Forces and energetics of intermittent swimming

Experiments are reported on intermittent swimming motions. Water tunnel experiments on a nominally two-dimensional pitching foil show that the mean thrust and power scale linearly with the duty cycle, from a value of 0.2 all the way up to continuous motions, indicating that individual bursts of activity in intermittent motions are independent of each other. This conclusion is corroborated by particle image velocimetry（PIV） flow visualizations, which show that the main vortical structures in the wake do not change with duty cycle. The experimental data also demonstrate that intermittent motions are generally energetically advantageous over continuous motions. When metabolic energy losses are taken into account, this conclusion is maintained for metabolic power fractions less than 1.

NUMERICAL MODELING OF 3-D TURBULENT TWO-PHASE FLOW AND COAL COMBUSTION IN A PULVERIZED-COAL COMBUSTOR

In the present paper, a multifluid model of two-phase flows with pulverized-coal combustion, based on a continuum-trajectory model with reacting particle phase, is developed and employed to simulate the 3-D turbulent two-phase hows and combustion in a new type of pulverized-coal combustor with one primary-air jet placed along the wall of the combustor. The results show that: (1) this continuum-trajectory model with reacting particle phase can be used in practical engineering to qualitatively predict the flame stability, concentrations of gas species, possibilities of slag formation and soot deposition, etc.; (2) large recirculation zones can be created in the combustor, which is favorable to the ignition and flame stabilization.

Understanding the Rayleigh instability in humping phenomenon during laser powder bed fusion process

The periodic undulation of a molten track’s height profile in laser-based powder bed fusion of metals(PBF-LB/M)is a commonly observed phenomena that can cause defects and building failure during the manufacturing process.However a quantitative analysis of such instabilities has not been fully established and so here we used Rayleigh–Plateau theory to determine the stability of a single molten track in PBF-LB/M and tested it with various processing conditions by changing laser power and beam shape.The analysis discovered that normalized enthalpy,which relates to energy input density,determines whether a molten track is initially unstable and if so,the growth rate for the instability.Additionally,whether the growth rate ultimately yields significant undulation depends on the melt duration,estimated by dwell time in our experiment.

Lattice Boltzmann Simulation of Blood Flow in Blood Vessels with the Rolling Massage

The rolling massage manipulation is a classic Chinese massage, which is expected to improve the circulation by pushing, pulling and kneading of the muscle. A model for the rolling massage manipulation is proposed and the lattice Boltzmann method is applied to study the blood flow in the blood vessels. The simulation results show that the blood flux is considerably modified by the rolling massage and the explicit value depends on the rolling frequency, the rolling depth, and the diameter of the vessel. The smaller the diameter of the blood vessel, the larger the enhancement of the blood flux by the rolling massage. The model, together with the simulation results, is expected to be helpful to understand the mechanism and further development of rolling massage techniques.

Effect of Switching on Metal-Organic Interface Adhesion Relevant to Organic Electronic Devices

Considerable efforts are currently being devoted to investigation of metal-organic, organic-organic and organic-inorganic interfaces relevant to organic electronic devices such as organic light emitting diode (OLEDs), organic photovoltaic solar cells, organic field effect transistors (OFETs), organic spintronic devices and organic-based Write Once Read Many times (WORM) memory devices on both rigid and flexible substrates in laboratories around the world. The multilayer structure of these devices makes interfaces between dissimilar materials in contact and plays a prominent role in charge transport and injection efficiency which inevitably affect device performance. This paper presents results of an initial study on how switching between voltage thresholds and chemical surface treatment affects adhesion properties of a metal-organic (Au-PEDOT:PSS) contact interface in a WORM device. Contact and Tapping-mode Atomic Force Microscopy (AFM) gave surface topography, phase imaging and interface adhesion properties in addition to SEM/EDX imaging which showed that surface treatment, switching and surface roughness all appeared to be key factors in increasing interface adhesion with implications for increased device performance.

