Strong Oscillations Research Articles

Characterising global ocean mesoscale eddy by AVISO and Haiyang-2 altimeter

ABSTRACT This work compares the Haiyang-2 (HY-2) altimeter data with the well-recognized satellite oceanographic data (AVISO) product in the global ocean in their capability of detecting mesoscale eddy. A well-established method to detect mesoscale eddies from HY-2 and AVISO was applied to the sea level anomaly (SLA) data on a global scale in 2021−2022. It is found that the number of mesoscale eddies derived from the HY-2 dataset is three times more than that from the AVISO dataset and with larger eddy kinetic energy (EKE), shorter lifespan and smaller amplitude. Besides, the monthly-averaged number of generated and terminated mesoscale eddies is similar and the temporal variation of SLA exhibits strong lunar oscillation for both datasets. Most mesoscale eddies are generated in the mid-latitude oceans due to the Coriolis force and the strong El Niño. In the western boundary of the global ocean, equator, Antarctic Ocean and other areas with strong currents, the mesoscale eddies have stronger SLA amplitudes, EKEs and vorticity. Furthermore, EKE is low at the centre and high at the eddy meander. The spatial distribution characteristics of vorticity are the same as those of EKEs in the western boundary of oceans, but the vorticity value of HY-2 data near the equator is larger than that of AVISO data. However, considering the large detection error of eddy near the equator, no further exploration will be conducted in this paper. Furthermore, mesoscale eddies tend to move in the east–west direction due to Coriolis force and wind (Ekman transport) effects and large-scale ocean circulation or the pathway of drifters also have a certain impact. Overall, similar to AVISO data, the HY-2 data is also reliable for mesoscale identification in global oceans.

Development of a Benchmark for Thermal Striping in Gas-Cooled Fast Reactors

This paper presents the development of a benchmark for predicting thermal striping through simulation. This work utilized large eddy simulation and will be used to benchmark future models. The testing domain was created using both the STRUCT and the Reynolds-averaged Navier-Stokes turbulence models and is based on an earlier design of the General Atomics Fast Modular Reactor upper plenum. The plenum features two adjacent, identical hexagonal bundles each with a center-placed axial rod drive, with a hot left coolant stream and a cold right coolant stream. The simulation solves the nondimensional Navier-Stokes equations, with temperature accounted as a passive scalar. First- and second-order flow statistics were obtained after 600 convective time units of averaging. The first-order statistics reveal that the hot jet is damped by a recirculatory flow from the near wall. At the same location, the second-order statistics show strong oscillations both in velocity and temperature. The power spectral density was utilized to determine that a low-frequency oscillation occurs here that is within the range of interest for thermal striping. Furthermore, proper orthogonal decomposition was used to identify coherent structures that confirm the oscillatory behavior, indicative of thermal striping. Overall, this benchmark can aid in the development of future models for predicting thermal striping in nuclear reactors, potentially leading to improved reactor safety and performance.

Pushing thermal conductivity to its lower limit in crystals with simple structures

Materials with low thermal conductivity usually have complex crystal structures. Herein we experimentally find that a simple crystal structure material AgTlI2 (I4/mcm) owns an extremely low thermal conductivity of 0.25 W/mK at room temperature. To understand this anomaly, we perform in-depth theoretical studies based on ab initio molecular dynamics simulations and anharmonic lattice dynamics. We find that the unique atomic arrangement and weak chemical bonding provide a permissive environment for strong oscillations of Ag atoms, leading to a considerable rattling behaviour and giant lattice anharmonicity. This feature is also verified by the experimental probability density function refinement of single-crystal diffraction. The particularly strong anharmonicity breaks down the conventional phonon gas model, giving rise to non-negligible wavelike phonon behaviours in AgTlI2 at 300 K. Intriguingly, unlike many strongly anharmonic materials where a small propagative thermal conductivity is often accompanied by a large diffusive thermal conductivity, we find an unusual coexistence of ultralow propagative and diffusive thermal conductivities in AgTlI2 based on the thermal transport unified theory. This study underscores the potential of simple crystal structures in achieving low thermal conductivity and encourages further experimental research to enrich the family of materials with ultralow thermal conductivity.

