Kind Of Instability Research Articles

Rockfall susceptibility analysis of the “San Michele Arcangelo” historic trail (Central Italy) based on virtual outcrops and multiple propagation models

The “San Michele Arcangelo” historic trail (Aurunci Regional Park, central Italy), located along the southern slope of Mt. Altino, which is highly prone to rockfalls, is hiked every year by thousands of faithful on pilgrimage who are exposed to such kinds of instabilities. To contribute to a better understanding of the condition and evolution of such phenomena, providing a susceptibility scenario able to support the adoption of mitigation measures, a specific analysis was completed based on field and literature data. Three-dimensional virtual outcrop models were obtained through both photogrammetric and iPad Pro LiDAR surveys to derive geomechanical features of outcropping rocks and estimate potential detaching block volume. Possible mechanisms of detachment were then analyzed using the Markland test method. Susceptibility to rockfall propagation and block deposition was analyzed using GIS processing and rockfall propagation simulations based on deterministic/stochastic models. Such models consist of a combination of a deterministic algorithm able to simulate the physics of rockfall movement during propagation and a stochastic treatment of input parameters based on random sampling within defined interval. Two different rockfall simulations were compared in this study using the Rockyfor3D™ (ecorisQ Association) software and the newly developed RocFall3© (RocScience) code. The use of iPad Pro LiDAR survey provides accurate and high-resolution point clouds, with high speed of acquisition and real-time processing of data. Discontinuity sets identified from 3D models have represented the input for kinematic analyses of slopes, allowing to identify potential failure mechanisms among wedge and planar sliding or toppling. Rockfall simulations indicate the potential for rock blocks propagation and deposition over the whole study area. Comparing the results of rockfall simulations with the geomorphological map of the area it is evident the control exerted by the hydrographic network on rockfall propagation as demonstrated by the presence of screes and slope deposits along the main channels. Understanding the potential susceptibility to both propagation and deposition of rock blocks along the trail provides useful indication for the assessment of appropriate mitigation measures to realize for the safer touristic fruition of the site.

Shape stability of a microbubble in a power–law liquid

The onset of non-spherical oscillations of a microbubble in an unbounded power–law liquid, important for biomedical ultrasound applications, is studied. Two sets of evolution equations are obtained from the equation of motion: a Rayleigh Plesset-type equation for the spherical oscillations and an equation for the non-spherical oscillations. The non-spherical oscillations are modeled using the perturbation method via the Legendre polynomials. Two kinds of instabilities, namely parametric and Rayleigh-Taylor instabilities, are investigated. A higher power–law index causes the damping of the oscillations for both spherical and non-spherical oscillations. The power–law index damping effect depends on the ultrasonic drive frequency. At natural frequency, the amplitude of the perturbations is high compared to the non-resonant cases. At a low consistency index, the damping effect of the power–law index decreases. Unlike Newtonian liquids, the viscosity of power–law liquids is affected by the frequency of the acoustic field, thereby affecting Rayleigh-Taylor instability.

An Experimental Database for the Analysis of Bursting of a Laminar Separation Bubble

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.

Deep Learning for Bengali Fake News Detection: Innovative Approaches for Accurate Classification

A vast quantity of data and information are available on the internet. Because the internet is so widely available and has resulted in a tremendous growth in the number of online news, people are interested in reading news from online news portals. Online news portals include things like Facebook, Twitter, WhatsApp, Telegram, Instagram, blogs, and more. Both the quantity of news-on-news websites and the number of readers are increasing. But how real is online news today is a matter of thought. A huge amount of fake news is being spread in newspapers and online due to various yellow journalists. Which is having an adverse effect on society. As a result, there are many kinds of instability, bad politics, etc. problems are being created in the country. If this situation continues, our country and society will go to hell. The only solution is to ensure that yellow journalists do not spread fake news. But despite all the vigilance, fake news will spread. We can solve this by using artificial intelligence, for example, by employing various machine learning and deep learning algorithms, we can identify bogus news and take precautions against it. In this paper, fake news is detected using 4 deep learning algorithms like RNN, LSTM, BiLSTM, GRU model and 1 machine learning algorithm BERT model. RNN has an accuracy of 94.58%, LSTM has an accuracy of 92.84%, BiLSTM has an accuracy of 94.29%, GRU has an accuracy of 93.22% and BERT has an accuracy of 95%. The BERT model has the highest accuracy of 95% among all models.

