43 research outputs found
Diffusion Mechanisms of Dissolved Gases in Transformer Oil Influenced with Moisture Based on Molecular Dynamics Simulation
Dissolved Gas Analysis (DGA) of insulating oil is widely used for diagnosing transformer incipient faults. Moisture is a major contaminant and degradation byproduct of transformer insulating oil. In this paper, molecular dynamics simulation was used to study the influence of moisture on the diffusion movement of dissolved gases in the insulating oil. Cycloalkanes (C20H42), alkanes (C20H38), and aromatic hydrocarbons (C20H26) are selected as the basic structural units in the constructed transformer oil simulation system. 0%, 1%, 3%, and 5% moisture molecules are added to insulating oil, respectively, and the insulating oil generates seven kinds of gas molecules through cracking. With an anhydrous model used as a benchmark, we researched the diffusion trajectory, the diffusion coefficient (D), free volume (VF), and the moisture-gas interaction energy of each gas molecule as a function of moisture content. Through this study, we found that the increase of moisture content enlarges the VFvalue of dissolved gas in insulating oil, which makes the gas in oil easier to diffuse. Besides, the moisture can also alter the mean square displacement (MSD) of dissolved gases; the mutual energy of molecules is mainly affected by the electrostatic interaction energy. This study can contribute to a better understanding of the influence of different moisture contents on the diffusion movement of dissolved gas in transformer oil from the micro level
Collective behaviors of isotropic phoretic microswimmers: from crystalline solids to active turbulence
Chemically phoretic microswimmers including Janus colloids and isotropic
active droplets have recently shown great potential for self-organizing into
active materials. However, the fascinating self-organization of these swimmers
remains enigmatic owing to an incomplete understanding of their collective
dynamics. We investigate a paradigmatic suspension of phoretic swimmers
represented by isotropic phoretic disks. By varying the activity and area
fraction of disks, we observe their diverse collective phenomena: formation of
crystalline solids resembling Wigner crystals, melting, gas-like dynamic
chaining, active transition and turbulence. Notably, altering the activity
alone leads to solid-fluid phase transition and subsequently the fluid's
laminar-turbulent transition. Our observations are based on large-scale
simulations explicitly resolving the many-body, long-range hydrochemical
interactions among the phoretic agents. We reproduce independent experimental
phenomena that have not been collectively captured by a single model,
emphasizing the importance of incorporating full physicochemical hydrodynamics.
We develop a predictive scaling delineating the solid-liquid transition and
further dissect it as a defect-mediated two-step melting scenario via a hexatic
phase. We demonstrate that the fluid phase exhibits oscillatory instability
with waves, transition due to wave-breaking clusters, and active turbulence
manifesting vortices. These progressive scenarios evidence a closer
phenomenological resemblance between active and classical fluids in their
laminar-turbulent transition than previously demonstrated. Our findings
elucidate the collective dynamics of phoretic agents, potentially facilitating
the design of active material systems by harnessing non-equilibrium phoretic
collectives
Physical Layer Security with Threshold-Based Multiuser Scheduling in Multi-antenna Wireless Networks
Physical Layer Security With Threshold-Based Multiuser Scheduling in Multi-Antenna Wireless Networks
Precision Higgs physics at the CEPC
The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS
and CMS Collaborations marked the beginning of a new era in high energy
physics. The Higgs boson will be the subject of extensive studies of the
ongoing LHC program. At the same time, lepton collider based Higgs factories
have been proposed as a possible next step beyond the LHC, with its main goal
to precisely measure the properties of the Higgs boson and probe potential new
physics associated with the Higgs boson. The Circular Electron Positron
Collider~(CEPC) is one of such proposed Higgs factories. The CEPC is an
circular collider proposed by and to be hosted in China. Located in a
tunnel of approximately 100~km in circumference, it will operate at a
center-of-mass energy of 240~GeV as the Higgs factory. In this paper, we
present the first estimates on the precision of the Higgs boson property
measurements achievable at the CEPC and discuss implications of these
measurements.Comment: 46 pages, 37 figure
Secure Multiuser Communications in Wireless Sensor Networks with TAS and Cooperative Jamming
In this paper, we investigate the secure transmission in wireless sensor networks (WSNs) consisting of one multiple-antenna base station (BS), multiple single-antenna legitimate users, one single-antenna eavesdropper and one multiple-antenna cooperative jammer. In an effort to reduce the scheduling complexity and extend the battery lifetime of the sensor nodes, the switch-and-stay combining (SSC) scheduling scheme is exploited over the sensor nodes. Meanwhile, transmit antenna selection (TAS) is employed at the BS and cooperative jamming (CJ) is adopted at the jammer node, aiming at achieving a satisfactory secrecy performance. Moreover, depending on whether the jammer node has the global channel state information (CSI) of both the legitimate channel and the eavesdropper’s channel, it explores a zero-forcing beamforming (ZFB) scheme or a null-space artificial noise (NAN) scheme to confound the eavesdropper while avoiding the interference to the legitimate user. Building on this, we propose two novel hybrid secure transmission schemes, termed TAS-SSC-ZFB and TAS-SSC-NAN, for WSNs. We then derive the exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput of both schemes to characterize the secrecy performance. Using these closed-form expressions, we further determine the optimal switching threshold and obtain the optimal power allocation factor between the BS and jammer node for both schemes to minimize the secrecy outage probability, while the optimal secrecy rate is decided to maximize the effective secrecy throughput for both schemes. Numerical results are provided to verify the theoretical analysis and illustrate the impact of key system parameters on the secrecy performance