Collapse Region Research Articles

Core-polarization corrected Dirac-Fock computations of one-electron spectra in the rubidium isoelectronic sequence: Mo VI through Pb XLVI

Core-polarization corrected Dirac-Fock computations were performed, under assumption of so-called one-electron spectrum over-imposed on multi-electron spectrum, to predict levels and oscillator strengths for lowest transitions of so far mostly unknown highly ionized spectra of the rubidium isoelectronic series from Tc VII through Pb XLVI. To evaluate the accuracy of our predictions, calculations were also made for Mo VI spectrum, where experimental and other theoretical data are available. Behavior of oscillator strengths along the isoelectronic sequence was studied, exhibiting some relativistic features for highly ionized systems. The effect of collapse of the 4p64f5/2,7/2 levels along the sequence leads to change in the level ordering between Rh IX (Z = 45) and Sn XIV (Z = 50) spectra. We also carefully monitored the influence of core polarization (CP) on both energy levels and oscillator strengths. For energy levels, the CP contributions are small and quickly diminish along the isoelectronic sequence. For oscillator strengths the CP influence is much more profound not only near the neutral end of the sequence but also for highly ionized spectra. Moreover, the relative CP contribution to oscillator strengths of the 4f5/2,7/2−5d3/2,5/2 transitions reaches extreme values in the vicinity of the collapse region, where the 4f5/2,7/2 levels fall below the 5d3/2,5/2 ones. Finally, the assumption of the so-called one-electron spectrum, i.e., arising from excitation of a single valence electron outside of the closed-shell core seems to be justified for all states in the spectra under consideration with exception of the 4f5/2,7/2 levels, which are heavily mixed with other states along the sequence.

Borromean Droplet in Three-Component Ultracold Bose Gases.

We investigate droplet formation in three-component ultracold bosons. In particular, we identify the formation of a Borromean droplet, where only the ternary bosons can form a self-bound droplet while any binary subsystems cannot, as the first example of Borromean binding due to a collective many-body effect. Its formation is facilitated by an additional attractive force induced by the density fluctuation of a third component, which enlarges the mean-field collapse region in comparison to the binary case and renders the formation of a Borromean droplet after incorporating the repulsive force from quantum fluctuations. Outside the Borromean regime, we demonstrate an interesting phenomenon of droplet phase separation due to the competition between ternary and binary droplets. We further show that the transition between different droplets and gas phase can be conveniently tuned by boson numbers and interaction strengths. The study reveals the rich physics of a quantum droplet in three-component boson mixtures and sheds light on the more intriguing many-body bound state formed in multicomponent systems.

Experimental study on unsteady characteristics of the transient cavitation flow

The characteristics of unsteady cavitation flow under different cavitation numbers are investigated experimentally. An optical test rig is designed to visualize the cavitation flow and the cavity length is defined based on high-speed camera technology. The development of cavitation flow can be divided into inception and fusion region, expansion region and collapse region. Due to the interaction between the re-entrant jet and the cavity interface, the cavitation bubbles near the solid wall collapse earlier than them in the downstream. The cavitation number is also the important factor determining the cavity length pulsation. Further analysis shows that the smaller the cavitation number, the smaller the frequency of cavity length fluctuations and the larger the mean cavity length. The cavitation intensity is stronger at lower cavitation number. Meanwhile, the cycle collapsing frequency of cavitation is around 220–320 Hz. With the decrease of cavitation number, the amplitude of cavitation pulsation increases while the corresponding collapsing frequency decreases.

An adaptable algorithm to predict the load-shortening curves of stiffened panels in compression

ABSTRACT An adaptable algorithm is proposed in this study, which can predict the complete load-shortening curve of a stiffened panel subjected to uniaxial longitudinal compression. The algorithm provides an extension to existing empirical formulae initially derived for predicting the ultimate compressive strength of stiffened panels. Based on observations from a series of nonlinear finite element analyses, the compressive load-shortening behaviour of stiffened panels is idealised with a linear pre-collapse response, an arc-shaped nonlinear ultimate collapse region and an asymptotic post-collapse decay. The algorithm allows direct modification of the elastic stiffness, ultimate strain, ultimate strength and post-collapse characteristics of the load shortening curve. This enables the load shortening curve to incorporate characteristics specific to the type of stiffened panel under analysis. The capability of the algorithm is demonstrated through an application with the simplified progressive collapse method to calculate the ultimate ship hull strength of four merchant ships and one naval vessel.