Plausibility and Implications of Converter-Driven Oscillations Induced by Unstable Long-Term Dynamics

For decades, synchronous generators have dominated power system dynamics and have been the foundation for the comprehension of the physical phenomena involved in power system stability. This paradigm is rapidly changing; large synchronous generators that contribute to system strength are being replaced by inverter-based generators. Consequently, new kinds of instabilities are emerging. Slow-interaction converter-driven instability is one example in which undamped oscillations occur when inverter-based generators operate in weak areas. Transmission system operators have observed the phenomenon under the scope of short-term dynamics following a disturbance during a weak operating condition. This paper suggests that converter-driven oscillations can occur even in the case when the system's initial operating condition is strong. It concludes that the phenomenon can be triggered by long-term dynamics that slowly weaken the system in which small-signal and short-term simulations show an ample stability margin. The outcome is a combination of unstable long-term dynamics and undamped oscillations. Furthermore, this work suggests that classical emergency controllers that are effective against long-term instability may not be successful if undamped oscillations are also present. A method for instability detection and counteraction is proposed based on Prony analysis and time-domain simulations on the IEEE test system for voltage stability assessment.

Effects of a subsonic gas flow on the wave blocking and energy of interfacial waves with a constant vorticity

Negative energy waves play a significant role in hydrodynamic instabilities of flow and are involved in all kinds of instability in flows because they can withdraw energy from the flow. Effects of subsonic gas flow and vorticity on the existence of negative energy for water waves as well as the occurrence of capillary-gravity wave blocking have been studied. The existence of blocking points and variation in group velocity for different flow parameters such as the vorticity and subsonic gas factor is studied. The results are in agreement with a previous experiment [Suastika et al., “Experimental study of wave blocking,” Coastal Eng. 1, 227 (2000)]. The present study aims to deal with the combined effect of a subsonic gas flow parallel to solid boundaries and a moving liquid layer with linear velocity profile to establish the existing condition for negative energy waves and the blocking waves, especially for waves propagating in the opposite direction of the current. As a limiting case of the present model, in the absence of aerodynamic effect, it is found that the incident and reflected interfacial waves propagate inside the stability domain; this means that the transformed wave is generated at the expense of the energy of incident wave.

Surface wave instability in the thermocapillary migration of a flat droplet

The surface wave instability (SWI) of thermocapillary migration is examined by linear stability analysis for a droplet on a unidirectional heated plane. Both a Newtonian fluid and an Oldroyd-B fluid are considered. The droplet, flattened by gravity, is susceptible to two kinds of instabilities: convective instability (CI), which is independent of surface deformation; and SWI, which occurs only when the Galileo number and the surface-tension number are not too large. The wavenumber of the latter is much smaller than that of the former, while the reverse is true for the wave speed. SWI is found at different Prandtl numbers (Pr), while its mode includes streamwise and oblique waves. Energy analysis suggests that the energy of the long-wave mode comes from the shear stress induced by the surface deformation, the energy source for the mode with finite wavelength is the work done by Marangoni forces, while the energy from the basic flow is only important in some cases at small Pr. For the Oldroyd-B fluid, a small elasticity slightly changes the critical Marangoni number of SWI, while larger elasticity changes the preferred mode from SWI to CI. The instability mechanism is discussed and comparisons are made with experimental results.

The influence of internal flow on fluidelastic instability of a flexible tube in tube bundles subjected to two-phase cross-flow

This paper investigates the fluidelastic instability of a flexible tube in a rotated triangular tube array subjected to two-phase cross-flow and internal flow. We first established a mathematical model of a flexible tube in the tube bundles conveying flow considering the effect of two-phase cross-flow. Then, the eigenvalue analysis was performed to study the influences of the internal flow on the threshold and mechanism of the fluidelastic instability of the tube system in two-phase cross-flow. The results indicate that the internal flow has a significant effect on the threshold of the fluidelastic instability of the tube system in two-phase cross-flow, and may cause different kinds of instabilities. And, the mechanism of the fluidelastic instability of the tube system subjected to two-phase cross-flow and internal flow is notably affected by the tube boundary condition.

