Characteristic Oscillation Frequency Research Articles

Accretion tori around rotating neutron stars II. Oscillations and precessions

The four characteristic oscillation frequencies of accretion flows (in addition to the Keplerian orbital frequency) are often discussed in the context of the time variability of black hole and neutron star (NS) low-mass X-ray binaries (LMXBs). These four frequencies are the frequencies of the axisymmetric radial and vertical epicyclic oscillations, and the frequencies of non-axisymmetric oscillations corresponding to the periastron (radial) and Lense-Thirring (vertical) precessions. In this context, we investigated the effect of the quadrupole moment of a slowly rotating NS and provide complete formulae for calculating these oscillation and precession frequencies, as well as convenient approximations. Simple formulae corresponding to the geodesic limit of a slender torus (and test-particle motion) and the limit of a marginally overflowing torus (a torus exhibiting a critical cusp) are presented, and more general approximate formulae are included to allow calculations for arbitrarily thick tori. We provide the Wolfram Mathematica code used for our calculations together with the C++ and PYTHON codes for calculating the frequencies. Our formulae can be used for various calculations regarding the astrophysical signatures of the NS super-dense matter equation of state. For instance, we demonstrate that even for a given fixed number of free parameters, a model that accounts for fluid flow precession matches the frequencies of twin-peak quasiperiodic oscillations observed in NS LMXBs better than a model that uses geodesic precession.

Study on the stability and ultra-low frequency oscillation suppression method of pumped storage power plant with dual units sharing one pipeline

This paper aims to investigate the stability of the pumped storage power plant (PSPP) with dual units sharing one pipeline (DUSOP) and the method of suppressing the ultra-low frequency oscillation (ULFO) characteristics. Firstly, four nonlinear models of pumped storage regulation system (PSRS) with DUSOP are established under the design scenarios of no surge tank, upstream surge tank (UST), downstream surge tank, and upstream and downstream dual surge tanks. Then the stability of the PSRS under different surge tank arrangements (STAs) in the long diversion system design scenarios (LDSDS) is comparatively analyzed. Finally, based on the concept of stability margin, the influence mechanism of the operating points on the ULFO characteristics is investigated in the LDSDS with UST, and the influence of the governor parameters, hydrodynamic parameters, and mechanical parameters on the damping ratio and frequency of the system is also analyzed. The results show that among STAs, the design scheme of arranging the UST is more reasonable for PSPP with LDSDS from the comprehensive consideration of the economy of the project and the stability of the system. For PSPP with UST, on the line of equal stability margin, increasing the proportional gain, reducing the unit inertia time constant, reducing the water flow inertia time constant, and appropriately increasing the area of the surge tank can increase the frequency and reduce the amplitude, which is conducive to the suppression of ULFO. In the stability domain range, increasing the proportional gain increases the frequency and the system stability increases and then decreases. Increasing the integral gain decreases the frequency and then increases the system stability. At the same time, the frequency of the unit can be changed by adjusting the parameters of the system to avoid resonance between the frequency of the unit and the frequency of the power grid and to ensure the stability and safety of the unit operation.

Observation of a phase transition from a continuous to a discrete time crystal

Discrete (DTCs) and continuous time crystals (CTCs) are novel dynamical many-body states, that are characterized by robust self-sustained oscillations, emerging via spontaneous breaking of discrete or continuous time translation symmetry. DTCs are periodically driven systems that oscillate with a subharmonic of the external drive, while CTCs are continuously driven and oscillate with a frequency intrinsic to the system. Here, we explore a phase transition from a continuous time crystal to a discrete time crystal. A CTC with a characteristic oscillation frequency ωCTC is prepared in a continuously pumped atom-cavity system. Modulating the pump intensity of the CTC with a frequency ωdr close to 2ωCTC leads to robust locking of ωCTC to ωdr/2 , and hence a DTC arises. This phase transition in a quantum many-body system is related to subharmonic injection locking of non-linear mechanical and electronic oscillators or lasers.

