Frequency Domain Analysis Method Research Articles

Vibration source analysis and structural optimization design of rotary table based on OTPA.

Due to the influence of coal rock shape, hardness, working environment and other factors in the cutting process of cantilever roadheader, the cutting head will produce irregular and violent vibration. As the rotary table of key stress components, its operation process stability, dynamic reliability and life affect the cutting efficiency and cutting stability of cantilever roadheader. In order to study the vibration characteristics of the rotary table in the cutting process, firstly, based on the theory of spatial force analysis and calculation, the spatial mechanical model of the rotary table of the cantilever roadheader is established. By solving the balance equation of the rotary table force system, the variation law of the load at the hinge ear of the rotary table with the cutting pitch angle and the horizontal angle is obtained. Secondly, based on the path transfer analysis method of working condition, the vibration data of cutting head, cutting cantilever, cutting lifting and rotary hydraulic cylinder under stable cutting condition are taken as input signals. By constructing the transfer path analysis model of rotary table working condition, the synthetic vibration of rotary table in cutting process is simulated, and the main vibration source of rotary table is determined. Then, the vibration contribution and contribution degree of each vibration excitation point to the hinge ear of rotary table are studied. By building a cutting test bench, the vibration response of rotary table in cutting process is tested to verify the correctness of the theoretical model.Thirdly, based on the frequency domain analysis method of random vibration fatigue life, combined with the S-N curve of the rotary table, the PSD curve at the maximum stress of the rotary table is obtained by modal excitation method, and the load data is imported into ANSYS nCode software to obtain the life cloud diagram and damage cloud diagram of the rotary table, and then the fatigue life of the rotary table under symmetrical cyclic load is solved. Finally, based on the response surface optimization analysis method, the maximum stress and maximum deformation of the rotary table are taken as the optimization objectives, and the aperture of each hinge ear of the rotary table is taken as the optimization variable. Based on Design Expert, a second-order regression model is established to realize the multi-objective optimization design of the key stress parts of the rotary table in the cutting process. The simulation results show that under the same cutting conditions, the maximum stress of the optimized rotary table is reduced by 15.82% year-on-year, and the maximum deformation is reduced by 24.70% year-on-year. The optimized rotary table structure can better adapt to the cutting process, which is beneficial to improve the service life of the rotary table and enhance its operation stability. The research results are beneficial to enrich the relevant research theory in the field of rotary table vibration of cantilever roadheader, and are beneficial to improve the service life of the rotary table and the efficiency of tunneling and mining.

Underwater radiated noise numerical analysis of polar transport vessels under ship-ice-water-air coupling continuous icebreaking based on the S-ALE algorithm

With the gradual opening of the Arctic channel, the underwater radiated noise induced by the ship-ice collision cannot be ignored. However, the underwater radiated noise induced by ship-ice collision is significantly different from that of ships navigating in open water. To explore the response rule of transient underwater radiation noise induced by ship-ice collision, this paper proposes a simplified numerical analysis method of ship-ice collision-induced transient underwater radiated noise in the full frequency band by combining the explicit dynamic method and acoustic analysis method. The ship-ice-water-air coupled collision analysis method based on the S-ALE algorithm is used to simulate and verify the ice load in the process of ship-ice continuous collision at different speeds. Then, the frequency-domain acoustic analysis method is used to predict the transient underwater radiated noise response in the full frequency band under icebreaking excitation and evaluate it by referring to the steady-state underwater radiated noise standard. The results show that the simplified analysis method can effectively predict the transient underwater radiation noise under icebreaking excitation. Under the icebreaking excitation, the radiation shape is irregularly scattered and the radiation direction is concentrated towards the bottom and bow of the ship with the frequency increase. The influence range of transient underwater radiated noise is mainly within 1000 Hz at different speeds, and the variation in the ship's speed affects the amplitude of the sound power level and the smoothness of the periodic response. The synthesized total sound power levels are 172.56 dB (V = 2.0kn), 174.15 dB (V = 2.5kn), 175.70 dB (V = 3.0kn), and 182.49 dB (V = 4.0kn), respectively. According to the comparison with the evaluation standard, it is found that the transient underwater radiation noise induced by ship icebreaking has a greater impact on marine organisms. It should give more detailed requirements within 10–1000 Hz when using relevant standards to evaluate underwater radiated noise induced by polar icebreaking. This paper can provide a reference for the research of underwater radiated noise induced by ship-ice collision and the formulation of relevant standards.

