Wave Head Research Articles

Effect of Combustor Outlet Geometry on Operating Characteristics of Disk-Shaped Rotating Detonation Engine

A disk-shaped rotating detonation engine with H2/air mixture was tested to identify the impact of combustor outlet geometry on the engine’s operating characteristics. Three combustor outlet diameters and five outlet lengths are employed in the experiments. Results show that with the increase of combustor convergent ratio, the propagation stability of the rotating detonation wave decreases, and the propagation velocity and pressure peak decrease slightly. When the convergent ratio increases to a certain value (1.70 in this study), a “platform zone” with a lower pressure value appears before the sharp rise of the dynamic pressure curve. The propagation mode varies with the increase of mass flow rate at different convergent ratios. As the mass flow rate increases, the wave head number in the combustor increases. But the change rule of propagation mode with mass flow rate is greatly affected by convergent ratio. Increasing the convergent ratio is conducive to the formation of multi-wave modes, and the critical mass flow rate for mode transition drops sharply. When the convergent ratio increases to 1.70, the unstable asymmetric dual-wave mode is obtained. With the increase in the convergent ratio, the engine’s operating range and operating stability decrease significantly. Finally, changing the combustor outlet length has little influence on the engine’s operating characteristics and detonation-wave parameters.

An Improved Arc Fault Location Method of DC Distribution System Based on EMD-SVD Decomposition

The influence of the control strategy of the power electronic converter obscures the fault characteristics of DC distribution networks. The existence of arc faults over an extended period of time poses a grave threat to the security of power grids and may result in electric shock, fire, and other catastrophes. In recent years, the method of fault localization based on the traveling wave method has been a popular topic of research in the field of DC distribution system protection. In this paper, the fault localization principle of the traveling wave method is described in depth, and the propagation characteristics of the traveling wave of fault current in the online mode network are deduced. We present a method for wave head calibration that combines empirical mode decomposition (EMD) and singular value decomposition (VMD). After the fault-traveling current signal has been subjected to EMD, the first eigenmode function is extracted and subjected to singular value decomposition (SVD). After SVD, the detail component can reflect the singularity of the signal. The point of the maximum value of the detail component signal corresponds to the moment when the faulty traveling wave head reaches the monitoring point. Finally, the DC distribution system is modeled based on the PSCAD/EMTDC simulation environment, and the fault location method is verified. The simulation results show that the method can effectively realize fault localization.

Microgrid line protection method using the cosine similarity of the time domain traveling waveform

For the existing microgrid line traveling wave protection method, the head information of traveling wave is hard to extract, which probably leads false action or non-action of protection. This paper proposes a transient-state protection method that utilizes cosine similarity of time domain voltage traveling wave to identify microgrid line faults. By deriving the time domain expression of the transient voltage traveling wave at both ends of the microgrid line, the conclusion can be summarized that the forward wave at the one end and the backward wave at the other end of the non-fault line are the same traveling wave, while the waves at both ends of the fault line are distinct. Based on this, fault discrimination is achieved by using the cosine similarity coefficient between the time domain voltage waveforms of the forward and backward waves. This method does not need to extract the initial wave head information of the traveling wave, which improves the reliability; and the speed of protection is enhanced by using the signal of transient traveling. The simulation results show that this protection method is independent of the microgrid operation state, fault location and fault type, which verifies the feasibility of the proposed method.

Fault branch determination method based on the amplitude information of the first wave head of the initial traveling wave of the fault

In order to solve the difficult problem of fault branch determination in multi-branch distribution networks, the transmission characteristics of fault traveling waves in the distribution line are analyzed. The variation law of fault traveling wave amplitude with fault distance and frequency is deduced theoretically, and the reasons for the difficult determination of fault branch in the distribution line are revealed. A fault branch determination method based on the energy spectral density of the amplitude of the first wave head of the fault initial traveling wave is proposed. The reference node and the reference end of distribution networks are defined. The ratio of the intrinsic amplitude matrix to the real fault matrix before offline is calculated. The fault branch determination matrix is established to realize the reliable determination of the fault branch. PSCAD is used to build a 10 kV multi-dominant network model to simulate various fault conditions to verify the fault branch determination method. The simulation results show that the method can effectively identify the fault branch and has strong reliability.

