测定不同形状物体空气阻力系数的实验

设计空气流速连续可调的小型风洞,对圆形平板、圆柱体、流线体的空气阻力系数进行测量.利用U型管中水柱的高度差表达孔板法中空气的流速,通过悬臂梁力传感器测量测试物受到的空气阻力,根据空气阻力和U型管中水柱的高度差的关系测定空气阻力系数.     

Effect of powder loading on the excitation temperature of a plasma jet in DC thermal plasma spray torch

A DC non-transferred mode plasma spray torch was fabricated for plasma spheroidization. The effect of powder-carrier gas and powder loading on the temperature of the plasma jet generated by the torch has been studied. The experiment was done at different input power levels; the temperature of the jet was within 5000–7000 K argon was used as plasma gas and powder-carrier gas. Nickel powder particles in the size range from 40 to 100 μm were processed. The temperature of the jet was estimated after flowing powder-carrier gas only into the plasma jet and with powder-carrier gas feeding powder into the flame. On introduction of powder-carrier gas and powder loading the temperature of the jet was found to decrease appreciably down to 11%. The temperature of the plasma jet was estimated using the Atomic Boltzmann plot method.     

Polydisperse effects in jet spray flames

A laminar jet polydisperse spray diffusion flame is analysed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, droplet size and initial spray polydispersity play a critical role in determining potential flame scenarios. Some qualitative agreement for lift-off height is found when comparing predictions of the theory and sparse independent experimental evidence from the literature.     

Non-Stokes drag coefficient in single-particle electrophoresis:New insights on a classical problem

We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field, and found it to be markedly different from that of the Stokes drag. The drag coefficient,along with the measured electrical force, yield a mobility-zeta potential relation that agrees with the literature. By using the measured mobility as input, numerical calculations based on the Poisson–Nernst–Planck equations, coupled to the Navier–Stokes equation, reveal an intriguing microscopic electroosmotic flow near the particle surface, with a well-defined transition between an inner flow field and an outer flow field in the vicinity of electric double layer's outer boundary.This distinctive interface delineates the surface that gives the correct drag coefficient and the effective electric charge. The consistency between experiments and theoretical predictions provides new insights into the classic electrophoresis problem,and can shed light on new applications of electrophoresis to investigate biological nanoparticles.     

Interparticle potential and drag coefficient in nematic colloids

Magneto-optic tweezers were used for measurements of liquid-crystal-mediated forces between spherical beads with tangential anchoring in thin nematic samples. Repulsive force, which results from the quadrupolar symmetry of defects around the immersed beads, decreases proportionally to 1/x6, with x being the bead separation. The velocity with which the particles are pushed apart also follows the same separation dependence. We thus find the effective drag coefficient gamma(eff) independent of x for surface-to-surface distances as small as 10% of the bead diameter.     

Electron drag of dislocations in thin metal plates

The electron force dragging dislocations in a thin metal plate of thickness d has been calculated and the dependence of the drag force F on the parameter $\text{d}\text{l}$ (l is the electron free path length) determined for the cases $\text{d}\text{l ? 1}$ and $\text{d}\text{l ? 1}$. With d ? l, the drag force depends upon the nature of electron scattering on the plate surface. The drag force dependence on the sample size has also been considered for a d.c. magnetic field H parallel to the plate faces. In the case of diffuse scattering the second derivative of F(H) reveals a square root singularity at the value of the magnetic field where the electron orbit diameter is equal to the plate thickness.     

Translation drag coefficient of a self-similar assembly of spheres immersed in an incompressible fluid.

On the basis of deterministic fractals and the Rotne-Prager hydrodynamic interaction tensor, we confirm the asymptotic as well as the finite size scaling of the friction coefficient lambda of a self-similar structure. The fractal assembly is made of N spheres with its dimension varying from D < 1 to D = 3. The number of spheres can be as high as N approximately O(10(4)). The asymptotic scaling behavior of the friction coefficient per sphere is lambda approximately N(1/D-1) for D > 1, lambda approximately (lnN)(-1) for D = 1, and lambda approximately N(0) for D < 1. The crossover behavior indicates that while in the regime of D > 1 the hydrodynamic screening effect grows with the size, for D<1 it is limited in a finite range, which decays with decreasing D.     