Experimental study on flow-induced vibration under laminar flow and turbulent flow transition state in helical coiled tube

This study investigates the vibration characteristics induced by single-phase flow at low velocities within helical coiled tube, encompassing laminar and turbulent flow transition regimes. Modal experiments were conducted to ascertain the natural frequencies in the radial and vertical directions within the helical coiled tube. The influence of fluid mass in tube on vibration was considered, revealing the fluid mass has a greater influence on the vibration frequency in the vertical direction. When the coil was filled with water, the radial natural frequency decreased by 0.1 Hz, while the vertical natural frequency decreased by 0.8 Hz.By simplifying the spiral coil into a spring system, the intrinsic causes of the influence of mass on the natural frequency are studied and verified with the experimental data. The results show that the radial natural frequency is only related to the local mass change of the pipe, while the vertical natural frequency is related to the static deformation of the system under the influence of gravity.By conducting single-phase flow induced vibration experiments inside the pipeline, the radial and vertical vibration response characteristics were obtained, and the modal experimental results were analyzed. Findings indicated larger vibration displacements when laminar flow prevailed within the tube, with radial RMS (Root Mean Square) at 10.1 μm and vertical RMS at 14.5 μm. After the transition to turbulent flow, significant reductions in vibration displacements were observed, reducing the radial RMS to 2.3 μm and vertical RMS to 5.5 μm.Based on statistical and frequency-domain analysis methods, the influence mechanism of secondary flow on the vibration of helical coils at low flow rates was revealed, and the importance of friction coupling was clarified. Moreover, this study considered the impact of fluid viscosity on vibration response and experimentally validated the critical role of friction coupling in exciting secondary flows.

The higher-order analysis of response for linear structure under non-Gaussian stochastic excitations with known statistical moments and power spectral density

The frequency-domain analysis method is a fundamental component of the random vibration analysis, in which the corresponding moment spectrum of excitations are the prerequisite. Nevertheless, the determination of the higher-order moment spectra for non-Gaussian stochastic excitations continues to pose a significant challenge in the existing research works. This paper introduces an accurate and efficient computational approach for determining higher-order moment spectra of non-Gaussian stochastic excitations with known statistical moments and power spectral density (PSD). Firstly, following the idea of the simulation method of non-Gaussian stochastic processes, the transformation model between the stationary non-Gaussian stochastic process and underlying Gaussian stochastic process is determined based on the known statistical moments, the PSD of the underlying Gaussian stochastic process is determined using the transformation model. Secondly, the approximate higher-order moment spectra models of stationary non-Gaussian stochastic processes are presented by the PSD of the underlying Gaussian process. Subsequently, the higher-order moment spectra models of non-Gaussian excitations are utilized to compute the higher-order moment spectra of response for the linear structure via the auxiliary harmonic excitation generalized method. Finally, three numerical examples are examined to assess the efficiency and accuracy of the proposed approximate models for the higher-order moment spectra of non-Gaussian stochastic processes.

