High-amplitude Waves Research Articles

Statistics of vector Manakov rogue waves.

We present a statistical analysis based on the height and return-time probabilities of high-amplitude wave events in both focusing and defocusing Manakov systems. We find that analytical rational or semirational solutions, associated with extreme, rogue wave (RW) structures, are the leading high-amplitude events in this system. We define the thresholds for classifying an extreme wave event as a RW. Our results indicate that there is a strong relationship between the type of RW and the mechanism which is responsible for its creation. Initially, high-amplitude events originate from modulation instability. Upon subsequent evolution, the interaction among these events prevails as the mechanism for RW creation. We suggest a strategy for confirming the basic properties of different extreme events. This involves the definition of proper statistical measures at each stage of the RW dynamics. Our results point to the need for redefining criteria for identifying RW events.

Effect of Filling Humidity on the Propagation of High-Amplitude Stress Waves through an Artificial Joint

Abstract The purpose of this article is to further investigate the seismic response of an artificial filled joint under high-amplitude stress waves considering the effect of filling humidity, following our earlier work on dry infill. A steel split Hopkinson pressure bar system is utilized to induce high-amplitude stress waves to the filled joint. In this study, the wet infill is a mixture composed of quartz sand, kaolinite clay, and water. It is found that when the water content is relatively low, i.e., 8.25 %, the seismic response of the joint with wet infill is similar to that of the joint with dry infill, as shown in the literature. When the stress wave amplitude increases, the infill is progressively compacted and the transmission coefficient increases. However, there exists a crushing deformation stage for the infill in which many particles are crushed, and the transmission coefficient decreases as the incident wave amplitude increases. The water in the infill can reduce the friction between grains, which may lead to the decrease of the joint stiffness. As a result, the transmission coefficient is smaller than the case with dry infill under similar loading conditions. When the water content is moderate, such as 16.75 %, particles are very difficult to crush and the infill dominantly experiences compaction even when loaded by very high-amplitude stress waves. Consequently, the transmission coefficient through the wet infill always increases with the increase of the incident wave amplitude. When the infill is fully saturated (water content = 25.0 %), it can only experience approximately elastic deformation, and few particles can be crushed. In this case, the transmission coefficient is independent of the incident wave amplitude. When the infill is dry or fully saturated, the transmission coefficient is insensitive to the amplitude of the incident wave.

Analysis of Coherent Structures in an Under-Expanded Supersonic Impinging Jet Using Spectral Proper Orthogonal Decomposition (SPOD)

The spatiotemporal dynamics of the coherent structures in an under-expanded supersonic impinging jet are studied using a spectral proper orthogonal decomposition technique. For this analysis, a large eddy simulation of an under-expanded supersonic impinging jet at a pressure ratio of 3.4 and a stand-off distance of 2 jet diameters at a Reynolds number of 50,000 is performed. The mean flow fields illustrate some striking features of this flow, such as an oblique shock, a stand-off shock, a Mach disk, and a recirculation bubble. The spectral proper orthogonal decomposition method is applied to time-resolved three-dimensional flow fields. The accumulative energy of modes within each azimuthal mode number reveals that the first three azimuthal modes contain most of the energy of the flow. The spectra of these azimuthal modes show that the flow exhibits a low-ranked behaviour with discrete frequencies at the optimal symmetric azimuthal mode while other two azimuthal modes have negligible contributions in this behaviour. Three peaks are observed in the spectra of the optimal symmetric azimuthal mode. The spatial fields of the streamwise velocity and pressure of these peaks show that the complex structures are consequences of the under-expansion, Mach disk, and the impingement. Strong hydrodynamic instabilities exist in the shear layer of the jet in the optimal azimuthal mode at each of these dominant frequencies. High-amplitude acoustic waves are also present in the near-field of the jet. These acoustic waves are strong at the nozzle lip, suggesting that a feedback loop linking these two processes exists for dominant frequencies in the optimal mode. High cross-spectrum density of near-field pressure fluctuations and streamwise velocity fluctuations near the nozzle lip at these frequencies confirms the hydro-acoustic coupling, which is necessary to close the feedback loop.

