Vortex Signals Research Articles

Improved empirical mode decomposition method based on amplitude-frequency characteristic of vortex signals.

The vortex flowmeter occupies a vital position in flow measurement with its unique advantages. It is essentially a fluid vibration instrument, and its measurement process is susceptible to interference, which seriously affects measurement accuracy. In particular, at low flow rates, it is an urgent problem to extract vortex signals from the complex noise. Among many signal processing methods, Empirical Mode Decomposition (EMD) is a time-frequency analysis method suitable for nonlinear, non-stationary signals. EMD can adaptively decompose noisy signals into noise and useful signal components arranged from high frequency to low frequency. For the above problems, an innovative, improved EMD method is proposed in this paper. The digital filter is designed according to the amplitude-frequency characteristic of vortex signals. After filtering, the vortex signal is adjusted to a fixed value, and high-frequency noise is filtered. According to the consistency of the filtered signal's amplitude, we design a decomposition stop criterion for EMD to process the output signal of the vortex sensor. This method not only maintains the characteristic of adaptive decomposition in EMD but also completes the automatic extraction of the vortex signal under complex noise. It provides a new comprehensive method for realizing high-precision and anti-interference vortex flowmeters.

Average capacity and ABER for perfect optical vortex system in weakly to strongly turbulent oceans

Based on the modified Rytov method, the reception probability spectrum, average bit error rate (ABER), and average channel capacity of the perfect optical vortex (POV) optical system using quadrantal amplitude modulation (QAM) are derived by introducing the mutual coherence function of POV in weak to strong turbulent seawater. The relationship between eddy topological charge, seawater turbulence characteristic parameters, optical system performance parameters, reception probability spectrum the bit error rate, and the average capacity of the POV optical system is numerically analyzed. The following important results are obtained: in long-distance turbulent seawater, the conclusion that the radius of bright ring POV remains almost constant in short-distance standing water and running water is still valid. 4-QAM is more suitable for signal modulation of the POV system in moderate to strong turbulent seawater. The increase of the vortex ring will lead to the increase of the crosstalk degree between vortex modes and the bit error rate of the POV system. Medium to strong turbulence changes the properties that the transmission ability of POV vortex signal mode in non-turbulent media is independent of the topological charge of the vortex. The transmission performance of vortex signals of the POV optical system can be improved by choosing a smaller half-beamwidth of signal, a higher ring radius to the half-beamwidth ratio of the signal, a lower topological charge of vortex mode, a longer signal wavelength, and higher SNR of the optical system.

Experimental investigation on droplet evolutions in co-flow around the bluff body

Annular mist flow widely exists in natural gas and other engineering fields. Vortex flowmeter has gradually become an effective means of wet gas measurement. However, the droplets in the flow field will influence the stability of the vortex. In this paper, the interaction between droplets and vortex was studied by experiments. A novel in-focus criterion was proposed to improve the accuracy of droplet measurement. Simultaneous acquisition of droplet size, velocity, and concentration was achieved. The spatial evolution of droplet parameters was obtained to determine the position where the droplets were evenly distributed. The bidirectional coupling mechanism between droplets and vortex was studied. Firstly, the influence of the vortex flow field on droplets was reflected by the characteristics of droplet distribution around the bluff body. Secondly, the effect of droplets on the vortex signal in the flow field was analyzed based on Stokes number Stp and liquid-phase load φl. When φl < 0.03, the probability distribution of droplet velocity downstream of the bluff body presents double peaks. The exchange momentum between the small droplets and the vortex is stronger, and vice versa for the larger droplets. Finally, the quality factor of vortex signals decreases from -2.09 to -2.18 with the increase in the volume concentration of droplets. It is further demonstrated that the increase in volume concentration of droplets will destroy the vortex structure and degrade the quality of the vortex signal.

