Fluctuation Characteristics Research Articles

Experimental investigation on the characteristics and alleviation of the upper part load pressure fluctuation in a Francis turbine

The Francis turbine is a renowned reaction hydraulic turbine known for its remarkable ability to convert the energy of flowing water into mechanical energy. Operating near the best efficiency point, the Francis turbine experiences the upper part load pressure fluctuation, which is characterized by an abrupt high amplitude increment and decay and can put the stability of a power plant at risk. This study experimentally investigated the amplitude-frequency characteristics of this special pressure fluctuation. The findings indicate that the most powerful amplitude reaches up to 15.35% of the tested water head in the vaneless space, and the characteristic frequency ranges from 1.6 to 2.3 times the rotating frequency. Notably, the study has found that the vortex rope in the draft tube exhibits a slender and spring-like within the upper part load region, and it looks like a spring stretching and compressing during the evolution of vortex rope, implying energy accumulation and release. Furthermore, a technical approach to alleviate high-amplitude pressure fluctuation in upper part load region is proposed that controls the velocity distributions at the runner outlet. A comparison of experimental test evidence shows that an impressive result is obtained with a reduction of the maximum amplitude from 10.75% to 3.52% in spiral casing, and from 15.35% to 3.03% in the vaneless space. In addition, the first-order frequencies at the spiral casing are primarily reduced, while the draft tube shows local increment, and the runner blade passage frequency becomes dominant in the vaneless space.

Spatial-temporal evolution and pressure fluctuation characteristics of the combined submerged vortex in a closed pump sump

Following the retrofit of the pump sump, there is a high risk of inducing the Combined Submerged Vortex (CSV), which consists of the roof-attached vortex (RAV) and floor-attached vortex (FAV). This vortex formation can lead to irregular pressure fluctuations, adversely affecting the unit's performance and compromising the stability of energy conversion. This study aims to investigate the spatiotemporal evolution of the CSV and its associated pressure fluctuation characteristics in a closed pump sump. High-speed visualization and pressure fluctuation tests are conducted on a transparent closed-loop test rig. The findings demonstrate that the spatiotemporal evolution of the CSV can be divided into three stages: developing, competing, and collapsing. The competing stage, in particular, has a significant impact on the FAV. During the CSV period, the dominant low frequencies induced by the RAV and FAV are 0.24 and 0.13 Hz, respectively, with the FAV exhibiting higher intensity than the RAV. The regions of high coherence between the RAV and FAV are primarily concentrated within the low-frequency range of 0.25–2 Hz, and the signals exhibit multiple phase differences. Furthermore, a vortex identification criterion for a closed sump is proposed based on Continuous Wavelet Transform.

Effects of magnetic field on the evolution of energy density fluctuations

We study the effects of a static and uniform magnetic field on the evolution of energy density fluctuations present in a medium. By numerically solving the relativistic Boltzmann-Vlasov equation within the relaxation time approximation, we explicitly show that magnetic fields can affect the characteristics of energy density fluctuations at the timescale the system achieves local thermodynamic equilibrium. A detailed momentum mode analysis of fluctuations reveals that a magnetic field increases the damping of mode oscillations, especially for the low momentum modes. This leads to a reduction in the ultraviolet (high momentum) cutoff of fluctuations and also slows down the dissipation of relatively low momentum fluctuation modes. We discuss the phenomenological implications of our study on various sources of fluctuations in relativistic heavy-ion collisions.

Energy loss and pressure fluctuation characteristics of coastal two-way channel pumping stations under the ultra-low head condition

The ultra-low head condition is a unique condition of the coastal two-way channel pumping stations, but the quantitative research on its hydraulic characteristics still needs to be improved. The entropy production method with computational fluid dynamics and the high-frequency pressure fluctuation test were employed to analyze the pumping device's internal energy loss and pressure fluctuation. The results show that the primary source of energy loss of the two-way channel pumping device under the ultra-low head condition is the turbulent dissipation caused by the velocity fluctuation of the impeller and guide vane, in which there are large irregular high entropy production areas on the pressure surface of the impeller and in the impeller channel. The high-frequency pressure fluctuation in the pumping device is mainly concentrated near the impeller, and the amplitude under the ultra-low head is 3.5 times the amplitude under the design head. Due to the dynamic and static interaction between the impeller and the guide vane, the frequency domain components of the pressure fluctuation at the outlet channel bell mouth are complex. Under the ultra-low head condition, the pressure fluctuation degree is intensified, the energy loss of each part is greater, and the low-frequency resonance should be prevented.

