On turbulent secondary flows in pipes of noncircular cross-section

The origin of turbulent secondary flow in pipes of noncircular cross section is examined from a theoretical standpoint. It is proven mathematically that secondary flows result from a nonzero difference in the normal Reynolds stresses on planes perpendicular to the axial flow direction. Furthermore, it is shown that the $\text{K-?}$ model of turbulence has no natural mechanism for the development of secondary flow while the currently popular second-order closure models do. The implications that this has on turbulence modeling are discussed briefly.     

Structure and photoluminescence of the a-Gex Se1-x, system

Alloys of this system in the composition range 0 < x < $\text{1}\text{2}$ are composed of Se chains, Se₈ rings, Ge(Se_{$\text{1}\text{2}$})₄ tetrahedral, Ge₂$\text{(Se}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}_6$ ethane-like and Ge(Se_{$\text{1}\text{2}$})₂ structural units in proportions varying to comply with the actual x values. The glass forming tendency has minima at the stoichiometric compositions GeSe₂ and GeSe and maxima at $\text{x =}\text{1}\text{6}$ and $\text{x =}\text{3}\text{7}$ where the fractions of two coexisting structural units are nearly equal, $\text{i.e.}$ the number product of different bounds (Se-Se, Ge-Se) is maximal. Within the $\text{1}\text{3}$ < x < $\text{1}\text{2}$ composition range, and in non-relaxed samples even at smaller x values, ethane-like 2 (Se₃Ge)-[Ge(Se_{$\text{1}\text{2}$})₃]_n -units probably also exist. Photoluminescence (PL) is most efficient in the stoichionetric compound GeSe₂ having a rigid three-dimensional structure. The PL band is strongly Stokes shifted and thus situated in the middle of the forbidden band except for x = 0.5 where the PL is due to band-toband transitions. Defects of the bonding configurations attributable either to distortions in the structure or to irregular under-and over-coordinated atoms formed during preparation or irradiation witn band gap photons serve as PL centres. The decrease of PL intensity (fatigue) during excitation is caused by the creation of non-radiative centres whose concentration can reach 10²⁰ cm^t3?. Different models accounting for most of the observations are coinpared and discrepancies between their predictions and the experimental data are pointed out.     

Flow separation in a viscous parabolic shear past a cylinder

Low Reynolds number parabolic shear flow past a circular cylinder, placed asymetrically with its centre at a distance $\text{k}$ from the shear axis, is investigated employing matching technique. The analysis of flow separation indicates coalescion of wakes with increasing $\text{k}$. The drag is governed by a parameter λ which changes sign depending on the magnitude of $\text{k}$, while the torque is proportional to $\text{k}$. The case of a rotating cylinder including free rotation is also discussed.     

On the stability of the solid-liquid interface during solidification under convection

The non-dendritic morphology of solid particles forming during rheocasting has been examined in terms of the stability of the solid-liquid interface under the influence of stirring using a constitutional supercooling approach. The theoretical analysis suggests that stability may be achieved if the effective solute partition ratio is less than $\text{2}\text{3}$, or when the particle's Reynolds number exceeds the value required for the laminar to turbulent transition.     

On the path independent integral-Ĵ

The path independent integral, $\text{J}\text{?}$, is presented as the rate of energy flux during crack extension. This integral is an extension of the J-integral proposed by Rice and includes the existence of a fracture process region and the effect of plastic deformations, body forces, thermal strains and inertial of material.It is shown that the $\text{J}\text{?}\text{-}\text{integral}$ can include as special cases other fracture mechanics parameters such as J by Rice, $\text{G}\text{?}$ by Eftis et al., J¹ by Blackburn or f by Strifors.A definition of the $\text{J}\text{?}\text{-}\text{integral}$ in a three-dimensional problem is presented and possibility of applying the $\text{J}\text{?}\text{-}\text{integral}$ as fracture criterion is discussed.     