燃烧基础研究的进展和挑战(英文)

超过80%的世界的能源转换是由燃烧方法来实现的.发展可利用替代燃料的清洁和高效的新型发动机是解决可持续能源发展的关键之一.在燃烧研究领域,实现这一目标的挑战是要揭示从燃料分子到发动机的多尺度燃烧过程中化学反应和火焰动力学机理,发展高效,定量的数值模拟方法和开发新的燃烧技术.该文从7个方面综述最近几年燃烧领域的基础燃烧研究的进展和挑战.它们包括低温清洁燃烧的发动机技术,极限条件下的燃烧机理和现象,替代燃料和混合燃料模型,多尺度化学反应模拟方法,高压燃烧反应动力学,基础燃烧的实验方法,和先进测量技术.该文首先介绍均值充量压缩点火(HCCI),反应控制压缩点火(RCCI)以及增压燃烧等新型发动机的概念,评述燃料特性和低温燃烧反应过程对湍流燃烧和发动机的影响,讨论发展基础燃烧研究的必要性.第二,综述燃料浓度分层燃烧,稀薄燃烧,冷炎燃烧,以及等离子体助燃等极限燃烧条件下的新的燃烧现象和火焰机制.第三,以航空煤油和生物柴油为例来讨论建立模拟真实燃料和替代燃料的混合燃料模型的方法.介绍活性基指数和输运加权的反应焓的概念并用来比较燃料的高温反应特性和评价燃料的分子结构对燃烧特性的影响.第四,评述详细化学反应机理简化的方法.介绍多时间尺度(MTS)的化学反应的模拟和动态关联性自适应机理简化(CO-DAC)的方法来提高详细化学反应机理的计算效率.第五,讨论高压燃烧的火焰传播速度的实验测量结果以及高压燃烧化学反应机理所存在的问题,并分析高压燃烧的关键组分和反应路径.第六,评述测量火焰速度和组分等基础燃烧实验方法和模型中的问题和误差来源.介绍一些改进测量方法和提高测量精度的方法.最后,介绍测量低温燃烧中的关键组分和自由基的测量方法和最新进展.

Novel concept of tailorable magnetic field and electron pressure distribution in a magnetic nozzle for effective space propulsion

Magnetic nozzle appears to be a practical solution for prolonged space missions.For efficient handling of the spaceship,an in-flight solution to customize the thrust from the magnetic nozzle is essential.Here a new concept of three-thick coils system is proposed for tailoring the magnetic field in-flight in accordance with electron pressure distribution.The role of peak position of the pressure and its axial gradient is also uncovered for realizing higher thrust.About three-fold increase in thrust is observed when the electron temperature is raised to w2.5 times of its original value at the exit plane.The set-up is optimized for its best performance and efficient use in the electric space propulsion sector with thrust approaching 5 mN.In particular,this can contribute to the attitude control or the precision pointing of the spacecraft,the technology for removal of space debris and manipulating the ion momentum flux lost to a wall or unsteady laser produced plasma flow in a magnetic nozzle.

Supergravity effects on flame propagation and structure in hydrogen/air mixtures

In the current work,we investigated hydrogen/air flame propagation under supergravity conditions.Results show that when gravity is in the same/opposite direction as flame propagation,it leads to acceleration/deceleration of the flame,and that such an effect could substantially modify the flame propagation and structure at high gravity levels.Furthermore,for the absolute and relative flame propagation speeds,the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field,while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature.The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio,pressure,and flame stretch.

Promoting Si-graphite composite anodes with SWCNT additives for half and NCM 811 full lithium ion batteries and assessment criteria from an industrial perspective

Single wall carbon nanotube(SWCNT)additives were formulated into(im-Si-graphite composite electrodes and tested in both half cells and full cells with high nickel cathodes.The critical role of small amount of SWCNT addition(0.2 wt%)was found for significantly improving delithiation capacity,first cycle coulombic efficiency(FCE),and capacity retention.Particularly,Si(10 wt%)-graphite electrode exhibits 560 mAh/g delithiation capacity and 92%FCE at 0.2 C during the first chargedischarge cycle,and 91%capacity retention after 50 cycles(0.5 C)in a half cell.Scanning electron microscope(SEM)was used to illustrate the electrode morphology,compositions and promoting function of the SWCNT additives.In addition,full cells assembled with high nickel-NCM811 cathodes and fim-Si-graphite composite anodes were evaluated for the consistence between half and full cell performance,and the consideration for potential commercial application.Finally,criteria to assess Si-containing anodes are proposed and discussed from an industrial perspective.

Single-lens dynamic z-scanning for simultaneous in situ position detection and laser processing focus control

Existing auto-focusing methods in laser processing typically include two independent modules,one for surface detection and another for z-axis adjustment.The latter is mostly implemented by mechanical z stage motion,which is up to three orders of magnitude slower than the lateral processing speed.To alleviate this processing bottleneck,we developed a single-lens approach,using only one high-speed z-scanning optical element,to accomplish both in situ surface detection and focus control quasi-simultaneously in a dual-beam setup.The probing beam scans the surface along the z-axis continuously,and its reflection is detected by a set of confocal optics.Based on the temporal response of the detected signal,we have developed and experimentally demonstrated a dynamic surface detection method at 140-350 kHz,with a controlled detection range,high repeatability,and minimum linearity error of 1.10%.Sequentially,by synchronizing at a corresponding oscillation phase of the z-scanning lens,the fabrication beam is directed to the probed z position for precise focus alignment.Overall,our approach provides instantaneous surface tracking by collecting position information and executing focal control both at 140-350 kHz,which significantly accelerates the axial alignment process and offers great potential for enhancing the speed of advanced manufacturing processes in three-dimensional space.

