The objective of this paper is to investigate transient cavitating flows around a hydrofoil via combined physical and numerical studies. The aims are to 1) investigate the periodic formation, breakup, shedding, and co...The objective of this paper is to investigate transient cavitating flows around a hydrofoil via combined physical and numerical studies. The aims are to 1) investigate the periodic formation, breakup, shedding, and collapse of the sheet/cloud cavities, 2) provide a better insight in the physical mechanism that governs the dynamics and structures of the sheet/cloud cavitation, 3) quantify the influence of cavitation on the surrounding flow structures. Results are presented for a Clark-Y hydrofoil fixed at an angle of attack of a=8° at a moderate Reynolds number, Re=7×105 , for sheet/cloud cavitating conditions. The experimental studies were conducted in a cavitation tunnel at Beijing Institute of Technology, China. The numerical simulations are performed by solving the incompressible, multiphase unsteady Reynolds-averaged Navier-Stokes (URANS) equations via the commercial code CFX using a transport equation-based cavitation model; a filter-based density corrected model (FBDCM) is used to regulate the turbulent eddy viscosity in both the cavitation regions near the foil and in the wake. The results show that numerical predictions are capable of capturing the initiation of the cavity, growth toward the trailing edge, and subsequent shedding in accordance with the quantitative features observed in the experiment. Regarding vapor shedding in the cavitating flow around the three-dimensional foil, it is primarily attributed to the effect of the re-entrant flow, which is formed due to the strong adverse pressure gradient. The results show strong correlation between the cavity and vorticity structures, demonstrating that the inception, growth, shedding, and collapse of sheet/cloud cavities are important mechanisms for vorticity production and modification.展开更多
Transmission spectra of coupled cavity structures （CCSs） in two-dimensional （2D） photonic crystals （PCs） are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performa...Transmission spectra of coupled cavity structures （CCSs） in two-dimensional （2D） photonic crystals （PCs） are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain （FDTD） method, and the results show that within a very short coupling distance of about 3λ, where ）, is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.展开更多
The objective of this work is to reveal the effect of a passive control method called wedge-type cavitating-bubble generator(WCG)on the cloud cavitation dynamics of National Advisory Committee for Aeronautics(NACA)66 ...The objective of this work is to reveal the effect of a passive control method called wedge-type cavitating-bubble generator(WCG)on the cloud cavitation dynamics of National Advisory Committee for Aeronautics(NACA)66 hydrofoil.The simulations are performed using the Partially-averaged Navier-Stokes(PANS)method coupled with the Zwart cavitation model.The proper orthogonal decomposition(POD)method is applied to extract the dominant flow structures.The results show that the WCG can induce the attached cavity to occur just behind the WCG instead of the hydrofoil leading edge.During the periodical time-evolution process of the unsteady cavity,it is found that the attached cavity with a larger scale around the hydrofoil with WCG has a rougher surface,accompanied with more shedding behaviors of small cavities.This is further illustrated by the POD modes,that is,the mode 1 and modes 2–4 present the large and small cavity vortex structures respectively.Meanwhile,the dominant frequencies of 50 Hz,47.5 Hz are given by the POD method respectively for the hydrofoils without and with WCG,which is in good agreement with that of FFT analysis.In addition,the correlation distribution of POD modal coefficients shows that the WCG can strengthen the vortex energy as well as the turbulence intensity.展开更多
本文介绍了在先进高效数值方法—多层快速多极子方法(Multilevel Fast Multipole Algorithm,MLFMA)分析框架下,旨在进一步提升数值求解能力和工程应用能力的研究成果,以及结合相关成果的应用研究;重点讨论了用于超电大目标(特别是含腔目...本文介绍了在先进高效数值方法—多层快速多极子方法(Multilevel Fast Multipole Algorithm,MLFMA)分析框架下,旨在进一步提升数值求解能力和工程应用能力的研究成果,以及结合相关成果的应用研究;重点讨论了用于超电大目标(特别是含腔目标)散射分析的各型相位提取(Phase Extracted,PE)基函数、用于金属-厚介质层复合结构散射建模的电流磁流混合场积分方程(Electric and Magnetic Current Combined Field Integral Equation,JMCFIE)、用于天线罩及雷达仓散射计算的多层介质散射的多层薄介质层(Thin Dielectric Sheet,TDS)边界条件方法、用于多尺度结构电磁散射建模的快速笛卡尔展开方法;并给出了相关应用实例.最后,对当前仍然存在的主要挑战和可能的对策进行了简略的讨论和展望.展开更多
The lattice structure image of a plasma standing wave in a Purcell cavity of silicon is observed. The plasma wave produced by the pulsed laser could be used to fabricate the micro-nanostructure of silicon. The plasma ...The lattice structure image of a plasma standing wave in a Purcell cavity of silicon is observed. The plasma wave produced by the pulsed laser could be used to fabricate the micro-nanostructure of silicon. The plasma lattice structures induced by the nanosecond pulsed laser in the cavity may be similar to the Wigner crystal structure. It is interesting that the beautiful diffraction pattern could be observed in the plasma lattice structure. The radiation lifetime could be shortened to the nanosecond range throughout the entire spectral range and the relaxation time could be lengthened for higher emission efficiency in the Purcell cavity, which results in the fact that the plasmonic emission is stronger and its threshold is lower.展开更多
