Three-dimensional Velocity Distribution Research Articles

Investigation of sheath structure in surface flashover induced by high-power microwave

Flashover is a major limiting factor for the transmission and miniaturization of high-power microwave (HPM) devices. We conducted a study to investigate the developmental process of surface flashover on HPM dielectric windows through particle-in-cell-Monte Carlo collision simulations. A one-dimensional spatial distribution and three-dimensional velocity distribution model is established, encompassing the entire process of surface flashover, which includes electrode field emission, single-surface multipactor, outgassing, and gas breakdown. The nonuniform mesh generation method is employed to enhance the simulation accuracy. The growth rates of electron and ion densities increase as gas pressure rises. Additionally, the discharge transitions gradually from multipactor to gas ionization dominance. Notably, a space-charge-limited (SCL)-like sheath occasionally forms during an rf cycle near the surface under intermediate background pressure (∼0.05 Torr). The SCL-like sheath cannot exist stably. Instead, it periodically disappears and appears as the rf electric field changes. The underlying physics are explained by the variations of the rf electric field, which lead to the variations in the surface charge density, thereby affecting the normal electric field. The normal electric field interacts with the spatial distribution of charged particles, ultimately leading to the formation of the SCL-like sheath. This work may facilitate a comprehensive understanding of the developmental processes of surface flashover.

Crustal structure beneath Central Kamchatka inferred from ambient noise tomography

In this study, we investigated the crustal structure beneath the area of Central Kamchatka (Kamchatka Peninsula, Russian Far East) limited by latitudes 52.8 and 54.1 degrees. This area comprises parts of the active Eastern Volcanic Front and two mountain ridges Sredinny and Ganalsky, as well as several backarc volcanoes. We used the continuous data from the permanent stations and the temporary network installed in 2019–2020 to perform ambient noise tomography. We implemented a two-step tomographic inversion algorithm that initially generated the two-dimensional maps of group velocities for periods ranging from 1 s to 30 s, then found the optimal 1D model of shear wave velocities and derived the three-dimensional S-wave velocity distribution. In the resulting model, we observe a prominent low-velocity anomaly below the active volcanoes of Avacha and Koryaksky, which is traced to depths of ∼40 km and may be associated with a zone of heated rocks suitable for petrothermal energy extraction. We propose that this anomaly represents a common magma pathway through the crust feeding both volcanoes. Beneath another active Zhupanovsky volcano, we observe three low-velocity layers at depths of 2 km, 8 km and 20 km indicative of a multilevel magma feeding system. We believe our findings will further the understanding of the complex processes occurring within these active volcanoes, which pose a tangible hazard to the relatively populated Petropavlovsk-Kamchatsky urban district. Beneath the Sredinny Ridge we reveal a high-velocity pattern at shallow depths, likely associated with consolidated magmatic structures. The low-velocity in the lower crust is interpreted as a high-temperature zone conserved here since the time when this area hosted an active volcanic arc. Upon further exploration, this high-temperature zone could prove to be a prospective source of geothermal energy. Below the Ganalsky Ridge, a prominent high-velocity anomaly extending to a depth of ∼15 km can be attributed to the Precambrian to Carboniferous metamorphic rocks of predominantly mafic composition.

How Long-lived Grains Dominate the Shape of the Zodiacal Cloud

Grain–grain collisions shape the three-dimensional size and velocity distribution of the inner Zodiacal Cloud and the impact rates of dust on inner planets, yet they remain the least understood sink of zodiacal dust grains. For the first time, we combine the collisional grooming method combined with a dynamical meteoroid model of Jupiter-family comets (JFCs) that covers 4 orders of magnitude in particle diameter to investigate the consequences of grain–grain collisions in the inner Zodiacal Cloud. We compare this model to a suite of observational constraints from meteor radars, the Infrared Astronomical Satellite, mass fluxes at Earth, and inner solar probes, and use it to derive the population and collisional strength parameters for the JFC dust cloud. We derive a critical specific energy of QD*=5×105±4×105Rmet−0.24 J kg−1 for particles from JFC particles, making them 2–3 orders of magnitude more resistant to collisions than previously assumed. We find that the differential power-law size index −4.2 ± 0.1 for particles generated by JFCs provides a good match to observed data. Our model provides a good match to the mass-production rates derived from the Parker Solar Probe observations and their scaling with the heliocentric distance. The higher resistance to collisions of dust particles might have strong implications to models of collisions in solar and exosolar dust clouds. The migration via Poynting–Robertson drag might be more important for denser clouds, the mass-production rates of astrophysical debris disks might be overestimated, and the mass of the source populations might be underestimated. Our models and code are freely available online.

