Journal of University of Chinese Academy of Sciences

Mahler measure of a two-variable polynomial

ZHANG Anhao, TANG Guoping

2024, 41 (2): 145-150. DOI: 10.7523/j.ucas.2022.060

In this paper, we express the Mahler measure of a two-variable polynomial P(x,y)=(x²+1)y²+2(x²+x)y+x(x²+1) as a linear sum of some Bloch-Wigner Dilogarithm functions. and prove that the Mahler measure of P(x,y) is rationally proportional to L'(χ_-3,-1):m(P(x,y))=5/2L'(χ_-3,-1).

Robust individualized subgroup analysis

ZHANG Xiaoling, REN Mingyang, ZHANG Sanguo

2024, 41 (2): 151-164. DOI: 10.7523/j.ucas.2022.037

Abstract ( 512 ) PDF (0KB) ( 2 )

Subgroup analysis of heterogeneous groups is a crucial step in the development of individualized treatment and personalized marketing strategies. Regression-based approaches are one of the main schools of subgroup analysis, a paradigm that divides predictor variables into two parts with heterogeneous and homogeneous effects and divides the sample into subgroups based on the heterogeneous effects. However, most of the existing regression-based subgroup analysis methods have two major limitations: First, they still consider the sample homogeneous within subgroups and do not fully consider individual effects; Second, the common contamination phenomenon of homogeneous effect variables is not taken into account, which will lead to large bias in the model results. To address these challenges, we propose a robust individualized subgroup analysis. We use a multidirectional separation penalty function to achieve individualized effects analysis for the heterogeneous part of the model and use γ-divergence to obtain robust estimates for the contaminated homogeneous part. We also propose an efficient alternating iterative two-step algorithm, combining coordinate descent and alternating direction method of multipliers (ADMM) techniques to implement this process. Our proposed method is further illustrated by simulation studies and analysis of a skin cutaneous melanoma dataset.

IDDES simulation of flow and mixing characteristics in a confined impinging jet reactor

JIN Linna, CAO Yuhui

2024, 41 (2): 165-175. DOI: 10.7523/j.ucas.2022.050

Abstract ( 296 ) PDF (0KB) ( 0 )

There exist various vortical structures in the turbulent flow field of confined impinging jet reactors, which have important effects on the flow characteristic and mixing performance. A numerical study was conducted on the flow field of a confined impinging jet reactor using the IDDES method based on SST k-ω turbulent model. Various large-scale vortical structures were identified, and their generation and evolution mechanisms were discussed. The effects of vortices on the phase-averaged temperature and heat flux fields were discussed to reveal the mixing mechanism. Results indicated that the alternating deflection of two jets leaded to unequal momentum between them, and the self-sustained oscillation of two jet shear layers caused the periodic motion of vortices, hence the stagnation point deviated from the mid-point periodically. Large-scale spanwise vortices were generated in the downstream region due to the development of K-H instability. The streamwise vortices in the impinging region only enhanced the thermal mixing near the channel axis, while the spanwise vortices could stimulate a large-scale heat transport and turbulent mixing in the downstream region.

Numerical simulation for thermal convection of cold water in the Taylor-Couette system with a rotating inner cylinder

TONG Jiawei, CAO Yuhui

2024, 41 (2): 176-187. DOI: 10.7523/j.ucas.2022.051

Abstract ( 267 ) PDF (0KB) ( 0 )