Clinical characteristics of acute encephalopathy with acute brain swelling: A peculiar type of acute encephalopathy

ObjectivesAcute encephalopathy has been observed with acute brain swelling (ABS) that is characterized by rapid progression to whole-brain swelling. The objective of this study was to describe the clinical characteristics of ABS. MethodsWe encountered four patients with ABS and retrospectively investigated their clinical data with a medical chart review. ResultsThree patients had seizure clustering or status epilepticus in the clinical course. Signs of elevated intracranial pressure (ICP) appeared 3–9 h after the first convulsive attack in three patients. In all patients, signs of brainstem involvement appeared 1–8 h after signs of elevated ICP. Mild hyponatremia that progressed after signs of elevated ICP appeared was noted in three patients. Brain CT revealed mild brain swelling in the initial phase, which rapidly progressed to whole-brain swelling. No focal abnormalities were detected on brain MRI in one patient. Continuous electroencephalography was initially normal, but in two patients, high-amplitude slow waves appeared with rapid changes before signs of brainstem involvement. Although recovery was achieved without sequelae in two patients, outcome was fatal for the other two. ConclusionsThe pathogenesis of ABS has yet to be clarified, but clinical features in our patients are not consistent with any established subtypes of acute encephalopathy. Therefore, we believe that ABS should be recognized as a new type of acute encephalopathy.

Single-phase convective heat transfer performance of wavy microchannels in macro geometry

The feasibility of attaining microscale heat transfer effects using macro geometries has been demonstrated. The concentric superposition of two macro geometries, manufactured through conventional machining, yielded an annular microchannel with a microscale gap of 300 μm. In this paper, sinusoidal wave geometrical profiles were introduced on the inner cylinder’s surface to enhance convective heat transfer and the overall energy efficiency, for a given heat transfer area. Experimental and numerical studies were conducted on steady-state, single-phase heat transfer, using distilled water as the coolant, with an operating Reynolds number range of 1300–4600. Results showed that the enhanced microchannels with higher wave amplitudes and shorter wavelengths performed better in terms of heat transfer, at the expense of heightened pressure losses. Overall, the highest-performing enhanced microchannel is capable of removing 51 percent more heat than the plain annular channel at a given pumping power. In addition, large wave amplitudes coupled with low operating Reynolds number yielded optimal heat transfer efficiency and vice versa. The wavy profiles promote heat transfer efficiency through flow perturbation and the reinitialization of boundary layers along the peaks while keeping pressure losses relatively low. New correlations for the average Nusselt number and friction factor were proposed for the wavy annular microchannels, which can be utilised for future compact heat exchanger designs, exhibiting enhanced microscale heat removal capabilities while employing relatively economical fabrication processes.

P.113 Down Syndrome: robust neurophysiological perspectives

Background: Down Syndrome (DS) has a mosaicism of presentations, but a number of common features. Cerebral evoked potentials (somatosensory, visual and auditory) can be higher in amplitude in DS. The aim of this study is to explore the value of the neurophysiological amplitude of three different modalities in DS individuals undergoing spinal surgery, or epilepsy evaluation. Methods: Standard procedure of EEG evaluation was conducted. We routinely monitor somatosensory (SSEP) and motor evoked potentials (MEP), using peripheral nerves stimulation and transcranial electrical stimulation during surgery. We report findings from 14 DS individuals age-matched to 14 individuals with idiopathic scoliosis Results: The amplitude of the SSEP is significantly higher in DS individuals than in age-matched controls using the same parameters. SSEP;10.2±2.5µV vs 2.4±2.3µV (p<0.05, paired t-test). The threshold for eliciting MEPs was also significantly lower in DS in comparison to controls, 175±20V vs 629±100V, (p<0.05, paired t-test). Interictal EEG showed high amplitude spike and waves, and greater intracortical coherence in DS with epilepsy than non-DS patients Conclusions: Robust neurophysiological findings showed high amplitude sensory evoked potentials, low threshold motor evoked potentials, and high amplitude spikes and wave, all reflect a common process of increased neuronal synchronicity and oscillatory behaviour in Down Syndrome.