Heat or mass transfer from single spheres in a low reynolds number flow

The problem of forced heat or mass transfer from a single sphere in low Reynolds number flow is studied. In Stokes flow the Nusselt number $\text{N}$ can depend only upon the Peclet number $\text{P}$. For $\text{P ?}$ a general “master-formula” is derived which involves Stokes resistance $\text{F}{}_0$. On the other hand, the$\text{P?l}$ limit requires different formulas, depending upon whether the tangential velocity vanishes at the sphere surface (rigid sphere) or not (slip flow and droplets). Although these differences do not disappear when first-order inertia effects are included, it is found that inertia enhances the heat or mass transfer in either case.     

Influence of particle size and structure of ZrO2 on microstructure development and dielectric constant of PbZr0.5Ti0.5O3

The synthesis of PbZr_0.5Ti_0.5O₃ ceramics from the raw materials was reinvestigated in order to find relations in the characterizations for the products in various stages of the preparation procedure. Techniques used were particle size measurements, X-ray powder diffractometry, density and dielectric constant measurements and scanning electron microscopy. The results show, that the particle size and structure of ZrO₂ determine the inhomogeneity, expressed as $\text{x}{}_{\mathrm{T}}\text{x}{}_{\mathrm{R}}$, of the calcination product. An inhomogeneous calcination product sintered at relatively low temperatures results in an inhomogeneous ceramic of low density. Using high sintering temperatures and long periods of time an inhomogeneous calcination product converts into a dense (>97%) and homogeneous ceramic.     

A microcontinuum analysis of the self propulsion of the spermatozoa in the cervical canal

The hydrodynamics of the self propulsion of a spermatozoa, swimming through the mucus filling the cervical channel, is investigated. The mucus is modeled as a micropolar fluid and the spermatozoa as a 2-dimensional sheet swimming at low Reynolds number between two rigid walls. The wavelengths of the propulsive waves passing down the sheet are assumed to be very large compared to the channel spacing, but the amplitude of the propulsive waves is arbitrary. Expressions for the propulsive velocity and the energy expended by the swimming sheet are obtained in terms of various parameters involved. The results are elaborated through graphs. It is found that both the propulsive velocity and the rate of working by the sheet increase as the value of the micropolar parameters $\text{N}$ increases and that of $\text{L}$ decreases.     

Crystal growth and structure of BiVO4

Large crystals of BiVO₄ were grown. Extensive twinning can be related to the ferroelastic transition at 528K. The structure of BiVO₄ was refined at 4.5, 295 and 566K from powder neutron diffraction data and at 295K from powder X-ray diffraction data by the Rietveld profile technique. Space groups of $\text{I}\text{4}{}_1\text{a}$ above and $\text{I}\text{2}\text{b}$ below 528K were confirmed. It is suggested that the transition in BiVO₄ is driven by the lone-pair cation Bi³⁺. The Bi-O polyhedron is regular above the transition but becomes significantly distorted below the transition. This lone-pair distortion increases on cooling from 295 to 4.5K. The V-O tetrahedron remains regular at all temperatures.     

The influence of compression loads and of dk/da on the crack propagation under variable amplitude loading

During the past 10–15yr the development of crack growth prediction models for variable amplitude loading conditions has reached a high standard. However, there still exist some uncertainties regarding the quantitative effect of compression loads and of the variation of the stress intensity with increasing crack length on the crack propagation.For the evaluation of the influences of compression loads and of $\text{d}\text{K/}\text{d}\text{a}$ simple loading histories with single peak overloads and with compression underloads were run in stress controlled tests as well as in tests where the (reference) stress intensity was kept constant. The test results showed that the influence of $\text{d}\text{K/}\text{d}\text{a}$ was significant for variable amplitude loading conditions, and that this influence has to be considered in realistic crack propagation prediction models.Compression stresses have a strong effect on the crack opening level. Of special importance is the magnitude of the compression load relative to the compression yield strength of a material.A straight forward analytical analysis of the stresses before and behind a propagating crack was performed and a quantitative equation has been derived for the consideration of the influence of compression loads on the crack opening level. The derived equation is compared to the presently availalbe analytical expressions.     

