Velocity Distribution Research Articles

Measurement of the 13S1→23S1 interval in positronium using field ionization of Rydberg states

We report a 40 parts per 109 measurement of the positronium 13S1→23S1 interval using pulsed two-photon optical spectroscopy. The transition is detected via field ionization of atoms excited from the 2S to the 20P Rydberg state. Precise Monte Carlo line-shape simulations allow for the accounting of effects such as Doppler and ac Stark shifts, while an optical heterodyne measurement of the excitation laser pulse is used to correct for laser frequency chirp. A value of 1233607210.5±49.6MHz is obtained. This scheme allows for the measurement of the velocity distribution of positronium atoms to correct for the second-order Doppler effect. This is a major source of systematic uncertainty expected for future measurements of this transition with a cw laser; thus, our technique paves the way toward a new generation of a high-precision determination of this interval in positronium. Published by the American Physical Society 2025

Compositional Analysis and Numerical Simulation of Slagging Process on a Water-Cooled Wall of an MSW Incinerator

The mechanism of slagging in municipal solid waste incinerators is complex, and the slagging process is simultaneously affected by the composition, temperature, and flue gas flow. In this study, slag samples on a water-cooled wall were first analysed, and the key components and fusion temperatures were measured. Second, a gas-phase combustion model of an incinerator was established, and the temperature and velocity distributions of the flue gas inside the incinerator were calculated. Based on the incineration process, coupled with a discrete-phase model, a numerical simulation model of the slagging process on the water-cooled wall of the incinerator was constructed, considering the transport and adhesion processes of ash particles. The influence of parameters such as the ash particle size and concentration on the degree of slagging on the water-cooled wall was analysed. Smaller ash particles were less likely to adhere to water-cooled walls, with approximately 2.72% of ash particles with a particle size of 10 mm adhering to water-cooled walls. The proportion of ash particles with a particle size of 50 mm adhering to water-cooled walls was approximately three times that of those with a particle size of 10 mm. As the concentration of ash particles increased, the number of ash particles adhering to the water-cooled wall increased, and the adhesion ratio decreased. These results are of great significance for optimising the operation of incinerators and reducing slagging rates.

Study of Fluid Flow Characteristics and Mechanical Properties of Aviation Fuel-Welded Pipelines via the Fluid–Solid Coupling Method

The welded pipeline structure of aircraft fuel is a complex and diverse entity, significantly influenced by fluid–solid coupling. The refined aviation fuel-welded pipeline model plays a pivotal role in the investigation of its fluid–solid coupling mechanical properties. However, the mechanical analyses of pipelines with welded structures frequently simplify or ignore the influence of the weld zone (WZ). Consequently, these analyses fail to reveal the complex interactions between different weld zones in detail. In this study, a comprehensive and precise fuel-welded pipeline refinement model is developed through the acquisition of microstructural dimensions and mechanical parameters of the weld zone via metallographic inspection and microtensile testing. Additionally, the influence of clamps and brackets under airborne conditions is fully considered. Furthermore, the numerical simulation results are compared and verified using modal and random vibration tests. This paper addresses the impact of diverse fluid characteristics on the velocity field, pressure field, and stress in disparate areas, and it also conducts an investigation into the random vibration characteristics of the pipeline. The results demonstrate that the fluid pressure and velocity exert a considerable influence on the fluid flow state and structural stress distribution within the pipeline. An increase in flow velocity and alteration to the pipeline geometry will result in a change to the local velocity distribution, which in turn affects the distribution of the fluid pressure field. The highest stresses are observed in the weld zone, particularly at the junction between the weld zone and the heat-affected zone (HAZ). In contrast, the stresses in the bend region exhibit a corrugated distribution in both the axial and circumferential directions. An increase in fluid pressure has a significant impact on the natural frequency of the pipeline. This study enhances our comprehension of the mechanical properties of aircraft fuel lines with fluid–solid coupling and provides a foundation and guidance for the optimal design of fuel-welded lines.

