2nd Harmonic Component Research Articles

Possible physical mechanisms of the high echogenicity of lipid coated nanobubbles

Lipid coated nanobubbles (NBs) have attracted a great level of interest as ultrasound (US) contrast agents due to their ability to extravagate through leaky tumor vasculature. Their linear resonance frequency is in the range of ∼50 M Hz–200 MHz, leading to confusion over their observed strong contrast in diagnostic US frequencies. By solving the Marmottant model, the dynamics of uncoated and lipid coated NBs and microbubbles (MBs) are studied over the frequency and pressure ranges (6–12 MHz, 0.1–1.2 MPa) generally used in diagnostic US. A novel bifurcation analysis in tandem with the analysis of the frequency component of the scattered pressure are conducted. Results show that despite the increased linear resonance frequency and viscous damping due to the lipid shell, buckling and rupture of the shell enhances the generation of the 2nd and 3rd harmonic resonances at pressures as low as 0.2 MPa, not observed with uncoated NBs. The generation of the harmonic resonances are concomitant with an abrupt increase in the 2nd and 3rd harmonic frequency component of the scattered pressure with their pressure threshold (PT) increasing with decreasing NBs size. For the same gas volume, and above the PT, the maximum non-destructive 2nd and 3rd harmonic powers of NBs can become higher than the 2–4 μm MBs. Similar to the lower subharmonic pressure threshold of MBs, the dynamic variation of the NBs effective surface tension due to buckling and rupture may be the potential reason behind the observed harmonic echogenicity.

Short-Time Adaline Based Fault Feature Extraction for Inter-Turn Short Circuit Diagnosis of PMSM via Residual Insulation Monitoring

Inter-turn short circuit (ITSC) is a common fault in electrical machines, which may result in further devastation to the whole winding and magnets. In this article, a method for real-time residual insulation capacity monitoring of ITSC fault in permanent magnet synchronous machines (PMSMs) is proposed, which is robust to speed/torque variation and independent of machine parameters and extra sensors. It is based on the theory that the ITSC fault initiates as an insulation deterioration among adjacent turns and the fault severity can be described by the residual insulation capacity being relevant to the 2nd harmonic component in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -axis currents. Thus, a short-time Adaline-based harmonic extraction method is proposed for the separation of the needed 2nd harmonic component from time-varying current signals, being used as the fault indicator afterward. Finally, the performance of the proposed method is experimentally evaluated on a PMSM and shows quite good performance in fast-tracking and detection of faults in various stages.

Inductance Characteristics of Flux-Switching Permanent Magnet Machine Based on General Air-Gap Filed Modulation Theory

Winding inductance is a crucial parameter of machines, which plays an essential role in flux-weakening capability, reluctance torque, power factor, etc. In this article, winding inductance characteristics of flux-switching permanent magnet (FSPM) machines are investigated from the perspective of air-gap modulation magnetic field. With the aid of the general air-gap field modulation theory, the air-gap magnetic field contributed by an arbitrary energized winding of FSPM machines is modeled and analyzed. Then, an analytical expression of winding inductance is derived based on winding function and single-phase magnetic field, through which the relationship between winding inductance and single-phase magnetic field harmonics is unveiled. It is found that the direct current (dc) and 2nd harmonic components of winding inductances in FSPM machines are caused by the dc and 2nd time-harmonic components of primitive harmonics, respectively, which are essentially different from that of traditional interior permanent magnet or reluctance machines. Finally, experiments on a prototype three-phase 12/10 FSPM machine are conducted to verify the theoretical analysis.

A New Approach based on Electromechanical Torque for Detection of Inter-Turn Fault in Permanent Magnet Synchronous Motor

Fault detection is an important issue for permanent magnet synchronous motors (PMSMs). In the initial stage, it is very crucial to detect stator winding inter-turn short-circuit failure, which is one of the most common types of faults. In this paper, a new approach based on electromechanical torque has been proposed to detect the stator inter-turn short circuit fault (ISCF) that occurs in surface-mounted permanent magnet synchronous motors (PMSMs). New fault signatures based on the torque signal that can be used in stator winding ISCF detection are tried to be found in the torque frequency distribution. Fast Fourier Transform (FFT) was used to extract the torque frequency components associated with the stator ISCF. It was found that the amplitudes of the 2nd and 4th harmonic components of the torque signal are distinctive features that can be used for stator winding ISCF detection in PMSM. With the proposed components of the 2nd and 4th harmonic of torque, an inter-turn fault can be easily detected at the initial stage. Both experimental results and simulation results for healthy and three different faulty states (2%, 12.5%, and 25% ISCF) at different load levels and different speeds are presented in this paper.

