Circular Frequency Research Articles

Effect of spanwise groove on the dynamic stall characteristics of an airfoil

Numerical simulations are conducted to investigate the effect of triangular groove on the dynamic stall characteristics of a NACA 0012 airfoil at a Reynolds number of 135,000. The right-angled triangular grooves are placed at either 10%, 25%, or 50% chord locations on the suction and have depths of 0.025c and 0.05c, measured normal to the surface of the airfoil. The solutions that are second order accurate in time and space are obtained using pressure-based finite volume solver and the 4-equation transition SST turbulence model viz. γ- Re θt is used to predict transition and viscous stresses accurately. The airfoil is in harmonic pitch motion about its quarter-chord with a maximum circular frequency of 18.67 rad/s. The results suggest that the presence of a groove, except for the deeper grove at 0.5c, quickens the dynamic stall, but with smaller rise in C l,max and a less severe fall in lift at the stall. The mean C l value during the downstroke is improved by up to 8% for the deeper groove at 0.25c, reducing the hysteresis in lift significantly. The grooves at 0.1c, 0.25c, and 0.5c also reduce the drag by 4%, 7%, and 9% during a complete cycle, with subsequent improvements of 54%, 69%, and 63% in the l/d ratio. The current finding can be thus used to enhance the performance of flapping wing MAVs, helicopter rotors, and wind turbine blades as these applications encounter the dynamic stall phenomena frequently.

Multitarget Position-Sensitivity Security Transmission with OAM-DM Based on FDA

The multitarget position-sensitivity security transmission scheme with orbital angular momentum (OAM) directional modulation (DM) waveform using the uniform circular frequency diverse array (FDA) is proposed. The transmitter employs FDA to generate dual-mode range-dependent OAM beam pattern, and the direction information of OAM beam is modulated into the signal. According to the modulation method, orthogonal carrier frequency can be designed to realize multitarget position-dependent OAM pattern. In the undesired position, the intensity pattern and the phase front of the radio beam vary randomly with the digital transmission sequence. Because modulation waveform is position-dependent, the technique offers security, as the signal can be purposely distorted in other positions. The composite dual-mode OAM signal makes it more difficult for eavesdroppers to demodulate correct messages. The receiver with a single antenna employs the phase compensation and helical phase factor to restore the correct digital signal in the desired position. Numerical results show that multitarget position-sensitivity OAM-DM technology based on FDA offers the security transmission scheme.

Influence of the internal varying density flow on the vibrations and fatigue damage of a top-tensioned riser undergoing vortex-induced vibrations

The top-tensioned risers have been widely used in the field of ocean engineering. They are usually used to transport fluid flows from seabed wells to surface platforms. Under the action of ocean currents, these risers are often subjected to vortex-induced vibrations (VIV), which could cause serious fatigue damage. When the riser carries oil and natural gas for the same time, the total fluid density will change with time and space and causes some effects on the riser. In this paper, a nonlinear dynamic model of a top-tensioned riser conveying varying density flow and simultaneously undergoing VIV is established based on the Hamilton's principle. The present model is numerically solved. The results are compared with experiments and CFD simulations, which are in reasonable agreement. With the present model, the influence of internal varying density flow on the vibrations and fatigue damage of a VIV riser is analysed in detail. The results demonstrate that when the internal fluid density fluctuates with a small circular frequency, the vibrations of the riser will be strongly affected. As the mean fluid density or the fluctuation amplitude of the internal varying fluid density increases, the fatigue damage of the riser will become larger.