Three-dimensional crescent-shaped ion velocity distributions created by magnetic reconnection in the presence of a guide field

By means of theory and particle simulations, crescent-shaped ion velocity distributions in the outflow region of symmetric magnetic reconnection in the presence of a guide magnetic field are investigated. Assuming a spatial one-dimensional electromagnetic field, a theoretical model accounting for the shape of crescents is derived. First, following the earlier theoretical models suggested by Bessho et al. [Geophys. Res. Lett. 43, 1828–1836 (2016)] and Zenitani et al. [J. Geophys. Res.: Space Phys. 122, 7396–7413 (2017)], we derive a theoretical model for 2D velocity distributions based on the conservation of the canonical momentum and the energy. The 2D theoretical model exhibits a two-dimensional structure of crescent-shaped velocity distributions and further demonstrates that no magnetic field reversal is required in the formation of crescents, although many researchers have considered that magnetic field reversal plays an essential role. Next, we construct a theoretical model for 3D velocity distributions based not only on the conservation of the canonical momentum in two directions and the energy but also on the conservation of the magnetic moment and the kinetic energy in the moving frame with the reconnection outflow speed, which are applied when the guide field ratio is high. The 3D theoretical model derives a three-dimensional structure of crescents by combination of an antiparallel magnetic field and a guide magnetic field comparable to or greater than the reconnection field. Both of the 2D and 3D theoretical models are consistent with ion velocity distributions found in our particle simulations.

The data assimilation of volumetric velocity and temperature data to extract boundary conditions for high-fidelity simulation of the indoor air

The contemporary challenge of the air conditioning sector is to continuously improve the provided comfort conditions while reducing the energy consumption of the devices. This necessity can only be satisfied by improving research and design methods. Therefore, on the one hand, researchers worldwide develop and utilize experimental methods for more accurate measurements. On the other hand, researchers develop numerical methods to investigate design iterations effectively and quickly.The main idea of this study is to create a novel data assimilation method to utilize all the advantages of experimental and numerical methods to develop a high-fidelity investigation method of an indoor air model.In the present study, the three-dimensional velocity distribution at the split air conditioning indoor unit's outlet section was measured by using the Stereo Particle Image Velocity Measurement method. The temperature distribution at the outlet section of the split air conditioning indoor unit was also determined using the Meshed Infrared Thermography method. The data from these experimental measurements were assimilated to generate a highly realistic boundary condition of the steady-state and time-dependent Computational Fluid Dynamics (CFD) models. This is achieved for the first time in the literature.The results of these analyses compared with conventional CFD and the aforementioned experimental methods. The study's results show that the novel data assimilation approach (3.13% average error) is significantly more accurate than the traditional average boundary condition approach (13.19% average error).

3D reconstruction of nonuniform and unsteady flow velocity distribution using nonlinear acoustic ray tracing and tomography

The nonuniform and unstable gas flow distribution at the air inlets of the boiler combustion chamber can negatively affect the energy efficiency and safety of the boiler. Accurately reconstructing the flow velocity inside the furnace in three dimensions can help the boiler automatically adjust the air supply to reduce unwanted energy waste and gas emissions.In this study, we propose a novel method for three-dimensional velocity reconstruction based on nonlinear acoustic ray tracing and tomography. The model provides the equation of acoustic ray motion in the flow field, which includes spatial and temporal parameters. During the process of measuring flow velocity, higher precision reconstruction of the three-dimensional velocity distribution was achieved by combining acoustic refractive paths based on derivative-free optimization iteration updates with radial basis functions that use Gaussian kernels.The computational time cost and reconstruction accuracy of the proposed algorithm were analyzed numerically and through simulations. The results indicate that the method incorporating acoustic ray tracing greatly improves the reconstruction accuracy of large gradient nonuniform flow fields. Additionally, we analyzed the impact of different numbers of sound tracing paths on reconstruction quality and computation time, providing a reference for the rational placement of microphones and the selection of the most efficient paths in boilers.Experimental results show that this method has good reconstruction capabilities for nonuniform flow fields and practical applications.