Deep Learning Method for Airfoil Flow Field Simulation Based on Unet++

ABSTRACTThis paper investigates the accuracy of U‐Net++ networks in predicting Reynolds‐Averaged Navier‐Stokes (RANS) solutions. The study employs the symbolic distance function (SDF) to represent geometry and flow conditions, utilizing parameterized airfoil data from the UIUC (University of Illinois at Urbana‐Champaign) airfoil datasets. The research assesses the performance of multiple trained neural networks in predicting pressure and velocity distributions. Specifically, the study examines the influence of varying network weights on solution accuracy. Through the optimization of the model, the research demonstrates that the mean relative error is below 1.72% for a range of previously unseen wing shapes, with a computational speedup factor of up to 1,000× in certain scenarios. The accuracy achieved by this model underscores the significant potential of deep learning‐based approaches as reliable tools for aerodynamic design and optimization.

Three-Dimensional Particle Tracking Velocimetry Investigation of Flow Dynamics Around Simplified Stones at Low Submergence: Implications for Instream Habitat

Shallow waterways such as rapids, tributaries and smaller streams can have important ecological functions in both free-flowing and regulated rivers. As more intermittent renewable energy is introduced to the energy system to reduce CO2 emissions, the operational conditions of hydropower plants are changing. This implies various flow scenarios that can lead to more locations with shallow depths and larger variations in water levels and velocities, resulting in increased impact on the riverine ecosystem. Accurate predictions of these impacts require an understanding of the flow dynamics near large roughness elements such as boulders or trees in shallow river regions. This study uniquely investigates the effect of relative submergence, i.e., water depth relative to boulder size, on the flow field, turbulence, and potential fish habitats around idealized stone shapes (hemispheres) in shallow open channel flow using time-resolved 3D particle tracking velocimetry. The results indicate that varying relative submergence significantly affects recirculation zones, velocity and vorticity distribution, as well as turbulent kinetic energy. Notably, larger regions of lower velocity downstream of the roughness elements were generated at lower submergences, which might be favorable for fish energy conservation. Valuable insights into ecohydraulic engineering and habitat restoration in shallow waterways can be gained by understanding the fundamental flow mechanisms at low submergence for the flow around large roughness elements.

Disk-locking Regulates Stellar Rotation in Young Clusters: Insights from NGC 2264

Abstract Many young clusters possess extended main sequences, a phenomenon commonly ascribed to stellar rotation. However, the mechanism behind their very wide stellar rotation distributions remains unclear. A proposed explanation is that magnetic star–disk interaction can regulate stellar rotation, i.e., protostars with longer disk lifetimes will eventually evolve into slow rotators, and vice versa. To examine this hypothesis, we took the star-forming region NGC 2264, as a test bed. We have studied its high-mass pre-main-sequence and zero-age main-sequence (ZAMS) stars. We find that, on average, diskless pre-main-sequence stars rotate faster than their disk-bearing counterparts. The stellar rotation distribution of the ZAMS stars is similar to evolved young clusters. We conclude that disk-locking may play a crucial role in the rotational velocity distribution of intermediate-mass early-type stars. We suggest that the observed wide stellar rotation distributions in many young clusters can occur in their early stages.

An analytical study for predicting incipient motion velocity of sediments under ice cover

This study investigates the critical impact of incipient sediment motion on sediment transport estimation and riverbed evolution prediction. In this research, we examine the effects of ice cover on the vertical distribution of flow velocity, establishing a mathematical relationship between the vertical average flow velocities in open channel and ice-covered flows. This leads to the derivation of a formula for incipient motion velocity under ice cover. Additionally, the study analyzes the riverbed evolution process under ice jam conditions. The proposed formula is applicable to both open channel and ice-covered flows, effectively capturing the characteristics of incipient sediment motion for non-cohesive and cohesive sediments. The calculated incipient motion velocities closely align with the empirical data from existing literature. The study reveals that the roughness of ice cover significantly influences the incipient motion velocity of sediment, with higher ratios of ice cover roughness to riverbed roughness promoting sediment initiation under more favorable hydraulic conditions. Furthermore, the riverbed beneath ice jams experiences significant scouring. Field observations indicate that when ice jams form in localized sections of the river, the displacement of the main flow can substantially increase flow velocity in areas away from the ice jam, leading to scouring in non-ice-jammed areas and sedimentation in ice-jammed areas. The uneven distribution of ice jam is likely a critical factor contributing to discrepancies between theoretical predictions and observed outcomes. The complexity and limited data associated with the initiation of cohesive sediments pose challenges in validating the proposed formula for these sediment types.

