Numerical and experimental study on the flow history effects of axial flow on the Couette–Taylor flow

In this research, experimental and numerical techniques are used to study the flow history effects of axial flow on the Couette–Taylor flow. For the experimental investigation, the flow is visualized using the PIV technique with reflective particles with a density of 1.62 g/cm³. Dispersed in a solution, the particles have a strong refraction index equal to 1.85. In this study, two protocols are adopted to study the effect of an axial flow superimposed on a Couette–Taylor flow, and of the history of the flow. The first one, the direct protocol, consists of imposing an azimuthal flow to the inner cylinder. In this case, when the regime is established, the axial flow is superimposed. The second protocol, the inverse protocol, consists of imposing first the axial flow in the gap of the system, after which an azimuthal flow is conveyed. The Couette–Taylor flow with axial flow is strongly dependent on the flow history (the protocol). Thus, the flow structures and development for different protocols are studied and analyzed here experimentally and numerically. In addition, from the numerical results, mathematical models for the two protocols are presented. For the direct protocol, a new relation between the axial Reynolds number, which stabilizes the Couette–Taylor flow, and the Taylor number is presented; for the inverse protocol, a new mathematical model for the critical Taylor number is developed as a function of the axial Reynolds number and also the first critical Taylor number without axial flow.     

Prograde and retrograde flow past cylindrical obstacles on a ��-plane

The problem of viscous prograde (eastward) and retrograde (westward) flow past a cylindrical obstacle on a ??-plane is considered. The barotropic vorticity equation is solved using a numerical method that combines finite difference and spectral methods. A modified version of the ??-plane approximation is proposed to avoid computational difficulties associated with the traditional ??-plane approximation. Numerical results are presented and discussed for flow past a circular cylinder at selected Reynolds numbers (Re) and non-dimensional ??-parameters ( ${\hat{\beta}}$ ) as well as for flow past an elliptic cylinder of a fixed aspect ratio (r?=?0.2) at inclination angles of ±15°, 90° and selected Re and ${\hat{\beta}}$ . In prograde flows, it is found that the ??-effect acts to suppress boundary-layer separation and to excite a standing Rossby lee wavetrain, as observed in previous works. In retrograde flows, the boundary-layer separation region is elongated and westward propagating Rossby waves are excited.     

Comparison of trust-region-based and evolutionary methods for optimization of flow geometries †

Optimization has become increasingly important in computer-aided engineering, and applications of engineering optimization to design, control, operations, and planning already exist. Since in the context of numerical flow simulation no information on the gradient information of the objective function is available, or is very difficult to obtain, in such optimization cases it is advantageous to use an optimization technique which does not directly depend on the derivative. The aim of this study is to investigate two optimization techniques, the trust-region-based method and the evolutionary algorithm technique, and compare them quantitatively with respect to efficiency, quality, and working strategy. A tool based on free-form deformation (FFD) is employed for the variation of flow geometry. This simulation tool is the parallel multigrid flow solver FASTEST, which uses a fully conservative finite-volume method for solving the incompressible Navier–Stokes equations on a non-staggered cell-centred grid arrangement. The optimization tools are investigated by considering the optimization of the connection of two pipes with respect to the minimization of the pressure drop. This problem can be considered as a representative test case for a practical three-dimensional flow configuration.     