A Novel Grey Seasonal Model for Natural Gas Production Forecasting

To accurately predict the time series of energy data, an optimized Hausdorff fractional grey seasonal model was proposed based on the complex characteristics of seasonal fluctuations and local random oscillations of seasonal energy data. This paper used a new seasonal index to eliminate the seasonal variation of the data and weaken the local random fluctuations. Furthermore, the Hausdorff fractional accumulation operator was introduced into the traditional grey prediction model to improve the weight of new information, and the particle swarm optimization algorithm was used to find the nonlinear parameters of the model. In order to verify the reliability of the new model in energy forecasting, the new model was applied to two different energy types, hydropower and wind power. The experimental results indicated that the model can effectively predict quarterly time series of energy data. Based on this, we used China’s quarterly natural gas production data from 2015 to 2021 as samples to forecast those for 2022–2024. In addition, we also compared the proposed model with the traditional statistical models and the grey seasonal models. The comparison results showed that the new model had obvious advantages in predicting quarterly data of natural gas production, and the accurate prediction results can provide a reference for natural gas resource allocation.

Estimation of global waste smartphones and embedded critical raw materials: An industry life cycle perspective

Considering the quarterly fluctuation characteristics of global smartphone shipment, a novel and improved Verhulst model was proposed for forecasting smartphone shipments under different industry life cycle scenarios. The number of waste smartphones between 2007 and 2039 was estimated via using the lifespan distribution of electronics. The weight of the disposals was then converted by using product data collected from 2766 sampled smartphone devices. It was found that under the scenario of maturity, approximately 126,000 tons critical raw materials (CRMs) were expected to be recovered by 2039. According to the CRMs lists published by China and the European Union, the recoverable amount compared to about 62,000 tons and 64,000 tons, respectively. Enhancing the recycling of waste smartphones worldwide, will provide a new option to ensure an important part of the stability of the CRMs supply chains.

Optimal Configuration of Hybrid Energy Storage Capacity in a Microgrid Based on Variational Mode Decomposition

The capacity configuration of the energy storage system plays a crucial role in enhancing the reliability of the power supply, power quality, and renewable energy utilization in microgrids. Based on variational mode decomposition (VMD), a capacity optimization configuration model for a hybrid energy storage system (HESS) consisting of batteries and supercapacitors is established to achieve the optimal configuration of energy storage capacity in wind–solar complementary islanded microgrids. Firstly, based on the energy mapping relationship between the frequency domain and time domain, the decomposition mode number K of VMD is determined based on the principle of minimum total mode aliasing energy. Then, considering the smoothing fluctuation characteristics of different energy storage components, the dividing point N of high frequency and low frequency in the unbalanced power between the source and load in the microgrid is selected to allocate charging and discharging power instructions for the battery and supercapacitor. Finally, taking the annual comprehensive cost of the HESS as the objective function, a hybrid energy storage capacity optimization configuration model is established, and the dividing point N is used as the optimization variable to solve the model in order to obtain the optimal configuration results. The case study results show that the proposed method is more economical and feasible than the existing energy storage configuration methods.