Evaluation of melt-water premixture formation due to Rayleigh-Taylor instabilities

A linear analysis of the stability of the three-layer stratified hydrodynamic system “water (top) – steam – melt (bottom)” has been performed. Kinematic and dynamic conditions on the “water – steam” and “steam – melt” interfaces are formulated, and on their basis a dispersion equation is obtained that relates the circular frequency of perturbations to the wavenumber. Analysis of this equation made it possible to determine the region of instability of this system and to find the wavenumber of the most fastly growing harmonics. The results obtained were used to estimate the size of bubbles formed at the interface between steam and water due to the development of the Rayleigh-Taylor instability. The obtained theoretical results are consistent with experimental observations in such systems. The heights of the melt splashes into water due to the collapse of the formed steam bubble were estimated. The obtained estimations demonstrate possibilities of the formation of melt-water mixture region during the spreading of the melt under a water layer, in which a strong steam explosion can occur.

Improving the mechanical-mathematical model of pneumatic vibration centrifugal fractionation of grain materials based on their density

This paper has substantiated the mechanical-mathematical modeling of the process of fractionation of grain material into fractions. It has been established that this could optimize the process parameters and would make it possible to design new or improve existing working surfaces of centrifugal separators. A mechanical-mathematical model of the pneumatic vibratory centrifugal separation of grain material by density has been improved. This research is based on the method of hydrodynamics of multiphase media. The improved mechanical-mathematical model takes into consideration the interaction between the discrete and continuous phases of grain material by introducing conditions of interaction at the interface of these phases. In the hydrodynamic modeling of the movement of the circular layer of seeds, the coefficient of dynamic viscosity of discrete and continuous phases was taken into consideration. It was established that the pneumatic vibratory centrifugal separation process parameters are critically affected by the circular frequency of rotation of the cylindrical working surface, the frequency and amplitude of its oscillations. As well as such process characteristics as the airflow rate, dynamic viscosity coefficient, the average thickness of a grain material layer, and the mean density of its particles. Rational values for the technical parameters of the grain material pneumatic vibratory centrifugal fractionation process in terms of density have been determined by using the improved mechanical-mathematical model. The amplitude and oscillation frequency of the working surface are in the ranges A=(35…50)·10–5 m, ω=15.0...15.6 rad/s. The circular rotation frequency of the working surface, ω=24...25 rad/s. The airflow rate, V=2 m/s. It was established that using the improved mechanical-mathematical model of fractionation makes it possible to improve the performance of a pneumatic vibratory centrifugal separator by 9 %. At the same time, the effectiveness of grain material separation could reach 100 %.

Improving image quality in transcranial magnetic resonance guided focused ultrasound using a conductive screen

Transcranial Magnetic Resonance guided Focused Ultrasound (TcMRgFUS) has been proven to be an effective treatment for some neurological disorders such as essential and Parkinson's tremor. However, magnetic resonance guidance at 3 Tesla (3T) frequencies and using the large hemispherical transducers required for TcMRgFUS results in artifactual low-signal bands that pass through key regions of the brain. The purpose of this work was to investigate the use of a circular conductive Radio Frequency (RF) screen, that is bent to have a 12 cm radius in one direction and positioned near the top or back of the head, to reduce or remove these artifactual low-signal bands in TcMRgFUS.The impact of using an RF screen to remove these low signal bands was studied in both imaging experiments and electromagnetic simulations. Hydrophone measurements of the acoustic transparency of the bronze 2 mm diameter square mesh screen used in the imaging studies were compared with temperature measurements with and without the screen in heating studies in the TcMRgFUS system.The imaging and simulation studies both show that for the different screen configurations studied in this work, RF screen removes the low-signal bands and increases both homogeneity and signal-to-noise ratio (SNR) throughout the region of the brain. Hydrophone and heating studies indicate that even a 2 mm wire mesh provides minimal attenuation to the ultrasound beam. Simulation results also suggest that a 1 cm mesh will provide adequate artifact suppression with even less ultrasound attenuation.An RF screen that disrupts the natural waveguide nature of the transducer in the 3T MR environment can change the electromagnetic field profile to reduce unwanted artifacts and provide an imaging region which has more homogeneity and higher SNR throughout the brain.