Tapered coils system for space propulsion with enhanced thrust: a concept of plasma detachment

A concept for plasma detachment in a magnetic nozzle is developed based on the detachment region which is found to decrease with the taper angle of the coils employed in the proposed flexible three coil setup.On tapering the coils while resulting in the same crosssectional area,the plasma plume outside the throat grows radially that leads to an enhancement in the thrust from 2.67 mN to 5 mN at the final detachment plane for a rise in the taper angle from 0 to 13
.The maximum thrust can reach about 9 mN when the middle coil is shifted closer to the right coil along with increasing middle-to-outer-coil diameter(inner)ratio from 1 to 3.Proposed three-tapered-coils arrangement for a magnetic nozzle turns out to be a robust candidate for space propulsion offering the ability to control plasma detachment and tune thrust in-flight simply via mechanical movements without changing the current.

Synchronization of Weighted Essentially Non-Oscillatory Methods

Weighted essentially non-oscillatory(WENO)methods have been developed to simultaneously provide robust shock-capturing in compressible fluid flow and avoid excessive damping of fine-scale flow features such as turbulence.Under certain conditions in compressible turbulence,however,numerical dissipation remains unacceptably high even after optimization of the linear component that dominates in smooth regions.Of the nonlinear error that remains,we demonstrate that a large fraction is generated by a“synchronization deficiency”that interferes with the expression of theoretically predicted numerical performance characteristics when the WENO adaptation mechanism is engaged.This deficiency is illustrated numerically in simulations of a linearly advected sinusoidal wave and the Shu-Osher problem[J.Comput.Phys.,83(1989),pp.32-78].It is shown that attempting to correct this deficiency through forcible synchronization results in violation of conservation.We conclude that,for the given choice of candidate stencils,the synchronization deficiency cannot be adequately resolved under the current WENO smoothness measurement technique.

Optical sensors for power transformer monitoring:A review

Power transformers play a vital role in power transmission systems from the energy source to the consumption center.In situ diagnostics of the performance of power transformers provides a variety of benefits to ensure reliable electricity transmission.Immunity to electromagnetic interference,high sensitivity,high insulation as well as small dimensions of optical sensing make it very attractive for applications in power transformer monitoring.This paper presents a critical review of various diagnostic methods for power transformers such as partial discharge,dissolved gases,temperature and other important sensing,and optical detection.The advantages and disadvantages of different monitoring methods are carefully discussed and assessed.Finally,the existing technical barriers and future prospects of optical monitoring methods for power transformers are presented.

Designing interfaces in energy materials applications with firstprinciples calculations

Materials for energy-related applications,which are crucial for a sustainable energy economy,rely on combining materials that form complex heterogenous interfaces.Simultaneously,progress in computational materials science in describing complex interfaces is critical for improving the understanding and performance of energy materials.Hence,we present an in-depth review of the physical quantities regulating interfaces in batteries,photovoltaics,and photocatalysts,that are accessible from modern electronic structure methods,with a focus on density functional theory calculations.For each energy application,we highlight unique approaches that have been developed to calculate interfacial properties and explore the possibility of applying some of these approaches across disciplines,leading to a unified overview of interface design.Finally,we identify a set of challenges for further improving the theoretical description of interfaces in energy devices.

Electromagnetic beam propagation in nonlinear media

We deduce a complete wave propagation equation that includes inhomogeneity of the dielectric constant and present this propagation equation in compact vector form. Although similar equations are known in narrow fields such as radio wave propagation in the ionosphere and electromagnetic and acoustic wave propagation in stratified media, we develop here a novel approach of using such equations in the modeling of laser beam propagation in nonlinear media. Our approach satisfies the correspondence principle since in the limit of zero-length wavelength it reduces from physical to geometrical optics.

变物性平面上边界层燃烧分析

本文通过解析方法分析了Pr与Sc不为1的情况下挥发燃料平面上平行氧化剂来流形成的边界层中的火焰面。对给定的物性,火焰位置和温度以及气化率可以被显式表达。同时也得到了和数值结果符合较好的挥发边界布拉休斯方程的近似解。