Strong Instability Research Articles

A Numerical Study of The Heat Transfer Intensification Using High Amplitude Acoustic Waves

The current practice in the efforts aiming to improve cooling conditions is to place emphasis on the application of non-stationary flow effects, such as the unsteady jet heat transfer or the heat transfer intensification by means of a high-amplitude oscillatory motion. The research presented in this paper follows this direction. A new concept is put forward to intensify the heat transfer in the cooling channels with the use of an acoustic wave generator. The acoustic wave is generated by a properly shaped fixed cavity or group of cavities. The sound generated by the cavity is a phenomenon analysed in various publications focused on the methods of its reduction. The phenomenon is related to the feedback mechanism between the vortices flowing from the leading edge and the acoustic waves generated within the cavity. The acoustic waves are generated by the interaction between the vortices and the cavity walls. Strong instabilities can be observed within a certain range of the free flow velocities. The investigations presented in this paper are oriented towards the use of the phenomenon for the purposes of the heat transfer process intensification. The first part of the work presents the numerical model used in the analysis, as well as its validation and comparison with empirical relations. The numerical model is constructed using the commercial CFD Ansys CFX-16.0 commercial program. The next part includes determining of the relationship between the amplitude of the acoustic oscillations and the cooling conditions within the cavity. The calculations are performed for various flow conditions.

Transmission of Vortex Solitons in Three-Dimensional χ(2) Helical-Periodically Poled Ferroelectric Crystals

It is well known that bright vortex solitons are unstable in the χ2 nonlinear media due to the strong azimuthal modulation instability. To solve this problem, a quadratic (χ2) LiNbO3 ferroelectric crystal with a special kind of helical-periodically poled structure is proposed. The proposed structure is designed by embedding topological charges into the crystal with a quasi-phase matching technique. Simulation results indicate that vortex solitons containing fundamental-frequency and second-harmonic waves can robustly propagate over a distance. Two types of vortex states are obtained: double vortices state and vortex–antivortex state. The dependence of effective area, propagation constants, and maximum light intensity on the control parameters are presented. These results provide a new solution for robust transmission of bright vortex solitons in a χ2 nonlinear media.

Experimental study of stability and pollutant emissions of turbulent biogas flames under microsecond pulsed plasma

Biogas is a mixture of methane (50–80%.vol), carbon dioxide (20–45%.vol), and in smaller proportions of N2, O2, H2S, and NH3. For a high rate of CO2 in the biogas, the combustion can generate strong instabilities and the flame can be very lifted from the burner and even blown. To overcome these problems of instabilities and flame extinction, the use of a microsecond pulsed plasma is considered for this study. This study aims to characterize the effects of microsecond pulsed discharges on the stabilization and emission characteristics of non-premixed swirling biogas/air flames at different operating conditions of combustion (CO2 rate, power, and equivalence ratio) and plasma (pulse repetition frequency and input electrical voltage). The plasma is performed above a coaxial burner with a swirler of the oxidizer. It is generated by a DC-pulsed power generator with a high voltage (HV) pulse duration of a few microseconds. The burner consists of two concentric tubes and a central metal rod used as a cathode for the plasma. The outer tube contains a swirler and supplies the oxidant flow, and the inner tube delivers the fuel (methane, biogas) around the central metal rod. High voltage pulses are applied on two anode tungsten wires that are symmetrically arranged from either side of the cathode to obtain two plasma zones in the stabilization region of the flame. The burner is placed in a parallelepiped combustion chamber with a volume of 100x48x48 cm3. Mass flow rates of air and fuel are regulated by thermal mass flow controllers (Brooks SLA5851S). OH* chemiluminescence measurements are done to describe the structure and the flame stability throughout the lift-off heights (Hf) variation. Results showed that plasma generated by microsecond HV pulses can improve flame stability by decreasing its Hf especially at low fuel velocities. In this regard, the optical emission spectra (OES) were visualized. The results reveal that the pulsed plasma generates chemically active species such as excited N2*, CH*, OH* molecules, and H* atoms, leading to improved flame stability. Their intensities' dependence on the pulse repetition frequency, the sight position of the OES, the concentration of CO2 in the fuel, and the fuel velocities were investigated in detail. Furthermore, the NOx and CO emissions in the burned gas are investigated over a wide range of repetition pulse frequencies. It is seen that the microsecond pulsed plasma slightly increases the NOx emission and reduces CO concentration in the flue gases.