Abstract P-524: CLINICAL CHARACTERIZATION TO IDENTIFY THE CASES WITH ACUTE ENCEPHALOPATHY WITH SEQUELAE AMONG THE CASES WITH FEBRILE STATUS EPILEPTICUS

Aims & Objectives: It is difficult to make a precise diagnosis among patients with febrile status epilepticus on admission to identify the cases of acute encephalopathy with sequelae. However, it is necessary to make an early diagnosis of cases with sequelae, in order to determine whether an aggressive treatment is required to alleviate the possibility of sequelae. Thus, we evaluated the clinical characteristics of the patients with febrile status epilepticus on admission. Methods We reviewed the medical charts of the patients admitted to our PICU from March in 2016 to December in 2017. We found 14 cases with febrile status epilepticus. Of which 4 developed sequelae. We evaluated the differences between the patients grouped with and without sequelae for the following data. 1.aEEG on admission 2.Regional SrO2 of both brain frontal lobe and liver on admission 3.Serum liver function and coagulation function Results All patients with sequelae and 2 cases without sequelae had high-amplitude slow waves on aEEG. All patients with sequelae and 1 case without sequelae showed that regional SrO2 of both frontal lobes were more than 10% higher than that of the liver. All patients with sequelae and 1 case without sequelae showed liver dysfunction with coagulopathy. Conclusions Based on this study, the combination of aEEG, regional SrO2 and liver dysfunction with coagulopathy could be useful to make an early diagnosis to identify the cases with sequelae. However, a larger number of patients should be included to clarify the results of this study.

Dexmedetomidine and propofol sedation requirements in an autistic rat model

BackgroundAutism is a challenging neurodevelopmental disorder. Previous clinical observations have suggested altered sedation requirements for children with autism. Our study aimed to test this observation experimentally in an animal model and to explore its possible mechanisms.MethodsEight adult pregnant female Sprague-Dawley rats were randomly divided into two groups. Four were injected with intraperitoneal sodium valproate on gestational day 12 and four were injected with normal saline. On postnatal day 28, the newborn male rats were subjected to the open-field test to confirm autistic features. Each rat was injected intraperitoneally with a single dose of propofol (50 mg/kg) or dexmedetomidine (0.2 mg/kg). The times to loss of righting reflex (LORR) and to return of righting reflex (RORR) were recorded. On the following day, all rats were re-sedated and underwent electroencephalography (EEG). Thereafter, the rats were euthanized and their hippocampal gamma-aminobutyric acid type A (GABAA) and glutamate N-methyl-D-aspartate (NMDA) receptor gene expressions were assessed.ResultsAutistic rats showed significantly longer LORR times and shorter RORR times than did the controls (median LORR times: 12.0 versus 5.0 min for dexmedetomidine and 22.0 versus 8.0 min for propofol; P < 0.05). EEG showed a low-frequency, high-amplitude wave pattern 2 min after LORR in the control rats. Autistic rats showed a high-frequency, low-amplitude awake pattern. Hippocampal GABAA receptor gene expression was significantly lower and NMDA gene expression was greater in autistic rats.ConclusionsThis study supports the clinical observations of increased anesthetic sedative requirements in children with autism and our biochemical analyses using GABAA and glutamate receptor gene expression highlight possible underlying mechanisms.

Koroška 8000 Himalayan expedition: digit responses to cold stress following ascent to Broadpeak (Pakistan, 8051m).

Cold-induced vasodilatation (CIVD) is a peripheral blood flow response, observed in both the hands and feet. Exercise has been shown to enhance the response, specifically by increasing mean skin temperatures (Tsk), in part due to the increased number of CIVD waves. In contrast, hypobaric hypoxia has been suggested to impair digit skin temperature responses, particularly during subsequent hand rewarming following the cold stimulus. This study examined the combined effect of exercise and hypobaric hypoxia on the CIVD response. We compared the CIVD responses in the digits of both the hands and feet of a team of alpinists (N = 5) before and after a 35-day Himalayan expedition to Broadpeak, Pakistan (8051m). Five elite alpinists participated in hand and foot cold water immersion tests 20 days before and immediately upon return from their expedition. The alpinists summited successfully without supplemental oxygen. Post-expedition, all alpinists demonstrated higher minimum Tsk in their hands (pre: 9.9 ± 1.1, post: 10.1 ± 0.7°C, p = 0.031). Four alpinists had either greater CIVD waves, and, consequently, higher mean Tsk in their hands, or higher recovery temperatures (pre: 26.0 ± 5.5°C post: 31.0 ± 4.1°C, p = 0.052), or faster rewarming rate (pre: 2.6 ± 0.5°Cmin-1 post: 3.1 ± 0.4°Cmin-1,p = 0.052). In the feet, the responses varied: 1/5 had higher wave amplitudes and 1/5 had higher passive recovery temperatures, whereas 3/5 had lower mean toe temperatures during cold exposure. The results of the cold stress test suggest after a 35-day Himalayan expedition, alpinists experienced a slight cold adaptation of the hands, but not the feet.