Comparison of Initiation and Temporal Evolution of Positive and Negative Ionic Wind Under Needle-to-Plate Electrode

Considerable research efforts have been devoted to the generation, characterization, and optimization of ionic wind produced by corona discharge. The current article tries to explain the mechanism that causes the different behaviors observed in negative and positive dc corona ionic wind. The steady-state and the transient state ionic wind produced by both polarities are studied through particle image velocimetry (PIV) technology. The results show that at transient state, a series of mushroom-like jets are produced in negative corona discharge ionic wind due to the effect of Trichel pulse. These jets merge with each other and combine momentum to form the wide structure at steady state. In comparison, the positive corona discharge current has no pulse, and the ionic wind has only one “I-shape” wave head during the initiation process. The dynamics of the jets is studied in detail. The formation of the jet series in negative corona, or single jet in the case of positive corona, is believed to play a key role in the generation of ionic wind.

Grounding electrode line fault location method based on simulation after test and deduction

• Fault characteristics of the grounding electrode line are weak. • Fault location by injecting signals into the line. • The initial pulse signal is eliminated after linear mode transformation. • Using simulation after test and deduction to obtain fault distance. • No need to calibrate the arrival time of the wave head. When a high-voltage direct current (HVDC) transmission system with the grounding electrode line (GEL) is in the balanced operation mode, the unbalanced current in the GEL and the neutral potential of the system are extremely small. In the GEL, after a fault incident, the additional fault component will be small, and fault characteristics will be weak. As the traditional passive fault location methods are challenging to apply, a GEL fault location method based on simulation after test and deduction (SATD) is proposed in this paper. Firstly, the pulse signal is injected into the GEL. Then, the mutation point of the pulse response signal is projected along the line to form a virtual fault point matrix. Based on the mentioned matrix, the fault location function is constructed, and the fault distance is obtained by the catastrophe point of the function. In this method, the bottleneck of precisely identifying the traveling wave (TW) arrival time in the time domain is broken, and the automated single-ended TW location of the GEL can easily be implemented. Many real time digital simulator (RTDS) tests validate the high accuracy and strong robustness of the method.

Fault Location Method of Multi-Terminal Transmission Line Based on Fault Branch Judgment Matrix

Aiming at the difficulty of fault location of multi-source transmission lines, this paper proposes a fault location method for multi-terminal transmission lines based on a fault branch judgment matrix. The fault traveling wave signal is decomposed by Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN), and the IMFs sensitive components that can characterize the fault characteristics of the target signals are selected by constructing a correlation-rearrangement entropy function. The arrival time of fault signals at the endpoint has been accurately calibrated by combining them with the Teager Energy Operator (TEO). To eliminate the influence of wave velocity and fault time on the location results, this paper proposes a two-terminal location method based on the line mode component to improve the location accuracy. On this basis, combined with the fault branch judgment matrix, the accurate location of multi-terminal transmission line faults is realized. This method has been shown to have high accuracy in detecting traveling wave heads, accurately judging fault branches, and producing a small error in fault location results. Compared with the existing multi-terminal transmission line fault location algorithm, it has obvious advantages and meets the needs of actual working conditions.

