Low-frequency Conditions Research Articles

Breeding behaviour of colour-aberrant Adélie penguins (Pygoscelis adeliae) at Cape Crozier, Ross Island, Antarctica

AbstractPlumage colour variation occurs widely among bird species and is often associated with individual fitness. More specifically, colouration can affect thermoregulatory ability, mate selection and conspicuousness during foraging. Colour aberrations can be caused by genetic mutations, dietary imbalances, environmental conditions or disease and are rare. Plumage variations have previously been noted in Adélie penguins, although without any follow-up to measure implications for behaviour or fitness. To assess how this low-frequency condition affects breeding in Adélie penguins, we monitored the breeding of several colour-aberrant Adélie penguins during the 2019–2020 nesting season at the large Cape Crozier, Ross Island colony (> 300,000 pairs). In total, we found 12 individuals with unusual plumage for a frequency of 1:50,000 breeding penguins. There were seven dark brown Adélie penguins, three progressive greying Adélie penguins, one dilute Adélie penguin and one brown Adélie penguin, of which five were female, three male and four of unknown sex. Six colour aberrants initiated breeding with a normal-coloured mate, and five raised at least one chick to crèche. The likelihood of breeding and breeding success of colour aberrants were similar to those of normal-coloured Adélie penguins, suggesting that colour aberrations do not negatively affect breeding.

Remarks on the Abraham–Minkowski problem, from the formal and from the experimental side

We analyze the Abraham–Minkowski problem known from classical electrodynamics from two different perspectives. First, we follow a formal approach, implying use of manifolds with curved space sections in accordance with Fermat’s principle, emphasizing that the resulting covariant and contravariant components of the photon four-momentum are a property linked to the Minkowski theory only. There is thus no link to the Abraham theory in that context. Next we turn to the experimental side, giving a brief account of older and newer radiation pressure experiments that clearly show how the Minkowski photon momentum is preferable under optical conditions. Under low-frequency conditions, where experimental detection of the individual oscillations predicted by the Abraham term are possible, the picture is however quite different.

Daily Myofibrillar Protein Synthesis Rates in Response to Low- and High-Frequency Resistance Exercise Training in Healthy, Young Men.

The impact of resistance exercise frequency on muscle protein synthesis rates remains unknown. The aim of this study was to compare daily myofibrillar protein synthesis rates over a 7-day period of low-frequency (LF) versus high-frequency (HF) resistance exercise training. Nine young men (21 ± 2years) completed a 7-day period of habitual physical activity (BASAL). This was followed by a 7-day exercise period of volume-matched, LF (10 × 10 repetitions at 70% one-repetition maximum, once per week) or HF (2 × 10 repetitions at ∼70% one-repetition maximum, five times per week) resistance exercise training. The participants had one leg randomly allocated to LF and the other to HF. Skeletal muscle biopsies and daily saliva samples were collected to determine myofibrillar protein synthesis rates using 2H2O, with intracellular signaling determined using Western blotting. The myofibrillar protein synthesis rates did not differ between the LF (1.46 ± 0.26%/day) and HF (1.48 ± 0.33%/day) conditions over the 7-day exercise training period (p > .05). There were no significant differences between the LF and HF conditions over the first 2 days (1.45 ± 0.41%/day vs. 1.25 ± 0.46%/day) or last 5 days (1.47 ± 0.30%/day vs. 1.50 ± 0.41%/day) of the exercise training period (p > .05). Daily myofibrillar protein synthesis rates were not different from BASAL at any time point during LF or HF (p > .05). The phosphorylation status and total protein content of selected proteins implicated in skeletal muscle ribosomal biogenesis were not different between conditions (p > .05). Under the conditions of the present study, resistance exercise training frequency did not modulate daily myofibrillar protein synthesis rates in young men.