Influence of Guide Vane Opening on the Runaway Stability of a Pump-Turbine Used for Hydropower and Ocean Power

Runaway is a common phenomenon in pump-turbines for hydropower and ocean power, accompanied by strong instability, which can easily lead to accidents. This study reveals the stability during the runaway process of a pump-turbine, mainly exploring the phenomenon of guide vane rejection happening in transition conditions when dealing with hydropower or ocean power. Through model experiments and computational fluid dynamics numerical simulations, the pressure pulsation when reaching runaway was compared under different guide vane opening angles. The amplitude of pressure pulsation measured in the experiment increases with the increase in guide vane opening, but there are also local changes in size and peak. The simulation results show that when the guide vane opening angle is 12 degrees, the vortex flow in the area between the guide vane and the runner of the unit increases, leading to instability. When the opening angle of the guide vane is between 12 and 20 degrees, the vortex flow intensity does not change much and the distribution becomes uniform, resulting in a decrease in the amplitude of pressure fluctuations. The pulsation of the flow field causes a dissipation of flow energy. Relationships can be found among velocity field, vortex intensity, and entropy production. This study is of great significance for ensuring the stable operation of pump-turbines.

Experimental Study on the Effects of Waves and Current on Ice Melting

Ice melting plays a crucial role in ocean circulation and global climate. Laboratory experiments were used to study the dynamic mechanisms of the influence of waves and currents on ice melting. The results showed that under near stable air temperature and water temperature conditions, the ice melting rate was significantly greater with waves than that without waves, as well as the higher the wave height, the greater the melting rate. This is related to the increase in the contact area between ice and water by waves. Further research was carried out to observe the flow field at different locations on the ice bottom, ice sides, and behind the ice by particle image velocimetry (PIV) and dyeing experiments. At different flow velocities, the changes in the side melting rate and bottom melting rate were not the same. Meltwater is attached to the bottom in the form of plume at low background flow velocity, which leads to the slowness of the heat exchange between the ice with a higher ambient temperature. Therefore, the melting of the ice bottom and the ice side were slower at low flow velocity. At high background flow velocity, there is strong shear instability and vortex at the ice bottom and behind the ice. The dissipation and mixing effects caused by vortices accelerate the melting of the ice bottom and the ice back. The thermodynamic factors, such as air temperature and water temperature, had significant impacts in the experiments. Further research needs to improve the accuracy of temperature control of experimental equipment. Computational fluid dynamics and sensitive tests of numerical simulation may also be carried out on ice melting.

Numerical study of the natural transition of water boundary layers over heated/cooled flat plates

The natural transition of water boundary layers on heated/cooled flat plates is investigated using numerical methods. The influences of wall temperature, oncoming flow velocity, and onco ming flow temperature on the transition are studied. Laminar basic flows are obtained by solving the Blasius equations, while water temperature is taken into consideration. Linear stability analyses of the basic flows are performed, and the transition positions are predicted using the eN method. The spectra of wall pressure fluctuations in the laminar region are also predicted. The results show that the heated wall produces a fuller velocity profile and a thinner boundary layer for the laminar flow and that a cooled wall causes the opposite effects. Consequently, the heated wall delays transition, while the cooled wall advances transition. As wall temperature increases, the effect of transition delay becomes stronger until the maximal delay effect occurs at an optimal wall temperature. Wall temperatures higher than the optimal value weaken the transition delay effect. This phenomenon of the heated wall is called transition reversal. A heated wall suppresses the wall pressure fluctuation, and a cooled wall has the opposite effect. The oncoming flow velocity has little influence on the non-dimensional transition position, while the oncoming flow temperature has a strong influence on it. At the same wall temperature, a higher oncoming flow temperature leads to a thicker boundary layer, stronger instability, and an earlier transition. A lower oncoming flow temperature leads to stronger sensitivity of the transition to the wall temperature.