Discrete-time stabilization of wave phenomena engineering of network-like continuum transfer processes

This work is a natural continuation of the authors’ research of flow phenomena in the direction of increasing the dimension of the network-like region of change in the spatial variable. The possibility of practical use of the stability analysis of the differential-difference scheme to solve the problem of stabilization of wave phenomena in the engineering of the process of transfer of continuous media through network-like carriers (pipelines, gas and oil pipelines, industrial carriers of gas and petroleum products) is shown. Namely, if the scheme is stable, then sufficiently small changes in the initial data of the mathematical model of the process under study entail small changes in the solution of the difference problem, i.e. in practice do not lead to undesirable consequences; if the scheme is unstable, then small changes in the initial data can lead to arbitrarily large changes in the solution. During the operation of the above-mentioned industrial structures, wave phenomena inevitably arise, the consequence of which are various kinds of instabilities, entailing destruction of one or another nature. The results obtained are used in the digitalization of modern technological processes for the movement of liquid media and gases.

Moving Contact Line Instability on Soluble Fibers

AbstractThis paper presents a new kind of instability when inserting a soluble fiber into liquid. After wetting and dissolving the fiber by the liquid, the moving contact line (MCL) spontaneously loses stability. Because the sculpted shape of the fiber looks like a Chinese pagoda, this instability is named as the pagoda instability (PI). The coupling of dissolution and wetting leads to other special phenomena, i.e., dissolving‐induced jet flow, and optimizes the fiber shape, etc. A criterion of PI is proposed and the competition between the interfacial energy and chemical potential to deduce the MCL motion and PI is shown. A phase diagram is used to summarize the final shapes of the fibers. By conducting atomic force microscopy (AFM) measurements, it is found that fibers with optimized shapes have a low adhesion force. Using the optimized fiber can decrease the influence of the capillary force during AFM measurements in humid environments by 70%.

Chaotic instability in the BFSS matrix model

Chaotic scattering is a manifestation of transient chaos realized by the scattering with non-integrable potential. When the initial position is taken in the potential, a particle initially exhibits chaotic motion, but escapes outside after a certain period of time. The time to stay inside the potential can be seen as lifetime and this escape process may be regarded as a kind of instability. The process of this type exists in the Banks-Fischler-Shenker-Susskind (BFSS) matrix model in which the potential has flat directions. We discuss this chaotic instability by reducing the system with an ansatz to a simple dynamical system and present the associated fractal structure. We also show the singular behavior of the time delay function and compute the fractal dimension. This chaotic instability is the basic mechanism by which membranes are unstable, which is also common to supermembranes at quantum level.

Geomechanical analysis of an oil field: Numerical study of wellbore stability and reservoir subsidence

Geomechanics as the knowledge of rock deformation and stability is an indispensable part of all field development plans. Conducting geomechanical analyses leads to a safer and more efficient operation otherwise different kinds of instability and distortion might occur. In this study, the geomechanical behavior of Ilam and Sarvak formations of an oil field in southwest of Iran was investigated. The research objectives can be summarized as wellbore stability evaluation and predicting the value of reservoir subsidence due to pressure drop as a result of reservoir fluid production. To fulfill these, a set of petrophysical logs run in the exploration well of this green field were collected. Next, using empirical correlations and statistical methods, required data for evaluating wellbore stability during drilling, specifying safe mud window to discover reservoir breakdown pressure, predicting the possibility of wellbore collapse in field lifetime, and assessing reservoir subsidence were determined. The results revealed that the average subsidence value as the consequence of production within 21 years is 0.275 ft Which is not significant. In terms of wellbore stability, it was concluded that all horizontal and vertical wells remain stable during this time period. Briefly to conclude, field development is not associated with alarming incidents from geomechanical aspect.

The Response of Matter to Spatially Distributed Transient Energy Addition: An Asymptotic Analysis”: Part 1, Inert Gases

ABSTRACT The response of an inert gas to energy deposition is modeled using nondimensional Euler equations. Asymptotic methodologies are employed to identify the diversity of thermo-mechanical responses to spatially resolved, transient energy deposition into a gas with an emphasis on the induced gasdynamics. Physical conditions compatible with familiar linear and non-linear acoustics are quantified in terms of several nondimensional parameters. Significantly, conditions are identified that are associated with nearly isobaric or constant density responses and with induced O(1) Mach numbers. These results have important implications for related phenomena in reactive gases, including various kinds of instabilities, shocks and detonation evolution.

