新型大涡数值模拟亚格子模型及应用

基于湍流大小尺度间动量输运的结构函数方程,提出了一种新的湍流大涡模型(LES)亚格子涡粘模式.新亚格子涡粘系数正比于纵向速度增量的扭率,它表征大小尺度湍流间的能量输运和耗散之比.新模式通过各向同性湍流直接数值模拟数据库的检验,并用于槽道湍流的大涡模拟计算,将所得结果与DNS结果进行了比较.

Pr数对湍流被动标量输运的影响

采用直接数值模拟的方法,研究分子Pг数对湍流被动标量输运的影响,并提供充分的证据证明,湍流Pг数明显依赖于分子Pг数.在算例中,湍流雷诺平均PгT数与分子Pг数的倒数呈线性关系;湍流亚格子Pгt数与分子Pг数的关系较为复杂,在分子Pг数为1附近时,湍流亚格子Pгt数出现极小值.

Understanding occupancy pattern and improving building energy efficiency through Wi-Fi based indoor positioning

Detailed visualisation and data analysis of occupancy patterns including spatial distribution and temporal variations are of great importance to delivering energy efficient and productive buildings. An experimental study comprising 24-h monitoring over 30 full days was conducted in a university library building.Occupancy profiles have been monitored and analysis has been carried out. Central to this monitoring study is the Wi-Fi based indoor positioning system based on the measured Wi-Fi devices' number and locations and data mining methods. Distinct from traditional occupancy and energy studies,more detailed informationrelated to the indoor positions and number of occupants has offered a better understanding of building user behaviour. The implication of the occupancy patterns for energy( e. g. lighting and other building services) efficiency is assessed,assisted with data from lighting sensors where needed. It is found occupancy patterns change dramatically with time. Also,the energy waste patterns have been identified through the method of data association rule mining. If the identified energy waste is removed,the total energy consumption can be reduced by 26. 1%. The indoor positioning information also has implications for optimizing space use,opening hours as well as staff deployment. The work could be extended to more rooms with diverse functions,other seasons and other types of non-domestic buildings for a more comprehensive understanding of building user behaviour and energy efficiency.

TRANSPORTATION OF PASSIVE SCALAR IN INHOMOGENEOUS TURBULENCE

The paper studies the transportation of passive scalar in the inhomogeneous turbulence by means of large eddy simulation. The prediction accuracy is verified by the well-known Comte-Bellot spectrum of the homogeneous turbulence. The mean transportation properties are predicted with satisfaction and the underestimation of the thermal flux by the phenomenological models has been disclosed. The high intermittence of the temperature fluctuations has been found in the inhomogeneous turbulence and it is the reason for the underestimation of turbulent thermal flux by the phenomenological model.

Flux variations of cosmic ray air showers detected by LHAASO-KM2A during a thunderstorm on June 10,2021

The Large High Altitude Air Shower Observatory(LHAASO)has three sub-arrays,KM2A,WCDA,and WFCTA.The flux variations of cosmic ray air showers were studied by analyzing the KM2A data during a thunderstorm on June 10,2021.The number of shower events that meet the trigger conditions increases significantly in atmospheric electric fields,with a maximum fractional increase of 20%.The variations in trigger rates(increases or decreases)were found to be strongly dependent on the primary zenith angle.The flux of secondary particles increased significantly,following a trend similar to that of shower events.To better understand the observed behavior,Monte Carlo simulations were performed with CORSIKA and G4KM2A(a code based on GEANT4).We found that the experimental data(in saturated negative fields)were in good agreement with the simulations,assuming the presence of a uniform electric field of-700 V/cm with a thickness of 1500 m in the atmosphere above the observation level.Due to the acceleration/deceleration by the atmospheric electric field,the number of secondary particles with energy above the detector threshold was modified,resulting in the changes in shower detection rate.