Improved accuracy and robustness of electron density profiles from JET's X-mode frequency-modulated continuous-wave reflectometers.

JET's frequency-modulated continuous wave (FMCW) reflectometers have been operating well with the current design since 2005, and density profiles have been automatically calculated intershot since then. However, the calculated profiles had long suffered from several shortcomings: poor agreement with other diagnostics, sometimes inappropriately moving radially by several centimeters, elevated levels of radial jitter, and persistent wriggles (strong unphysical oscillations). In this research, several techniques are applied to the reflectometry data analysis, and the shortcomings are significantly improved. Starting with improving the equilibrium reconstruction that estimates the background magnetic field, adding a ripple correction in the reconstructed magnetic field profile, and adding new inner-wall reflection positions estimated through ray-tracing, these changes not only improve the agreement of reconstructed profiles to other diagnostics but also solve density profile wriggles that were present during band transitions. Other smaller but also persistent wriggles were also suppressed by applying a localized correction to the measured beat frequency where persistent oscillations are present. Finally, the burst analysis method, as introduced by Varela etal. [Nucl. Fusion 46 S693 (2006)], has been implemented to extract the beat frequency from stacked spectrograms. Due to the strong suppression of spurious reflections, the radial jitter that sometimes would span several centimeters has been strongly reduced. The stacking of spectrograms has also been shown to be very useful for stacking recurring events, like small gas puff modulations, and extracting transport coefficients that would otherwise be below the noise level.

Walking modulates visual detection performance according to stride cycle phase

Walking is among our most frequent and natural of voluntary behaviours, yet the consequences of locomotion upon perceptual and cognitive function remain largely unknown. Recent work has highlighted that although walking feels smooth and continuous, critical phases exist within each step for the successful coordination of perceptual and motor function. Here, we test whether these phasic demands impact upon visual perception, by assessing performance in a visual detection task during natural unencumbered walking. We finely sample visual performance over the stride cycle as participants walk along a smooth linear path at a comfortable speed in a wireless virtual reality environment. At the group-level, accuracy, reaction times, and response likelihood show strong oscillations, modulating at approximately 2 cycles per stride (~2 Hz) with a marked phase of optimal performance aligned with the swing phase of each step. At the participant level, Bayesian inference of population prevalence reveals highly prevalent oscillations in visual detection performance that cluster in two idiosyncratic frequency ranges (2 or 4 cycles per stride), with a strong phase alignment across participants.

Across-wind response characteristics of tall-square towers in urban flow: An experimental study focused on the aeroelastic effects

To fully use the land resource in the urban regions of metropolises, high-rise buildings of large aspect ratio maybe preferred for the new constructions. To systematically investigate the across-wind responses of these quite slender structures, the urban boundary layer flow of high turbulence intensity was simulated in the wind tunnel, and aeroelastic tests realized by the pivot model setup were carried out for the square cross section towers of various heights resulting in large aspect ratios (λ = 12, 16, 20). No appreciable aeroelastic behaviors have been observed for the λ = 12 tower even for a very low damping ratio (0.57%). This is in line with the many previous studies for the tall-square tower with an aspect ratio not higher than 10, likely due to the very high turbulence intensity in the urban terrain, mitigating the occurrence of possible aeroelastic behaviors. In contrast, the λ = 16 and (especially) the λ = 20 towers exhibit strong oscillations known as the across-wind galloping, evidenced by the monotonic increase in the amplitude–velocity curve and the clear characteristics of simple-harmonic oscillations. Moreover, it is observed that for a certain range of low damping ratios, these large oscillations were initiated at the same wind speed very close to the vortex-resonance wind speed. The interaction mechanism between vortex induced vibration and galloping is thus supposed responsible for this observation (even in urban flow). Finally, peak across-wind responses at various damping ratios were provided as well, serving as a useful reference for the future design of similar tall-square towers.

