Instability Processes Research Articles

Turbulence Characteristics in the Mixing Layer of a Submerged Cavitating Jet at High Reynolds Numbers

In this paper, unsteady and time-averaged turbulence characteristics in a submerged cavitating jet with a high Reynolds number are studied using large eddy simulation. The simulation is validated by comparing the vapor distribution using CFD and a high-speed photography experiment. The results indicate that the currently used numerical method can predict the evolution of the cavitation cloud in the jet accurately. The instantaneous and time-averaged flow fields of the submerged jet with three different cavitation numbers are studied. Comparing the frequency spectral of jets with different cavitation numbers, it is found that, for a fixed location, the frequency increases with the decrease in the cavitation number. Comparing the vorticity distribution at different streamwise locations, the instability process of the ring-shapes vortexes is revealed. Comparing the shape of the cavitation cloud and the vortexes in the jet finds that their spatial distribution and the temporal evolution are similar, indicating that the dynamic characteristics of the vortex and the cavitation affect each other. For the currently investigated cavitating jets, the Reynolds number increases with the decrease in the cavitation number. However, the spreading rate is lower for the jet with higher Reynolds numbers here. This is means that the momentum exchange between the jet and submerging water is reduced by the cavitation phenomenon.

Beyond third-order optical nonlinearities in liquid suspensions of metal-nanoparticles and metal-nanoclusters

Various nonlinear optical (NLO) phenomena observed in liquid suspensions of metal nanoparticles (NPs) and metal nanoclusters (NCs) have received considerable attention in the past few years. For example, multiphoton absorption, modulational instability, optical solitons, and parametric processes, benefit from research on soft-matter systems. Thus, many results are still being published providing support for earlier theoretical predictions and new discoveries. In particular, colloids containing NPs and NCs of noble-metal atoms present outstanding NLO responses and have been exploited to study for studying various phenomena and applications. Indeed, when illuminated with intense optical fields, such colloids can exhibit effective high-order nonlinearities (HONs) with large magnitudes which can dominate their NLO behavior. In this article, we review experiments with liquid suspensions of noble-metal NPs and noble-metal NCs, in which an appropriate methodology has been applied to study effective HONs for different sizes and morphologies of metal-nanostructures. In addition, a nonlinearity management procedure is discussed as a valuable tool to control for controlling the NLO response of metal-colloids through constructive and destructive interference among the nonlinearities of different orders. Because of the strong NLO behavior of metal-colloids, various phenomena associated to effective HONs were investigated such as: self-action effects, saturated multiphoton absorption, NLO scattering, bright and vortex spatial solitons, and operation of Random Lasers. Perspectives for further studies and photonic applications of metal-colloids exploring their effective nonlinearities are also discussed.

Soliton Solutions of Klein–Fock–Gordon Equation Using Sardar Subequation Method

The Klein–Fock–Gordon equation (KFGE), defined as the equation of relativistic wave related to NLEEs, has numerous implications for energy particle physics and is useful as a model for several types of matter, with deviation in the basic stuffs of particles and in crystals. In this work, the Sardar subequation method (SSM) is used for finding the solution of this KFGE. The advantage of SSM is that it provides many different kinds of solitons, such as dark, bright, singular, periodic singular, combined dark–singular and combined dark–bright solitons. The results show that the SSM is very reliable, simple and can be functionalized to other nonlinear equations. It is verified that all the attained solutions are stable by modulation instability process. To enhance the physical description of solutions, some 3D, contour and 2D graphs are plotted by taking precise values of parameters using Maple 18.