Fault Diagnosis of Dry Gas Seal Operation Status Based on Acoustic Emission Monitoring

A dedicated test bench is employed to record acoustic emission signals from dry gas seals under various operating conditions. Time-domain and frequency-domain analysis methods are utilized to process and analyze the acoustic emission signals during start/stop, stable operation, and two common fault states (end-face defects and compensation spring failure). Furthermore, feature recognition research is conducted. A method for identifying the operational states of seals (low-speed friction, gradual detachment, stable operation) based on the root mean square (RMS) was established, with transition points at speeds of 100 and 1000 RPM, respectively. Additionally, spectral analysis is conducted using Fourier transform to determine the frequency band of acoustic emission signals (240–320 kHz) generated during contact wear of dry gas seals. Investigation into two typical faults of dry gas seals reveals that the RMS value of the acoustic emission signal gradually increases with the rotational speed during the operation of dry gas seal end-face defects. This is attributed to the insufficient dynamic pressure effect on the end face, resulting in long-term wear and tear. When the dry gas seal compensates for spring failure, the RMS value of the acoustic emission signal initially increases, then decreases, and finally increases again as the speed increases. It reaches the stable operating inflection point when the end-face speed is 800 r/min.

Evaluation of indicators of railway-induced annoying vibration based on building measurements developed by TOD: a discussion

The main negative impact of the residential buildings located on the cover structure of a Transit Oriented Development (TOD) metro depot is that residents would suffer from the train-induced annoying vibrations. Arguments accordingly arose on the possibility of evaluating indicators controversy for this application that acceleration data was more captured in the past while velocity would be more related to structure born noise. Therefore, this paper provided a relative comparison based on a measurement campaign that both velocity and acceleration sensors were installed together at the same time. The collected vibration accelerations and velocities are analyzed and discussed using the methods of transmission loss, coherence, and time and frequency domain analysis. The results indicated that even there was no difference for identifying the resonance frequency of structure, but the transit mobility and human annoyance evaluation were obviously different by using velocity or acceleration. Attempts to establish a relationship between the two indicators using the calculus method failed because they showed the largest error at the peak frequency at the same vibration level, which would have affected the evaluation results and the judgment on the choice of mitigation measures.

Vibration source analysis and structural optimization design of rotary table based on OTPA.

Due to the influence of coal rock shape, hardness, working environment and other factors in the cutting process of cantilever roadheader, the cutting head will produce irregular and violent vibration. As the rotary table of key stress components, its operation process stability, dynamic reliability and life affect the cutting efficiency and cutting stability of cantilever roadheader. In order to study the vibration characteristics of the rotary table in the cutting process, firstly, based on the theory of spatial force analysis and calculation, the spatial mechanical model of the rotary table of the cantilever roadheader is established. By solving the balance equation of the rotary table force system, the variation law of the load at the hinge ear of the rotary table with the cutting pitch angle and the horizontal angle is obtained. Secondly, based on the path transfer analysis method of working condition, the vibration data of cutting head, cutting cantilever, cutting lifting and rotary hydraulic cylinder under stable cutting condition are taken as input signals. By constructing the transfer path analysis model of rotary table working condition, the synthetic vibration of rotary table in cutting process is simulated, and the main vibration source of rotary table is determined. Then, the vibration contribution and contribution degree of each vibration excitation point to the hinge ear of rotary table are studied. By building a cutting test bench, the vibration response of rotary table in cutting process is tested to verify the correctness of the theoretical model.Thirdly, based on the frequency domain analysis method of random vibration fatigue life, combined with the S-N curve of the rotary table, the PSD curve at the maximum stress of the rotary table is obtained by modal excitation method, and the load data is imported into ANSYS nCode software to obtain the life cloud diagram and damage cloud diagram of the rotary table, and then the fatigue life of the rotary table under symmetrical cyclic load is solved. Finally, based on the response surface optimization analysis method, the maximum stress and maximum deformation of the rotary table are taken as the optimization objectives, and the aperture of each hinge ear of the rotary table is taken as the optimization variable. Based on Design Expert, a second-order regression model is established to realize the multi-objective optimization design of the key stress parts of the rotary table in the cutting process. The simulation results show that under the same cutting conditions, the maximum stress of the optimized rotary table is reduced by 15.82% year-on-year, and the maximum deformation is reduced by 24.70% year-on-year. The optimized rotary table structure can better adapt to the cutting process, which is beneficial to improve the service life of the rotary table and enhance its operation stability. The research results are beneficial to enrich the relevant research theory in the field of rotary table vibration of cantilever roadheader, and are beneficial to improve the service life of the rotary table and the efficiency of tunneling and mining.

