High Frequency Waves Research Articles

Real gas effects in sound wave propagation through two-phase systems

We study sound wave propagation through a two-phase system of gas with dispersed liquid droplets. The key element of the study is a combination of real gas effects, entering the model via a suitable equation of state, with steady and unsteady contributions to the drag force and heat transfer. This feature makes the model applicable for arbitrary pressures and temperatures. In the cases of low and high wave frequencies, ω, analytical solution is derived. At low ω, the model yields a generalization of the homogenous flow approximation to real gases. At high ω, the speed of sound tends to its value in the real gas in the absence of droplets while the attenuation coefficient diverges as ω. The model predicts the phenomenon of resonant attenuation demonstrated by the maximum of the growth rate of attenuation coefficient when ω is close to the eigen frequency of particle relaxation. In the absence of droplets (single-phase limit) for the gas satisfying the van der Waals equation of state, the model yields the corresponding states principle for the speed of sound: The ratio of the actual speed of sound to its ideal-gas value is the universal function of reduced density and temperature. This ratio demonstrates a nonmonotonous behavior of the speed of sound as a function of density reflecting the competition between the repulsive and attractive terms in the intermolecular interaction potential.

Uptake-leak balance of SR Ca2+ determines arrhythmogenic potential of RyR2R420Q+/− cardiomyocytes

Mutations of the RyR2 are channelopathies that can predispose to life threatening catecholaminergic polymorphic ventricular tachycardias (CPVTs) during exercise or stress. However, the cellular and molecular mechanisms that are causal for the arrhythmias downstream of the β-adrenergic receptor (β-AR) activation are not defined. They may be specific and different for each particular RyR2 mutation. Obvious possibilities are the phosphorylation of the mutated RyR2s or the stimulation of the SR Ca2+ pump (SERCA), which could increase SR Ca2+ loading. Potentially arrhythmogenic Ca2+ signals, such as Ca2+ waves, were recorded and analyzed from WT and RyR2R420Q+/− mouse cardiomyocytes with confocal microscopy after field stimulation at 1 Hz. In RyR2R420Q+/− cardiomyocytes we found a higher occurrence and frequency of Ca2+ waves, particularly upon β-AR stimulation with isoproterenol. This was accompanied by a shorter latency to the first spontaneous wave. Wave velocity from raw traces, as well as amplitude and decay time constant (τ) analyzed in de-skewed traces were comparable in both cell types. To obtain further insight into the role of the SERCA we selectively stimulated SERCA in permeabilized myocytes using Fab fragments of a PLB antibody (2D12). Surprisingly, SERCA stimulation alone resulted in considerably higher wave frequencies than when mimicking β-AR stimulation with cAMP, particularly in RyR2R420Q+/− cardiomyocytes. This may be a consequence of some protective SR Ca2+ unloading resulting from the SR Ca2+ leak via phosphorylated RyR2s in cAMP. Spark-to-spark recovery analysis suggested a remarkably higher Ca2+ release sensitivity in RyR2R420Q+/− cells, both in control and upon β-AR stimulation. Together these findings suggest that the fine balance between SR Ca2+ loading via SERCA and the Ca2+ leak via mutated and phosphorylated RyR2s is an important determinant for the overall cellular arrhythmogenicity prevailing in the RyR2R420Q+/− myocytes.