Links between precipitation patterns over eastern tropical South America and productivity in the western tropical South Atlantic Ocean during the last deglacial

The possibility of a collapse (or marked weakening) of the Atlantic Meridional Overturning Circulation (AMOC) within a few centuries boosted the interest of the scientific community on the responses of different compartments of the climate system to past AMOC collapses (or marked weakening). The most recent examples of such periods are Heinrich Stadial 1 (HS1,18-15 ka BP; near-collapsed AMOC) and the Younger Dryas (YD, 12.9-11.7 ka BP; markedly weakened AMOC). Here, we present data from elemental ratios, spectral reflectance, organic biomarkers, sea surface temperatures and planktonic foraminiferal assemblage from two marine sediment cores recovered from the western tropical South Atlantic Ocean spanning the last ca. 20 kyr. The recovered sediments record the responses of eastern tropical South American hydroclimate, western tropical South Atlantic productivity and sea surface temperatures to changes in AMOC strength associated with HS1 and the YD. Our data show that both periods were characterized by wetter conditions over eastern tropical South America associated with increased continental runoff compared to the Last Glacial Maximum. High sea surface temperatures during HS1 and the YD are in line with marked AMOC decreases that led to heat accumulation in the surface layer of the western South Atlantic Ocean, facilitating increased precipitation over eastern tropical South America. Our data demonstrate for the first time that intense runoff during HS1, and hence nutrient transfer to the ocean, affected the pelagic ecosystem, likely increasing primary productivity in the otherwise oligotrophic western tropical South Atlantic Ocean. Our study demonstrates a tight coupling between marine and terrestrial processes in the eastern South American realm during periods of strong climate instability.

Distinctly Different Dynamical Processes in Maintaining the Intraseasonal NAO+ and NAO−

AbstractPositive feedback between high‐frequency eddies and low‐frequency processes is believed to play an essential role in the NAO evolution. In this study, however, it is found that the previously well‐studied upscale forcing to the intraseasonal scale window is mostly limited during the phase NAO−, but almost disappears during NAO+. The maintenance of the intraseasonal NAO+ is by a strong barotropic instability of the basic flow, which has been overlooked previously. The divergence of the Reynolds stress tensor that decelerates the basic flow is found to account for this asymmetry. For NAO−, there also exist strong interactions between the basic flow and the intraseasonal component, but the energy transfer is in a dipolar form, which, if integrated over space, contributes insignificantly as a whole. These new findings from a dynamics viewpoint shed insight deeper into NAO, a climate mode playing a key role in the Northern Hemisphere warming.

Lorenz Energy Cycle: Another Way to Understand the Atmospheric Circulation on Tidally Locked Terrestrial Planets

In this study, we employ and modify the Lorenz energy cycle (LEC) framework as another way to understand the atmospheric circulation on tidally locked terrestrial planets. It well describes the atmospheric general circulation in the perspective of energy transformation involved with several dynamical processes. We find that on rapidly rotating, tidally locked terrestrial planets, the mean potential energy (P M) and eddy potential energy (P E) are comparable to those on Earth, as they have similar steep meridional temperature gradients. The mean kinetic energy (K M) and eddy kinetic energy (K E) are larger than those on Earth, related to stronger winds. The two conversion paths, P M → P E → K E and P M → K M → K E, are both efficient. The former is associated with strong baroclinic instabilities, and the latter is associated with Hadley cells. On slowly rotating, tidally locked terrestrial planets, weak temperature gradients in the free atmosphere and strong nightside temperature inversion make P M and P E much smaller than on Earth. Meanwhile, a large day–night surface temperature contrast and small rotation rate make the overturning circulation extend to the globe, so that the main conversion path is P M → K M → K E. This study shows that the LEC analyses improve the understanding of the atmospheric circulation on tidally locked terrestrial planets.

Strong instability of ground state standing waves for the Hartree equation with a partial/complete harmonic potential

In this paper, we consider the strong instability of standing waves for the Hartree equation with a partial/complete harmonic potential where . By establishing the corresponding profile decomposition theory, we first obtain the variational characterization of ground state solutions. Then, we deduce that there exists such that for all , the ground state standing wave is strongly unstable by blow‐up. Our results give an improvement for some recent results.

