Vortex Movement Research Articles

Effect of Water Vapor Transport on a Typical Rainstorm Process in the Arid Region of Southern Xinjiang: Observations and Numerical Simulations

There are frequent and intensive periods of heavy rain in the arid areas of southern Xinjiang. This study uses a typical rainstorm process in the South Xinjiang Basin to investigate the weather, physical mechanisms, mesoscale characteristics, and income and expenditure characteristics of water vapor sources, analyzing them using the observation data from southern Xinjiang regional automatic stations, ERA5 reanalysis data, multi-source satellite data, and WRF numerical simulation results. The study results show that torrential rain processes occur in the double-body distribution of the South Asian High in the upper troposphere, which is “high in the east and low in the west,” with “two ridges and one trough” in the middle layer. The development and movement of the low vortex, the configuration of low-level convergence and high-level divergence, and vertical upward movement provide favorable dynamic conditions for heavy rain. Additionally, the Black Sea, the Caspian Sea, the Aral Sea, the Arabian Sea, and the Bay of Bengal are important water vapor sources for this rainstorm. The water vapor reaches the South Xinjiang Basin along westward, southwest, and eastward paths. It is mainly imported into the South Xinjiang Basin from 500 to 300 hPa on the southern border and 700–500 hPa on the west, north, and east borders, and exported from 500 to 300 hPa on the eastern border. The simulation results show that the change in water vapor content significantly influences the precipitation intensity and range. The water vapor transport at the southern boundary contributes the most precipitation during the rainstorm. As the water vapor in the rainstorm area increases (decreases), the ascending motion is strengthened (weakened), the low-level convergence and high-level divergence are strengthened (weakened), the water vapor transport to the middle and high levels increases (decreases), and the precipitation increases (decreases).

Effects of the Reynolds number and reduced frequency on the aerodynamic performance and dynamic stall behaviors of a vertical axis wind turbine

Dynamic stall is the main reason for the low power efficiency of vertical axis wind turbines (VAWTs) at low tip speed ratios, where the utilizable wind kinetic energy is also high. However, the determining factors behind VAWT dynamic stall have not been adequately investigated. Therefore, we carefully investigated VAWT dynamic stall under the different Reynolds numbers (Re) and reduced frequencies (k). The unsteady flow characteristics are identified using transitional URANS simulations. Although the blade undergoes significant vortex movements during each revolution, the output power is primarily determined by the aerodynamic responses close to the onset of dynamic stall. Increasing Re and k can impact dynamic stall behaviors significantly and also improve VAWT performance effectively. As both Re and k increase, the onset of dynamic stall is progressively postponed to a higher angle of attack, effectively suppressing the separated flow. Dynamic stall behaviors are therefore changed from abrupt stall to moderate stall with attenuated laminar separation bubble (LSB) bursting and dynamic stall vortex (DSV) shedding. An increase in Re hardens the LSB and prevents the DSV formation due to strong inertial forces, particularly at low Re. On the other hand, increasing k slows the flow-field transitions and delays the onset of LSB bursting and DSV shedding, particularly at high k.

Flow Structures and Charge Behaviors of Electroconvective Silicone Flows Induced by a Surface Dielectric Barrier Injection Actuator Subjected to Bipolar Pulsed DC Signals

Surface dielectric barrier injection (SDBI) actuators have received increasing interest due to the presence of dielectric barriers, which can greatly improve the applied voltage. The success of SDBI actuators in driving silicone oil makes silicone flow a very promising application in the field of microfluidics. However, polarity-dependent flows that are different from conventional cases are observed experimentally under bi-polar pulsed DC signals. To interpret this specific phenomenon, a new model of electrohydrodynamic (EHD) wall jet considering both electrochemical injection and the initial presence of an electrical double layer (EDL) at the dielectric solid/liquid interface is developed for the first time based on the finite element method. The results show that the flow field in the simulation is consistent with the experimental bidirectional flow field. The silicone vortex movement is accompanied by charge injection, migration and accumulation in the near-wall region. Through studies of the effect of signal parameters on flow behavior, including voltage amplitude, duty cycle, frequency and waveform, a square wave signal with a frequency of 0.2 Hz proves to be the most efficient in generating a high-velocity silicone flow.