Study on Energy Density Distribution and Mass Fractal of Deep Saturated Coal Samples under Uniaxial Stepwise High Stressed Loading and Unloading Path

The geostress in deep coal roadway is high and affected by mining frequently; under the influence of water-rock interaction, the bearing capacity of coal continues to deteriorate. Based on the characteristics of short water absorption process and low moisture content of deep coal samples in this experiment, the threshold range of quantitative division of water absorption stage of coal samples is given, and the structural characteristics of deep coal samples that are more susceptible to cracks are analyzed. The actual loading process is simulated through uniaxial stepwise high stressed loading and unloading experiments, the gradual development of cracks was the root cause of instability of coal, and saturated water mainly aggravates the instability process through these cracks; the stepwise high stressed loading and unloading path makes cracks more likely to occur in the sample. After each level of loading and unloading, the average stiffness is approximately linear trend degradation. By calculating the energy density of deep coal samples in each stage before the peak, the change of energy density indexes between saturated and dried samples is different from relative shallow samples, and the energy density indexes do not decrease but increases instead. However, the proportion of each energy is generally positive and inverted “U”-type distribution. The change trend of plastic energy in proportion to dissipation energy can be used as the judgment feature of coal sample instability. The instability of deep coal samples is mainly caused by the gradual dissipation of plastic energy to generate cracks and divided into fragments with large and long edge, resulting in low mass fractal dimension. In engineering practice, the loading times should be reduced and the suitable pressure grouting should be conducted timely to delay the deterioration process of crack development in deep water-bearing coal under high stress disturbance.

How accounting creates performative moments and performative momentum

PurposeThis paper aims to analyse the longitudinal performative effects of accounting, focusing on how accounting shapes the stability/instability of economic frames over time.Design/methodology/approachTo explore the performative effects of accounting over time, a longitudinal case study narrates the transformation of a large, listed manufacturing company's financial strategy over 20 years. Using extensive document collection, the authors trace the shift from an “industrial” frame to a “shareholder value” frame in the mid-1990s, followed by the gradual entrenchment of this shareholder value frame until its decline in the wake of the financial crisis in 2008.FindingsOur findings show how accounting has different performative temporalities, capable of precipitating sudden shifts between different economic frames and stabilising an ever-more entrenched and narrowly defined enactment of a specific frame. We conceptualise these different temporalities as performative moments and performative momentum respectively, explaining how accounting produces these performative effects over time. Moreover, in contrast to extant accounting research, the authors provide insight into the performative role of accounting not only in contested but also “cold” situations marked by consensus regarding the overarching economic frame.Originality/valueOur paper draws attention to the longitudinal performative effects of accounting. In particular, the analysis of how accounting entrenches and refines economic frames over time adds to prior research, which has focused mainly on the contestation and instability of framing processes.

Stability analysis of supermassive primordial stars: a new mass range for general relativistic instability supernovae

ABSTRACT Observed supermassive black holes in the early Universe have several proposed formation channels in part because most of these channels are difficult to probe. One of the more promising channels, the direct collapse of a supermassive star has several possible probes including the explosion of a helium-core supermassive star triggered by a general relativistic instability. We develop a straightforward method for evaluating the general relativistic radial instability without simplifying assumptions and apply it to population III supermassive stars taken from a post Newtonian stellar evolution code. This method is more accurate than previous determinations and it finds that the instability occurs earlier in the evolutionary life of the star. Using the results of the stability analysis, we perform 1D general relativistic hydrodynamical simulations, and we find two general relativistic instability supernovae fuelled by alpha capture reactions as well as several lower mass pulsations, analogous to the puslational pair instability process. The mass range for the events ($2.6-3.0\times 10^4{\rm M}_\odot$) is lower than that had been suggested by previous works ($5.5\times 10^4{\rm M}_\odot$), because the instability occurs earlier in the star’s evolution. The explosion may be visible to among others JWST, while the discovery of the pulsations opens up additional possibilities for observation.

A discontinuous deformation analysis of the internal shear restraint mechanism in the sliding instability process of compound slopes

Because of the geometric turning characteristics of its basal sliding surface, the compound slope will undergo internal shear deformation during the sliding process. However, few investigations have been conducted into the accurate location and formation process of the internal shear surface, as well as the internal shear restraint mechanism and its influence on slope stability. Therefore, relevant research was carried out based on the Vajont landslide, which is a typical representative of the compound slope. Firstly, the mechanical equilibrium on the internal shear fracture surface is further taken into account in the Rigid Limit Equilibrium Method (RLEM), and the accurate location of the shear surface was obtained by solving the equilibrium equations on the three sliding surfaces simultaneously. Additionally, a sensitivity analysis of the landslide's mechanical characteristics was conducted. Then, using the Discontinuous Deformation Analysis (DDA) method with hydraulic coupling, the entire instability process of the Vajont landslide generated by reservoir impoundment is simulated, as well as the formation process of the shear surface. The results reveal that the locations of the shear surface derived by the two methods agree well. Meanwhile, an objective shear restraint mechanism exists inside the compound slope, which contributes to its overall stability.