Decadal Change in the Linkage between QBO and the Leading Mode of Southeast China Winter Precipitation

Abstract In this study, observational and model datasets are used to analyze winter precipitation and its leading empirical orthogonal function (EOF1) mode over Southeast China. EOF1 displays a dominant monosign pattern during the last 60 years; however, its major impacting factors have a decadal transition near the mid-1990s. The first principal component (PC1) is related to El Niño–Southern Oscillation (ENSO) after the mid-1990s and to the quasi-biennial oscillation (QBO) before the mid-1990s. An enhanced ENSO–precipitation relationship is associated with stronger ENSO-induced tropical zonal circulation and the westward shift of ENSO-induced SST over the tropical Pacific after the mid-1990s compared to before the mid-1990s. Negative correlation coefficients between QBO and precipitation are evident before the mid-1990s but are no longer statistically significant after the mid-1990s. This change originates from the interaction between QBO’s subtropical influences and the Holton–Tan effect. The QBO’s subtropical influence and the Holton–Tan effect lead to a zonal pressure gradient and meridional wind anomalies over East Asia before the mid-1990s, which further influence the meridional transport of water vapor and precipitation over Southeast China. However, the Holton–Tan effect is enhanced after the mid-1990s. Downward stratospheric polar vortex signals and the QBO’s subtropical influence cause a meridional pressure gradient over East Asia, and thus the relevant moisture flux divergence lacks statistical significance. The above results indicate that the subtropical response to QBO and the Holton–Tan effect should be considered together when using the QBO signal to improve forecasts of winter precipitation over East Asia. Significance Statement Southeast China winter precipitation (SCWP) is often attributed to variation in lower-atmospheric dynamics and sea surface temperature, such as El Niño–Southern Oscillation (ENSO). Few studies focus on the role of the mid- to upper atmosphere. In this study, we diagnose the influence of the stratospheric quasi-biennial oscillation (QBO) on SCWP. Both observational and modeling analyses indicate a strong decadal change in the QBO–SCWP relationship, which limits the use of the QBO in seasonal forecasts of SCWP. This decadal change originates from the strength of the QBO’s modulation of the stratospheric polar vortex. Our results provide a new perspective on the use of the mid- to upper atmosphere in seasonal forecasts of SCWP.

Increased Persistence in Winter-to-Spring Precipitation Anomalies over South China since the Late 1990s and the Possible Mechanisms

Abstract This study reveals that South China precipitation (SCP) anomalies tend to persist well from winter to the following spring after the late 1990s, favoring long-lasting drought or flood events over South China. Mechanism analysis indicates that the interdecadal changes in El Niño–Southern Oscillation (ENSO) and the preceding November central Asian snow cover could contribute to the increased persistence of winter-to-spring SCP anomalies. ENSO has a stable impact on winter SCP, whereas its impact on spring SCP is significantly enhanced after the late 1990s. With a weakened intensity and faster decay rate in the recent two decades, the ENSO-related spring SST anomalies over the tropical Pacific are relatively weaker, inducing a weakened and more southward-located western North Pacific anticyclone. This further leads to an interdecadal migration of the spring rainfall belt anomaly, consequently favoring the persistence of winter-to-spring SCP anomalies after the late 1990s. Additionally, the impacts of November central Asian snow cover on winter and spring SCP are both strengthened after the late 1990s. In the most recent two decades, the snow-cover-related cooling effect has become stronger, which induces winter cyclonic anomalies over Lake Baikal, favoring increased winter SCP. In addition, increased snow cover excites upward-propagating waves from the troposphere to the stratosphere, consequently weakening the stratospheric polar vortex. In spring, the stratospheric polar vortex signals propagate downward and result in a negative Arctic Oscillation in the troposphere, favoring more spring SCP. Therefore, central Asian snow cover is also conductive to the persistence of winter-to-spring SCP anomalies after the late 1990s.

Selective excitation of optical vortex modes with specific charge numbers in band-tuned topological waveguides.

We propose a method for selectively propagating optical vortex modes with specific charge numbers in a photonic integrated circuit (PIC) by using a topological photonic system. Specifically, by performing appropriate band tuning in two photonic structures that comprise a topological waveguide, one specific electromagnetic mode at the Γ point of a band diagram can be excited. Based on theoretical analysis, we successfully propagated optical vortex modes with specific charge numbers over a wide range in the C band in the proposed topological waveguide. The proposed method could be useful in controlling optical vortex signals at the chip level in future orbital angular momentum multiplexing technologies.

Analyzing the optical modes and vortex signals in an optical fiber by means of side illumination

This article discusses a new method for instrumentally (experimentally) finding the mode-distribution parameters of an optical signal propagating along an optical fiber. The proposed method is based on illuminating the optical fiber from the side, followed by investigating the image for the linear and nonlinear (with the Kerr component) cases and then recalculating the mode parameters using the proposed technique. Unlike known methods of analyzing mode distributions that operate only with end irradiation of the optical fiber and are implemented only at definite points of a line with an intermittent signal transmitted along it, the proposed method does not assume breakage of the optical line and can be used at any point along it. The measurement layout is based on the well-known experimental apparatus, intended to obtain the refractive-index profile of an optical fiber. In addition to the known layout, it is proposed to make measurements at various angular positions of the optical fiber of interest and at various longitudinal positions along it. The proposed method can be implemented when the signal that propagates along the optical fiber is characterized by increased intensity, and this makes it possible to record the nonlinear (Kerr) component of the main refractive-index profile.