Etude des transformations allotropiques de Mg2NiH4

Three allotropic varieties of Mg₂NiH₄ with monoclinic, orthorhombic and cubic symmetry have been identified. The unitcell dimensions are : $\text{a}\text{= 12.99}\text{A}\text{?}$, $\text{b}\text{= 6.390}\text{A}\text{?}$, $\text{c}\text{= 6.598}\text{A}\text{?}$ and $\text{β = 93.22}\text{A}\text{?}$ for the monoclinic form. The orthorhombic structure whose parameters are : $\text{a}\text{= 6.499}\text{A}\text{?}$, $\text{b}\text{= 6.415}\text{A}\text{?}$ et $\text{c}\text{= 6.589}\text{A}\text{?}$, represents a slight distortion from the cubic one. Under pressure, an irreversible transformation from the monoclinic to the cubic variety has been observed. The transition temperature is 245°C at 1 bar. On the other hand, the orthorhombic form transforms reversibly into the cubic one about 235°C under 1 bar pressure. The orthorhombic-monoclinic transition occurs only at very high pressure.     

Fatigue analysis of an edge crack specimen: Hysteresis strain energy density

Accumulative damage model based on the hysteresis strain energy density is proposed for predicting fatigue crack growth. Investigated is the application of sinusoidal loading on an edge crack whose growth rates are obtained by specifying the number of cycles, Δ$\text{N}$, for each growth step. The corresponding increment of crack growth, Δ$\text{a}$, is calculated by having the accumulated local strain energy density to reach certain critical value, $\text{(dW/dV)}{}_{\mathrm{c}}$. As it is to be expected, each growth increment Δ$\text{a}$ increases up to the point of unstable rapid fracture. The growth rate $\text{da/dN}$ versus a data are generated from the nonlinear incremental theory of plasticity. Because of the complexities involved in the stress and subcritical crack growth analysis, the finite element procedure is adopted such that the grid pattern is readjusted for each step of crack growth. Results for the edge crack specimen are displayed graphically and compared with those for the center cracked specimen made of the same material. The different growth characteristics are discussed and expected because material damage by fatigue is sensitive to changes in load history, specimen geometry and crack configuration. Insight into these nonlinear effects provides a means for establishing the range of applicability of the linear fatigue growth models. Discussed in particular are the $\text{da/dN}$ vs δ$\text{k}{}_1$ and AS relations where the linear theory of elasticity is used to calculate Δ$\text{K}{}_1$ and Δ$\text{S}$.     

Numerical simulation of the separated fluid flows at large Reynolds numbers

The variety of flow regimes (steady separated, periodically separated-‘Karman vortex street’, unsteady turbulent) and their characteristic peculiarities (separation and reattachment points, secondary separation, boundary layer, instability of the shear mixing layer, etc.) require the construction of effective numerical methods, which will be able to simulate adequately the considered flows. MERANGE ? SMIF–a splitting method for physical factors of incompressible fluids¹-is used for calculations of the steady and unsteady fluid flows past a circular cylinder in a wide range of Reynolds numbers (10° < Re < lo6). The finite-difference scheme for this method is of second order accuracy in the space variables, has minimal numerical viscosity and is also monotonic. Use of the Navier-Stokes equations with the corresponding transformation of Cartesian co-ordinates allows the calculations to be made by one algorithm both in a boundary layer and out of it. The method allows calculations at Re = ∞ cc and simulation of d‘Alembert’s paradox. Some results on the classical problem of the flow around a circular cylinder for a wide range of Reynolds numbers are discussed. The crisis of the total drag coefficient and the sharp rise of the Strouhal number are simulated numerically (without any turbulence models) for the critical Reynolds numbers (Re ≈ 4 × 10⁵), and are in a good agreement with experimental data.     

KTi6Nb5O25 : A member of a series of chemically twinned rutile oxides (AM3O9) (M2O4)n

A new oxide KTi₆Nb₅O₂₅ with a structure related to that of KTi₂Ta₅O₁₅ (1) has been prepared and described. It crystallizes in the orthorhombic system with unit-cell dimensions $\text{a}\text{= 6.611(4)}\text{A}\text{?}$, $\text{b}\text{= 8.880(8)}\text{A}\text{?}$, $\text{c}\text{= 30.154(18)}\text{A}\text{?}$ and space group Cmcm. The tunnel structure of this oxide has been studied from powder data. It is built up from rutile slabs whose width is determined by strings of six octahedra. This compound may be described as the fourth member of a chemically twinned rutile series (AM₃O₉) (M₂O₄)_n, KTi₂Ta₅O₁₇ being the second member of the series.     

