- 标题
- 摘要
- 关键词
- 实验方案
- 产品
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Numerical problems in coupling photon momentum (radiation pressure) to gas
摘要： Radiation pressure (RP; or photon momentum absorbed by gas) is important in a tremendous range of astrophysical systems. But we show the usual method for assigning absorbed photon momentum to gas in numerical radiation-hydrodynamics simulations (integrating over cell volumes or evaluating at cell centres) can severely underestimate the RP force in the immediate vicinity around unresolved (point/discrete) sources (and subsequently underestimate its effects on bulk gas properties), unless photon mean free paths are highly resolved in the fluid grid. The existence of this error is independent of the numerical radiation transfer (RT) method (even in exact ray-tracing/Monte Carlo methods), because it depends on how the RT solution is interpolated back onto fluid elements. Brute-force convergence (resolving mean free paths) is impossible in many cases (especially where UV/ionizing photons are involved). Instead, we show a 'face-integrated' method – integrating and applying the momentum fluxes at interfaces between fluid elements – better approximates the correct solution at all resolution levels. The 'fix' is simple and we provide example implementations for ray-tracing, Monte Carlo, and moments RT methods in both grid and mesh-free fluid schemes. We consider an example of star formation in a molecular cloud with UV/ionizing RP. At state-of-the-art resolution, cell-integrated methods underestimate the net effects of RP by an order of magnitude, leading (incorrectly) to the conclusion that RP is unimportant, while face-integrated methods predict strong self-regulation of star formation and cloud destruction via RP.
关键词： galaxies: active,galaxies: formation,hydrodynamics,methods: numerical,stars: formation
更新于2025-09-23 15:22:29
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Lost and found sunquake in the 6 September 2011 flare caused by beam electrons
摘要： The active region NOAA 11283 produced two X-class ?ares on 6 and 7 September 2011 that have been well studied by many authors. The X2.1 class ?are occurred on September 6, 2011 and was associated with the ?rst of two homologous white light ?ares produced by this region, but no sunquake was found with it despite the one being detected in the second ?are of 7 September 2011. In this paper we present the ?rst observation of a sunquake for the 6 September 2011 ?are detected via statistical signi?cance analysis of egression power and veri?ed via directional holography and time–distance diagram. The surface wavefront exhibits directional preference in the north-west direction We interpret this sunquake and the associated ?are emission with a combination of a radiative hydrodynamic model of a ?aring atmosphere heated by electron beam and a hydrodynamic model of acoustic wave generation in the solar interior generated by a supersonic shock. The hydrodynamic model of the ?aring atmosphere produces a hydrodynamic shock travelling with supersonic velocities toward the photosphere and beneath. For the ?rst time we derive velocities (up to 140 km s?1) and onset time (about 50 s after ?are onset) of the shock deposition at given depths of the interior. The shock parameters are con?rmed by the radiative signatures in hard X-rays and white light emission observed from this ?are. The shock propagation in the interior beneath the ?are is found to generate acoustic waves elongated in the direction of shock propagation, that results in an anisotropic wavefront seen on the solar surface. Matching the detected seismic signatures on the solar surface with the acoustic wave front model derived for the simulated shock velocities, we infer that the shock has to be deposited under an angle of about 30? to the local solar vertical. Hence, the improved seismic detection technique combined with the double hydrodynamic model reported in this study opens new perspectives for observation and interpretation of seismic signatures in solar ?ares.
关键词： Sun: X-rays, gamma rays,radiative transfer,Sun: ?ares,Sun: helioseismology,hydrodynamics
更新于2025-09-23 15:21:21
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In-fiber Dispersion Compensated Polarizer and Mode-locked Fiber Laser Application
摘要： Microplasma jets propagating in a helium flow surrounded by open air at ambient temperature have attracted the attention of many researchers for their putative ability to deliver high fluxes of reactive oxygen and nitrogen species to a target situated some centimeters away. This is of particular interest in biomedical applications. In this paper, we use Schlieren images to show that ignition of the plasma jet causes a reduction in the length of the laminar zone of the helium flow.
关键词： Atmospheric-pressure plasmas,hydrodynamics
更新于2025-09-23 15:21:01
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Interface Kinematics of Laser Impact Welding of Ni and SS304 Based on Jet Indentation Mechanism
摘要： With the application of high-velocity impact welding (HVIW) technology, the research on the mechanism of the formation and evolution of interface waveforms has become an important research direction in HVIW. Understanding the mechanism of interface motion can help control and improve HVIW. Laser impact welding (LIW), a branch of HVIW, lacks research on the formation and evolution of interface waveforms. Therefore, LIW experiments of Ni and SS304 are carried out. The jet indentation mechanism and the smoothed particle hydrodynamics method are used to simulate and analyze the formation and evolution of the LIW interface waveform. Results show that the interface materials exhibit ?uid behavior during LIW. The jet indentation mechanism can explain the formation and evolution of waveforms in LIW. Jet velocity, normal stress, and ?yer horizontal welding speed are the main factors a?ecting the variation of the interface waveform size in LIW. At the collision point, the smaller the horizontal welding speed of the ?yer, the larger the wavelength; the smaller the jet velocity, the smaller the amplitude. In addition, the propagation of the normal stress deviating from each other at the welding interface along the welding direction is the fundamental cause of springback and cracking.