Fast electrons produced by lower hybrid wave and effects on plasma–wall interactions

Abstract Lower hybrid wave (LHW) current drive plays a crucial role in sustaining steady-state (SS) discharges on the Experimental Advanced Superconducting Tokamak (EAST). Hotspots frequently form on the wave antenna and guard limiters during SS operations. Although both experimental and theoretical studies suggest that fast electrons could be responsible for these hotspots, the underlying mechanisms of fast electron generation under typical EAST operational parameters and their impact on the hotspots remain unresolved. In this work, particle-in-cell simulations are used to investigate the interactions between LHWs and electrons in front of the antenna, taking into account the realistic incident power spectra and localized field effects. The results show that, due to resonance overlap, fast electrons are produced through resonant interactions between electrons and LHW components with a high parallel refractive index (N ∥). The velocity distribution function in velocity space is found to significantly depend on plasma parameters near the antenna, such as q 95, electron temperature, and input power. These fast electrons notably enhance the sheath potential on the guard limiters and increase the heat flux to the wall.

Impact of Rock Cuttings on Downhole Fluid Movement in Polycrystalline Diamond Compact (PDC) Bits, Computational Fluid Dynamics, Simulation, and Optimization of Hydraulic Structures

The flow occurring at the bottom of a polycrystalline diamond compact (PDC) drill bit involves a complex process made up of drilling fluid and the drilled rock cuttings. A thorough understanding of the bottom-hole flow conditions is essential for accurately evaluating and optimizing the hydraulic structure design of the PDC drill bit. Based on a comprehensive understanding of the hydraulic structure and fluid flow characteristics of PDC drill bits, this study integrates computational fluid dynamics (CFD) with rock-breaking simulation methods to refine and enhance the numerical simulation approach for the liquid–solid two-phase flow field of PDC drill bits. This study further conducts a comparative analysis of simulation results between single-phase and liquid–solid two-phase flows, highlighting the influence of rock cuttings on flow dynamics. The results reveal substantial differences in flow behavior between single-phase and two-phase conditions, with rock cuttings altering the velocity distribution, flow patterns, and hydraulic performance near the bottom-hole region of the drill bit. The two-phase flow simulation results demonstrate higher accuracy and provide a more detailed depiction of the bottom-hole flow, facilitating the identification of previously unrecognized issues in the hydraulic structure design. These findings advance the methodology for multiphase flow simulation in PDC drill bit studies, providing significant academic and engineering value by offering actionable insights for optimizing hydraulic structures and extending bit life.

Numerical analysis of aerodynamic performance characteristics of NACA 2412 and NACA 4412 at Re = 4000000

This paper conducts a numerical analysis of the two-dimensional aerodynamic characteristics of the NACA 2412 and NACA 4412 aerofoils at a Reynolds number of Re = 4106. Employing the fluid simulation software Ansys Fluent, which is based on the finite volume method, for numerical analysis. Both the geometric model and mesh of the aerofoil are established by using Fluent Meshing, and the simulation calculation is performed based on the pressure solver. Based on the Navier-Stokes equations, the SST k- turbulence model and coupled algorithm are employed for the simulation processing. The paper systematically compares the aerodynamic characteristics of NACA 2412 and NACA 4412, including the pressure and velocity distributions, as well as the variations in the lift coefficient, drag coefficient, and lift-to-drag ratio concerning the angle of attack. The results indicate that at Re = 4106, the peak value of the lift-to-drag ratio of the NACA 2412 aerofoil is attained under the 8 condition, for the NACA 4412 aerofoil, the lift-to-drag ratio reaches its peak value at a 6 angle of attack. Additionally, the stall angles of both types of aerofoils under the studied operating conditions are 16.

