High Frequency Fluctuations Research Articles

Carboniferous glaciotectonized sediments in the southernmost Paraná Basin, Brazil: Ice marginal dynamics and paleoclimate indicators

Carboniferous glaciogenic strata (Itararé Gp.) in the southernmost Paraná Basin, Brazil exhibit soft-sediment deformation features previously interpreted as glaciotectonism. These sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation, depositional environments, and paleoclimate in this region during the Carboniferous. Five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts, a medium sandstone facies, a fine grained silt/clay rhythmite and mudstone facies with dropstones and diamictite pellets, a sandy clinoform facies, and a folded sandstone with interbedded mudstone facies. The depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice-proximal, transitional terrestrial-to-estuarine setting. Rb/K values from the rhythmites reflect a transition from a freshwater to brackish environment and the Chemical Index of Alteration (CIA) of the rhythmites ranges from 65 to 73, reflecting a relative increase in the degree of chemical weathering through time. Deformation features include widespread folding, thrust faults, hydrofractures, décollement surfaces, and piggyback fold-thrust complexes. The deformation is interpreted as evidence of a push-moraine complex formed by at least two complete ice advance/retreat cycles. The occurrence of décollement surfaces, plastically deformed proglacial sediments, and hydrofractures indicate a dynamic, warm-based or polythermal glacier. Abundant outwash sediments, rhythmites with dropstones, and the shift to average CIA values all support a temperate paleoclimate. Deformation structures indicate a NW direction of ice shove that is in agreement with the regional-scale hypothesis that a NNW flowing lobe extended out of Uruguay during the Carboniferous and terminated in the southernmost Paraná Basin. This study demonstrates that late Paleozoic glaciation in this region was more dynamic than previously understood, with high frequency fluctuations in ice marginal positions.

High-Frequency Fluctuations in Post-stenotic Patient Specific Carotid Stenosis Fluid Dynamics: A Computational Fluid Dynamics Strategy Study

PurposeScreening of asymptomatic carotid stenoses is performed by auscultation of the carotid bruit, but the sensitivity is poor. Instead, it has been suggested to detect carotid bruit as neck’s skin vibrations. We here take a first step towards a computational fluid dynamics proof-of-concept study, and investigate the robustness of our numerical approach for capturing high-frequent fluctuations in the post-stenotic flow. The aim was to find an ideal solution strategy from a pragmatic point of view, balancing accuracy with computational cost comparing an under-resolved direct numerical simulation (DNS) approach vs. three common large eddy simulation (LES) models (static/dynamic Smagorinsky and Sigma).MethodWe found a reference solution by performing a spatial and temporal refinement study of a stenosed carotid bifurcation with constant flow rate. The reference solution left( {Delta x = 1.92 times 10^{ - 4} ;{text{m}},; Delta t = 5 times 10^{ - 5} ;{text{s}}} right) was compared against LES for both a constant and pulsatile flow.ResultsOnly the Sigma and Dynamic Smagorinsky models were able to replicate the flow field of the reference solution for a pulsatile simulation, however the computational cost of the Sigma model was lower. However, none of the sub-grid scale models were able to replicate the high-frequent flow in the peak-systolic constant flow rate simulations, which had a higher mean Reynolds number.ConclusionsThe Sigma model was the best combination between accuracy and cost for simulating the pulsatile post-stenotic flow field, whereas for the constant flow rate, the under-resolved DNS approach was better. These results can be used as a reference for future studies investigating high-frequent flow features.

A new method for mitigating frequency fluctuations in ships with electrical propulsion

Electrical networks of ships have less generation capacity compared to conventional large power systems. This leads to high frequency fluctuations and maybe blackout, especially in ships with electrical propulsion. Fundamentally, all of these ships have load reduction ability in critical conditions. Normally, load reductions are initiated with a time delay and this leads to a high frequency drop. Furthermore, load reductions need a real-time load management technique which can be used to decrease misdiagnosis and miscalculation probability. In some cases, the frequency may not violate its predefined limits but it may have high fluctuations that lead to malfunction of the sensitive equipment especially in military applications. In this paper, after extracting an electromechanical model of the whole system, frequency is regulated continuously by adding a frequency controller. Basic controller of the propulsion drive is investigated using the direct torque control (DTC) method. The frequency controller is designed to have minimum effect on normal operation of the drive and contributes in mitigating fluctuations only when response of the diesel generators is poor.