The mechanisms of yield and plastic flow in HgTe and Cd x Hg1−x Te

Single crystals of HgTe and Cd _x Hg_1–x (0.18<x<0.30), oriented for single slip, have been deformed in four-point bending at strain rates 10^–4 sec^–1 and temperatures from –11 to +84° C for HgTe, and 20 to 195° C for Cd _x Hg_1–x Te. At the lowest temperatures, the stress-strain curve exhibits a sharp yield relaxation and subsequent zero work hardening regime, as commonly observed for other semiconductors. Experiments show that the yielding mechanism is that proposed by Johnston and Gilman for LiF. Possible explanations for the post-yield zero work hardening phenomenon are discussed. The influence of composition, temperature and strain rate on the stress-strain behaviour are reported. At 20° C, the upper and lower yield stresses ( _uy and _1y ) increase with increasingx in qualitative agreement with our earlier hardness results. For Cd_0.2Hg_0.8Te, _1y varies with temperature,T, at a strain rate of 10^–4 sec^–1, according to _1y exp (Q/kT) whereQ is 0.16 eV. For HgTe the comparable value is 0.11 eV. Atx=0.25 and constant temperature, _1y depends on strain rate as _1y ^1/n wheren is 4. The stress level for deformation of Cd_0.2Hg_0.8Te at 10^–4 sec^–1 and 20° C is 2–3 kg mm^–2, comparable with that for InSb at 300° C or Si at 1000° C. Strain rate cycling tests on Cd _x Hg_1–x Te give values of activation volumeV* around 10b³ at 20° C, independent of plastic strain (up to 2–3%), suggesting that deformation in these alloys is controlled by the Peierls mechanism, as observed in other II–VI compounds.     

Aluminium and Al–4·5Cu alloy end chill: structural observation and heat flow analysis

Abstract

End chill experiments were performed on aluminium and Al–4·5Cu (wt-%) in order to study the effect of melt superheat (20–150 K), chill material (copper, iron, or sand), and specimen length (890–230 mm) on the type and size of macrostructure. Increasing melt superheat increases the length of columnar zone, which is shorter for the alloy than for the commercial purity metal. The columnar fraction increases with the thermal conductivity of the chill material and the heat transfer coefficient. The results are correlated with the temperature gradient, solidification rate, and growth rate obtained from a heat flow model. The columnar to equiaxed transition is found to occur at a critical temperature gradient and growth rate. These critical values differ with alloy composition. The grain size of columnar and equiaxed grains is found to follow a power relationship with solidification rate.

MST/1709     

Pressure drop of slush nitrogen flow in converging–diverging pipes and corrugated pipes

Cryogenic slush fluids such as slush hydrogen and slush nitrogen are solid–liquid, two-phase fluids. As a functional thermal fluid, there are high expectations for use of slush fluids in various applications such as fuels for spacecraft engines, clean-energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. Experimental flow tests were performed using slush nitrogen to elucidate pressure-drop characteristics of converging–diverging (C–D) pipes and corrugated pipes. In experimental results regarding pressure drop in two different types of C–D Pipes, i.e., a long-throated pipe and a short-throated pipe, each having an inner diameter of 15 mm, pressure drop for slush nitrogen in the long-throated pipe at a flow velocity of over 1.3 m/s increased by a maximum of 50–60% as compared to that for liquid nitrogen, while the increase was about 4 times as compared to slush nitrogen in the short-throated pipe. At a flow velocity of over 1.5 m/s in the short-throated pipe, pressure drop reduction became apparent, and it was confirmed that the decrease in pressure drop compared to liquid nitrogen was a maximum of 40–50%. In the case of two different types of corrugated pipes with an inner diameter of either 12 mm or 15 mm, a pressure-drop reduction was confirmed at a flow velocity of over 2 m/s, and reached a maximum value of 37% at 30 wt.% compared to liquid nitrogen. The greater the solid fractions, the smaller the pipe friction factor became, and the pipe friction factor at the same solid fraction showed a constant value regardless of the Reynolds number. From the observation of the solid particles’ behavior using a high-speed video camera and the PIV method, the pressure-drop reduction mechanisms for both C–D and corrugated pipes were demonstrated.     

Constitutive equations for inhomogeneous plastic flow and application to Lüders band propagation

When deformation is not macroscopically homogeneous, the structure variables that define the mechanical state of a material depend on position and their space derivatives appear in the constitutive equations for plastic deformation. The general form of these equations in uniaxial deformation is written for the case of a single structure variable. Constitutive equations for deformation by Lüders bands are derived in which the density of mobile dislocations plays the role of the structure variable. The equations exemplify the general form of a constitutive law for inhomogeneous deformation. Strain and strain rate profiles in a tensile specimen traversed by a steady state Lüders band are obtained by integration of the constitutive equations and the band parameters are determined. Comparison with experimental results is quite favourable in spite of the simplifications introduced.     

