Traveling Wave Characteristics Research Articles

Fluidic undulation effects on carangiform swimmers propelled by internal active bending moments

With different shapes and material properties, fish all achieve undulatory swimming gait under the action of internal active muscle stimulation and external fluid forces. Such locomotion can be decomposed into deformation affected by internal and external forces in the body frame and overall translation and rotation solely determined by fluid forces. In order to revisit the undulatory swimming gait, we investigate the hydrodynamic performance of two-dimensional flexible carangiform swimmers with varying stiffnesses and thicknesses, which are driven by the active internal bending moments, and employ the complex orthogonal decomposition and Fourier decomposition methods to quantitatively measure and analyze the proportion of undulation. It is found that standing wave deformation characteristics are prominently observed along fish-like bodies with high stiffness, whereas traveling wave characteristics are more evident in bodies with lower stiffness. The self-propelled fish body demonstrates lateral oscillation and rotation around its center of mass, namely, the heaving and pitching movement, particularly in specimens with high stiffness. The present analysis shows that the heaving and pitching locomotion induced by the fluid significantly increase the traveling wave proportion by modulating the amplitude and phase of the left and right traveling waves viewed in forward frame. We called it fluidic undulation effects (FUE), which is different from the undulation of body deformation. This effect is more pronounced for large stiffnesses and thin airfoils. The standing wave deformation observed with a large stiffness transforms into a traveling wave propulsion pattern, with its traveling wave index even slightly surpassing that of a small-stiffness pattern. Although the efficiency of the standing wave deformation is low, it facilitates a faster forward speed (body lengths per stroke). The positive impact of the FUE on the swimming performance is also confirmed by restricting the recoil motions of the lateral translation and rotation of the body. Furthermore, we observe that there is no undulatory swimming gait that has both the highest energy efficiency and the highest speed.

Traveling Wave Characteristics Based Pilot Protection Scheme for Hybrid Cascaded HVDC Transmission Line

Traveling Wave Characteristics Based Pilot Protection Scheme for Hybrid Cascaded HVDC Transmission Line

Investigation on Free Vibration of Rotating Cylindrical Shells with Variable Thickness

In order to improve the performance and efficiency of the rotating cylindrical shell (RCS), one of the effective ways reduces the mass of the RCS. The scientific and effective method is to design the variable thickness of RCS (VTRCS) along the axial direction in response to this demand. Then, the vibration traveling wave characteristics of VTRCS are investigated by Chebyshev–Ritz method. The displacement field of the cylindrical shell is expanded in the form of the product of the Chebyshev polynomial and the boundary function. The kinetic and potential energies of the VTRCS are calculated based on the Sanders shell theory considering the effects of Coriolis forces and centrifugal forces. Also, the frequency equation of the VTRCS is obtained according to the Ritz method. The accuracy of the modeling method is verified by comparing the present results with literature that had done, and the convergence of the calculated results is studied. Finally, the free-vibration traveling wave characteristics of the VTRCS in different thickness variations are compared, and the parameters such as the rotational speed, thickness variation parameters, and aspect ratio on the free-vibration traveling wave characteristics of the VTRCS are discussed. The influence of thickness variation on the vibration characteristics is analyzed, which has significance for the lightweight design for VTRCS.

Traveling Wave Characteristics of a Rotating Functionally Graded Laminated Cylindrical Shell with General Boundary Conditions

The traveling wave modes of rotating functionally graded laminated cylindrical shells with general boundary conditions are studied by using different admissible displacement functions. Firstly, the effective temperature-dependent material properties of the functionally graded materials (FGMs) are described by the Voigt model and the Sigmoid-type volume fraction. Subsequently, considering Love’s thin shell theory, the motion equations of the FGMs shell subjected to the Coriolis force, the centrifugal inertial force, the hoop initial tension and the internal thermal force are derived. Thereafter, the displacement functions are expanded by six different sets of admissible displacement functions. The Rayleigh–Ritz method is utilized to derive the eigenvalue equations of the rotating FGMs laminated shells. In the end, the convergence rate and computational efficiency of the six polynomial functions are compared and parameter study is carried out. Results indicate that the Legendre orthogonal polynomials (LOP) are the best ones in terms of convergence rate and computational efficiency for predicting the traveling wave modes. The patterns of [Formula: see text] expressed by Sigmoid law are more sensitive to the modal frequency of the shell than the ceramic volume fraction exponents. The stiffness of the axial springs plays a more significant role in the traveling wave modes than the stiffness of other springs.