Weak Grid Intertie WEGS With Hybrid Generalized Integrator for Power Quality Improvement

The wind energy generating system (WEGS) in this paper, aims to provide the required active peak power without high frequency fluctuations even under variable wind speed conditions. The generator speed is adjusted according to the intermittent wind speeds through two voltage source converters (VSCs) connected back to back, namely, machine-side VSC (MSVSC) and utility grid side VSC (UGVSC) across the dc-link capacitor. In this paper, the hybrid generalized integrator control is proposed for switching UGVSC and providing dc offset rejection and immunity against oscillatory errors due to subharmonics, thereby improving the power quality (PQ). Fuzzy logic controller (FLC) is implemented for the speed control of the salient pole synchronous generator (SG) driven by the wind turbine. The FLC provides the tracking of the reference speed under high overshoot transient conditions and narrow bandwidth. The switching of MSVSC is obtained by field oriented control. The dynamic performance is improved by the wind feed-forward term, which reduces the oscillation, ensuring balanced and sinusoidal grid currents. The generated power from the WEGS is fed to the grid. The weak grid conditions, namely, grid voltage unbalance, voltage sag, voltage swell, and grid voltage distortion, are considered. The performance of the system is tested on a laboratory prototype. Test results provide the effectiveness of the system with increased wind penetration and performance under weak grid conditions. Moreover, improving the PQ, the grid current total harmonic distortion meets the requirement of IEEE-519 standard, and is found to be less than 5%.

Phytoplankton adaptation in ecosystem models

We compare two different approaches to model adaptation of phytoplankton through trait value changes. Both consider mutation and selection (MuSe) but differ with respect to the underlying conceptual framework. The first one (MuSe-IBM) explicitly considers a population of individuals that are subject to random mutation during cell division. The second is a deterministic multi-compartment model (MuSe-MCM) that considers numerous genotypes of the population and where mutations are treated as a transfer of biomass between neighboring genotypes (i.e., a diffusion of characteristics in trait space). Focusing on the adaptation of optimal temperature, we show model results for different scenarios: a sudden change in environmental temperature, a seasonal variation and high frequency fluctuations. In addition, we investigate the effect of different shapes of thermal reaction norms as well as the role of alternating growth and resting phases on the adaptation process. For all cases, the differences between MuSe-IBM and MuSe-MCM are found to be negligible. Both models produce a number of well-known and plausible features. While the IBM has the advantage of including more mechanistic (i.e., probabilistic) processes, the MCM is much less computationally demanding and therefore suitable for implementation in three-dimensional ecosystem models.

Evaluation of turbulence-related high-frequency tidal current velocity fluctuation

Within the development needed for economy viability of tidal stream energy, adaptability of laboratory converters to sea flow conditions is a milestone. The objective of this work is to investigate the high frequency fluctuations in current velocity magnitude and direction related to the turbulent nature of the flow and present a new method for their prediction. With this purpose, high frequency data measured by two ADV (32 Hz) and two ADCP (8 Hz) at four different points in the sea area surrounding Goto Islands (Japan) are analyzed. The data were divided in short-time samples (3-min data for ADV and 5-min data for ADCP) and treated separately. Velocity magnitude fits a normal distribution, with prediction levels higher than 95% for a margin of error of 0.25 m/s when comparing different percentiles between 0.1 and 99.9. Flow direction is analyzed in terms of opening angle between two representative percentiles equidistant from the median (99.9-0.1, 95-5, …), giving as a result a leptokurtic distribution, more outlier-prone than normal. Empirically, for opening angles 99.9-0.1, 97.7-2.3 and 95-5, slopes of 6.79 (6 in normal distribution), 4.17 (4) and 3.38 (3.29) were found, with results similar to a theoretical normal distribution for narrower angles. The new prediction method for high frequency fluctuations is based in this direct correlation between velocity magnitude and direction fluctuations with turbulence intensity and transverse turbulence intensity, respectively. These two parameters can be estimated indirectly by numerical models, giving rise to a tool for the prediction of turbulence-related high frequency fluctuation.