Numerical study on the mixing and combustion characteristics of a liquid kerosene jet in a scramjet combustor

We numerically investigate the mixing and combustion process of a kerosene jet injected into a scramjet combustor with dual-cavity at different global fuel equivalence ratios using a compressible two-phase flow large-eddy simulation based on the Eulerian–Lagrangian approach. The results indicate that the calculated wall pressure distribution is basically consistent with the experimental result. The high-temperature zone and intermediate product CO distribution zone shown by the numerical simulation agree with the flame position observed by the experiment. When the global equivalent ratio is 0.149, the combustion is in the weak combustion mode. The flame only exists in the downstream cavity, which is mostly the premixed flame. The combustion efficiency is low, and the corresponding total pressure loss is small. When the global equivalent ratio is 0.187, the combustion is in the intense combustion mode, and the flame has a strong instability characteristic. The numerical simulation reproduces the flame flashback phenomenon that the flame backpropagation from the downstream region of cavity C1 to the upstream region of the liquid jet. During the flame flashback process, the flame gradually changes from premixed flame dominated to premixed flame and diffusion flame jointly dominated. In the weak combustion mode, the interaction between flame and droplet is weak, and droplet evaporation is less affected by combustion. While in the intense combustion mode, droplets evaporate more violently, the spray penetration depth increases, and some liquid droplets are transported upstream of the jet by the wall reflux zone.

Cosmogenic-nuclide data from Antarctic nunataks can constrain past ice sheet instabilities

Abstract. We apply geologic evidence from ice-free areas in Antarctica to evaluate model simulations of ice sheet response to warm climates. This is important because such simulations are used to predict ice sheet behaviour in future warm climates, but geologic evidence of smaller-than-present past ice sheets is buried under the present ice sheet and therefore generally unavailable for model benchmarking. We leverage an alternative accessible geologic dataset for this purpose: cosmogenic-nuclide concentrations in bedrock surfaces of interior nunataks. These data produce a frequency distribution of ice thickness over multimillion-year periods, which is also simulated by ice sheet modelling. End-member transient models, parameterized with strong and weak marine ice sheet instability processes and ocean temperature forcings, simulate large and small sea-level impacts during warm periods and also predict contrasting and distinct frequency distributions of ice thickness. We identify regions of Antarctica where predicted frequency distributions reveal differences in end-member ice sheet behaviour. We then demonstrate that a single comprehensive dataset from one bedrock site in West Antarctica is sufficiently detailed to show that the data are consistent only with a weak marine ice sheet instability end-member, but other less extensive datasets are insufficient and/or ambiguous. Finally, we highlight locations where collecting additional data could constrain the amplitude of past and therefore future response to warm climates.

Vortex Solitons in Quasi-Phase-Matched Photonic Crystals.

We report solutions for stable compound solitons in a three-dimensional quasi-phase-matched photonic crystal with the quadratic (χ^{(2)}) nonlinearity. The photonic crystal is introduced with a checkerboard structure, which can be realized by means of the available technology. The solitons are built as four-peak vortex modes of two types, rhombuses and squares (intersite- and onsite-centered self-trapped states, respectively). Their stability areas are identified in the system's parametric space (rhombuses occupy an essentially broader stability domain), while all bright vortex solitons are subject to strong azimuthal instability in uniform χ^{(2)} media. Possibilities for experimental realization of the solitons are outlined.