Factors Leading to Heavy Rainfall in Southern Taiwan in the Early Mei-Yu Season of 2020

Abstract This paper examines the meteorological factors that led to the record-breaking heavy precipitation event in Taiwan in the early 2020 mei-yu season (15–31 May). The extreme amount of rainfall (an average of 135.9 mm per station) during the 36-h period around 22 May (hereafter Y20R) also set a record. Compared to climatology, the Pacific subtropical high was stronger and the southwesterly monsoonal flow was more intense during the first half of the 2020 mei-yu season, resulting in a stronger moisture conveyor belt over the northern Indo-China Peninsula. The record-breaking precipitation in Y20R was mainly caused by the eastward movement of a southwest vortex (SWV) generated in southwestern China. When the eastern portion of the SWV touched northern Taiwan, its associated west-southwesterly winds and the large-scale southwesterly monsoonal flow transported moisture toward the Taiwan Strait. The moisture-laden southwesterly flow was lifted by the stationary mei-yu front, leading to the heavy rainfall in southern Taiwan. When the SWV passed through northern Taiwan, it became the dominant weather system that enhanced the west-southwesterly winds and transported moisture from South China to Taiwan. The front moved southward through the Taiwan Strait during this period, with its location greatly determining the pattern of rainfall in southern Taiwan. In summary, the most critical factors leading to heavy rainfall in southern Taiwan are the strong 850-hPa southwesterly winds and moisture fluxes associated with the SWV. The other key factors include, in order of sensitivity to rainfall, the distance of the front, the distance of the SWV, the frontal speed, and the intensity of the SWV.

On the harmonic evolute of time-like Hasimoto surfaces in Lorentz–Minkowski space

The movement of a thin vortex in a thin viscous fluid by the motion of a curve propagating in Lorentz–Minkowski space [Formula: see text] is described by the vortex filament or smoke ring equation and can be viewed as a dynamical system on the space curves in [Formula: see text]. This paper investigates the harmonic evolute surfaces of time-like Hasimoto surfaces in [Formula: see text]. Also, we discuss the geometric properties of these surfaces, namely, we obtain the Gaussian and mean curvatures of the first and second fundamental forms. As a verification, we construct a concrete example for the meant surfaces to demonstrate our theoretical results.

Mechanisms Governing the Formation and Long-Term Sustainment of a Northeastward Moving Southwest Vortex

From 10 July to 12 July 2021, a long-lived (~66 h) southwest vortex (SWV), moved from Southwest China to Northeast China and caused a series of heavy rainfall events. This SWV case was rarely seen, as its lifespan was much longer than the SWVs’ mean lifespan, and the vast majority of SWVs showed a quasi-stationary behavior. It was found that the SWV formed and sustained in favorable background environments, which were characterized by a strong upper-level divergence (related to the South Asia High), a notable middle-tropospheric warm advection (related to a shortwave trough), and a vigorous low-level jet. The SWV showed remarkable interactions with a middle-tropospheric mesoscale vortex. The strong southwesterly wind in the eastern section of a deep shortwave trough east of the Tibetan Plateau acted as the steering flow for the northeastward movement of both vortices. Vorticity budget showed that the convergence-related vertical stretching dominated the SWV’s formation and development; the convection-related upward transport of cyclonic vorticity was the most favorable factor for the SWV’s sustainment, whereas, during the decaying stage, the SWV dissipated mainly due to the tilting effects and the net export transport of cyclonic vorticity. Backward trajectory analyses showed that most of the air particles that formed the SWV (at its formation time) were sourced from the lower troposphere. These air particles mainly ascended and experienced a rapid increase in cyclonic vorticity during the SWV’s formation stage. The topography of the Yunnan–Guizhou Plateau was crucial for the SWV’s formation, as around a half of the air particles (that formed the SWV) came from this region. Most of these air particles enhanced in their cyclonic vorticity and convergence when they descended along the topography of the plateau.