A fully implicit edge/face centered discontinuous Galerkin / mixed finite element scheme for the advection-dispersion equation

Mixed Finite Element (MFE) method is a robust numerical technique for solving elliptic and parabolic partial differential equations (PDEs). However, MFE can generate solutions with strong unphysical oscillations and/or large numerical diffusion for hyperbolic type PDEs. For its part, Discontinuous Galerkin (DG) finite element method is well adapted to solve hyperbolic systems and can accurately reproduce solutions involving sharp fronts. Therefore, the combination of DG and MFE is a good strategy for solving hyperbolic/parabolic problems such as advection – diffusion/dispersion equations. The classical formulation of the two methods is based on operator and time splitting allowing for separate solutions to advection with an explicit scheme and to dispersion with an implicit scheme. However, this kind of approach has the following drawbacks: (i) it lacks efficiency, as two systems with different unknowns are solved at each time step, (ii) it induces errors generated by the splitting, (iii) it can be CPU wise-expensive because of the CFL constraint, and (iv) it cannot be employed for steady-state transport simulations.To overcome these difficulties, we develop in this work a fully implicit edge/face centered DG-MFE formulation where the two methods share the same unknowns. In this formulation, the DG method is developed on lumping regions associated with the mesh edges/faces instead of mesh elements. Thus, the traces of concentration at mesh edges/faces, which are the Degrees Of Freedom (DOF) of the hybrid-MFE, are also part of the DOFs of the DG. The temporal discretization is based on the Crank–Nicolson method for both advection and dispersion. Numerical tests are performed to validate the new scheme by comparison against an analytical solution and to show its ability to handle steady-state transport simulations.The procedure is developed for 2D triangular meshes but can easily be extended to other 2D and 3D shape elements.

Synchrotron X-ray phase-contrast imaging of ultrasonic drop atomization

Ultrasonic atomization is employed to generate size-controllable droplets for a variety of applications. Here, we minimize the number of parameters dictating the process by studying the atomization of a single drop pending from an ultrasonic horn. Spatiotemporally resolved X-ray phase-contrast imaging measurements show that the number-median sizes of the ejected droplets can be predicted by the linear Navier–Stokes equations, signifying that the size distribution is controlled by the fluid properties and the driving frequency. Experiments with larger pendant water drops indicate that the fluid–structure interaction plays a pivotal role in determining the ejection onset of the pendant drop. The atomization of viscoelastic drops is dictated by extended ligament formation, entrainment of air, and ejection of drop-encapsulated bubbles. Existing scaling laws are used to explain the required higher input amplitudes for the complete atomization of viscoelastic drops as compared to inviscid drops. Finally, we elucidate the differences between capillary wave-based and cavitation-based atomization and show that inducing cavitation and strong bubble oscillations quickens the onset of daughter drop ejection but impedes their size control.

The impact of the Madden-Julian oscillation on spring and autumn afternoon diurnal convection in Sri Lanka

This study examines the impact of strong Madden-Julian Oscillation (MJO) phases (P1–P8) on diurnal rainfall patterns focusing on Afternoon Diurnal Convection (ADC) events in Sri Lanka during 2001–2020 spring and autumn. Daily mean rainfall increases (decreases) during the P2-to-P3 (P6-to-P7) MJO phases in both seasons, while the diurnal rainfall amplitude peaks during the P2-to-P3 (P8-to-P1) MJO phases in spring (autumn). ADC events also occur more frequently and intensely during MJO P2-to-P3 (P8-to-P1) in spring (autumn). The MJO’s modulation of diurnal rainfall amplitude and ADC events is more apparent in autumn than in spring. Active MJO phases enhance the westward propagation of diurnal rainfall associated with ADC events, sustained by moisture flux convergence and enhanced upward motion. The prevailing mid-to-upper level easterly wind, combined with deep convection over Sri Lanka, contributes to a more pronounced westward propagation during the P2-to-P3 (P8-to-P1) phases for ADC events in spring (autumn).