A generalized minimax-concave penalty based compressive beamforming method for acoustic source identification

Acoustic source identification using compressive beamforming has been extensively investigated and applied in numerous fields. In this paper, a generalized minimax-concave (GMC) penalty based compressive beamforming method is proposed. The method solves acoustic inverse problems using the GMC penalty, by which the capacity of sound field reconstruction in low-frequency and low-signal-to-noise ratio environments can be enhanced. In addition, the problem of systematic underestimation of source strength can be relieved with the benefit of penalty homogeneity, and a balance can be reached between the sparsity-induced capacity and computing complexity. Moreover, the convergence of the proposed method and parameter influences were studied through numerical simulations. The results show that the method is robust within a relatively wide range of parameters, and the computational efficiency is within the limits of acceptability. Furthermore, the performance of the proposed method was compared with two other methods using simulated and experimental data. The results indicate that the proposed method performs better than existing algorithms under low-frequency and strong-noise conditions, and the spatial resolution in acoustic maps and the capacity of source strength estimation can be improved.

Fitting of Resistance Model of Underwater Electrodes

This paper measures and analyzes the resistance of a circuit formed by underwater metal electrodes under a low-frequency condition. The conditions for ignoring displacement current and the existence of Ohm’s law are discussed. On this basis, three expressions of resistance are proposed based on the measured data, and the three expressions are analyzed according to their different application emphases. The results show that under partial constraints, the three resistance expressions proposed can accurately calculate the resistance of the circuit formed by underwater metal electrodes.

A Deep Neural Network Model for Predicting Electric Fields Induced by Transcranial Magnetic Stimulation Coil

This article proposes a deep neural network (DNN) model to predict the electric field induced by a transcranial magnetic stimulation (TMS) coil under high-amplitude and low-frequency current pulse conditions. The DNN model is comprised of an input layer with 6 neurons, three non-linear hidden layers with a total of 1088 neurons, and a linear single output layer. The model is developed in Google Colaboratory environment with TensorFlow framework using six features including coil turns of single wing, coil thickness, coil diameter, distance between two wings, distance between head and coil position, and angle between two wings of coil as the inputs and electric field as the output. The model performance is evaluated based on four verification statistic metrics such as coefficient of determination (R2), mean squared error (MSE), mean absolute error (MAE), and root mean squared error (RMSE) between the simulated and predicted values. The proposed model provides an adequate performance with $\text{R}^{2} =0.766$ , MSE = 0.184, MAE = 0.262, and RMSE = 0.429 in the testing stage. Therefore, the model can successfully predict the electric field in an assembly TMS coil without the aid of electromagnetic simulation software that suffers from an extensive computational cost.

Soil Medium Electromagnetic Scattering Model for the Study of Wireless Underground Sensor Networks

In recent years, there has been keen interest in the area of Internet of Things connected underground, and with this is the need to fully understand and characterize their operating environment. In this paper, a model, based on the Peplinski principle, for the propagation of waves in soils that takes into account losses attributable to the presence of local inhomogeneity is proposed. In the work, it is assumed that the inhomogeneities are obstacles such as stones or pebbles, of moderate size, all identical and randomly distributed in space. A new wave number is obtained through a combination of the multiple scattering theory and the Peplinski principle. Since the latter principle considers the propagation in a homogeneous medium (without obstacles), the wave number it provides is inserted into the one resulting from the former, the multiple scattering theory. The effective wave number thus obtained is compared numerically with that of Peplinski alone on the one hand and with that of multiple scattering alone on the other hand. The phase velocity and the loss tangent are analyzed against the particle concentration at the low‐frequency Rayleigh limit condition (ka≲0.1) and against the frequency at two particle concentrations (c = 0.2 and c = 0.4), two particle radii (a = 0.55 cm and a = 1.10 cm), and 5% and 50% volumetric water content of the soil. Path losses are also compared to each other to examine the effects on transmission of soil containing obstacles. The results obtained suggest that the proposed model has better accuracy in estimating the wave number than previously used schemes.

Modular Multilevel Converter With Self-Energy Equalization for Medium Voltage AC Drive Applications

The modular multilevel converter (MMC) is a promising candidate for high-/medium-voltage high-power applications such as medium voltage ac drives due to its modularity, reliability, and scalability. The main challenge of a conventional MMC is low-frequency operation. Capacitor voltage ripple is inversely proportional to the operating frequency. Hence, operating at low frequency necessitates employing large submodule capacitances to limit voltage ripple or the utilization of energy equalization modules (EEMs). The first option negatively affects converter lifetime, and the second option requires a large number of semiconductor devices and isolating transformers. In this article, a modified MMC topology with self-energy equalization is proposed to ensure converter operation with balanced capacitor voltages with low ripple during zero-/low-frequency conditions. Operating with balanced capacitor voltages with low ripple is achieved by maintaining the same energy level in the involved upper and lower arms. A low device count alternative to the EEMs approach is proposed in this work. Detailed illustration of the operational concept and design of the associated passive components is presented. The performance of the proposed medium voltage ac drive is elucidated through simulation and experimental results.