Experimental investigation of vortex-induced vibration of a flexible pipe in bidirectionally sheared flow

The existence of a bidirectionally sheared flow field caused by internal solitary waves has recently been confirmed according to a field survey in the South China Sea. A model test of a tensioned flexible pipe in bidirectionally sheared flow was performed in an ocean basin. The model was 28.41mm in diameter and 7.64m in length. The purpose of the model test was to understand the response performance and obtain benchmark data of vortex-induced vibration (VIV) in bidirectionally sheared flow. The test was performed on a rotating test rig to simulate bidirectionally sheared flow conditions. Fiber Bragg Grating (FBG) strain sensors were arranged along the test pipe to measure bending strains, and the modal analysis approach was used to determine the VIV response. Reduced velocities based on the tested first natural frequency in water reached 30.79. The cross-flow and in-line VIV amplitudes, response frequencies in statistics and time-domain analysis, traveling wave characteristics, phase synchronization and trajectories are presented in this article. The maximum root mean square (RMS) VIV amplitudes in the test reached 0.51 diameters and 0.18 diameters in the cross-flow and in-line directions, respectively. A relative low Strouhal number of 0.10 was found in the CF direction.

Fault Analysis and Traveling Wave Protection Based on Phase Characteristics for Hybrid Multiterminal HVDC Systems

By fully combining the advantages of line commutated converter (LCC) high-voltage direct current (HVDC) and modular multilevel converter (MMC)-HVDC systems, hybrid multiterminal HVDC (hybrid-MTDC) transmission technology has become a new technical means to overcome the shortcomings of conventional two-terminal HVDC systems. However, the conditions of high fault resistances and more sophisticated network topologies have brought considerable challenges to the existing traveling wave (TW) protection principles. In this article, a parallel hybrid-MTDC system is taken as the research object, and its mathematical analysis model is established. Based on the mathematical model, the TW characteristics that the phase of the high-frequency voltage TW during the occurrence of an internal and external fault approaches approximately 90° and 0°, respectively, are clarified. Therefore, considering the influences of reflection and refraction of TWs and line propagation, the phase-characteristics-based criterion is constructed. Using the proposed criterion, a protection scheme for hybrid-MTDC systems is proposed, which realizes the identification of line faults with high fault resistance. The proposed method is verified in detail with the electromagnetic transient simulation model using PSCAD/EMTDC, and the test results show that the proposed protection scheme is feasible and has high sensitivity, reliability, and adaptability to different operation modes.

Single‐pole fault protection for MMC‐HVDC transmission line based on improved transient‐extracting transform

Due to the overload limitation of the converters, the main protection for DC lines of High Voltage Direct Current based on Modular Multilevel Converter (MMC-HVDC) requires ultra high speed for fault detection. The protection principles of single-terminal traveling-wave can meet this requirement. Such protection principles which are usually constructed by transmission theory of fault traveling-wave have a clear physical concept and do not depend on boundary components. Almost all of these principles are dependent on accurately detecting the arrival time of the traveling-waves. Thus, it is urgent to research a new time-frequency analysis method with a high time resolution to identify the arrival time of traveling-wave. This paper introduces Transient-Extracting Transform (TET) to detect traveling-wave and improves it fully considering the fault traveling-wave characteristics of DC lines. The time resolution of improved TET is greatly increased and the arrival time of traveling-wave is detected more accurately. Using the improved TET, a protection scheme for single-pole fault based on the arrival time difference between aerial-mode traveling-wave and zero-mode traveling-wave is studied. The theoretical analysis and simulation results show that the performance of detecting traveling-wave arrival time by the improved TET is higher than wavelet transform. Under the same fault condition, the time resolution of wavelet transform is only tens microsecond level. However, the time resolution of the improved TET increases to several microseconds. The accuracy of traveling-wave detection by the improved TET is 95% higher than existing works. The simulation results also indicate that the single-pole protection scheme using the improved TET can accurately distinguish between internal faults and external faults. There is a better ability to resist fault resistance for the protection scheme, and it is not affected by DC boundary elements.