关键词： SS304,Ni,Laser impact welding,Smoothed particle hydrodynamics,Jet indentation mechanism,Interface waveforms
更新于2025-09-23 15:21:01
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[IEEE 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Chicago, IL, USA (2019.6.16-2019.6.21)] 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Inkjet Printing and Characterization of Titanium Dioxide and Platinum Electrodes For Dye-sensitized Solar Cells (DSSCs)
摘要： We have bridged the unsteady and steady mechanisms of underwater propulsion in nature and engineering by colocating their flapping and steady lifting surfaces in an outwardly conventional propulsor. The feasibility is indicated by the observation of overlap in the mechanisms in propulsion density versus displacement volume from 0.1 to 1 m . Such an overlap also exists between natural and engineered flyers. A novel, 0.7-m diameter, propulsor has been built where the fins, twistable along their span (0 to 30 ), can either slosh (where roll, pitch, and twist of the fins vary independently) or spin (where the rotational rate, fin pitch, and twist vary independently). Here, we discuss the origin of the novelty of the propulsor, the production of small thrust by slosh and propeller (prop) modes, the control of thrust amplitude by spanwise twisting of the fin, and the abrupt reversing of thrust. The tow speeds are low and close to the minimum induced velocity required for thrust onset by the flapping mechanism in the present propulsor—0 to 0.09 m/s, the fin chord 8, 250, and Reynolds number and shaft input power being 1 W. Time-averaged measurements show that thrust is more sensitive to pitch amplitude than to twisting during hovering, an effect that is reversed during slow towing due to the reduction in the spanwise variation of angle of attack. During towing, twist is more effective in the slosh mode than in the prop mode. Steady and quasi-steady thrust modeling is done to compare with prop- and slosh-mode measurements, respectively. The departures of the models are interpreted to mean that the beneficial effects of twist on the leading edge vortex (LEV) augment slosh forces and the rotational effects are detrimental to prop forces. We present simultaneous videography of fins during twisting and thrust reversal, and of thrust time trace as direct evidence of the relationship of cause and effect. Spanwise fin twisting is used to show that near-zero levels of thrust (0 to 1 N in steps of approximately 0.1 N) can be produced in both the slosh and prop modes and can be controlled merely by twisting the fins while keeping all other fin parameters unchanged. Transient-free reversal of the thrust direction has been achieved in the slosh mode while maintaining the same absolute value of thrust. However, thrust reversal in the prop mode is not transient free. This prop-mode transient is weaker due to the change in sign of the pitch angle but a change in the direction of the hub rotation produces a large spike and the reasons are discussed. Fine thrust control with individual fin hydrodynamics at the source that involves the lowest change in inertia is smoother. Smooth thrust reversibility is clearly identified as a unique property of flapping fin hydrodynamics. The mechanism overlap occurs because both fin modes have similar low transitional Reynolds numbers. Dynamical system models of unsteady hydrodynamics and control are shown to be similar suggesting that animal swimmers control vortex shedding ion-by-ion and animal-like motion control is theoretically possible with the proposed propulsor in the slosh mode but not in the prop mode.
关键词： hydrodynamics,oscillating propulsor,nonlinear control,force transient,Bioinspiration
更新于2025-09-23 15:19:57
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Effects of large-scale floating (solar photovoltaic) platforms on hydrodynamics and primary production in a coastal sea from a water column model
摘要： An improved understanding of the effects of floating solar platforms on the ecosystem is necessary to define acceptable and responsible real-world field implementations of this new marine technology. This study examines a number of potential effects of offshore floating solar photovoltaic (PV) platforms on the hydrodynamics and net primary production in a coastal sea for the first time. Three contrasting locations within the North Sea (a shallow and deeper location with well-mixed conditions and a seasonally stratifying location) have been analysed using a water column physical–biogeochemical model: the General Ocean Turbulence Model coupled with the European Regional Seas Ecosystem Model – Biogeochemical Flux Model (GOTM-ERSEM-BFM). The results show strong dependence on the characteristics of the location (e.g. mixing and stratification) and on the density of coverage with floating platforms. The overall response of the system was separated into contributions by platform-induced light deficit, shielding by the platforms of the sea surface from wind and friction induced by the platforms on the currents. For all three locations, light deficit was the dominant effect on the net primary production. For the two well-mixed locations, the other effects of the platforms resulted in partial compensation for the impact of light deficit, while for the stratified location, they enhanced the effects of light deficit. For up to 20 % coverage of the model surface with platforms, the spread in the results between locations was relatively small, and the changes in net primary production were less than 10 %. For higher percentages of coverage, primary production decreased substantially, with an increased spread in response between the sites. The water column model assumes horizontal homogeneity in all forcings and simulated variables, also for coverage with floating platforms, and hence the results are applicable to very-large-scale implementations of offshore floating platforms that are evenly distributed over areas of at least several hundreds of square kilometres, such that phytoplankton remain underneath a farm throughout several tidal cycles. To confirm these results, and to investigate more realistic cases of floating platforms distributed unevenly over much smaller areas with horizontally varying hydrodynamic conditions, in which phytoplankton can be expected to spend only part of the time underneath a farm and effects are likely to be smaller, spatial detail and additional processes need to be included. To do so, further work is required to advance the water column model towards a three-dimensional modelling approach.