Suppression of Wind Waves in the Presence of Swell: A Physical Modeling Study

AbstractLaboratory measurements of the interaction between high‐frequency (HF) wind‐waves and low frequency (LF) paddle waves are presented. The measurements were made in a wind‐wave flume with wind and paddle waves propagating in the same direction. The primary objective is to examine the physical mechanisms proposed to explain the suppression of wind‐waves due to presence of swell (here paddle waves). For this purpose, the precise time scale of the temporal transition from wind‐only to wind‐plus‐paddle conditions is examined. The results reveal that the majority of wind‐wave suppression occurs too quickly to have been caused by reduced wind‐input, instead indicating that enhanced HF wave dissipation is the primary suppression mechanism. The spatial variation in HF wave energy along the LF wave phase indicates that HF suppression mainly occurs on the LF wave crest, and high on the windward face, which are the locations that experience the highest wind velocities. This observation also suggests that the reduced wind‐input is not the primary suppression mechanism. Quantification of HF wave suppression versus a broad range of wind velocities, paddle wave conditions and fetch did not reveal any critical dependence of suppression on LF wave breaking or wind separation at LF wave crests. It is concluded that suppression occurs primarily due to enhanced dissipation of HF waves near LF wave crests. This mechanism is proposed to be coupled to the wind velocity, where an increased wind velocity near LF wave crests exceeds the level of forcing that HF waves of low C/u* can withstand without breaking.

High-frequency Waves in Chromospheric Spicules

Using high-cadence observations from the Hydrogen-alpha Rapid Dynamics camera imaging system on the Dunn Solar Telescope, we present an investigation of the statistical properties of transverse oscillations in spicules captured above the solar limb. At five equally separated atmospheric heights, spanning approximately 4900–7500 km, we have detected a total of 15,959 individual wave events, with a mean displacement amplitude of 151 ± 124 km, a mean period of 54 ± 45 s, and a mean projected velocity amplitude of 21 ± 13 km s−1. We find that both the displacement and velocity amplitudes increase with height above the solar limb, ranging from 132 ± 111 km and 17.7 ± 10.6 km s−1 at ≈4900 km, and 168 ± 125 km and 26.3 ± 14.1 km s−1 at ≈7500 km, respectively. Following the examination of neighboring oscillations in time and space, we find 45% of the waves to be upwardly propagating, 49% to be downwardly propagating, and 6% to be standing, with mean absolute phase velocities for the propagating waves on the order of 75–150 km s−1. While the energy flux of the waves propagating downwards does not appear to depend on height, we find the energy flux of the upwardly propagating waves decreases with atmospheric height at a rate of −13,200 ± 6500 W m−2/Mm. As a result, this decrease in energy flux as the waves propagate upwards may provide significant thermal input into the local plasma.

Three-dimensional (3D) semi-analytical solution of wave-induced fluid resonance in narrow gaps of caisson-type breakwaters

This study presents a 3D semi-analytical solution for wave-induced fluid resonance in narrow gaps of caisson-type breakwaters. The energy dissipation near the breakwater is modelled by introducing a quadratic pressure loss condition on the gap entrance and exit. By introducing this treatment of nonlinear boundary, the pressure loss due to flow separation induced by the sudden contraction and expansion of a fluid domain can be accounted. The present semi-analytical solution is validated by previous analytical results. The dimensionless energy loss coefficient is calibrated with available experimental data and is found to be insensitive to the incident wave angle θ0 (θ0 ≤ π/12) and wave frequency. The analytical predictions are in good agreement with experimental data. With calibrated coefficient, the effects of wave and geometrical parameters on hydrodynamic quantities are investigated. The results show that the gap resonance leads to large wave energy dissipation and wave force, and the wave resonant frequencies are insensitive with dimensionless gap width l/a ranging from 26.6 to 56.6. The abrupt changes of wave reflection and transmission coefficients at high wave frequency are mainly due to the occurrence of multiple reflection and transmission waves and the wave frequency at the occurrence of multiple waves is always larger than resonant wave frequency.

Modeling waves over the Changjiang River Estuary using a high-resolution unstructured SWAN model