Manipulating nanoparticles based on a laser photothermal trap.

A method of efficient directional optical manipulation of nanoparticles based on a laser photothermal trap is proposed, and the influence mechanism of external conditions on the photothermal trap is clarified. Through optical manipulation experiments and finite-element simulations, it is determined that the main cause of gold nanoparticle directional motion depends on the drag force. The laser power, boundary temperature, and thermal conductivity of the substrate at the bottom of the solution and liquid level essentially affect the intensity of the laser photothermal trap in the solution and then affect the directional movement and deposition speed of gold particles. The result shows the origin of the laser photothermal trap and the three-dimensional spatial velocity distribution of gold particles. It also clarifies the height boundary of photothermal effect onset, which clarifies the boundary between light force and photothermal effect. In addition, nanoplastics are manipulated successfully based on this theoretical study. In this study, the movement law of gold nanoparticles based on the photothermal effect is deeply analyzed through experiments and simulations, which is of significance to the theoretical study of the optical manipulation of nanoparticles using the photothermal effect.

Electromagnetic particle simulation of secondary electron multipactor characteristics in inner surface of 110 GHz microwave output window

The secondary electron multipactor on the inner surface of the output window is one of the main factors limiting the power capacity of high power microwave. Therefore, it is of great significance to carry out relevant research. In this work, the process of secondary electron multipactor and the resulting loss of power are numerically simulated by using the electromagnetic particle model with one-dimensional spatial distribution and three-dimensional velocity distribution at the microwave frequency of 110 GHz. The influences of microwave electric field at the surface and dielectric material type on the power loss are studied. The simulation results show that the electron number density is higher than the critical cut-off number density after the secondary electron multipactor has reached the steady state, but the microwave electric field does not show obvious change. This is because the electrons in a very high electrostatic field are mainly concentrated in the domain of several micrometers near the dielectric surface, which is far less than the corresponding skin depth. The electron number density in the multipactor steady state increases with the microwave electric field increasing, but the ratio of the power loss to the microwave power at the surface increases slowly. After the multipactor reaches the steady state, the number density of electrons near the sapphire surface is the highest, followed by the number density near the crystal quartz surface, and the number density near the fused quartz surface is the lowest, so the corresponding power loss decreases successively. In order to verify the accuracy of the model, the simulated value of the multipactor threshold is compared with the experimental data, and the difference between them is discussed.

Derivation and Verification of Gaussian Terrain Wake Model Based on Wind Field Experiment

Aiming at the problem where the current engineering wake model does not describe the wind speed distribution of the wake in the complex terrain wind farm completely, based on the three-dimensional full wake model (3DJGF wake model), this paper proposed a wake model that can predict the three-dimensional wind speed distribution of the entire wake region in the complex wind farm, taking into account the Coanda effect, wind shear effect, and wake subsidence under the Gaussian terrain. Two types of Doppler lidar were used to conduct wind field experiments, and the inflow wind profile and three-dimensional expansion of the wake downstream of the wind turbine on the Gaussian terrain were measured. The experimental results showed that the wake centerline and terrain curve showed similar variation characteristics, and the near wake profile was similar to a super-Gaussian shape (asymmetric super-Gaussian shape) under low-wind-speed conditions, while the near wake profile presented a bimodal shape (asymmetric bimodal shape) under high-wind-speed conditions. The predicted profiles of the Gaussian terrain wake model were compared with the experimental data and the three typical wake models. The comparison results showed that the newly proposed Gaussian terrain wake model fit well with the experimental data in both near wake and far wake regions, and it had better performance in predicting the wake speed of the Gaussian terrain wind farm than the other three wake models. It can effectively predict the three-dimensional velocity distribution in the whole wake region of complex terrain.

