Numerical investigation of the scale effects of pump-jet propulsor with a pre-swirl stator

In this study, the performance of a pump-jet propulsor (PJP) with pre-swirl stator in open water is numerically investigated. Both full-scale and model-scale configurations are considered. The Reynolds-averaged Navier–Stokes equations and shear stress transport\ua0\u1d458−\u1d714 turbulence model are used in the numerical calculation. The computational domain is discretized using structured grids, and a rotating grid is affixed to the rotor to deal with the relative motion between the rotor and stationary components. The mesh quality is determined based on a grid uncertainty analysis. The numerical method is validated using model-scale experimental data. The simulation results reveal the influences of the scale size on the hydrodynamic performance and the distributions of the velocity, pressure and vorticity under three advance coefficients. With the increase in the advance coefficients, the scale influences on the efficiency become more obvious, and the efficiency of the full-scale PJP is always higher than that of the model-scale PJP. The full-scale configuration is found with a more significant instability in the gap vortex development, because it presents larger interaction between tip leakage vortex (TLV) and the inner wall of the duct. As the main velocity increases, the TLV shedding is delayed. Finally, the development process of gap vortices is analyzed for the difference operation conditions

A Modified Normalized Difference Impervious Surface Index (MNDISI) for Automatic Urban Mapping from Landsat Imagery

Impervious surface area (ISA) is a key factor for monitoring urban environment and land development. Automatic mapping of impervious surfaces has attracted growing attention in recent years. Spectral built-up indices are considered promising to map ISA distributions due to their easy, parameter-free implementations. This study explores the potentials of impervious surface indices for ISA mapping from Landsat imagery using a case study area in Boston, USA. A modified normalized difference impervious surface index (MNDISI) is proposed, and a Gaussian-based automatic threshold selection method is used to identify the optimal MNDISI threshold for delineating impervious surfaces from background features. To evaluate its effectiveness, comparison analysis is conducted between MNDISI and the original NDISI using Landsat images from three sensors (TM/ETM+/OLI-TIRS) acquired in four seasons. Our results suggest that built-up indices are sensitive to image seasonality, and summer is the best time phase for ISA mapping. With reduced uncertainties from automatic threshold selection, the MNDISI extracts impervious surfaces from all Landsat images in summer with an overall accuracy higher than 87% and an overall Kappa coefficient higher than 0.74. The proposed method is superior to previous index-based ISA mapping from the enhanced thermal integration and automatic threshold selection. The ISA maps from the TM, ETM+ and OLI-TIRS images are not significantly different. With enlarged data pool when all Landsat sensors are considered and automation of threshold selection proposed in this study, the MNDISI could be an effective built-up index for rapid and automatic ISA mapping at regional and global scales

Parameter design oriented analysis of the current control stability of the weak-grid-tied VSC

This paper studies the dynamic behaviors of weak-grid-tied VSCs with simplified transfer functions, which provides an accurate stability analysis and useful indications for tuning system parameters. A reduced-order multi-input multi-output (MIMO) transfer function that contains four single-input single-output (SISO) transfer functions for the weak-grid-tied VSC is first presented. It is found that the four SISO transfer functions share the same equivalent open-loop transfer function, i.e., the same stability conclusion. The Bode plots of the equivalent open-loop transfer function show that the inner current loop behaves as a band-pass filter whose maximum gain is approximately at the frequency of the PLL's bandwidth. By stability criterion, the harmonic amplification and instability occur when its maximum gain exceeds 0dB caused by high PLL's bandwidth, large grid impedance or high active power. It is also found that the target system is less stable when it works as an inverter than as a rectifier, due to the risk of the local positive feedback in the inverter mode. An effective criterion is further proposed to guide the selection of a proper PLL's bandwidth to ensure the stability of the VSC system. Simulation results validate the correctness of the analysis and the efficacy of the criterion

Exploring Format Consistency for Instruction Tuning

Instruction tuning has emerged as a promising approach to enhancing large language models in following human instructions. It is shown that increasing the diversity and number of instructions in the training data can consistently enhance generalization performance, which facilitates a recent endeavor to collect various instructions and integrate existing instruction tuning datasets into larger collections. However, different users have their unique ways of expressing instructions, and there often exist variations across different datasets in the instruction styles and formats, i.e., format inconsistency. In this work, we study how format inconsistency may impact the performance of instruction tuning. We propose a framework called "Unified Instruction Tuning" (UIT), which calls OpenAI APIs for automatic format transfer among different instruction tuning datasets. We show that UIT successfully improves the generalization performance on unseen instructions, which highlights the importance of format consistency for instruction tuning. To make the UIT framework more practical, we further propose a novel perplexity-based denoising method to reduce the noise of automatic format transfer. We also train a smaller offline model that achieves comparable format transfer capability than OpenAI APIs to reduce costs in practice

Smart grid power load type forecasting: research on optimization methods of deep learning models

