Institute Of Mechanics,Chinese Academy of Sciences
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Breakup and coalescence behaviors of oil droplets in complex swirling flow system
No full textDroplet migration, coalescence, and breakup are common phenomena in cyclone systems. This study explores and provides insights into the breakup and coalescence behaviors of droplet groups in a swirling flow field through a combination of theoretical analyses and numerical simulations. A cylindrical cyclone with a tangential inlet was used to construct a stable swirling flow field, which was then comprehensively studied. The swirl intensity initially increased and then started decreasing, and the internal and external swirls coexisted. The breakup and coalescence behaviors of oil droplets in a complex swirling flow field were simulated, and their coalescence mode and breakup region were investigated in detail. Based on the energy-dissipation rate of the turbulence, the swirl-flow field was divided into strong- and weak-dissipation regions comprising overflow, separation, and underflow. Sauter mean diameter and energy-dissipation rate were evident in these: the overflow part exhibited a linear relationship, the separation part exhibited a quadratic linear relationship, the underflow part remained stable under low Reynolds numbers, and a quadratic linear relationship was observed at high Reynolds numbers. Based on the critical Weber number of droplets, a factor "g" is proposed to predict the droplet size, which has an exponential relationship with the Reynolds number
真空高效电弧熔炼炉系统
No full text本发明属于电弧熔炼炉设备技术领域，针对现有技术中存在的气密性差的技术问题，本发明的目的在于提供真空高效电弧熔炼炉系统，包括氩气罐、电弧炉、扩散泵、真空泵，氩气罐通过气体管路和电弧炉连接，通过氩气罐向电弧炉内充氩气，电弧炉的顶端通过摇杆底座和摇杆连接，电弧炉的底端通过坩埚连接铜帽，电弧炉的侧部通过管路和扩散泵连接，电弧炉通过气体管路和真空泵连接，通过开启真空泵控制电弧炉抽真空。提高了该系统的气密性，本发明所提综合系统设计的提升熔炼炉的高真空抽气时间从30分钟每次到1分钟每次。巨大节约了科研工作者的时间
Thermochemical kinetic model for high altitude and low altitude high-speed flow and radiation prediction based on the State-to-State model
No full textThe reasonable thermochemical kinetic model is a key prerequisite for accurately predicting the non-equilibrium flow field and radiation of high-speed atmospheric reentry vehicles. A vibrational and electronic specific State-to-State kinetic model is first established to couple with post-shock flow equations to analyze the thermochemical non-equilibrium flow characteristics. It is found that the vibrational and electronic energy levels non-Boltzmann metric are large at the flight altitude higher than 50 km, which is thus considered a demarcation line for constructing the low- and high-altitude models. Therefore, in this study, two new physics-based thermochemical kinetic models are constructed from the State-to-State model using the multi-group maximum entropy method for low-altitude and high-altitude regions, respectively, across the entire flight velocity domains. By analyzing the differences in post-shock flow physical characteristics under various flight altitudes, single-group and multi-group functions are, respectively, adopted to reconstruct the internal energy levels distributions within the flow field for the low-altitude and high-altitude chemical models. Compared to the State-to-State model, the low-altitude and high-altitude models significantly reduce the number of solved species and chemical reactions, while maintaining computational accuracy of post-shock non-equilibrium flow, with calculated thermodynamic parameters error less than 5% and chemical parameters error below 10%. Finally, the low-altitude and high-altitude chemical models constructed in this study exhibit better predictive capability in reproducing ground experimentally measured electron number density and predicting reentry flight heat flux and radiation compared with the classical Park's and Gupta's models, demonstrating the promising applicability of developed models. This study provides a new, reasonable chemical model for the entire flight altitude and velocity domain that can be used in computational fluid dynamics simulations to predict non-equilibrium flow and radiation in high-speed reentry vehicles