Application of Impact Elastic Wave Method in Quality Inspection of Concrete Lining

As the main part of inter-basin water transfer project, water tunnel plays an important role in long-distance water conveyance. In water tunnel, the lining quality problem is one of the important potential safety hazards of the tunnel engineering. It is very necessary to detect the internal defects of concrete lining. In this paper, a multi-parameter comprehensive interpretation method based on impact elastic wave is proposed to quickly detect the internal defects of lining. The common offset time domain analysis method, frequency domain analysis method and two-dimension Rayleigh wave method are used for data processing. The detection method has the characteristics of single data acquisition and multi-parameter joint interpretation. The physical model of concrete lining is tested and compared with the result of GPR method to verify the accuracy of this method. The coring results show the reliability and stability of this method in practical application.

Supraharmonic mitigation in microgrid and electric vehicle charging station through multilevel converter

<span>The problem of supraharmonics (SH) in a microgrid (MG) system connected to an electric vehicle (EV) charging station is discussed in this work. SH, or high-frequency harmonics which occur beyond the typical harmonic spectrum, can cause problems with power quality (PQ) and equipment failure. A multilevel converter (MLC)-based solution is proposed together with a frequency domain analysis method to lessen this issue. Using frequency domain methods like the fast fourier transform (FFT), the suggested method precisely measures and examines the SH content in the MG and EV charging station. Based on the findings, an MLC control method that actively lowers the SH is created. The efficiency of the suggested strategy in lowering SH levels and ensuring PQ is assessed through experimental validation. The development of SH analysis and mitigation techniques as a result of this research makes it easier to safely and effectively integrate the infrastructure for EVs with renewable energy sources (RES).</span>

CFI-LFENet: Infusing cross-domain fusion image and lightweight feature enhanced network for fault diagnosis

Data-driven fault diagnosis has become a hot topic of research in recent years, due to its wide applicability, high accuracy, and ease of modeling. In data-driven fault diagnosis, feature extraction is usually performed through a combination of one-dimensional time domain or frequency domain analysis methods. However, the constraints of data format and diagnostic methods give rise to three primary issues: inadequate utilization, expression, and mining of fault features, thereby impeding the broader application of data-driven methods. This paper presents a fault diagnosis framework that integrates cross-domain fusion images and a lightweight feature-enhanced network to address these issues. We leverage the temporal and frequency features of raw time series data, transforming them into images. This approach not only offers a greater amount of fault information but also highlights the inherent features more prominently. In terms of diagnosis methods, a novel feature-enhanced network model is proposed. This model takes images generated by a cross-domain image fusion module as inputs and then goes through a customized feature extraction module, a feature fusion module, a feature enhancement module, and a classification module to achieve high-precision and fast fault diagnosis. The effectiveness of this framework is validated on two publicly available datasets. The results show that the average accuracy of this fault diagnosis framework can reach 100%, surpassing the findings of existing research. The experiments on noise resistance further showcase the superiority of the proposed framework. It is expected that the framework presented in this study will provide a new perspective and direction for fault diagnosis.