Seasonal and Interannual Variations of Global Tides in the Mesosphere and Lower Thermosphere Neutral Winds: I. Diurnal Tides

AbstractIn the mesosphere and lower thermosphere, diurnal tides are responsible for the dynamics and structures at low latitudes since they have largest amplitudes there. Based on the 20‐year (2002−2021) observations from Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler interferometer (TIDI), we investigate the seasonal variations of three diurnal tidal components (DW1, DE3, DW2), and their responses to stratospheric quasi‐biennial oscillation (SQBO) and solar cycle globally. The results show that: (a) DW1, DE3 and DW2 show prominent semiannual (SAO) and annual oscillations (AO) in their peak regions where the ratio of their annual mean amplitude to their maximum annual mean amplitude is larger than 0.8. DW1 also exhibit strong terannual oscillations (TAO) especially in meridional wind. (b) The responses of the amplitudes of seasonal variations of DW1, DE3 and DW2 to SQBO and solar cycle are comparable in magnitude to those of their annual mean amplitudes (for response to solar cycle, even stronger in some cases), and thus cannot be neglected. (c) In their respective peak regions, the responses of annual mean amplitudes of these diurnal tidal components to SQBO are uniformly positive except DW2, and their responses to solar cycle are uniformly negative. For these diurnal tides, the amplitudes of the dominant seasonal variations exhibit consistent response patterns with those of annual means to the SQBO/solar cycle. (d) Empirical formulas are given, which well describe the seasonal and interannual variations of dominant diurnal tidal components in their peak regions.

Disentangling the Unprecedented Yangtze River Basin Extreme High Temperatures in Summer 2022: Combined Impacts of the Reintensified La Niña and Strong Positive NAO

Abstract During July and August of 2022, the Yangtze River Basin (YRB) experienced its most extreme high temperature (EHT) event since 1979, resulting in large numbers of human casualties and severe economic losses. This paper reveals the spatial and temporal features of the EHT over the YRB (YRB-EHT) in 2022 and disentangles its extreme nature from a historical perspective. Results showed the following: 1) The record-breaking YRB-EHT was directly caused by the adiabatic heating associated with an anomalous barotropic high pressure (or heat dome) and descending motion in the region. The intensified and westward-shifted western North Pacific subtropical high and eastward-extended South Asian high played critical roles in the formation of the heat dome and descending motion anomaly. 2) Convection anomalies over the tropical Atlantic and Pacific induced by the reintensified La Niña–like Pacific sea surface temperature anomaly pattern, along with the strong positive North Atlantic Oscillation (NAO), were the key contributing factors to the formation of the barotropic high pressure anomaly and YRB-EHT. 3) A physics-based empirical simulation model constructed using the factors of the NAO and tropical convection successfully reproduced the historical year-to-year variation of YRB temperatures, as well as the extreme in 2022, implying that the unprecedented 2022 YRB-EHT had universal dynamic origins. This study highlights the importance of the combined impacts of tropical and extratropical forcings in the record-breaking YRB-EHT in 2022, and thus may provide useful clues for seasonal predictions of summer mean or extreme temperatures in the YRB. Significance Statement Extreme high temperatures hit the Yangtze River Basin (YRB) in summer 2022, causing large numbers of human casualties and severe economic losses. Here, from a historical perspective, we disentangle the dynamic origins of these high temperatures over the YRB. Based on evidence from both observations and model simulations, the present study highlights the importance of the combined impacts of tropical and extratropical routes of influence on these extremely high temperatures in the YRB, and thus may provide useful clues for seasonal predictions of YRB summer mean or extreme temperatures.

