ALBERT

Description: Tipping in multistable systems occurs usually by varying the input slightly, resulting in the output switching to an often unsatisfactory state. This phenomenon is manifested in thermoacoustic systems. The thermoacoustic instability may lead to the disintegration of rocket engines, gas turbines and aeroengines, so it is necessary to design control measures for its suppression. It was speculated that such unwanted instability states may be dodged by changing the bifurcation parameters quickly enough, and compared with the white noise discussed in [1], colored noise with nonzero correlation time is more practical and important to the system. Thus, in this work, based on a fundamental mathematical model of thermoacoustic systems driven by colored noise, the corresponding Fokker–Planck–Kolmogorov equation of the amplitude is derived by using a stochastic averaging method. A transient dynamical behavior is identified through a probability density analysis. We find that both a relatively higher rate of change of parameters and change in the correlation time of the noise are beneficial to dodge thermoacoustic instability, while a relatively large noise intensity is a disadvantageous factor. More interestingly and importantly, power-law relationships between the maximum amplitude and the noise parameters are uncovered, and the probability of successfully dodging a thermoacoustic instability is calculated. These results serve as a guidance for the design of engines and to propose an effective control strategy, which is of great significance to aerospace-related fields.

Description: Thermoacoustic instability has been an important challenge in the development of high-performance combustion systems, as it can have catastrophic consequences. The process of a sudden change in the dynamical behavior of a thermoacoustic system from a low- to high-amplitude thermoacoustic instability actually entails as a tipping point phenomenon. It has been found that when rate-dependent parameters are considered, a tipping-delay phenomenon may arise, which helps in the control of undesirable states that give rise to thermoacoustic instabilities. This work aims at understanding rate-dependent tipping dynamics of the thermoacoustic system with both time-varying parameters and a non-Gaussian Lévy noise. The latter better describes the severe operating environment of such systems than simpler types of noise. Through numerical simulations, the tipping dynamical behavior is analyzed by considering the rate-dependent parameters coupled with the main parameters of the Lévy noise, including the stability and skewness indices and the noise intensity. In addition, we investigate the effectiveness of early warning indicators in rate-dependent systems under Lévy noise excitation and uncover a relationship between warning measures and the rate of change in the parameters. These results inform and enlighten the development and design of power combustion devices and also provide researchers and engineers with effective ideas to control thermoacoustic instability and the associated tipping dynamics. Thermoacoustic systems are closely related to propulsion combustion devices such as solid and liquid rocket motors, aero engines, and gas turbines. A thermoacoustic instability arises when there is a positive feedback between the non-constant exothermic rate in the combustion chamber and the acoustic field. Thermoacoustic instabilities lead to self-excited large amplitude pressure oscillations in the combustion chamber. These typically undesirable pressure oscillations can lead to catastrophic consequences such as structural damage due to excessive heat transfer and vibration, ballistic anomalies in the engine, damage to electronic equipment in aircrafts and satellites, and even to rocket launch missions due to engine disintegration. Therefore, the occurrence of thermoacoustic instability has been a major problem faced during the development of high-performance combustion systems. So far, studies of the thermoacoustic instability are limited to the excitation of Gaussian noise, which has limitations in describing large jumps. However, the effect of extreme severe operating environments on thermoacoustic instability cannot be ignored. Therefore, it is crucial to introduce a more appropriate noise portrayal in the study: non-Gaussian Lévy noise. Considering that the control parameters in real industrial thermoacoustic systems are time-varying, this work provides referenceable control strategies and early warning signals for thermoacoustic instability avoidance based on the rate-dependent mathematical model of thermoacoustic systems.