Numerical study of turbulent bubbly upflow: effect of density ratio
No full textIn this study, we conducted interface-capturing high-resolution simulations of a bubbly upflow in a vertical channel to investigate the bubble distribution and its interaction with surrounding turbulence, focusing on the effects of the density ratio. A bulk Reynolds number was used for all simulations. The influence of density ratio on vortex structures and turbulence statistics differed between the near-wall and core regions of the channel. Adding 5.43 gas caused an increase in wall friction. By applying a generalised FIK identity to analyse wall friction, it was determined that the drag rise in the bubbly channel was mostly due to the near-wall region. Visualisation of the bubble and vortex structures showed that small bubbles near the wall induced larger magnitude of Reynolds shear stress and increased wall friction. Bubble behaviour near the wall region was similar for density ratios above 30, leading to wall friction saturation. In the core region, large deformable bubbles created wake vortices due to slip velocity between liquid and gas phases. Wake vortices help large bubbles absorb smaller bubbles and maintain their sizes. As the density ratio increased, the slip velocity increased owing to greater difference in the gravitational acceleration between liquid and gas phases, resulting in corresponding increase in wake intensity and velocity fluctuations. However, quadrant analysis showed that Q1 and Q3 events increased together with Q2 and Q4 events in the core region, cancelling out any net effect of wake vortices on Reynolds shear stress or wall friction
A deep learning-based segmentation method for multi-scale and overlapped bubbles in gas-liquid bubbly flow
No full textIn the realm of fluid dynamics, gas-liquid bubbly flow represents a prevalent and significant multiphase flow phenomenon. With the advancement of imaging technology, high-speed photography combined with image processing techniques has become a common method for measuring bubbly flows. To overcome the challenges posed by multi-scale and overlapping bubbles in gas-liquid bubbly flows, a deep learning-based method for precise bubble contour segmentation and trajectory tracking has been developed. This approach involves specific optimizations and enhancements to the one-stage object detection model "You-Only-Look-Once version 8", leading to a bubble segmentation algorithm that strikes a balance between speed and precision. Omni-dimension dynamic convolution and high-resolution feature layer pyramid level 2 (P2) were integrated into the model to extract more precise spatial and texture information, enhancing precision and facilitating the detection of small-sized bubbles. Additionally, to address the issue of severe bubble overlap in images, the bubble spatially enhanced attention module was developed to capitalize on detailed texture, thereby achieving the segmentation of severely overlapping bubbles. Based on the improved detection model, combined with the Botsort tracking algorithm, vanishing bubble re-identification as well as continuous tracking of severely occluded bubbles are realized. The model achieves inference speeds of 0.427 s on central processing unit and 0.03 s on graphics processing unit (GPU), respectively, facilitating its application in efficiently processing large comprehensive datasets
The many faces of vibrational energy relaxation in N<sub>2</sub>(<i>v</i>) + O(<SUP>1</SUP>D) collisions: Dynamics on <SUP>1</SUP>Π and <SUP>1</SUP>Δ potential energy surfaces
