Large Frequency Range Research Articles

Numerical Prediction of the Vertical Responses of Planing Hulls in Regular Head Waves

The evaluation of the hydrodynamic performance of planing vessels has always been one of the most attractive study fields in the maritime agenda. Resistance and self-propulsion studies have been performed using experimental and numerical methods by researchers for a long time. As opposed to this, the seakeeping performance of planing hulls is assessed with 2D approximation methods, but limitedly, while the experimental campaign is not cost-effective for several reasons. With this motivation, pitch and heave transfer functions and accelerations were obtained for a monohedral hull and a warped hull using a state of art commercial Reynolds-averaged Navier–Stokes (RANS) solver, in this study. Moreover, 2-DOF (degree of freedom) dynamic fluid–body interaction (DFBI) equations were solved in a coupled manner with an overset mesh algorithm, to find the instantaneous motion of the body. After verification, obtained numerical results at three different Froude numbers and a sufficiently large wave frequency range were compared with the experiments. The results showed that the employed RANS method offers a very accurate prediction of vertical motions and accelerations for planing hulls.

Spectropolarimetric Properties of Swift J1818.0–1607: A 1.4 s Radio Magnetar

Abstract The soft-gamma repeater Swift J1818.0−1607 is only the fifth magnetar found to exhibit pulsed radio emission. Using the Ultra-Wideband Low receiver system of the Parkes radio telescope, we conducted a 3 hr observation of Swift J1818.0−1607. Folding the data at a rotation period of P = 1.363 s, we obtained wideband polarization profiles and flux density measurements covering radio frequencies between 704 and 4032 MHz. After measuring, and then correcting for the pulsar’s rotation measure of 1442.0 ± 0.2 rad m−2, we find the radio profile is between 80% and 100% linearly polarized across the wide observing band, with a small amount of depolarization at low frequencies that we ascribe to scatter broadening. We also measure a steep spectral index of across our large frequency range, a significant deviation from the flat or inverted spectra often associated with radio-loud magnetars. The steep spectrum and temporal rise in flux density bears some resemblance to the behavior of the magnetar-like, rotation-powered pulsar PSR J1119−6127. This leads us to speculate that Swift J1818.0−1607 may represent an additional link between rotation-powered pulsars and magnetars.

Structural, DFT, vibrational spectroscopic, thermal, electrical and magnetic characterizations of hydrothermally grown CoCO3 microcrystals

The minerals such as the CoCO3 can be utilized for the fabrication of lithium-grafted batteries and for a variety of food applications. In this study, we have successfully synthesized Cobalt carbonate crystals (CoCO3) using hydrothermal synthesis method. The synthesized micro-crystals consist of hexagonal structure symmetry. The XRD shows the single-phase hexagonal (R-3c) crystal with lattice constants a = 4.661 A, b = 4.661 A, and c = 14.96 A. EDAX spectroscopy analysis confirms the presence of all the elements of sample. FE-SEM analysis suggests nearly rod-shaped crystals with ~ 29 µm average crystalline size. The Fourier Transform Infrared spectrum suggested existence of O–H, C–O etc. bonding. The TGA is carried out to study the thermal stability. The dielectric relaxation is well studied over large angular frequency range. Complex impedance spectrum of the synthesized microcrystals consists of single semi-circular arc due to the presence of grain only. The magnetic study has suggested paramagnetic nature of the sample. The theoretical study such as the density function theory has been carried out using the Quantum ESPRESSO. A reasonably good agreement is observed between experimentally obtained data and DFT data for optimized structures.