A torque ripple reduction method using field current injection for doubly salient motor

Doubly salient motor (DSM) is widely used owing to attractively in high-speed operation capability. However, there is a shortcoming of torque ripple for the non-sinusoidal nonlinear structure, for the purpose of solving this problem, this paper presents a torque ripple reduction strategy by field current injection for doubly salient motor. Initially, the armature current control method of the DSM adopts the multi-sine superposition (MSWS) driving scheme, combined with finite element and Fourier analysis methods to establish the output torque and torque ripple model, where the torque of the DSM is mainly composed of the 1st, 2nd, and 4th harmonic component, and the torque ripple is mainly composed of 3rd harmonics component. Then, a field current injection approach is presented, and calculate the magnitude and phase of the injected component, which serves as a quantitative analysis foundation for torque ripple suppression. Finally, the experimental results illustrate the superiority of the presented method in torque ripple optimization.

Vortex generator design and numerical investigation for wake non-uniformity and cavitation fluctuation pressure reduction

Aiming at the problem that the exciting force induced by the propeller is excessive and the hull vibration noise exceeds certain limits for a certain surface ship, a type of triangular vortex generator is designed in this paper which is installed on the surface of ship stern to improve the quality of ship wake and solve the vibration problem. Firstly, the size and installing position of the vortex generator are designed based on the ship body lines. Then, the numerical calculation is conducted both for the bare hull and “hull-propeller-vortex generator” system to investigate the effectiveness of the vortex generator. With the installation of the vortex generator, the wake changes mainly occur in the region near the 12 o'clock. The velocity in the high wake area gains effective acceleration and the circumference transition of wake becomes more moderate which contributes to the smooth transition of the blade unsteady cavitation in circumference direction. What's more, the changes of wake can delay the circumference angle of the blade cavitation collapse. It increases the distance between the positions where the blade cavitation collapsed and the ship bottom shell. It is also beneficial to reduce the amplitude of fluctuating pressure. The calculated results show that the fluctuation pressure energy is spread from 1st, 2nd to 3rd or more high-order harmonic components with help of vortex generator. So, the level of fluctuation pressure drops significantly for the 1st and 2nd harmonic components which are the main exciting sources for the hull vibration.

A Compensation Method for the 2nd Harmonic Component in the Input Currents of Three-Phase Current-Fed PFC Rectifier with Sawtooth Carrier PWM Scheme

A Compensation Method for the 2nd Harmonic Component in the Input Currents of Three-Phase Current-Fed PFC Rectifier with Sawtooth Carrier PWM Scheme

A novel phase-locked loop applied on frequency fluctuation of EAST power supply

The excellent performance of synchronous phase locked technology is the basis of improving the power quality of Experimental Advanced Superconducting Tokamak (EAST) power system. The conventional synchronous reference frame phase-locked loop (SRF-PLL) with low pass filter (LPF) has a poor filter capacity for the 2nd harmonic component as the fundamental frequency fluctuation due to the high-power, high-parameter operation of the EAST. To improve accuracy of the phase estimation, adaptive notch filter (ANF) will be introduced into the LSRF-PLL to reject the disturbance of 2nd harmonic component, which can adaptively track the fluctuation of fundamental frequency with least mean square (LMS) algorithm. A novel composite algorithm of LPF and ANF SRF-PLL (LASRF-PLL) is presented to suppress the disturbance of the frequency fluctuation and harmonic pollution. At the last, the performance of the proposed LASRF-PLL is finally validated by simulation results of MATLAB software.