Schlieren measurements of shock train flow fields in a supersonic cylindrical isolator at Mach 2

AbstractIn a supersonic cylindrical isolator at Mach 2, the structures and frequency characteristics of shock train flow fields were experimentally studied by the schlieren measurement method. According to the design principle of parallel light through schlieren windows in a cylindrical duct, a high-precision conformal optical window pair was designed and integratively processed before. Based on a self-built pipeline structure with conformal windows in a direct-connect wind tunnel under adjustable back-pressure conditions, the shock surfaces in a cylindrical isolator at Mach 2 were first captured by the schlieren method. Then, the schlieren photographs were corrected by a nonlinear image transformation algorithm for the restoration of real shock train structures, and the experimental results were compared with numerical simulation results quantitatively. Finally, the shock train positions were calculated by an image recognition algorithm to analyze the self-excited oscillation frequency characteristics of shock train structures. The methods and experiments in this study enriched optical observation methods of supersonic flows through non-rectangular cross-section isolators in scramjet. Graphical abstract

Pulsar Timing Array signature from oscillating metric perturbations due to ultra-light axion

Abstract A coherently oscillating ultra-light axion can behave as dark matter. In particular, its coherently oscillating pressure perturbations can source an oscillating scalar metric perturbation, with a characteristic oscillation frequency which is twice the axion Compton frequency. A candidate in the mass range 10(-24,-21) eV can provide a signal in the frequency range tested by current and future Pulsar Timing Array (PTA) programs. Involving the pressure perturbations in a highly nonlinear environment, such an analysis demands a relativistic and nonlinear treatment. Here, we provide a rigorous derivation of the effect assuming weak-gravity and slow-motion limit of Einstein's gravity in zero-shear gauge and show that dark matter's velocity potential determines the oscillation phase and frequency change. A monochromatic PTA signal correlated with the velocity field would confirm the prediction, for example, by cross-correlating the PTA results with the future local velocity flow measurements.

Spectral and temporal features of GX 13+1 as revealed by AstroSat

ABSTRACT GX 13+1, a neutron star low-mass X-ray binary that exhibits the properties of both atoll and Z sources, is studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traces a ν shaped track in its hardness-intensity diagram (HID). Spectral modelling of the data in the 0.7–30.0 keV energy range, with the model – $\tt {constant}$ × $\tt {tbabs}$ × $\tt {thcomp}$ × $\tt {bbodyrad}$ + $\tt {relxillNS}$, yields orbital inclination angle (θ) of 77$\substack{+10\\-8}$°. Flux resolved spectral analysis reveals the ν shaped pattern in the plots of spectral parameters kTe, kTbb, and Γ versus Fbol, closely resembling the pattern traced in LAXPC HID. This indicates changes in the spectral properties of the corona and the boundary layer/accretion disc. Assuming that the accretion disc truncates at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 is calculated to be 5.10 × 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer is estimated to be 5.70 km, which results in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations (QPOs) at 56 ± 4 and 54 ± 4 Hz are detected in Regions D and E of HID, respectively. The frequencies of these QPOs are similar to the characteristic frequency of horizontal branch oscillation and these do not exhibit a positive correlation with mass accretion rate.

On the One‐Dimensional Transition State Theory and the Relation between Statistical and Deterministic Oscillation Frequencies of Anharmonic Energy Wells

AbstractThe transition state theory allows the development of approximated models useful to study the non‐equilibrium evolution of systems undergoing transformations between two states (e.g., chemical reactions). In a simplified 1D setting, the characteristic rate constants are typically written in terms of a temperature‐dependent characteristic oscillation frequency , describing the exploration of the phase space. As a particular case, this statistical oscillation frequency can be defined for an arbitrary convex potential energy well. This value is compared here with the deterministic oscillation frequency of the corresponding anharmonic oscillator. It is proved that there is a universal relationship between statistical and deterministic frequencies, which is the same for classical and relativistic mechanics. The independence of this relationship from the adopted physical laws gives it an interesting thermodynamic and pedagogical meaning. Several examples clarify the meaning of this relationship from both physical and mathematical viewpoints.

Constraints on the rotating self-dual black hole with quasi-periodic oscillations

An impressive feature of loop quantum gravity (LQG) is that it can elegantly resolve both the big bang and black hole singularities. By using the Newman-Janis algorithm, a regular and effective rotating self-dual black hole (SDBH) metric could be constructed, which alters the Kerr geometry with a polymeric function P from the quantum effects of LQG geometry. In this paper, we investigate its impact on the frequency characteristics of the X-ray quasi-periodic oscillations (QPOs) from 5 X-ray binaries and contrast it with the existing results of the orbital, periastron precession and nodal precession frequencies within the relativistic precession model. We apply a Monte Carlo Markov Chain (MCMC) simulation to examine the possible LQG effects on the X-ray QPOs. We found that the best constraint result for the rotating self-dual geometry from LQG came from the QPOs of X-ray binary GRO J1655-40, which establish an upper bound on the polymeric function P less than 6.15 × 10-3 at 95% confidence level. This bound leads to a restriction on the polymeric parameter δ of LQG to be 0.66.