How do children fall asleep? A high-density EEG study of slow waves in the transition from wake to sleep

IntroductionSlow waves, the hallmarks of non-rapid eye-movement (NREM) sleep, are thought to reflect maturational changes that occur in the cerebral cortex throughout childhood and adolescence. Recent work in adults has revealed evidence for two distinct synchronization processes involved in the generation of slow waves, which sequentially come into play in the transition to sleep. In order to understand how these two processes are affected by developmental changes, we compared slow waves between children and young adults in the falling asleep period. MethodsThe sleep onset period (starting 30s before end of alpha activity and ending at the first slow wave sequence) was extracted from 72 sleep onset high-density EEG recordings (128 electrodes) of 49 healthy subjects (age 8–25). Using an automatic slow wave detection algorithm, the number, amplitude and slope of slow waves were analyzed and compared between children (age 8–11) and young adults (age 20–25). ResultsSlow wave number and amplitude increased linearly in the falling asleep period in children, while in young adults, isolated high-amplitude slow waves (type I) dominated initially and numerous smaller slow waves (type II) with progressively increasing amplitude occurred later. Compared to young adults, children displayed faster increases in slow wave amplitude and number across the falling asleep period in central and posterior brain regions, respectively, and also showed larger slow waves during wakefulness immediately prior to sleep. ConclusionsChildren do not display the two temporally dissociated slow wave synchronization processes in the falling asleep period observed in adults, suggesting that maturational factors underlie the temporal segregation of these two processes. Our findings provide novel perspectives for studying how sleep-related behaviors and dreaming differ between children and adults.

Abnormal coherence and sleep composition in children with Angelman syndrome: a retrospective EEG study

BackgroundAngelman syndrome (AS) is a neurodevelopmental disorder characterized by intellectual disability, speech and motor impairments, epilepsy, abnormal sleep, and phenotypic overlap with autism. Individuals with AS display characteristic EEG patterns including high-amplitude rhythmic delta waves. Here, we sought to quantitatively explore EEG architecture in AS beyond known spectral power phenotypes. We were motivated by studies of functional connectivity and sleep spindles in autism to study these EEG readouts in children with AS.MethodsWe analyzed retrospective wake and sleep EEGs from children with AS (age 4–11) and age-matched neurotypical controls. We assessed long-range and short-range functional connectivity by measuring coherence across multiple frequencies during wake and sleep. We quantified sleep spindles using automated and manual approaches.ResultsDuring wakefulness, children with AS showed enhanced long-range EEG coherence across a wide range of frequencies. During sleep, children with AS showed increased long-range EEG coherence specifically in the gamma band. EEGs from children with AS contained fewer sleep spindles, and these spindles were shorter in duration than their neurotypical counterparts.ConclusionsWe demonstrate two quantitative readouts of dysregulated sleep composition in children with AS—gamma coherence and spindles—and describe how functional connectivity patterns may be disrupted during wakefulness. Quantitative EEG phenotypes have potential as biomarkers and readouts of target engagement for future clinical trials and provide clues into how neural circuits are dysregulated in children with AS.

A detailed examination of laser-ion acceleration mechanisms in the relativistic transparency regime using tracers

We present a particle-in-cell study of linearly polarized laser-ion acceleration systems, in which we use both two-dimensional (2D) and three-dimensional (3D) simulations to characterize the ion acceleration mechanisms in targets which become transparent to the laser pulse during irradiation. First, we perform a target length scan to optimize the peak ion energies in both 2D and 3D, and the predictive capabilities of 2D simulations are discussed. Tracer analysis allows us to isolate the acceleration into stages of target normal sheath acceleration (TNSA), hole boring (HB), and break-out afterburner (BOA) acceleration, which vary in effectiveness based on the simulation parameters. The thinnest targets reveal that enhanced TNSA is responsible for accelerating the most energetic ions, whereas the thickest targets have ions undergoing successive phases of HB and TNSA (in 2D) or BOA and TNSA (in 3D); HB is not observed to be a dominant acceleration mechanism in the 3D simulations. It is in the intermediate optimal regime, both when the laser breaks through the target with appreciable amplitude and when there is enough plasma to form a sustained high density flow, that BOA is most effective and is responsible for the most energetic ions. Eliminating the transverse laser spot size effects by performing a plane wave simulation, we can isolate with greater confidence the underlying physics behind the ion dynamics we observe. Specifically, supplemented by wavelet and FFT analyses, we match the post-transparency BOA acceleration with a wave-particle resonance with a high-amplitude low-frequency electrostatic wave of increasing phase velocity, consistent with that predicted by the Buneman instability.