Design and Optimization of a Fault Tolerant Modular Permanent Magnet Assisted Synchronous Reluctance Motor With Torque Ripple Minimization

This article proposes a new asymmetric modular permanent magnet assisted synchronous reluctance motor (PMaSynRM) to offer low torque ripple and high reliability. First, the stator is segmented into three modules without magnetic coupling. The influence of flux gaps between modules is investigated on torque performance. For the torque improvements, the multilevel optimization strategy is conducted, which simultaneously enhances the fault-tolerant capability. Besides, the torque ripple reduction methods are proposed with the asymmetric rotor and shifted end-slots. The analytical formulae of asymmetric flux barrier angle are derived. Then, the asymmetric flux barrier is designed to reduce the 12th harmonic, and the end-slots are shifted to minimize the additional 2nd harmonic component caused by flux gaps. Finally, the enhanced fault-tolerant capability of the proposed modular PMaSynRM is also evaluated. The effectiveness of the proposed approach is verified by both simulation and experiment.

Winding Fault Diagnosis and Failure Prognosis Technique for Brushless Synchronous Generator

Winding fault diagnosis and failure prognosis technique has been developed based on d-q model of brushless synchronous generator (BLSG) considering actual winding insulation degradation curve of machine in inter-turn fault, damper bar and field currents for various severities of inter-turn winding faults have been estimated using extended Kalman filter. For validation of damper bar currents, 2 D transient electromagnetic analyses using finite element method have been done, and 2nd harmonic component of damper bar currents for various severities of inter-turn fault in stator windings have been computed by Fourier transformation. The winding insulation degradation curve of machine has been determined by accelerated life test, and used for estimation of remaining useful life of machine under inter-turn fault, assuming a pre-defined failure threshold. Actual and predicted life of the BLSG has been compared. Wireless sensors using rotor telemetry system have been mounted on the rotor of BLSG for experimental validation of field currents under winding faults. The 2nd harmonic of damper bar currents increase with increase in severities of winding faults and do not get affected by diode failures in rotating rectifier, hence this can be used as a robust fault indicator for diagnosis and prognosis of inter-turn winding fault in BLSG.

Single- and Two-Phase Open-Circuit Fault Tolerant Control for Dual Three-Phase PM Motor Without Phase Shifting

A fault tolerant control strategy for dual three-phase permanent magnet synchronous motor (DTP-PMSM) with 0° phase shifting between two windings is proposed in this paper. When a DTP-PMSM suffers from single- or two-phase open-circuit fault, to maintain the torque performance, current in faulty phase must be compensated by other healthy phases, which causes asymmetric self- and mutual inductances. Therefore, the 2nd harmonic component can be observed in the dq -axis currents and torque ripple also increases. To analyse this harmonic component in two types of faults, post-fault model of no-phase-shifting DTP-PMSM based on vector space decomposition (VSD) method is built in this paper. Then, proportion-integral-resonant (PIR) controller which can be used to suppress the specific periodic disturbance is applied to compensate the 2nd harmonic components. As the resonant controller is rarely dependent on motor parameters, phase and amplitude of current harmonic, the PIR controller can be applied in no-phase-shifting DTP-PMSM to suppress the harmonic caused by single- and two-phase open-circuit faults. The experimental results validate the effectiveness of the proposed fault tolerant control under single- and two-phase open-circuit faulty conditions.

Numerical simulation of wake field and propulsion performance in a four-screw ship

Computational studies using the Reynolds Averaged Navier–Stokes (RANS) method on viscous wake fields and the propulsion performance of a four-screw ship were conducted. The self-propulsion experimental investigation was carried out in a towing tank to analyze the propulsion performance of the four-screw ship. The characteristics of the wake field and hydrodynamic performance of the inside and outside propellers were compared in detail. The computational fluid dynamics results and experimental data showed good agreement. There were considerably different distribution patterns of the wake field between inside and outside the propeller disc, especially in the zone behind the brackets. The mean axial velocity at different radii of the inside propeller is generally smaller than that of the outside propeller, which results in the time-averaged loads of the inside propeller becoming larger than those of the outside one. Harmonics analysis indicates that in the considered range of the outer radii, the 1st and 2nd order harmonic components of the inside propeller disc are larger than those of the outside one, which causes the fluctuation amplitudes of the loads on the inside propeller to become higher than those of the outside one.