Vibration Analysis and Motion Control Method for an Under-Actuated Tower Crane

In this paper, we developed a solution for controlling a tower crane thought as a no-rigid system, and therefore able to have deformation and, during the motion, vibrations. Particularly, large tower cranes show high structural dynamics. Under external excitations, the payload tends to sway around its vertical position and this motion is coupled to the resulting dynamic vibration of the crane structure. These induced vibrations may cause instability and serious damage to the crane system. Furthermore, the energy stored in the flexible structure of a tower crane causes vibrations in the structure during the acceleration and deceleration of slewing movements. A crane operator perceives these vibrations as an unstable speed of the boom. Such behavior involves the control of the crane, particularly precise positioning and manual control of the crane movement at low pivoting speed. We define an Elastic model of the Slewing crane and analyze the bending and Torsional elasticity of the Tower, and the Jib Elasticity. With an approximated method, we calculate the natural wavelengths of the crane structure in the slewing direction. We consider the tower crane as a nonlinear under-actuated system. The motion equations are obtained considering both the normal vibration modes of the tower crane and the sway of the payload. An elastic model of the Slewing crane is achieved, modeling the crane jib as an Euler-Bernoulli beam. Even the payload dynamic is considered, developing an Anti-sway solution by the equation of the movement. We define an iterative calculation of the sway angles and obtain the corresponding velocity profiles, implementing two kinds of solution: an input-shaping control in open-loop, to be used with automatic positioning, and a “command smoothing” method in open-loop, used for reducing the sway of the payload with the operator control. These solutions lead to a reduction of the vibrations of the crane structure. As a consequence, the tower crane is not subject to the strong horizontal and vertical oscillations during the motion of the elastic structure.

Disc corona radii and QPO frequencies in black hole binaries: testing Lense– Thirring precession origin

ABSTRACT Stellar-mass black hole binary systems in the luminous X-ray states show a strong quasi-periodic oscillation (QPO) in their Comptonized emission. The frequency of this feature correlates with the ratio of a disc to Comptonized emission rather than with total luminosity. Hence, it changes dramatically during spectral transitions between the hard and soft states. Its amplitude is also strongest in these intermediate states, making them an important test of QPO models. However, these have complex spectra which generally require a disc and two separate Comptonization components, making it difficult to uniquely derive the spectral parameters. We build a new energy-conserving model of the accretion flow, SSsed model, which assumes a fixed radial emissivity but with a changing emission mechanism. This is similar to the agnsed model in xspec but tuned to be more suitable for stellar mass black holes. It uses a combination of the disc luminosity and temperature to constrain the inner radius of the (colour temperature corrected) blackbody disc, separating this from the more complex Comptonization spectra emitted inwards of this radius. We show a pilot study of this model fit to hundreds of RXTE spectra of the black hole binary XTE J1550 − 564. We show that the derived disc radius tightly anticorrelates with the central frequencies of the low-frequency QPO detected in the same observations. The relation is consistent with the quantitative predictions of Lense–Thirring precession of the entire inner Comptonization regions for the assumed system parameters. This supports the scenario that low-frequency QPOs are caused by Lense–Thirring precession.

Anomalous Hall effect in Nd-doped Bi1.1Sb0.9STe2 topological insulator single crystals.

Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantum anomalous Hall effect, which has been observed in magnetic-topological-insulator-based devices. In this work, we report successful doping of rare-earth element Nd into Bi1.1Sb0.9STe2 bulk-insulating topological insulator single crystals, in which the Nd moments are ferromagnetically ordered at ∼100 K. Benefiting from the in-bulk-gap Fermi level, electronic transport behaviors dominated by the topological surface states are observed in the ferromagnetic region. At low temperatures, strong Shubnikov-de Haas oscillations with a nontrivial Berry phase are observed. The topological insulator with long range magnetic ordering in Nd-doped Bi1.1Sb0.9STe2 single crystals provides a good platform for quantum transport studies and spintronic applications.