Power System Voltage Stability analysis with Renewable power Integration

The purpose of this research is to find the loading limit of a power system before hitting voltage instability and to assess the margin to voltage instability of a system consisting of a wind farm. An index called Bus Apparent Power Difference Criterion (BSDC) is used to find maximum loadable point. The measure depends on the way that in the region of the voltage collapse no extra apparent power can be delivered to the affected bus. The analysis is performed combination of wind power injection at different wind speeds and line outages in the network. In the feasibility and siting studies of wind farms the steady state analysis with network contingencies give the utility or the developer a sense of network condition upon the injection of power in the network. However, the extent of voltage stability impacted due to load growth in the system is not assessed. The research paper makes way to assess the impact on voltage stability margin with obtaining the maximum loadable point of the system and assessing the best suited bus to integrate a wind farm into the system.

Assessment of muscular tone of the tongue using a digital measure spoon in a healthy population: A pilot study.

The study of the muscles of the tongue forms part of a basic evaluation of upper airway function that includes swallowing, speaking and chewing. It is important because the upper airway presents a region of collapse during sleep. Through the action of the dilator muscles, mainly the genioglossus, such collapse can be prevented. In this study, we present a simple tool that can be used to measure the strength of the tongue. This tool may provide an easy way to measure tongue function and allow a simple evaluation of pathologies that affect the tone of the tongue. We have carried out 20 tongue strength measurements using the Tongue Digital Spoon (TDS) in a healthy adult population, using the Iowa Oral Performance Instrument (IOPI) as the gold standard. To validate the procedure, we performed replicate measurements on 20 individuals aged 20–70 years. We found a mean TDS measurement of 115.99 g/cm2 in young subjects, 98.47 g/cm2 in middle-aged subjects and 84.23 g/cm2 in the elderly. There was a significant difference in the measurements between young and elderly participants. There was also a significant correlation between TDS and IOPI measurements (Pearson correlation coefficient, r = 0.69, P < 0.001). We found the TDS to be a useful tool in daily clinical practice for the measurement of the strength of the tongue in the healthy population. It has potential application in oropharyngeal monitoring and rehabilitation.

Interaction Between a Southwestward Propagating MSTID and a Poleward Moving WSA‐Like Plasma Patch on a Magnetically Quiet Night at Midlatitude China Region

AbstractIn this paper, we first investigate the effects of a postevening Weddell Sea Anomaly (WSA)‐like plasma patch on a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID) over Xinglong, China (40.4°N, 117.6°E; Mlat. ~30.4°N). We found that some of this kind of WSA‐like plasma patches moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they traveled westward into China. During 2012–2016, these plasma patches frequently occurred during the period of May–August with a monthly occurrence rate of over 20%, causing nighttime plasma density enhancements (NPDEs) in midlatitudes of China. Over 50% of these structures were accompanied by concurrent MSTID and Es. We propose that an intense polarization electric field (PEF) associated with an MSTID/Es from the more northern regions of EIA could frequently drive these plasma patches poleward. On the night of 27 June 2014 (Kp &lt; 2−), either an MSTID and an intense Es layer (the maximum foEs &gt;12 MHz) occurred at the onset of the poleward motion of a WSA‐like plasma patch, lending credence to this possibility. The observed plasma patch was pushed poleward to interact with the observed MSTID, causing some poleward extending C‐shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one. We attributed those poleward extending airglow depletions to the interaction between the MSTID and the plasma patch because of the secondary gradient drift instability and those of airglow enhancements to the redistribution of plasma.

Computational investigation of the cavitation vortex dynamics in flow over a three-dimensional hydrofoil by a new transport-based model

Cavitation is of significant practical interest due to its unsteady features which could induce destructive effects such as drastic drop in efficiency, noise, vibration, and corrosion for propulsion systems, rudders and other hydraulic machinery. A thorough understanding of the hydrodynamics in the cavitating flow past a three-dimensional hydrofoil makes indicators for an improved control performance of these hydraulic systems. Hence, a computational investigation of the cavitating flow over the Delft Twist-11 hydrofoil was performed with special emphasis on the cavitation vortex dynamics. A new transport-based cavitation model was proposed and compared with the conventional Schnerr–Sauer model through the predictions of cavitation characteristics, for which available experimental data were also provided. The results show that, as compared with the Schnerr–Sauer model, the proposed model can predict closer engineering parameters, including time-mean lift coefficient and vapor shedding frequency with the experiments. In addition, more reasonable structure and dynamics of the three-dimensional unsteady sheet/cloud cavitation patterns, including the cavity growth, break-off, and collapse downstream are captured by the proposed model. With the help of the vorticity transport equation in a variable density flow, further analysis of the flow field predicted by the proposed model reveals that cavitation promotes the production of vortex as well as the flow instabilities. Vorticity production in the cavitating flow is mainly induced by the terms of vortex stretching and vortex dilatation, while the baroclinic torque only contributes in the region of shedding and collapse of the cloud cavity and the contribution of the viscous diffusion term is negligible as compared with the other three terms. The main significance of this study is that it demonstrates the potential of a robust transport-based cavitation model to investigate the unsteady dynamics in the cavitating flow past a three-dimensional twisted hydrofoil and expected to make sense for other hydraulic machinery.