Fault Ranging Method for Overhead-Cable Hybrid Distribution Lines Based on CEEDMAN Energy Relative Entropy

According to the parameter characteristics of cable-overhead line and the folding and reflection process of traveling waves when the wave impedance is discontinuous, a traveling wave abrupt change extraction method based on the empirical mode decomposition of fully adaptive noise ensemble (CEEDMAN) energy relative entropy is proposed for the first time. Through the combination of CEEDMAN and energy relative entropy, the transient traveling wave signals are generated when reflection and refraction occur, and the energy value of traveling waves will change abruptly, and the reflected and refracted wave heads will be captured by the change of relative energy. Combined with the magnitude difference of zero sequence current, the fault area can be determined. Traveling waves have different wave speeds in overhead lines and cable lines, and fault ranging is performed by determining the corresponding propagation speed. Finally, simulation verifies that the method can accurately determine the fault zone with high ranging accuracy under the conditions of different fault locations, different transition resistances and different initial phase angles.

Study on current sharing characteristics of multi-column parallel arresters

The ideal working conditions of a multi-column parallel arrester is to share the energy generated when it is impacted, otherwise it will lead to a breakdown of the arrester and the forced shutdown of the HVDC system, resulting in significant economic losses. This paper analyzes the current sharing characteristics of multi-column parallel arresters under different current waveforms, different current amplitudes, different temperatures, and different cumulative impulse times. The results show that the current waveform, current amplitude, and temperature difference between the columns all affect the current sharing characteristics. The current deviation coefficient is positively correlated with the current wave head time. The closer the current value is to the balancing point, the smaller the deviation coefficient. The temperature difference between the columns has the greatest influence on the current deviation coefficient. As the temperature difference increases, the current deviation coefficient may exceed 10%. Finally, based on experimental conclusions, some suggestions are put forward to modify the operation rules of large-capacity multi-column parallel arresters, which provides an important reference for subsequent projects.

Two-terminal fault combination location method on MMC-HVDC transmission lines based on ensemble empirical mode decomposition

In the HVDC transmission system, the main factors that affect the accuracy of traditional two-terminal fault traveling wave distance measurement are the selection of appropriate wave velocity and whether the wave head can be identified. The line dispersion and transition resistance of the DC transmission will make it more difficult to determine the time when the wavehead reaches the two ends of the two sides, and the boundary elements of both terminal of the line will cause distortion of the wave head. Based on fault traveling waves characteristics of the time–frequency domain, we propose a method of Two-Terminal Fault Combination Location (TTFCL) on High Voltage Direct Current based on MMC (MMC-HVDC) transmission line. The method combines the natural frequency fault location method and the traveling wave fault location method. The first step of the walking wave natural frequency method is used for rough ranging, so that the approximate time range of the initial traveling wave of fault to reach the busbar can be obtained, and then can be calculated the rough position range of the fault point. Then the second step of accurate ranging is carried out, using the Ensemble Empirical Mode Decomposition (EEMD) algorithm to decompose the processed signal, extract the traveling wave component of the high-frequency, identify the traveling wave of initial head recognition and calibration, obtain the time parameters required for fault ranging, and calculate the more accurate fault point location by the two-ended fault ranging formula. In PSCAD/EMTDC, we built a simulation model of MMC-HVDC system line fault, and the simulation experiments are compared with different fault types and locations, and the results show that the combined fault ranging method eliminate the wave speed influence on accurate positioning and effectively identifies the wave head, which further improves the ranging accuracy.

Study on the Influencing Factors of Modal Transformation of Continuous Rotating Detonation Wave Propagation

In order to study the propagation mode of rotating detonation wave, the detailed reaction mechanism of hydrogen-oxygen with 9 components and 19 steps is used, and the finite rate chemical reaction model is used for numerical calculation. After ignition, there is a stable collision period in the flow field for a certain period, and the collision extinguishing and new detonation wave generation occur during the mode conversion. By adding nitrogen or argon to the hydrogen-oxygen mixture for dilution, when the dilution ratio increases, the activity of the gas mixture decreases, the height of the combustible gas layer increases, and the temperature of the combustion product after the detonation wave decreases away from the head of the detonation wave, which will reduce the mixture that is burned in advance. There are different degrees of reflected shock at the outlet of the combustion chamber, and the upward detonation wave will cause the height change of the detonation wave head and the instability of the flow.