ПРЕДВАРИТЕЛЬНЫЕ РЕЗУЛЬТАТЫ ЛАБОРАТОРНЫХ ИССЛЕДОВАНИЙ ЭВОЛЮЦИИ НЕФРОНТАЛЬНЫХ ВИХРЕЙ В ДВУСЛОЙНОЙ ВРАЩАЮЩЕЙСЯ ЖИДКОСТИ

In this work, by means of laboratory modeling, the patterns of evolution (baroclinic instability and/or viscous degeneration) of mesoscale non-frontal baroclinic eddies, or open ocean eddies (without a core with the water of different density) were revealed. The experiment was provided in a two-layer salinity-stratified aquatic environment in a cylindrical tank located on a rotating platform. Using the original application of the “cylinder method”, single axisymmetric cyclonic and anticyclonic eddies were produced in the upper layer, and their evolution was traced over 100 or more platform rotation periods (laboratory days). It has been established that the vortices lose their stability if the value of the defining parameter representing the ratio of the Burger and Froude numbers is less than the critical one. In this case, eddies either fall apart into two or more secondary vortices (strong instability) or acquire an elongated shape (weak instability). The slope of the bottom and its roughness increase the stability of eddies. For values of the determining parameter greater than the critical one, the vortices are stable and gradually degenerate due to viscous friction. In the case of a large difference in salinity (density) between the layers, the bottom roughness does not have a significant effect on the rate of upper layer eddy damping, which, apparently, is determined by the friction in this layer. A parametrization is proposed that describes the exponential law of eddy decay, which is tested on the example of the Black Sea mesoscale eddy.

Motion stability analysis of cage of rolling bearing under the variable-speed condition

The fluctuation of the cage rotating speed will lead to the strong instability of the cage movement of the rolling bearing, which will seriously affect the operation performance and service life of the bearing. In this paper, a nonlinear dynamic model of a cylindrical roller bearing is established considering the dynamic contact relationship between the rollers, the raceways, and the cage. The variation of cage motion characteristics and its internal mechanism under the uniform, acceleration, deceleration, and harmonic speed fluctuation are discussed, respectively. The effects of angular acceleration, angular deceleration, amplitude, and period of fluctuation on cage stability are analyzed. The results show the unstable motion of the cage mainly happens at the initial stage of the acceleration process and the later stage of the deceleration process. Appropriate increases in angular acceleration and deceleration can help to improve the stability of the cage movement, but excessive speed fluctuations should be avoided to adversely affect the stability of the cage. The guiding force between the cage and the outer raceway plays an important role in the stability of the cage under variable-speed conditions. The results of this paper can provide a theoretical basis for the design and failure analysis of the cage.

Light bullets in Su-Schrieffer-Heeger photonic topological insulators

We introduce a different class of thresholdless three-dimensional soliton states that form in higher-order topological insulators based on a two-dimensional Su-Schrieffer-Heeger array of coupled waveguides. The linear spectrum of such structures is characterized by the presence of a topological gap with corner states residing in them. We find that a focusing Kerr nonlinearity allows families of light bullets bifurcating from the linear corner states to exist as stable three-dimensional solitons, which inherit topological protection from their linear corner counterparts and, remarkably, survive even in the presence of considerable disorder. The light bullets exhibit a spatial localization degree that depends strongly on the array dimerization, and may feature large temporal widths in the topological gap near the bifurcation point, thus drastically reducing the otherwise strong instabilities caused by higher-order effects.

Local current shrinkage induced by the MARFE in L mode discharges on EAST tokamak

In this study, the multifaceted asymmetric radiation from the edge (MARFE) was observed in high-density discharges or during impurity gas injection on the EAST tokamak. The MARFE onset indicated by spectral and radiation signals can also be detected by the POlarimeter-INTerferometer (POINT) diagnostic, which measures the horizontal line-integrated density and the Faraday rotation. The fluctuation amplitude of the density signal resulting from the MARFE oscillation increases with the edge safety factor, which is consistent with the thermal instability theory. By combining density and the Faraday rotation, the local current shrinkage in the MARFE region is observed during the MARFE movement. The density and the current profile calculated by the POINT become more peak during the MARFE, which may lead to a strong magnetohydrodynamic instability that can result in disruption.