Velocity Gradient in the Presence of Self-gravity: Identifying Gravity-induced Inflow and Determining Collapsing Stage

Abstract Understanding how star formation is regulated requires studying the energy balance between turbulence, magnetic fields, stellar feedback, and gravity within molecular clouds. However, identifying the transition region where the gravity takes over remains elusive. Recent studies of the Velocity Gradient Technique (VGT), which is an advanced tool for magnetic field studies, reveal that the gradients of spectroscopic observables change their directions by 90° with respect to the magnetic fields in the regions of gravitational collapse. In this study, we perform 3D MHD numerical simulations. We observe that star formation successfully proceeds in strongly magnetized and fully ionized media. We confirm that the self-gravity induces the change of gradients’ orientation and gradients’ high amplitude. We explore two ways of identifying collapsing self-gravitating regions through the double-peak feature in the histogram of gradients’ orientation and the curvature of gradients. We show that velocity gradients’ morphology and amplitude can be synthetically used to trace the convergent inflows. By comparing with the column density Probability Density Functions method, we show that VGT is a powerful new tool for studying the gas dynamics and tracing magnetic field in star-forming regions. By analogy with VGT, we extend the Intensity Gradient Technique (IGT) to locate the gravitational collapsing region and shocks. We demonstrate that the synergy of VGT and IGT can determine the collapsing stages in a star-forming region.

Investigation of failure region around the wellbore based on the extended Mogi-Coulomb criterion for rock matrix

ABSTRACT In order to investigate the failure region around wellbore under a given mud pressure, three different strength criteria are fit to triaxial experimental data, then the strength criterion with the smallest fitting error is selected to analyse wellbore stability. Based on the investigate of rock strength criteria, a prediction model for the shear failure region around borehole is established. The influence of wellbore trajectory on the shape and position of shear failure region around wellbore, and collapse pressure variation at wellbore wall is analysed. Results indicated that, the area of shear collapse around the well predicted by Mohr-Coulomb criterion (MC) is elliptic, while the same results predicted by extended Mogi-Coulomb criterion (EMG-C) is approximately quadrilateral. The intermediate main stress decreases the collapse pressure and its anisotropy at the wellbore wall, but increases the failure region around the wellbore. The EMG-C takes into account the effect of intermediate principal stress and the nonlinear failure characteristics of rocks in high stress environment, and the prediction position of the shear collapse region around the borehole is quite different from the shape predicted by MC criterion.

Experimental Characterization of an Embossed Capacitive Micromachined Ultrasonic Transducer Cell.

Capacitive Micromachined Ultrasonic Transducer (CMUT) is a promising ultrasonic transducer in medical diagnosis and therapeutic applications that demand a high output pressure. The concept of a CMUT with an annular embossed pattern on a membrane working in collapse mode is proposed to further improve the output pressure. To evaluate the performance of an embossed CMUT cell, both the embossed and uniform membrane CMUT cells were fabricated in the same die with a customized six-mask sacrificial release process. An annular nickel pattern with the dimension of 3 m × 2 m (width × height) was formed on a full top electrode CMUT to realize an embossed CMUT cell. Experimental characterization was carried out with optical, electrical, and acoustic instruments on the embossed and uniform CMUT cells. The embossed CMUT cell achieved 27.1% improvement of output pressure in comparison to the uniform CMUT cell biased at 170 V voltage. The fractional bandwidths of the embossed and uniform CMUT cells were 52.5% and 41.8%, respectively. It substantiated that the embossed pattern should be placed at the vibrating center of the membrane for achieving a higher output pressure. The experimental characterization indicated that the embossed CMUT cell has better operational performance than the uniform CMUT cell in collapse region.