Introduction: In the field of power systems, power load type prediction is a crucial task. Different types of loads, such as domestic, industrial, commercial, etc., have different energy consumption patterns. Therefore, accurate prediction of load types can help the power system better plan power supply strategies to improve energy utilization and stability. However, this task faces multiple challenges, including the complex topology of the power system, the diversity of time series data, and the correlation between data. With the rapid development of deep learning methods, researchers are beginning to leverage these powerful techniques to address this challenge. This study aims to explore how to optimize deep learning models to improve the accuracy of load type prediction and provide support for efficient energy management and optimization of smart grids.Methods: In this study, we propose a deep learning method that combines graph convolutional networks (GCN) and sequence-to-sequence (Seq2Seq) models and introduces an attention mechanism. The methodology involves multiple steps: first, we use the GCN encoder to process the topological structure information of the power system and encode node features into a graph data representation. Next, the Seq2Seq decoder takes the historical time series data as the input sequence and generates a prediction sequence of the load type. We then introduced an attention mechanism, which allows the model to dynamically adjust its attention to input data and better capture the relationship between time series data and graph data.Results: We conducted extensive experimental validation on four different datasets, including the National Grid Electricity Load Dataset, the Canadian Electricity Load Dataset, the United States Electricity Load Dataset, and the International Electricity Load Dataset. Experimental results show that our method achieves significant improvements in load type prediction tasks. It exhibits higher accuracy and robustness compared to traditional methods and single deep learning models. Our approach demonstrates advantages in improving load type prediction accuracy, providing strong support for the future development of the power system.Discussion: The results of our study highlight the potential of deep learning techniques, specifically the combination of GCN and Seq2Seq models with attention mechanisms, in addressing the challenges of load type prediction in power systems. By improving prediction accuracy and robustness, our approach can contribute to more efficient energy management and the optimization of smart grids

Spatial-temporal heterogeneity of landscape ecological risk in Yushenfu Mining Area from 1995 to 2021

As a strong human disturbance, coal mining has affected the ecosystem service function and economic value on mining area. However, there is a lack on the comparing for the long-term spatial scale evolution of landscape ecological risk after mining development based on different landforms in the same climate environment. Therefore, the spatial and temporal evolution characteristics of landscape ecological risk were explored on loess hilly and sandy land in the Yushenfu Mining Area based on the Landsat data from 1995—2021 with the construction of landscape ecological risk index and spatial statistical analysis methods. The results showed that: ① There was no significant changing between loess hilly and sandy land for the ecological risk pattern from 1995 to 2000. From 2000 to 2010, the low level ecological risk area changed to a higher level in the loess hilly region. In 2010, the proportion of medium-high, medium and medium-low ecological risk areas was 70% in the loess hilly area, while the high ecological risk area in the sand-covered area increased but not significant, and it was still dominated by medium-low ecological risk and account for 31%. Since 2010, the landscape ecological pattern tended to homogenization, and the landscape ecological risk gradually stabilized to a low-medium, medium and high-medium on loess hilly area, and the proportion of these three risk levels was 74% in 2021. The sandy landscape also formed a low, medium and low-medium ecological risk, and the proportion of the three risk levels was 77%, and decreased and stabilized gradually. ② From 1995 to 2021, the landscape ecological risks showed obvious spatially clustered distribution characteristics, and presented a clear hotspots and coldspots in Yushenfu Mining Area. The ecological risk hotspots were mainly located in the loess hilly area in the northeast and southeast of the study area where coal has been developed for a long time and the geological environment has been seriously damaged, and the ecological risk coldspots were mainly located in the sand-covered areas in the central part of study area which are still under resource exploration and survey. ③ Human disturbance was the most important factor affecting the landscape ecological risk in Yushenfu Mining Area, the determining force of q values in the loess area and the sand-covered area were 0.49−0.72 and 0.38−0.55, respectively, followed by vegetation coverage and temperature in the loess hilly area, and vegetation coverage and air temperature had a similar effects on landscape ecological risk in the sand-covered area, and the least factor was elevation for ecological risk on both landforms, the q values were less than 0.10. ④ The results of spatial and temporal landscape ecological risk indicated that the landscape pattern should be optimized according to the characteristics of different landforms in the process of ecological restoration in Yushenfu Mining Area, and the mining scheme should be optimized to reduce the surface ecological damage and the active restoration strategy should be actively carried out in the loess hilly area. Whilethe natural restorationcan be implemented to ensure the ecological stability on the sand-covered mining area

Mitigation of oscillations in three phase LCL-filtered grid converters based on proportional resonance and improved model predictive control

For three phase LCL -filtered gird converters, this paper designs a robust control strategy to reduce high frequency and subsynchronous or supersynchronous oscillations. Two components, namely the grid side inductor component and the LC filter component, constitute a three phase LCL -filtered grid converter. Model predictive control (MPC) with a disturbance observer is used to control the interconnection voltage of the LC filter. Proportional resonance (PR) control regulates the grid side current. It is possible to combine MPC with PR's advantages. The dynamic performance is enhanced by MPC's intrinsic ability to achieve active damping without extra control and reduce modulation latency. In addition to achieving zero steady state error, PR control greatly simplifies the control process when compared to the overall MPC of the entire grid converter. By analyzing the frequency response of the transfer function and output impedance, it is possible to determine that the proposed control has a sufficient phase margin and that, even when the system and control parameters change, the grid converters' output impedance is always resistive or inductive at the entire frequency, suppressing subsynchronous and high frequency oscillations. To further reduce the oscillations and harmonics, an improved MPC control framework and a feedback compensation mechanism are proposed. The effectiveness and reliability of the proposed control in current tracking, harmonic suppression, and response to grid impedance variations are verified by comparative analysis of simulation results