Influence of Two-Frequency Radiation Intensity Fluctuations on the Output Signal of a Vortex Optical Fiber Forming OAM Address in Polyharmonic Sensor Technology

A schematic diagram of a RoF radio-optic system with vortex signals is presented, in which the radio frequency is determined by the difference between the wavelengths of two lasers. It is assumed that the generation of a vortex signal can be performed through a vortex fiber-optic periodic structure, which can be obtained using a technology similar to the manufacture of long-period fiber Bragg gratings. The parameters of the grating are modeled assuming that the fundamental light-guide mode (LP01) is applied to the specified vortex element, and the higher-order mode (LP11) is reflected. It was found that the distortion of the vortex signal can be reduced by introducing apodization and chirping of this periodic structure. The following optimal parameters have been estimated: the apodization and chirp multiplier functions, at which the distortions of the amplitude and phase of the vortex signal, as well as the appearance of an unwanted angle distortion, will be minimal. It is shown that such gratings can be exploited in addressed sensors systems using the orbital angular momentum (OAM) of a lightwave as a unique sensor address.

Wet gas metering using a vortex cross-correlation meter based on fluctuating pressure measurement

A solution for wet gas metering was developed with a vortex meter by using pressure sensors and correlation technology. For the fluctuating pressure measurement, the micro pressure sensors, especially the probe-transducer systems, were designed to acquire reliable vortex signals. For the experiments, an annular mist flow loop with a film metering system was established, and the tests were conducted on various pressure, gas velocity and liquid flowrate conditions. Then, the dimensionless convection velocity, named convection coefficient, and the peak value of the normalized cross-correlation function, named correlation coefficient were calculated. A piecewise function was developed for the convection coefficient with modified Weber number where the segmentation was determined by the correlation coefficient. Finally, the wet gas measurement model was developed and realized by iterative algorithm. Under the present conditions, the ±5% error bands contain 91% of the relative errors for gas flowrate prediction, the mean absolute percentage error is 2.54%, and the uncertainty of gas flowrate prediction is 2.85%. The primary advantage of the proposed method is that the convection coefficient is almost independent of liquid fraction, no extra systems are necessary for watercut measurement, hence providing a cost-effective solution for wet gas metering.

Wingtip vortex structure in the near-field of swept-tapered wings

Wingtip vortices are an important phenomenon in fluid dynamics due to their complex and negative impacts. Despite numerous studies, the current understanding of the inner vortex is very limited; thus, a basis for the design of effective wingtip geometry and vortex manipulation is narrow. This work examines the structure of the trailing vortex shed from a swept-tapered wing, analogous to a commercial aircraft topology. Stereoscopic particle imaging velocimetry has been utilized to compare the vortex structure and development through several angles of attack at various downstream stations for a fixed Reynolds number (Re = 1.5 × 106). After correcting for vortex meander through helicity-based spatial localization of the vortex core, relationships between the vortex core velocity/vorticity fields, core shape, and turbulent properties have been examined. Subsequently, the vortex is found to exhibit a layered structure with slow linear rates of dissipation indicative of laminar diffusion mechanisms, despite being a turbulent vortex. The turbulent kinetic energy distribution in the vortex signals that relaminarization of the inner core occurs. Consideration of the streamline curvature around the core, via examination of the local Richardson number, indicated that a laminar core structure had formed within which large-scale turbulent eddies could not contribute to the turbulent diffusion of vorticity away from the core. The normalized circulation within the vortex core has been shown to exhibit self-similar behavior typical of the fully developed axisymmetric vortices.