On the ion and electron temperature recovery after the ELM-crash at ASDEX upgrade

The access to fast measurements, i.e. Δt ≈ 100 µs, of the ions and the electrons during an entire edge localized cycle (ELM) reveals asymmetries in the recovery of the maximum edge gradients. Different magnetic fluctuations are found to correlate with the saturation of the edge ion temperature (Ti), electrons temperature (Te) and density (ne) gradients. In particular, while ∇Ti and ∇ne clamp roughly 3.0 ms after the ELM-crash together with the onset of mid-frequency (f ≲ 50 kHz) magnetic fluctuations, ∇Te recovers to the pre-ELM conditions only after 7.0 ms and saturates with the appearance of high frequency fluctuations (f ≈ 200 kHz). The effect of electron temperature gradient modes (ETGs) and of energy losses induced by ionization of neutrals are discussed as possible reasons for the delayed recovery of ∇Te. The onset and the suppression of ETGs qualitatively follow the requirements of an increased electron heat transport. However, gyro-kinetic simulations are necessary to quantify the impact of ETGs. On the other hand, the impact of the neutral ionization during the density build-up as an electron energy loss channel is measured to be small compared to the total electron energy. The dominant terms in the electron energy balance are instead the radiative power and the ion-electron heat exchange.

Demodulation Technique Based on Laser Interference for Weak Photo-Acoustic Signals on Water Surface

To detect underwater sound-generating targets, a water surface acoustic wave laser interference and signal demodulation technique is proposed in this paper. The underlying principle of this technique involves casting a laser beam onto the water surface disturbed by an underwater acoustic source and creating interference between lights reflected by the surface and reference lights. A data acquisition and processing system was employed to obtain water surface acoustic wave information from the interference signals by means of demodulation, thus allowing detection of the underwater target. For the purpose of this study, an interference detection platform was set up in an optical dark chamber. High-frequency water surface fluctuations were introduced in the reference optical path as the phase generated carriers to create laser interference signals in two different paths, which received demodulation based on an improved arc tangent demodulation algorithm and characteristic ratio algorithm, respectively, in view of their different frequencies. Water surface wave information was then derived from such low-frequency and high-frequency signals. According to test results, in the frequency range of 200 Hz–10 kHz, the frequency detection accuracy was better than 1 Hz. The amplitude measurements exhibited high repeatability, with a standard deviation lower than 2.5 nm. The theory proposed in this paper is therefore experimentally verified with good results.

Power stabilization of a diode laser with an acousto-optic modulator.

Laser power fluctuations can significantly reduce the device performances in various applications. High frequency fluctuations impact the signal-to-noise ratio, while slow variations can reduce the device repeatability or accuracy. Here we report experimental investigations on the power stabilization of a diode laser with an acousto-optic modulator. In the frequency domain, the relative power noise is reduced at the level of 2.2 × 10-8 Hz-1/2 in the range 1-100 kHz. The slow variations are studied in the time domain. The relative Allan standard deviation is measured at the level of 6 × 10-7 at 100 s averaging time. Above 100 s, the instability increases and reaches 2 × 10-6 at 10 000 s.

Fewer-axis grinding methodology with simultaneously guaranteeing surface accuracy and grinding force for large optical SiC mirror

This paper presents a grinding force control strategy with simultaneously guaranteeing surface accuracy for large optical SiC mirror. A fewer-axis grinder with toric wheel is used to ensure the high stiffness during the grinding process. According to the geometric properties of the contact area at the grinding point between the grinding wheel and the workpiece, the relationship between the grinding force, the feed rate, and grinding depth is derived. Furthermore, the dynamic grinding process model is established by considering the deformation of grinding machine and contour deviation of toric wheel. The contour deviation of toric wheel is also identified by the Fourier transform and analysis of the high frequency fluctuation of grinding force. Based on the grinding principle of toric wheel and the above grinding process model, the force-controlled trajectory planning method for toric grinding is introduced. To ensure the workpiece surface accuracy and decrease the subsurface damage simultaneously, the force-controlled method by adjusting the feed rate and keeping the grinding depth unchanged is proposed. The strategy is successfully validated by grinding experiments of SiC mirror on a high-precision fewer-axis grinding machine.