关键词： North Sea,hydrodynamics,GOTM-ERSEM-BFM,floating solar platforms,primary production,coastal sea
更新于2025-09-23 15:19:57
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On experimental and numerical study of the dynamics of a liquid metal jet hit by a laser pulse
摘要： In this paper, we present an experimental and numerical study of the laser pulse impact on the liquid metal jet target. The jet motion was recorded with the stroboscopic ultrafast shadow photography. Simulations were carried out in a two-step approach. At the first step, we simulated laser interaction with the target using the radiation hydrodynamics code 3DLINE, which accounts for a number of effects: liquid–gas phase transition, dynamics of ionization, radiation transfer, laser reflection, refraction and absorption. However, this code cannot be used on a deeply refined mesh near the liquid surface and does not account for the surface tension, which strongly affects liquid motion on the microsecond timescale after the laser pulse ends. Therefore, for the second step we employed the OpenFOAM solver, based on the volume-of-fluid method, which overcomes these limitations. The simulated target dynamics is found to be in a fairly good agreement with the experiment.
关键词： Laser pulse,Fragmentation,Plasma,Hydrodynamics,Liquid metal jet
更新于2025-09-23 15:19:57
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[IEEE 2019 IEEE 16th India Council International Conference (INDICON) - Rajkot, India (2019.12.13-2019.12.15)] 2019 IEEE 16th India Council International Conference (INDICON) - Design of two-element Circularly Polarized Bubbled Antenna for Vehicular Communication
摘要： We have bridged the unsteady and steady mechanisms of underwater propulsion in nature and engineering by colocating their flapping and steady lifting surfaces in an outwardly conventional propulsor. The feasibility is indicated by the observation of overlap in the mechanisms in propulsion density versus displacement volume from 0.1 to 1 m . Such an overlap also exists between natural and engineered flyers. A novel, 0.7-m diameter, propulsor has been built where the fins, twistable along their span (0 to 30 ), can either slosh (where roll, pitch, and twist of the fins vary independently) or spin (where the rotational rate, fin pitch, and twist vary independently). Here, we discuss the origin of the novelty of the propulsor, the production of small thrust by slosh and propeller (prop) modes, the control of thrust amplitude by spanwise twisting of the fin, and the abrupt reversing of thrust. The tow speeds are low and close to the minimum induced velocity required for thrust onset by the flapping mechanism in the present propulsor—0 to 0.09 m/s, the fin chord 8, 250, and Reynolds number and shaft input power being 1 W. Time-averaged measurements show that thrust is more sensitive to pitch amplitude than to twisting during hovering, an effect that is reversed during slow towing due to the reduction in the spanwise variation of angle of attack. During towing, twist is more effective in the slosh mode than in the prop mode. Steady and quasi-steady thrust modeling is done to compare with prop- and slosh-mode measurements, respectively. The departures of the models are interpreted to mean that the beneficial effects of twist on the leading edge vortex (LEV) augment slosh forces and the rotational effects are detrimental to prop forces. We present simultaneous videography of fins during twisting and thrust reversal, and of thrust time trace as direct evidence of the relationship of cause and effect. Spanwise fin twisting is used to show that near-zero levels of thrust (0 to 1 N in steps of approximately 0.1 N) can be produced in both the slosh and prop modes and can be controlled merely by twisting the fins while keeping all other fin parameters unchanged. Transient-free reversal of the thrust direction has been achieved in the slosh mode while maintaining the same absolute value of thrust. However, thrust reversal in the prop mode is not transient free. This prop-mode transient is weaker due to the change in sign of the pitch angle but a change in the direction of the hub rotation produces a large spike and the reasons are discussed. Fine thrust control with individual fin hydrodynamics at the source that involves the lowest change in inertia is smoother. Smooth thrust reversibility is clearly identified as a unique property of flapping fin hydrodynamics. The mechanism overlap occurs because both fin modes have similar low transitional Reynolds numbers. Dynamical system models of unsteady hydrodynamics and control are shown to be similar suggesting that animal swimmers control vortex shedding ion-by-ion and animal-like motion control is theoretically possible with the proposed propulsor in the slosh mode but not in the prop mode.