The Grid Voltage’s Asymmetrical Fault Detection Method based on Variable-Step LMS Adaptive Notch Filter Algorithm

For wind turbine systems (WTS), the grid voltage fault detection algorithm determines the response speed of WTS to grid voltage fault. This paper proposed a sort of voltage asymmetrical fault detection method, to detect negative-sequence component in grid voltage, variable-step (VS) least mean square (LMS) adaptive notch filter (ANF) is applied to eliminate the 2nd-harmonic frequency component. The application of ANF can separate negative-sequence component from grid voltage, therefore the voltage asymmetrical fault can be detected; the self-tuning of controllable parameters of ANF can be realized by the LMS adaptive algorithm, which is convenient and efficient in practical application; furthermore, because the variable step size adjustment is adopted, the contradiction between convergence rate and steady-state error of ANF can be solved. The MATLAB/Simulink simulation and FFT analysis results are in conformity with the theorical analysis, which proves the feasibility of the method.

Nonlinear post-flutter behavior and self-excited force model of a twin-side-girder bridge deck

The nonlinear aeroelastic behavior of a typical bluff bridge section, i.e., open twin-side-girder bridge deck, was investigated through a series of spring-suspended sectional model (SSSM) tests in the post-flutter state. When the reduced wind speed exceeded linear aeroelastic limits, the sectional model underwent significant limit cycle oscillation (LCO) characterized by a quasi-harmonic torsional vibration with slight heave-torsion coupling effect. The stable amplitudes of post-flutter LCOs increased in a roughly linear way with reduced wind speed. The nonlinear self-excited force (NSEF) was measured by a novel force measurement technique and validated by comparing the calculated post-flutter responses with experiments. The measured NSEF contained significant 2nd and 3rd order nonlinear harmonic components. Based on the measured NSEF signals and equivalent energy principle, a NSEF model suitable for large amplitude of vibration was proposed and then verified by comparing its predicted response with experimental results. The identified aerodynamic parameters further indicated that the mechanism of post-flutter LCOs was due to the negative aerodynamic damping provided by the linear term as an energy source to drive the growth of vibration amplitude and the nonlinear positive aerodynamic damping provided by the 3rd order angular velocity term as a stabilizing factor of self-limitation phenomenon.

Subcritical vibrations of a large flexible rotor efficiently reduced by modifying the bearing inner ring roundness profile

Abstract Undesired vibration of a rotor is harmful for industrial processes and decreases the machine lifetime. In industrial processes, large rotors are commonly operated below the critical speed. In the subcritical speed range, the bearings are an important excitation source of vibration. The bearing inner ring based excitation is observed at a frequency, which is the rotor rotating frequency multiplied by the number of undulations in the roundness profile of the roller race of the bearing inner ring. Subcritical resonance occurs, when the bearing excitation frequency equals the natural frequency of the rotor system. The present study investigates the effect of the bearing inner ring roundness profile on the subcritical vibrations of a flexible rotor. The roundness profile of the bearing inner ring was measured, while installed on the rotor shaft. The roundness profile was modified to five different geometries to investigate different excitation cases. Finally, the roundness error of the inner ring was minimized. The rotor subcritical vibration was measured with each bearing inner ring geometry in the horizontal and vertical directions. The analysis was focused on the 2nd, 3rd and 4th harmonic vibration components, occurring at 1/2, 1/3 and 1/4 of the critical rotational speed. The results clearly suggest that the roundness profile of the roller race of the bearing inner ring affects the rotor subcritical vibration significantly. The increased waviness components of the bearing inner ring roundness profile increased the corresponding subcritical vibration amplitude. Minimizing the roundness error decreased the subcritical vibration substantially.

Delay-Encoded Harmonic Imaging (DE-HI) in Multiplane-Wave Compounding.