No full textComplete datasets of rate coefficients for the vibrational quenching of molecular nitrogen by collision with electronically excited atomic oxygen O(D-1) over a wide temperature range are calculated for the first time. Such data are important ingredients in the modeling of non-local thermal equilibrium conditions that characterize the atmosphere, media of astronomical interest, and cold and hot plasmas, where O(D-1), also formed when O-2 molecules break, represents a significant fraction of the gas mixture. To this end, we developed analytical potential energy surfaces (PESs) for the (1)Pi and (1)Delta electronic states of the N-2-O(D-1) system to accurately describe the interaction in the long, medium, and first repulsive range of intermolecular distances, the most effective regions in inelastic collisions under a variety of conditions of interest. The derived PESs are used to calculate the vibration-to-translation (V-T) and vibration-to-electronic (V-E) energy transfer rates by mixed quantum-classical dynamics and by the Landau-Zener formulation, respectively. In addition, the datasets are extended to cover the entire N-2 vibrational ladder by using the Gaussian process regression. The results show that at low temperatures, where V-E relaxation dominates, N-2 vibrational quenching by O(D-1) collisions is faster than by O(P-3) collisions
俯仰角对扑翼飞行与发声的影响
No full text扑翼俯仰角的变化对昆虫的扑翼飞行与发声有着显著影响。本文以使用扑翼扑动产生的声波（也称翼音）进行交流的昆虫为研究对象，初步分析和讨论了扑翼执行3种俯仰角模式（正弦、梯形和快速上仰）时的飞行与发声性能。通过重叠网格方法求解不可压缩Navier-Stokes方程得到扑翼周围的流场；利用气动力模拟得到的数据，通过Ffowcs Williams-Hawkings方程预测翼音。结果表明，不同的俯仰角模式可以适用不同的场景。当执行正弦俯仰角变化时，扑翼具有较高的飞行效率和较小的噪声；当执行梯形俯仰角变化时，扑翼可以提供更多的升力，且具有更高的发声效率以进行翼音交流；当执行快速上仰的俯仰角变化时，扑翼可以同时具有较高的飞行效率和发声效率。因此，未来在微型飞行器的设计中，可以采用不同的扑翼俯仰角变化模式以满足不同的需求
固体力学工程导向型“三步走”教学探索——以“接触力学”为例
No full text传统力学课程的教学主要依赖课堂讲授和习题练习，存在学生参与度有限、理论与实际应用联系不够紧密等问题，落后于新时代教育的发展。为了提高固体力学导论课程的教学效果，更好地帮助学生理解固体力学的基本概念、基本研究思想和基本理论，夯实基础知识，文章提出了以工程导向型教学为理念，强化工程思维的固体力学导论课程改革方案。以接触力学部分的讲授内容为例，首先通过理论教学建立知识体系；其次通过典型例题讲解，培养学生分析问题、解决问题的能力；最后拓展工程案例，在工程应用中检验学生的学习效果。希望通过这种“三步走”的教学方法可以促进学生全面发展，提高人才培养质量
Expanding the Electrochemical Stability Window: Unraveling the Role of Solvent Polarity and a WiSE-Compatible Strategy
No full textExpanding the electrochemical stability window (ESW) of aqueous batteries significantly enhances their safety and energy density, addressing performance limitations and elevating their position in energy storage systems. Over the past decade, water-in-salt electrolyte (WiSE) has led to groundbreaking advancement in this field. However, a pressing question arises: can we further broaden the ESW through novel approaches? This study delves into this question, leveraging atomistic simulation along with ESW estimation and WiSE continuum theory to uncover that interfacial solvent polarity, subtly modulated by adding minor organic solvents, expands the ESW as well as promotes ion intercalation and transport. The strategy of incorporating minor organic solvents is compatible with WiSE, which not only advances our comprehension but also forges new research paths for post-WiSE era aqueous battery innovation. More importantly, our study provides a systematic way for theoretically estimating ESW and analyzing its enhancement mechanism in aqueous batteries
Structure function of helicity in compressible homogeneous isotropic turbulence
No full textHelicity plays an essential role in the interscale dynamics of turbulence. This paper focuses on the compressible effects on helicity cascades via structure functions. We first investigate the spatial-local dynamics of helicity, revealing that helicity is enhanced by pressure gradients along vorticity lines. Then, structure functions are used to describe the multiscale dynamics. The scaling law r(2/3) for helicity remains valid in compressible turbulence, as helicity is independent of the compressive components. Additional pressure and divergence terms are introduced in the new third-order relation for helicity in compressible turbulence. The pressure term is related to the pressure gradients along the vorticity lines and plays a dominant role in the dissipative range. The divergence term is mainly induced by the mixed structure function of divergence and helicity, significantly contributing to inverse helicity cascades in the inertial range