Hybrids as mirrors of the past: genomic footprints reveal spatio-temporal dynamics and extinction risk of alpine extremophytes in the mountains of Central Asia.

Hybridization is one of the key processes shaping lineage diversification, particularly in regions that experienced strong climate oscillations. The alpine biome with its rich history of glacial-interglacial cycles and complex patterns of species distribution shifts offers an excellent system to investigate the impact of gene flow on population dynamics and speciation, important issues for evolutionary biology and biodiversity conservation. In this study, we combined genomic data (DArTseq), chloroplast markers, and morphology to examine phylogenetic relationships and the permeability of species boundaries and their evolutionary outcomes among the alpine extremophilic species of Puccinellia (Poaceae) in the Pamir Mountains, a part of the Mountains of Central Asia biodiversity hotspot. We determined the occurrence of interspecific hybrids between P. himalaica and P. pamirica, which demonstrated almost symmetric ancestry from their parental species and did not show signals of introgression. According to our integrative revision, the natural hybrids between P. himalaica and P. pamirica should be classified as Puccinellia ×vachanica (pro species). Using approximate Bayesian computation for population history inference, we uncovered that P. himalaica hybridized with P. pamirica independently in multiple localities over the Holocene. Hybrids inherited the fine-scale genetic structure from their parental species, which developed these patterns earlier, during the Late Pleistocene. Hybridization had different consequences for the involved parental lineages, likely playing an important role in a continuing decline of P. himalaica in the Pamir Mountains over the Holocene. Our results show that P. himalaica should be considered a critically endangered species in the Pamir Mountains and could also be retreating across its entire range of distribution in High Mountain Asia. Using a comparative phylogeographic framework, we revealed the risk of extinction of a cold-adapted alpine species in a global biodiversity hotspot. This study highlights that genomics could unravel diversity trends under climate change and provides valuable evidence for conservation management.

Mixed quantum/classical calculations of rotationally inelastic scattering in the CO + CO system: a comparison with fully quantum results.

An updated version of the CO + CO potential energy surface from [R. Dawes, X. G. Wang and T. Carrington, J. Phys. Chem. A 2013, 117, 7612] is presented, that incorporates an improved treatment of the asymptotic behavior. It is found that this new surface is only slightly different from the other popular PES available for this system in the literature [G. W. M. Vissers, P. E. S. Wormer and A. Van Der Avoird, Phys. Chem. Chem. Phys. 2003, 5, 4767]. The differences are quantified by expanding both surfaces over a set of analytic functions and comparing the behavior of expansion coefficients along the molecule-molecule distance R. It is shown that all expansion coefficients behave similarly, except in the very high energy range at small R where the PES is repulsive. That difference has no effect on low collision-energy dynamics, which is explored via inelastic scattering calculations carried out using the MQCT program which implements the mixed quantum/classical theory for molecular energy exchange processes. The validity of MQCT predictions of state-to-state transition cross sections for CO + CO is also tested by comparison against full-quantum coupled-states calculations. In all cases MQCT gives reliable results, except at very low collision energy where the full-quantum calculations predict strong oscillations of state-to-state transition cross sections due to resonances. For strong transitions with large cross sections, the results of MQCT are reliable, especially at higher collision energy. For weaker transitions, and lower collision energies, the cross sections predicted by MQCT may be up to a factor of 2-3 different from those obtained by full-quantum calculations.