A high resolution unstructured SWAN model has been implemented for the Changjiang River Estuary (CRE). Five different winds are adopted to assess their quality over the CRE, including ERA5, CFSv2, GFS, CCMP-NRT and a newly available high resolution (9 km) wind product from APRCP (Asia-Pacific Regional Coupled Prediction System). The performance of the five winds, together with four wind input source functions is evaluated by comparing with satellite altimeter observations and multiple in-situ observations. Systemic difference is presented in the four input source functions, with the JANSSEN and KOMEN packages outperformingWST in terms of significant wave height (Hs) over the CRE. The default ST6 has comparative skill as JANSSEN during fair weather conditions but tends to predict higher Hs during extreme weather events. All winds have the tendency to underestimate the altimeter observed Hs during fair weather conditions. Part of the negative bias attributes to the overestimation of altimeter observations, while the boundary conditions only play a minor role, given a relatively large model domain extending to about 135° E. The CFSv2/GFS wind has the best performance during fair weather conditions, but overestimates the extreme wave height during typhoon events. The ERA5, on the other hand, underestimates Hs during both fair and extreme weather conditions. In terms of extreme waves, CCMP-NRT generally agrees best with observations. The high resolution APRCP behaves significantly different from other wind forcings and shows poorer performance in wave simulation. But high resolution winds have the advantage in resolving the detail wind structure and show better performance in reproducing the high frequency wave fluctuations adjacent to the CRE. In addition, the deviation due to different wind forcings is more distinct than the different input source functions. Nevertheless, given a certain wind forcing, calibrations would further improve the model performance. This study provides a good start to build a hindcast and forecast system for the CRE region.

Two-dimensional simulations of solar-like models with artificially enhanced luminosity

Artificially increasing the luminosity and the thermal diffusivity of a model is a common tactic adopted in hydrodynamical simulations of stellar convection. In this work, we analyse the impact of these artificial modifications on the physical properties of stellar interiors and specifically on internal gravity waves. We perform two-dimensional simulations of solar-like stars with the MUSIC code. We compare three models with different luminosity enhancement factors to a reference model. The results confirm that properties of the waves are impacted by the artificial enhancement of the luminosity and thermal diffusivity. We find that an increase in the stellar luminosity yields a decrease in the bulk convective turnover timescale and an increase in the characteristic frequency of excitation of the internal waves. We also show that a higher energy input in a model, corresponding to a larger luminosity, results in higher energy in high frequency waves. Across our tests with the luminosity and thermal diffusivity enhanced together by up to a factor of 104, our results are consistent with theoretical predictions of radiative damping. Increasing the luminosity also has an impact on the amplitude of oscillatory motions across the convective boundary. One must use caution when interpreting studies of internal gravity waves based on hydrodynamical simulations with artificially enhanced luminosity.

DYNAMICS OF HEMOREOLOGIC, BIOCHEMICAL AND IMMUNOLOGICAL INDICATORS IN PATIENTS WITH ACUTE HEMORRHOID THROMBOSIS UNDER THE INFLUENCE OF COMPLEX TREATMENT, INCLUDING PARENTERAL OZONE THERAPY

The purpose of this work was to determine the effectiveness of acute hemorrhoid thrombosis treatment using the "Surgitron" device in combination with intravenous parenteral ozone therapy. Patients aged from 26 to 67 years, who were divided into groups, agreed to participate in the study. The main group included 34 (53.1%) patients who underwent treatment according to the generally accepted method with "Surgitron" device in combination with intravenous parenteral ozone therapy. The second group included 30 (46.8%) patients who underwent thrombectomy. To analyze the results, data on the duration of the pain syndrome, immunological changes and coagulogram parameters were used. Most pathological conditions are accompanied by inflammation, hypoxia, collateral thrombosis, activation of damage processes by lipid peroxidation products, disturbed cellular and humoral immunity, decreased local immune protection, increased incidence of inflammatory complications, changes in the hemostasis system, since damage to the blood vessel wall causes immediate vasoconstriction of both the damaged vessel itself and adjacent capillaries and arterioles, which leads to blood flow slowdown in the area of damage to the external and internal hemorrhoidal plexus. Under normal conditions, endothelial cells are responsible for the antithrombotic interaction between blood and tissues, maintaining the liquid state of the blood. They produce such anticoagulants as glucosoaminoglycans, heparin sulfates, thrombomodulin, nitric oxide, which promotes vasodilation, improves blood rheology. In our studies, the content of fibrinogen A, fibrinogen B (products of fibrin degradation), the prothrombin index decreased in patients, and the thrombin clotting time decreased. Damage to endothelial cells in the anal-hemorrhoidal zone is caused by endotoxins, leading to inflammatory edema, which results in intoxication. Ozone's detoxifying effect counter-balances these complications. This work presents and analyses the results of acute hemorrhoid thrombosis treatment with radio wave exposure, which provided non–contact incision and hemostasis of soft tissues by high frequency waves (3.8–4.0 Hz), in combination with intravenous ozone therapy to enhance the analgesic and anti-inflammatory effects characterized by oxygen supply and oxidation of mediators involved in passing nociceptive signals in the central nervous system. When carrying out the comparative analysis of the patients' treatment results, the use of ozone against the background of thrombectomy showed a positive effect.