Electromagnetic transient characteristics and overvoltage suppression strategy of renewable energy-oriented power system connected by gas-insulated transmission lines

New energy systems, such as wind power and photovoltaic and distributed power supply, in the power generation and grid-connected transmission system basically will use stronger insulation design, higher operational safety, and smaller footprint GIL equipment, while the new energy transmission system of the smart grid development trend requires accompanying the electronic transformer equipment to promote a large area. However, the current GIL equipment used in new energy transmission systems has caused great electromagnetic interference to the electronic equipment due to the compact layout of the equipment and the transient signals generated by normal operation, which has seriously restricted the development of smart grids and became a major obstacle to the grid connection of new energy systems. In order to study the characteristics and developmental law of transient electromagnetic interference signal of GIL equipment in new energy transmission and transformation system, this study proposes a dynamic arc model based on an optimized arc extinguishing criterion, verifies the correctness of the improved dynamic arc model through theoretical analysis, simulates calculation combined with the test measurement data, and compares and analyzes the simulation results with test measurement data to study the transient electromagnetic interference source signal during arc burning duration. The simulation results and test data are compared and analyzed to study the pulse steepness, size of the wave head, and overall waveform development law of the transient electromagnetic interference source signal during the arc burning process. It is concluded that the dynamic arc model based on the optimized arc extinguishing criterion can be better used to study the transient electromagnetic interference signal in the GIL equipment. Meanwhile, the transient electromagnetic interference signal has the characteristics of large amplitude and high frequency, the maximum overvoltage can reach 1.9 p.u., the UHF can reach 30 MHz, and the wave head rise time is about 12 ns. This study provides theoretical support for subsequent research on suppressing transient electromagnetic interference source signals, improving the anti-electromagnetic interference performance of electronic devices, and is dedicated to solving the obstruction problem in the process of grid connection of new energy transmission and substation systems.

Cable Fault Location in VSC-HVDC System Based on Improved Local Mean Decomposition

Aiming at the problem of low positioning accuracy caused by modal aliasing and noise interference in DC cable fault location analysis of a VSC-HVDC system, a double-ended fault location method for flexible DC cables based on improved local mean decomposition is proposed. Firstly, the local mean decomposition (LMD) is used to decompose the six-mode voltage signal to obtain the product function (PF) component; then, to overcome the problem that the instantaneous frequency function of the LMD is limited by the extreme value, the Hilbert transform is performed on the PF1 to obtain the instantaneous frequency curve, and the arrival time of the voltage traveling wave head is determined from the mutation information. Finally, the fault distance is obtained by using the principle of double-ended traveling wave fault location. Different fault conditions are simulated, analyzed, and compared with wavelet transform and Hilbert–Huang transform. The results show that the proposed method has a positioning error within 1%, and it is less affected by interference noise and transition resistance.

Revealing the Effect of Skull Deformation on Intracranial Pressure Variation During the Direct Interaction Between Blast Wave and Surrogate Head.

Intracranial pressure (ICP) during the interaction between blast wave and the head is a crucial evaluation criterion for blast-induced traumatic brain injury (bTBI). ICP variation is mainly induced by the blast wave transmission and skull deformation. However, how the skull deformation influences the ICP remains unclear, which is meaningful for mitigating bTBI. In this study, both experimental and numerical models are developed to elucidate the effect of skull deformation on ICP variation. Firstly, we performed the shock tube experiment of the high-fidelity surrogate head to measure the ICP, the blast overpressure, and the skull surface strain of specific positions. The results show that the ICP profiles of all measured points show oscillations with positive and negative change, and the variation is consistent with the skull surface strain. Further numerical analysis reveals that when the blast wave reaches the measured point, the peak overpressure transmits directly through the skull to the brain, forming the local positive ICP peak, and the impulse induces the local inward deformation of the skull. As the peak overpressure passes through, the blast impulse impacts the nearby skull supported by the soft and incompressible brain tissue and extrudes the skull outward in the initial position. The inward and outward skull deformation leads to the oscillation of ICP. These numerical analyses agree with experimental results, which explain the appearance of negative and positive ICP peaks and the synchronization of negative ICP with surface strain. The study has implications for medical injury diagnosis and protective equipment design.