Comparison of aerodynamic characteristics of NACA 2415 and NACA 4415 Aerofoils

This study compared the two-dimensional aerodynamic characteristics of NACA 2415 and NACA 4415 at different angles of attack. In this study, the Reynolds number is 2.3e6, under which the velocity of the air is 33 m/s. The mesh of C-type is established based on ANSYS Meshing; to simplify the calculation, the chord length is set to 1 m. The fluid simulation is done by ANSYS Fluent, and the simulation calculation is performed based on the pressure solver. The N-S equation is used as the governing equation. Velocity inlet and pressure outlet are set as the boundary condition, and the k-omega SST viscous model is chosen for the simulation processing. This study compares the aerodynamics of NACA 2415 and NACA 4415 at different angles of attack, including the pressure and velocity distribution, lift and drag coefficient, and lift-drag ratio. The results of this research would help optimize wing design and develop new aerofoil and testing.

Computational simulation of the Aerodynamic performance of NACA2412 and NACA25112 in low Reynolds numbers

The present study introduces the basic definition of aerofoil and NACA series aerofoil. Then numerically investigates the two-dimensional aerodynamic characteristics of NACA 2412 and NACA 25112 aerofoils at a Reynolds number of 2. 0106. This study utilises the fluid dynamics simulation tool ANSYS Fluent, which is based on the finite volume method, to acquire the necessary data. The geometric model and grid of aerofoil were built with ANSYS Meshing and simulated with the pressure-based solver. In addition, the experimental environment of this study is the inlet speed of 30m/s and the air pressure of 1ATM. The study based on the Nabier-Strokes control equation completes simulation processing with SSTK-Omega. The aerodynamic characteristics of NACA2412 and NACA25112 were compared, including the pressure, velocity distribution map and the lift resistance coefficient change with the angle of attack. These experimental results are presented in the form of graphs. This experiment can be used for subsequent wind tunnel experiments or for developing a new aerofoil.

Improved Particle Spray Algorithm for Modeling Globular Cluster Streams

Abstract Stellar streams that emerge from globular clusters (GCs) are thin stellar structures spread along the orbits of progenitor clusters. Numerical modeling of these streams is essential for understanding their interaction with the host galaxy's mass distribution. Traditional methods are either computationally expensive or oversimplified, motivating us to develop a fast and accurate approach using a particle spray algorithm. By conducting a series of N-body simulations of GCs orbiting a host galaxy, we find that the position and velocity distributions of newly escaped stream particles are consistent across various GC masses and orbital parameters. Based on these distributions, we develop a new algorithm that avoids computing the detailed internal cluster dynamics by directly drawing tracer particles from these distributions. This algorithm correctly reproduces the action space distribution of stream particles and achieves a 10% accuracy in stream morphology and velocities compared to N-body simulations. To facilitate broader use, we have implemented this algorithm in galactic dynamics codes agama, gala, galax, and galpy.

Influence of Processing Parameters on Flow Behaviour of Ultra-Large-Section Beam Blank Continuous Casting Mould

The complex cross-sectional shape of oversized beam blanks and the size effect of ultra-large-section beam blanks create severe issues related to the surface and internal quality of the castings. To ensure quality and control in the production of ultra-large-section beam blanks, a numerical and physical model of molten steel flow in the three-port submerged entrance nozzle (SEN) mould, with section dimensions of 1300 × 510 × 140 mm, was established. This model was created using numerical simulations and NSGA-II genetic algorithm optimisation, and the impact of the casting speed and SEN immersion depth on the mould’s flow behaviour was investigated. The results showed that a deeper SEN immersion depth resulted in, a greater impact depth of the molten steel, and the surface flow velocity decreased. Both the impact depth and the surface flow velocity of the molten steel increased with increasing casting speed. The physical simulation and numerical simulation of the molten steel flow form and flow velocity distribution in the mould were in good agreement with each other, thus verifying the accuracy of the numerical simulation. The process parameters derived from this study were all within an appropriate range, which can help to improve the quality of continuously cast beam blanks. This also provides guidance for selecting optimal parameters for actual continuous casting production processes.