Description: Tipping is a phenomenon in multistable systems where small changes in inputs cause huge changes in outputs. When the parameter varies within a certain time scale, the rate will affect the tipping behaviors. These behaviors are undesirable in thermoacoustic systems, which are widely used in aviation, power generation and other industries. Thus, this paper aims at considering the tipping behaviors of the thermoacoustic system with the time-varying parameters and the combined excitations of additive and multiplicative colored noises. Transient dynamical behaviors for the proposed thermoacoustic model are implemented through the reduced Fokker-Planck-Kolmogorov equation derived by a standard stochastic averaging method. Then, the tipping problems of the rate-dependent thermoacoustic systems with random fluctuations are studied by virtue of the obtained probability density functions. Our results show that the rate delays the value of the tipping parameter compared to the one with the quasi-steady assumption, which is called as a rate-dependent tipping-delay phenomenon. Besides, the influences of the initial values, the rate, the changing time of the parameters, and the correlation time of the noises on the rate-dependent tipping-delay phenomenon are analyzed in detail. These results are of great significance for research in related fields such as aviation and land gas turbines.

Description: Humanity's transformation of the nitrogen cycle has major consequences for ecosystems, climate and human health, making it one of the key environmental issues of our time. Understanding how trends could evolve over the course of the 21st century is crucial for scientists and decision-makers from local to global scales. Scenario analysis is the primary tool for doing so, and has been applied across all major environmental issues, including nitrogen pollution. However, to date most scenario efforts addressing nitrogen flows have either taken a narrow approach, focusing on a singular impact or sector, or have not been integrated within a broader scenario framework – a missed opportunity given the multiple environmental and socio-economic impacts that nitrogen pollution exacerbates. Capitalizing on our expanding knowledge of nitrogen flows, this study introduces a framework for new nitrogen-focused narratives based on the widely used Shared Socioeconomic Pathways that include all the major nitrogen-polluting sectors (agriculture, industry, transport and wastewater). These new narratives are the first to integrate the influence of climate and other environmental pollution control policies, while also incorporating explicit nitrogen-control measures. The next step is for them to be used as model inputs to evaluate the impact of different nitrogen production, consumption and loss trajectories, and thus advance understanding of how to address environmental impacts while simultaneously meeting key development goals. This effort is an important step in assessing how humanity can return to the planetary boundary of this essential element over the coming century.

Description: This Focus Issue brings together recent notable advances in the field of dynamical systems and complexity science on the subject of the non-linear collective behavior of power grids and energy systems. Due to the ongoing energy transition, mandated by the epochal challenge that is the decarbonization of human activity, our energy infrastructure is evolving rapidly. This challenges us to understand and rethink current and potential future power grids. In response to this urgent need, the last decade has seen the emergence of a new direction of theoretical research on complex power grids. There is a long tradition of studying models of the complex power grid in the electrical engineering and control theory literature, with the foundational second-order Kuramoto model appearing first in 1981 in the paper by Bergen and Hill,1 leading up to the seminal synchronization results of Dörfler et al.2 and the basin of attraction of the synchronous state of Menck et al.3 The new direction that has been developing using dynamical systems theory complements this by bringing in a new systemic perspective and considering heads on the novel challenge posed by stochastic4 and decentral power generation. This theory- and data-driven research achieved notable insights. A subjective, non-exhaustive list of highlights include (i) the network-wide dynamic response patterns emerging from fluctuating power feed-in;5–7 (ii) time-delayed control mechanisms to reduce grid frequency variation;8 (iii) a deeper understanding of the network structure’s impact on stability;9–13 (iv) new insights into power flows and cascades14,15 and the introduction of dual graphs for new analytic results in this context;16,17 and (v) data-driven and extensive stochastic analysis on renewable energies and power grid frequencies.18–20 The methods and topics in this Focus Issue not only cover further developments in the directions of these highlights but also contribute to a number of new emerging directions that are gaining importance. This includes full range of dynamical systems theory aspects, from delays and control systems, over stochastic systems, and time series analysis to bifurcation theory and multilayer complex networks. We will present the contributions to this Focus Issue grouped according to these themes: multistability and bifurcation structure (Sec. II A), cascades and line failures (Sec. II B), variability and stochastic drivers of power grids (Sec. II C), and control and delays (Sec. II D). Beyond these established areas, we discuss the most important emerging and cross-cutting themes in Sec. III.

Description: Anthropogenic climate change is expected to affect global river flow. Here, we analyze time series of low, mean, and high river flows from 7250 observatories around the world covering the years 1971 to 2010. We identify spatially complex trend patterns, where some regions are drying and others are wetting consistently across low, mean, and high flows. Trends computed from state-of-the-art model simulations are consistent with the observations only if radiative forcing that accounts for anthropogenic climate change is considered. Simulated effects of water and land management do not suffice to reproduce the observed trend pattern. Thus, the analysis provides clear evidence for the role of externally forced climate change as a causal driver of recent trends in mean and extreme river flow at the global scale.