Efficacy of supine trans-abdominal pronated shock head ESWL for treatment of distal ureteric stones: A pilot study.

To evaluate the efficacy of extracorporeal shockwave lithotripsy (ESWL) in supine trans-abdominal approach for distal ureteric calculi and to determine the variables that could affect the outcome results. Between November 2019 and January 2021, 172 patients with a solitary distal ureteric calculus were treated with ESWL in supine position with a pronated shock wave head against the anterior abdominal wall. The outcome of treatment evaluated and the clinical and radiological findings, as well as stone characteristics, were reviewed and correlated with the stone-free rate (SFR). Overall SFR was 84.84% (140/165). Those with stone clearance, 83.57% (117/140) cleared after one session and 16.42% (23/140) needed more than one session. Only three factors had a significant impact on SFR, that is stone length, stone width, stone attenuation. For stone length ⩽ 9 mm SFR was 93.44% (114/122) compared to 60.46% (26/43) for stone length of 10-12 mm (p < 0.00001). There were statistically significant increased SFR for stones with attenuation of <0.0001 and for the mean stone width of 8.02 mm (p < 0.00001). Stone localisation was easy with good patient comfort. Supine trans-abdominal ESWL is a new, effective and safe form of treatment for distal ureteric calculi. The stone length, stone width along with stone attenuation were the only significant predictors of stone free status in supine position.

Fault Location Method in Nonsolid-Earthed Network Based on Spatial Domain Image Fusion and Convolution Neural Network

Given that the single-terminal traveling wave location method has significant errors, a novel fault location method based on the spatial domain image fusion and convolutional neural network (CNN) is proposed. Firstly, the three-phase traveling wave can be decoupled by the phase-mode transformation matrix for obtaining the line-mode component of the traveling wave. Secondly, the 1D line-mode traveling wave can be converted into a 2D image by the Gramian angular field (GAF). The 1D line-mode component can be mapped into the color, point, line, and other characteristic parameters of the 2D image. In order to expand the invisible information of the line-mode traveling wave, the images obtained by the Gramian angular summation field (GASF) and Gramian angular difference field (GADF) are weighted and fused. Finally, the CNN can be used to autonomously mine the characteristic parameters of the weight-fusion image and realize fault location. The simulation results show that the proposed method does not need to be considered in the traveling wave head and the traveling wave speed. The localization method is not affected by fault time, fault distance, or transition resistance factors. It possesses high reliability with an absolute range error of no more than 200 m.

Study of thrust vector control for the rotating detonation model engine

The detonation wave in a rotating detonation engine is highly adaptable to the incoming flow, making the wave easier to control. In this study, a numerical simulation method is used to analyze the working process and flow field structure of a rotating detonation model engine with dual cavity injection of an H2/air mixture by controlling the injection pressure ratio of the dual cavity and the number of detonation wave heads. It is found that the rotating detonation engine offers the possibility to control the thrust vector with two different modes. The first is a one-cycle alternate control mode with a small injection pressure ratio. Here two deflections occur in different directions occur across one detonation wave propagation cycle, but the overall deflection direction is in the low-pressure region. The second is a one-way control mode, with a large injection pressure ratio, and the deflection direction towards the low-pressure region. For the multi wave-mode, it belongs to one-way control mode because of constant deflection direction in the low-pressure area. From the perspective of thrust distribution along the circumference, the one-way control strategy satisfies the ability of a rotating detonation thrust vector control.