A robust and self-powered tilt sensor based on annular liquid-solid interfacing triboelectric nanogenerator for ship attitude sensing

Ship's attitude information is of great significance for navigation safety, cargo handling monitoring, and operation management. It is highly desired to develop an electromechanical integrated tilt sensor which can work well under harsh environment. In this paper, a robust and self-powered tilt sensor based on annular liquid-solid interfacing triboelectric nanogenerator (TENG) is to be proposed and systematically investigated. The TENG tilt sensor is composed of an annular polytetrafluoroethylene (PTFE) tube with copper electrodes segment disposed on the surface and the internal liquid which is encapsulated in PTFE tube with no air bubble. Studies on electrode width and liquid type are conducted to reasonably design the TENG sensor structure and improve its sensitivity. By conducting the durability test, the TENG tilt sensor demonstrates stable output performance and high sensitivity characteristics in low-frequency and low-amplitude inclination conditions. Moreover, the TENG tilt sensor hardly fails due to parts wear since there are no moving parts in it. The advantage of durability, maintenance-free, and independent with the external environment are highlighted in a harsh environment with high humidity, high salinity and strong magnetic field. Therefore, the present TENG tilt sensor is of practical significance for improving the automation and intelligence of ships.

Vortex-induced swing (VIS) motion for energy harvesters and flowmeters

Vortex-induced vibration (VIV) has been widely studied in the fields of vibration control, building construction, and underwater vehicles. Recently, researchers began utilizing the VIV phenomenon for flow energy harvesting. Here, we describe that vortex shedding causes periodic rotational motions and explore these vortex-induced swing (VIS) motions for harvesting flow energy and measuring flow speed. An arc-bluff structure was constructed to enlarge the VIS motions, and a phenomenological model was developed using the Van der Pol equation. Swing characteristics when flow velocities were in the range of (0.15, 0.45) m/s were assessed. Experiments showed that the maximum swing amplitude of the device is 120°, and it converges to 80° as the water velocity increases. The frequency, amplitude, and initial angle curve of the VIS device can be used to represent the water speed vector. The proposed flowmeter showed a flow rate sensitivity of 7∼10 Hz/(m·s−1) in the experimental range. Energy harvester prototypes demonstrated a peak-peak output of 3.28 V in water with a velocity of 0.45 m/s. The present work provides an approach for the flow measurement and energy harvesting under low-speed and low-frequency conditions.

Separation event-constrained optimal power flow to enhance resilience in low-inertia power systems

Weakly interconnected power systems face separation risk during extreme weather events or due to cascading failures. These contingencies may become particularly severe in low-inertia grids, where the sub-regions resulting after system split need to procure enough resources to respond to low (importing areas) and high (exporting areas) frequency conditions. This paper introduces a separation event-constrained optimal power flow (SECOPF) with considerations of inertia and frequency response resources allocation in each balancing area to be capable to withstand, besides the largest generation and load contingencies, also the loss of selected interconnectors. The model, cast as mixed integer linear program, is tested on a reduced version of the Australian power system and validated via dynamic simulation. The results show how the system's frequency resilience to extreme events can be effectively enhanced for both high- and low-frequency conditions that arise in different areas after an interconnector's trip . This is achieved by co-optimising the interconnector flow magnitudes along with the distribution of frequency response resources and inertia levels in each area.