Effect of Slope on the Frequency and Height of Fire Whirls

Fire whirls are reported to occur frequently in the wilderness and in urban areas due to the influence of ambient winds. Fire whirls that occur on sloped fuel surfaces are common in the wilderness and have received less attention despite their potential to significantly alter fire behavior. Particularly in terms of frequency and height, previous studies have been performed on flatlands but less so on slopes. This paper presents an experimental study of fire whirls in sidewind line fires, focusing on the frequency of occurrence and the height of fire whirls. Regarding the effect of a side wind, it is shown that a side wind increases the frequency of occurrence, while the velocity component parallel or perpendicular to the line fire has a competing effect. In contrast, an increase in the slope reduces the height of the fire whirl; this phenomenon has been justified on the basis of experimental data from our work and the literature and explained in terms of the mechanism of vortex generation and movement.

Helicity characteristics of cyclonic vortexes and their effect on convection in a wide‐ranging extreme rainstorm in China

AbstractOn 20–22 July 2012, a severe rainstorm occurred in the Sichuan basin of Southwest China and in the Beijing area of North China. This rainstorm was related to the activities of a Tibetan Plateau vortex (TPV) and Southwest China vortex (SWCV). By using radiosonde, satellite brightness temperature, and NCEP_FNL data, we investigated the helicity characteristics nearly the vortexes and their effect on convection. Results showed that (1) strong precipitation in the Sichuan basin was mainly related to the interaction between the TPV and SWCV, while strong precipitation in the Beijing area was related to the northward movement of a cyclonic vortex caused by a split in the SWCV. (2) During the occurrence of the rainstorm, four mesoscale convection systems (MCSs) were observed. Their vertical structure showed a positive vorticity–negative divergence in the lower levels, and negative vorticity–positive divergence in the upper levels, accompanied by vertical upward movement. This was an important factor in the development and maintenance of MCSs, as well as one of the mechanisms for producing heavy precipitation. On this basis, we further discussed the effect of helicity on the MCSs in the atmospheric environment with rotational characteristics. Results showed that the increase in negative water vapour helicity and storm‐relative helicity were more likely to cause a strong development of MCSs.

Large Ensemble Simulation for Investigating Predictability of Precursor Vortices of Typhoon Faxai in 2019 With a 14‐km Mesh Global Nonhydrostatic Atmospheric Model

AbstractTyphoon Faxai hit Japan in 2019 and severely damaged the Tokyo metropolitan area. To mitigate such damages, a good track forecast is necessary even before the typhoon formation. To investigate the predictability of the genesis and movement of a precursor vortex and its relationship with the synoptic‐scale flow, 100‐member ensemble simulations of Typhoon Faxai were performed using a 14‐km mesh global nonhydrostatic atmospheric model, which started from 16 different initial days (i.e., 1,600 members in total). The results show that the model could predict an enhanced risk of a Faxai‐like vortex heading toward Japan 2 weeks before landfall, which was up to 70%. The reason for the enhancement was a rapid increase in the members reproducing a precursor vortex from 15 to 12 days before landfall in Japan. In addition, the upper‐tropospheric vortex played an essential role in the track simulation of Faxai.

Glitches due to quasineutron-vortex scattering in the superfluid inner crust of a pulsar

We revisit the mechanism of vortex unpinning caused by the neutron-vortex scattering \cite{prad1} in the inner crust of a pulsar. The strain energy released by the crustquake is assumed to be absorbed in some part of the inner crust and causes pair-breaking quasi-neutron excitations from the existing free neutron superfluid in the bulk of the inner crust. The scattering of these quasi-neutrons with the vortex core normal neutrons unpins a large number of vortices from the thermally affected regions and results in pulsar glitches. We consider the geometry of a cylindrical shell of the affected pinning region to study the implications of the vortex unpinning in the context of pulsar glitches. We find that a pulsar can release about $\sim 10^{11} - 10^{13}$ vortices by this mechanism. These numbers are equivalent to the glitch size of orders $\sim 10^{-11} - 10^{-9}$ for Vela-like pulsars with the characteristic age $\tau \simeq 10^4$ years. We also suggest a possibility of a vortex avalanche triggered by the movement of the unpinned vortices. A rough estimate of the glitch size caused by an avalanche shows an encouraging result.