Echo Frequency Estimation Technology for Passive Surface Acoustic Wave Resonant Sensors Based on a Genetic Algorithm

Passive wireless surface acoustic wave (SAW) resonant sensors are widely used in measuring pressure, temperature, and torque, typically detecting sensing parameters by measuring the echo signal frequency of SAW resonators. Therefore, the accuracy of echo signal frequency estimation directly affects the performance index of the sensor. Due to the exponential attenuation trend of the echo signal, the duration is generally approximately 10 μs, with conventional frequency domain analysis methods limited by the sampling frequency and data points. Thus, the resolution of frequency estimation is limited. Here, signal time-domain fitting combined with a genetic algorithm is used to estimate SAW echo signal frequency. To address the problem of slow estimation speed and poor timeliness caused by a conventional genetic algorithm, which needs to simultaneously estimate multiple parameters, such as signal amplitude, phase, frequency, and envelope, the Hilbert transform is proposed to remove the signal envelope and estimate its amplitude, and the fast Fourier transform subsection method is used to analyze the initial phase of the signal. The genetic algorithm is thereby optimized to realize the frequency estimation of SAW echo signals under a single parameter. The developed digital signal processing frequency detection system was monitored in real time to estimate the frequency of an SAW echo signal lasting 10 μs and found to have only 100 sampling points. The proposed method has a frequency estimation error within 3 kHz and a frequency estimation time of less than 1 s, which is eight times faster than the conventional genetic algorithm.

Frequency-domain hydroelastic stress analysis of floating structures under spatially inhomogeneous wave field

This paper presents a frequency-domain method for the stress analysis of flexible floating structures exposed to a spatially inhomogeneous wave field. A hydroelastic model is developed for the continuous structure, wherein discrete rigid modules are connected by elastic beams with equivalent stiffness. The inhomogeneous wave field is divided into wave regions characterized by constant wave parameters. By assigning distinct wave parameters to modules based on their respective positions, the global hydroelastic equation is established to capture the wave inhomogeneity. To compute the stress, external loads in the hydroelastic equation, comprising hydrodynamic and hydrostatic restoring pressures, as well as inertia forces, are mapped onto the finite element model of the continuous structure, enabling stress calculation by the quasi-static method. Variations in wave directions and heights along the longitudinal direction of the structure are introduced with the aim of exploring the effect of the wave inhomogeneity on the stress. A deliberately flexible barge is constructed to evaluate the stress resulting from global structural vibrations. The results demonstrate a significant contribution of wave inhomogeneity to the stress, particularly during structural resonance frequencies. Moreover, an exclusive phenomenon is observed whereby the inhomogeneous wave field induces stress due to anti-symmetric vibrations under beam sea conditions.

Research on Compressive Tests of Iron Tailing and Cement-Modified Earth Materials from Northwest China

The strength of earth-modified materials is an important mechanical indicator of raw soil building materials. Iron tailings are a rich mineral waste resource with basic properties similar to river sand and can be used as a modified material for raw soil construction. To increase the utilization rate of iron tailings and improve the mechanical properties of earth materials, modification of earth materials were analyzed using iron tailings and cement. Through a single lattice theory design method, 10 different blending ratios of modified earth specimens were designed, with each group comprising six 100 mm cubic specimens. Earth specimens were used as the control group and compression tests carried out under standard curing conditions, with analysis of the failure mode, compressive strength, deformation, and load–displacement curve for each specimen. Also, the reasons for specimen failures were examined, including the influence of strength, dispersion, and implications of load–displacement curves. The optimal mix ratio of modified materials was determined using frequency domain analysis method. The results showed that the load–displacement curves of materials with different material ratios were basically similar and the slope and discreteness of the curves quite different. Increased tailings and cement content improved the compressive strength of earth materials and increased the discreteness of the material. Through frequency analysis, it was obtained that when the content of tailings was 12.1%–19.5%, cement content 13.9%–19.1%, and earth 65.5%–69.9%, there was a 95% of the guaranteed rate of compressive strength of iron tailings and cement-modified earth materials between 6 and 9 MPa. Iron tailings can, thus, be used as the preferred material for modifying earth materials; single lattice theory can be used as the design method for mixing experiments, and frequency domain analysis can be an effective calculation method for optimizing mixing design. Adding cement and iron tailings to modify the raw soil formula can provide reference for the selection of basic materials for raw soil construction.