Flow behaviour concerning bank stability in the presence of spur dike – A review

Abstract Flow in streams and rivers typically erodes the banks, causing channel bank migration laterally, resulting in loss of nearby land, modification in channel morphology, excessive sediment transport, and water quality degradation. A spur dike is a hydraulic structure placed at the channel bank projecting outward to guide or divert the flow away from the bank, thus protecting it from erosion. The stability of the riverbed and banks is influenced by turbulent characteristics such as three-dimensional velocity distribution, turbulent kinetic energy, Reynolds shear stress, turbulent intensity, and bed shear stress. The researchers found that these turbulence parameters are instrumental in sediment movement along the channel's bed and from its banks. Spurs dikes are a significant river training structure provided along the river bank to protect from erosion. Several aspects related to spur dikes, such as their geometry, physical features, design considerations, flow and scour patterns, etc., are critically reviewed in this paper. Despite the numbers of literature in the field of turbulent characteristics and scour depth around spur dike, the role of vegetation and the effect of seepage around spur dike still remains an unexplored area. These knowledge gaps of spur dikes in field conditions are discussed for future studies.

An enhanced mesoscale model for laminar and turbulent three-dimensional simulations of the gas phase flow with concentration mixing in structured packing columns

This work presents a mesoscale model (MSM) simulating the laminar and turbulent gas phase fluid dynamics and concentration mixing in structured packings, used in absorption and distillation columns or static mixers. The MSM aims to provide computationally efficient predictions of the three-dimensional distributions of pressure, velocity and concentration in the structured packing’s gas phase. The MSM is successfully calibrated and validated with virtual experiments obtained from DNS (direct numerical simulations) of a single structured packing section featuring concentration mixing under uniform and maldistributed velocity inlet profiles. The packing section scenarios exhibit a variety of physical effects, like backflow, high-velocity fronts and blockage at the packing’s borders to the wall, which are all predicted correctly by the MSM. The physical effects have an impact on concentration mixing and velocity smoothing and are particularly interesting for applications of structured packings in dividing-wall columns or static mixers. Finally, the MSM is set up to simulate cylindrical columns with alternately rotated structured packing sections, including wall regions and wipers. MSM simulations of three packed columns of varying diameter are compared against experimental data and correctly predict the pressure drop dependence on the diameter, demonstrating the MSM’s potential for scale-up predictions.

Revolutionizing Our Understanding of Particle Energization in Space Plasmas Using On-Board Wave-Particle Correlator Instrumentation

A leap forward in our understanding of particle energization in plasmas throughout the heliosphere is essential to answer longstanding questions in heliophysics, including the heating of the solar corona, acceleration of the solar wind, and energization of particles that lead to observable phenomena, such as the Earth’s aurora. The low densities and high temperatures of typical heliospheric environments lead to weakly collisional plasma conditions. Under these conditions, the energization of particles occurs primarily through collisionless interactions between the electromagnetic fields and the individual plasma particles with energies characteristic of a particular interaction. To understand how the plasma heating and particle acceleration impacts the macroscopic evolution of the heliosphere, impacting phenomena such as extreme space weather, it is critical to understand these collisionless wave-particle interactions on the characteristic ion and electron kinetic timescales. Such understanding requires high-cadence measurements of both the electromagnetic fields and the three-dimensional particle velocity distributions. Although existing instrument technology enables these measurements, a major challenge to maximize the scientific return from these measurements is the limited amount of data that can be transmitted to the ground due to telemetry constraints. A valuable, but underutilized, approach to overcome this limitation is to compute on-board correlations of the maximum-cadence field and particle measurements to improve the sampling time by several orders of magnitude. Here we review the fundamentals of the innovative field-particle correlation technique, present a formulation of the technique that can be implemented as an on-board wave-particle correlator, and estimate results that can be achieved with existing instrumental capabilities for particle velocity distribution measurements.