Low-frequency unsteadiness of shock-wave/boundary-layer interaction in an isolator with background waves

Low-frequency unsteadiness is investigated through wind tunnel experiments and numerical simulations of the internal flow in a supersonic isolator with background waves generated by a 14° wedge in a freestream with a Mach number of 2.94. The power spectra, coherence, and phase analyses of high-frequency pressure signals and schlieren images provide a local and global description of the unsteadiness. The upstream mechanism exhibits a significant influence on the unthrottled flow field. In the weak interactions of small separation flow, the pressure fluctuation between two adjacent incident points has a strong correlation in a large frequency range, while only large-amplitude shock oscillations are exhibited in the pressure fluctuations at the boundary layer. The downstream mechanism dominates the asymmetric shock motion in the throttled flow field. The profiles of the power spectrum and standard deviation both exhibit two peaks at the upstream and downstream peripheries of the wall separation patterns. Two types of oscillations can be identified through the pressure data, and type III is established from the analysis of schlieren images. The oscillation behavior of the three types is obtained through the power spectral analysis of a series of schlieren snapshots. The frequency of the occurrence and the one-cycle amplitude of different oscillation types are significantly different. By combining the coherence and phase analyses with the corresponding schlieren images and pressure data, the feedback mechanism of the three oscillation types is determined. This study combines the low-frequency unsteadiness in supersonic internal flows with the multiple separation regions caused by complex background waves.

Wideband Frequency-Tunable Parity-Time Symmetric Optoelectronic Oscillator Based on Hybrid Phase and Intensity Modulations

To achieve a wideband frequency tunability, a parity-time symmetric optoelectronic oscillator (PT-OEO) based on hybrid phase modulation (PM) and intensity modulation (IM) is proposed and experimentally demonstrated. In the PT-OEO, the hybrid PM and IM is realized with an arbitrary modulation chirp, by adjusting the polarization state of the polarization-modulated optical signal. Thus, a reconfigurable dispersion-induced power fading is formed to implement a tunable coarse oscillation mode selection. Meanwhile, the parity-time (PT) symmetry breaking is introduced to achieve a fine-mode selection. The incorporation of the two kinds of mode selection mechanisms ensures a widely tunable PT-OEO, even in the absence of a high quality-factor (Q-factor) microwave/optical filter. Experimental results indicate a large tunable frequency range from 16 to 30 GHz for the generated signal. The side-mode suppression ratio of the generated signal is about 40 dB, and the phase noise measured at 18.7 GHz is evaluated as −116 dBc/Hz @ 10-kHz offset frequency.

Data-driven and machine learning identification of seismic reference stations in Europe

SUMMARYThe growing seismic networks and the increasing number of permanent seismic stations can help in improving the physical basis of seismic hazard assessment. For this purpose, the definition of reference site conditions is of great significance. If a reliable estimate of the reference ground motion is known, its modification at any given site can be modelled with respect to that reference site. Since the choice of a well-characterized reference site is not straightforward, mainly due to the high variability in the shallow layers, such choices prove to be affected by large uncertainties. While proxy parameters like the average S-wave velocity over the uppermost 30 m (vS30) might help in characterizing reference site conditions, such parameters are neither available at all sites nor do they allow concluding that the site is not affected by amplification and attenuation effects. In this study, we identify prospective reference sites across Europe in a harmonized and fully data-driven way. All analysis is based on freely available geological and geophysical data and no on-site measurements or site-specific proxies are required. The study accounts for both the influence of amplification and attenuation in a large frequency range. To address the key conceptual issues, we verify our classification based on machine learning techniques in which the influence of the individual site characterization parameters is investigated. Our study indicates that around 250 sites in Europe over more than 2000 investigated are not affected by local site effects and can de facto be considered as reference sites based on the criteria applied.

Strong anion–anion hydrogen bond in the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate

This work discusses the remarkable differences in the mid- and far-infrared regions of the spectrum of the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], in comparison to ionic liquids based on other anions. The infrared spectrum unveils the occurrence of anion–anion hydrogen bond in [C2C1im][HSO4]. The band related to the anion OH stretching mode, ν(OH), covers a large frequency range (2000–3400 cm−1) and exhibits the spectral pattern of strongly hydrogen bonded systems. The far-infrared (FIR) spectrum of [C2C1im][HSO4] also exhibits the signature of the anion–anion interaction, that is, the stretching mode of the hydrogen bond, ν(O…O), at ~170 cm−1. Hydrogen bond between [HSO4]− anions also occurs in ionic liquids containing imidazolium cation with longer alkyl chain ([C4C1im]+) or tetraalkylammonium cation. The vibrational frequencies of ν(OH) and ν(O…O) modes satisfy empirical correlations that characterize the anion–anion interaction in [C2C1Im][HSO4] as a strong hydrogen bond.