Numerical study of the thermocapillary instability in a thin liquid–air film

It is well known that thermal field would cause instability on a two-fluid interface due to the Marangoni effect. This phenomenon is also referred to as thermocapillary flow. A thin liquid/air film experiences thermocapillary instability when it is confined between hot and cold plates. The periodic micro/nano-patterns can generally be observed in the film. Therefore, the thermocapillary instability can be used to fabricate micro/nano-polymeric structures. The present paper proposes a fully nonlinear numerical model based on the phase field method to study the dynamic process of thermocapillary instability. Interfacial evolution and nonlinear effects of the thin liquid film are especially investigated. The impact of the key parameters, e.g., the Marangoni and Reynolds numbers, on the stability are also explored. In addition to the single-mode analysis, the thermocapillary instability is also studied in the multi-mode condition. The conventional single-mode approach facilitates the stability analysis of different wavelengths, while the multi-mode method describes the experiments in a more practical way.

Mobility, Nonstationary Density, and Robbery Distribution in the Tourist Metropolis

This study looks at the spatial distribution of robbery against residents as a function of nonstationary density and mobility patterns in the most densely populated city in Spain, Barcelona. Based on the geographical coordinates of mobile devices, we computed two measures of density of the ambient population and the tourist presence, for work days, weekends, and holidays in 2019. Negative binomial regressions are then estimated to analyse whether these measures are correlated with the risk of robbery, controlling for land use and the characteristics of the social environment. The model reveals that residents’ chances of being exposed to robbery in Barcelona depend on the social relevance and tourism attractiveness of certain places at particular times of the year. Our results disclose two sources of social disorganization as stronger predictors of the occurrence of robbery in Barcelona, respectively linked to structural processes of residential instability and daily and seasonal mobility patterns. On the one hand, we found that the effect of the density of international tourists on the outcome variable is mediated by residential volatility, which is assumed to be associated with housing shortages in neighbourhoods where short-term vacation rentals are widespread. On the other hand, the ability to exert effective social control is significantly undermined in urban areas, where the ambient population and the volume of tourists outnumber the resident population, thus increasing incidents of robbery victimization. The implications of these findings for urban policy and crime prevention in the Catalan capital are discussed.

Measurement of electromagnetic waves from runaway electrons.

Electromagnetic waves emitted during a tokamak discharge can be partially ascribed to coupling with plasma waves. In particular, in the presence of runaway electrons, the electromagnetic waves deliver information, otherwise inaccessible, about kinetic instabilities excited by the fast particles. Experiments aimed at studying radio frequency emissions from runaway electron scenarios during different stages of plasma discharge have been carried out at the Frascati Tokamak Upgrade. Frequencies in the range of lower hybrid and whistler waves have been explored, in the presence of relativistic electrons with different energies, ranging from a few to tens of MeV. A pronounced sensitivity of the radio frequency measurements in detecting driven instabilities is observed, providing the possibility to exploit this kind of technique as a monitor of the instability processes and for studies of the fast electron activity. In particular, in this work, we propose a simplified analysis of the frequency scaling of a specific family of kinetic instabilities arising at the lower hybrid frequency range during the current ramp-up stage. The study is performed with respect to the density profile and the wave vector coupling conditions and is aimed at obtaining a rough estimate of the most likely radial location of the interaction between the runaway electron beam and plasma waves at the emission times of the observed signals.