Stabilizing deep tomographic reconstruction: Part A. Hybrid framework and experimental results

SummaryA recent PNAS paper reveals that several popular deep reconstruction networks are unstable. Specifically, three kinds of instabilities were reported: (1) strong image artefacts from tiny perturbations, (2) small features missed in a deeply reconstructed image, and (3) decreased imaging performance with increased input data. Here, we propose an analytic compressed iterative deep (ACID) framework to address this challenge. ACID synergizes a deep network trained on big data, kernel awareness from compressed sensing (CS)-inspired processing, and iterative refinement to minimize the data residual relative to real measurement. Our study demonstrates that the ACID reconstruction is accurate, is stable, and sheds light on the converging mechanism of the ACID iteration under a bounded relative error norm assumption. ACID not only stabilizes an unstable deep reconstruction network but also is resilient against adversarial attacks to the whole ACID workflow, being superior to classic sparsity-regularized reconstruction and eliminating the three kinds of instabilities.

Instability attenuation and bifurcation studies of a non-ideal rotor involving time-delayed feedback

In non-ideal vibratory system, the excitation is a nonlinear function of system response. The dynamic behavior of such system is often characterized by an energy source with limited power. The study of instability phenomena in non-ideal rotor driven through a non-ideal energy source is of considerable current interest. The non-ideal rotor system often gets destabilized on exceeding a critical input power near the resonance. This kind of instability is termed as Sommerfeld effect marked with nonlinear jump phenomena. This paper investigates the attenuation of nonlinear jump phenomena and numerical study of bifurcations of a non-ideal unbalanced rotor system with internal damping using time-delayed feedback via active magnetic bearings. The results show that the time delay indeed plays a critical role on the suppression of the jump phenomena. Following, some new insights are also revealed through a numerical study of saddle node, Hopf and trans-critical bifurcations with time delay as a bifurcation parameter. The transient analysis confirms the results obtained analytically through the steady-state consideration.

Phase-field modeling of wormhole formation and growth in carbonate matrix acidizing

Acidic fluid flow in geologic formations leads to mineral dissolution and, under certain circumstances, to localized dissolution forming a dendritic pattern, known as wormhole. Such patterns of conduits and caves are often observed in karstic aquifer and deliberately engineered in oil and gas well stimulation with acid injection. Two different kinds of instability are thought to cause wormholes. One is heterogeneous property in rocks. The other is a process itself such as reaction infiltration or viscous fingering. To assess each one properly, we need to separate one from the other. While most numerical studies rely on randomly seeded material heterogeneities to induce wormholes, Daccord and Lenormand (1987a) demonstrated that even water injection into a homogeneous plaster can form wormholes. Here, we focus on the process instability driven by reaction-infiltration in homogeneous materials. We apply a phase-field approach, which diffuses a sharp interface in a continuous manner and show that it is capable of simulating wormhole without random seeds by accounting for the energy expenditure in the dissolution topology. We verified the model against the sharp interface counterpart in one-dimensional simulations. We then performed the two-dimensional simulations to qualitatively validate wormhole formation and growth patterns against acid injection experiments on carbonate rocks under radial flow conditions. Our simulation results indicate that the injected acid is rapidly consumed near the acid entry point at low injection rates while the live acid becomes available at the tip of the dissolved cavity under high rates and thus wormhole starts to grow resulting in much faster acid breakthrough.