The thermal convection of cold water has attracted considerable attention due to its relevance to engineering applications, such as the phase change cool storage device. However, the natural convection of cold water in the vertical annulus inherently has a minimum heat transfer rate due to the density inversion phenomenon of water near 4 ℃. It is feasible to reduce the negative effect of density inversion phenomenon on convective heat transfer by imposing a slow axial rotation on the inner cylinder. In order to better understand the heat transfer enhancement induced by the low-speed rotation of inner cylinder, three-dimensional numerical simulation was carried out to investigate the thermal convection of cold water near its density maximum in a finite vertical annulus with a heated rotating inner cylinder over a wide range of Rayleigh and Reynolds numbers (10⁴≤Ra≤10⁶ and Re≤150) for various density inversion parameters. The radius ratio and aspect ratio of the annulus were 0.5 and 8, respectively. Results indicated that the combination of centrifugal and buoyancy forces led to multiple three-dimensional flow patterns in the cold water, which were distinct from the conventional Taylor-Couette flow under the Oberbeck-Boussinesq approximation. Furthermore, the transition of flow regimes in the rotating system was generally beneficial to heat transfer enhancement. However, at relatively high rotation speeds, the increase of Ra could result in the non-monotonic change of the overall heat transfer rate.

HI distribution in dwarf starburst galaxy NGC 5253

WO Jiayi, TSAI Chao-Wei, ZUO Pei, CHEN Tianlu

2024, 41 (2): 188-194. DOI: 10.7523/j.ucas.2022.040

Abstract ( 309 ) PDF (0KB) ( 0 )

Neutral hydrogen (HI) gas is the fundamental material for star formation that drives the evolution of a galaxy. For starburst dwarf galaxy NGC 5253, previous study has observed HI gas distributed asymmetrically with ~7 kpc extension toward north-east direction using the radio interferometry. This extension is likely indicating the dynamical connection between NGC 5253 and its nearby spiral galaxy M 83. To investigate the possible HI bridge between these two galactic systems, we acquire the HI mapping data around NGC 5253 in radio L-band from the Robert C. Byrd Green Bank Telescope. We have detected the 7 possible HI gas clouds between NGC 5253 and M 83. These HI gas cloud candidates distribute mainly in the north-east direction extending over 60 kpc from NGC 5253. These clouds could play an important role in triggering intensive star formation activity in the dwarf galaxy NGC 5253.

Spatiotemporal variation in the maximum leaf area index of temperate grassland in northern China and its response to climate change

FENG Yiming, ZHANG Na, YUE Rongwu, YAN Zhihui, LI Zhenyu, LI Xiaofan, Erridunqimuge

2024, 41 (2): 195-211. DOI: 10.7523/j.ucas.2022.072

Abstract ( 509 ) PDF (0KB) ( 0 )

There might be great differences in spatiotemporal variation in leaf area index (LAI) of different grassland types; the responses of LAI with different annual variations to climate change are probably distinct. To explore these differences, from the long-term LAI data and meteorological data and the grassland type data, we obtained the temporal and spatial varying characteristics of the maximum LAI of different types of temperate grassland in northern China from 1981 to 2017, and explored the responses of LAI with different annual variations to the changes of air temperature and precipitation. The results showed that the average annual maximum LAI of temperate grassland was (0.76±1.07) m²/m², the higher in the east and the lower in the west. The meadow steppe had the highest LAI ((2.73±1.20) m²/m²) and the steppe desert had the lowest LAI ((0.13±0.17) m²/m²). From 1981 to 2017, the average annual maximum LAI showed a significantly increasing trend. The maximum LAI increased significantly for 32.52% of the area and decreased significantly for 6.31% of the area. The areas with a significantly increasing LAI were greater than those with a significantly decreasing LAI for all the grassland types. The annual average maximum LAI was positively correlated with the annual total precipitation from January to August and the annual mean air temperature for July and August. Both the significant decrease and increase of the maximum LAI was mainly affected by the significant increase of annual mean air temperature for July and August. There existed a critical threshold of the rising rate of air temperature for July and August that influenced the variation in LAI; for the grassland types that accounted for the larger areas, this threshold was 0.042-0.043 ℃/a. LAI increased with the rising air temperature as the rising rate of air temperature was lower than this threshold; conversely, LAI decreased. These results are expected to provide important scientific basis for grassland utilization, protection and restoration in the context of climate change.