Computational Fluid Dynamics Analysis of a Unique Biodiesel Production System based on Hydrodynamic Cavitations

Purpose: The study aims to improve biodiesel production through hydrodynamic cavitation, focusing on increased energy efficiency, higher yields, and reduced production time. By optimising the production process, especially with the use of waste cooking oil. Design/Methodology/Approach: A computational fluid dynamics (CFD) analysis was performed to model the flow of biodiesel within a multi-plate system featuring several orifices. Theoretical input and output velocities calculations were derived from experimental mass flow rates. The CFD analysis provided pressure distribution along the pipe walls, which was utilised to assess the lifespan and damage cycles of the cavitation pipe and to study the behaviour of fluid turbulence. Research Limitation: The research focuses on simulating flow behaviour in acrylic pipes within designated boundary conditions. Findings: The research showed that producing biodiesel through hydrodynamic cavitation markedly lowers production expenses and duration while enhancing energy efficiency and output. Computational fluid dynamics (CFD) analysis offered valuable information about the distribution of pressure, velocity, and turbulence, aiding in optimising the cavitation pipe design. Practical Implication: An optimised design for cavitation chambers has the potential to enhance the cost-effectiveness and efficiency of biodiesel production, positioning it as a useful substitute for traditional fuels. Social Implication: Embracing this technology can help mitigate environmental pollution by repurposing waste cooking oil and lessening reliance on non-renewable petroleum fuels, thereby supporting global sustainability objectives. Originality/Value: This study introduces an innovative method for producing biodiesel using hydrodynamic cavitation and CFD analysis to improve pipe design and production parameters. It addresses a significant gap in boosting biodiesel's economic and environmental feasibility, offering a scalable and sustainable alternative for fuel production. .

Sound-vortex conversion on droplets: Kinematic model of sound activated vortices

The rapid pace of urbanization across the globe has led to the proliferation of various urban infrastructure. They generate aerodynamic noise, posing significant challenges to urban planning and residents' comfort. Despite Lighthill's mathematical analogy, current noise control applications rely heavily on wind tunnel tests and computer simulations. Physical mechanisms behind aeroacoustic noise have yet to be validated and elucidated experimentally. This work observed the geometry of sound-activated vortices and determined their streamline and velocity distribution. The theoretical results are consistent with experimental observations, offering a kinematic model for sound activated vortices. The work experimentally improves the understanding of aeroacoustic noise and provides a kinematic model for the development of noise control strategies.

The physical and chemical structure of Sagittarius B2

Context. The giant molecular cloud complex Sagittarius B2 (Sgr B2) in the central molecular zone of our Galaxy hosts several high-mass star formation sites, with Sgr B2(M) and Sgr B2(N) being the main centers of activity. This analysis aims to comprehensively model each core spectrum, considering molecular lines, dust attenuation, and free-free emission interactions. We describe the molecular content analysis of each hot core and identify the chemical composition of detected sources. Aims. Using ALMA’s high sensitivity, we aim to characterize the hot core population in Sgr B2(M) and N, gaining a better understanding of the different evolutionary phases of star formation processes in this complex. Methods. We conducted an unbiased ALMA spectral line survey of 47 sources in band 6 (211-275 GHz). Chemical composition and column densities were derived using XCLASS, assuming local thermodynamic equilibrium. Quantitative descriptions for each molecule were determined, considering all emission and absorption features across the spectral range. Temperature and velocity distributions were analyzed, and derived abundances were compared with other spectral line surveys. Results. We identified 65 isotopologs from 41 different molecules, ranging from light molecules to complex organic compounds, originating from various environments. Most sources in the Sgr B2 complex were assigned different evolutionary phases of high-mass star formation. Conclusions. Sgr B2(N) hot cores show more complex molecules such as CH3OH, CH3OCHO, and CH3OCH3, while M cores contain lighter molecules such as SO2, SO, and NO. Some sulfur-bearing molecules are more abundant in N than in M. The derived molecular abundances can be used for comparison and to constrain astrochemical models. Inner sources in both regions were generally more developed than outer sources, with some exceptions.