On the similarity of electromagnetic and gravitational fields

It is well known that gravitational quantities are in many ways similar to electromagnetic quantities. The masses that create a gravitational field are similar in this to electric charges. A gravitational static field, like an electrostatic field, is characterized by potential and intensity. Vortex and rotational movements of masses are often and quite justifiably compared with a magnetic field. According to existing concepts, there are four types of different fundamental physical interactions: strong, weak, electromagnetic and gravitational. Strong and weak interactions are manifested inside atomic nuclei and between elementary particles, electromagnetic – between charges, and gravitational acts between massive bodies, becoming the main one in the case of astronomical objects. The article presents a comparison of mechanical and gravitational quantities from the condition of their similarity to electromagnetic quantities. Visual systemically possible patterns involving field gravitational quantities similar to electromagnetic ones are given.

Effects of rotation on collection characteristics of fine particles by droplets

Removing particles dispersed in fluid through drops is widely presented in various fields, and the critical factor is particles captured by droplets. Drop rotation effects play a dominant role in the capture process. However, their influences on collection characteristics remain unclear. Thus, a particle collection model was developed to simultaneously consider rotation and translation effects on fine particles captured by an individual droplet. The finite volume method was used to solve for flow field and collection efficiency, and the proposed model was verified by comparison with experimental and published results. The Liutex method was used to identify the vortex structure, on which dimensionless droplet rotation rates ranged from 0 to 0.1. Velocity, drag coefficient, radial position, and captured particle velocity distribution and collection efficiency were also investigated in relation to the rotation effect. The results show that the established model is reasonable. Vortex strength increases with increased rotation speed where the increment can be up to 480, and fluid rotation strength depends on the competitive relation between the increase in the rotation rate and the vortex movement. Radial velocity increases in regions where the angle between the positive x axis and the normal vector of drop surface ranges from 115° to 180° but decreases in regions where the angle ranges from −180° to −120°, and corresponding regions produce a comparative relation for improving particle capture. Increasing the rotation rate can increase the drag force coefficient by about 0.025, hindering droplet–particle collision. Average radial velocity of particles with higher than 3.7 mm/s is necessary at high rotation rates, while collection efficiency decreases at increased droplet rotation rates.

Vortex Pinning in Neutron Stars, Slipstick Dynamics, and the Origin of Spin Glitches

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using three-dimensional dynamical simulations. Strong pinning occurs for certain lattice orientations of an idealized, body-centered-cubic lattice and occurs generally in an amorphous or impure nuclear lattice. The pinning force per unit length is ∼1016 dyn cm−1 for a vortex–nucleus interaction that is repulsive and ∼1017 dyn cm−1 for an attractive interaction. The pinning force is strong enough to account for observed spin jumps (glitches). Vortices forced through the lattice move with a slipstick character; for a range of superfluid velocities, the vortex can be in either a cold, pinned state or a hot, unpinned state, with strong excitation of Kelvin waves on the vortex. This two-state nature of vortex motion sets the stage for large-scale vortex movement that creates an observable spin glitch. We argue that the vortex array is likely to become tangled as a result of repeated unpinnings and repinnings. We conjecture that during a glitch, the Kelvin-wave excitation spreads rapidly along the direction of the mean superfluid vorticity and slower in the direction perpendicular to it, akin to an anisotropic deflagration.

Analysis of Intraseasonal Oscillation Features of Winter Cold Precipitation Events in Southern China

Based on the daily minimum air temperature (Tmin) data and the daily precipitation data from the NCC/CMA combined with the NCEP/DOE reanalysis data, the intraseasonal features and circulations of the winter cold precipitation events (CPEs) in southern China under the influence of strong Madden–Julian oscillation (MJO) were explored. The results show that: (1) Winter temperatures in southern China are characterized by intraseasonal oscillations (ISOs) of 10–30-d and 30–60-d, with six CPEs under strong MJO all occurring during these two intraseasonal scales in cooling phases. The invasion of cold air coupled with the availability of appropriate moisture conditions in southern China is more conducive to the CPEs. (2) A cyclone and anticyclone lying to the east of the Ural Mountains and the northwest of Lake Baikal at 925-hPa gradually move southeastward. The merging of the low-frequency (LF) blocking highs over the Ural Mountains and the North Pacific Ocean at 500-hPa leads to the contraction and southward movement of the LF cold vortex. The following anomalous northerly winds steer the cold air towards southern China. The cold advection is the dominant term in the cooling process, while the adiabatic cooling accompanied with ascending motion is also beneficial to the cooling process. (3) MJO has some effect on the LF blocking highs and the cold vortex in the mid-high latitudes and induces the CPEs over southern China. The joint effect of mid-high and low latitudes on the 30–60-d intraseasonal oscillation scale can have a significant impact on the cooling and precipitation processes of CPEs.