Observation of the Crab Nebula with LHAASO-KM2A−a performance study

A sub-array of the Large High Altitude Air Shower Observatory(LHAASO),KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV.Even though the detector construction is still underway,half of the KM2A array has been operating stably since the end of 2019.In this paper,we present the KM2A data analysis pipeline and the first observation of the Crab Nebula,a standard candle in very high energy γ-ray astronomy.We detect γ-ray signals from the Crab Nebula in both energy ranges of 10-100 TeV and>100 TeV with high significance,by analyzing the KM2A data of 136 live days between December 2019 and May 2020.With the observations,we test the detector performance,including angular resolution,pointing accuracy and cosmic-ray background rejection power.The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE=(1.13±0.05stat±0.08sys)×10^(-14).(E/20 TeV)-309±0.06stat±0.02syscm^(-2) s^(-1) TeV^(-1).It is consistent with previous measurements by other experiments.This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena,such as cosmic PeVatrons,might be discovered.

Effects of plane strain on inhomogeneous turbulence

The influence of the mean plane strain on the turbulence transportation is investigated by large eddy simulation (LES) in the shearless turbulence mixing layer. It is found that the mean strains enhance the turbulent fluctuations in the mixing region. Compression in the inhomogeneous direction can greatly increase the transport of turbulent kinetic energy by triple correlation terms, while stretching in the inhomogeneous direction decreases the turbulence transportation. The gradient diffusion models for turbulent transportation are evaluated and it is found that the inter-mittency consideration can improve the prediction ability of the gradient-type models for the triple correlation terms.

Numerical studies towards practical large-eddy simulation

大旋涡的模拟开发和确认为狂暴的内部流动的改进模拟被介绍。数字方法根据二竞争标准被建议：数字质量(精确并且光谱特征) ，并且适应性到复杂配置。首先，方法在学术测试用例上被测试，以便与基本研究缩短。一致结果用能适应的有限体积方法被获得，与更高的顺序移流流动，含蓄的格子过滤和鈥渓o w 花费鈥 ? 砍改进 Smagorinsky 模型。这分析特别地集中于吝啬的流动，变化，二点的关联和系列。而且，指数的平均是为在有确定的摇摆的复杂几何学的 LES 实现的一个有希望的工具，这被显示出。最后，方法的适应性被申请在 turbomachine 表明到片尖端清理流动的一个配置代表。

Geometrical reconstruction of fluorescence events observed by the LHAASO experiment

The LHAASO-WFCTA experiment,which aims to observe cosmic rays in the sub-EeV range using the fluorescence technique,uses a new generation of high-performance telescopes.To ensure that the experiment has ex-cellent detection capability associated with the measurement of the energy spectrum,the primary composition of cosmic rays,and so on,an accurate geometrical reconstruction of air-shower events is fundamental.This paper de-scribes the development and testing of geometrical reconstruction for stereo viewed events using the WFCTA(Wide Field of view Cherenkov/Fluorescence Telescope Array)detectors.Two approaches,which take full advantage ofthe WFCTA detectors.are investigated.One is the stereo-angular method,which uses the pointing of triggered SiPMs in the shower trajectory,and the other is the stereo-timing method,which uses the triggering time of the fired SiPMs.The results show that both methods have good geometrical resolution:the resolution of the stereo-timing method is slightly better than the stereo-angular method because the resolution of the latter is slightly limited by the shower track length.

Performance of LHAASO-WCDA and observation of the Crab Nebula as a standard candle

The first Water Cherenkov detector of the LHAASO experiment(WCDA-1)has been operating since April 2019.The data for the first year have been analyzed to test its performance by observing the Crab Nebula as a standard candle.The WCDA-1 achieves a sensitivity of 65 mCU per year,with a statistical threshold of 5 cr.To accomplish this,a 97.7%cosmic-ray background rejection rate around 1 TeV and 99.8%around 6 TeV with an ap proximate photon acceptance of 50%is achieved after applying an algorithm to separate gamma-induced showers.The angular resolution is measured using the Crab Nebula as a point source to be approximately 0.45°at 1 TeV and better than 0.2°above 6 TeV,with a pointing accuracy better than 0.05°.These values all match the design specifications.The energy resolution is found to be 33%for gamma rays around 6 TeV.The spectral energy distribution of the Crab Nebula in the range from 500 GeV to 15.8 TeV is measured and found to be in agreement with the results from other TeV gamma ray observatories.