Dynamic mechanism and river evolution under coupling effects of surficial and tectonic processes: a case study of Qinghai Lake and Daotang River

MIAO Yu, ZHANG Huai, SHI Yaolin

2024, 41 (2): 212-221. DOI: 10.7523/j.ucas.2022.056

Abstract ( 424 ) PDF (0KB) ( 0 )

Since the late Cenozoic, the geodynamic mechanism of the transformation of Qinghai Lake from the external lake to the endorheic lake is still an open problem. The Daotang River is an important channel for transforming from external-flow to the endorheic-flow type of Qinghai Lake. Its evolution records the prominent landform transition event. Based on the newly developed numerical calculation program for geomorphic evolution with finite volume method, in this work, we conduct a series of the landscape evolution models of the Daotang River under the combined influence of mountain uplift and river downcutting, and have a quantitative analysis of the effects of uplift rate and river undercutting coefficient on river backflow patterns. We attempt to explore the mechanism of landform transition events that could provide evidence for the formation process of Daotang River basin and the origin of Qinghai Lake. Our modeling results indicate that the reorganization of the river system and backward flow of the river is jointly controlled by the rapid mountain uplift from the Riyue Mountain active fault and the river undercutting coefficient. The mountain uplift rate is the controlling factor that affects the reorganization of the river system in the Daotang River basin. When rising mountains block the river, the river incision coefficient is the factor that controls the rate of reorganization that occurs in the basin. The result recognizes they have enlightening significance for further understanding the dynamic mechanism of river evolution under the regional tectonic deformation and the coupled surface processes.

Numerical simulation of temperature recovery process after enhanced geothermal system shutdown

LIU Hanqing, HU Caibo, ZHAO Guiping

2024, 41 (2): 222-230. DOI: 10.7523/j.ucas.2022.067

Abstract ( 267 ) PDF (0KB) ( 0 )

During the operation of the enhanced geothermal system (EGS), the temperature of the artificial reservoir in the hot dry rock (HDR) will rapidly decline, and the external heat supply will be unable to keep up with the system’s rate of heat extraction. At the same time, it will also have a significant impact on the artificial reservoir’s fluid pressure. The predecessors conducted extensive research on the temperature field and fluid pressure field changes during HDR exploitation process, but little attention has been paid to the temperature recovery process and fluid pressure field changes of artificial reservoirs after production has ceased, this process is critical for the site selection of new geothermal wells and the reuse of abandoned geothermal wells. The numerical simulation software COMSOL Multiphysics is used in this paper to establish a two-dimensional thermal-hydraulic coupling numerical model of the two-well EGS during operation and after shutdown, considering the contribution of radioactive heat generation and the influence of different thermal conductivities. According to the findings, heat convection is the primary mode of heat exchange during EGS operation, and the temperature of the artificial reservoir declines rapidly. The EGS has reached the end of its operational life after 19 years, while the artificial reservoir has lost its exploitation value after 68 years. The natural recovery process of artificial reservoir temperature lasts 10 000 years after geothermal exploitation is terminated. The primary technique of heat transfer and temperature recovery is conduction. The higher the thermal conductivity, the sooner the artificial reservoir temperature recovers.

Uncertainty-based credit assignment for cooperative multi-agent reinforcement learning

YANG Guangkai, CHEN Hao, ZHANG Mingyi, YIN Qiyue, HUANG Kaiqi

2024, 41 (2): 231-240. DOI: 10.7523/j.ucas.2022.047

Abstract ( 415 ) PDF (0KB) ( 0 )

In recent years, multi-agent cooperation under partially observable conditions has attracted extensive attention. As a general paradigm to deal with such tasks, centralized training with decentralized execution faces the core problem of credit assignment. Value decomposition is a representative method within this paradigm. Through the mixing network, the joint state action-value function is decomposed into multiple local observation action-value functions to realize credit assignment, which performs well in many problems. However, the single point estimation of the mixing network parameters maintained by these methods lacks the representation of uncertainty and is thus difficult to effectively deal with the random factors in the environment, resulting in convergence to the suboptimal strategy. To alleviate this problem, this paper performs Bayesian analysis on the mixing network and proposes a method based on uncertainty for multi-agent credit assignment, which guides the credit assignment by explicitly quantifying the uncertainty of parameters. Considering the complex interactions among agents, this paper utilizes the Bayesian hypernetwork to implicitly model the arbitrary complex posterior distribution of the mixing network parameters, to avoid falling into the local optima by specifying the distribution type a priori. This paper compares and analyzes the performance of representative algorithms on multiple maps in StarCraft multi-agent challenge (SMAC) and verifies the effectiveness of the proposed algorithm.

