The geometry of the „cones“ used in cone- and-plate viscometers

Summary The geometry of eighteen cones used in two commercial cone- and-plate viscometers has been determined by measuring the profiles along four radii at right angles to each other. It is found that the cones are not truly conical and that there is considerable uncertainty in assigning a value to the cone angle and to the position of the apex. Large differences were also found between the calculated mean values and those quoted by the manufacturers. Thus the latter values should not be used without confirmation. The effect of these results on viscosity determination is discussed.     

Determination of normal-stress differences in steady shear flow

Rheologica Acta - A new wide-gap concentric cylinder apparatus has been constructed for the measurement of pressures generated in Couette flow; diaphragm-capacitance pressure transducers in the...     

Determination of normal stress differences and hole pressure errors in shear flow using a plate-and-truncated-cone apparatus

Rheologica Acta - On the basis of assumptions (i)-(v), an eq. [3] is derived which enables one to determine the normal stress difference p11-p33 from values of pressure p(r) (measured by means of...     

Aerodynamics of indirect thrust measurement by the impulse method

The aerodynamic aspects of indirect thrust measurement by the impulse method have been studied both experimentally and numerically. The underlying basic aerodynamic principle is outlined, the phenomena in subsonic, supersonic and arc-heated jets are explored, and factors affecting the accuracy of the method are studied and discussed. Results show that the impulse method is reliable for indirect thrust measurement if certain basic requirements are met, and a simple guideline for its proper application is given.     

Evaluation of thrust measurement techniques for dielectric barrier discharge actuators

Despite its popularity in the recent literature, plasma actuators lack a consistent study to identify limitations, and remedy thereof, of various thrust measurement techniques. This paper focuses on comparing two different experimental techniques commonly used to measure the global, plasma-induced thrust. A force balance is used to make a direct measurement of the thrust produced, which is then compared with a control volume analysis on data obtained through particle image velocimetry. The local velocity measured by particle image velocimetry is also validated with a fine-tip pressure probe. For the direct thrust measurements, the effect of varying the actuator plate length upon which the induced flow acts is investigated. The results from these tests show that the length of the actuator plate is most influential at higher voltages with the measured thrust increasing as much as 20?% for a six times reduction in the length of the plate. For the indirect thrust measurement, the influence of the control volume size is analyzed. When the two methods are compared against each other, good agreement is found when the control volume size has a sufficient downstream extent. Also, the discharge length is optically measured using visible light emission. A linear correlation is found between the discharge length and the thrust measurements for the actuator configurations studied. Finally, the energy conversion efficiency curve for a representative actuator is also presented.     

Two-degree-of-freedom vortex-induced vibrations using a force assisted apparatus

We report results from two-degree-of-freedom vortex-induced vibration tests on a flexibly mounted, rigid, smooth cylinder in cross-flow. The tests are performed for six in-line natural frequency to transverse natural frequency ratios. The Reynolds number based on diameter ranged from 11 000 to 60 000. To reduce structural damping in both directions, an apparatus utilizing two linear motors was used. Increasing the in-line to transverse frequency ratio caused a shift in the peak amplitude response to increasingly higher reduced velocities; and at a frequency ratio of 1.9, two distinct response peaks appear, in agreement with earlier experiments by Sarpkaya in 1995. Other comparisons are made with the low mass-damping, two-degree-of-freedom experiments by Jauvtis and Williamson in 2004. The frequency ratio affects the phase lag between transverse and in-line oscillations and hence the shape of the cylinder orbital.     

喷水推进器推力的CFD计算方法研究

简要介绍获取喷水推进器推力的理论、试验及数值计算(CFD)方法,重点研究采用动量流量法和壁面积分法计算喷水推进器推力的CFD方法。采用多块网格技术,用六面体结构化网格和四面体非结构化网格相结合的混合网格离散计算区域,采用稳态多参考系方法求解RANS方程,对喷水推进器进水流道、叶轮、导叶体和喷口所组成的整个流场进行数值计算。计算中采用了k-ε湍流模型和标准壁面函数,对用动量流量法计算推力方法中所需的假想流管分界面和进口面的求取做了分析,将两种方法计算的推力与厂商提供的推力特性曲线进行了对比。结果表明,采用CFD计算和分析方法来研究喷水推进系统推力性能是可行、可信的。     