Envelope analysis links oscillatory and arrhythmic EEG activities to two types of neuronal synchronization

Traditionally, EEG is understood as originating from the synchronous activation of neuronal populations that generate rhythmic oscillations in specific frequency bands. Recently, new neuronal dynamics regimes have been identified (e.g. neuronal avalanches) characterized by irregular or arrhythmic activity. In addition, it is starting to be acknowledged that broadband properties of EEG spectrum (following a 1/f law) are tightly linked to brain function. Nevertheless, there is still no theoretical framework accommodating the coexistence of these two EEG phenomenologies: rhythmic/narrowband and arrhythmic/broadband. To address this problem, we present a new framework for EEG analysis based on the relation between the Gaussianity and the envelope of a given signal. EEG Gaussianity is a relevant assessment because if EEG emerges from the superposition of uncorrelated sources, it should exhibit properties of a Gaussian process, otherwise, as in the case of neural synchronization, deviations from Gaussianity should be observed. We use analytical results demonstrating that the coefficient of variation of the envelope (CVE) of Gaussian noise (or any of its filtered sub-bands) is the constant 4π−1≈0.523, thus enabling CVE to be a useful metric to assess EEG Gaussianity. Furthermore, a new and highly informative analysis space (envelope characterization space) is generated by combining the CVE and the envelope average amplitude. We use this space to analyze rat EEG recordings during sleep-wake cycles. Our results show that delta, theta and sigma bands approach Gaussianity at the lowest EEG amplitudes while exhibiting significant deviations at high EEG amplitudes. Deviations to low-CVE appeared prominently during REM sleep, associated with theta rhythm, a regime consistent with the dynamics shown by the synchronization of weakly coupled oscillators. On the other hand, deviations to high-CVE, appearing mostly during NREM sleep associated with EEG phasic activity and high-amplitude Gaussian waves, can be interpreted as the arrhythmic superposition of transient neural synchronization events. These two different manifestations of neural synchrony (low-CVE/high-CVE) explain the well-known spectral differences between REM and NREM sleep, while also illuminating the origin of the EEG 1/f spectrum.

Dynamics of liquid–liquid flows in horizontal pipes using simultaneous two–line planar laser–induced fluorescence and particle velocimetry

Experimental investigations are reported of stratified and stratified–wavy oil–water flows in horizontal pipes, based on the development and application of a novel simultaneous two–line (two–colour) technique combining planar laser–induced fluorescence with particle image/tracking velocimetry. This approach allows the study of fluid combinations with properties similar to those encountered in industrial field–applications in terms of density, viscosity, and interfacial tension, even though their refractive indices are not matched, and represents the first attempt to obtain detailed, spatiotemporally–resolved, full 2–D planar–field phase and velocity information in such flows. The flow conditions studied span mixture velocities in the range 0.3–0.6 m/s and low water–cuts up to 20%, corresponding to in situ (local) Reynolds numbers of 1750–3350 in the oil phase and 2860–11,650 in the water phase, and covering the laminar/transitional and transitional/turbulent flow regimes for the oil and water phases, respectively. Detailed, spatiotemporally–resolved in situ phase and velocity data in a vertical plane aligned with the pipe centreline and extending across the entire height of the channel through both phases are analysed to provide statistical information on the interface heights, mean axial and radial (vertical) velocity components, (rms) velocity fluctuations, Reynolds stresses, and mixing lengths. The mean liquid–liquid interface height is mainly determined by the flow water cut and is relatively insensitive (up to 20% the highest water cut) to changes in the mixture velocity, although as the mixture velocity increases the interfacial profile transitions gradually from being relatively flat to containing higher amplitude waves. The mean velocity profiles show characteristics of both laminar and turbulent flow, and interesting interactions between the two co–flowing phases. In general, mean axial velocity profiles in the water phase collapse to some extent for a given water cut when normalised by the mixture velocity; conversely, profiles in the oil phase do not. Strong vertical velocity components can modify the shape of the axial velocity profiles. The axial turbulence intensity in the bulk of the water layer amounts to about 10% of the peak mean axial velocity in the studied flow conditions. In the oil phase, the axial turbulence intensity increases from low values to about 10% at the higher Reynolds numbers, perhaps due to transition from laminar to turbulent flow. The turbulence intensity showed peaks in regions of high shear, i.e., close to the pipe wall, and at the liquid–liquid interface. The development of the mixing length in the water phase, and also above the liquid–liquid interface in the oil phase, agrees reasonably well with predicted variations described by the von Karman constant. Finally, evidence of secondary flow structures both above and below the interface exists in the vertical velocity profiles, which is of interest to explore further.