EEG monitoring during anesthesia in children aged 0 to 18\u2009months: amplitude-integrated EEG and age effects

BackgroundThe amplitude-integrated EEG (aEEG) is a widely used monitoring tool in neonatology / pediatric intensive care. It takes into account the amplitudes, but not the frequency composition, of the EEG. Advantages of the aEEG are clear criteria for interpretation and time compression. During the first year of life, the electroencephalogram (EEG) during sedation / anesthesia changes from a low-differentiated to a differentiated EEG; higher-frequency waves develop increasingly. There are few studies on the use of aEEG during pediatric anesthesia. A systematic evaluation of the aEEG in defined EEG stages during anesthesia / sedation is not yet available. Parameters of pediatric EEGs (power, median frequency, spectral edge frequency) recorded during anesthesia and of the corresponding aEEGs (upper and lower value of the aEEG trace) should be examined for age-related changes. Furthermore, it should be examined whether the aEEG can distinguish EEG stages of sedation / anesthesia in differentiated EEGs.MethodsIn a secondary analysis of a prospective observational study EEGs and aEEGs (1-channel recordings, electrode positions on forehead) of 50 children (age: 0–18 months) were evaluated. EEG stages: A (awake), Slow EEG, E2, F0, and F1 in low-differentiated EEGs and A (awake), B0–2, C0–2, D0–2, E0–2, F0–1 in differentiated EEGs.ResultsMedian and spectral edge frequency increased significantly with age (p < 0.001 each). In low-differentiated EEGs, the power of the Slow EEG increased significantly with age (p < 0.001). In differentiated EEGs, the power increased significantly with age in each of the EEG stages B1 to E1 (p = 0.04, or less), and the upper and lower values of the aEEG trace increased with age (p < 0.001). A discriminant analysis using the upper and lower values of the aEEG showed that EEG epochs from the stages B1 to E1 were assigned to the original EEG stage in only 19.3% of the cases. When age was added as the third variable, the rate of correct reclassifications was 28.5%.ConclusionsThe aEEG was not suitable for distinguishing EEG stages above the burst suppression range. For this purpose, the frequency composition of the EEG should be taken into account.

An in-depth analysis on the Quasi-Longitudinal approximations applied to ionospheric ray-tracing, oblique and vertical sounding, and absorption

For the phase refraction index of high frequency (HF) waves in the ionospheric medium exists a well-established theory. However, under the Quasi-Longitudinal (QL) conditions, scientific literature presents various formulas that are not equivalent and that, in some cases, give rise to wrong results. In the present study, further consequences of Booker’s rule are discussed, illustrating the validity ranges of the above-mentioned approximate formulas; and the different regimes for applying such QL formulas are described, along with the consequences in simulating the ionospheric HF ray-tracing, oblique and vertical sounding, and absorption.