Broadband tunable focusing lenses by acoustic coding metasurfaces

Acoustic tunable focusing lens has important applications in acoustic fast imaging and three-dimensional displays. Previously developed metasurface lenses suffer from narrow bandwidth and fixed geometric configuration, thereby limiting their further practical applications. Here, we design an acoustic coding metasurface (ACM) lens to achieve a broadband tunable focusing in air. This simple ACM lens only includes two coding bits: a space-folding structure is designed to act as the bit ‘1’ and an air unit with the same size is used as the bit ‘0’. By adjusting the coding sequence of the ACM, the focus location of the ACM lens can be modified efficiently and be steered in the whole working plane. Moreover, the focusing behaviors of the ACM lens are well performed in a large frequency range of ∼3.1 to ∼4.6 kHz. Our ACM lens may be beneficial to applications that traditionally need different and complicated structures for tunable acoustic focusing.

Study on connecting tube dynamics for transient pressure measurement

The present study evaluates the dynamic response of connecting tubes for transient pressure measurement. A systematic study is conducted to quantify the amplitude and phase distortion of connecting tubes of diameter 1, 2 and 3 mm with different lengths (10–50 cm). The experimental measurements and theoretical predictions have been carried out with both air and water as the working medium to cover a wide range of frequencies. The study highlights the underdamped nature of all the systems studied. The natural frequency of the system increases with an increase in the tube diameter and a decrease in tube length. The difference in natural frequency obtained from the experimental results and theoretical prediction is less for the smaller tube diameter (d = 1 mm) and more pronounced for the larger tube diameter. Larger tube diameters are recommended to avoid amplitude and phase distortion errors, especially in the low-frequency range. However, resonance effects are more pronounced for larger tube diameters. The phase response of larger tube diameters remains close to zero over a large range of frequency (0–0.8 times the natural frequency); hence, this range is more suitable for applications where phase information is more important than amplitude. This study is useful for compensating the amplitude and phase distortion error encountered in transient pressure measurements.

Full-Spectrum High-Resolution Modeling of the Dielectric Function of Water.

In view of the vital role of water, exact knowledge of its dielectric function over a large frequency range is important. We report on currently available measurements of the dielectric function of water at room temperature (25 °C) across the full spectrum: microwave, IR, UV, and X-ray (up to 100 eV). We parameterize the complex dielectric function of water with two Debye (microwave) oscillators and high resolution of IR and UV/X-ray oscillators. We also report dielectric parameters for ice-cold water with a microwave/IR spectrum measured at 0.4 °C, while taking the UV spectrum at 25 °C (assuming negligible temperature dependence in UV). We employ van der Waals dispersion interactions to contrast our model of ice-cold water with earlier models. Air bubbles in water and dissolved gas molecules show attraction toward interfaces rather than repulsion. The van der Waals interaction promotes complete freezing rather than supporting a thin layer of water on ice. We infer that premelting is driven by charge and ion adsorption. Density-based extrapolation from warm to cold water of the dielectric function is satisfactory in microwave but poor (40% error) at IR frequencies.

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

We determine the mantle attenuation (1/Qμ) structure beneath 70 Myr seafloors in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: Qμ=1400 in the shallow layer, Qμ=110 in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, Qμ observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric Qμ measured at higher frequency (∼3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (VS) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (fe) and determine, at each depth, which values of fe predict Qμ and VS that can fit the NoMelt Qμ and VS values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in fe by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in fe reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ∼70-km depth.

Influence of the microstructure of two-dimensional random heterogeneous media on propagation of acoustic coherent waves.