Characterizing the Water Storage Variation of Kusai Lake by Constructing Time Series from Multisource Remote Sensing Data

In September 2011, Zhuonai Lake (ZL) in the northeast of Hoh Xil (HX) on the Qinghai–Tibet Plateau (QTP) broke out. The outburst event seriously changed the environmental hydraulics in this region. Due to the insufficient temporal resolution of observations, it is challenging to assess the impact of this event on short-period variations of water volumes in three lakes downstream of ZL. Combining multisource remote sensing data, we constructed long and high-temporal-resolution time series for the lake level, area, and lake water storage (LWS) of Kusai Lake (KL) to characterize the variabilities before and after the outburst. The water level, area, and LWS time series contain 1051 samples from 1990 to 2022, with uncertainties of 0.16 m, 2.5 km2, and 0.016 km3, respectively. The accuracies verified using the Database for Hydrological Time Series of Inland Waters (DAHITI) are 0.26 m, 2.64 km2, and 0.08 km3 for water level, area, and LWS, respectively. We characterized the LWS variations during the past 30 years based on the high temporal resolution LWS time series. Before the outburst, the 1-year and 3.5-year variations dominated the LWS time series, and there was no obvious semi-annual signal. After the outburst, the 3.5-year variation disappeared, and a strong semi-annual oscillation was observed. From 2012 to 2015, the periodic LWS variations in KL were disturbed by the ZL outburst and the subsequent outflow of KL led by the outburst. Regular cyclic signals have been restored since 2016, with an amplified annual fluctuation. By analysis, precipitation, evaporation, and glacier area change are excluded as driving factors of the pattern change in LWS variations of KL. It can be concluded that the altered recharge pattern of KL triggered by the outburst directly resulted in the observed changes in TWS behavior. For the first time, we identified the periodic patterns of LWS variations of KL during the past 30 years and revealed that the ZL outburst event significantly influenced these patterns. This finding contributes to the comprehensive understanding of the effects of the ZL outburst on downstream lake dynamics. Furthermore, the presented procedure for constructing long and high-resolution time series of LWS allows for monitoring and characterizing the short-period variabilities of Tibetan lakes that lack hydrological data.

Fuel-Cell Performance and Stability during Liquid-Water Removal Cycles

Water management is crucial for achieving high-performance proton-exchange-membrane fuel cells (PEMFCs), as it helps keep the electrolyte hydrated while avoiding performance degradation and cell shutdown due to liquid-water accumulation. According to ex-situ measurements [1-3], accumulation of liquid water in gas-diffusion layers (GDLs) of PEMFCs is a transient process that can be accompanied with oscillations in capillary pressure and saturation. To understand how liquid-water accumulation and drainage impact PEMFC performance hysteresis and stability, a transient cell-level model that accounts for electrode structure and composition and is computationally efficient is needed. A number of volume-averaged models that describe the electrode structure through pore-size distribution have been developed in the past [4-7], but they are steady-state and thus cannot predict dynamic PEMFC performance. Existing transient models have also not been used to analyze cyclic liquid-water accumulation [8-10].In this work, a transient two-phase 2D PEMFC model is developed in the open-source fuel-cell modeling software OpenFCST [11] and applied to analyze PEMFC performance hysteresis and stability during liquid-water accumulation and drainage cycles. The model accounts for the electrode structure through a mixed-wettability pore-size-distribution framework and incorporates a novel dynamic boundary condition to describe the experimentally observed cyclic liquid-water accumulation. Results of the numerical simulations are compared to transient current-density and resistance data at two polarization scan rates and during voltage steps measured with an in-house single-channel cell at multiple operating conditions. This work demonstrates how high breakthrough pressure and rapid liquid-water removal from GDLs may cause highly unstable fuel-cell operation with strong hysteresis and oscillations in the polarization curve (such as those in the figure below) that closely resemble experimental reports [12]. Numerical simulations also reveal the existence of a scan rate that maximizes polarization hysteresis due to a match between the time scale of GDL flooding and of a complete voltage sweep. Fast-scan polarization sweeps are shown most suitable for detecting catalyst-layer flooding that depends on its wettability and occurs within single seconds in contrast to GDL flooding that takes hundreds of seconds. Overall, this work brings more attention to the transient analysis of fuel-cell performance under wet conditions.Figure: Polarization-curve oscillations caused by cyclic liquid-water removal from the cathode GDL. Similar fluctuations have been experimentally observed in [12]. Transient graphs show the dynamics of current density and cathode GDL saturation. Operating conditions are 60 °C, 90% RH, 1.5 atm; scan rate is 0.5 mV/s. References J. T. Gostick et al., J. Electrochem. Soc. 157.4 (2010) C. Quesnel et al., J. Phys. Chem. C 119.40 (2015) D. Ziegler, B.Sc. thesis, Hochschule Mannheim / University of Alberta (2020) A. Z. Weber et al., J. Electrochem. Soc. 151.10 (2004) M. Eikerling, J. Electrochem. Soc. 153.3 (2006) V. Mulone and K. Karan, Int. J. Hydrog. Energy 38.1 (2013) Zhou et al., J. Electrochem. Soc. 164.6 (2017) R. J. Balliet and J. Newman, J. Electrochem. Soc. 158.8 (2011) I. V. Zenyuk et al., J. Electrochem. Soc. 163.7 (2016) A. Goshtasbi et al., J. Electrochem. Soc. 166.7 (2019) M. Secanell et al. ECS Transactions 64.3 (2014) C. Ziegler and D. Gerteisen, J. Power Sources 188.1 (2009) Figure 1

