Difference of the W/C ratio,porosity and microscopical aspect between the upper boundary paste and the lower boundary paste of the aggregate in concrete

The author compared the W/C ratio, pore volume and microscopical aspect of the upper paste of coarse aggregate with those at the lower paste. Based on experiments using red ink, it can be said that the W/C ratio is partially increased near the lower surface of coarse aggregate in concrete. The total pore volume of the hardened paste near the lower surface of coarse aggregate is confirmed to be larger than that near the upper. With a scanning electron microscope, a larger number of fragile calcium hydroxide crystals was observed near the lower boundary of coarse aggregate than near the upper.     

Thermocapillary-Buoyancy Convection in Annular Two-layer System with a Radial Temperature Gradient

In order to understand the characteristics of thermocapillary-buoyancy convection in the annular two-layer system of 5cSt silicone oil and HT-70 with a radial temperature gradient, a set of two-dimensional numerical simulations are carried out using the finite-volume method. The annular two-layer system is heated from the outer cylindrical wall and cooled at the inner cylindrical wall. The radius ratio and the aspect ratio of the system are 0.2 and 0.05–0.1, respectively. Results show that the flow is steady for sufficiently small Marangoni number. When the Marangoni number exceeds a critical value, the additional cells appear near the interface and the cold inner wall, and then an unsteady multi-cellular structure is developed. The temperature fluctuation wave propagates from the cold inner wall to the hot outer wall. And the critical Marangoni number decreases with the increase of the aspect ratio.     

Dependence of flow characteristics of rectangular cylinders near a wall on the incident velocity

Numerically simulated results are presented for a family of rectangular cylinders with aspect ratios r ₁ (=b/a with height a and width b) ranging from 0.1 to 1.0 (square cylinder) to gain a better insight into the dependency of the aerodynamic characteristics on the operational dimensionless parameters, namely Reynolds number Re and aspect ratio r ₁. This work describes the flow from a long cylinder of rectangular cross-section placed parallel to a wall and subjected to a uniform shear flow. The flow is investigated in the laminar Reynolds number range (based on the incident stream at the cylinder upstream face and the height of the cylinder) at cylinder to wall gap height 0.5 times the cylinder height. The governing unsteady Navier?CStokes equations are solved numerically through a finite volume method on a staggered grid system using QUICK scheme for convective terms. The resulting equations are then solved by an implicit, time-marching, pressure correction-based SIMPLE algorithm for Reynolds number up to 1,000. The critical Reynolds numbers at which vortex shedding from the cylinder is started are specified for both the cases: far from the wall and near to the wall. It is reported that the vortex shedding from the rectangular cylinder of lower aspect ratio r ₁ (???0.25) becomes regular and insensitive to the Reynolds number, while the aerodynamic characteristics of the rectangular cylinders with higher aspect ratio r ₁ (???0.5) are strongly dependent on the Reynolds number.     

An experimental study of bubble behavior in a temperature gradient near a heated wall under low-gravity and microgravity conditions aboard NASA DC9 airplane

The behavior of a single bubble and a pair of bubbles under microgravity conditions has been investigated using the NASA-DC9 aircraft in order to understand the effects of various parameters and to control the bubble behavior in space. Silicone oil was used as the test liquid, and a nitrogen gas bubble was injected from the top wall under different experimental conditions. In an isothermal case, two different microgravity conditions were achieved by either fixing the experimental apparatus to the aircraft floor or freely floating the apparatus in the aircraft cabin. The bubble behavior was found to be clearly influenced by the quality of the microgravity environment, and variations of the bubble aspect ratio with the Bond number were presented. The results indicate that there is a critical Bond number of the order of 10⁻¹ which determines the bubble shape deformation. In the free-floating experiments, a temperature gradient was imposed on the liquid around the bubble near the heated top wall. Marangoni convection was expected to occur around the bubble and the bubble behavior was studied under various temperature gradients. The bubble aspect ratio was found to decrease with an increase in the Marangoni number. A theoretical model for the relation between the Marangoni flow around the bubble and the aspect ratio is proposed based on simple assumptions. Visualization of Marangoni convection around the bubbles using the photochromic dye activation method was successfully performed. The aspect ratios predicted by the model agreed with the experimental results reasonably well. Direct measurements of surface velocity are, however, necessary to further evaluate the validity of the model.     