The development of ultrafast ultrasound imaging brings great opportunities to improve imaging technologies such as shear wave elastography and ultrafast Doppler imaging. In ultrafast imaging, several tilted plane or diverging wave images are coherently combined to form a compounded image, leading to trade-offs among image signal-to-noise ratio (SNR), resolution, and post-compounded frame rate. Multiplane wave (MW) imaging is proposed to solve this trade-off by encoding multiple plane waves with Hadamard matrix during one transmission event (i.e. pulse-echo event), to improve image SNR without sacrificing the resolution or frame rate. However, it suffers from stronger reverberation artifacts in B-mode images compared to standard plane wave compounding due to longer transmitted pulses. If harmonic imaging can be combined with MW imaging, the reverberation artifacts and other clutter noises such as sidelobes and multipath scattering clutters should be suppressed. The challenge, however, is that the Hadamard codes used in MW imaging cannot encode the 2nd harmonic component by inversing the pulse polarity. In this paper, we propose a delay-encoded harmonic imaging (DE-HI) technique to encode the 2nd harmonic with a one quarter period delay calculated at the transmit center frequency, rather than reversing the pulse polarity during multiplane wave emissions. Received DE-HI signals can then be decoded in the frequency domain to recover the signals as in single plane wave emissions, but mainly with improved SNR at the 2nd harmonic component instead of the fundamental component. DE-HI was tested experimentally with a point target, a B-mode imaging phantom, and in-vivo human liver imaging. Improvements in image contrast-to-noise ratio (CNR), spatial resolution, and lesion-signal-to-noise ratio ( l SNR) have been achieved compared to standard plane wave compounding, MW imaging, and standard harmonic imaging (maximal improvement of 116% on CNR and 115% on l SNR as compared to standard HI around 55 mm depth in the B-mode imaging phantom study). The potential high frame rate and the stability of encoding and decoding processes of DE-HI were also demonstrated, which made DE-HI promising for a wide spectrum of imaging applications.

Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser.

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

Impact of Different Static Air-Gap Eccentricity Forms on Rotor UMP of Turbogenerator

Theoretical analysis and numerical FEM calculations, together with segmental experiment studies, are used to study the impact of the static air-gap eccentricity forms on the rotor unbalanced magnetic pull (UMP) of turbogenerator. The universal expression of the magnetic flux density under different forms of SAGE is firstly deduced, based on which the detailed UMP formulas for the normal condition and three SAGE cases are obtained, respectively. Then the exciting characteristics of the UMP for each SAGE form to generate vibrations are analyzed. Finally, numerical FEM calculations and segmental experiments are carried out to investigate the effect of SAGE forms on the rotor UMP, taking the SDF-9 type non-salient-pole fault simulating generator as the object. It is shown that, no matter what kind of SAGE occurs, amplitude increments at each even harmonic component of the UMP and the rotor vibration, especially the 2nd harmonic component, will be brought in. Meanwhile, the UMP keeps directing to the very position where the minimum radial air-gap is. Among the different SAGE forms, the rotor offset has the most sensitive effect on the rotor UMP and vibration, while the stator ellipse deformation has the weakest impact.

Stub-Loaded Wilkinson Power Divider with Performance Enhancement

Microstrip Wilkinson power dividers with harmonic suppression and size reduction are investigated. It is found that by loading reactive components at the middle of high impedance transmission lines (TLs), both size reduction and harmonic suppression can be achieved. Analyses and designs of such a kind of power divider are formulated in this paper. To demonstrate the design methodology, two power dividers centered at 1.8 GHz are optimally designed and confirmed by experiments. As compared with conventional Wilkinson power divider, the proposed power divider exhibits 55.6% size reduction, and high suppressions are achieved for 2nd and 3rd harmonic components. Both simulations and measurements are presented with good agreement.

The influence of higher harmonic flow forces on the response of a curved circular cylinder undergoing vortex-induced vibration

Vortex-induced vibration (VIV) of a curved circular cylinder (a quarter of a ring, with no extension added to either end) free to oscillate in the crossflow direction was studied experimentally. Both the concave and the convex orientations (with respect to the oncoming flow direction) were considered. As expected, the amplitude of oscillations in both configurations was decreased compared to a vertical cylinder with the same mass ratio. Flow visualizations showed that the vortices were shed in parallel to the curved cylinder, when the cylinder was free to oscillate. The sudden jump in the phase difference between the flow forces and the cylinder displacement observed in the VIV of vertical cylinders was not observed in the curved cylinders. Higher harmonic force components at frequencies twice and three times the frequency of oscillations were observed in flow forces acting on the vertical cylinder, as well as the curved cylinder. Asymmetry in the wake was responsible for the 2nd harmonic force component and the relative velocity of the structure with respect to the oncoming flow was responsible for the 3rd harmonic force component. The lock-in occurred over the same range of reduced velocities for the curved cylinder in the convex orientation as for a vertical cylinder, but it was extended to higher reduced velocities for a curved cylinder in the concave orientation. Higher harmonic force components were found to be responsible for the extended lock-in range in the concave orientation. Within this range, the higher harmonic forces were even larger than the first harmonic force and the structure was being excited mainly by these higher harmonic forces.