Quasinormal modes of acoustic black hole

In this paper the electromagnetic and massive scalar perturbations of the Schwarzschild acoustic black hole are studied and their characteristic oscillation frequencies, so-called quasinormal modes are calculated. It has been shown that the odd- and even-parity electromagnetic perturbations are isospectral. We have shown that small value of a tuning parameter (ξ<2) of the acoustic black hole decreases the frequency and damping rate of perturbations. A quasi-resonance modes of the massive scalar perturbations that corresponds to possible maximum value of mass of the scalar field to generate the quasinormal modes have been determined. The mass of the scalar field makes the perturbations longer-lived. The time-domain analysis of the perturbations have shown that the acoustic black hole is stable against electromagnetic and scalar perturbations.

Study on stability margin of ultra-low frequency oscillation in hydropower system

In recent years, the ultra-low frequency oscillation of power system caused by hydropower units has occurred frequently. It is very important to study the ultra-low frequency oscillation characteristics of hydropower stations for the stable operation of power system. It is worth further research on how to determine the system stability margin in engineering practice. Based on the established model of single machine system with load, the analysis of the characteristic equation of closed-loop transfer function of single machine system shows that the ultra-low frequency oscillation will occur when the ratio of proportional parameter to integral parameter is too small. Based on the single machine with load system model and the equivalent single machine with load model of multi machine system, the ultra-low frequency oscillation mode of the system is analyzed by eigenvalue analysis method. According to the requirements of the damping ratio index, the security domain of the system ultra-low frequency oscillation is constructed, and the risk and stability margin of the system ultra-low frequency oscillation are evaluated by the characteristic values of the system ultra-low frequency oscillation mode.

Ribonucleic acid genome mutations induced by the Casimir effect

In this paper, we investigate the Casimir effect within a virus RNA, particularizing the study to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Then, we discuss the possibility of occurring damage or mutation in its genome due to the presence of quantum vacuum fluctuations inside and around the RNA ribbon. For this, we consider the geometry and the nontrivial topology of the viral RNA as having a simple helical structure. We initially compute the non-thermal Casimir energy associated to that geometry, considering boundary conditions that constrain the zero point oscillations of a massless scalar field to the cylindrical cavity containing a helix pitch of RNA ribbon. Then we extend the obtained result to the electromagnetic field and, following, we calculate the probability of occurring damage or mutation in RNA by using the normalized inverse exponential distribution, which suppresses very low energies, and consider cutoff (threshold) energies corresponding to UV-A and UV-C rays, surely responsible by mutations. Then, by taking into account UV-A, we arrive at a mutation rate per base per infection cycle, which in the case of the SARS-CoV-2 is non-negligible. We find a maximum value of this mutation rate for an RNA ribbon radius, applying it for SARS-CoV-2, in particular. We also calculate a characteristic longitudinal oscillation frequency for the helix pitch value corresponding to the local minimum of the Casimir energy. Finally, we consider thermal fluctuations of classical and quantum nature and show that the corresponding probability of mutation is completely negligible for that virus. Therefore, we conclude that only the nontrivial topology and the geometric attributes of the RNA molecule contribute to the possible mutations caused by quantum vacuum fluctuations in the viral genome.

Combining the Tait equation with the phonon theory allows predicting the density of liquids up to the Gigapascal range

Predicting the density of liquids at ultrahigh pressures in the case when only the data measured at ambient pressure are available is a long-standing challenge for thermodynamic research. In this work, we archived this goal for molecular liquids by applying the half-sum of the Tait equation and the Murnagnan equation in the form coordinated with Tait’s at low pressure for predicting the density of molecular liquids up to the pressures more than 1 GPa with uncertainty comparable with the experimental one. It is shown that the control parameter, which is needed in addition to the initial density and the isothermal compressibility can be found using the speed of sound and the density at ambient pressure and has a clear physical interpretation in terms of the characteristic frequency of intermolecular oscillation mimicking the limiting frequency of Debye’s theory of heat conductivity of solids. This fact is discussed as arguing in favour of the modern phonon theory of liquid thermodynamics and expands it range of applicability to the volumetric properties of liquids at temperatures far below the critical one. The validity of the model is illustrated with the case study of classic Bridgman’s dataset as well as with some examples of ultrahigh-pressure data obtained by the diamond anvil cell and shock wave compression methods.