Stability of Thermocapillary Convection in Annular Pools with Low Prandtl Number Fluid

In order to understand the stability characteristics of thermocapillary convection in an annular pool with low Prandtl number (Pr = 0.011) fluid, a linear stability analysis had been performed. The annular pool was heated from the outer cylindrical wall and cooled at the inner wall. Bottom and top surface were adiabatic. The results show that the critical Marangoni number and the critical wave number decrease with the increase of the aspect ratio, which is defined as the ratio of the depth to the gap width of the annular pool. When the aspect ratio exceeds 0.12, the critical wave number keeps almost constant. As the radius ratio increases, the critical Marangoni number decreases slightly, while the critical wave number increases. In the gravity condition, the effect of the buoyancy on the critical Marangoni number and wave number increases gradually with the increase of the aspect ratio.     

Effect of aspect ratio on the compaction characteristics and micromorphology of copper powders by magnetic pulse compaction

Magnetic pulse compaction (MPC) technology has the advantages of near net shape, high efficiency and density. In this paper, the effect of aspect ratio on the deformation characteristics of copper powders compacted by MPC technology was studied. Specifically, the compaction velocity, volume strain, micromorphology and micro hardness of compacts at various aspect ratios were obtained. Results showed that the compaction velocity and volume strain increased and then decreased with the increase of aspect ratio. The maximum compaction velocity reached 10.28 m/s at aspect ratio of 1.2. At the same compaction moment, the volume strain was decreased as the aspect ratio increased. Then the relationship between compaction pressure and volume strain was revealed. The aspect ratio range of the optimal compaction effect was 0.6–1.0, and the fluctuation degree of relative density was only 1.03%. The aspect ratio had less effect on the micromorphology distribution in the axial direction of the compacts. The micro hardness value of the upper surface was higher than that of the lower surface, and the edge position was greater than the center position.     

Effect of Surface Evaporation on Steady Thermocapillary Convection in an Annular Pool

In order to understand the effect of surface evaporation on thermocapillary convection in an annular pool, a series of numerical simulation on thermocapillary convection of the fluids with Prandtl number from 0.01 to 50 in the pure vapor environment were carried out. The results show that thermocapillary convection is always coupled with the evaporation process on the free surface. With the increase of evaporation Biot number, the surface temperature decreases, and the evaporation mass flux near the hot wall increases obviously. However, near the cold wall, the evaporation mass flux increases first, and then decreases. When Marangoni number is small, the total evaporation mass rate at free surface increases with the increase of evaporation Biot number; when Marangoni number is larger, it increases first and then approaches a constant value. The aspect ratio of the annular pool has a positive influence on the thermocapillary convection strength and the total evaporation mass rate. With the increase of Prandtl number, the surface temperature rises gradually and the evaporative mass flux increases, and the thermocapillary convection cell moves gradually toward the outer wall and the free surface. This effect decreases with the increase of evaporation Biot number When evaporation Biot number is smaller, the total evaporation mass rate increases with the Prandtl number; when Biot number is larger, Prandtl number has little impact on the total evaporation mass rate.     

Aerodynamic properties of a wing performing unsteady rotational motions at low Reynolds number

Summary The aerodynamic forces and flow structures of a wing of relatively small aspect ratio in some unsteady rotational motions at low Reynolds number (Re=100) are studied by numerically solving the Navier-Stokes equations. These motions include a wing in constant-speed rotation after a fast start, wing accelerating and decelerating from one rotational speed to another, and wing rapidly pitching-up in constant speed rotation. When a wing performs a constant-speed rotation at small Reynolds number after started from rest at large angle of attack (=35°), a large lift coefficient can be maintained. The mechanism for the large lift coefficient is that for a rotating wing: the variation of the relative velocity along the wing-span causes a pressure gradient and hence a spanwise flow which can prevent the dynamic stall vortex from shedding. When a wing is rapidly accelerating or decelerating from one rotational speed to another, or rapidly pitching-up during constant speed rotation, even if the aspect ratio of the wing is small and the flow Reynolds number is low, a large aerodynamic force can be obtained. During these rapid unsteady motions, new layers of strong vorticity are formed near the wing surfaces in very short time, resulting in a large time rate of change of the fluid impulse which is responsible for the generation of the large aerodynamic force.     