Device-free multi-target localization method using phase-shift-based compressive sensing

SHENG Jinfeng, LI Ning, GUO Yan, CHEN Cheng, LI Huajing

2024, 41 (2): 241-248. DOI: 10.7523/j.ucas.2022.066

Abstract ( 387 ) PDF (0KB) ( 0 )

Device-free localization(DFL),as a new localization technology,is a hotspot in passive sensing such as security monitoring,intrusion detection and contact tracking.It locates the target by analyzing the shadow effect of passive target on wireless link.Phase is an important characteristic of wireless signal,which is more fine-grained than signal strength.To improve the localization performance,we use phase information of wireless links and propose a device-free multi-target localization method using phase shift based compressive sensing.In this method, the phase shift of received signal is taken as the observation data, and the sparse vector of target position is recovered by variational Bayesian inference.Simulation results show that in the monitoring area of 6.5 m×6.5 m,the average localization error of RSS method is 0.579 0 m, while the average localization error of this method is 0.254 7 m,the localization accuracy is improved by more than one times,and it can achieve robust DFL.

VDE-SAT downlink channel modeling method

DING Ganghui, LI Zongwang, XIE Zhuochen, LIANG Xuwen

2024, 41 (2): 249-256. DOI: 10.7523/j.ucas.2022.062

Abstract ( 323 ) PDF (0KB) ( 0 )

The scattering characteristics of sea surface lead to the complex and changeable channel model of VDES maritime communication. In the VDE-SAT downlink, low elevation communication leads to more serious multipath effect caused by wave reflection. To solve this problem, a VDE-SAT downlink channel modeling method based on spherical coordinate system was proposed to analyze the influence of wave reflection on signal receiving characteristics according to different sea conditions. Firstly, the statistical results of sea surface scattering characteristics are parameterized and combined with the influence of earth curvature, a geometric model of satellite-ship communication based on spherical coordinate system is established. Based on this, the effective diffuse reflection area of sea surface is obtained. Secondly, the signal multipath propagation model of Satellite-Ship communication link is established. Based on G1139 protocol, the power distribution of received signals is analyzed according to the channel parameters of each path, as well as the receiving characteristics of signals at different sea levels. The simulation results show that the multipath channel is mainly specular reflection in good sea state and diffuse reflection in bad sea state. Compared with convolutional codes, Turbo codes have better anti-multipath performance.

Power performance measurement based on nacelle mounted Lidar

LIANG Zhi, SHI Yu, ZHANG Zhe, HU Fei

2024, 41 (2): 257-267. DOI: 10.7523/j.ucas.2022.036

Abstract ( 439 ) PDF (0KB) ( 0 )

The power performance measurement (PPM) of wind turbine is an objective method to evaluate the performance of wind turbine, which is the basis for the optimization of the wind turbine, the calculation of annual energy production and other related topics. Traditional testing method usually installs the ground-based equipment, such as the meteorological mast or ground-based lidar, which are more constrained by the on-site conditions. The nacelle mounted Lidar (NML), which can be installed on the top roof on wind turbine nacelle, could overcome the constraints of the on-site conditions. In recent years, NML starts to be used in PPMs, especially for offshore wind turbines. In this paper, the PPM had been conducted by using NML, and the differences were analyzed. The results showed that the wind speed accuracy of NML was consistent with the meteorological mast, the correlation coefficient of the two devices was 0.994, and the slope and offset by linear regression were 0.979 and 0.084, respectively; the scattering point of PPM by NML was more concentrated, and NML was always measuring the wind speed exactly in front of the wind turbine with the yawing of the wind turbine nacelle, which was better representative of the wind speed. The AEP assessment results showed that the NML was 1.73% overestimated relative to the met mast, and the overall evaluation error range was smaller due to the lower dispersion of NML. The NML had a better representation of wind speed than met mast, with less uncertainty, and had value for the application of PPM of wind turbine.