Multiple scattering of waves arises in all fields of physics in either periodic or random media. For random media the organization of the microstructure (uniform or nonuniform statistical distribution of scatterers) has effects on the propagation of coherent waves. Using a recent exact resolution method and different homogenization theories, the effects of the microstructure on the effective wave number are investigated over a large frequency range (ka between 0.1 and 13.4) and high concentrations. For uniform random media, increasing the configurational constraint makes the media more transparent for low frequencies and less for high frequencies. As a side but important result, we show that two of the homogenization models considered here appear to be very efficient at high frequency up to a concentration of 60% in the case of uniform media. For nonuniform media, for which clustered and periodic aggregates appear, the main effect is to reduce the magnitude of resonances and to make network effects appear. In this case, homogenization theories are not relevant to make a detailed analysis.

Moments of Inertia of SDIBs in A = 190 Mass Region using VMI Model

A lot of identical bands are known at present in the Normal Deformed (ND) region. In our study of the occurrence and properties of identical bands in Super-Deformed (SD) nuclei we first applied the modified Variable Moment of Inertia (VMI) model to extract the band-head spin of Super-Deformed bands. The calculated transition energies, level spins and dynamic moment of inertia are systematically examined. Then, in the framework of theoretical model several identical bands are identified. The kinematic and dynamic moment of inertia have been calculated for the six pairs of Super-Deformed Identical Bands (SDIBs) which was not reported earlier in the literature. Thus, the results are significant. In all the cases J(2) is significantly higher than J(1) over a large range of frequency.

Controlled generation of periodic vortical gusts by the rotational oscillation of a circular cylinder and attached plate

Understanding the interaction of vortical gusts and unsteady wing kinematics is crucial to the operation of small aircraft in urban or turbulent environments. Studies investigating this interaction require adequate control of a vortical gust generator to consistently produce the desired vortical gusts for study. Previous studies have focused on the pitching or heaving of airfoils to generate vortical gusts. The current study focuses on the characterization of the vortex shedding process for a rotationally oscillating circular cylinder with an attached one-diameter long plate. When this vortical gust generator is actuated in relatively small sinusoidal motions, von Karman vortices can be generated in a controllable and repeatable fashion over a large range of oscillation frequencies that are significantly different than the natural shedding frequency of the static cylinder, and without the generation of any additional unwanted vorticity. Fine tuning of the actuation parameters allows for the control of the vortex strength, frequency, and transverse location.

Implementation of MOIDS Algorithm for Optimal Parameter Selection of SFPT for Modern Radars

Modern radars require signals of the large frequency range to attain relatively greater range resolution. Frequency stepping is the technique to convert the narrowband signal, i.e., a train of pulses into a wideband signal to achieve high accuracy in range resolution measurements. Due to the cause of constant step in frequency of successive pulses, grating lobes appeared when the period of pulse multiplied by step in frequency exceeds unity as is needed in modern radars. Hence to achieve the best resolution, grating lobes height, sidelobes level, and mainlobe width have to be minimum. In this paper an attempt has been made to diminish grating lobes, minimize sidelobes and reduce mainlobe width using MOIDSA to find the parameters of Stepped Frequency Pulse Train (SFPT) mechanism. The compromise between various lobes is obtained by using three dimensional Pareto fronts for different ranges of SFPT parameters.

Frequency- and angle-dependent scattering of a finite-sized meta-structure via the relaxed micromorphic model

In this paper, we explore the use of micromorphic-type interface conditions for the modeling of a finite-sized metamaterial. We show how finite-domain boundary value problems can be approached in the framework of enriched continuum mechanics (relaxed micromorphic model) by imposing continuity of macroscopic displacement and of generalized tractions, as well as additional conditions on the micro-distortion tensor and on the double-traction. The case of a metamaterial slab of finite width is presented, its scattering properties are studied via a semi-analytical solution of the relaxed micromorphic model and compared to a direct finite-element simulation encoding all details of the selected microstructure. The reflection and transmission coefficients obtained via the two methods are presented as a function of the frequency and of the direction of propagation of the incident wave. We find excellent agreement for a large range of frequencies going from the long-wave limit to frequencies beyond the first band-gap and for angles of incidence ranging from normal to near-parallel incidence. The present paper sets the basis for a new viewpoint on finite-size metamaterial modeling enabling the exploration of meta-structures at large scales.