Light absorption efficiencies of gold nanoellipsoid at different resonance frequency

Light absorption efficiencies (defined as the ratio of the absorption cross section to the total extinction cross section at each resonance wavelength) of gold nanodisk and nanorod were calculated based on quasi-static approximation. The absorption efficiency solely depends on the frequency of surface plasmon resonance. With increasing resonance wavelength, the absorption efficiencies change in the same fashion for both nanodisk and nanorod. However, the resonance absorption at short wavelength is easy to be obtained by gold nanodisk, whereas the resonance absorption at longer wavelength is easy to be obtained by gold nanorod. High absorption efficiency (>98%) can be obtained in the visible region by increasing the aspect ratio of gold nanodisk. Although the longitudinal absorption efficiency of gold nanorod is relative lower by increasing the aspect ratio, the absorption efficiency is also tunable in the near infrared region, which makes it potentially useful in silicon solar cells and vivo applications. Jian Zhu and Jian-Jun Li have contributed equally to this work.     

Experiments on two immiscible fluids in spherical Couette flow

This paper deals with the experimental instabilities analysis of spherical Couette flow. We consider the flow of two immiscible fluids superimposed between concentric spheres when the outer sphere is fixed and the inner one rotates. The working fluids have rather different viscosities and thus different Reynolds numbers. The obtained results are compared with a reference case of filled gap using one fluid (Γ _max = 20). Experiments are performed for different aspect ratio values, and Laser photometric technique is used for visualization. Our analysis is mainly focused on the type of instabilities and their relationship with the laminar-turbulent transition regime. We intend to explore the combined effects of the aspect ratio and the interaction between the two superposed fluids on the appearance of different instability evolutions. The evolution of the phase velocity for different aspect ratio of heavy fluid Γ _HF = H _HF/d is presented. The immiscible fluids are separated by a liquid–liquid interface (water–oil). In order to control instability occurrence, Taylor number variation is presented versus aspect ratio. Instability phenomena are found to be the same as for the nominal case for large heavy fluid aspect ratios. The first equatorial symmetry breaking of the flow is observed for a critical value Γ _c  = 13 where the Taylor vortex flow is introduced with three stationary cells. For the same aspect ratio, the interaction of the immiscible fluids leads to the appearance of gravitational waves near the equatorial zone. A surface cell, starting before the appearance of Taylor vortices, is detected in the light fluid for low aspect ratios. This cell of Ekman type has not been observed before, to our best knowledge, in spherical Couette flow.     

ELLIPTICAL-ARC SURFACE CRACKS IN ROUND BARS

The crack aspect ratio influence on the stress-intensity factor of elliptical-arc edge flaws in solid round bars under tension or bending loading is discussed. The relative crack depth ξ=a/D between the depth a of the crack's deepest point and the bar diameter D ranges from 0.1 to 0.6, while the aspect ratio a/b of the flaw is made to vary from 0.0 (straight crack front) to 1.0 (circular-arc crack front). For each value of ξ being considered, the maximum stress-intensity factor is attained in correspondence to the deepest point on the crack front in the case of a/b= 0.0, while it is attained near the external surface in the case of a/b= 1.0. For intermediate aspect ratios, the stress-intensity factor presents the maximum value at the crack's deepest point for ξ≤ξ_CR and near the free surface for ξ≥ξ_CR, with the critical relative crack depth ξ_CR getting lower and lower as the parameter a/b is increased.     