A Comparison study of distributions of molecular flow field in spherical and pillar systems

The distribttions of the molecular flow field in spherical and pillar system are simulated by Monte Carlo method in this paper. It is shown that as the spherical container is replaced by a pillar container in calibration or measurement, the effective measurement area for smaller systematic error is around the equator of the pillar container within 1/5 of the pillar height. By comparing and analyzing the measurement system os the dynamic flow method it is firmly believed that the systematic error using foreflow measurement method is smaller than the one using forepressure measurement method. The revision coefficients P=Q/C_2, P=C_1P_1/(C_1 C_2) for spherical and pillar measurement system are obtained separately. The results in this paper provide reliable basis for the design and application of various vacuum system.     

Activation energies for superplastic tensile and compressive flow in microduplex α/ß copper alloys

Logarithmic stress against strain rate curves have been determined at various temperatures for a superplastic commercial/ nickel-silver alloy strained in tension, and a laboratory prepared microduplex alloy of nominally similar composition strained in compression. The shapes of the curves were found to be affected by grain growth at high temperatures and strain softening at low temperatures. After taking these factors into account, it was apparent that with decreasing strain rate in both alloys a change in deformation mechanism occurred giving rise to a Region I of low strain-rate sensitivity. By confining activation energy (Q) measurements to temperatures at which steady-state deformation occurred, it was found thatQ for Region II was very similar to that measured for grain boundary diffusion in the phase of a nickel-silver alloy of similar composition, whileQ for Region I was substantially higher than that for lattice diffusion. Values of strain-rate sensitivity andQ were found to be similar for each direction of applied stress.     

Vibrations of an infinitely long flowing liquid jet—Influence of spin and axial flow

For an infinitely long liquid column the influence of axial flow velocity and spin has been investigated. The results are exhibited for axisymmetric mode m=0 and asymmetric modes m=1 and 2. A frictionless liquid shows with the increase of axial flow an increase of the frequency in flow direction and a decrease of the oscillation frequency in the opposite flow direction for axisymmetric motion. It also means that a larger surface tension, larger diameter or larger density of the liquid column exhibit the same behavior. For asymmetric motions the effect of axial velocity w₀ is the opposite. With increasing axial wave length the natural frequencies decrease. At certain axial speed magnitudes both waves move in flow direction with different magnitude. The effect of increasing spin is a decrease of natural frequencies and an instability for smaller axial wave lengths. Viscous effects show usually smaller oscillation frequencies.     

Triboelectric separation of a starch-protein mixture – Impact of electric field strength and flow rate

Triboelectric separation is a method for separating dry particulate systems due to their different electrostatic chargeability. Previous applications are limited to the separation of coarse powders. The aim of the present study is to examine the influence of the flow conditions and the influence of the electric field strength on the separation efficiency of starch and protein particles. Very fine organic powders are separated in a simple bench scale electrostatic separator to extend this technique to powders below 50?µm. The influence of different gas flow rates in the turbulent flow regime on particle charging and subsequent separation is investigated.As an organic model substrate, a mixture of barley starch and whey protein was used. The tribocharger consists of a PTFE charging tube and a rectangular separation chamber where an electric field is applied between two electrodes. The particles are conveyed through the charging tube and charged by frictional contact with the tube wall. It is shown that different gas flow rates at a turbulent flow regime in the charging tube did not change the separation characteristics. In contrast, increasing electrical field strength increases separation efficiency of protein particles regardless of gas flow conditions. The proportion of starch at the anode is the same for all the investigated parameters.     