Sex differences in brain oscillatory activity during sleep and wakefulness in obstructive sleep apnea.

Epidemiological studies consistently show a male predominance in obstructive sleep apnea (OSA). Hormonal differences, breathing control, upper airway anatomy and fat distribution have been proposed as causes of gender differences in OSA. Clinical manifestations are accentuated in men, although white matter structural integrity is affected in women.To the best of our knowledge, no previous studies have explored gender differences in the electrical brain activity features of OSA. Polysomnography was performed on43 patientswith untreated OSA (21 women, 22 men),and power spectral density (1-50Hz) was compared between groups across sleep and wakefulnessat two levels of OSA severity. Severe versus moderate OSA showed decreased power for fast frequencies (25-29Hz) during wakefulness. OSA men displayed decreased power of a large frequency range (sigma, beta and gamma) during sleep compared with women. Comparisons of men with severe versus moderate OSA presented significantly decreased sigma power during non-rapid eye movement (NREM) sleep, but significantly increased delta activity during REM sleep. Meanwhile, women with severe versus moderate OSA showed no significant power differences in any condition. These findings indicated a different evolution of brain oscillations between OSA men and women with significant impairment of brain activity related to cognitive processes. Our study emphasizes the importance of understanding the differential effects of sleep disorders on men and women in order to develop more precise diagnostic criteria according to gender, including quantitative electroencephalogram (EEG) analysis tools.

Photonic-assisted single system for microwave frequency and phase noise measurement

We propose a photonic-assisted single system for measuring the frequency and phase noise of microwave signals in a large spectral range. Both the frequency and phase noise to be measured are extracted from the phase difference between the signal under testing and its replica delayed by a span of fiber and a variable optical delay line (VODL). The system calibration, frequency measurement, and phase noise measurement are performed by adjusting the VODL at different working modes. Accurate frequency and phase noise measurement for microwave signals in a large frequency range from 5 to 50 GHz is experimentally demonstrated.

A Simple High-Gain Millimeter-Wave Leaky-Wave Slot Antenna Based on a Bent Corrugated SIW

We propose a bent corrugated substrate integrated waveguide (BCSIW) structure that can be used to design a high gain leaky wave antenna (LWA). The design removes the need for a metallic via fabrication process needed for standard substrate integrated waveguides (SIW) and displays superior performance to previously reported structures. We use simulations to compare the performance of the proposed BCSIW LWA to equivalent standard SIW and corrugated SIW (CSIW) structures, as well as experimentally characterize a fabricated BCSIW LWA. Simulation results show that the BCSIW structure can help improve the impedance bandwidth of a slotted LWA by about 14.7% while still maintaining high gain (about 13.2-17.4 dBi) as compared to an LWA based on a CSIW structure. Measurement results indicate that the proposed BCSIW LWA has a wide impedance bandwidth (32.6%) and a high peak gain (12-16.2 dBi) throughout a large frequency range from 22 to 29.2 GHz with a large beam angle range from -69° to -10°.

Experimental study of noise barrier boards with increased acoustic performance by utilizing Helmholtz resonator effects

Noise barriers are widely adopted to control environmental noise, but their performance may be decreased by the too reflective surface of their material. The noise is reflected back towards the noise source and can reflect back again from parallel wall in case of highway applications. To improve the performance of barriers, an array Helmholtz resonators (HRs) is proposed to be formed in the barrier surface. The current investigation proves that by using Helmholtz resonator arrays in the surface of solid plastic material, it is possible to considerably improve the absorption of the material. In case the material has porous interlayer the formation of the resonator cavities is relatively simple. It is proved in the invstigation that by selecting the number and the depth of the resonator necks high absorption values can be reached over the large frequency range. Furthermore, experimental results demonstrate that the use of HRs to improve absorption do not decrease the sound insulation properties of the investigated material.