Plasma acceleration on multiscale temporal variations of electric and magnetic fields during substorm dipolarization in the Earth’s magnetotail

Magnetic field dipolarizations are often observed in the magnetotail during substorms. These generally include three temporal scales: (1) actual dipolarization when the normal magnetic field changes during several minutes from minimum to maximum level; (2) sharp Bz bursts (pulses) interpreted as the passage of multiple dipolarization fronts with characteristic time scales < 1 min, and (3) bursts of electric and magnetic fluctuations with frequencies up to electron gyrofrequency occurring at the smallest time scales (≤ 1 s). We present a numerical model where the contributions of the above processes (1)-(3) in particle acceleration are analyzed. It is shown that these processes have a resonant character at different temporal scales. While O+ ions are more likely accelerated due to the mechanism (1), H+ ions (and to some extent electrons) are effectively accelerated due to the second mechanism. High-frequency electric and magnetic fluctuations accompanying magnetic dipolarization as in (3) are also found to efficiently accelerate electrons.

Initiation and sustainment of tokamak plasmas with local helicity injection as the majority current drive

Local helicity injection (LHI) is a non-solenoidal current drive capable of achieving high- tokamak startup with non-invasive current injectors in the plasma scrape-off layer. The choice of injector location within the edge region is flexible but has a profound influence on the nature of the current drive in LHI discharges. New experiments on the Pegasus ST with injection in the high-field-side, lower divertor region produce plasmas dominated by helicity injection current drive, static plasma geometry, and negligible inductive drive. Peak plasma current up to 200 kA, and a sustained plasma current of 100 kA for up to 18 ms, is demonstrated. Maximum achievable plasma current is found to scale approximately linearly with the effective loop voltage from LHI. A newly-observed MHD regime for LHI-driven plasmas in which large-amplitude fluctuations at 20–50 kHz are abruptly reduced on the outboard side results in improved current drive. A simultaneous increase in high frequency fluctuations (>400 kHz) inside the plasma edge suggests short wavelength turbulence as an important current drive mechanism during LHI.

High-frequency climate fluctuations over the last deglaciation in the Alboran Sea, Western Mediterranean: Evidence from calcareous plankton assemblages

A high resolution study, with a centennial scale resolution, has been performed on the calcareous plankton assemblage (coccolithophores and planktonic foraminifera) at Ocean Drilling Program Site 976, Alboran Sea (Western Mediterranean), focusing on the interval between 20 and 9 ka, in order to reconstruct changes in surface and subsurface water dynamics and productivity. The biotic surface water proxies integrate the extremely detailed (multi-decadal scale) geochemical data set and the pollen record already available at the core, thus providing a complete paleoenvironmental/paleoceanographic reconstruction. The results highlight the sensitivity of the calcareous plankton in recording stadial/interstadial phases and higher-frequency climatic events, that produced changes in sea water features. Wind-induced upwelling and river discharge during the Last Glacial Period favored nutrient availability and moderate productivity. During Heinrich Stadial 1 (HS1), the arrival of cold and fresher waters from iceberg melting in the North Atlantic, significantly hampered productivity, at its lowest values of the whole investigated interval. Calcareous plankton behavior supports the hypothesis that HS1 is a composite event, marked by three phases: HS1a characterized by the coldest SST, polar Atlantic water inflow into the basin and a first step toward a dry climate on the continent; HS1b recording enhanced freshwater inflow and drought on land, and HS1c indicating reduced polar water influx. The Bølling-Allerød (BA) interstadial sees the highest rate of productivity in the entire interval and accompanies the deposition of Organic Rich Layer 1; during the BA coccolithophore assemblage also marks higher frequency changes in hydrographic conditions, apparently in relation with Greenland Interstadial I (GI-I) high variability. The Younger Dryas Stadial (YD) is characterized by refunding cold and dry conditions, although not as severe as during HS1, and by enhanced detrital input in surface water during drier conditions on land. The Holocene onset records climate amelioration both in the marine and continental environment, although coccolithophore productivity is not as high as during the BA. The rapid shifts in climate sensitive taxa support the hypothesis that each of the climate phases-subphases is characterized by a complex pattern forced by strong interaction between ice-sheet instability, meltwater discharge entering the Gibraltar Strait, North Atlantic westerlies and moisture availability, clearly impacting over surface water temperature, productivity and land masses.