New Insights on the Formation and Breaking Mechanism of Convective Cyclonic Cones in the South Adriatic Pit during Winter 2018

Abstract The deepwater formation in the northern part of the South Adriatic Pit (Mediterranean Sea) is investigated using a unique oceanographic dataset. In situ data collected by a glider along the Bari–Dubrovnik transect captured the mixing and the spreading/restratification phase of the water column in winter 2018. After a period of about 2 weeks from the beginning of the mixing phase, a homogeneous convective area of ∼300-m depth breaks up due to the baroclinic instability process in cyclonic cones made of geostrophically adjusted fluid. The base of these cones is located at the bottom of the mixed layer, and they extend up to the theoretical critical depth Zc. These cones, with a diameter on the order of internal Rossby radius of deformation (∼6 km), populate the ∼110-km-wide convective site, develop beneath it, and have a short lifetime of weeks. Later on, the cones extend deeper and intrusion from deep layers makes their inner core denser and colder. These observed features differ from the long-lived cyclonic eddies sampled in other ocean sites and formed at the periphery of the convective area in a postconvection period. So far, to the best of our knowledge, only theoretical studies, laboratory experiments, and model simulations have been able to predict and describe our observations, and no other in situ information has yet been provided.

Investigation of progressive settlement of sandy cobble strata for shield tunnels with different burial depths

The tunnel face stability is a key issue to ensure the engineering safety. Combined with model tests and numerical simulations, this paper focuses on progressive settlement characteristics of sandy cobble strata for shield tunnels during the instability process. Burial depths are the main parameter in this study, and ratios of burial depth and tunnel diameter were set as 0.5, 1.0 and 2.0, respectively. The results show that the surface settlement value decreases with the increase of the burial depth ratio. In addition, the time of local instability is negatively correlated with the burial depth ratio. The soil disturbance range of tunnel face instability with relatively large burial depth is significantly smaller than that with small burial depth. This provides a reference for the research on the strata deformation caused by the instability of the tunnel face in the sandy cobble strata.

Analytical model for Rayleigh-Taylor instability in conical target conduction region

This work builds an isobaric steady-state fluid analytical-physical model of the plasma conduction region in a conical target. The hydrodynamic instability in the double-cone ignition scheme[21] for inertial confinement fusion (ICF) proposed by Zhang is studied with the built model. With this idealized model, the relevant parameters, such as density, temperature, and length of the plasma in the conduction region of the conical target under long-pulse conditions are given. The solution of the proposed analytical model dovetails with the trend of the numerical simulation. The model and results in this paper are beneficial for discussing how to attenuate Rayleigh–Taylor instability in ICF processes with conical and spherical targets.

Second harmonic generation as a probe of parametric spin wave instability processes in thin magnetic films

We have explored the dynamic response of in-plane magnetized thin permalloy films excited by microwave fields of high amplitudes (up to 3 Oe) at 1 GHz. The response was detected using a microstrip line by measuring the second harmonic signal generated by the dynamic components of the uniform magnetization. The data measured at ferromagnetic resonance showed the threshold effect of the Suhl parametric instability process. With the increase of the microwave power above the threshold value, the dynamic response revealed an intricate nonlinear behavior, including the emergence of an additional threshold. This second threshold can be explained in terms of the ``stage by stage'' process of parametric spin wave excitation following the $S$ theory of Zakharov, L'vov, and Starobinets.

Nutrition program, physical activity and gut microbiota modulation: a randomized controlled trial to promote a healthy lifestyle in students with vitamin D3 deficiency.