U-net-based pseudoseismic imaging for the short-offset transient electromagnetic method

AbstractShort-offset transient electromagnetic (SOTEM) is an innovation of the artificial-source electromagnetic method, and detection is conducted in the near-source area using an electromagnetic formation wave so that the signal has strong strength and large bandwidth. Inspired by the parallelism of the propagation of seismic body waves and EM formation waves, this paper intends to implement pseudoseismic imaging (PSI) on SOTEM data to give play to the high resolution of SOTEM. Traditionally, to perform the wave-field transform (WFT), a set of ill-posed linear equations needs to be solved and the solving process has strong ambiguity and instability. This paper presents a new method for performing WFT based on U-Net, and a nonlinear mapping from the EM diffusion field to the pseudoseismic wave field under the same velocity model is established by training the network. To eliminate the error that might result when the time‒depth transform is conducted for the wave-field record, another U-Net is used to transform the wave-field record into a depth-offset profile to accomplish the whole PSI process for SOTEM. Simulation and measured data are used to validate the effect of the networks, and the results indicate that this approach is highly feasible, thus providing a new strategy for using SOTEM to detect sedimentary strata.

Experimental Study on the Mechanical Behaviors of Aeolian Sand Treated by Microbially Induced Calcite Precipitation (MICP) and Basalt Fiber Reinforcement (BFR).

Aeolian sand flow is a major cause of land desertification, and it is prone to developing into a dust storm coupled with strong wind and thermal instability. The microbially induced calcite precipitation (MICP) technique can significantly improve the strength and integrity of sandy soils, whereas it easily leads to brittle destruction. To effectively inhibit land desertification, a method coupled with MICP and basalt fiberreinforcement (BFR) was put forward to enhance the strength and toughness of aeolian sand. Based on a permeability test and an unconfined compressive strength (UCS) test, the effects of initial dry density (ρd), fiber length (FL), and fiber content (FC) on the characteristics of permeability, strength, and CaCO3 production were analyzed, and the consolidation mechanism of the MICP-BFR method was explored. The experiments indicated that the permeability coefficient of aeolian sand increased first, then decreased, and subsequently increased with the increase in FC, whereas it exhibited a tendency to decrease first and then increase with the increase in FL. The UCS increased with the increase in the initial dry density, while it increased first and then decreased with the increase in FL and FC. Furthermore, the UCS increased linearly with the increase in CaCO3 generation, and the maximum correlation coefficient reached 0.852. The CaCO3 crystals played the roles of providing bonding, filling, and anchoring effects, and the spatial mesh structure formed by the fibers acted as a bridge effect to enhance the strength and brittle damage of aeolian sand. The findings could supply a guideline for sand solidification in desert areas.

Late quaternary geomorphological evolutionary dynamics of the Araripe sedimentary plateau, northeast of Brazil

The stratigraphic and geochronological data, compared to the set of ages already known for the semi-arid Northeast of Brazil, allowed us to understand part of the geomorphological dynamics of the sub-humid sector of the Araripe Plateau, in the south of the State of Ceará. Paleoenvironmental records were recovered through the study of Quaternary deposits generated by climatic inputs, in the range of 72.0 ka BP to the Early Holocene. Except for section CSM-C, it was observed that all other debris flows found do not exceed the MIS 3/LMG in age, and that morphogenetic events linked to gravitational flows are constrained until the Lower Holocene, coinciding with the findings in other sectors of the Northeast of Brazil such as the Borborema Highlands. Thus, as in other locations, it appears that the depositional processes and the formation of successive colluvial ramps in the Araripe Plateau occurred at times of strong climatic instability, as periods of intense rainfall disrupted the pedo-geomorphic equilibirum of the landscape. Despite the undeniable predominance of semi-arid conditions for the Brazilian Northeast during the Quaternary, the results obtained suggest that such conditions were neither homogeneous nor constant, and that in the last 40.0 ka. BP the Northeast experienced climatic fluctuations with abrupt oscillations, on millennial scales, from dry to wetter conditions and vice versa, which apparently coincide with the displacement of the ITCZ to more southern positions in line with the Heinrich events in the Northern Hemisphere. From the end of the Lower Holocene towards the Present, drainage began to actively dissect colluvial sediments, in response to continued changes in the base level around the Plateau. This scenario led to a longitudinal reworking of the sediments along valley bottoms and channels, forming fluvial plains. This aspect points to the fact that the surface dynamics of the landscape, at least from the Middle Holocene onwards, was rather defined by changes in the base level (and ultimately in land use), and erosion of colluvial deposits, than by the entry of new episodes of climatically conditioned sedimentation of great magnitude.