The effect of substrate amplitude and wavelength on gravity-driven film flow inside an inclined corrugated pipe

An experimental investigation was conducted to determine the effect of substrate amplitude and wavelength on gravity-driven film flow inside an inclined corrugated pipe. Nine different geometries were examined, with substrate amplitude and wavelength varied independently. A statically deformed free surface occurred for all conditions. The amplitude of the statically deformed free surface depended on substrate amplitude and wavelength, with phase shift unaffected by changes in substrate geometry for many conditions under investigation. Fluctuations in free surface elevation were enhanced at low substrate amplitude and intermediate substrate wavelength. Notable reductions in transient free surface behavior were observed for conditions that resulted in a positive phase shift. Transient free surface behavior developed into periodic traveling waves without applied external forcing. Frequency selection for traveling waves was strong, and traveling waves were detected for a majority of the conditions examined. The frequency, phase velocity, and wavelength of the traveling waves showed a potential dependence on substrate geometry; however, there were ranges of substrate amplitude and wavelength for which traveling wave characteristics remained unaffected by changes in substrate geometry. An examination of the amplitude of the statically deformed free surface and transient free surface fluctuations revealed that waviness is a potentially suitable method for combining the effects of substrate amplitude and wavelength on film flow in corrugated pipes. The comparison of amplitudes highlighted a possible link between the statically deformed free surface and the emergence of transient behavior and traveling waves. Length scales proposed in our original work showed promise for characterizing some results.

The impact of memristive coupling initial states on travelling waves in an ensemble of the FitzHugh–Nagumo oscillators

A ring of the FitzHugh–Nagumo (FHN) oscillators coupled via memristive elements is studied. The impact of the memristive element initial states on the travelling wave characteristics is established for the self-oscillatory and excitable dynamics of the individual oscillators. It is shown that the initial state effect is especially evident in the excitable regime. In such a case the initial distribution of the memristive element states affects on the wave spatial profile as well as on the frequency of the pulse excitation. The study of the ensemble with ideal memristor coupling is complemented by the exploration of the model implying the interaction through the memristor characterized by ‘forgetting’ the initial states over time. It is demonstrated through this approach that the dependence of the oscillation characteristics on the memristor initial states persists in the presence of the weak memristor imperfectness.

Fast Mutual-Speed-Up Protection Adaptive to Dead-Zone Grounding-Fault Identification for VSC-MTDC

Local-measurement-based protection elements are essential for the safety and stability of the VSC-MTDC. However, when a DC line fault occurs, the resistive coverage for each side of a protection element is not always identical. As a result, a certain side of the protection element may not be sufficiently sensitive to trip the fault, especially in scenarios of communication failure. To solve this problem, a mutual-speed-up protection strategy is proposed to identify dead-zone grounding faults. First, the time domain characteristics and frequency domain characteristics of the fault-induced primary traveling wave and those of the second traveling wave caused by the opening operation of the DC CB are analyzed. Then, based on the above-mentioned difference, a time difference criterion (TDC) and zero-crossing criterion (ZCC) are proposed, working in conjunction to discriminate the remote-end dead-zone fault and the external fault at the head of the adjacent DC line. Simulation results indicate that the proposed strategy works without communication and is sufficiently fast to operate before the blocking of the converter, thus making it a powerful supplemental strategy for existing main protection elements.

A Single-End Protection Scheme for Hybrid MMC HVDC Grids Considering the Impacts of the Active Fault Current-Limiting Control

In the hybrid modular multilevel converter (MMC) based high voltage direct current (HVDC) systems, the fault current can be actively suppressed by the converter itself, which endows a smaller requirement for current-limiting reactors (CLR) and a larger time margin for fault detection algorithms, comparing with the half-bridge MMC. But the robustness to fault resistance and noise disturbance of existing boundary protection schemes will be deteriorated with small CLRs. Moreover, the fast response of the fault current-limiting control will change the output DC voltage of hybrid MMC, which affects the fault characteristics and may cause mal-operation of existing protection algorithms. Thus, a single-end protection scheme considering the impacts of the active current-limiting control is proposed for the hybrid MMC based DC grids. The traveling-wave characteristics under different fault stages are analyzed to evaluate the impacts of the fault current-limiting control. In addition, a coordination protection strategy versus different fault conditions is adopted to improve reliability. Various cases in PSCAD/EMTDC are simulated to verify that the proposed method is robust to fault resistance, fault distance, power reversal, AC faults, and immune to noise.