关键词： oscillating propulsor,Bioinspiration,hydrodynamics,nonlinear control,force transient
更新于2025-09-23 15:19:57
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Atomic Diffusion Behavior and Interface Waveform on the Laser Shock Welding of Aluminum to Nickel
摘要： Atomic diffusion behavior and interface waveform characteristics and formation mechanism during laser shock welding were investigated by using a molecular dynamics (MD) model and smooth particle hydrodynamics (SPH) modeling. The MD simulation showed that the diffusion coef?cient of Al atom was larger than that of the Ni atom. Ni atom is easily diffused deeply into the Al lattice during impact welding. The SPH simulation showed that the wavelength and amplitude of the welding interface increased with loading speed, and SPH simulations at different loading speeds demonstrated that the movement direction of the Ni wave peak is the same as the welding direction, whereas the movement direction of the Al wave peak is opposite to the welding direction. The effective plastic strain and temperature were mainly distributed at the interface waveform. The shear stress of the composite and substrate foil is in opposite direction near the collision point, and the pressure near the collision point was as high as about 10 GPa. Energy-dispersive spectroscopy line scanning analysis showed the presence of a 2.5-lm-thick element diffusion layer at the wavy interface between Al and Ni, verifying the element diffusion between Al and Ni in the MD simulation.
关键词： laser shock welding,molecular dynamics,smoothed particle hydrodynamics,welding characteristics
更新于2025-09-23 15:19:57
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Conical vs Gaussian Terahertz Emission from Two-Color Laser-Induced Air Plasma Filaments
摘要： We have bridged the unsteady and steady mechanisms of underwater propulsion in nature and engineering by colocating their flapping and steady lifting surfaces in an outwardly conventional propulsor. The feasibility is indicated by the observation of overlap in the mechanisms in propulsion density versus displacement volume from 0.1 to 1 m . Such an overlap also exists between natural and engineered flyers. A novel, 0.7-m diameter, propulsor has been built where the fins, twistable along their span (0 to 30 ), can either slosh (where roll, pitch, and twist of the fins vary independently) or spin (where the rotational rate, fin pitch, and twist vary independently). Here, we discuss the origin of the novelty of the propulsor, the production of small thrust by slosh and propeller (prop) modes, the control of thrust amplitude by spanwise twisting of the fin, and the abrupt reversing of thrust. The tow speeds are low and close to the minimum induced velocity required for thrust onset by the flapping mechanism in the present propulsor—0 to 0.09 m/s, the fin chord 8, 250, and Reynolds number and shaft input power being 1 W. Time-averaged measurements show that thrust is more sensitive to pitch amplitude than to twisting during hovering, an effect that is reversed during slow towing due to the reduction in the spanwise variation of angle of attack. During towing, twist is more effective in the slosh mode than in the prop mode. Steady and quasi-steady thrust modeling is done to compare with prop- and slosh-mode measurements, respectively. The departures of the models are interpreted to mean that the beneficial effects of twist on the leading edge vortex (LEV) augment slosh forces and the rotational effects are detrimental to prop forces. We present simultaneous videography of fins during twisting and thrust reversal, and of thrust time trace as direct evidence of the relationship of cause and effect. Spanwise fin twisting is used to show that near-zero levels of thrust (0 to 1 N in steps of approximately 0.1 N) can be produced in both the slosh and prop modes and can be controlled merely by twisting the fins while keeping all other fin parameters unchanged. Transient-free reversal of the thrust direction has been achieved in the slosh mode while maintaining the same absolute value of thrust. However, thrust reversal in the prop mode is not transient free. This prop-mode transient is weaker due to the change in sign of the pitch angle but a change in the direction of the hub rotation produces a large spike and the reasons are discussed. Fine thrust control with individual fin hydrodynamics at the source that involves the lowest change in inertia is smoother. Smooth thrust reversibility is clearly identified as a unique property of flapping fin hydrodynamics. The mechanism overlap occurs because both fin modes have similar low transitional Reynolds numbers. Dynamical system models of unsteady hydrodynamics and control are shown to be similar suggesting that animal swimmers control vortex shedding ion-by-ion and animal-like motion control is theoretically possible with the proposed propulsor in the slosh mode but not in the prop mode.
关键词： hydrodynamics,oscillating propulsor,nonlinear control,force transient,Bioinspiration
更新于2025-09-19 17:13:59