Metal solidification–nucleation–rate model under coupling effects of shearing flow and vibration

This paper proposes the influence factors of coupling effects of shearing flow and vibration on diffusion coefficient and critical nucleation energy during metal solidification. Based on this proposal, a metal solidification–nucleation–rate model under coupling effects of shearing flow and vibration is established. Verification experiment using Al–7Si alloy is carried out. When vibration frequency and melt flow velocity are zero, the results calculated by the above model agree with that calculated by Turnbull’s theory. The results calculated by the above model under coupling effects of shearing flow and vibration agree with the experimental results, with the error within 0·2–14·3%. So the established model can calculate and explain the nucleation rate of melt under coupling effects of shearing flow and vibration.     

Experimental and numerical investigation of effects of particle shape and size distribution on particles’ dispersion in a coaxial jet flow

In this study, an experimental and a numerical investigations are performed to investigate the effect of particle’s shape and size distribution on its dispersion behavior. Firstly, particle dispersion of pulverized coal and spherical polymer particles is observed by Particle Image Velocimetry (PIV) technique in the experiment. Secondly, a simulation is performed to analyze the particle dispersion in detail. Spherical and spheroidal motion models are applied to particle’s movement to investigate the shape effect. Furthermore, monodisperse and polydisperse for particles are applied to investigate the size distribution effect on the dispersion. Experimental results show that in the jet turbulence flow, pulverized coal particles, which have complex shapes and various sizes, have quite different dispersion behavior compared to spherical particles. In terms of the results of the simulation, this difference is mainly caused by the size distribution effect. Although particle’s shape affects the dispersity, it is weakened by the size distribution effect.     

Stability of Bödewadt flow

The stability of the flow produced over an infinite stationary plane in a fluid rotating with uniform angular velocity at an infinite distance from the plane is considered. The basic flow is an exact solution of the Navier-Stokes equations making it amenable to theoretical study. An asymptotic investigation is presented in the limit of large Reynolds number. It is shown that the stationary spiral instabilities observed experimentally can be described by a linear inviscid stability analysis. The prediction obtained for the wave angle of the disturbances is found to agree well with the available experimental and numerical results.     

Hydromagnetic turbulent shear flow—II

Hydromagnetic turbulent shear flow between two infinite uniformly porous planes in the presence of (i) axial and (ii) transverse magnetic field was studied in Flow—I by adopting the semi-empirical approach first initiated by Kampe de Feriet[1] and developed later on by Pai[2,3] for studying turbulent shear flow through channels. In this paper (Flow—II) the same approach has been used for obtaining the mean distributions for velocity, magnetic field and temperature compatible with similar distributions for the corresponding laminar flow for hydromagnetic turbulent shear flow through a porous walled annular channel in the presence of a uniform radially transverse magnetic field.     

Hydromagnetic turbulent shear flow—I

Equations have been derived for describing fully developed hydromagnetic turbulent shear flow of viscous, incompressible, electrically conducting fluid between two infinitely-extending uniformly porous planes. Two types of magnetic field alignments have been considered (i) axial, i.e. parallel to the planes and (ii) transverse. Mean distributions for velocity, magnetic field and temperature have been obtained by adopting the semi-empirical approach first initiated by Kampe-de-Feriet[1] and developed later on by Pai[2,3] in the study of hydrodynamic turbulent shear flow through channels.     

Effect of amphiphilic coupling agent on heat flow and dielectric properties of flax–polypropylene composites

The study on heat transport in composites is of fundamental importance in engineering design and for tailoring thermal and mechanical behaviour of materials. In this study, the thermal conductivity and thermal diffusivity of flax reinforced polypropylene (PP) composites were determined at room temperature. Chemical modification in the form of a biodegradable zein coating was applied to the flax nonwovens. The effect of fibre loading and chemical modification on the thermo-physical properties was investigated. Dielectric permittivity studies were also evaluated and the dielectric constant of fibre reinforced composites was found to be higher than that of PP. The heat flow and crystallinity effects of the composites were also determined by differential scanning calorimetric (DSC) studies. Zein modification of the flax fibres resulted in a decrease of thermal conductivity and diffusivity which was attributed to a decrease in velocity and mean free path of phonons due to increase in interfacial adhesion.