Description: The underwater positioning of marine acoustic datum is mainly based on the principle of spatial distance intersection positioning. As the limitation of the observation structure, the observation error is easy to accumulate in the vertical direction. If the accurate depth of marine acoustic datum can be obtained and used for depth constrained positioning, it will help to further improve the positioning accuracy of marine acoustic datum. The depth gauge based on the principle of pressure measurement has been widely concerned due to the advantage of autonomous depth measurement. However, the effect of pressure-depth conversion on the positioning error of marine acoustic datum under depth constraint has not drawn enough scholarly attention. This paper focuses on the evaluation of the pressure-depth conversion accuracy of different seawater physical models and conducts an experimental analysis based on Argo buoy data. The results of the experiment show that the TEOS-10 model has the highest conversion accuracy, and using accurate profile information of temperature, salinity, and pressure parameters to correct the depth data is helpful to improve the accuracy of marine acoustic datum positioning.

Description: In order to systematically and thoroughly study the crust-mantle structure and deep geodynamic processes of basins, mountains and plateaus of western China, we proposed and led the implementation of the ANTILOPE Project (Array Network of Tibetan International Lithospheric Observation and Probe Experiments) in 2003. So far, we have completed four 2D broadband arrays, ANTILOPE-I to ANTILOPE-IV, on the Tibetan Plateau, and deployed two 3D broadband arrays, ANTILOPE-V and ANTILOPE-VI, at the eastern and western Himalayan syntaxis, respectively. In addition, we included in our study framework nine comprehensive geophysical observation profiles previously obtained from the Junggar Basin, Tienshan Orogenic Belt, Tarim Basin, Altyn Orogenic Belt, and Qaidam Basin. Through the implementation of the ANTILOPE Project, we collected a large amount of high-quality, comprehensive first-hand observational data from western China (including the basin-mountain system surrounding the Tibetan Plateau in the northwest and the Tibetan Plateau in the southwest). The fine crust-mantle structure systematically reveals the deep geodynamic processes of the basin-mountain-plateau geosystem in western China. The up-to-date main research progress can be summarized as follows. The structure and properties of the basement of the Junggar Basin have been determined, and the basement structural framework has been optimized. A new intracontinental orogenic model of lithospheric subduction with crustal interlayer intrusion in the Tienshan Orogenic Belt has been established, which reveals the fate of the 44% shortened Tienshan lithosphere after the India-Eurasia collision and the conversion mechanism from ocean-continent subduction to continent-continent collision and subduction. Our results reveal the basin-mountain contact relationship between the Tarim Basin, Altyn Orogenic Belt and Qaidam Basin. We have obtained the deep geometric, kinematic and geodynamic evidence for the clockwise rotation of the Tarim Basin, and determined the collision boundary between the Indian and the Eurasian Plates under the Tibetan Plateau. We also found that the current Tibetan Plateau consists of the Indian Plate in the south, the Eurasian Plate in the north, and the giant crush zone-also called the "Tibetan Plate"-between them. For the first time, the respective lithospheric bottom boundaries are determined; two end-member models of plateau deformation are corrected; and the constraints of deep structures on the surface topography are established. Our result systematically reveals the changing pattern and controlling factors of the horizontal advancing distance and the subduction angle of the Indian Plate along the Himalayan Orogenic Belt. By combining a huge observation network with comprehensive geophysical detection technologies, the ANTILOPE Project adopts different methods, including geophysical, geological and geochemical methods, to reveal the subduction of the Indian continent, the development of the giant crush zone in Tibet, the clockwise rotation of the Tarim Block, the accelerated closure of the western water vapor channel, and the advance of aridification and desertification in northwest China and their constraints on surface topography, oil and gas resources, and environmental variations. The above results have promoted the development of the Earth system theory in the Tibetan Plateau. © 2021, Editorial Office of Earth Science Frontiers. All right reserved.