The models prescribed by the IEC standard for the generation of wind fields, as well as intermittent modifications of the original models, are used as inflows in LES. The behaviour of the wind fields ...is investigated with regard to higher-order moments. It is shown that all models reach an intermittent state in the LES. This intermittent state can be determined by the intermittency parameter and by higher orders of the moments. It can be seen that the Kaimal model generates significantly lower intermittency than the other models. The convergence length is significantly reduced with a correct intermittency description in the wind fields.
Sediment transport by wind or water near the threshold of grain motion is dominated by rare transport events. This intermittency makes it difficult to calibrate sediment transport laws, or to define ...an unambiguous threshold for grain entrainment, both of which are crucial for predicting sediment transport rates. We present a model that captures this intermittency and shows that the noisy statistics of sediment transport contain useful information about the sediment entrainment threshold and the variations in driving fluid stress. Using a combination of laboratory experiments and analytical results, we measure the threshold for grain entrainment in a novel way and introduce a new property, the “shear stress variability”, which predicts conditions under which transport will be intermittent. Our work suggests strategies for improving measurements and predictions of sediment flux and hints that the sediment transport law may change close to the threshold of motion.
Plain Language Summary
Sediment transport by wind or water is intermittent―Displaying long periods of low transport followed by sudden large transport events. Intermittency is commonly seen as unwanted noise that makes it difficult to predict sediment flux. We introduce a mathematical model for sediment transport near the threshold of motion which provides a possible explanation for why and when intermittency occurs, and show that the noisy statistics of intermittent sediment transport encode information about the threshold of sediment motion and the variations in driving fluid forces. This knowledge can help improve the accuracy of sediment transport predictions, which are central to many engineering and geological applications. The ingredients necessary for intermittency―Noise near a transition―apply to many systems beyond sediment transport.
Key Points
Sediment transport near the threshold of motion is intermittent, making it hard to measure average flux, which is crucial for predictions
We use bifurcation theory in the presence of multiplicative noise to understand and describe the intermittency
Applying this to flume experiments, we find a new way of measuring the threshold of motion and predicting when intermittency will be present
For initial datum of finite kinetic energy, Leray has proven in 1934 that there exists at least one global in time finite energy weak solution of the 3D Navier-Stokes equations. In
this paper we ...prove that weak solutions of the 3D Navier-Stokes equations are not unique in the class of weak solutions with finite kinetic energy. Moreover, we prove that Holder
continuous dissipative weak solutions of the 3D Euler equations may be obtained as a strong vanishing viscosity limit of a sequence of finite energy weak solutions of the 3D
Navier-Stokes equations.
Near the threshold of grain motion, sediment transport is “on‐off” intermittent, characterized by large but rare bursts separated by long periods of low transport. Without models that can account for ...the effects of intermittency, measurements of average sediment flux can be in error by up to an order of magnitude. Despite its known presence and impact, it is not clear whether on‐off intermittency arises from the grain activity (the number of moving grains) or grain velocities, which together determine the sediment flux. We use laboratory flume experiments to show that the on‐off intermittency has its origins in the velocity distributions of grains that move by rolling along the bed, whereas grain activity is not on‐off intermittent. Incorporating the types of intermittency we identify into stochastic models of sediment transport could yield improved predictions of sediment flux, including physically based estimates of the uncertainty in time‐averaged sediment flux.
Plain Language Summary
Sediment, such as sand and gravel, is transported across the surface of the Earth and other planets by wind and water. Predicting the amount of sediment that can be transported given a certain flow rate is crucial for predicting how a landscape will change over time. For low flow rates, little to no grain motion occurs. Just beyond the flow rate required for motion, sediment transport occurs mostly in rare bursts. This so‐called “on‐off” intermittency creates difficulties when trying to measure the average transport rate, which must be done over longer time periods as the bursts become larger but less frequent. While on‐off intermittency has been identified in previous studies of sediment transport, there is currently no understanding of its physical origin. We use a series of experiments in a small laboratory river to show that the on‐off intermittency is due to large fluctuations in the speed of the grains rolling on the bed, and that the sediment transport becomes less bursty as more grains are lifted off the river bed and into the fluid. Our results will pave the way for better measurements and predictions of sediment transport in rivers.