Spatio-Temporal Evolution of Inland Lakes and Their Relationship with Hydro-Meteorological Factors in Horqin Sandy Land, China

In the inland closed area of Horqin Sandy Land, China, lakes are the most important source of water, and they maintain the regional hydrological balance and ecosystem health. Clarifying long-term continuous changes of inland lake surface area and water storage in the sandy land is thus of great significance to the management of water resources in arid and semi-arid areas. This study estimated changes in the surface area and water storage of small lakes in the sandy land during 1984–2021 using a multiple index threshold method and an empirical equation based on Shuttle Radar Topography Mission (SRTM) DEM (digital elevation model) data and Landsat 5/7/8 images. Hydro-meteorological variables were also incorporated to explore their potential relationship with changes in the lake elements. The lakes in the sandy land resemble stars or dots, with distinct inhomogeneity. Permanent lakes account for the majority of the total lake area, mostly distributed in the center and east of the study area, whereas most seasonal lakes are small water bodies, with surface areas of 0.1–0.5 km2 and irregularly distributed. Lake surface area and water storage underwent frequent changes during the 38 years, and could be divided into three characteristic fluctuation phases. From 1984 to 1999, the lake surface area fluctuated greatly, and the water storage increased by approximately 0.3732 Gt, with an average expansion rate of 0.0572 Gt/y. Specifically, lakes with a surface area >2 km2 primarily accounted for the expansion. From 1999 to 2004, the lake area shrank sharply to the lowest point, and the water storage capacity decreased by approximately 0.4003 Gt. From 2004 to 2021, the lake surface area and water storage tended to be stable. Annual fluctuations of lake surface area and water storage were mostly affected by precipitation and evapotranspiration, followed by vapor pressure deficit, wet day frequency, and temperature, which have significant periodicity and hysteresis.

Research on price transmission in Chinese mining stock market: Based on industry

The strong impact of COVID-19 on the global mining market has caused severe fluctuations in the prices of mineral products and mining stocks. Meanwhile, geopolitical conflicts have exacerbated risks in minerals trade and mining stock transactions. In the face of uncertainties in the international economic landscape and volatility of stock prices, China, as the world's major mineral trading country, has become increasingly linked between its stock market and the mining economy. To clarify the characteristics of mining stock price fluctuations and the evolution of the transmission relationships, and identify the key nodes and main paths of price transmission, we select 100 Chinese mining stocks from January 2019 to October 2022, distinguish them according to the industry category, and use Granger causality test, minimum spanning tree model and complex network analysis method to study. The results show that: (1) Chinese mining stock prices have risen significantly since 2020, and there has been a “decoupling” phenomenon within the stock market, that is, the linkage between some mining stocks has weakened. (2) The stock price fluctuation characteristics and transmission effects of different mining industries are obviously different. Precious metal minerals (PM) have the most dramatic changes in price fluctuations, the most prominent hedging characteristics, and the rapid price response ability, which is the first to accept price transmission. rare earth and rare metal minerals (RE) are sensitive to price fluctuations and are usually the “leader” of the transmission path. Bulk non-ferrous minerals (BNFM) have the most stable price fluctuations and are closely related to other stocks, which is a “transit warehouse” in the transmission path. (3) The price transmission mechanism of Chinese mining stock market has gradually stabilized, and the main transmission paths of “Coal→Agricultural minerals (Agri)→BNFM→Steel” and “PM, Core minerals for new energy (NEM), and RE→BNFM” have been formed in 2022.

Transient simulation of reversible pump turbine during pump mode's starting up

A pumped storage power station is an electric power system that utilizes low-peak electricity to pump water into a high-altitude reservoir and releases water to generate electricity during high-peak electricity demand. The pump mode's starting up impacts the operational stability and lifespan of the reversible pump turbine (RPT). Especially, the pressure pulsation signal during the pump mode's starting up is a typical non-stationary signal in RPT, which is an important basis for judging the operation status of the unit. In this study, the time-varying boundary conditions are used to calculate the pressure fluctuation and flow pattern evolution for a prototype RPT. This paper analyzed and compared the time-frequency characteristics of pressure fluctuations by continuous wavelet transform (CWT) and by variational mode decomposition (VMD). And the internal flow of RPT was also visualized using entropy generation theory. In this study, the effect of the penalty coefficient on the VMD algorithm is investigated in the form of a signal-noise ratio (SNR). With the increase of the penalty coefficient, the SNR of the signal gradually decreases and tends to be stable. Several modal components with strong physical significance and center frequency separation are obtained. Influenced by the boundary conditions that change with time, the pressure fluctuation presents a five-stage trend of “descending - ascending - descending - ascending - descending”. There are three main types of high-flow energy dissipation (FED) areas during this process of the RPT. The research and analysis in this paper will provide a good reference for the stable and safe operation of RPT and pumped storage power stations.