Numerical Simulation Analysis on Hydraulic Optimization of the Integrated Pump Gate

Based on the Reynolds time mean N-S equation and standard k-ε turbulence model and using Computational Fluid Dynamics technology, this study aims to integrate the brake pump to carry out numerical simulation. Through the adoption of different arrangements of impending height and spacing, the hydraulic characteristics of the full tubular pump unit are analyzed. The two-dimensional streamline, velocity and pressure distribution, and three-dimensional streamline, axial velocity and vorticity distribution of the front pool of each scheme are displayed. The results show that the recommended pump installation height is 0.8 Dd, the maximum limit value of the pump station design specification; in the dual-pump mode, the recommended pump spacing is 2.00 Ds.

Simple model for flow field division and flow structure calculation in a vortex tube

The flow pattern and energy transfer process in the vortex tube are extremely complex, resulting in the difficulty of the direct calculation and prediction of the flow structure and temperature separation performance of a vortex tube. In view of this, research idea of dividing the vortex tube flow field into six regions was proposed, corresponding simplifications were made in different regions according to the flow features, and fluid parameters were coupled at the boundaries. An equation for the axial velocity component in the hot tube region was developed based on the characteristic that the axial velocity profiles under different conditions present similar patterns, the calculation method of reverse flow boundary which separates the working fluid into hot and cold fluids was described, and a set of relatively simple calculation methods for the internal flow field of vortex tubes was established based on the partition model. Three-dimensional velocity distributions inside the vortex tube under different working conditions were computed and analyzed. Further, the model was evaluated and validated through experimental data. The results showed good agreements of calculated data and experimental velocity fields. This work proposed a convenient calculation method for the estimation of the flow behavior inside a vortex tube and provide reliable predictions.

The ESO-VLT MIKiS Survey Reloaded: Velocity Dispersion Profile and Rotation Curve of NGC 1904* *Based on observations collected at the European Southern Observatory, Cerro Paranal (Chile), under proposal 193.D-0232 (PI: Ferraro) and 0104.D-0636 (PI: Ferraro).

We present an investigation of the internal kinematic properties of M79 (NGC 1904). Our study is based on radial velocity measurements obtained from the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters for more than 1700 individual stars distributed between ∼0.″3 and 770″ (∼14 three-dimensional half-mass radii) from the center. Our analysis reveals the presence of ordered line-of-sight rotation with a rotation axis almost aligned along the east–west direction and a velocity peak of 1.5 km s−1 at ∼70″ from the rotation axis. The velocity dispersion profile is well described by the same King model that best fits the projected density distribution, with a constant central plateau at σ 0 ∼ 6 km s−1. To investigate the cluster rotation in the plane of the sky, we have analyzed the proper motions provided by the Gaia EDR3, finding a signature of rotation with a maximum amplitude of ∼2.0 km s−1 at ∼80″ from the cluster center. Analyzing the three-dimensional velocity distribution for a subsample of 130 stars, we confirm the presence of systemic rotation and find a rotation axis inclination angle of 37° with respect to the line of sight. As a final result, the comparison of the observed rotation curves with the results of a representative N-body simulation of a rotating star cluster shows that the present-day kinematic properties of NGC 1904 are consistent with those of a dynamically old system that has lost a significant fraction of its initial angular momentum.

Experimental and numerical analysis of velocity distribution in a compound meandering channel with double layered rigid vegetated flood plains

Vegetation is one of the major topographic features that is encountered along and across the margins and flood plains of many rivers systems. This vegetation creates a most complex flow mechanism in the compound river bed channels; therefore, a detailed analysis is required to observe the flow and vegetation interactions to understand the hydrodynamic aspects in the river systems. This paper studies the effect of double-layered rigid vegetation in a meandering channel on the flow characteristics at two relative depth conditions, of 0.34, 0.45 which creates an alternate emergent and submerged flow situation. The three-dimensional velocity distribution was captured using micro-ADV. The concept of two relative depth conditions allowed us to capture record and classify the velocity zones between the short and tall vegetation. Compared to the flow in the main channel, flood plains registered relatively lower velocity values due to the resistance offered by the vegetation along the flood plains, which consequently led to the increase in main channel flow velocities. Velocities compared to the non-vegetated meandering channels; the highest velocity readings were recorded at the centerline of the main channel. Numerical analysis was also conducted using the CFD codes in fluent. The vegetation geometry is modelled as cylindrical dowels of diameter 10 mm and two-variable heights of 7.5 cm and 15 cm at two relative depth flow conditions. The experimental results were numerically compared using the k-ϵ model along with grid sensitivity tests. The final simulated numerical results were found to be close and in good agreement with experimental values.