Nonlinear forced vibration and resonance analysis of rotating stiffened FGM truncated conical shells in a thermal environment

In this paper, a semi-analytical method is presented for nonlinear vibration analysis of rotating stiffened truncated conical shells with functionally graded materials (FGM) in the thermal environment subjected to harmonic excitation. Considering Coriolis acceleration and the centrifugal force, the governing equations of stiffened rotating FGM truncated conical shells are extracted utilizing the classical shell theory regarding the nonlinear von Karman relationships and the smeared stiffeners technique. The partial differential equations are discretized to a set of ordinary nonlinear equations by employing Galerkin’s approach. The condensation method is utilized to reduce the equations of motion into the transverse direction. For analyzing the nonlinear resonance behavior, we use the multiple scales method. According to this approach, the primary resonance of the nonlinear vibration of the stiffened rotating FGM truncated conical shells is examined. Besides, linear free vibration of the stiffened rotating shell is analyzed to investigate the natural circular frequency. Numerical results are studied to investigate the influence of shell rotating speed on the frequency response curve. Also, the effects of force amplitude, volume–fraction index, temperature changes, and different vertex angles on the frequency response curve are examined for various rotating speeds.

Three-Dimensional Parameter Estimation of Uniform Circular Frequency Diverse Array Radar With Two-Stage Estimator

Frequency diverse array (FDA) radar has been studied by many researchers due to the range-angle dependent beampattern which is different from the conventional phased array radar. However, the contribution of previous studies has received little attention within the circular frequency diverse array radar and no known empirical research has focused on investigating the three-dimensional (3D) parameter estimation (azimuth, elevation, and range). In this paper, we proposed a two-stage estimation algorithm for 3D parameter estimation of uniform circular frequency diverse array (UC-FDA) radar and firstly demonstrate the capability and performance of 3D parameter estimation. The first step of the two-stage estimation is the estimation of azimuth and elevation from the beamspace which is transformed from the element space by the beamformer. The second step is to substitute the angle obtained in the first step into the transmitting steering matrix for the range estimation. Cramér-Rao bound (CRB) that is used to evaluate the 3D parameter estimation performance is derived to compare. Theoretical analysis and numerical simulation demonstrate that the UC-FDA radar has superior 3D parameter estimation performance and the effectiveness of the proposed two-stage estimation algorithm. The research findings can contribute to a better 3D localization method and represent an additional step towards developing multi-dimensional localization and interference suppression for UC-FDA radar.

A Combined Energy Method for Flutter Instability Analysis of Weakly Damped Panels in Supersonic Airflow

A combined energy method is proposed to investigate the flutter instability characteristics of weakly damped panels in the supersonic airflow. Based on the small damping assumption, the motion governing partial differential equation (PDE) of the panel aeroelastic system, is built by adopting the first-order piston theory and von Karman large deflection plate theory. Then by applying the Galerkin procedure, the PDE is discretized into a set of coupled ordinary differential equations, and the system reduced order model (ROM) with two degrees of freedom is obtained. Considering that the panel aeroelastic system is non-conservative in the physical nature, and assuming that the panel exhibits a single period oscillation on the flutter occurrence, the non-conservative energy balance principle is applied to the linearized ROM within one single oscillation period. The obtained result shows that the ROM modal coordinate amplitudes ratio is regulated by the modal damping coefficients ratio, though each modal damping coefficient is small. Furthermore, as the total damping dissipation energy can be eliminated due to its smallness, the He’s energy balance method is applied to the undamped ROM, therefore the critical non-dimensional dynamic pressure on the flutter instability occurrence, and the oscillation circular frequency amplitude relationship (linear and nonlinear form) are derived. In addition, the damping destabilization paradoxical influence on the system flutter instability is investigated. The accuracy and efficiency of the proposed method are validated by comparing the results with that obtained by using Routh Hurwitz criteria.