University students are a high-risk group for stress, consumption of junk food and significant weight gain over a short period. Inadequate vitamin D intake has been linked to many health issues, including chronic headache, apathy, aggression and depression. Furthermore, vitamin D deficiency led to dysbiosis of the gut microbiota. The purpose of our study was to estimate the effect of 90-days healthy lifestyle programs along with gut microbiota modulation in university students with vitamin D<inf>3</inf> deficiency. In this randomized controlled trial, 130 students (18-25 years old) with vitamin D deficiency were recruited. Both the standard care group (N.=65) and the intervention group (N.=65) received a 3-months course of individually selected nutrition program and physical activity (8000-10,000 steps daily). The intervention group received an additional treatment with synbiotic Acidolac and vitamin D<inf>3</inf> for 3 months. The psycho-emotional status of the participants was assessed by a validated questionnaire that examined situational anxiety. In all students, blood pressure, anthropometric variables, as well as laboratory metabolic parameters, were recorded. In both groups, vitamin D<inf>3</inf> deficiency was associated with instability and lability of mental processes, mood swings, bad sleep, high rates of stuck and agitation for any problem. Combined therapy (diet, physical activity and synbiotic) induced a significant improvement in the psycho-emotional state of students. The 90-days therapy vitamin D<inf>3</inf> increased the level of vitamin D<inf>3</inf> in serum in the intervention group. Lastly, we observed a decrease in the body weight, body mass index, waist circumference and fatty mass, only in students included in the interventional group. Nutrition program, physical activity, vitamin D<inf>3</inf> intake and gut microbiota modulation led to both the improvement in vitamin D levels in serum and emotional harmonization.

Simulation of Es Layer Modulated by Nonlinear Kelvin–Helmholtz Instability

AbstractIn this study, a two‐dimensional numerical model of the nonlinear Kelvin–Helmholtz (KH) instability is used to investigate the nonlinear evolution processes of the ion layer, which is associated with the middle and low latitude E region irregularities. Based on simulation results, the two‐stage process in the evolution of the nonlinear KH instability is found in KH vortex morphology. The simulated nonlinear KH instability process presents multiple striation structures, including horizontal structures of the primary ion layer vortex with a size of a few kilometers and the secondary KH vortex with a size of 2 km. The polarization electric field is induced in the horizontal structure of the sporadic E (Es) layer associated with the vortex structure of neutral wind. The polarization electric field presents a similar tendency to the quasi‐periodic vortex structure. The horizontal distorted Es layer structure and anomalously large polarization electric field can provide the initial condition and lead to plasma instability, which can further develop to meter‐scale irregularities of QP echoes.

Modulation instability of two-dimensional Bose-Einstein condensates with helicoidal and a mixture of Rashba-Dresselhaus spin-orbit couplings

The modulational instability (MI) process is exclusively studied in a two-component Bose-Einstein condensate (BEC) which includes Rashba-Dresselhaus (RD) spin-orbit (SO) and helicoidal SO couplings. A generalized set of two-dimensional (2D) Gross-Pitaevskii (GP) equations is derived. The tunability of the helicoidal gauge potential is exploited to address BECs dynamics in a square lattice. The MI growth rate is derived, and parametric analyses of MI show the dependence of the instability on interatomic interaction strengths, the RD SO coupling, and helicoidal SO coupling, which combines the gauge amplitude and the helicoidal gauge potential. Direct numerical simulations are carried out to confirm the analytical predictions. Trains of solitons are obtained, and their behaviors are debated when the RD SO parameters are varied under different combinations between the gauge amplitude and the helicoidal gauge potential. The latter gives a potential way to manipulate the trapping capacities of the proposed BEC model.

Influence of relative density on deformation and failure characteristics induced by tunnel face instability in sandy cobble strata

This paper aims at influence of relative density on deformation and failure characteristics for a shield tunnel in sandy cobble strata. A series of model tests were performed for tunnels with different relative densities of 0.35, 0.55, and 0.75. The evolution law of the stratum deformation during the instability process of the tunnel face was analyzed. The soil disturbance range of tunnel face instability with different relative densities was explored. The results show that relative density has a significant impact on the extent of surface subsidence. The stratum deformation caused by the instability of tunnel face in the medium dense and dense sandy cobble strata only occurs significantly behind the tunnel face, while the stratum with loose sandy cobble has significant stratum deformation behind and in front of the tunnel face. The method of improving the relative density can be used to reduce the deformation of the sandy cobble strata.