A dynamic range extension system for LHAASOWCDA-1

Purpose The main scientific goal of LHAASO-WCDA is to survey gamma-ray sources with energy from 100 GeV to 30 TeV.To observe high-energy shower events,especially to measure the energy spectrum of cosmic rays from 100 TeV to 10 PeV,a dynamic range extension system(WCDA++)is designed to use a 1.5-inch PMT with a dynamic range of four orders of magnitude for each cell in WCDA-1.Method The dynamic range is extended by using these PMTs to measure the effective charge density in the core region of air shower events,which is an important parameter for identifying the composition of primary particles.Result and Conclusion The system has been running for more than one year.In this paper,the details of the design and performance of WCDA++are presented.

Prospects for a multi-TeV gamma-ray sky survey with the LHAASO water Cherenkov detector array

The Water Cherenkov Detector Array(WCDA) is a major component of the Large High Altitude Air Shower Array Observatory(LHAASO), a new generation cosmic-ray experiment with unprecedented sensitivity, currently under construction. WCDA is aimed at the study of TeV γ-rays. In order to evaluate the prospects of searching for TeV γ-ray sources with WCDA, we present a projection of the one-year sensitivity of WCDA to TeV γ-ray sources from TeVCat using an all-sky approach. Out of 128 TeVCat sources observable by WCDA up to a zenith angle of 45°, we estimate that 42 would be detectable in one year of observations at a median energy of 1 TeV. Most of them are Galactic sources, and the extragalactic sources are Active Galactic Nuclei(AGN).

Line-of-shower trigger method to lower energy threshold for GRB detection using LHAASO-WCDA

Purpose Observation of high energy and very high emission from Gamma Ray Bursts(GRBs)is crucial to study the gigantic explosion and the underline processes.With a large field-of-view and almost full duty cycle,the Water Cherenkov Detector Array(WCDA),a sub-array of the Large High Altitude Air Shower Observatory(LHAASO),is appropriate to monitor the very high energy emission from unpredictable transients such as GRBs.Method Nevertheless,the main issue for an extensive air shower array is the high energy threshold which limits the horizon of the detector.To address this issue a new trigger method is developed in this article to lower the energy threshold of WCDA for GRB observation.Result The proposed method significantly improves the detection efficiency of WCDA for gamma-rays around the GRB direction at 10-300 GeV.The sensitivity of the WCDA for GRB detection with the new trigger method is estimated.The achieved sensitivity of the quarter WCDA array above 10 GeV is comparable with that of Fermi-LAT.The data analysis process and corresponding fluence upper limit for GRB 190719C is presented as an example.

Reconstruction of Cherenkov image bymultiple telescopes of LHAASO-WFCTA

Introduction One of main scientific goals of the Large High Altitude Air Shower Observatory(LHAASO)is to accurately measure the energy spectra of different cosmic ray compositions around the‘knee’region.The Wide Field-of-View(FoV)Cherenkov Telescope Array(WFCTA),which is one of the main detectors of LHAASO and has 18 telescopes,is built to achieve this goal.Multiple telescopes are put together and point to connected directions for a larger FoV.Method Telescopes are deployed spatially as close as possible,but due to their own size,the distance between two adjacent telescopes is about 10 m.Therefore,the Cherenkov lateral distribution and the parallax between the two telescopes should be considered in the event building process for images crossing over the boundaries of FoVs of the telescopes.An event building method for Cherenkov images measured by multiple telescopes of WFCTA is developed.The performance of the shower measurements using the combined images is evaluated by comparing with showers that are fully contained by a virtual telescope in simulation.Results and conclusion It is proved that the developed event building process can help to increase the FoV of WFCTA by 30%while maintaining the same reconstruction quality,compared to the separate telescope reconstruction method.