A Self-consistent, Time-dependent Treatment of Dynamical Friction: New Insights Regarding Core Stalling and Dynamical Buoyancy

Abstract Dynamical friction is typically regarded as a secular process, in which the subject (“perturber”) evolves very slowly (secular approximation) and has been introduced to the host over a long time (adiabatic approximation). These assumptions imply that dynamical friction arises from the LBK torque with nonzero contribution only from purely resonant orbits. However, dynamical friction is only of astrophysical interest if its timescale is shorter than the age of the universe. In this paper we therefore relax the adiabatic and secular approximations. We first derive a generalized LBK torque, which reduces to the LBK torque in the adiabatic limit, and show that it gives rise to transient oscillations due to nonresonant orbits that slowly damp out, giving way to the LBK torque. This is analogous to how a forced, damped oscillator undergoes transients before settling to a steady state, except that here the damping is due to phase mixing rather than dissipation. Next, we present a self-consistent treatment that properly accounts for time dependence of the perturber potential and circular frequency (memory effect), which we use to examine orbital decay in a cored galaxy. We find that the memory effect results in a phase of accelerated, super-Chandrasekhar friction before the perturber stalls at a critical radius, R crit, in the core (core stalling). Inside R crit the torque flips sign, giving rise to dynamical buoyancy, which counteracts friction and causes the perturber to stall. This phenomenology is consistent with N-body simulations, but has thus far eluded proper explanation.

Ultralow Scattering and Broadband Metasurface Using Phase Adjustable FSS Elements Embedded With Lumped Resistors

This letter presents a novel phase-cancellation and absorptive metasurface (PCAM) with ultralow scattering and broadband features. The PCAM is formed by two circular frequency selective surface elements with lumped resistors, which can not only reduce reflection amplitudes by transforming the incident waves into heat but also adjust reflection phase responses simultaneously. The two electromagnetic scattering mechanisms, phase cancellation and absorption, cooperate in the same band and single layer. This is the first proposed ultralow backscattering performance where the monostatic radar cross section (RCS) is reduced by more than 20 dB in 5.0–10.9 GHz. Simultaneously, an RCS reduction over 10 dB is achieved at a broad frequency band of 4.1–12.0 GHz under normal incidence. Both the analysis and measurements are demonstrated in detail. The good agreement between simulated and measured results validates the proposed PCAM.

Visualization of user interactions with a pressure mapping mobile application for wheelchair users at risk for pressure injuries

ABSTRACT Pressure injuries for individuals with spinal cord injuries (SCI) are correlated with mortality and are a leading cause for rehospitalizations. The Assisted Weight Shift (AW-Shift©) is a mobile pressure mapping application designed to provide users with a live map view and reminders to perform weight shifts. Novel visualization techniques were used to understand daily distributions of user interaction with AW-Shift©. The date and time of system interactions were recorded for six participants with SCI over 7 days (five males/one female, five manual users/one power user, 55.3 ± 17.3 years old, 10.6 ± 6.5 years since injury). Circular frequency plots were created to visualize the time and frequency participants brought the app to the foreground of their phone and received alerts and reminders to complete weight shifts. While some participants used the system regularly throughout the day, others primarily used it before 8am; highlighting the system’s importance for regular spot checks and morning wheelchair setup. Participant adherence to weight shift reminders was low suggesting the live pressure map may be more useful. Circular frequency plots can be used by clinicians to more easily review large amounts of patient data. Future work will investigate raw pressure mat data and create a closed-loop weight shift detection algorithm.

Energy and frequency ripple in devices with inertial excitation of oscillations.

We consider vibration devices that consist of softly vibration-isolated rigid bodies subjected to vibrations transmitted by means of inertial vibration exciters (unbalanced rotors) driven into rotation by electric motors. Typically, when designing such devices, it is assumed that the rotors rotate uniformly with a certain circular frequency and the body performs small harmonic oscillations with the same frequency. The present work, using a second-order approximation of their nonlinear coupled differential equations, shows that the rotor and the oscillating body keep exchanging energy. At the same time, the angular velocity of the rotor oscillates with the working frequency as well as with its multiple frequencies during each revolution. As a result, the acceleration of the oscillating body also acquires harmonics with multiple frequencies. This may cause both unwanted and beneficial resonance phenomena. We obtain formulae describing the magnitudes of these ripples. We show that the magnitude of oscillations of the angular frequency can also be estimated using energy considerations. Such estimates are provided for the three most common schemes of dynamic devices. Available experimental data confirm the main conclusions of the theory. We discuss both the harmful effects of these phenomena as well as their possible applications. The latter include design of bi-harmonic vibration exciters and exciters based on vibrational resonance. This article is part of the theme issue 'Vibrational and stochastic resonance in driven nonlinear systems (part 2)'.