Signals for defocus arise from longitudinal chromatic aberration in chick

Chicks respond to two signals from longitudinal chromatic aberration (LCA): a wavelength defocus signal and a chromatic signal. Wavelength defocus predicts reduced axial eye growth in monochromatic short-wavelength light, compared to monochromatic long-wavelength light. Wavelength defocus may also influence growth in broadband light. In contrast, a chromatic signal predicts increased growth when short-wavelength contrast > long-wavelength contrast, but only when light is broadband. We aimed to investigate the influence of blue light, temporal frequency and contrast on these signals under broadband conditions. Starting at 12 to 13 days-old, 587 chicks were exposed to the experimental illumination conditions for three days for 8h/day and spent the remainder of their day in the dark. The stimuli were flickering lights, with a temporal frequency of 0.2 or 10 Hz, low (30%) or high contrast (80%), and a variety of ratios of cone contrast simulating the effects of defocus with LCA. There were two color conditions, with blue contrast (bPlus) and without (bMinus). Stimuli in the “bPlus” condition varied the amounts of long- (L), middle- (M_) and double (D-) cone contrast, relative to short- (S-) and (UV-) cone contrast, to simulate defocus. Stimuli in the “bMinus” condition only varied the relative modulations of the L + D vs. M cones. In all cases, the average of the stimuli was white, with an illuminance of 777 lux, with cone contrast created through temporal modulation. A Lenstar LS 900 and a Hartinger refractometer were used to measure ocular components and refraction. Wavelength defocus signals with relatively high S-cone contrast resulted in reduced axial growth, and more hyperopic refractions, under low-frequency conditions (p = 0.002), in response to the myopic defocus of blue light. Chromatic signals with relatively high S-cone contrast resulted in increased axial growth and more myopic refractions, under high frequency, low contrast, conditions (p < 0.001). We conclude that the chromatic signals from LCA are dependent on the temporal frequency, phase, and relative contrast of S-cone temporal modulation, and recommend broadband spectral and temporal environments, such as the outdoor environment, to optimize the signals-for-defocus in chick.

Shift Detection in Hydrological Regimes and Pluriannual Low-Frequency Streamflow Forecasting Using the Hidden Markov Model

Improved water resource management relies on accurate analyses of the past dynamics of hydrological variables. The presence of low-frequency structures in hydrologic time series is an important feature. It can modify the probability of extreme events occurring in different time scales, which makes the risk associated with extreme events dynamic, changing from one decade to another. This article proposes a methodology capable of dynamically detecting and predicting low-frequency streamflow (16–32 years), which presented significance in the wavelet power spectrum. The Standardized Runoff Index (SRI), the Pruned Exact Linear Time (PELT) algorithm, the breaks for additive seasonal and trend (BFAST) method, and the hidden Markov model (HMM) were used to identify the shifts in low frequency. The HMM was also used to forecast the low frequency. As part of the results, the regime shifts detected by the BFAST approach are not entirely consistent with results from the other methods. A common shift occurs in the mid-1980s and can be attributed to the construction of the reservoir. Climate variability modulates the streamflow low-frequency variability, and anthropogenic activities and climate change can modify this modulation. The identification of shifts reveals the impact of low frequency in the streamflow time series, showing that the low-frequency variability conditions the flows of a given year.

Differential effects of the temporal and spatial distribution of audiovisual stimuli on cross-modal spatial recalibration.

Visual input constantly recalibrates auditory spatial representations. Exposure to isochronous audiovisual stimuli with a fixed spatial disparity typically results in a subsequent auditory localization bias (ventriloquism aftereffect, VAE), whereas exposure to spatially congruent audiovisual stimuli improves subsequent auditory localization (multisensory enhancement, ME). Here, we tested whether cross-modal recalibration is affected by the stimulation rate and/or the distribution of audiovisual spatial disparities during training. Auditory localization was tested before and after participants were exposed either to audiovisual stimuli with a constant spatial disparity of 13.5° (VAE) or to spatially congruent audiovisual stimulation (ME). In a between-subjects design, audiovisual stimuli were presented either at a low frequency of 2Hz, as used in previous studies of VAE and ME, or intermittently at a high frequency of 10Hz, which mimics long-term potentiation (LTP) protocols and which was found superior in eliciting unisensory perceptual learning. Compared to low-frequency stimulation, VAE was reduced after high-frequency stimulation, whereas ME occurred regardless of the stimulation protocol. In two additional groups, we manipulated the spatial distribution of audiovisual stimuli in the low-frequency condition. Stimuli were presented with varying audiovisual disparities centered around 13.5° (VAE) or 0° (ME). Both VAE and ME were equally strong compared to a fixed spatial relationship of 13.5° or 0°, respectively. Taken together, our results suggest (a) that VAE and ME represent partly dissociable forms of learning and (b) that auditory representations adjust to the overall stimulus statistics rather than to a specific audiovisual spatial relationship.