Sensitivity of third-generation interferometers to extra polarizations in the stochastic gravitational wave background

When modified theories of gravity are considered, at most six gravitational wave polarization modes are allowed and classified in tensor modes, the only ones predicted by General Relativity (GR), along with additional vector and scalar modes. Therefore, gravitational waves represent a powerful tool to test alternative theories of gravitation. In this paper, we forecast the sensitivity of third-generation ground-based interferometers, Einstein Telescope and Cosmic Explorer, to non-GR polarization modes focusing on the stochastic gravitational wave background. We consider the latest technical specifications of the two independent detectors and the full network in order to estimate both the optimal signal-to-noise ratio and the detectable energy density limits relative to all polarization modes in the stochastic background for several locations on Earth and orientations of the two observatories. By considering optimal detector configurations, we find that in 5 years of observation the detection limit for tensor and extra polarization modes could reach $h_0^2\Omega^{T,V,S}_{GW} \approx 10^{-12}-10^{-11}$, depending on the network configuration and the stochastic background (i.e., if only one among vector and scalar modes exists or both are present). This means that the network sensitivity to different polarization modes can be approximately improved by a factor $10^3$ with respect to second-generation interferometers. We finally discuss the possibility of breaking the scalar modes degeneracy by considering both detectors angular responses to sufficiently high gravitational wave frequencies.

On non-diffractive cones

A subject of recent interest in inverse problems is whether a corner must diffract fixed frequency waves. We study the related question of which cones $[0,\infty) \times Y$ do not diffract high frequency waves. We prove that if $Y$ is analytic and does not diffract waves at high frequency then every geodesic on $Y$ is closed with period $2\pi$. Moreover, we show that if $\operatorname{dim} Y=2$, then $Y$ is isometric to either the sphere of radius $1$ or its $\mathbb{Z}^2$ quotient, $\mathbb{RP}^2$.

Modeling wave climates and wave energy attenuation in marsh terrace environments in the northern Gulf of Mexico

Marsh terracing is a coastal restoration technique that has been implemented in Louisiana and Texas for almost 30 years. Marsh terraces are segmented ridges of soil built in inland coastal ponds for the objectives of creating new marsh, disrupting fetch, and dissipating wind-driven wave energy, which causes marsh erosion. Despite widespread implementation, no numerical modeling studies have been conducted to understand the effect of terraces on wave climate in marsh environments. The objectives of this study were to: 1) simulate wave climates in marsh terrace fields using the Simulating WAves Nearshore (SWAN) model and evaluate the agreement between modeled and observed wave conditions and 2) assess the effectiveness of marsh terraces at reducing significant wave height by comparing wave climates in terraced sites with hypothetical unterraced sites. The SWAN model was used to simulate wind-waves at two terrace fields in coastal Louisiana. Model validation was performed using data collected with in situ wave instruments and wind data measured by NOAA stations located 25 km to the east and 62 km to the northeast of the study area. Results from this study modeled and measured small (0.03–0.14 m) and high–frequency waves (0.80–1.29 s) in the marsh terrace fields. Model validation revealed agreement between modeled and observed data, particularly for significant wave height and direction. Additionally, a comparison of wave climates in terraced and unterraced marsh environments indicated an average significant wave height reduction of 45% (min:18%, max: 84%) when terraces were present. Overall, the model was able to evaluate wave climates in marsh terrace fields and assess the effectiveness of this restoration technique in reducing significant wave height. The model's performance in this low energy and complex environment (shallow water, nonlinear shoreline, marsh fragmentated) is encouraging given the lack of research assessing wave parameters in marsh terrace environments.

About the combination of high and low frequency methods for impact detection on aerospace components

This paper presents an analysis of state-of-the-art of impact detection techniques for aerospace structural components as well as a study about the combination of two promising approaches for localizing an incidental impact event on a typical metallic aerospace structural component as test article. In the aeronautical scenario, some typical damaging events that may occur during service life are runway bird-strike, tool drop and debris impact. The last two cases produce generally high-frequency vibrations that are usually well predicted by ultrasonic techniques. The impacts from birds on the other hand produces vibrations in the lower or modal frequency range. The present work is focused on the possible combination of two methodologies: the first one, related to impacts inducing low-frequency vibrations, is based on the implementation of a Neural Network, while the second one, related to impacts inducing higher-frequency stress waves, is based on an acoustic source localization approach. Both numerical and experimental analyses were implemented on the same isotropic aluminum flat panel, and a possible combination of the experimental sensors arrangement will be discussed within the paper. The results have confirmed the positive performance of the neural network, opening to a more extended experimental campaign mainly oriented to the definition of the system precision, possible fault reconstruction and optimization in the data handling and reduction of computational effort. On the other hand, the main advantage of the acoustic emission formulation is that it does not require the knowledge of the wave velocity profile in the panel. Dependence of the guided wave velocity on the signal frequency for isotropic plates and, also on the wave propagation direction for anisotropic plates are the two major obstacles for acoustic source localization in a plate. Both these obstacles are avoided in this latter formulation.