基金supported by the National Natural Science Foundation of China (Grant Nos. 11172040, 50979004)
文摘The objective of this paper is to investigate transient cavitating flows around a hydrofoil via combined physical and numerical studies. The aims are to 1) investigate the periodic formation, breakup, shedding, and collapse of the sheet/cloud cavities, 2) provide a better insight in the physical mechanism that governs the dynamics and structures of the sheet/cloud cavitation, 3) quantify the influence of cavitation on the surrounding flow structures. Results are presented for a Clark-Y hydrofoil fixed at an angle of attack of a=8° at a moderate Reynolds number, Re=7×105 , for sheet/cloud cavitating conditions. The experimental studies were conducted in a cavitation tunnel at Beijing Institute of Technology, China. The numerical simulations are performed by solving the incompressible, multiphase unsteady Reynolds-averaged Navier-Stokes (URANS) equations via the commercial code CFX using a transport equation-based cavitation model; a filter-based density corrected model (FBDCM) is used to regulate the turbulent eddy viscosity in both the cavitation regions near the foil and in the wake. The results show that numerical predictions are capable of capturing the initiation of the cavity, growth toward the trailing edge, and subsequent shedding in accordance with the quantitative features observed in the experiment. Regarding vapor shedding in the cavitating flow around the three-dimensional foil, it is primarily attributed to the effect of the re-entrant flow, which is formed due to the strong adverse pressure gradient. The results show strong correlation between the cavity and vorticity structures, demonstrating that the inception, growth, shedding, and collapse of sheet/cloud cavities are important mechanisms for vorticity production and modification.
文摘Transmission spectra of coupled cavity structures （CCSs） in two-dimensional （2D） photonic crystals （PCs） are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain （FDTD） method, and the results show that within a very short coupling distance of about 3λ, where ）, is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.
基金Project supported by the National Natural Science Foundation of China(Grant No.52076108).
文摘The objective of this work is to reveal the effect of a passive control method called wedge-type cavitating-bubble generator(WCG)on the cloud cavitation dynamics of National Advisory Committee for Aeronautics(NACA)66 hydrofoil.The simulations are performed using the Partially-averaged Navier-Stokes(PANS)method coupled with the Zwart cavitation model.The proper orthogonal decomposition(POD)method is applied to extract the dominant flow structures.The results show that the WCG can induce the attached cavity to occur just behind the WCG instead of the hydrofoil leading edge.During the periodical time-evolution process of the unsteady cavity,it is found that the attached cavity with a larger scale around the hydrofoil with WCG has a rougher surface,accompanied with more shedding behaviors of small cavities.This is further illustrated by the POD modes,that is,the mode 1 and modes 2–4 present the large and small cavity vortex structures respectively.Meanwhile,the dominant frequencies of 50 Hz,47.5 Hz are given by the POD method respectively for the hydrofoils without and with WCG,which is in good agreement with that of FFT analysis.In addition,the correlation distribution of POD modal coefficients shows that the WCG can strengthen the vortex energy as well as the turbulence intensity.
文摘本文介绍了在先进高效数值方法—多层快速多极子方法(Multilevel Fast Multipole Algorithm,MLFMA)分析框架下,旨在进一步提升数值求解能力和工程应用能力的研究成果,以及结合相关成果的应用研究;重点讨论了用于超电大目标(特别是含腔目标)散射分析的各型相位提取(Phase Extracted,PE)基函数、用于金属-厚介质层复合结构散射建模的电流磁流混合场积分方程(Electric and Magnetic Current Combined Field Integral Equation,JMCFIE)、用于天线罩及雷达仓散射计算的多层介质散射的多层薄介质层(Thin Dielectric Sheet,TDS)边界条件方法、用于多尺度结构电磁散射建模的快速笛卡尔展开方法;并给出了相关应用实例.最后,对当前仍然存在的主要挑战和可能的对策进行了简略的讨论和展望.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11264007 and 61465003)
文摘The lattice structure image of a plasma standing wave in a Purcell cavity of silicon is observed. The plasma wave produced by the pulsed laser could be used to fabricate the micro-nanostructure of silicon. The plasma lattice structures induced by the nanosecond pulsed laser in the cavity may be similar to the Wigner crystal structure. It is interesting that the beautiful diffraction pattern could be observed in the plasma lattice structure. The radiation lifetime could be shortened to the nanosecond range throughout the entire spectral range and the relaxation time could be lengthened for higher emission efficiency in the Purcell cavity, which results in the fact that the plasmonic emission is stronger and its threshold is lower.