Spectrum study on unsteadiness of shock wave–vortex ring interaction

Shock oscillation with low-frequency unsteadiness commonly occurs in supersonic flows and is a top priority for the control of flow separation caused by shock wave and boundary layer interaction. In this paper, the interaction of the shock caused by the compression ramp and the vortex rings generated by a micro-vortex generator (MVG) in a supersonic flow at Ma = 2.5 is simulated by the implicit large eddy simulation method. The analysis of observation and the frequency of both the vortex ring motion and the shock oscillation is carried out. The results show that the shock produced by a compression ramp flow at Ma = 2.5 has a dominant non-dimensional low frequency, which is around St = 0.002, while the vortex rings behind the MVG have a dominant high frequency which is around St = 0.038. The dominant low frequency of the shock, which is harmful, can be removed or weakened through the shock-vortex ring interaction by the vortex rings which generate high frequency fluctuations. In the shock and vortex ring interaction region, a dominant high frequency St = 0.037–0.038 has been detected rather than the low frequency St = 0.002, which indicates that the vortex ring is stiff enough to break or weaken the shock. This analysis could provide an effective tool to remove or weaken the low frequency pressure fluctuation below 500 Hz, which has a negative effect on the flight vehicle structures and the environmental protection, through the high frequency vortex generation.

Mechanism of high amplitude low frequency fluctuations in a pump-turbine in pump mode

Pumped storage technology has become the most important energy storage technology in industry today. The instabilities of pump-turbines as key parts of pumped storage power plants have become critical issues in development of pump storage technology. High-amplitude pressure fluctuations are one of such instabilities. In this study, unsteady numerical simulations were carried out in the pump mode of a pump-turbine at using the shear stress transport (SST) k-ω turbulence model. Performance characteristics agree well with experimental data. Unsteady characteristics of typical operating points were obtained. The results show that there are high-amplitude pressure fluctuations at the partial operating points. The frequency characteristics and the propagation law of an at-large partial operating point (0.74QBEP) were determined using time and frequency domain analysis methods (bispectrum, coherence) in combination with the flow field. The analysis results reveal that high-amplitude pressure fluctuations at point 0.74QBEP result from the rotation of Dean Vortices in the draft tube. Due to the rotation of the Dean Vortices, the blocking intensity of the two regions in the guide vane and the stay vane passages are periodically changed at a frequency 0.58fn, which results in a shock phenomenon in the guide/stay vanes.

Transfer Entropy as a Tool for Hydrodynamic Model Validation.

The validation of numerical models is an important component of modeling to ensure reliability of model outputs under prescribed conditions. In river deltas, robust validation of models is paramount given that models are used to forecast land change and to track water, solid, and solute transport through the deltaic network. We propose using transfer entropy (TE) to validate model results. TE quantifies the information transferred between variables in terms of strength, timescale, and direction. Using water level data collected in the distributary channels and inter-channel islands of Wax Lake Delta, Louisiana, USA, along with modeled water level data generated for the same locations using Delft3D, we assess how well couplings between external drivers (river discharge, tides, wind) and modeled water levels reproduce the observed data couplings. We perform this operation through time using ten-day windows. Modeled and observed couplings compare well; their differences reflect the spatial parameterization of wind and roughness in the model, which prevents the model from capturing high frequency fluctuations of water level. The model captures couplings better in channels than on islands, suggesting that mechanisms of channel-island connectivity are not fully represented in the model. Overall, TE serves as an additional validation tool to quantify the couplings of the system of interest at multiple spatial and temporal scales.