Experimental Study and Fatigue Analysis of Vortex-Induced Vibration of Umbilical Cable Considering Internal Friction

In order to investigate the effect of internal friction of umbilical cable on its vortex-induced vibration (VIV) responses, the experimental study on VIV of bond umbilical cable (BUC) and un-bond umbilical cable (UBUC) was carried out in an experimental tank. A current generator in the laboratory simulated the uniform current, and the current velocities were observed in real time by using a Doppler Velocimeter. In addition, different sizes of top tension were applied to the umbilical cable model. The VIV responses of the umbilical cable model were measured by using Fiber Bragg grating (FBG) strain sensors. The displacement responses of umbilical cable model were reconstructed based on the experimental strain data processed by modal superposition method. In this paper, the traveling wave characteristics, the spatial-temporal distribution characteristics of frequency and fatigue damage of the BUC and UBUC under VIV are studied. The experimental results show that there are obvious differences between BUC and UBUC in the response characteristics of VIV. The UBUC appears the traveling wave sooner than BUC, but its standing wave characteristics are more obvious than those of BUC at high velocities. Compared with BUC, the spatial-temporal distribution of UBUC frequencies appears wide-band distribution sooner, but has narrower bandwidth in the “lock-in” state. The level of fatigue damage of BUC was approximately the same as that of UBUC.

Simulation Study on Transmission Line Fault and Traveling Wave Transmission Characteristics

Fault traveling wave characteristics and traveling wave transmission characteristics are the key factors affecting the positioning accuracy of traveling wave positioning technology. This paper takes a common 110 kV transmission line as the research object, establishes the electromagnetic transient simulation model through ATP-EMTP software, and simulates the lightning strike, lightning shielding, metal grounding and high resistance grounding faults of the line, and obtains the difference of fault traveling wave current and voltage characteristics under different types of faults. On this basis, the attenuation law of each modulus traveling wave is studied. Finally, the application of each modulus traveling wave in fault location is analyzed.

Acoustic field alteration in a 100 Hz dual acoustic driver straight tube travelling wave thermoacoustic heat pump for thermoacoustic heat transport control

A dual acoustic driver thermoacoustic heat pump was constructed to investigate the effect of acoustic field control on travelling wave thermoacoustic heat transport at a frequency of 100 Hz. The acoustic field was controlled by varying the phase difference and magnitude between the two drivers. Variation of phase difference demonstrated the change of both acoustic power flow direction and standing-travelling wave characteristics. Maximum temperature differences obtained between the ends of the regenerator were 23 °C and 19 °C when the acoustic power was flowing in the negative and positive directions, respectively. It has been shown that matching impedances on the cold and hot side of the regenerator influence the thermoacoustic heat pumping characteristics. Where monodirectional acoustic flow was observed on both sides of the regenerator, the impedance angles also coincided well. Variation of magnitude displayed a trend in which activation of an opposing acoustic driver at weaker power enhanced acoustic power flow and heat transport. A maximum temperature difference between the hot and cold sections of 23 °C is obtained when the input electric power of the opposing acoustic driver was 17% of the power of the initial acoustic driver where the impedances and impedance angles on both ends of the regenerator are matched but the acoustic field travelling wave characteristics are still sufficient for travelling wave device operation.

An Improved Non-Unit Traveling Wave Protection Method With Adaptive Threshold Value and Its Application in HVDC Grids

The performance of the traditional derivative-based non-unit traveling wave protection (TWP) in high voltage dc (HVDC) grids shows the problem of low-sensitivity when high-impedance faults occur. In this article, the characteristics of the fault initial traveling wave are analyzed first, two conclusions can be drawn: i) the distortion degree of the fault initial traveling wave is related to the fault location, ii) the amplitude of the fault initial traveling wave is related to the fault impedance. Then a zero-sequence current fitting method is proposed to obtain parameters respectively reflecting fault impedance and fault distance. When a fault occurs, a fault information extraction method based on the fitting parameters will give the estimation interval of the fault impedance, which provide reference for the adaptive threshold value selection of TWP. In this way, the influence of the fault impedance on TWP can be eliminated. A ±400 kV modular multilevel converter (MMC) HVDC grid is built in PSCAD/EMTDC. The verification of the proposed method considers different installation positions of the current limit inductor. The simulation results show that the proposed method can fast and correctly identify the dc line faults and effectively improves the protection sensitivity in detecting high-impedance faults.

Influence of Superconducting Fault Current Limiters on Traveling Wave Based Protection

Superconducting fault current limiters (SFCLs) have been used in electric power transmission systems to limit the current magnitudes when faults occur. Installation of SFCLs inevitably alters transmission line properties and thus may affect measurements in traveling wave based protection. In this paper, we investigate the impact of SFCLs on the performance of the traveling wave based protection schemes in high-voltage transmission systems. An SFCL and a 230 kV high-voltage transmission line are modeled into a simulation system in the electromagnetic transients program, version alternative transients program. The transmission line is represented by a frequency dependent model so that traveling wave characteristics can be effectively captured. We simulate faults on the transmission line, with the incoming and reflected traveling waves recorded and thoroughly analyzed. Also, the influence of SFCL stray capacitances on traveling wave shape is investigated. The results show that the traveling wave current magnitudes are attenuated by the SFCL and the shape of the traveling wave is slightly affected by SFCL stray capacitances. Possible consequences in protection performance are discussed and future research directions are suggested.