Online Measurement of Wet Gas by Modeling Frequency and Amplitude Characteristics of a Vortex Flowmeter

In this article, an accurate online measurement of gas flowrate in wet gas was developed by modeling the frequency and amplitude characteristics of a vortex flowmeter in annular mist flow. The low-frequency modulation behaviors of the vortex signals were analyzed by the ridge method based on continuous wavelet transform (CWT). It indicates that the instability of frequency and amplitude of the signals both enhance with the increase of liquid loading; and therefore, a feature extraction method based on ridge-averaged was proposed and validated by the tests. A film extraction device was developed to measure the flowrate of the liquid film and further obtain the droplet mass loading. Then, a uniform linear equation for the overreading was obtained experimentally on the basis of the scaling parameter analysis in particle-laden flow. A uniform exponential equation for the dimensionless signal amplitude of the sensor output was developed with droplet mass loading. Finally, a wet gas measurement model with the simultaneous equations from these two correlations was established and the iterative solution algorithm was given. In the present cases, the percentage errors (PEs) of the gas flowrate predicted by the modeling are within ±1.5% and the mean absolute PE is 0.37%. The method proposed in this article can achieve accurate measurement of gas flowrate by using only one vortex flowmeter and, hence, creates a simple inexpensive metering system for wet gas flow.

Vortex channel modelling for the radio vortex system

In this paper, a free-space vortex channel model of the radio vortex system is proposed to describe the propagation characteristics of vortex signals over the vortex channel. The key idea is to combine the Laguerre-Gaussian (LG) modes in the optical field with the free-space propagation model in the radio field. The proposed free-space vortex channel model is derived from the electric field expression of the LG modes and the freespace channel transfer function of the freespace propagation model theoretically. Simulation results verify that the proposed model could reflect the vortex channel characteristics better than the currently used free-space propagation model.

Radio vortex for future wireless broadband communications with high capacity

As a potential and promising technique for future wireless broadband communications, the OAM technique has attracted a significant attention due to its high capacity. In this article, a radio system based on OAM, called an RV system, is proposed. Specifically, a vortex modulation method and an azimuthal angle integral method are developed for the transmitter and receiver of the proposed RV system, respectively. Different from the conventional radio system, vortex signals are generated by the vortex modulation method and are multiplexed to provide remarkable improvement of capacity without increasing the bandwidth in the proposed RV system. Simulation results are conducted to verify that the proposed RV system could achieve higher capacity than the traditional radio system in free space.

A magnetic origin of cuprate superconductivity? A MaxEnt-μSR view

The fundamental physics of cuprate superconductivity is still much deliberated after three decades of research. In contrast to phononic or polaronic roots, some major theories promote a magnetic origin. In this perspective, we review cuprate magnetism, as probed by muon-spin-rotation [Formula: see text] in [Formula: see text] (RBCO), [Formula: see text] (Bi2212) and [Formula: see text] (Tl2223). Site-search RBCO studies show that muons localize and probe in locations away from the superconducting CuO2 planes. Maximum entropy (MaxEnt, ME) analysis of transverse field [Formula: see text] data of [Formula: see text] (GdBCO) indicates that the muon probes in an undisturbed insulating environment, allowing [Formula: see text] to detect (weak) magnetic features in these cuprates. Concerning Varma’s predicted loop currents, MaxEnt has shown weak [Formula: see text] signals for GdBCO in zero field above and below the critical temperature, [Formula: see text]; these are near the predicted [Formula: see text] Oe. Concerning Zhang’s predicted antiferromagnetism (AF) connected to the vortex cores, we have observed Lorentzian relaxation of cuprate vortex signals below half [Formula: see text], consistent with AF-broadening effects. For both Bi2212 and Tl2223, Lorentzians describe the [Formula: see text] vortex signals much better below [Formula: see text] than Gaussians, indicating that extra AF fields occur near and in the vortex cores. In sum, both our MaxEnt-[Formula: see text] (ME-[Formula: see text]) studies point toward magnetic roots of cuprate superconductivity.

Andreev reflection, a tool to investigate vortex dynamics and quantum turbulence in 3He-B.

Andreev reflection of quasiparticle excitations provides a sensitive and passive probe of flow in superfluid (3)He-B. It is particularly useful for studying complex flows generated by vortex rings and vortex tangles (quantum turbulence). We describe the reflection process and discuss the results of numerical simulations of Andreev reflection from vortex rings and from quantum turbulence. We present measurements of vortices generated by a vibrating grid resonator at very low temperatures. The Andreev reflection is measured using an array of vibrating wire sensors. At low grid velocities, ballistic vortex rings are produced. At higher grid velocities, the rings collide and reconnect to produce quantum turbulence. We discuss spatial correlations of the fluctuating vortex signals measured by the different sensor wires. These reveal detailed information about the formation of quantum turbulence and about the underlying vortex dynamics.