Dynamical response of a rotating cantilever pipe conveying fluid based on the absolute nodal coordinate formulation

Abstract A dynamical model of a rotating cantilever pipe conveying fluid is derived based on the absolute nodal coordinate formulation. The free vibration and dynamical response of the system are investigated in this paper. Based on the absolute nodal coordinate method and the extended Lagrangian equation proposed by Irschik for the nonmaterial system, the motion equation of the rotating flexible cantilever pipe conveying fluid is built. The influence of the rotational angular velocity and flow velocity on the natural frequency of the system is analyzed. The critical nondimensional circular frequency of in-plane vibration and critical nondimensional flow velocity are investigated. The static deformation is shown under different flow velocities and angular velocities. The nonlinear transient analyses of a rotating flexible cantilever pipe are completed with the variation of parameters. During the rotation, the Coriolis force of fluid acting on the pipe has a great effect on the static deformation.

Effect of electronic excitation on nuclear magnetic shielding as illustrated by nuclear spin optical rotation (NSOR) in liquids

Near resonance effect of NSOR generates new resonant light shift ω well separated from the ground state optical chemical shift ω 0 , from which the shielding in the excited state can be obtained. • The near resonance effect on the optical chemical shift in NSOR is theoretically studied. • The dressed state theory is employed to explore the near resonance effect. • New optical chemical shift line in NSOR spectrum due to excited state molecule is estimated. • Possibility of detecting the new optical chemical line is discussed. Conventional NMR is generally a technique for electronic ground state molecules. In this paper, the effect of electronic excitation on nuclear magnetic shielding is theoretically illustrated by Nuclear Spin Optical Rotation (NSOR) in liquids. The near-resonance effect on the optical chemical shift in NSOR is explored by using the dressed state theory. The results indicate that in the NSOR spectrum of a molecule with nucleus I = 1/2, such effect can induce a new optical chemical shift line with the circular frequency different from that of the ground state. From it the NMR shielding of a molecule in the electronic excited state can be got directly and used to analyze excited molecule structure in liquids. For multi-nuclei molecules, the result applies to each nucleus.

Quantum Mechanics from the Energy Circulation Theory — Wave Function Showing an Energy Location in the 3D Real Space

The Schrödinger equation is one of the cores in quantum mechanics, but bears a contradiction. In the process to obtain the energy and momentum operators, the relation [Formula: see text] is used for [Formula: see text]. However, when they are applied to the Hamiltonian equation, the kinetic energy is set as [Formula: see text]. Based on the Energy Circulation Theory, we examine in this paper the quantization of motions of a particle. We clarify in which situation and for what energy we can use the relation [Formula: see text]. We derive a wave equation de novo to provide wave functions representing a concrete motion and energy distribution of a particle. The Schrödinger equation has a similar form by chance but the mass in our new equation is that of energy quantum expressed by [Formula: see text], which is common for any energies of any particles and decided only by the moving speed. A solution shows an energy location in the 3D real space even if it is expressed in complex. When a motion of a particle gets in circle, its circular frequency becomes quantized. In an atom, an electron circulates around the hidden dimensional axis, and the circulation can further rotate. We propose the quantization conditions for the electron orbiting, and derive the wave functions in concrete for S and P orbitals, which are different from current perceptions. We also demonstrate that the uncertainty principle is not valid for a motion of a particle.