Pressure fluctuation and cellularization characteristics of 2-ethylfuran spherical expanding flame

The pressure fluctuation and cellularization characteristics of 2-ethylfuran (2-EF) liquid biofuel were investigated. The study was based on experiments in a constant volume combustion chamber (CVCC) under different initial temperatures (373, 403, and 433 K), initial pressures (1, 2, and 4 bar), and equivalence ratios (0.7–1.4). The spectrum analysis of the explosion pressure showed that the amplitude of pressure fluctuations was more intense with an increase in the equivalence ratio and initial pressure. Moreover, a three-dimensional reconstruction algorithm was constructed to analyze the evolution of cellular structures. It was found that both crack expansion and cell formation caused the irregular evolution of the flame front, which was more obvious at high equivalence ratios and initial pressures. Finally, the flame’s pressure fluctuation before reaching the quartz window was analyzed and combined with flame cellularization, which showed that the early pressure curve and cellularization parameters (cell number and crack length) had similar fluctuations, and the amplitude of the pressure fluctuation was influenced mainly by the initial pressure and the extent of flame cellularization.

Operating characteristics of a linear range extender with different energy consumption topologies

The free-piston linear generator is a new energy transformation technology with compact structure, multi-fuel adaptability, low friction loss, and simple modular configuration. To advance the practical process of the free-piston linear generator, several different topologies of electrical energy consumption are applied to the system, and the generating performance of the system is investigated in three-phase resistive load topology, uncontrolled rectification-resistive load topology, uncontrolled rectification-battery topology, and uncontrolled rectification-direct current to direct current converter -battery topology. The influence of the different topologies on the generator is mainly in the output type and output power (periodic, fluctuating). Furthermore, the impact of the topology on the operating characteristics of the free-piston linear generator, which mainly brings about changes in the characteristics of the current amplitude current fluctuations and generator force near the dead center, is explored. The current amplitude can influence the frequency and damp properties significantly and may cause system shutdown because of the improper matching of the generator and the load terminal. Changes in the generator force in the low-velocity region near the dead center have less impact on the output and thermodynamic processes of the system. The impact on system performance increases as the changes expand to the high-velocity region.

Low-Temperature Magnetic Fluctuations Investigated by 125Te-NMR on the Uranium-Based Superconductor UTe2

To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe2, we measured the NMR Knight shift K and the nuclear spin–lattice relaxation rate 1/T1 in H || a by 125Te-NMR on a 125Te-enriched single-crystal sample. 1/T1T in H || a is much smaller than 1/T1T in H || b and c, and magnetic fluctuations along each axis are derived from the 1/T1T measured in H parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe2 is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the qa = 0 ferromagnetic fluctuations.

The Fluctuation Characteristics and Periodic Patterns of Potato Prices in China

The aim of this paper was to provide a more scientific and effective analysis of the fluctuation pattern of the Chinese potato market by extracting the characteristics of the price fluctuation cycle to effectively grasp the characteristics of price changes in the potato market, thus promoting the stable and healthy development of the Chinese potato industry, and to expand the application scenarios of the EEMD model to provide a reference for the study of price fluctuation patterns in other agricultural markets. This study used an ensemble empirical modal decomposition (EEMD) model to examine time-series data on Chinese wholesale potato market prices from January 2005 to December 2021. The results showed that (1) Chinese wholesale potato market prices are characterized by some rigidity, with sharp changes in growth rates; (2) Chinese wholesale potato market prices are dominated by short- and medium-term fluctuations, and the decomposed components can better reflect the characteristics of the original series fluctuations; (3) Chinese wholesale potato market monthly prices have long- and short-term fluctuations with a 6- and 19-month cycle, and are dominated by short-term high-frequency fluctuations; (4) monthly price fluctuations in the Chinese wholesale potato market are more intense in high-frequency than low-frequency fluctuations, and there is a strong correlation between high- and low-frequency fluctuations in precipitation, temperature and potato prices. Finally, suggestions were made for creating and improving a national potato price information platform and strengthening the information early warning mechanism; improving the potato production interest linkage mechanism and enhancing potato farmers’ ability to cope with market and natural risks; and improving the potato reserve system and potato storage facilities.