Graphene@poly(dopamine)-Ag core–shell nanoplatelets as fillers to enhance the dielectric performance of polymer composites

A new type of core–shell graphene@poly(dopamine)-Ag nanoplatelets was developed to improve the dielectric properties of thermoplastic polyurethane (TPU) nanocomposites. Dopamine was oxidatively polymerized to form a polydopamine (PDA) layer on the surface of graphene nanosheets (Gns), and then Ag nanoparticles were chemically reduced by silver nitrate and uniformly embedded on the surface of PDA. This obtained Gns@PDA-Ag core–shell nanoplatelets were filled into TPU to prepare a flexible dielectric material. The microstructure and dielectric performances were tested. Results indicated that the Gns@PDA-Ag core–shell nanoplatelets were evenly dispersed in TPU matrix. Moreover, the addition of Gns@PDA-Ag core–shell nanoplatelets could effectively improve the dielectric constant of the composite. Under low-frequency test conditions, the dielectric constant of the TPU/Gns@PDA-Ag (4 wt%) nanocomposite was 117.81, which was about 14 times higher than that of pure TPU. Theoretical analysis shows that the PDA shell not only promoted the uniform dispersion of Gns in TPU matrix, but also enhanced the interface bonding between the nanoplatelets and the TPU matrix. Moreover, introducing ultra-small Ag nanoparticles had effectively decreased the dielectric loss and conductivity of TPU/Gns@PDA-Ag composites due to its coulomb blockade and quantum trapped carrier structure effects, which resulted in the improvement of energy storage characteristics of composite.

Assessing Physiological Stress Responses to Word Retrieval in Individuals with Aphasia: A Preliminary Study

Objective: Word retrieval difficulty can be distressing for individuals with aphasia. However, physiological stress responses to word retrieval are relatively unstudied. This study aims to measure the physiological response (i.e., heart rate and respiratory rate) and perceived stress in individuals with aphasia during a naming task to understand whether word retrieval may be a stressor to them. Method: An aphasia group (n = 7) and a healthy group (n = 38) participated in 4 experimental conditions, including rest, counting, and high-frequency and low-frequency word conditions, while their physiological parameters were measured throughout the sessions. Stress ratings were also obtained for each condition. Results: Significant differences in heart rate were found while participants were engaged in speech and/or language tasks, compared to the rest condition, for both groups. Significant differences in respiratory rate were found between the rest and low-frequency word conditions in the aphasia group. No significant difference was found in heart rate between 2 groups, but a difference in respiratory rate was found between 2 groups in the low-frequency word condition. There were some discrepancies between stress level and physiological parameters. Conclusions: Individuals with aphasia perceived greater stress in the naming tasks, however, the perceived stress was not necessarily parallel to the physiological parameters.

Phased Array Detection Parameters for Electron Beam Welds in CFETR Vacuum Vessel Port Stubs

The China Fusion Engineering Testing Reactor (CFETR) vacuum vessel (VV) port stub was formed by electron beam welding (EBW). In this article, the phased array ultrasonic testing (PAUT) method was introduced for testing the quality of the weld. First, the acoustic field simulation and analysis of the EBW of 316L austenitic stainless steel were carried out with different frequency probes, then the microstructure of electron beam welds were observed by microscopic metallographic to analyze the influence of the type and influence of probe, finally fabricating the reference test blocks according to the NB/T47013-2015 standard to conducting the PAUT experiment. The test results show that the type of probe has less influence on the signal-to-noise ratio (SNR) and localization of the detection of large-thickness austenitic stainless steel electron beam welds under low-frequency conditions. The SNR of EBW detection was gradually decreased as the frequency of the probe increases under the same type of probe. In the direction of depth of the EBW, with the increase in the depth, the weld microstructure size becomes finer, so the sector scan image of $\phi ~1$ -mm side drilled hole (SDH) near the upper weld was severely distorted, while the sector scan image of $\phi ~1$ -mm SDH near the lower EBW was relatively good.