Fault location of hybrid three-terminal HVDC transmission line based on improved LMD

To reduce the impact of traveling wave velocity, wave head recognition, and LMD (Local Mean Decomposition) end effect and mode mixing on location accuracy, this paper presents a fault location method based on improved LMD for hybrid three-terminal HVDC transmission lines, which is not affected by wave velocity. Firstly, the improved LMD is used to get the time when the fault traveling wave first arrives at the four measuring terminals. Then the fault branch is identified according to the ratio of the time of the fault traveling wave arriving at the measuring terminal. Finally, the equation is listed according to the principle of traveling wave location and the measurement equation which is not affected by the wave velocity is deduced. A hybrid three-terminal HVDC transmission system model is built by PSCAD to simulate various faults and output fault data under various fault conditions. MATLAB is used to process the fault data and verify the location algorithm. The simulation results show that the location method proposed in this paper is not affected by fault distances, fault types, infeed levels and transition resistances. In addition, it has higher accuracy than LMD based on CSI (Cubic Spline Interpolation) algorithm, wavelet packet location algorithm and double-ended location method of using wave velocity calculation.

Numerical simulations for predicting wave force effects on dynamic and motion characteristics of blended winged-body underwater glider

The Blended winged-body (BWB) shape of underwater glider (UG) has its unique advantages and gained remarkable attention in recent research. As BWB UG is autonomous underwater vehicle (AUV), the assessment of external disturbances and their effect on UG motion characteristics is of significant importance in order to accurately predict UG dynamics and navigation along the glide path. Being buoyancy driven slow speed vehicles, Waves play dominant role in disturbing UG dynamics near ocean surface, therefore better control systems and vehicle design can be achieved by studying of influence of wave parameters on the motion dynamics of the UG. Present work evaluates the underwater effects of wave forces on motion characteristics of BWB UG. The Wave forces acting on BWB UG are calculated in time domain by panel method. Series of simulations were performed considering Airy wave theory and head sea conditions. MATLAB curve fitting technique is used to derive analytical formulations for wave forces and moments on BWB UG obtained by panel method. The BWB hydrodynamics coefficients are calculated using Computational Fluid Dynamics (CFD). The fourth order Runge-Kutta method is used to solve motion dynamics model coupled with wave force analytical formulation in time domain. It is observed that the results of numerical simulations are reasonable and the wave force of different frequencies and amplitudes affect the UG stability and navigation along the glide path.

Formation rate of extreme mass ratio inspirals in active galactic nuclei

Extreme mass ratio inspirals (EMRIs) are important sources for space-borne gravitational wave detectors, such as the laser interferometer space antenna and TianQin. Previous EMRI rate studies have focused on the ``loss cone'' scenario, where stellar-mass black holes (sBHs) are scattered into highly eccentric orbits near the central massive black hole (MBH) via multibody interaction. In this work, we calculate the rate of EMRIs of an alternative formation channel: EMRI formation assisted by the accretion flow around accreting massive black holes. In this scenario, sBHs and stars on inclined orbits are captured by the accretion disk, and then subsequently migrate towards the MBH, under the influence of density wave generation and head wind. By solving the Fokker-Planck equation incorporating both sBH-sBH--sBH-star scatterings and sBH--star-disk interactions, we find that an accretion disk usually boosts the EMRI formation rate per individual MBH by $\mathcal{O}({10}^{1}--{10}^{3})$ compared with the canonical loss cone formation channel. Taking into account that the fraction of active Galactic nuclei (AGNs) is $\ensuremath{\sim}\mathcal{O}({10}^{\ensuremath{-}2}\ensuremath{-}{10}^{\ensuremath{-}1})$, where the MBHs are expected to be rapidly accreting, we expect EMRI formation assisted by AGN disks to be an important channel for all EMRIs observed by space-borne gravitational wave detectors. These two channels also predict distinct distributions of EMRI eccentricities and orbit inclinations with respect to the MBH spin equatorial plane, which can be tested by future gravitational wave observations.