Study on the Trade-Off Synergy Relationship of “Production-Living-Ecological” Functions in Chinese Counties: A Case Study of Chongqing Municipality

The trade-off and coordinated development of the “production-living-ecological” functions (PLEFs) of an area is an important basis for the optimization of the spatial pattern of the land, and helps to promote the harmonious symbiotic relationship between human beings and nature. This paper combines ecological niche theory, a coupling coordination model, and a trade-off synergy model to construct an evaluation index system for Chinese county PLEFs. Quantitative methods were used to measure spatiotemporal evolution characteristics, trade-off synergy of PLEFs in 38 counties in Chongqing, China, and the coupling coordination degree between PLEFs. The results showed that the ecological niche width of the “production-ecological” function revealed an overall growth trend. However, there was a mismatch in regional development of the “production-ecological” function, showing dislocation characteristics of “high in the west and low in the east” and “high in the east and low in the west.” The niche width of the life function is similar to the comprehensive niche width of PLEFs, showing the characteristics of fluctuation and partition change. PLEFs and both the aforementioned functions showed distribution characteristics of “high in the west and low in the east,” with the whole moving towards the stage of coordinated coupling, of which the “production-living” function has the highest coupling level. The functional coupling coordination degree of “production-living-ecological” is generally manifested as “high in the west and low in the east,” and changing from the primary stage of imbalance to well-coordinated development. The “production-ecological” and “living-ecological” functions are in low-level imbalance in the primary and moderate coordination stages. Additionally, the evolution trend of the “production-ecological” and the “living-ecological” functions are similar, showing alternating and fluctuating development characteristics. Overall, in the past 20 years, Chongqing’s “living-production” function has changed from a trade-off model to a collaborative development relationship, and the “living-ecological” function is generally based on a collaborative development relationship. The “ecological function” is manifested as a trade-off constraint relationship. Moreover, the coordinated development level of “living-production,” “living-ecological,” and “production-ecological” functions in the central urban area has been greatly improved, while counties have gradually shown different degrees of trade-offs.

Research on Operation Control Strategy of Wind and Solar Storage Systems Considering High Ratio of New Energy Access

High proportion of distributed new energy generation, DG) Connect to the grid to mitigate problems such as fossil energy shortages and the threat of global climate change. However, distributed new energy has the characteristics of fluctuation and uncertainty of output, so it will bring some difficulties to energy consumption after the high proportion is connected to the power grid. Given the above problem, the uncertainty of wind and solar output is considered in this paper to ensure that accuracy and the stability of the power grid operation after a large amount of distributed energy is accessed, with 8,760 hour-by-hour power generation data throughout the year as an input. Three kinds of wind and solar storage system operation control strategies are compared. Using storage battery energy storage method and multiple energy storage methods on the fly surplus storage, coordinated control of the system, and setting different discharge priorities in the energy storage portion, increase real-time and ordered energy consumption.

A Waveguide Model of the Developed Turbulent Boundary Layer

A study of the developed turbulent boundary layer that emerges when incompressible viscous fluid flows around a plate at a zero angle of attack and with zero longitudinal pressure gradient is presented. The waveguide approach is used for describing the turbulent boundary layer; in this approach, turbulent fluctuations are related with Tollmien–Schlichting waves that are in three-wave resonance. To study the original nonlinear system of equations, an estimate of hydrodynamic quantities is proposed that does not violate the generally accepted approach in the boundary layer but leads to the appearance of a new small parameter—the ratio of the thickness of the boundary layer momentum loss to the damping length of the least damped mode of the Tollmien–Schlichting waves. Equations for the coherent and stochastic parts of fluctuations are obtained on the basis of the method of multiple scales. The dispersion characteristics of waves of the least damped mode on the profile of the average longitudinal velocity of the developed turbulent boundary layer are determined, and the conditions for the multiple three-wave resonance of this mode of the Tollmien–Schlichting waves are analyzed. For the coherent part of the fluctuations, the fluctuation characteristics are compared with the known numerical results.