Hybrid Modular Multilevel Converter With Arm-Interchange Concept for Zero-/Low- Frequency Operation of AC Drives

DC-AC Modular Multilevel Converter (MMC) is a promising candidate for high-power AC drive applications. The main challenge of operating the MMC in AC drive applications is its performance during low-frequency conditions, as high voltage ripple is associated with the low-frequency operation, which results in voltage stresses on the involved semiconductor devices. To keep capacitor voltages balanced and bounded in low-frequency operation, different techniques have been proposed in literature, which can be classified into software and hardware approaches. In this paper, a hybrid MMC with arm-interchange concept is proposed to ensure operating successfully during low-frequency conditions. The hybrid MMC consists of two-stages. The first stage is a front-end Integrated Gate Commutated Thyristor (IGCT)-based H-bridge converter, while the second stage is a Full-Bridge Sub-Module (FBSM)-based MMC, where FBSMs is able to generate positive, zero, and negative voltage states. Detailed illustration and design of the hybrid MMC are introduced. Simulation results of the proposed converter are presented to show the effectiveness during low-frequency and normal frequency conditions in AC drive applications. Finally, a scaled-down prototype is employed for experimental validation.

Low-frequency resonant vibration energy harvester using piezoelectric stacks with force magnification frames

The piezoelectric stack energy harvester (PSEH) is often used to transform ambient large amplitude vibrational energy, such as road vibrations and pressure fluctuations, because of its ability to withstand large external excitation force. However, due to the high natural frequency of the piezoelectric stack, it is difficult to generate resonance phenomenon, which leads to low energy efficiency for the energy harvester. In this paper, a piezoelectric stack energy harvester with force magnification frame (FMF) is proposed and studied for energy harvesting from low-frequency pressure ripples. First, a force magnification frame is designed to enhance the pressure transferring ability. The FMF is able to, not only amplify the exciting force acting on the piezoelectric stack but also decrease the natural frequency of the PSEH. Based on finite element method (FEM) analysis, the natural frequency of the PSEH with FMF is obtained. The natural frequency of the PSEH is successfully reduced, and the open-circuit voltage output of the PSEH under low-frequency resonance conditions is much higher than for off-resonance conditions. The results validate that the PSEH with FMF is efficient for lower frequency vibration environments.

Analysis on Absorption Sound Acoustic Panels from Egg Tray with Corn Husk and Sugar Cane

In the contemporary world, one of the areas that have gained growing attention entails sound control. The motivation has been to respond to the increasing demand to improve sound waves in various contexts. One of the sound absorbing agents entails natural fiber, a platform that has led to the growing development of sound absorbers. Hence, this study’s central purpose is to examine the potentiality of sound absorber panels developed from materials such as egg trays with corn husks, egg trays with sugar cane, and egg trays combined. For the selected materials, two tests that are implemented include reverberation room test towards achieving noise reduction coefficients (NRCs) and the impedance tube test – towards achieving the sound absorption coefficients (SACs). For the impedance experiment, findings indicate that the frequency of the absorption values ranges from 0 Hz to 1000 Hz. For the respective forms of the acoustic panels, the SAC values are seen to be 0.9600, 0.9700, and 0.8900; findings that are achieved under low-frequency conditions. In relation to the case of NRC, the values obtained for the selected panels include 0.5500, 0.5900, and 0.5000. Indeed, the absorption aspect is seen to be pronounced when the experimental platform entails a combination of egg trays with sugar cane stacks. Regarding the results gained from the case of the reverberation room test, the echo time is likely to be shortened by the acoustic panel. Whereas the maximum time for the egg tray panel is observed to be 4.800 seconds, the case of an empty room depicts a maximum of 6.280 seconds. For the egg tray panels with corn skins and sugar cane, the results point to the maximum times of 3.990 seconds and 4.210 seconds respectively. Hence, it is concluded that the egg tray panel with corn husks depicts lower NRC values. Overall, it is evident that the three materials exhibit a promising state of eco-friendliness, complemented by the affirmation that in the market, they are easily available