Stability and Complexity Analysis of Temperature Index Model Considering Stochastic Perturbation

A temperature index model with delay and stochastic perturbation is constructed in this paper. It explores the influence of parameters and stochastic factors on the stability and complexity of the model. Based on historical temperature data of four cities of Anhui Province in China, the temperature periodic variation trends of approximately sinusoidal curves of four cities are given, respectively. In addition, we analyze the existence conditions of the local stability of the temperature index model without stochastic term and estimate its parameters by using the same historical data of the four cities, respectively. The numerical simulation results of the four cities are basically consistent with the descriptions of their historical temperature data, which proves that the temperature index model constructed has good fitting degree. It also shows that unreasonable delay parameter can make the model lose stability and improve the complexity. Stochastic factors do not usually change the trend in temperature, but they can cause high frequency fluctuations in the process of temperature evolution. Stability control is successfully realized for unstable systems by the variable feedback control method. The trend of temperature changes in Anhui Province is deduced by analyzing four typical cities.

High performance cutting of Zr-based bulk metallic glass: a review of chip formation

Zr-based bulk metallic glass (Zr-based BMG) has a unique amorphous structure, and therefore presents high tensile strength, high elastic limit, low elastic modulus, as well as improved wear and corrosion resistance. For this reason, Zr-based BMG is emerging as a promising material for applications in the aviation, automotive, and healthcare industries. However, Zr-based BMG is classed as an extremely difficult-to-cut material. At very low cutting speeds, lamellar chips are formed accompanied by high frequency fluctuations. At higher cutting speeds, dazzling light is emitted during BMG chip formation and severe oxidation, crystallization, and melting of the working material occur, and consequently, causing premature failure of the tool, loss of amorphous properties, and poor machining accuracy. The purpose of this paper is to investigate chip formation of Zr-based BMG. Based on a survey of existing works, lamellar chip morphology and its underlying mechanism are reviewed and compared with crystalline metals. The key factor affecting light emission during chip formation is uncovered. Further, the influence of oxidation, crystallization, and melting on chip formation is investigated.

Experimental investigation of effect of spacer on two phase turbulent mixing rate in subchannels of pressure tube type BWR

Abstract Turbulent mixing rate between adjacent subchannels in a two-phase flow has been known to be strongly dependent on the flow pattern. The most important aspect of turbulent motion is that the velocity and pressure at a fixed point do not remain constant with time even in steady state but go through very irregular high frequency fluctuations. These fluctuations influence the diffusion of scalar and vector quantities. The Advanced Heavy Water Reactor (AHWR) is a vertical pressure tube type, heavy water moderated and boiling light water cooled natural circulation based reactor. The fuel bundle of AHWR contains 54 fuel rods set in three concentric rings of 12, 18 and 24 fuel rods. This fuel bundle is divided into number of imaginary interacting flow channel called subchannels. Alteration from single phase to two phase flow situation occurs in reactor rod bundle with raise in power. The two phase flow regimes like bubbly, slug-churn, and annular flow are generally encountered in reactor rod bundle. Prediction of thermal margin of the reactor has necessitated the investigation of turbulent mixing rate of coolant between these subchannels under these flow regimes. Thus, it is fundamental to estimate the effect of spacer grids on turbulent mixing between subchannels of AHWR rod bundle.

NMR spectroscopy up to 35.2 T using a series-connected hybrid magnet

The National High Magnetic Field Laboratory has brought to field a Series-Connected Hybrid magnet for NMR spectroscopy. As a DC powered magnet it can be operated at fields up to 36.1T. The series connection between a superconducting outsert and a resistive insert dramatically minimizes the high frequency fluctuations of the magnetic field typically observed in purely resistive magnets. Current-density-grading among various resistive coils was used for improved field homogeneity. The 48mm magnet bore and 42mm outer diameter of the probes leaves limited space for conventional shims and consequently a combination of resistive and ferromagnetic shims are used. Field maps corrected for field instabilities were obtained and shimming achieved better than 1ppm homogeneity over a cylindrical volume of 1cm diameter and height. The magnetic field is regulated within 0.2ppm using an external 7Li lock sample doped with paramagnetic MnCl2. The improved field homogeneity and field regulation using a modified AVANCE NEO console enables NMR spectroscopy at 1H frequencies of 1.0, 1.2 and 1.5GHz. NMR at 1.5GHz reflects a 50% increase in field strength above the highest superconducting magnets currently available. Three NMR probes have been constructed each equipped with an external lock rf coil for field regulation. Initial NMR results obtained from the SCH magnet using these probes illustrate the very exciting potential of ultra-high magnetic fields.