Numerical simulation of aerosol collection in filters with staggered parallel rectangular fibres

 In this paper, filters with rectangular fibres arranged in a staggered and parallel array and placed transverse to the flow are studied numerically. A two- dimensional flow field is obtained by solving Navier–Stokes equations with the control volume method. Periodic boundary conditions are introduced in the calculation. In order to achieve higher accuracy, a second-order upwind scheme is adopted and a fine mesh is arranged near the fibre and the symmetrical plane of the flow field where large gradients in velocity are expected. Particle trajectories are calculated by solving the corresponding Lagrangian equation of motion to obtain the collection efficiency of a single rectangular fibre, in which positions of the approaching particles on the inlet plane of the flow field are randomly distributed according to the Monte-Carlo principle. The simulation considers all the important mechanisms of particle capture including interception, inertial impaction and Brownian motion. Effects of fibre aspect ratio, filter packing density, particulate size and Reynolds number on the collection efficiency are numerically determined. The volumetric packing density ranges from 0.4 to 4% and the particle diameter is from 0.01 μm to 2 μm. Reynolds number based on the height of computational domain varies from 20 to 100 and the aspect ratio is from 0.1 to 10. Simulations with and without Brownian motion are carried out for different Reynolds numbers, packing densities and aspect ratios and the results show that Brownian effects are significant for particles smaller than 1 μm. Received 25 May 2001     

Sensitivity and optimization of a high-Q sapphire dielectric motion-sensing transducer

A high-Q sapphire dielectric motion sensing transducer that operates at microwave frequencies has been developed. The device uses cylindrical whispering gallery modes of quality factor greater than 10 (5) at room temperature and greater than 10(8) at 4 K. The tuning coefficient of the transducer resonance frequency with respect to displacement was measured to be of the order of a few MHz/mum. An electromagnetic model that predicts the resonant frequency and tuning coefficient has been developed and was verified by experiment. We implemented the model to determine what aspect ratio and what dielectric mode is necessary to maximize the sensitivity. We found that the optimum mode type was a TM whispering gallery mode with azimuthal mode number of about 7 for a resonator of 3 cm in diameter. Also, we determined that the tuning coefficients were maximized by choosing an aspect ratio that has a large diameter with respect to the height. By implementing a microwave pump oscillator of SSB phase noise -125 dBc/Hz at 1 kHz; offset, we have measured a sensitivity of order 10 (-16) m/ radicalHz. We show that this can be improved with existing technology to 10(-18) m/ radicalHz, and that in the near future this may be further improved to 10(-19) m/ radicalHz.     

Microfluidic synthesis of Fe nanoparticles

A microfluidic reactor process is demonstrated for the synthesis of worm-like Fe nanoparticles (NPs) at room temperature. These high aspect ratio shaped Fe NPs can be further transformed into large ellipsoid species with refined crystal structures and increased aggregation by way of a sonication process, evidenced by their transmission electron microscope (TEM) images, selected area electron diffraction (SAED), X-ray Diffraction (XRD), X-ray absorption near K-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS). Due to the compensation effects on the magnetic properties of NPs from their increased size, enhanced magnetic coupling, refined crystal structure and the reduced aspect ratio, the sonication-treated Fe NPs show a slightly enlarged coercivity (Hc), an enhanced saturation magnetization (Ms) and a slight suppress in the increased blocking temperature (Tb).     

纳米管场发射的电场计算

利用电磁场理论计算出单根纳米管场发射时其尖端附近的电势和电场分布。结果表明 ,纳米管尖端表面电场非常强 ,随着距离尖端表面的距离的增加 ,电场迅速下降 ;尖端附近的场强与纳米管场发射有低的阈值电压相符合。计算还给出在保持极板间距离和电压不变的情况下 ,纳米管长径比越大 ,尖端电场越强 ,因此 ,具有更低的阈值电压     

Effect of the vicinal character of substrate on nucleation of nanoislands in lattice-mismatched systems

A model of the nucleation of three-dimensional nanoislands by the Stranski-Krastanov mechanism on a lattice-mismatched vicinal substrate is proposed. It is shown that the work of island formation at the step edge is a function of two variables: number of particles in the island and its aspect ratio. The activation barrier for the nucleation corresponds to a saddle point of the work of formation. The vicinal character of the surface leads to a change in the work of island formation and significantly decreases the nucleation activation barrier. Data on the nucleation activation barrier and energetically favorable aspect ratio, which are necessary to simulate particular systems, are obtained.     