Control strategy study and cascade control system design for space thermionic nuclear reactor

Nuclear reactors are gaining increasing attention for use in spacecraft owing to their long life and high power-density. Because the dynamic characteristics and operation strategy of a space nuclear reactor (SNR) are different from those of a traditional reactor, its control strategy must be investigated to design an appropriate control system. In this study, the dynamic characteristics of the time and frequency domains are analyzed. Based on this analysis, a control strategy is proposed. The cascade control method is applied to the design of control systems, including electric power and coolant temperature control systems. The controller parameters are tuned using the frequency-domain analysis method. To verify the control performance, step and ramp disturbances of the electric power and coolant temperature setpoints are introduced, and comprehensive comparisons are made. Additionally, the startup process is simulated; it reveals that the cascade control systems perform better compared with the traditional proportional-integral-derivative (PID) control. Moreover, the control performance of the wide-range power regulation is smooth and steady. Thus, the control strategy and cascade control system designed in this study can be applied to the thermionic nuclear reactors in space.

A NARX Model-Based Condition Monitoring Method for Rotor Systems.

In this study, we developed a data-driven frequency domain analysis method for rotor systems using the NARX (Nonlinear Auto-Regressive with eXternal input) model established by system vibration signals. We propose a model-based index of fault features calculated in a multi-frequency range to facilitate condition monitoring of rotor systems. Four steps are included in the proposed method. Firstly, displacement vibration signals are collected at multiple monitored rotating speeds. Secondly, the collected signals are processed as output data and the corresponding input data is generated. Then, NARX models are developed with input and output data to characterize the rotor system. Finally, the NRSF (Nonlinear Response Spectrum Function)-based nonlinear fault index is calculated and compared to the healthy condition. An experimental application to the misaligned rotor system is also demonstrated to verify its effectiveness. Our results indicate that the value of the index directly reflects the severity of the misaligned fault.

Comparative Study of Furuta Pendulum Based on LQR and PID Control

Furuta pendulum is an underdriven and unstable controlled object with one input and two outputs, and has an open-loop zero point at the origin of the complex plane. In this paper, a nonlinear dynamic model of the Furuta pendulum is established by the Eulerian-Lagrangian method, and a series-connected PID control strategy is designed by using the root locus method and the frequency domain analysis method based on the local linearization model at the steady-state operating point. Besides, an LQR control strategy is designed based on the discrete state-space model. In addition, a comparative analysis of the PID control strategy and the LQR control strategy is conducted, and it is found that the LQR control strategy can achieve stable control of the outer loop in the case of unstable inner loop. Finally, the effectiveness of the two control schemes is verified by real-time control experiments.

Milling tool wear prediction: optimized long short-term memory model based on attention mechanism

To improve the prediction accuracy of milling tool wear, a prediction method based on Attention-LSTM is proposed. In the training phase, first, the data are pre-processed by truncation, downsampling, and the Hampel filtering method, and then features are extracted by the time domain, frequency domain, and time-frequency domain analysis methods. Second, a deep neural network is designed to describe the complex nonlinear function between features and tool wear. Last, aiming at the insufficient prediction accuracy due to the LSTM lacking feature extraction and enhancement, the Attention mechanism is introduced to optimize the model. The results suggest that this prediction method provides an efficient strategy for milling tool wear prediction.

Association of heartbeat complexity with survival in advanced non-small cell lung cancer patients.