Chordwise flexible aft-tail suppresses jet-switching by reinstating wake periodicity in a flapping foil

The effect of a chordwise flexible aft-tail of a rigid heaving aerofoil on the dynamical transitions of the trailing-wake is studied here. The two-way coupled fluid–solid dynamics is simulated using an in-house fluid–structure interaction (FSI) platform, comprising a discrete forcing immersed boundary method based incompressible Navier–Stokes solver, weakly coupled with a finite difference method based structural solver. The FSI dynamics is studied in comparison to the corresponding rigid tail configuration. For the latter, mild jet-switching due to quasi-periodic movement of the wake vortices gives way to vigorous jet-switching as the dynamics transitions to a state of intermittency, where the quasi-periodic behaviour gets interspersed with chaotic windows. Introduction of a moderately flexible tail regularises this intermittent dynamics, eliminating jet-switching. The wake exhibits a deflected reverse Kármán pattern with fluctuating angles, governed by quasi-periodicity. With a highly flexible tail (very low rigidity), the wake shows almost a symmetric reverse Kármán street as periodicity is restored. Flexibility of the aft-tail is next controlled by changing its length, and flow is regularised and periodicity retained for moderate rigidity for increased length. Different dynamical states are established through robust nonlinear dynamical tools. The underlying flow-field behaviour, instrumental in suppressing the jet-switching phenomenon, is identified through a detailed investigation of the near-field vortex interactions dictated by the dynamics. A suite of measures has also been derived from the unsteady flow field to quantify the interactions of the key near-field vortices with a view to understanding the mechanism of switching and its subsequent suppression through flexibility.

Heavy precipitation events in northwestern China induced by northeastward plateau vortex: Two cases comparison

The plateau vortex is one of the dominant weather systems that modulate summer rainfall in northwestern China, a typical arid/semi-arid area worldwide. Two heavy rainfall cases in the summer of 2012 and 2013, accompanying the northeastward movement of plateau vortexes, are selected here. The role of plateau vortexes in these two cases is explored by diagnosing their moist potential vorticity, helicity, and convective clouds characteristics, using numerous data (ERA-Interim reanalysis, multi-sourced precipitation data, FY-2E satellite images). The plateau vortexes enhanced convective instability in the rainfall area by entraining upper-level dry and cold air, manifested as salient vertical gradients of moist potential vorticity and steep pseudo-equivalent potential temperature isolines. The rainfall occurred eastward relative to the vortex movement track and intensified on the left of cloud black body temperature (TBB) with larger gradients at the middle to late stages of the vortex life span. The generation of moisture helicity is dictated by the water vapor transportation, with the negative value concentrated in the water vapor-rich areas. Notably, the two cases differ in tracks, life spans, water vapor sources, and precipitation amounts, which may be attributed to large-scale circulation background, vertical vortex structure, and moisture condition.

Optical Investigation of the Influence of In-cylinder Flow and Mixture Inhomogeneity on Cyclic Variability in a Direct-Injection Spark Ignition Engine