A new single-phase flow algorithm for enabling water level regulation in surge shafts

Surge shafts play a role in stabilizing pressure in long-distance water pipeline systems. However, to accurately simulate its regulatory function in three-dimensional numerical simulations, a multiphase flow model that demands substantial computational resources is necessary. Through secondary development of software, the implementation of a single-phase flow surge shaft model was achieved. By comparing the calculation results of the multiphase flow model, the accuracy and feasibility of the single-phase flow calculation method were verified. The results indicate that the single-phase simulation method could accurately capture the water level variation pattern. Deviations within an acceptable range only occur during water level drawdown. Furthermore, minor differences in velocity distribution between the two methods exist solely within the impedance hole channel, and the pressure field was in good agreement. The pressure pulsation deviation of all monitoring points was below 2.5%, and in the long pipeline, it was less than 0.5%. The correlation coefficients of key monitoring points, especially those inside the pipeline, were higher than 0.98, indicating a high correlation and confirming the high reliability of the simplified single-phase flow method for practical applications. This study demonstrates that the approach is a viable and accurate tool for simulating surge shaft behavior in pipeline systems.

INFLUENCE OF GRID DENSITY AND ROTATION MODEL ON PROPELLER OPEN WATER CFD SIMULATION AND UNCERTAINTY

To investigate and quantify the effect of grids as well as different numerical models on the propeller open water simulation results and to evaluate the confidence level of the simulation results, numerical simulation of open water and uncertainty assessment are necessary. The propeller open water performance was calculated using the SST <i>k-ω</i> method with MRF (Multiple Reference Frames) and SM (Sliding Mesh) models under different grid densities. Following ITTC (International Towing Tank Conference) (2021) procedures, CFD (Computational Fluid Dynamics) uncertainty evaluation and flow field analysis of the numerical models were carried out. The principles of the "verification" and "validation" steps were introduced. Three grids of different densities were numerically simulated to obtain the thrust coefficient <i>K</i><sub>T</sub>, torque coefficient <i>K</i><sub>Q</sub> and open water efficiency η at different advance coefficient, J. CFD calculation result uncertainty analysis was implemented. Comparisons were made of near-wall Y+ distribution, wakefield, velocity distribution, and surface pressure distribution at <i>J</i>=0.6. Results show that: 1) the trends of the three grids' results were consistent, and the SM model was more sensitive to grids, but its converged results were closer to the true value; 2) near-wall Y+ distribution, wakefield and velocity distribution showed noticeable differences due to grid discrepancies, while pressure distributions were approximate. Calculation results monotonically converged in "verification" and were validated in "validation". Thus, calculation results meet application requirements and model settings can provide an engineering reference.

Numerical Study of Supercritical Flow over Mesh-panel Bottom Rack Used to Estimate the Dividing Discharge Streamline in Clear Water Conditions

Bottom racks are critical for diverting flow for small hydropower projects. This study investigates the hydraulic behavior of a mesh-panel bottom rack, comprising longitudinal and transverse bars, under clear-water conditions using a numerical model developed with the Flow-3D finite volume code. The research focuses on the discharge dividing streamline (DDS) and derives an empirical regression equation for its position based on model results. The numerical model is calibrated and validated against experimental data from Bina and Saghi (2017), yielding average relative errors of 3.4% for diverted discharge (Qd) and 1.9% for remaining discharge (Qr). The pressure distribution and vertical flow velocity analyses reveal that negative pressure around the transverse bars enhances diverted discharge. The vertical velocity distribution shows that outflow begins after approximately 30% of the rack length. The deep velocity distribution extracted from the Flow-3D model matches experimental data well, achieving a coefficient of determination (R² = 0.98) and a relative error (Er = 0.32%). This study proposes a regression relation to estimate the normalized DDS position, facilitating easy determination of diverted discharge in river diversion projects by measuring the approaching flow depth. Keywords: Bottom rack, Diverted discharge ratio (DDR), Discharge dividing streamline (DDS), Flow-3D numerical model, Hydraulic behavior, Vertical flow velocity.