Harmonic State-Space Based AC-Side Impedance Model of MMC and High Frequency Oscillation Characteristics Analysis

Recently, high-frequency oscillation of the modular multilevel converter (MMC) based high-voltage direct current (HVDC) projects has attracted great attentions. In order to analyze the small-signal stability, this paper uses the harmonic state-space (HSS) method to establish a detailed frequency domain impedance model of the AC-side of the HVDC transmission system, which considers the internal dynamic characteristics. In addition, the suggested model is also used to assess the system's high-frequency oscillation mechanism, and the effects of the MMC current inner loop control, feedforward voltage links, and control delay on the high-frequency impedance characteristics and the effect of higher harmonic components. Finally, three oscillation suppression schemes are analyzed for the oscillation problems occurring in actual engineering, and a simplified impedance model considering only the high-frequency impedance characteristics is established to compare the suppression effect with the detailed impedance model to prove its reliability.

Analysis of quality of «Puma» soybean seeds by luminescent and spectroscopic methods

The seed shell of soybeans performs a protective function and participates in the vital processes of the seed. Soybean seeds are characterized by a unique chemical composition. The priority direction in the development of technologies for the production of foodstuffs from soybeans is the control of the quality indicators of their seeds shells. The use of a spectrofluorimeter and an IR-Fourier spectrometer allows you to study the coating material of soybean quickly and without destruction. For the purposes of comparative spectral characterization, studies were carried out on the soybean seed envelope of the untreated and treated modifier mixture. Main characteristics of luminescence excitation spectrum are determined including recording wavelength, excitation spectrum region, excitation wavelength intensity. Parameters glow attenuation are revealed by spectral-kinetic studies of phosphorescence of soybean seeds shell. IR spectroscopic analysis of milled and whole soybean seeds, as well as modifiers, allows us to interpret groupings that have characteristic oscillation frequencies. The main spectral characteristics of the soybean seed shell are determined, including wave numbers, incident radiation wavelength, transmission, and oscillation character. Modifiers adsorbed on surface of soybean seed shell are identified. The type of adsorption of the modifier shell is defined.

Size effects in stress propagation and dynamics of dislocations: Fe–Ni–Cr steel

Movement of line dislocations in fcc steel 310S is found to depend on the size of nanometer sized structures, based on modeling within molecular dynamics (MD). The effect is attributed to time (and size) dependencies of pressure propagation into the medium interior. The observation is crucial in interpreting any MD studies of pressure effects since these are governed by time-dependent internal virial stresses. In particular, velocity of dislocations scales better with value of local internal shear component of virial stress S xy than with external shear pressure applied. Dynamics of stress penetration is described well within the model of damped harmonic oscillator, where characteristic oscillation frequency depends on number of crystallographic layers in direction along the wave propagation while the speed of stress propagation is the speed of sound. The minimal stress required for dislocation movement (Peierls stress) is determined to be 0.75 GPa. Pressure and temperature effects on dislocation movement are systematically investigated.

What can clozapine’s effect on neural oscillations tell us about its therapeutic effects? A scoping review and synthesis

Clozapine, a drug effective in treatment resistant schizophrenia, can modulate the brain’s electrical activity as measured by an electroencephalogram (EEG). Past reviews have focused on synthesizing literature related to epileptiform activity or rate of seizures in clozapine treated individuals. The aim of this review was to determine whether clozapine’s mediated effects on measurements related to neural oscillations can inform its therapeutic effects. Here, literature pertaining to studies that implemented pre-post designed investigations of clozapine and measured frequency characteristics of neural oscillations in individuals with schizophrenia were reviewed. The synthesis of findings suggests that while clozapine is associated with alterations in all neural oscillations, slower waves (delta and theta) are consistently increased in power by clozapine. We then further discuss potential mechanisms that may underlie these effects of clozapine. Future research can implement the findings of this review to motivate hypothesis-driven investigations into clozapine responsiveness biomarkers.

Resting State EEG Directed Functional Connectivity Unveils Changes in Motor Network Organization in Subacute Stroke Patients After Rehabilitation.