Injectivity of Multiple Slugs in Surfactant Alternating Gas Foam EOR: A CT Scan Study

SummaryA surfactant alternating gas (SAG) process is often the injection method for foam, on the basis of its improved injectivity over direct foam injection. In a previous study, we reported coreflood experiments on liquid injectivity after foam flooding and liquid injectivity after injection of a gas slug following steady-state foam. Results showed that a period of gas injection is important for the subsequent liquid injectivity. However, the effects of multiple gas and liquid slugs were not explored.In this paper, we present a coreflood study of injectivities of multiple gas and liquid slugs in an SAG process in a field core. Nitrogen and surfactant solution are either coinjected or injected separately into the sandstone core sample. The experiments are conducted at an elevated temperature of 90°C with a backpressure of 40 bar. Differential pressures are measured to quantify gas and liquid injectivities. Computed tomography (CT) scanning is applied to relate water saturation to mobility.During the injection of a large gas slug following foam, a bank in which foam completely collapses or greatly weakens forms near the inlet and propagates slowly downstream. During the subsequent period of liquid injection, liquid flows through the collapsed-foam bank much more easily than further downstream. Beyond the collapsed-foam region, liquid first imbibes into the whole cross section. In this region, liquid flows mainly through a finger of high liquid saturation. Our CT results suggest a revision of our earlier interpretation; the process of gas dissolution does not merely follow fingering but is evidently directly involved in the fingering process.Our results suggest that, in radial flow, the small region of foam collapse very near the well greatly improves injectivity. The subsequent gas and liquid slugs behave near the wellbore, affecting injectivity, in a way similar to the first slugs. Thus, the behavior and modeling of the first gas slug and first subsequent liquid slug is representative of near-well behavior in an SAG process. The trends observed in our previous work are reproduced in a low-permeability field core.

A pressure decomposition framework for aeroacoustic analysis of turbulent jets

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e, not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise.

Three flavor neutrino conversions in supernovae: slow & fast instabilities

Self induced neutrino flavor conversions in the dense regions of stellar core collapse are almost exclusively studied in the standard two flavor scenario. Linear stability analysis has been successfully used to understand these flavor conversions. This is the first linearized study of three flavor fast instabilities. The 'fast' conversions are fascinating distinctions of the dense neutrino systems. In the fast modes the collective oscillation dynamics are independent of the neutrino mass, growing at the scale of the large neutrino-neutrino interaction strength (105 km−1) of the dense core. This is extremely fast, in comparison to the usual 'slow' collective modes driven by much smaller vacuum oscillation frequencies (100 km−1). The three flavor analysis shows distinctive characteristics for both the slow and the fast conversions. The slow oscillation results are in qualitative agreement with the existing nonlinear three flavor studies. For the fast modes, addition of the third flavor opens up possibilities of influencing the growth rates of flavor instabilities when compared to a two flavor scenario.

Prediction of the Collapse Region Induced by a Concealed Karst Cave above a Deep Highway Tunnel

The presence of a concealed karst cave above a deep highway tunnel may cause the collapse of the rock mass between the karst cave and tunnel during excavation. Rock mass collapse threatens the safety of tunnel construction personnel. A prediction method of the collapse region induced by a concealed karst cave above a deep highway tunnel is proposed on the basis of the upper bound theorem of limit analysis. An analytical expression of the collapse surface is derived from the variational principle. Using the analytical expression of the collapse surface, the shapes of the collapse surfaces are plotted for different rock mass parameters. Moreover, the minimum safe distance between the karst cave and tunnel is defined, and the computational equation of the minimum safe distance is derived. The proposed method is applied in a highway tunnel excavated in a karst terrain as a case study. Based on geological survey report parameters, the shape of the collapse surface and the minimum safe distance between the karst cave and tunnel are obtained. Finally, the collapse surface of the rock mass provided by the proposed approach is compared with that provided by numerical simulation, and the favorable result comparison shows that the proposed method is valid.

Collapse mode of rock mass induced by a concealed karst cave above a deep cavity

Reliable prediction of the potential collapse region of rock mass is a key challenge for deep underground cavity excavation, especially if a concealed karst cave is located above the excavated cavity. Because of the effect of the blast vibration, a possible collapse would occur at a certain region between the concealed karst cave and the excavated cavity. This paper aims to study the roof collapse of a deep buried cavity induced by a concealed karst cave base on a two-dimensional failure mechanism by using upper bound theorem. The failure mechanism is constituted by arbitrary curves which is similar to the collapse observed in an actual cavity excavation. The shape and range of the collapse block is determined by virtual work equation in conjunction with variational approach. The results obtained by the presented approach are approximate with the numerical results provided by finite difference code, which indicates that the proposed method in this work is valid.