Phase-field modelling of directional melting of lamellar and rod eutectic structures

The melting of 2D lamellar and 3D rod eutectic structures was studied by multi-phase-field simulations. A binary model eutectic system was investigated in a standard directional solidification setup. Switching from solidification to melting was realised by inverting the pulling speed of the sample. Steady state melting profiles were obtained as the long-time solutions of the evolution equations solved in the minimal representative domains of the periodic structures. Series of simulations were performed using different volume fractions of the initial solid phases, different values of the temperature gradient, and different widths of the simulation domain. It was found that melting occurs with a nearly flat interface if the average composition of the initial solid structure is equal to the eutectic composition. If the volume fraction of the solid phases is changed and the average composition becomes off-eutectic, then the melting positions of the phases decouple: the phase of sub-eutectic amount will melt near the eutectic temperature, while the phase of super-eutectic amount will melt near its liquidus temperature corresponding to the off-eutectic mean composition. The lamellar/rod spacing imposed by the width of the simulation domain has only minor effect on the interface temperature. Besides the steady states, we could observe two kinds of instabilities in the regime of non-planar melting both in 2D and 3D. By increasing the lamellar spacing, oscillations may appear around the trijunction and along the phase boundaries. By decreasing the lamellar spacing and increasing the pulling speed, the lamellae/rods of the phase that protrude deeper in the melt become thinner and may eventually break up to a series of small spherical particles before melting completely.

Stability of operator-difference schemes with weights for the hyperbolic equation in the space of summable functions with carriers in the network-like domain

This work is a natural continuation of the authors’ research on flow phenomena in the direction of increasing the dimensionality of the network-like domain of change of the space variable. The possibility of practical use of the analysis of the stability of operator-difference schemes to solve the issue of stability (stabilization) of wave phenomena in the engineering of the process of transferring continuous media through network-like carriers (water pipelines, gas and oil pipelines, industrial carriers of petroleum products) is shown. Namely, if the scheme is stable, then sufficiently small changes in the initial data of the mathematical model of the process imply small changes of the solution of the difference problem, i. e. in practice do not lead to undesirable aftereffects. If the schema is unstable, then small changes to the initial data can lead to arbitrarily large changes of the solution. In the process of exploitation of industrial constructions of network-like carriers, wave phenomena inevitably arise, the consequence of which are various kinds of instabilities that entail destruction of one nature or another. It is possible to avoid or essentially reduce such undesirable oscillations using the analysis of the stability properties of the mathematical model of the wave process. The obtained results are used in the algorithmically and digitalization of modern technological processes of the movement of fluid media and gases.

How to increase the physics output per MW.h for FCC-ee?

The efficiency of colliders for physics is largely determined by their luminosity, while most of the energy consumed by high-energy e^+e^- colliders is proportional to the total beam current. Thus, the energy efficiency is mainly determined by the specific luminosity that needs to be maximized. One of the most effective ways to achieve this is by using the Crab waist collision scheme, which implies a large Piwinski angle (LPA). A distinctive feature of the FCC-ee is the great influence of beamstrahlung (radiation in the field of an opposite bunch) on beam dynamics. At low energies, this manifests itself in a significant increase in the energy spread and bunch length, at high energies, in a limitation of the beam lifetime. The collision of intense bunches with LPA and beamstrahlung can also lead to various kinds of instabilities limiting the luminosity. Here, we discuss the main aspects to consider when optimizing the parameters of the FCC-ee collider at different energies and explain the choice of basic parameters such as RF voltage, lattice functions at IP, bunch intensity, etc. We will also pay attention to open issues requiring further study and identify some key points for the next phase of this project.

Aero-elastic behavior of open-type one-way tensioned membrane structure models

Wind tunnel tests were conducted on an aero-elastic model of an open-type one-way tensioned membrane to investigate the aero-elastic instability mechanism of membrane structures. The structural response and wind velocities above the membrane were measured and analyzed. Complementary simulations were carried out to explain the interaction between the wind and the structure. The results indicate that the aero-elastic instability of the membrane is a kind of vortex induced vibration. This kind of instability is characterized by a sudden increase in the amplitude, or a sudden change in the dominant vibration mode, when the coming flow velocity exceeds a certain value. The vortices generated around the time-averaged deformation of the membrane lead to vortex induced vibration when the dominant frequency of the vortices is close to a certain natural frequency of the membrane. When the aero-elastic instability occurs, the frequency of the vortices is locked in by the structural vibration within a certain coming flow velocity range, and the total damping ratio of the vibration decreases dramatically and becomes nearly zero. Finally, the critical wind velocities are defined, and the reduced critical wind velocities corresponding to the first and second aero-elastic instabilities are 0.84 and 2.3 approximately.