Brain plasticity and functional reorganization are mechanisms behind functional motor recovery of patients after an ischemic stroke. The study of resting-state motor network functional connectivity by means of EEG proved to be useful in investigating changes occurring in the information flow and find correlation with motor function recovery. In the literature, most studies applying EEG to post-stroke patients investigated the undirected functional connectivity of interacting brain regions. Quite recently, works started to investigate the directionality of the connections and many approaches or features have been proposed, each of them being more suitable to describe different aspects, e.g., direct or indirect information flow between network nodes, the coupling strength or its characteristic oscillation frequency. Each work chose one specific measure, despite in literature there is not an agreed consensus, and the selection of the most appropriate measure is still an open issue. In an attempt to shed light on this methodological aspect, we propose here to combine the information of direct and indirect coupling provided by two frequency-domain measures based on Granger’s causality, i.e., the directed coherence (DC) and the generalized partial directed coherence (gPDC), to investigate the longitudinal changes of resting-state directed connectivity associated with sensorimotor rhythms α and β, occurring in 18 sub-acute ischemic stroke patients who followed a rehabilitation treatment. Our results showed a relevant role of the information flow through the pre-motor regions in the reorganization of the motor network after the rehabilitation in the sub-acute stage. In particular, DC highlighted an increase in intra-hemispheric coupling strength between pre-motor and primary motor areas, especially in ipsi-lesional hemisphere in both α and β frequency bands, whereas gPDC was more sensitive in the detection of those connection whose variation was mostly represented within the population. A decreased causal flow from contra-lesional premotor cortex towards supplementary motor area was detected in both α and β frequency bands and a significant reinforced inter-hemispheric connection from ipsi to contra-lesional pre-motor cortex was observed in β frequency. Interestingly, the connection from contra towards ipsilesional pre-motor area correlated with upper limb motor recovery in α band. The usage of two different measures of directed connectivity allowed a better comprehension of those coupling changes between brain motor regions, either direct or mediated, which mostly were influenced by the rehabilitation, revealing a particular involvement of the pre-motor areas in the cerebral functional reorganization.

Micromagnetic prediction strain and current co-mediated spindynamics in skyrmion-based spin-torque nano-oscillator

In this article, we theoretically investigate the influence of strain on the oscillation frequency characteristics of a skyrmion-based spin-torque nano-oscillator (STNO) using micromagnetic simulations. The system consists of an ultrathin nanopillar magnetic film with interfacial Dzyaloshinskii–Moriya interactions and a ring-shaped region to offer a tunable spatial strain source. The applied strain gives rise to an extra effective field in the magnetic film through magnetoelastic coupling and forms a strong confined potential for skyrmion dynamics. Our simulation results show that the operation frequency of the STNO can be effectively modulated by the strain. Furthermore, we introduce a simplified analytical expression of the skyrmion gyration dynamics to explain the micromagnetic simulation results. Our results will be useful in the development of wide-range frequency-tunable microwave emitters and frequency-shift keying for wireless communication.

Adaptive Control Strategy with Threshold of Virtual Inertia and Virtual Damping for Virtual Synchronous Generator

Virtual Synchronous Generator (VSG) control strategy can provide inertia and damping support for the power system by simulating the operating characteristics of synchronous generator, and improve the frequency response of the power system. First, analyse the influence of various parameters on the output characteristics of the system by establishing a mathematical model of VSG. Secondly, to fully utilize the flexible and controllable characteristics of VSG control strategy, the basic principle of VSG parameter adaptation is obtained through the analysis of the angular frequency oscillation characteristics of synchronous generators, and an adaptive control strategy of virtual inertia and virtual damping parameter is proposed, which can better track changes in frequency, and set the action threshold for adaptive control. Finally, compare the frequency and active power response between the traditional VSG control strategy and the adaptive one and verify the effectiveness of the proposed control strategy.

Flow-measurements in the wake of an oscillating sessile droplet using laser-Doppler velocity profile sensor

Abstract Although relevant in many technical applications, the removal of sessile droplets on surfaces by an (air-) flow is still hard to predict. The flow around the droplet has not been investigated in detail so far but knowledge on the flow structure is essential for the assessment of appropriate drag force correlation. Small droplets (in the range of 5–40 µl) on PMMA substrate in an air flow are investigated. Due to the small size of the droplet the laser-Doppler velocity profile sensor with frequency shift is applied to measure the flow in the wake of the oscillating, still adhering droplet. Thanks to the high spatial and temporal resolution of Laser-Doppler velocity profile sensor and its capability to measure bidirectional flows, the flow behind the droplet can be precisely analyzed. Additional hot-film measurements are used to investigate the temporal behavior of the flow. Complementary, numerical simulation is performed applying a modified VOF (Volume-of-Fluid) method. The combination of the experimental and numerical data gives new insight in the wake flow structure of sessile droplets: With increasing Reynolds number, a backflow can be detected in the wake of the droplet. A separated shear layer stemming from the upper side of the droplet leads to a vortex shedding with formation of a recirculation region in the temporal mean. In contrast to rigid hemispheres, the movement of the air-liquid interface of the droplet leads to an internal flow which is driven by the outer flow structure and vice versa. This is a hint that drag coefficients of sessile droplets cannot be simply derived by analyzing flows of rigid hemispherical structures. Additionally, droplet contour and wake flow exhibit the same characteristic oscillation frequency. The corresponding Strouhal number is almost constant at 0.03 compared to the Strouhal number of a rigid hemisphere of 0.28. Therefore, it can be assumed that an aeroelastic self-excitation effect may be present that eventually leads to droplet movement.