In this study, a single-cylinder direct-injection spark-ignition research engine with full optical access was used to investigate the influence of the flow field and fuel/air mixing on cyclic variability, in particular in the early flame propagation. The engine was operated under lean-burn conditions at 1500 rpm. Two different injection strategies were compared, port-fuel injection (PFI) and direct injection (DI), the latter with early and late injection split about 2:1 in fuel mass. High-speed particle image velocimetry captured the flow in the tumble plane in the compression stroke. The velocity fields and the movement of the tumble vortex are analyzed. Simultaneously, a second camera detected the chemiluminescence of the flame, and the projected area of the line-of-sight-integrated flame luminosity was extracted through morphological image processing. By combining pressure-based combustion analysis and high-speed optical diagnostics, the early flame propagation and the flow field are correlated. In separate experiments the equivalence ratio was imaged for the DI at selected crank angles and correlated with CA10 to learn about the influence of mixture inhomogeneity on early flame propagation. With PFI, the flow near the spark plug just before ignition is closely related to the subsequent speed of combustion. The combustion-relevant flow features can be traced back in time to about –90 °CA. In contrast, the chosen DI scheme results in a decorrelation of flow and flame, and the equivalence ratio distribution at ignition becomes more important. For both flow and mixture fields, regions of high correlation with early-combustion metrics are typically associated with gradients in the multi-cycle average fields.

Magneto-optical measurements of mesoscopic Nb superconducting structures using a ferromagnetic metal indicator layer

Imaging local magnetic fields produced by nano- and micrometer-scale superconductors has become a vital tool that can not only reveal crucial information on the vortex dynamics and order parameters of the superconducting materials but also visualize the working mechanism of superconducting devices made for quantum information. Here, we performed measurements of the magnetic field distributions of mesoscopic superconducting structures with various geometries by combining a thin ferromagnetic metal layer as a magneto-optical sensing element that responds to the environmental magnetic fields and a custom-made sensitive Sagnac interferometer. The sensitivity of the technique enables the observation of magnetic flux jumps due to individual vortex entries into nanostructured superconductors. In addition, with the control of incident power at a focused laser spot, we induce thermally driven movement of vortices that leaves a trace of a microscopic optical heating pattern.

Vortex technique to track 3D object displacement in CGH

In this work, we introduce and test a technique to track the movement of objects from computer-generated holographic videos, without the need for reconstruction. To achieve this, we first generated holographic videos of moving 3D objects using a random phase layer-based approach. Then, we analyzed the hologram phase to determine the locations of the optical vortices. We used the core properties of each vortex to track its movement over all the frames of the video, and then establish a relationship between the object and the vortex movement. We also tested the vortex tracking over a pseudo-phase obtained by performing the Laguerre–Gauss transform of the hologram, which enables more precise control over the number of optical vortices that can be tracked. Our proposal was tested with holographic videos generated with one and two objects, using two different hologram generation approaches.

Tracking the Early Movements of Northeast China Cold Vortices Using FY-3D MWTS-2 Observations of Brightness Temperature

The Northeast China cold vortex (NCCV) often occurs in spring and summer, causing extreme weather such as rainstorm and hail in Northeast China. The brightness temperature (TB) observations of Microwave Temperature Sounder-2 (MWTS-2) on board Fengyun-3D (FY-3D), which can provide atmospheric temperature in various vertical layers, are firstly limb-corrected and then applied to track the origin and movement of four NCCV cases in June and July 2019. Results show that a cold core is observed at the location of NCCVs in TB observations of channels 4 and 5, whose peak weighting function (WF) altitudes are 700 and 400 hPa, respectively, indicating the cold structure of NCCVs in the middle and lower troposphere. The TB observations of channels 6 and 7, which measure the atmospheric temperature around 250 and 200 hPa, respectively, capture a warm core structure of NCCVs in the upper troposphere and lower stratosphere. Being less affected by the low-level cloud and rain, TB observations of channels 6 and 7 are applied to identify and track the warm cores of NCCVs. The NCCV tracks of movement identified by MWTS-2 observations compare well with those determined by the 500 hPa geopotential height and the 300 hPa potential vorticity (PV) anomaly from the ERA5 reanalysis. Both clearly show that the NCCVs were originated from high latitudes, then moved southeastward, and finally entered Northeast China. The entire process took several days. Therefore, TB observations of MWTS-2 can be used to identify the precursors of NCCVs and monitor their appearances, developments, and movements in time. With the flourishing development of Fengyun satellite series in China as well as the already existing 40 years of microwave sounder observations worldwide, this research provides a new way to investigate the synoptic and climatological features of NCCVs.