Quasi 18 h wave activity in ground-based observed mesospheric H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O over Bern, Switzerland

Abstract. Observations of oscillations in the abundance of middle-atmospheric trace gases can provide insight into the dynamics of the middle atmosphere. Long-term, high-temporal-resolution and continuous measurements of dynamical tracers within the strato- and mesosphere are rare but would facilitate better understanding of the impact of atmospheric waves on the middle atmosphere. Here we report on water vapor measurements from the ground-based microwave radiometer MIAWARA (MIddle Atmospheric WAter vapor RAdiometer) located close to Bern during two winter periods of 6 months from October to March. Oscillations with periods between 6 and 30 h are analyzed in the pressure range 0.02–2 hPa. Seven out of 12 months have the highest wave amplitudes between 15 and 21 h periods in the mesosphere above 0.1 hPa. The quasi 18 h wave signature in the water vapor tracer is studied in more detail by analyzing its temporal evolution in the mesosphere up to an altitude of 75 km. Eighteen-hour oscillations in midlatitude zonal wind observations from the microwave Doppler wind radiometer WIRA (WInd RAdiometer) could be identified within the pressure range 0.1–1 hPa during an ARISE (Atmospheric dynamics Research InfraStructure in Europe)-affiliated measurement campaign at the Observatoire de Haute-Provence (355 km from Bern) in France in 2013. The origin of the observed upper-mesospheric quasi 18 h oscillations is uncertain and could not be determined with our available data sets. Possible drivers could be low-frequency inertia-gravity waves or a nonlinear wave–wave interaction between the quasi 2-day wave and the diurnal tide.

Analysis of Pressure Wave Development in a Thermo-acoustic Engine and Sensitivity Study

Thermoacoustic engines (TAE) are special devices that convert thermal energy into mechanical energy in the form of a high decibel acoustic wave. These devices have a huge potential to revolutionize fields like refrigeration, if efficiently designed. They are simple in construction, having two main parts, i.e. the stack and the resonator tube, and work on a process similar to the Stirling cycle. Although, this technology exists for several years, there is still no strong design available that can be used for actual practical purposes. They have been tested through experiments and analytical techniques and recently through CFD based numerical simulations. In this work, a TAE has been analyzed through CFD simulations using a commercial package. Taking a step further from the existing research related to TAEs, a sensitivity study has been carried out. The study aims to analyze the pressure wave development in a TAE, and its dependency on the temperature gradient at the stack and the length of the resonator. It is seen that higher temperature gradient causes a higher pressure amplitude and early pressure wave development. The critical temperature gradient is found in the range 200 – 300 K. With resonator length, it is noticed that, very small resonator length results in failure, while very long resonator length causes delay in pressure wave initiation although it provides high amplitude. Medium lengths are better since they provide high pressure amplitude and faster wave initiation. Analysis of frequency showed that temperature gradient has minor effect whereas length has major effects.

Propagation and dissipation of elasto-plastic stress waves in two dimensional ordered granular media

We investigate elasto-plastic wave propagation in ordered two-dimensional granular media consisting of contacting metallic granules. A Hopkinson bar generates a high-amplitude wave pulse – sufficient to yield the contact points between granules – and is used to load two configurations of brass beads: a square packing and a hexagonal packing. Selected beads that have embedded piezoelectric sensors are used to measure the arrival time and magnitude of the elasto-plastic wave in situ. Postmortem optical microscopy of the bead contact points elucidates the propagation of plasticity by allowing measurement of the residual plastic contact area. Results show that the elasto-plastic wave propagates in similar directions to a purely elastic wave, but with decaying amplitude due to the plastic deformation. It is found, however, that even small amounts of disorder in a nominally perfect 2D packing significantly affects both wave transit time and peak force. To explore the effects of such disorder further, the dynamic behavior of the 2D granular packing is numerically modeled using a molecular dynamics simulation that incorporates an elasto-plastic contact model and allows for a variability of particle size that incorporates bead diameter tolerance. The results qualitatively confirm the experimental findings of the significance of even limited amounts of disorder on wave arrival time and peak amplitude. This result has significant implications on the feasibility of designing 2D ordered granular media for practical applications.