Previous studies have shown that the predictive value of traditional linear (time domain and frequency domain) heart rate variability (HRV) for the survival of patients with advanced non-small cell lung cancer (NSCLC) is controversial. Nonlinear methods, based on the concept of complexity, have been used to evaluate HRV, providing a new means to reveal the physiological and pathological changes in HRV. This study aimed to assess the association between heartbeat complexity and overall survival in patients with advanced NSCLC. This study included 78 patients with advanced NSCLC (mean age: 62.0 ± 9.3 years). A 5-min resting electrocardiogram of advanced NSCLC patients was collected to analyze the following HRV parameters: time domain indicators, i.e., standard deviation of the normal-normal intervals (SDNN) and root mean square of successive interval differences (RMSSD); frequency domain indicators, i.e., total power (TP), low frequency power (LF), high frequency power (HF), and the ratio of LF to HF (LF/HF); nonlinear HRV indicators characterizing heartbeat complexity, i.e., approximate entropy (ApEn), sample entropy (SampEn), and recurrence quantification analysis (RQA) indexes: mean diagonal line length (Lmean), maximal diagonal line length (Lmax), recurrence rate (REC), determinism (DET), and shannon entropy (ShanEn). Univariate analysis revealed that the linear frequency domain parameter HF and nonlinear RQA parameters Lmax, REC, and DET were significantly correlated with the survival of advanced NSCLC patients (all p < 0.05). After adjusting for confounders in the multivariate analysis, HF, REC, and DET were found to be independent prognostic factors for the survival of patients with advanced NSCLC (all p < 0.05). There was an independent association between heartbeat complexity and survival in advanced NSCLC patients. The nonlinear analysis method based on RQA may provide valuable additional information for the prognostic stratification of patients with advanced NSCLC and may supplement the traditional time domain and frequency domain analysis methods.

Experimental Investigation on Velocity Fluctuation in a Vaned Diffuser Centrifugal Pump Measured by Laser Doppler Anemometry

Turbulent flow, mainly originating from the rotor-stator interaction (RSI), is closely associated with the normal and safe operation of the centrifugal pump. In the current research, to clarify turbulent flow in the centrifugal pump with a vaned diffuser, the non-intrusive LDA (Laser Doppler Anemometry) system is applied to measure velocity pulsation signals at different regions when the pump operates at various flow rates. Time and frequency domain analysis methods are combined to investigate the velocity signals, and the velocity distribution around the volute tongue region is reconstructed from twenty measuring points. Results show that the velocity spectrum is characterized by the discrete components at the blade passing frequency and its higher harmonics, and it is caused by the RSI between the impeller and the diffuser. For the points in the volute spiral and diffusion sections, due to the significantly reduced RSI effect, the velocity spectrum shows an evident difference from comparison with the points between the impeller and diffuser, and the blade passing frequency is not always the dominant frequency. The comparison of velocity amplitudes and RMS* (root mean square of velocity) values at different points proves that the measuring position and flow rate affect velocity pulsations. As observed from velocity distribution reconstructed by LDA signals, high velocity regions are developed downstream of the diffuser channel for all the measured flow rates.

Multi-Inverter Resonance Modal Analysis Based on Decomposed Conductance Model

The Norton equivalent model based on the transfer function and the frequency domain analysis method for inverter resonance analysis lacks a comprehensive analysis of the resonant characteristics, and more information about the resonant key components and the degree of participation cannot be obtained. In this paper, a decomposed conductance model is proposed to characterize the resonance characteristics of the multi-inverter grid-connected system and the effect of the equivalent control link of the inverter on the resonance in more detail by combining the modal analysis method and the sensitivity analysis method. Firstly, based on αβ coordinates, the conductance division is carried out for the dual-loop inverter control link with the voltage external loop and current internal loop using capacitor-current feedback damping, and the inverter model based on the decomposition conductance is derived. The mathematical model of the multi-inverter grid-connected system is then established. Secondly, the resonance characteristics of the system are analyzed by combining the modal and frequency domain analysis methods when the number of inverters, inverter parameters, and grid-side impedance are changed. Thirdly, the degree of involvement of the system components, especially the equivalent control link of the inverter in resonance conditions, is determined in combination with the proposed model and the sensitivity analysis method, which is the basis for proposing an effective suppression strategy. Finally, a simulation model is built to verify the proposed method and the analysis results.