Experimental investigation of spray characteristics of a liquid jet in a turbulent subsonic gaseous crossflow

Two perforated plates with different solidity ratios, S=50% and 67%, were used to investigate the effect of the velocity fluctuations of a subsonic gaseous crossflow on the spray characteristics of a liquid jet including droplet size and velocity distributions. The experiments were conducted over a range of jet-to-crossflow momentum flux ratio of q=16.5-172, and two gas Weber numbers of ?We_g=2.7 and 5.9, corresponding to the enhanced capillary breakup and bag breakup regimes, respectively. The experimental results of this study revealed that the distribution of droplets size associated with a turbulent and a uniform crossflow for each specific breakup regime were approximately identical. The bimodal and single peak distributions of droplets size, respectively, associated with enhanced capillary and bag breakup regimes were generally consistent with the literature reports. However, the transition of the liquid primary breakup regime from enhanced capillary to bag breakup mode was delayed in a turbulent crossflow compared to its uniform counterpart. The general behavior of droplets size-velocity profiles were also consistent with the literature reports. Nonetheless, complex variations in the distribution of droplets velocity when changing the crossflow turbulence intensity were observed and linked with the presence of instabilities on the liquid jet's surface. Finally, the present experiments allowed shedding more light on the reason why the breakup mechanisms of a liquid jet in a conventional uniform crossflow should not be generalized to predict the distinct breakup process of a liquid jet in a turbulent crossflow.     

Influence of a plasma jet on different types of tungsten

The influence of a plasma producing nonstationary thermal loads akin to edge-localized modes in a tokamak on different types of tungsten is investigated. Tungsten is irradiated by a jet of a hydrogen plasma generated in a plasma gun. The plasma density and velocity are on the order of 10²² m^?3 and 100–200 km/s, respectively, and the irradiation time is 10 μs. Two plasma flux densities, 0.70 and 0.25 MJ/m², are used. Structural modifications in irradiated single-crystal and hot-rolled tungsten samples, as well as in V-MP and ITER_D_2EDZJ4 tungsten powders, are examined. It is found that the plasma generates a regular crack network with a period of about 1 mm on the surface of the single-crystal, hot-rolled, and V-MP powder samples, while the surface of the ITER_D_2EDZJ4 powder is more cracking-resistant. The depth of the molten layer equals 1–3 μm, and the extension of intense thermal action is 15–20 μm. The material acquires a distinct regular structure with a typical grain size of less than 1 μm. X-ray diffraction analysis shows that irradiation changes the crystal lattice parameters because of the melting and crystallization of the surface layer. The examination of the V_MP tungsten powder after cyclic irradiation by a plasma with different energy densities shows that high-energy-density irradiation causes the most significant surface damage, whereas low-energy-density irradiation generates defects that are small in size even if the number of cycles is large.     

Control of spray spot shape in cold spray technology. Part 2. Spraying process

The present paper studies high-velocity heterogeneous flows produced with nozzle designs unconventional for cold spray; the process of coating deposition was studied under these conditions. The possibility of using unconventional gas dynamic tools (swirling of main flow, nozzles with permeable profiles and with slots in the supersonic part of nozzle) for control of particle distribution in the supersonic jet is investigated: this might be useful for obtaining a proper shape of the spray spot. These experiments offered a method of gas-dynamic design for the spray spot shape, which extends the possibilities of cold spray technique.     

Spin-echo measurements of the conduction electrons self-diffusion coefficient in a metal

The principle possibility of spin echo measurements of conduction electron orbital motion parameters is experimentally demonstrated, using metallic lithium as an example.     

Electrical conductivity of a metal powder struck by a spark

Equivalent resistance of a polydisperse powder layer struck by an electric spark is evaluated. Particles were computationally created and mixed using discrete element method; the mixing protocol homogenized particle size distribution within the sample. The conductivity was determined from the equivalent resistance network for the simulated powder bed. A plasma streamer attached to a particle on top of the sample. For each particle contact, an electrical breakdown was assumed; each individual contact resistance was calculated considering its geometry and plasma resistivity. Results of calculations compare well with the available experimental data.     

