Numerical analysis of swirling turbulent droplet-laden flow and heat transfer in a sudden pipe expansion

The effect of swirling intensity on the structure and heat transfer of a turbulent gas–droplet flow after a sudden pipe expansion has been numerically simulated. Air is used as the carrier phase, and water, ethanol, and acetone are used as the dispersed phase. The Eulerian approach is applied to simulate the dynamics and heat transfer in the dispersed phase. The gas phase is described by a system of Reynolds-averaged Navier-Stokes (RANS) equations, taking into account the effect of droplets on mean transport and turbulent characteristics in the carrier phase. Gas phase turbulence is predicted using the second-moment closure. A swirling droplet-laden flow is characterized by an increase in the number of small particles on the pipe axis due to their accumulation in the zone of flow recirculation and the action of the turbulent migration (turbophoresis) force. A rapid dispersion of fine droplets over the pipe cross-section is observed without swirling. With an increase in swirling intensity, a significant reduction in the length of the separation region occurs. The swirling of a two-phase flow with liquid droplets leads to an increase in the level of turbulence for all three types of liquid droplets investigated in this work due to their intensive evaporation. It is shown that the addition of droplets leads to a significant increase in heat transfer in comparison with a single-phase swirling flow. The greatest effect of flow swirling on heat transfer intensification in a two-phase gas-droplet flow is obtained for the droplets of ethanol and water and smallest effect is for the acetone droplets.     

Hydrodynamics of a flexible cylinder under modulated vortex-induced vibrations

Both amplitude modulation and frequency modulation of Vortex-induced Vibration (VIV) are observed in a recent model test of a flexible cylinder under oscillatory flow, but its hydrodynamics has not yet been broached in detail. This paper employs the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible cylinder under modulated VIV. The FF-LS method’s applicability to accurately identify time-varying hydrodynamic coefficients is demonstrated through an elastically mounted rigid cylinder under flow with a given modulated motion. Furthermore, we propose a framework to predict instantaneous amplitude (envelope) and frequency using time-varying hydrodynamic coefficients to establish their analytical relationship. This prediction method is further extended to a highly tensioned flexible cylinder through Fourier series expansion in the spatial domain. By performing the identification procedure for all sampled data of a flexible cylinder undergoing oscillatory flow, we obtain the corresponding time-varying hydrodynamics in the cross-flow direction considering the amplitude and frequency modulation. The results show that, under modulated VIV, hydrodynamic coefficients of the flexible cylinder also show time-varying characteristics. We further investigate differences between identified hydrodynamic coefficients and those obtained from the database of a cylinder with modulated motion under flow. Prediction results using these identified time-varying coefficients reveal that the time-varying excitation coefficients mainly influence the amplitude modulation, and the time-varying added-mass coefficients contain the major information of frequency modulation. These results further suggest including the temporal derivative of the instantaneous amplitude as one determining parameter in building databases to improve the prediction of modulated VIV.     

Unidimensional continuous-variable quantum key distribution with discrete modulation

In this paper, we proposed an unidimensional continuous-variable quantum key distribution (CV-QKD) protocol with discrete modulation, which waives the necessity in one of the quadrature modulations and further simplified the implementation of the CV-QKD protocol. On the basis of the Heisenberg uncertainty relation, we analyze the boundary between the unphysical and physical region. Besides, we utilize a novel proof approach to achieve a lower bound valid of transmission distance. This scheme shows an available method to further simplified the implementation of the QKD.     

Recent Advances in Spatial Self-Phase Modulation with 2D Materials and its Applications

In recent years, spatial self-phase modulation (SSPM) with two-dimensional (2D) materials has attracted the attention of many researchers as an emerging and ubiquitous nonlinear optical effect. In this review, the state of the art of 2D material-based SSPM is summarized. SSPM measures or tunes the nonlinearity of 2D materials, and it is also an effective approach to study the band structure of 2D materials. Several modified forms of SSPM, such as high-order, white-light-excited, vector field excited, and optically nonlinearly enhanced SSPM are also presented. Subsequently, the physical origin of the SSPM formation mechanism is compared and analyzed. Furthermore, the applications of SSPM with 2D materials, including passive photonic devices, generation of Bessel beams, and identifying the mode of the orbital angular momentum, are listed. Finally, several urgent problems of the SSPM with 2D materials, potential applications, and prospects for future development are presented.     

用浓度调制光谱法测量水汽辉光放电中OH和H2O的布居数变化

本文使用波长为2.8 μm的分布式反馈激光器来测量水汽辉光放电产生的物种. 使用浓度调制光谱仅观察到OH自由基和瞬态H₂O分子的吸收光谱. 吸收峰的强度和方向随解调相位而变化,但是H₂O的一个吸收峰的方向始终与其它峰相反. OH和H₂O的不同光谱取向反映了能级中粒子数量的增加或减少. 如果在放电过程中可以检测到更多的瞬态物种,则可以更好地研究H₂O的激发、电离和分解动力学. 研究表明,浓度调制光谱的解调相位关系可以用来研究分子能级的布居数变化.     