Temperature distribution and pressure oscillation characteristics of di-tert-butyl peroxide overpressure relief in a confined space

Pressure relief can effectively reduce the damage caused by the thermal runaway reaction of organic peroxides in the chemical process. To further understand the overpressure relief mechanism of di- tert -butyl peroxide (DTBP), pressure vessel test system and numerical simulation method were combined to systematically analyze the dynamic relief characteristics of DTBP in a confined space. The three-dimensional spatial distribution and variation law of temperature, pressure, and airflow velocity during the relief process were also revealed. The results show that a high-speed under-expanded jet is generated when DTBP overpressure relief occurs, and low temperature and negative pressure are produced inside the vessel and near the vent. The airflow velocity first increases and then decreases during the relief process, with a maximum value of 487 m/s. Both the internal temperature and pressure show prominent damping oscillation characteristics, and the oscillation period is between 2.1 ms and 2.3 ms. The pressure oscillation curves at different positions present different peak shapes, and the closer to the vent, the smaller the pressure amplitude. The consistency between the experimental results and simulation data also verifies the reliability of the numerical model.

Three-Dimensional Velocity Distribution Measurement Using Ultrasonic Velocity Profiler with Developed Transducer

This study describes an ultrasonic velocity profiler that uses a new ultrasonic array transducer with unique 5-element configuration, with all five elements acting as transmitters and four elements as receivers. The receivers are designed to reduce the amount of uncertainty. As the fluid moves through this setup, four Doppler frequencies are obtained. The multi-dimensional velocity information along the measurement line can be reconstructed. The transducer has a compact geometry suitable for a wide range of applications, including narrow flow areas. The transducer’s basic frequency and sound pressure are selected and evaluated to be compatible with the application. First, to confirm the measurement ability, the measurement of the developed system in two-dimensional flow is validated by comparing it to the theoretical data. The uncertainty of measurement was within 15%. Second, the three-dimensional measurement in turbulent and swirling flow is proved experimentally to check the applicability of the proposed technique.

Breakup model of oscillating drops in turbulent flow field

Following the drop oscillation breakup mechanism, a theoretical model for drop breakup probability is proposed based on the three-dimensional Maxwell velocity distribution. The model considers both the interfacial energy increase constraint and viscous energy increase constraint. The model shows that for low-viscous drops, the breakup probability is determined by the Weber number (WeL), and for intermediate or high viscous drops, the breakup probability is determined by the combined influence of the Weber number (WeL) and the Ohnesorge number (Oh). By combining the theoretical model of drop breakup time constructed in our previous work, the breakup frequency model is obtained based on the statistical description framework. The accuracy and generality of the model were then validated using the direct experimental data. Moreover, effects of the drop diameter, turbulent energy dissipation rate, and interfacial tension on the predicted drop breakup frequency were analyzed in detail.

Cross-Scale Flow Field Analysis of Sealing Chamber and End Face Considering the CO2 Real Gas Effect

The dry gas seal (DGS) is a non-contacting, gas-lubricated mechanical face seal commonly used in rotating machinery. Traditional analyses of DGSs treat the end face as an independent factor by setting the end-face inlet as boundary conditions, but limited attention is focused on the sealing chamber of the DGS. Using the finite volume method and the shear stress transport (SST) k-ω model, the coupling between the millimeter-scale sealing chamber and the micrometer-scale end face are simulated with regard to the real gas effect of CO2. The three-dimensional distributions of velocity, pressure and temperature in the cross-scale flow field are investigated under different working conditions. Moreover, the boundary parameters of the end-face inlet are modified by response surface methodology with a central composite rotatable design. The results demonstrate that the real gas effect of CO2 leads to an increased total inlet flow. When the pressure reaches 10.3 MPa, the relative difference is 51.90% compared to ideal gas. Minor temperature and pressure changes occur in the sealing chamber when the dry gas seal is in operation. However, the inlet temperature of the end face Tf increases and the inlet pressure of the end face pf decreases. These research results provide a reliable reference for engineering practice.