Heat Transfer Due to Thermoelastic Wave Propagation in a Porous Rod

Abstract This paper investigates the propagation of thermoelastic waves in a homogeneous, linear, and isotropic porous solid. For physical and mathematical simplicity, one-dimensional wave propagation in a porous solid rod is considered to explain the concept of heat transfer caused by motion. The solutions of governing equations show that the transfer of heat in a porous rod is not only due to the conduction but also produced by the local particle displacement phenomenon. It is observed that the time-averaged transfer of heat depends on the circular frequency, porosity, thermal conductivity, thermal relaxation, specific heat, and other material coefficients.

Numerical algorithm for searching for layouts of electroelastic bodies with external electric circuits for obtaining the best damping properties

This paper presents an algorithm which allows finding such layouts of electromechanical systems that provide the best vibration damping whether for one mode or a set of vibration modes within some continuous frequency ranges. The basis for the algorithm is the problem solution about natural vibrations. An elastic structure with a piezoelectric element located on its surfaces, which electrodes are connected to a passive external electric circuit, is treated as an electromechanical system. The piezoelectric elements shunted with an electric circuit are the devices where energy dissipation occurs, thus leads to damping of vibrations. A change in damping properties of such systems can be reached by a proper choice of parameters of the electric circuits and corresponding location of the piezoelectric element, which provides the highest energy withdrawal into the electric circuit. The paper presents the mathematical formulation of the natural vibrations problem for piecewise-homogeneous electroelastic bodies shunted with passive external electric circuits. Within the proposed mathematical statement, the problem solution of natural vibrations for such objects is based on values of complex natural vibration frequencies. Real parts of the complex natural vibration frequencies are the circular frequency of vibrations, and the imaginary parts are the damping indices of vibrations. We proposed techniques aimed at determining the location of the piezoelectric element and selecting parameters of shunting the external electric circuit. These approaches are based on values of the complex natural vibration frequencies obtained as results of solving the natural vibrations problem. The proposed approach is demonstrated using a specimen of a thin-walled shell in the semi-cylinder form. The piezoelectric element has a form of a segment of a ring made of PZT-4 piezoceramics. Electrodes of the piezoelectric element are connected to the series resonant RL -circuit. The formulated problem is solved numerically using the finite element method and ANSYS commercial package.

Time‐variant focusing range‐angle dependent beampattern synthesis by uniform circular frequency diverse array radar

Time‐variant focusing range‐angle dependent beampattern synthesis by uniform circular frequency diverse array radar

Características del circular de cordón umbilical simple en neonatos de partos eutócicos atendidos en servicio de primer nivel

Introducción: El cordón umbilical, principal componente que interviene en el intercambio gaseoso y nutricional, de la placenta al feto, además este tenga una óptima oxigenación para un desarrollo adecuado, se describió la presencia de cordón umbilical alrededor del cuello y pecho del feto, considerándolo como riesgo para la vida fetal. El objetivo del trabajo es: Identificar las características del circular de cordón umbilical simple en cuello de neonatos nacidos por parto eutócicos en un servicio de primer nivel de atención. Metodología: El trabajo es cuantitativo, descriptivo, transversal y retrospectivo. La muestra estuvo conformada por 506 neonatos nacidos de partos eutócicos. Resultados: De 506 recién nacidos 37.5% presentaron circular de cordón simple en cuello, mujeres 48% y hombres 52%. En relación con el peso al nacer: bajo 3%, sobre peso 4% y normal 93%. Y con el número de gestación de la madre se encontró; primigestas 20%, secundigestas 36% y multigestas 44%. Discusión: La frecuencia de circular de cordón en esta población de estudio es elevada en relación con otras, siendo frecuente; en neonatos hombres, multigestas y secundigestas. Sin diferencias marcadas entre el peso de los recién nacidos. No se encuentra asociación con resultados perinatales adversos.
 Conflictos de Intereses
 Los autores declaramos no tener conflictos de interés para el presente estudio