Machine Learning Approaches for Reconfigurable Intelligent Surfaces: A Survey

Next-generation wireless networks must handle a growing density of mobile users while accommodating a rapid increase in mobile data traffic flow and a wide variety of services and applications. High-frequency waves will perform an essential role in future networks, but these signals are easily obstructed by objects and diminish over long distances. Reconfigurable intelligent surfaces (RISs) have attracted considerable interest because of their potential to improve wireless network capacity and coverage by intelligently changing the wireless propagation environment. Consequently, RISs possess potential technology for the sixth generation of communication networks. Machine learning (ML) is an effective method for maximizing the possible advantages of RIS-assisted communication systems, particularly when the computational complexity of operating and deploying RIS increases rapidly as the number of interactions between the user and the infrastructure starts to grow. Since ML is a promising strategy for improving a network and its performance, the application of ML in RISs is expected to open new avenues for interdisciplinary studies as well as practical applications. In this paper, we extensively investigate the ML algorithms used in RISs. We provide a brief overview of RISs, a summary of ML methods with RIS architecture, and a comparison of the available methodologies to explain the combination of these two technologies. Moreover, the significance of open research topics is emphasized to provide sound research directions.

Experimental investigations on the ultrasonic vibration-assisted hard turning of AISI 52100 steel using coated carbide tool

Ultrasonic vibration-assisted turning (UVAT) is a hybrid and eco-friendly machining technique. The UVAT is performed by superimposing high frequency and low amplitude sound waves on the cutting tool in conventional turning. In the present work, a comparative evaluation of the machining performance is presented during conventional turning (CT) and ultrasonically vibration-assisted turning (UVAT) of AISI 52100 hardened steel (62 HRC). Hard turning experiments were performed using a PVD coated TiAlSiN carbide tool considering the effect of process parameters such as cutting speed, feed, and depth of cut. The performance is measured in terms of cutting force and surface roughness. This study observed a better surface finish during of around 8.29% in UVAT when compared to conventional turning. Better surface finish achieved under UVAT could be attributed to better chip breaking and disentanglement of the chip around the tool during UVAT against long curly chips produced that further entangled on the tool while CT. These entangled long curly chips affected the surface finish. On the other hand, no significant difference is observed in cutting force while hard turning under CT and UVAT. This is mostly due to the fact that, although UVAT has less cutting force at slower cutting speeds, but it behaves very identically with CT when cutting speed approaches tangential velocity. This study finds further scope for machinability studies considering the effect of vibrations superimposed on a tool in feed, tangential, and radial directions.

Glass Package for Radar MMICs Above 150 GHz

This work presents a novel sensor packaging and a novel transition concept for radar applications above 150 GHz based on glass material. By using laser induced deep etching (LIDE) technology, glass vias and cavities are fabricated without degrading the mechanical stability of glass, as micro-cracks are completely avoided. Especially at high millimeter wave (mm-wave) frequencies, precise structuring on low dielectric loss materials and a high integration density are essential for low loss transitions. In this paper, an ultra compact FMCW radar monolithic microwave integrated circuit (MMIC) at 160 GHz is used to demonstrate this packaging technology. In addition, the high frequency signal is guided by a low loss transition to a deposed antenna via a dielectric waveguide (DWG) providing the antenna front end with mechanical flexibility. Thus, using plated through glass vias (TGVs) and a circulating solder ring, the package is hermetically sealed. The optical transparent glass package has a size of only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${5.8}\,\rm {mm}\times {4.4}\,\rm {mm}\times {0.9}\,\rm {mm}$</tex-math></inline-formula> . A minimum measured insertion loss of 2.85 dB at 162 GHz from chip to DWG is achieved. The complete radar system with a range resolution of 12 mm is demonstrated via radar measurement.