Tuning the electronic properties of hydrogen passivated C3N nanoribbons through van der Waals stacking

The two-dimensional (2D) C₃N has emerged as a material with promising applications in high performance device owing to its intrinsic bandgap and tunable electronic properties. Although there are several reports about the bandgap tuning of C₃N via stacking or forming nanoribbon, bandgap modulation of bilayer C₃N nanoribbons (C₃NNRs) with various edge structures is still far from well understood. Here, based on extensive first-principles calculations, we demonstrated the effective bandgap engineering of C₃N by cutting it into hydrogen passivated C₃NNRs and stacking them into bilayer heterostructures. It was found that armchair (AC) C₃NNRs with three types of edge structures are all semiconductors, while only zigzag (ZZ) C₃NNRs with edges composed of both C and N atoms (ZZCN/ CN) are semiconductors. The bandgaps of all semiconducting C₃NNRs are larger than that of C₃N nanosheet. More interestingly, AC-C₃NNRs with CN/CN edges (AC-CN/CN) possess direct bandgap while ZZ-CN/CN have indirect bandgap. Compared with the monolayer C₃NNR, the bandgaps of bilayer C₃NNRs can be greatly modulated via different stacking orders and edge structures, varying from 0.43 eV for ZZ-CN/CN with AB′-stacking to 0.04 eV for AC-CN/CN with AA-stacking. Particularly, transition from direct to indirect bandgap was observed in the bilayer AC-CN/CN heterostructure with AA′-stacking, and the indirect-to-direct transition was found in the bilayer ZZ-CN/CN with ABstacking. This work provides insights into the effective bandgap engineering of C₃N and offers a new opportunity for its applications in nano-electronics and optoelectronic devices.     

FPGA-Implemented Fractal Decoder with Forward Error Correction in Short-Reach Optical Interconnects

Forward error correction (FEC) codes combined with high-order modulator formats, i.e., coded modulation (CM), are essential in optical communication networks to achieve highly efficient and reliable communication. The task of providing additional error control in the design of CM systems with high-performance requirements remains urgent. As an additional control of CM systems, we propose to use indivisible error detection codes based on a positional number system. In this work, we evaluated the indivisible code using the average probability method (APM) for the binary symmetric channel (BSC), which has the simplicity, versatility and reliability of the estimate, which is close to reality. The APM allows for evaluation and compares indivisible codes according to parameters of correct transmission, and detectable and undetectable errors. Indivisible codes allow for the end-to-end (E2E) control of the transmission and processing of information in digital systems and design devices with a regular structure and high speed. This study researched a fractal decoder device for additional error control, implemented in field-programmable gate array (FPGA) software with FEC for short-reach optical interconnects with multilevel pulse amplitude (PAM-M) modulated with Gray code mapping. Indivisible codes with natural redundancy require far fewer hardware costs to develop and implement encoding and decoding devices with a sufficiently high error detection efficiency. We achieved a reduction in hardware costs for a fractal decoder by using the fractal property of the indivisible code from 10% to 30% for different n while receiving the reciprocal of the golden ratio.     

交叉相位调制对非线性光纤环镜中光脉冲传输的影响

基于耦合非线性薛定谔方程,采用分步傅里叶方法分析了在单波长信道和WDM（波分复用）4波长信道中XPM（交叉相位调制）对高速NOLM（非线性光纤环镜）光开关中脉冲传输的影响.数值计算表明：XPM造成NOLM中信号脉冲串扰与畸变,并造成NOLM开关性能的下降.WDM系统NOLM光纤环中的同向传播的各波长之间的XPM串扰比相向脉冲之间的XPM效应的影响更大,并且波长越短输出功率越低,中间信道受XPM的影响所导致的脉座现象和走离效应比两侧波长信道严重.     

无线光通信中脉冲位置调制信号误码率的蒙特卡罗仿真

分析了在无线光通信链路中对激光脉冲应用脉冲位置调制的误码率,介绍了蒙特卡罗仿真方法的基本思想及其处理概率问题的方法步骤,并用蒙特卡罗仿真方法得到了脉冲位置调制信号的误码率.仿真结果表明,脉冲位置调制信号的误码率随入射光强的增大而减小,可用雪崩光电二极管对其进行探测以得到良好的接收性能.这对探讨脉冲位置调制光通信链路的系统特性和无线光通信接收系统的设计具有一定的参考价值.     

ABX3型钙钛矿光伏材料的结构与性质调控

近年来,钙钛矿太阳能电池由于其效率高、制造成本低、工艺简单等特点受到广泛关注,成为目前太阳能电池领域的研究热点。在钙钛矿太阳能电池中,无机-有机杂化ABX₃材料非常重要。它既作为光吸收材料,同时又作为载流子传输材料,因此它的光电性质直接影响到太阳能电池的效率。本文综述了调控无机有机金属卤化物ABX₃型钙钛矿光伏材料结构和性质的几种途径。