Computationally lean algorithm of novel optimal FIR adaptive filter for vortex signal extraction

In a vortex flowmeter vortices are generated by a bluff body, inserted in the path of flow, which has a piezoelectric sensor embedded in it. This piezosensor develops a signal having a fundamental frequency that is proportional to flow. The flow measurement relies completely on extraction of true vortex signal and estimation of the correct frequency. A novel adaptive FIR filter has been designed and implemented using low power computational resource (8.25mW), which gives better results than an existing contemporary system when tested on an industrial flow rig. Further more a comparative study of autocorrelation, EMD Scales filter and proposed algorithm is carried on the good and bad vortex signals. From this comparative study it is seen that proposed algorithm is effective for bad vortex signals and low flowrates where vortex signals are weak.

Experimental investigations of the location of a piezoelectric probe in a vortex flow sensor

Three fundamental problems about the vortex flow sensor—bluff body design, signal detection method and signal processing method—have been mainly discussed by a number of researchers in recent years. However, another significant thing is that the position for vortex signal detection, the location of a piezoelectric pressure probe in a vortex flow sensor, is seldom investigated, which is the focus of this paper. It is known that the quality of vortex signals is directly related to the probe's location. Therefore, the low limit of measurement for a vortex flow sensor can be expanded by searching the optimum position for detection. Experiments were carried out in a water channel to analyze the pressure signals downstream of the bluff body. The vortex signal intensity, frequency, signal-to-noise ratio and uncertainty of the Strouhal number at each position were discussed. Finally, the optimum regions for vortex signal detection (probe location) for three bluff bodies were given: the distance between the bluff body and the position for detection is 50–75 mm when the width of the bluff body d = 14 mm, the distance is 75–125 mm when d = 22.5 mm and the distance is 75–150 mm when d = 28 mm. Meanwhile, numerical simulations on flow field were adopted to explain the experimental results.

Sheet-current distribution in a dc SQUID washer probed by vortices

We present a novel method, based on vortex imaging by low-temperature scanning electron microscopy (LTSEM), to directly image the sheet-current distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex signals are simply related to the scalar stream function describing the vortex-free circulating sheet-current distribution J. Unlike previous inversion methods that infer the current distribution from the measured magnetic field, our method uses pinned vortices as local detectors for J. Our experimental results are in very good agreement with numerical calculations of J.

Plausibility of antiferromagnetism in and around the vortex cores of Bi2212 and Tl2223

Muon-spin-resonance (μSR) vortex data of underdoped Bi2Sr2CaCu2Ox (Bi2212) and Tl2Ba2Ca2Cu3Oy (Tl2223) have been analyzed to search for antiferromagnetism in and around their vortex cores. The temperature-dependent magnetic field distributions in the vortex states are obtained by maximum-entropy μSR analysis. The grain-boundary signal is well fitted by a narrow Gaussian. For the vortex μSR signals, deviations from a Gaussian are observed well below Tc, especially at the high-field side associated with the vortex core. For both Bi2212 and Tl2223, Lorentzians fit the μSR vortex signals better below 0.4 Tc, indicating that extra magnetic fields occur near and in the vortex cores. An estimated Neél temperature is 0.4 Tc. Our vortex-μSR results are compared with those of RBa2Cu3O7−δ (R=Eu,Ho,Y). The relevance to a potential magnetic origin of cuprate superconductivity is discussed.

Plausiblity of Antiferromagnetism in and near RBCO Vortex Cores: An ME-μSR Analysis

Maximum-Entropy (ME) analysis is applied to muon-spin-rotation (μSR) data of RBa 2 Cu 3 O y (RBCO; R = Ho , Eu) vortex states. Our focus is on the prospect of antiferromagnetism (AF) associated with the vortex core. The ME transforms are fitted by two μSR signals: the vortex signal and the grain-boundary (GB) signal. Below Tc, the GB signals are well fitted by Gaussians for both EuBCO (Tc = 94 K ) and underdoped HoBCO (Tc = 66 K ). Below 0.5Tc, EuBCO and HoBCO vortex signals are best fitted using Lorentzians, instead of expected Gaussians. An estimate for an effective Neél temperature of an AF vortex core is about 30 K. Our AF-core search results for RBCO vortex states are discussed considering a magnetic origin of cuprate superconductivity.