Calculation of the Characteristics of Traveling Waves in Layered Media

A new method is considered for the calculation of traveling-wave characteristics in a layered medium. It is mainly based on the transfer-matrix method [2–10] and the generalized reflection-transmission coefficient (GRTC) method [11–14] for the calculation of characteristics. Introducing an impedance tensor of rank 2, Ẑ (z), in the boundary-value stress problem and utilizing in this way the boundary conditions and the Lame equation, we obtain a system of differential equations which is solved by the fourth-order Runge-Kutta method. Specifying a model of the layered medium in terms of known constants, we find the traveling-wave characteristic γ as a function of frequency ω . Given γ, we easily calculate the dispersion curves, which closely fit the GRTC method. Compared with the GRTC method, the impedance-tensor method fully solves the dispersion curve of the normal-mode traveling waves, and it is applicable to both a homogeneous planar Earth model and a model with a slow layer.

Nonlinear dust-ion acoustic periodic travelling waves in a magnetized plasma with two temperature superthermal electrons and stationary charged dust grains

In this research, the nonlinear propagation of dust-ion acoustic (DIA) periodic travelling waves in a dusty plasma consisting of cold ions, stationary charged dust grains, and two temperature superthermal electrons is theoretically studied. A nonlinear Zakharov-Kuznetsov equation, which describes nonlinear dust-ion acoustic waves, is derived using a reductive perturbation method. Furthermore, the bifurcation theory has been employed to study the nonlinear propagation of DIA periodic travelling wave solutions. In the proposed model, the co-existence of both compressive and rarefactive DIA periodic travelling waves are found. The numerical investigations illustrate that the characteristics of nonlinear DIA periodic travelling waves strongly depend on the temperature ratio, both the concentration and the superthermality of cold electrons, the ion cyclotron frequency, the direction cosines of wave vector k along z axis, and the concentration of dusty grains. The present investigation can help in better understanding of nonlinear DIA periodic travelling waves in astrophysical environments with two temperature superthermal electrons such as Saturn's magnetosphere.

Three-dimensional VIV prediction model for a long flexible cylinder with axial dynamics and mean drag magnifications

This study presents a three-dimensional phenomenological model for the analysis and prediction of vortex-induced vibration (VIV) of a long flexible circular cylinder in uniform flows, by taking into consideration the two salient effects of cylinder axial dynamics and mean drag magnifications. A semi-empirical theoretical model consists of structural nonlinear equations of coupled cross-flow, in-line and axial motions excited by vortex-induced hydrodynamic lift and drag forces. These forces are governed by two distributed and coupled wake oscillators describing the variations of lift and drag coefficients. This reduced-order model contains cubic and quadratic nonlinearities associated with geometric displacement couplings and fluid-structure interactions. A numerical finite difference approach combined with a frequency domain and modal analysis is used for analyzing the highly nonlinear three-dimensional VIV responses. Several key features of flexible cylinder VIV are captured in the case of varying flow velocities. In particular, the dynamic responses exhibit a transition from standing to travelling wave characteristics, spatial dual 2:1 resonances with multi modal contributions, variable figure-of-eight trajectories, higher harmonic components, mean in-line/axial displacements and mean stresses as a result of VIV-amplified mean drag forces. Model validations are also performed through comparisons with published experimental results. Parametric studies highlight the individual and combined effects of geometric and hydrodynamic nonlinearities together with the axial dynamics which contribute to the space-time varying responses, frequencies, dominant modes and modal contributions, transverse bending and longitudinal stresses. Both quantitative and qualitative discrepancies are noticeable when the prediction model disregards these effects.

Sandwich structures with periodic assemblies on elastic foundation under moving loads

Wave propagation in sandwich structures with periodic assemblies on elastic foundation under external moving load is studied. Two types of periodic assemblies are considered, namely, a periodic core, and a core with periodically embedded resonators. Wave numbers and traveling wave characteristics are analyzed by introducing a moving coordinate system. Depending on whether the bandgap (attenuation zone) exists, the perturbation due to the external moving load can/cannot propagate through the structure. The bandgap properties of both types of structures in the velocity field are also investigated. It is found that the location and range of the bandgap can be tailored by the physical properties of the core/resonator.