Simultaneous observation on the post-sunset occurrence of travelling ionospheric disturbance alongside ionospheric irregularities over the East Africa low latitude region

Using a data set from the network of more than 30 GNSS receivers within the East Africa sector; a study on the transient wave-like total electron content (TEC) modulation alongside the post-sunset irregularities has been presented. The study period is during the years 2019 to 2021 and the year 2014. The TEC modulation was detected from the quasi-periodic variation of detrended TEC and occurs nearly on a day-to-day basis at post-sunset hours just like the post-sunset ionospheric irregularities. The wavelike TEC modulation is likely due to gravity waves generated at the lower atmosphere that propagates into the F region and is quantified as travelling ionospheric disturbance (TIDs). The dominant quasi-periods of these TIDs are between 10 and 50 min. The TIDS, just like the irregularities, were confined in the latitudinal range of 0–9.4oS magnetic latitudes within the East Africa sector. A statistical analysis of simultaneous observations of the post-sunset TIDs shows that the day-to-day variations are identical to that of the ionospheric irregularity occurrence rate. The TIDs exhibit a relatively strong wave oscillation with multiple wavelengths preceding/accompanying the occurrence of irregularities. The simultaneous observation of TIDs and irregularities shows that the wavelike seeding perturbation associated with gravity waves from the lower atmosphere combined with strong seeding forces (unknown source) could facilitate the observed growth rate of the Rayleigh Taylor Instability mechanism that is associated with the formation of ionospheric irregularities. Additional experiments and tropospheric observation may be required to ascertain the source of gravity waves that are linked to TEC perturbation during nighttime hours.

Pulsational Velocity Variations of the Radial Mode sdBV Star BPM 36430

Hot subdwarf B stars are core-helium burning objects that were once red giants stripped of their outer H envelopes due to binary interactions. Many exhibit pulsations that can be used to determine fundamental stellar parameters like mass and radius. High-cadence TESS photometry of the sdB star BPM 36430 revealed it to be a hybrid-mode pulsator showing several g-mode pulsations and a single strong p-mode oscillation. The latter is likely a radial mode oscillation, given its period (342 s) and high photometric amplitude (2%). Using time-series spectroscopy from the Goodman spectrograph on the 4.1 m SOAR telescope, we detect a sinusoidal radial velocity variation with a semi-amplitude of 4.4 ± 0.4 km s−1 and period consistent with photometry of the p-mode oscillation. This detection provides further evidence the 342 s mode is radial in nature, and that the Baade–Wesselink method can be used with additional observations to constrain the stellar mass and radius.