Single photon point cloud denoising method based on density and local statistics

PAN Chao, LI Lianghai, CAO Haiyi, ZHAO Yiming, YUAN Yifei, HAN Xiaoshuang

2024, 41 (2): 268-274. DOI: 10.7523/j.ucas.2022.058

Abstract ( 375 ) PDF (0KB) ( 0 )

In this paper, aiming at a 64-channel airborne single-photon LiDAR system developed by Beijing Research Institute of Telemetry, a two-dimensional profile point cloud denoising method based on density and local statistics was proposed. First, the elevation range of the point cloud was determined; then a modified DBSCAN algorithm was utilized for coarse denoising; finally, the statistical outlier removal algorithm was adapted for fine denoising and the valid signal point cloud was obtained. The experimental result shows that the method proposed in this paper can adapt to different surface types, the root mean square error of elevation is about 0.27 m, and the accuracy is 90.87%, which is better than the conventional point cloud denoising methods, and can meet the technical requirements of domestic airborne single-photon LiDAR to obtain high-precision three-dimensional surface contours.

Solving quadratic assignment problem based on actor-critic framework

LI Xueyuan, HAN Congying

2024, 41 (2): 275-284. DOI: 10.7523/j.ucas.2022.031

Abstract ( 720 ) PDF (0KB) ( 0 )

The quadratic assignment problem (QAP) is one of the NP-hard combinatorial optimization problems and is known for its diverse applications in real life. The current relatively mature heuristic algorithms are usually problem-oriented to design customized algorithms and lack the ability to transfer and generalize. In order to provide a unified QAP solution strategy, this paper abstracts the flow matrix and distance matrix of QAP problem into two undirected complete graphs and constructs corresponding correlation graphs, thus transforming the assignment task of facilities and locations into node selection task on the association graph. Based on actor-critic framework, this paper proposes a new algorithm ACQAP(actor-critic for QAP). Firstly, the model uses a multi-headed attention mechanism to construct a policy network to process the node representation vectors from the graph convolutional neural network; Then, the actor-critic algorithm is used to predict the probability of each node being output as the optimal node. Finally, the model outputs an action decision sequence that satisfies the objective reward function within a feasible time. The algorithm is free from manual design and is more flexible and reliable as it is applicable to different sizes of inputs. The experimental results show that on QAPLIB instances, the algorithm has stronger transfer and generalization ability under the premise that the accuracy is comparable to the traditional heuristic algorithm, while the assignment cost for solving is less compared to the latest learning-based algorithms such as NGM, and the deviation is less than 20% in most instances.

Crystal facet effects during the dissolution of ZnO single crystals in the presence of natural organic matter

TAN Xiaonan, MA Jiahai

2024, 41 (2): 285-288. DOI: 10.7523/j.ucas.2022.030

Abstract ( 384 ) PDF (0KB) ( 0 )

The polar ZnO single crystals (0001) face and non-polar ZnO single crystals (101-0) face were used to investigate the differences in dissolution rates and corrosion morphology of different crystalline facets in the presence of natural organic matter (NOM). The experimental results show that the dissolution rate of specific crystalline facets of ZnO single crystals is affected by whether the other crystalline facets are in contact with the solution, and lateral contact with the solution will accelerate the dissolution rate of the top. The dissolution rate of (0001) face is higher than that of (101-0) face in the presence of NOM. Different NOMs will have different effects on the dissolution and corrosion morphology of ZnO crystalline facets, which is related to the properties of NOM itself.

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