillations. Under fuel-rich conditions with relatively low heat release rates and low flow rates, the flames exhibited quasi-periodic single-mode oscillations. At elevated flow rates, these oscillations were modulated by low-frequency flame flickering instabilities, which created sidebands around the primary oscillation frequency. At higher heat release rates, the flickering instability further triggered mode splitting, eventually leading to multi-mode oscillations. Regime diagrams of the flame oscillation modes, as well as the stability boundaries, were obtained under various fuel flow rates. These findings can be useful for both fundamental research on flame dynamics and practical applications of CO2-moderated oxy-combustion.

physics.flu-dyn↗

arXiv2025

Precision Thermometry of Flat Flames Using Spatially Resolved Multi-Color Laser Absorption Spectroscopy of Carbon Dioxide

This work developed an accurate and robust absorption-based method for spatially resolved measurements of gas temperatures in flames and reacting flows, with typical single-measurement uncertainties on the order of 1\%. This method exploits narrow-linewidth laser absorption of hot CO$_2$ molecules, which can be generated from combustion or artificially seeded into the flow. A collinear dual-laser setup allowed for periodic scans over tens of CO$_2$ absorption transitions near the $\nu_3$ bandhead every 100 $\mu s$, from which gas temperatures (as well as CO$_2$ concentrations) were determined with high sensitivity and robustness. Spatially resolved measurements were achieved using an electrically driven high-speed beam scanning system consisting of a 2-D galvo scanner and a pair of off-axis parabolic mirrors. An effective spatial resolution of 1 mm was achieved at a planar field measurement speed of 200 Hz and a volumetric field measurement speed of 2 Hz. A physically constrained nonlinear inference framework was also developed for the quantitative analysis of the measurement data. Proof-of-concept experiments were performed on axisymmetric flames stabilized on a Mckenna burner at various equivalence ratios and flow rates, and the results agreed asymptotically with the theoretical value of the adiabatic flame temperature. An additional experiment on a flame of complex geometry demonstrated an excellent level of resolution, precision, and contrast achieved by the current thermometry method. This method promises to provide good utility in future combustion studies due to its high performance metrics and relative ease of use.

physics.optics↗

arXiv2019

Information-Driven Design for Shock Tube / Laser Absorption Studies of Fundamental Rate Constants in Combustion, with Application to Methanol Pyrolysis

Shock tube experiments, paired with precision laser diagnostics, are ideal venues to provide kinetics data critically needed for the development, validation and optimization of modern combustion kinetics models. However, to design sensitive, accurate, feasible and information-rich experiments that may yield such data often requires sophisticated planning. This study presents a mathematical framework and quantitative approach to guide such experimental design, namely a method to pin-point the optimal conditions for specific experimentation under realistic constraints of the shock tubes and diagnostic tools involved. For demonstration purpose, the current work is focused on a key type of shock tube kinetic experiments -- direct determination of fundamental reaction rate constants. Specifically, this study utilizes a Bayesian approach to maximize the prior-posterior gain in Shannon information of the rate constants to be inferred from the intended experiment. Example application of this method to the experimental determination of the CH$_3$OH + H (k$_1$) and CH$_2$O + H (k$_2$) rate constants is demonstrated in shock tube/laser absorption studies of the CH$_3$OH pyrolysis system, yielding new recommended rate constant expressions (over 1287 K - 1537 K) as: k$_1$ = $ 2.50 \times 10^6 (T/K)^{2.35} exp(-2975 K /T) \, cm^3mol^{-1}s^{-1} \pm 11.4\%$ and k$_2$ = $7.06 \times 10^7 (T/K)^{1.9} exp(-1380 K/T) \, cm^3mol^{-1}s^{-1} \pm 9.7 \%$. Potential extension to other types of kinetic studies, e.g. prediction of combustion benchmarks such as ignition delay times and species yields, and global uncertainty minimization of generic reaction models, are also briefly discussed.

physics.chem-ph↗