Full-field strain measurement using a luminescent coating

In this paper we describe an optical-based technique, called strain sensitive skin (S³), for measuring in-plane strain data on structural members under static load. The technique employs a coating consisting of a luminescent dye and polymer binder that is applied to the surface of a test part via conventional aerosol techniques. Proper illumination stimulates the dye, which in turn emits higher wavelength luminescence. The excitation and emission intensities have different wavelengths; therefore, enabling optical filtering to separate the two signals. The optical strain response is intensity based. A network of randomized microcracks within the binder scatters the waveguided luminescence from the excited dye molecules. The amount of scattered luminescence is related to the changes in the microcrack openings and orientations via mechanical strain. Various calibration tests show the optical strain response to be proportional to the sum of in-plane principal strains. With this new experimental testing tool, full-field high-resolution strain measurements can be acquired. The optical strain response of this new sensor is minimally dependent on viewing and lighting directions, rendering the technique viable to imaging and determining strain fields for three-dimensional complex geometries.     

A novel dynamic friction experiment using a modified kolsky bar apparatus

A novel dynamic friction experiment using the Kolsky bar concept was developed. The technique is complementary to the plate impact and other macroscopic friction experiments in the sense that sliding velocities and pressures not attainable otherwise can be investigated. The experimental results reported in this article show that the technique provides accurate and repeatable measurement of time-resolved friction. The apparatus is simpler and easier to operate than the plate impact facility. However, it cannot achieve the same level of contact pressure. Several material pairs have been investigated. In particular, the kinetic friction coefficient of Ti-6Al-4V sliding against WC/Co (cermet) and 4340 steel sliding against WC/Co were measured and compared with the values reported by Prakash and Clifton in 1993. Atomic force microscopy is used to characterize the surface topography before and after the friction tests.     

The measurement of square channel velocity profiles using a microcomputer-based image analysis system

A technique was developed to perform automated velocity measurements from a sequence of particle images. A very thin sheet of laser light allows determination of essentially two-dimensional velocity profiles in very small conduits. A four image sequence was captured by a microcomputer-based frame grabber. After thresholding to eliminate particles not perfectly centered in the sheet of light, the sequence of pseudo-colored images of a given particle is used to determine its velocity. By measuring several two-dimensional velocity profiles across a square channel, the complete three-dimensional velocity profile was assembled. The experimentally measured velocity profile agrees closely with the known theoretical velocity profile for flow in a square channel.     

The measurement of the material parameters of viscoelastic fluids using a rotating sphere and a rheogoniometer

Summary In this work, measurement of the flow field around a rotating sphere has been used to obtain the material parameters of a second-order Rivlin-Ericksen fluid. Experiments were carried out with a Laser-Doppler anemometer to obtain the velocity distribution and usingGiesekus' analysis, the material parameters for the second-order fluid were obtained.

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Zusammenfassung In dieser Untersuchung wird die Ausmessung des Strömungsfeldes um eine rotierende Kugel dazu verwendet, um die Stoffparameter einer Rivlin-Ericksen-Flüssigkeit zweiter Ordnung zu erhalten. Die Experimente zur Bestimmung der Geschwindigkeitsverteilung werden mit einem Laser-Doppler-Anemometer durchgeführt, und zur Auswertung der Parameter der Flüssigkeit zweiter Ordnung wird eine Analyse vonGiesekus benutzt.

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Notations A ₁,A₂ Rivlin-Ericksen tensor - A ² Parameter used in eq. [12] - a Radius of the sphere - B Parameter used in eq. [12] - I Unit tensor - m ₀(₁ – ₂)/a², parameter used by ref. (8) - N ₁,N₂ First and second normal stress difference - p Isotropic pressure - Radial distance from the centre of the rotating body - S ₁,S₂ Stress tensor - v _r,v,v Velocity components in a spherical coordinate system - ₀,₁,₂ Material parameters used in eq. [2] - Shear rate - _a Apparent voscosity - ₀ Zero-shear viscosity - Angle measured from the axis of rotation - Fluid density - Stream function - Shear stress - Angular velocity With 3 figures     