Influence of small-scale turbulence on internal flamelet structure

Direct numerical simulation data obtained from a highly turbulent (Kolmogorov length scale is less than a laminar flame thickness by a factor of about 20) lean hydrogen–air complex chemistry flame are processed, with the focus of the study being placed on flame and flow characteristics conditioned to instantaneous local values cFx,t of the fuel-based combustion progress variable. By analyzing such conditioned quantities, the following two trends are documented. On the one hand, magnitudes of fluctuations of various local flame characteristics decrease with increasing the combustion progress variable, thus implying that the influence of small-scale (when compared to the laminar flame thickness) turbulence on internal flamelet structure is reduced as the flow advance from unburned reactants to combustion products. On the other hand, neither local turbulence characteristics (conditioned rms velocities, total strain, and enstrophy) nor local characteristics of flame–turbulence interaction (flame strain rate) decrease substantially from the reactant side to the product side. To reconcile these two apparently inconsistent trends, the former is hypothesized to be caused by the following purely kinematic mechanism: residence time of turbulence within a large part of a local flamelet is significantly shortened due to combustion-induced acceleration of the local flow in the direction normal to the flamelet. This residence-time reduction with increasing cF is especially strong in the preheat zone (cF<0.3) and the residence time is very short for 0.3<cF<0.8. Therefore, small-scale turbulence penetrating the latter zone is unable to significantly perturb its local structure. Finally, numerical results that indirectly support this hypothesis are discussed.

An assessment model for urban resilience based on the pressure-state-response framework and BP-GA neural network

It has been widely appreciated that urban resilience is one of the core goals of urban development. Various approaches for evaluating the level of urban resilience have been developed recently. However, previous urban resilience assessment studies have mainly concentrated on the economy, society, infrastructure, and ecological environment, with very few considering the characteristics of the urban resilience regression process. Therefore, this research proposes a new assessment framework for urban resilience from the perspective of “pressure-state-response” to address this issue. And then, the methods of the BP neural network, genetic algorithm, Moran's index and the center of gravity model are combined to establish the assessment model of urban resilience. 31 provinces in Mainland China are selected as a case study to demonstrate the application of the assessment model. The calculation results indicate that the urban resilience level of all provinces in China is rising, and the provincial urban resilience development shows the characteristics of fluctuation. The trend of urban resilience shifted from north to south from 2013 to 2019, consistent with China's economic center of gravity moving from north to south. This study develops a new angle for evaluating urban resilience and provides effective policies toward urban resilience.

Application of VMD-LSTM in Water Quality Prediction

Water is an indispensable prerequisite and essential factor for human life and social development. Change in water quality is closely related to water pollution. Water quality prediction is the basic work to control water environmental pollution. Therefore, the establishment of effective models is of great value to social economy, environmental protection and human health. This paper aims at the problem that the prediction error of relative maximum and relative minimum values of Long Short-Term Memory (LSTM) in dealing with time series is large, combined with the advantages of Variational Mode Decomposition(VMD) to extract signal fluctuation characteristics and reduce noise interference, Taking the Dissolved oxygen(DO) and the Total phosphorus(TP) as the main research objects, A combined model based on VMD-LSTM is proposed. The three indexes of Mean Absolute Error(MAE), Root Mean Square Error(RMSE) and Mean Absolute Percentage Error(MAPE) were used to judge. Experiments show that the VMD-LSTM model has improved all indicators compared with the LSTM model. DO prediction results respectively reduced by 38.8%, 41.4% and 44.4%, TP prediction results respectively reduced by 30.4%, 34.4% and 32.2%, the validity of the model is verified.