Transverse-energy correlations and jet shape in collisions of ultrarelativistic nuclei

Measurement of angular correlations between energy fluxes is one of the promising methods for analyzing the structure of events in ultrarelativistic heavy-ion collisions. The possibility of diagnosing the rescattering and the energy loss of hard partons in dense QCD matter is studied here on the basis of an analysis of the transverse-energy correlation function. It is shown that, if events are chosen for an analysis in a special way (that is, if at least one jet is required to have a transverse energy above some threshold) and if the procedure of background subtraction is applied in each event, the energy correlation function is sensitive to the parton energy loss and the angular spectrum of gluons emitted in a medium. The transverse-energy correlation function calculated for all events reflects the global structure of the transverse energy flux: it is independent of the azimuthal anisotropy of the energy flux for central collisions and is sensitive to the azimuthal anisotropy of the energy flux, reproducing all of its Fourier harmonics for noncentral collisions, but the coefficients of these harmonics are squared. A special correlation function in the vicinity of the maximum energy deposition in each event makes it possible to study changes in the jet shape. Within the conventional scenario of the scattering of hard jet partons on accompanying medium constituents, the correlation function is independent of the rapidity position of the jet axis and becomes much broader (symmetrically in the rapidity and azimuthal angle) than in proton-proton collisions. In the case of scattering on slow medium constituents, the broadening of the correlation function depends on the rapidity position of the jet axis and, in relation to the preceding scenario, increases sizably for jets of rather high rapidity.     

Resistive switching in a negative temperature coefficient metal oxide memristive one-port

A current-controlled memristive one-port was constructed from cobalt monoxide (CoO) using a traditional solid reaction method at 1150 °C in argon atmosphere. Hysteretic current–voltage (I–V) characteristics and resistance switching were investigated in the as-obtained Ag/CoO/Ag cell. Dependences of the I–V loop on voltage range (0 to 10, 11, and 12 V), voltage scan rate (0.1, 1, and 10 V/s), and temperature (323, 373, and 423 K) were reported. A thermistor model for materials with negative temperature coefficient (NTC) was proposed for explanation of the mechanism. An ideal NTC thermistor-based memristive one-port would broaden the applications of memristors and memristive devices.     

Electrothermal efficiency, temperature and thermal conductivity of plasma jet in a DC plasma spray torch

A study was made to evaluate the electrothermal efficiency of a DC arc plasma torch and temperature and thermal conductivity of plasma jet in the torch. The torch was operated at power levels from 4 to 20 kW in non-transferred arc mode. The effect of nitrogen in combination with argon as plasma gas on the above properties was investigated. Calculations were made from experimental data. The electrothermal efficiency increased significantly with increase in nitrogen content. The plasma jet temperature and thermal conductivity exhibited a decrease with increase in nitrogen content. The experiment was done at different total gas flow rates. The results are explained on the basis of dissociation energy of nitrogen molecules and plasma jet energy loss to the cathode, anode and the walls of the torch     

On the behavior of spray combustion in a turbulent spatially-evolving jet investigated by direct numerical simulation

Detailed investigations of turbulent spray combustion are very challenging due to the complexity of the underlying physicochemical processes. Experimentally, laboratory-scale burners are increasingly used to investigate these processes and support model development. One ultimate objective of these studies would be to deliver suitable benchmark data. In the present paper, the focus is similar but relying exclusively on direct numerical simulations. Conditions close that found in lab-scale burners are considered in the simulations, so that direct comparisons will ultimately become possible. The current analysis concentrates on the temporal evolution of temperature and concentrations of OH, CH₂O, and CH₄. The profiles of these variables show very complex features, therefore separate zones corresponding to characteristic physicochemical regimes have been tracked in time and space. It is found that, based on the temperature profile, four different zones coexist in the domain, associated to different degrees of competition between evaporation and reaction. It is observed that high concentrations of CH₂O and CH₄ can be used to delineate between three characteristic locations: 1) the evaporation zone; 2) close to the jet tip, at high temperatures; and 3) regions where evaporated droplets are entrained by mixing. This study demonstrates that direct numerical simulation of small spray burners can be used to deliver important information and to contribute useful benchmark data.