An unsteady lifting-line theory

A lifting-line theory is developed for wings of large aspect ratio undergoing time-harmonic oscillations, uniformly from high to low frequencies. The method of matched asymptotic expansions is used to enforce the compatibility of two approximate solutions valid far from and near the wing surface. The far-field velocity potential is expressed as a distribution of normal dipoles on the wake, and its expansion near the wing span leads to an expression for the oscillatory downwash. The near-field flow is two-dimensional. A particular solution is obtained from strip theory and a homogeneous component is added to account for the spanwise hydrodynamic interactions. The compatibility of the inner and outer solutions leads to an integral equation for the distribution of circulation along the wing span. In the zero-frequency limit it reduces to that in Prandtl's lifting-line theory, and for high frequencies it tends to the two-dimensional strip theory. Lift computations are presented for an elliptic and a rectangular wing of aspect ratio A = 4.     

Numerical Simulation of Thermocapillary Convection in a Shallow Rectangular Cavity Under the Action of Combining Horizontal Temperature Gradient with Vertical Heat Flux

In order to understand the influence of the vertical heat flux on thermocapillary convection, we conducted a series of unsteady two-dimensional numerical simulations of thermocapillary convection in a differently heated shallow rectangular cavity with vertical heat flux on the bottom by means of the finite volume method. The cavity was filled with the 1cSt silicone oil (Prandtl number Pr = 13.9) and aspect ratio is 30. It is found that a small vertical heat flux has slightly influence on the flow pattern of stable or unstable thermocapillary convection. However, the critical Marangoni number increases first, and then decreases with the increase of the heat flux. And the flow pattern of the oscillatory thermocapillary convection transits from a series of the rolls rotating clockwise and moving from the cold wall to the hot wall to the single roll near the hot wall and a series of rolls near the cold wall, further, two series of rolls moving from the hot wall and cold wall towards the hot spot with the maximum temperature. With the increase of the Marangoni number, the period and the wavelength of the oscillatory thermocapillary convection increase, but the wave speed decreases.     

Separation characteristics of fluid flow inside two parallel plates with wavy surface

Separation characteristics of fluid flow inside two parallel wavy plates for steady-laminar flow is investigated numerically in the present study. Governing equations are discretized using control volume based finite-volume method with collocated variable arrangement. SIMPLE algorithm is used and SIP solver is applied for solution of system of equations. Effect of surface waviness (defined by amplitude to average interwall spacing ratio, a/H) and aspect ratio (defined by wavelength to average interwall spacing ratio, w/H) on separation characteristics of fluid flow is presented. The present work has been carried out for surface waviness a/H=0-0.3, aspect ratio w/H=1.5-2.25. A critical Reynolds number (Re_c) is used to identify the appearance of first separation of fluid flow in the channel. Critical Reynolds (Re_c) number is calculated for wide range of surface waviness and aspect ratio. The structure of separation bubble depends strongly on waviness of the surface and aspect ratio for a particular Reynolds number and changes little with wave number (n). Finally pressure drop characteristics is presented in terms of average friction factor as a function of Reynolds number.     

水下航行体通气超空泡形态及阻力特性试验研究

利用中速空泡水洞开展了通气超空泡试验,并利用已有的经验公式得到了通气空化数,对通气超空泡形态及阻力特性进行了研究。研究表明,通气超空泡的长度和长细比都随通气空化数的增大而减小,航行体阻力系数则随长细比增大而减小。在流速恒定情况下,随通气率的增大,通气空化数逐渐减小,呈指数规律衰减。通气空化数越小,超空泡航行体的减阻效果越好,速度越大,超空泡减阻效果越明显。随通气空化数的减小,超空泡航行体尾部沾湿区域阻力系数也逐渐减小。