Temperature measurement of a curved surface using thermographic phosphors

An optical technique for surface temperature measurement based on the fluorescent emission of rare earth ion-doped phosphors was demonstrated in an experiment with a heated cylinder in crossflow. In this experiment, a uniform heat flux was imposed by applying a constant voltage across the thin stainless steel cylinder surface to produce surface temperatures between 24°C and 55°C. The fluorescent emission of a thermographic phosphor, lanthanum oxysulfide doped with europium (La₂0₂S:Eu³⁺) deposited on the surface, was recorded to determine the temperature distribution at the curved surface. When excited by ultraviolet radiation, the phosphor emits a spectrum containing certain emission lines, the intensities of which vary with temperature. For a single temperature-sensitive line, ratios of the intensity at a reference temperature to the intensity at different temperatures were correlated as a function of surface temperature. The use of intensity ratio correlations avoids complications due to geometric (viewing angle) effects. Digitized images of the cylinder permitted calculation of surface temperatures and local Nusselt numbers. Differences between surface temperatures measured by calibrated thermocouples and temperatures determined from the phosphor technique were at most 1.2°C.     

The development and use of a torsional Hopkinson-bar apparatus

A technique is described by means of which torsional waves of large, essentially constant amplitude can be generated in an elastic bar. Waves with rise times of order 25 μs and maximum angular velocities of order 10³ rad.s^?1 have been achieved and used to test tubular specimens at shear-strain rates up to 15×10³ s^?1. Results are presented for mild steel tested at 2×10³ s^?1, and it is shown that the flow stress correlates well with the trend found at lower rates using conventional methods. The measured drop of stress at yield, however, was considerably smaller in the present tests than in earlier work; this is attributed to the generation of flexural waves which reach the specimen at the same time as the torsional wave.     

Stability of a thick elastic plate under thrust

When a rectangular plate of incompressible neo-Hookean elastic material is subjected to a thrust, bifurcations of the flexural or barreling types become possible at certain critical values of the compression ratio. The states of pure homogeneous deformation corresponding to these critical compression ratios are states of neutral equilibrium. Their stability is investigated on the basis of an energy criterion, without restriction on the thickness of the plate.The critical state corresponding to the lowest order flexural mode is found to be stable (unstable) if the aspect ratio (thickness/length) is less (greater) than about 0.2. Agreement with the classical Euler theory is established in the limiting case of small aspect ratio.     

Three-dimensional wake structure measurement using a modified PIV technique

Three-dimensional vortical structures have been measured in a circular-cylinder wake using particle imaging velocimetry (PIV) for the Reynolds number range of 2×10³ to 1×10⁴. The PIV was modified, compared with the conventional one, in terms of its light sheet arrangement to capture reliably streamwise vortices. While in agreement with previous reports, the presently measured spanwise structures complement the data in the literature in the streamwise evolution of the near-wake spanwise vortex in size, strength, streamwise and lateral convection velocities, shedding new light upon vigorous interactions between oppositely signed spanwise structures. The longitudinal vortices display mushroom patterns in the (x, z)-plane in the immediate proximity to the cylinder. Their most likely inclination in the (x, y)-plane is inferred from the measurements in different (x, z)-planes. The longitudinal vortices in the (y, z)-plane show alternate change in sign, though not discernible at x/d > 15. They decay in the maximum vorticity and circulation rapidly from x/d = 5 to 10 and slowly for x/d > 10, and are further compared with the spanwise vortices in size, strength and rate of decay.     

Enhanced-displacement measurement using a generalized formulation for double-aperture specklegrams

A generalized form for the intensity distribution at the observation plane of a spatially filtered double-aperture specklegram is derived using classical Fresnel diffraction theory. The simplified formulas obtained by previous authors are found to be a special case of the general formula. In addition, both analytical and experimental results show that a variation in sensitivity of the double-aperture method exists within any given halo in the filter plane, even in the central D.C. halo. Also, in the D.C. halo, analytical and experimental work has shown that displacements can be measured parallel to the embedded grid present on the image, thereby demonstrating that a moiré analogy is inadequate to fully describe the phenomena. Finally, a comparison of experimental data to theoretical results based on the generalized formulation demonstrates (a) that the limits proposed by the authors for measuring displacements by use of the double-aperture method are valid, and (b) that the double-aperture method can be extended to measure displacements beyond those predicted by a moiré interpretation.