Key Points
We use grain tracking data from laboratory experiments to separately study the statistics of grain velocities and grain activity
We show that on‐off intermittency has its origins in the velocity distributions of grains, not in the grain activity
On‐off intermittency comes from grains rolling on the bed, and disappears as more and more grains are lifted into the bulk of the flow
Abstract
Randomness and intermittency are crucial challenges in photovoltaic (PV) power prediction. Most studies concentrate on addressing the randomness of PV power, and tend to overlook the ...intermittency that leads to sample imbalance, which negatively affects prediction accuracy. To address the sample imbalance, a novel approach called segment imbalance regression (SIR) is proposed. The SIR method proactively exploits the inherent imbalanced nature of samples by investigating the interactions among neighbouring samples, which leads to dynamical assigning weights. Through focused training and segmental prediction, SIR selectively retains the outside information while focusing segment inside, which enhances the gradient descent process and ultimately leads to improved training performance. With crisscross optimization (CSO), SIR demonstrates its performance sufficiently with an average RMSE reduction of 21.17% and 40.76% in the multi-step prediction and day-ahead prediction cases, respectively.
The twin problems of closure and dissipation have been barriers to the analytical solution of the Navier–Stokes equation for fluid flow by top-down methods for two centuries. Here, the statistical ...multifractal analysis of airborne observations is used to argue that bottom-up approaches based on the dynamic behaviour of the basic constituent particles are necessary. Contrasts among differing systems will yield scale invariant turbulence, but not with universal analytical solutions to the Navier–Stokes equation. The small number of publications regarding a molecular origin for turbulence are briefly considered. Research approaches using suitable observations are recommended.
The implementation of renewable energy sources (RES) in isolated power systems, as is the case of islands, constitutes both a challenge and an opportunity. The intermittency of some RES, namely wind ...and solar, originates problems of grid stability and a mismatch between power demand and supply. The interconnection between isolated power systems can decrease the RES variability and, thereby, minimize the problems associated with their intermittency. In this work, the endogenous resources of the islands of Pico and Faial, in Azores, were characterized and their power systems modelled. A scenario considering the interconnection between the power systems of the two islands is proposed with the objective of increasing the share of RES power in the total power production. The scenario was modelled using EnergyPLAN and the results show that RES penetration can increase 50 percentage points by 2030 in relation to the current situation. The implementation of this scenario requires additional annual costs of about 1.29 M€. Additionally, based on the knowledge gained from this study, measures that may lead, in the long run, to the complete elimination of the overall use of fossil fuels in both islands are presented and discussed.
•Interconnection between isolated power systems can decrease RES variability and intermittency.•RES potential of Pico and Faial islands were characterized and their power systems modelled.•Interconnection of the power systems of these islands increase RES penetration by 50 pp.•This interconnection leads to additional annual costs of 1.29 M€.•Measures that lead, in the long run, to complete elimination of fossil fuels in the islands are discussed.
Driven by global concerns about the climate and the environment, the world is opting for renewable energy sources (RESs), such as wind and solar. However, RESs suffer from the discredit of ...intermittency, for which energy storage systems (ESSs) are gaining popularity worldwide. Surplus energy obtained from RESs can be stored in several ways, and later utilized during periods of intermittencies or shortages. The idea of storing excess energy is not new, and numerous researches have been conducted to adorn this idea with innovations and improvements. This review is a humble attempt to assemble all the available knowledge on ESSs to benefit novice researchers in this field. This paper covers all core concepts of ESSs, including its evolution, elaborate classification, their comparison, the current scenario, applications, business models, environmental impacts, policies, barriers and probable solutions, and future prospects. This elaborate discussion on energy storage systems will act as a reliable reference and a framework for future developments in this field. Any future progress regarding ESSs will find this paper a helpful document wherein all necessary information has been assembled.
•The impacts of wind power intermittency on power system are summarized from different aspects.•The measurements of wind power intermittency are reviewed based on numerous studies.•New definitions ...and metrics are proposed to measure intermittency.•The mitigation solutions for intermittency are concluded from different aspects.•The further studies about wind power intermittency are discussed.
Environmental issues and the prospect of an energy crisis inspire humans to exploit wind power. However, with the increase of wind power penetration level, operating power systems securely and reliably is a serious challenge due to the inherent nature of wind power intermittency. Wind power intermittency has been the major barrier for large scale wind power integration. This paper reviews past research on wind power intermittency, including its impacts on power system, how it is measured, and mitigation solutions. It has been found that as wind power integration increase, the system reverses and costs consequently increase, while the system reliability and CO2 reductions decrease. In order to mitigate wind power intermittency, studies on intermittency measurements and mitigation solutions are necessary. Existing measurements of wind power intermittency are summarized firstly. Considering the limitations of existing methods, new definitions and metrics are proposed based on our study. Then, various wind power intermittency mitigation solutions are comprehensively reviewed, including wind farms, generation-side, demand-side and energy storage. In the final part of this paper, the further work on wind power intermittency is discussed in detail. In summary, wind power intermittency can be effectively mitigated using various technological and managerial approaches based on an in-depth understanding of intermittency.
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