Formation Mechanism of Mud Bank Along the Southwest Coast of India

Mud bank formation during the southwest monsoon along the southwest coast of India remains an enigma to the researchers and coastal community in spite of several earlier studies. The present study attempts to unravel the mystery through a high-frequency, season-long time-series observation at Alappuzha, located at the southern part of the west coast of India, a region of frequent occurrence of mud bank. Using 7-month-long weekly time-series observation, we identified strong winds and high waves associated with onset of the southwest monsoon and subsequent three episodic atmospheric low-pressure events (LPEs).With the help of in situ time-series data, we show that the strong winds and high waves associated with southwest monsoon pre-conditions the near shore bottom sediment to bring it into suspension. The high amplitude waves associated with the southwest monsoon, while propagating from the deep water to shallow water region, interact with the bottom initiating bottom-sediment movement and its suspension due to wave refraction and shoaling. The sporadic occurrence of the atmospheric LPEs enhances the process of suspension of bottom sediment in the near shore region leading to the formation of fluid mud. Simulations with a cohesive sediment transport model yielded realistic estimates of sediment transport, in the presence of an onshore current, a pre-requisite for transporting the fluid mud toward the coast. The prevailing onshore upwelling current during the southwest monsoon provides the favorable pre-requisite conditions for transporting the fluid mud through depression channel network towards the coast. Once sufficient quantity and thickness of fluid mud is accumulated in the near shore region, it acts as a wave damper for subsequent high monsoon waves, as indicated by the time-series wave data, leading to the formation of tranquil mud bank region. Depression channel networks extending from the shelf to the coast off Alappuzha, Kochi, Ponnani, Beypore, and Ullal were found in the bathymetric charts, thus explaining why mud banks occur only at few locations in spite of the prevalence of similar monsoon conditions.

Experimental measurements and bristle friction modeling of nonlinear hysteresis loops and harmonic generation in rock fractures

Frictional properties affect the propagation of high-amplitude seismic waves across rock fractures. Laboratory evidence suggests that these properties can be measured in active seismic surveys, potentially offering a route to characterizing friction in situ. We present experimental results from a subresonance torsional modulus and attenuation apparatus that utilizes micron-scale sinusoidal oscillations to probe the nonlinear stress-strain relation at a range of strain amplitudes and rates. Nonlinear effects are further quantified using harmonic distortion; however, time series data best illuminate underlying physical processes. The low-frequency stress-strain hysteretic loops show stiffening at the sinusoid’s static ends, but stiffening is reduced above a threshold frequency. This shape is determined by harmonic generation in the strain; the stress signal has no harmonics, confirming that the fractured sample is the source of the nonlinearity. These qualitative observations suggest the presence of rate-dependent friction and are consistent between fractures in three different rock types. We propose that static friction at the low strain rate part of the cycle, when given sufficient “healing” time at low oscillation frequencies, causes this stiffening cusp shape in the hysteresis loop. While rate-and-state friction is commonly used to represent dynamic friction, it cannot capture static friction or negative slip velocities. So, we implement a bristle friction model, which describes this process and produces similar results.

Flaw detection of wellbore systems by combining time reversal methods and nonlinear acoustic measurements

A wellbore system consisting of a steel pipe cased in cement is susceptible to cracking and delamination. Such flaws greatly reduce the integrity of the wellbore and can lead to significant leakage. To resolve the location and extent of the defects, we have combined a unique time reversal method with nonlinear acoustic evaluation. The time reversal method generates a compact high-amplitude wave only in a localized area that can be used to probe the acoustic properties spot by spot of a three dimensional region. Additionally, the nonlinear acoustic signal an object exhibits is far more sensitive to damage and defects than conventional linear analysis. The nonlinear behavior can be locally determined by observing the changes in elastic properties as the amplitude of the time-reversed focus wave is increased. The experimental setup consist of an array of piezoelectric transducers for generating the focused energy and a laser vibrometer mounted on a scanning frame for focusing and measuring nonlinearity on th...