Maximising the hydrodynamic performance of offshore oscillating water column wave energy converters

This paper provides a thorough examination and discussion of a design process developed to improve the hydrodynamic performance of an asymmetrical offshore Oscillating Water Column (OWC) Wave Energy Converter (WEC). The resulting WEC geometry was based on a column-stabilised semi-submersible platform where an asymmetrical OWC chamber was integrated into the moonpool of the platform, resembling both, a purpose built WEC, or an existing offshore structure retrofitted for wave energy conversion. The performance of a 1:36 scale model of the OWC WEC was designed using a validated computational fluid dynamics (CFD) method and experimentally tested to evaluate the effect of the external support structure on the hydrodynamic performance of the device. Detailed analysis included physical and numerical decay tests to quantify the natural period of the OWC moonpool, wave and OWC-structure interactions, turbine damping coefficients and hydrodynamic capture width ratios. The obtained results revealed that the addition of the external support structure improved the OWC Capture Width Ratio (CWR) from 0.849 at kd ∼ 2.53 to 1.541 at kd ∼ 2.99 (81.5% increase). It was also observed that the external support structure shifted the peak performance towards higher frequency waves.

Laboratory-scale investigation of a periodically forced stratified basin with inclined endwalls

We present results of numerical simulations of a stratified reservoir with a three-layer stratification, subject to an oscillating surface shear stress. We investigate the effect of sloped endwalls on mixing and internal wave adjustment to forcing within the basin, for three different periods of forcing. The simulations are carried out at a laboratory scale, using large-eddy simulation. We solve the three-dimensional Navier–Stokes equations under the Boussinesq approximation using a second-order-accurate finite-volume solver. The model was validated by reproducing experimental results for the response of a reservoir to surface shear stress and resonant frequencies of internal waves. We find interesting combinations of wave modes and mixing under variation of the forcing frequencies and of the inclination of the endwalls. When the frequency of the forcing is close to the fundamental mode-one wave frequency, a resonant internal seiche occurs and the response is characterized by the first vertical mode. For forcing periods twice and three times the fundamental period, the dominant response is in terms of the second vertical mode. Adjustment to forcing via the second vertical mode is accompanied by the cancellation of the fundamental wave and energy transfer to higher-frequency waves. The study shows that the slope of the endwalls dramatically affects the location of mixing, which has a feedback on the wave field by promoting the generation of higher vertical modes.

Low-frequency Whistler Waves Modulate Electrons and Generate Higher-frequency Whistler Waves in the Solar Wind

Abstract The role of whistler-mode waves in the solar wind and the relationship between their electromagnetic fields and charged particles is a fundamental question in space physics. Using high-temporal-resolution electromagnetic field and plasma data from the Magnetospheric MultiScale spacecraft, we report observations of low-frequency whistler waves and associated electromagnetic fields and particle behavior in the Earth’s foreshock. The frequency of these whistler waves is close to half the lower-hybrid frequency (∼2 Hz), with their wavelength close to the ion gyroradius. The electron bulk flows are strongly modulated by these waves, with a modulation amplitude comparable to the solar wind velocity. At such a spatial scale, the electron flows are forcibly separated from the ion flows by the waves, resulting in strong electric currents and anisotropic ion distributions. Furthermore, we find that the low-frequency whistler wave propagates obliquely to the background magnetic field ( B 0), and results in spatially periodic magnetic gradients in the direction parallel to B 0. Under such conditions, large pitch-angle electrons are trapped in wave magnetic valleys by the magnetic mirror force, and may provide free perpendicular electron energy to excite higher-frequency whistler waves. This study offers important clues and new insights into wave–particle interactions, wave generation, and microscale energy conversion processes in the solar wind.