The Near Fault Observatory community in Europe: a new resource for faulting and hazard studies

The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS, http://www.epos-eu.org) Thematic Communities, today consisting of six research infrastructures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. Earthquakes respond to complex natural systems whose mechanical properties evolve over time. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. Hence, NFOs are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows generation of innovative scientific products. The NFOs usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surface and sometimes below grade. These dense and modern networks of multi-parametric sensors are sited at and around active faults, where moderate to large earthquakes have occurred in the past and are expected in the future. They continuously monitor the underlying Earth instability processes over a broad time interval. Data collected at each NFO results in an exceptionally high degree of knowledge of the geometry and parameters characterizing the local geological faults and their deformation pattern. The novel data produced by the NFO community is aggregated in EPOS and is made available to a diverse set of stakeholders through the NFO Federated Specific Data Gateway (FRIDGE). In the broader domain of the Solid Earth sciences, NFOs meet the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and a societal issue. It represents the EPOS concept and objective of aggregating and harmonising the European research infrastructures capabilities to facilitate broader scientific opportunity. The NFOs are at the cutting edge of network monitoring. They conduct multidisciplinary experiments for testing multi-sensor stations, as well as realise robust and ultra-low latency, transmission systems that can routinely accommodate temporary monitoring densification. The effort to continuously upgrade the technological efficiency of monitoring systems positions the NFO at the centre of marketing opportunities for the European enterprises devoted to new sensor technology. The NFOs constitute ideal test beds for generating expertise on data integration, creating tools for the next generation of multidisciplinary research, routine data analysis and data visualization. In particular focus is often on near-real time tools and triggering alarms at different levels are tested and implemented, strengthening the cooperation with the Agencies for risk management. NFOs have developed innovative operational actions such as the Testing Centre for Earthquake Early Warning and Source Characterisation (CREW) and detailed fast ground shaking and damage characterization. Complementing the recent growth of modern laboratory and computational models, the NFOs can provide interdisciplinary observations of comparable high resolution to describe the behaviour of fault slip over a vast range of spatial and temporal scales and aiding to provide more accurate earthquake hazard characterizations.

Wavenumber lock-in and spatial parametric resonance in convection

Analysis of convection driven by spatial periodic heating shows a wealth of responses due to Prandtl number dependence. The primary convection is delineated by the heating wavenumber and the secondary convection is characterized by the critical wavenumber established by an instability process. The resulting two-wavenumber dynamical system involves spatial parametric resonance leading to the wavenumber lock-in and the Rayleigh–Bénard (RB) instability producing unlocked states. Heating conditions leading to the co-existence of both types of states have been identified. Transition from the locked to the unlocked states produces states with a wide range of wavelengths and diverse patterns of movement. The possible secondary states are driven by a competition between the RB mechanism and the spatial parametric resonance. The relative strengths of these mechanisms change with Prandtl number $Pr$ resulting in four types of system responses. In the type A response occurring for $Pr&gt;0.4$ , parametric resonance dominates for the heating wavenumber $\alpha &lt; 4.5$ resulting in the pattern lock-in between the primary and secondary convections whereas RB occurs for $\alpha &gt; 10$ . There is a wealth of possible convection patterns in the in-between zone where a small change of $\alpha$ results in a major change of flow pattern. In the type B response, which occurs for $0.19&lt; Pr&lt; 0.4$ , the RB effect dominates eliminating the lock-in. The type C response, which occurs for $0.08 &lt; Pr&lt; 0.19$ , is similar to type A but stronger spatial modulation extends the range of dominance of parametric resonance up to $\alpha =7$ while the RB effect dominates for $\alpha &gt;10$ as in the type A response. A wealth of possible patterns occurs in the transition zone ( $7&lt;\alpha &lt;10$ ). In the type D response, which occurs for $Pr&lt;0.08$ , the strong spatial modulation results in the formation of two separate critical stability curves, one resulting from the dominance of the spatial parametric resonance leading to the lock-in effect, and the other one corresponding to the dominance of the RB effect producing unlocked states. No continuous transition between both states can occur. Conditions where both distinct states can arise simultaneously were identified. Morphing between different patterns of secondary convection may occur in response to small changes in the heating pattern in types A and C, while such processes are not possible in types B and D.