Far-infrared reflection spectra of AgInSe2 and AgInTe2

The reflectivity of polarized light was measured, with a Michelson interferometer in the range between 10cm^?1 and 400cm^?1, on an artificial plane surface cut from oriented single crystal of AgInSe₂ with $\text{E}\text{?}\text{∥}\text{c}\text{?}$ and $\text{E}\text{?}\text{⊥}\text{c}\text{?}$, and on a natural (112) surface of AgInTe₂ with $\text{E}\text{?}\text{∥{111}}$ and $\text{E}\text{?}\text{⊥{111}}$. The results were analyzed by the Kramers-Kroning method and the least-squares fitting of the classical dispersion formula. The main features of the spectra and the form of the polarization vectors of some modes are discussed.     

On the constitutive relations for anisotropic materials—The crystal class D6h

We consider constitutive relations of the form W(B,…, C) where W is a scalar-valued polynomial function which is invariant under the group $\text{{D}{}_{\mathrm{6h}}\text{}}$ which defines the symmetry of the crystal class $\text{D}{}_{\mathrm{6h}}$. We make no restriction as to the number or kind of quantities appearing as arguments of $\text{W}$. We list the multilinear elements of an integrity basis for functions of quantities φ, φ',…, x, y,… where φ, φ',…, x,y,… form carrier spaces for the irreducible representations Γ₁,…, Γ'₆ of $\text{{D}{}_{\mathrm{6h}}\text{}}$. The elements of an integrity basis for functions of B,…, C are readily determined once these multilinear elements are given. We give examples and tables to facilitate the use of the results obtained here and in [1].     

PM flake material prepared by mechanical chopping of rapidly quenched metallic ribbon

Ductile metallic ribbons prepared by melt spinning can be easily cut into flakes suitable for $\text{P}\text{M}$ processing by a mechanical chopper designed to tear off ribbon flakes by combining a rotating blade and a bed knife. Details of the design features are described.     

Characterization of transition temperature behavior of HY130 steel by the JIc fracture toughness parameter

The objectives of this study were to determine whether the $\text{J-R}$ curve approach could be used to evaluate the ductile to brittle temperature performance of a high yield strength structural steel (HY130) and to demonstrate that the single specimen unloading compliance method is applicable to evaluate $\text{J}{}_{\mathrm{Ic}}$ values and $\text{J-R}$ curves for compact specimens tested at temperatures from ?192 to 150°C. The major conclusions of this work are that $\text{J}{}_{\mathrm{Ic}}$ and the complete $\text{J-R}$ curve can be obtained using the single specimen method over the above temperature range and that $\text{J}{}_{\mathrm{Ic}}$ does define a ductile to brittle transistion temperature for HY130 steel which should be more valuable for structural design than that found from Charpy $\text{V}$ or dynamic tear specimens because it is based on a fracture toughness parameter. The comparison of the ${}_{\mathrm{Ic}}$ transition temperature and that from Charpy $\text{V}$ specimens shows that the Charpy $\text{V}$ transition temperture is more conservative for the HY130 steel tested. In transitional $\text{J}{}_{\mathrm{I}}$ specimens which demonstrated a ductile crack tearing followed by a brittle failure, scanning microscope stereo pair fractography showed that the transition from ductile to brittle behavior was very gradual in comparison to the distinct crack tips obtained on ductile specimens broken in a brittle fashion at a cryogenic temperature.     

Observations par microscopie electronique de fautes ordonnees dans l'oxyde ReO2 monoclinique liees a la transformation structurale Re2O5 → ReO2

The formation of monoclinic ReO₂ from Re₂O₅ disproportion is studied by means of transmission electron microscopy and diffraction. The habit of single crystals of monoclinic oxide can be characterized as polygonal plates compatible with the space group $\text{P}\text{2}{}_1\text{c}$. A possible structural mechanism is described, involving the formation of cation vacancies and their regular arrangement, according to planar ordered faults observed on patterns.