Co-Nonsolvency Response of a Polymer Brush: A Molecular Dynamics Study

Using molecular dynamics simulations, we study the response of a polymer brush exposed to co-nonsolvent (CNS), which acts as a preferential solvent for the polymer. We investigate a broad range of attractions between CNS and monomers and of grafting densities over the full range of cosolvent volume fractions. We compare our simulation results with the recently proposed adsorption–attraction model for co-nonsolvency in polymer brushes. The brush layer collapses with increasing CNS concentration into a more compact layer and followed by a reswelling toward sufficiently high CNS concentrations. As the strength of attraction of CNS toward the brush increases, the collapse transition becomes discontinuous. Increasing the grafting density leads to higher sensitivity with respect to CNS but also to a weaker collapse behavior as predicted from the analytical model. In the narrow collapse region, two states of the brush layer coexist, as can be expected from the type-II phase-transition behavior. The coexistence states are identified by analyzing the density profiles. Here, a dense layer is formed at the substrate, which is covered by a swollen layer. Both collapse and coexistence behaviors are even more pronounced in polydisperse brushes, indicating the robust nature of the co-nonsolvency effect in polymer brushes.

Experimental investigation on the cavitation performance in a venturi reactor with special emphasis on the choking flow

Experiments were conducted to investigate the performance of choked cavitating flow in a transparent venturi reactor at different pressure ratios by high speed camera technique. Cavitation images of various flow conditions and corresponding pressure variations were analyzed to study the development performance of the cavitating flow. This work provides a thorough understanding of the evolution of cavitation regions, collapse mechanism, averaged wall pressure and pressure pulsation at different pressure ratios. The cavitation regions in venturi under choking flow condition can be divided into inception and developing region, fusion region and collapse region. Further analyses on the processed images reveal that cavitation performance can be approximately divided into two sections by a transition pressure ratio of 0.71. At smaller pressure ratios (pr < 0.71), the re-entrant jet governs the cavitation dynamics and induces complex flow regions. And the core position of collapse region moves upstream fast with the increase of pressure ratio. At larger pressure ratios (pr > 0.71), cavitation dynamics is governed by the shock wave. And the core position of collapse region moves upstream slowly with the increase of pressure ratio. The spectrum distribution of cavitation cloud in collapse region is concentrated and the peak frequency increases slowly at small pressure ratios. However, the spectrum distribution of cavitation cloud is relatively decentralized at large pressure ratios. The measured pressure data indicates that averaged pressure in diffuser goes through three stages at different slopes with the increase of pressure ratio under choking flow condition. In addition, the variations of pressure pulsation intensity (PPI) influenced by cavitation regions, collapse mechanisms, back pressure and collapse distance are exhibited in the present work.

Mechanical Properties of Riverbank Soil in the Jingjiang Reach and the Characteristics of Flow in Pocket Collapse Area

Based on analyzing the field data and the experimental results, Mechanical properties of riverbank soil in the Jingjiang Reach and the characteristics of flow in the region of pocket collapse are discussed. It is calculated that riverbank of the Jingjiang Reach is characterized by a typical composite structure, covering the low liquid limit cohesive upper layer and the sandy lower layer. The cohesion of the cohesive soil increases first and then decreases with the increase of water content. The internal friction angle generally decreases with the increase of water content. The flow structure in pocket collapse area shows the intensive three-dimensional characteristics. The surface and vertical average velocity, the range and strength of the reflux increase with the increase of upstream flow discharge. Under different conditions of flow discharge, the vertical average velocity in pocket collapse area has a distributed feature of nearly linear or tiny oscillation along the relative direction of water depth. The research can provide the technical support for prevention measures of bank collapse.

Ultrafast dynamics of magnetic vortices and pulse collapse in a laser-under dense plasma interaction

The energy of an intense ultrashort laser pulse interacting with high density (still under dense) plasma is typically transformed into electron heating and in excitation of nonlinear coherent structures such as magnetic vortices, solitons, or post-solitons. Using 33 TW 30 fs laser pulses and a high-density nitrogen gas jet, we experimentally investigated magnetic vortices in the laser pulse collapse region where electrons are efficiently accelerated and heated. Those vortices, which are associated with rapidly decaying magnetic fields, are found to be immobile and persist for several picoseconds. A collisionless plasma was formed due to the quasi-static field ionization of the gas associated with the hot and fast electron currents. The evolution dynamics of such nonlinear plasma phenomena have been monitored by using a 30-fs probe laser beam through employing the polarimetric and shadowgraphic techniques. Our experimental results are also supported by particle-in-cell simulations.