bluuebunny/crossref_metadata_2025 huggingface dataset:
Activation energy for precipitation hardening and softening in aluminium alloys calculated using hardness and resistivity data
Authors: Sha W.
Published: April 2006

Three types of alloys have been studied, Al–Mg, Al–Zn, and Al–Si. The first type of alloys include Al–11.1 wt% Mg, Al–4.5 at% Mg and Al–2.5 at% Mg. The second type includes low Zn contents at 1.3% and 3.2% (at%). The third type includes alloys with Si contents of 1.37%, 2.83% and 6.81% (at%). The results have demonstrated that hardness and resistivity data can be treated to reveal the activation energy of processes, for intervals when there is enough experimental measurement data. Where relevant, the calculation results of the activation energy values are discussed. The errors due to linear regression are given. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Structural, morphological, optical, and nonlinear optical properties of fluorine‐doped zinc oxide thin films deposited on glass substrates by the chemical spray technique
Authors: Castañeda L. , Morales‐Saavedra O. G. , Acosta D. R. , Maldonado A. , de la L. Olvera M.
Published: April 2006

Fluorine‐doped zinc oxide thin films (ZnO:F) were deposited on sodocalcic glass substrates from an aged solution using a chemical spray technique. The effect of substrate temperature (T S) on the morphological, optical, and nonlinear optical (NLO) properties of the ZnO:F thin films was studied. These films were analysed using X‐ray diffraction, scanning electron microscopy, profilometry, optical transmittance, optical fluorescence, and second (SHG) and third harmonic generation (THG) techniques. A polycrystalline structure with a preferential growth along the ZnO(002) plane was found in all cases, with a corresponding average crystal size of less than 32 nm for the films. The morphology of the surfaces and some optical properties were found to be dependent on T S. The films deposited at low T S (400 °C) showed a uniform surface covered with needle‐like grains as well as SHG and THG response. In contrast, for the films deposited at 525 °C, a porous surface with irregular‐shaped grains was obtained and a substantial reduction of the SHG and THG response was detected. NLO χ (2)eff coefficients one order of magnitude larger than the χ (2)11 component of an α‐quartz reference crystal were measured for all ZnO:F thin films. The χ (3) values of the samples, under resonant conditions, were in the interesting range of 10–12 esu. Finally, a typical optical transmittance higher than 85% and a band gap (E g) of 3.4 eV were evaluated for films deposited under optimal conditions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Intrinsic ferroelectric hysteresis behaviours for heterostructures
Authors: Chew K.‐H. , Ishibashi Y. , Shin F. G.
Published: April 2006

Intrinsic ferroelectric hysteresis behaviours for a heterostructure of interface between two different ferroelectrics of second‐order transition are studied using the Landau–Ginzburg theory. We show that the existence of interfacial coupling could lead to a new polarization reversal mechanism. A detailed description of the polarization reversal mechanism of the heterostructure is discussed based on the hysteresis loops and the spatial variation of polarizations under various electric field strengths. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Computer analysis of an influence of oxygen vacancies on the electronic properties of the SnO2 surface and near‐surface region
Authors: Izydorczyk Weronika , Adamowicz Boguslawa , Miczek Marcin , Waczynski Krzysztof
Published: April 2006

An influence of the surface stoichiometric defects on the electronic and electrical properties of SnO2 layers has been studied by means of computer simulation. The surface potential and in‐depth potential profiles in the accumulation region have been rigorously computed at various temperatures (from 400 to 900 K). The behaviour of both the surface and bulk Fermi level position versus temperature has been analysed. Furthermore, in‐depth profiles of carrier concentration and the surface potential as well as the SnO2 layer conductance for different surface donor density of state have been calculated. It has been found that the surface reduction and bulk donor concentration (in the range from 3 × 1014 to 3 × 1018 cm–3) are strongly correlated with the development of near‐surface accumulation region and thus modification of the layer electrical parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Laser assisted fabrication of Co on Ti–6Al–4V for bio‐implant application
Authors: Majumdar J. Dutta , Ganesan S. M. , Manna I. , Nath A. K.
Published: April 2006

In the present study, attempts have been made to fabricate a Co layer on the surface of Ti–6Al–4V substrate by laser assisted fabrication technique with an objective to develop compositionally graded hip and femoral prostheses. Laser processing was carried out by melting of Co powder (of 25 μm particle size) with a continuous wave CO2 laser and depositing it on Ti–6Al–4V substrate in a layer by layer fashion using Ar as shrouding environment to avoid oxidation. The process variables were applied power density, scan speed and number of layers. During the development of 1st layer, laser power and scan speeds were varied to develop a compositionally graded interface, following which the successive layers were formed. A detailed microstructural study of the fabricated layers was carried out to understand the influence of laser parameters on microstructure. X‐ray diffraction study and energy dispersive spectroscopic analysis were undertaken to see if non‐equilibrium cooling associated with the process has caused formation of any new phase or segregation of elements in the microstructure. Following characterization, the mechanical property (wear resistance) and electrochemical property of the fabricated components have been evaluated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Design of Multi‐Component Reactions: From Libraries of Compounds to Libraries of Reactions
Authors: Mironov Maxim A.
Published: April 2006

Multi‐Component Reactions (MCRs) constitute a methodology to shorter syntheses of natural products or complex molecules similar to natural. Due to the large number of accessible compounds and other advantages, this type of chemistry has become very popular in the community of combinatorial chemists. In the last 5 years combinatorial chemistry is has evolved from the synthesis of libraries based on one scaffold to the synthesis of structurally diverse compounds, better known as diversity‐oriented synthesis. Following this trend, the design of MCRs is becoming a tool kit for chemists to explore chemistry space broadly with complex and diverse structures. Various approaches to the creation of novel MCRs including “union of MCRs”, “substrate design”, and “reaction operator” are discussed here.
Matrix‐assisted laser desorption/ionization time‐of‐flight/time‐of‐flight tandem mass spectra of poly(butylene adipate)
Authors: Rizzarelli Paola , Puglisi Concetto , Montaudo Giorgio
Published: April 2006

Matrix‐assisted laser desorption/ionization time‐of‐flight/time‐of‐flight tandem mass spectrometry (MALDI‐TOF/TOF‐MS/MS) was employed to analyze four poly(butylene adipate) (PBAd) oligomers and to investigate their fragmentation pathways as a continuation of our work on the MALDI‐TOF/TOF‐MS/MS study of synthetic polymers. MALDI‐TOF/TOF‐MS/MS analysis was performed on oligomers terminated by carboxyl and hydroxyl groups, methyl adipate and hydroxyl groups, dihydroxyl groups, and dicarboxyl groups. The sodium adducts of these oligomers were selected as precursor ions. Different end groups do not influence the fragmentation of sodiated polyester oligomers and similar series of product ions were observed in all the MALDI‐TOF/TOF‐MS/MS spectra. According to the structures of the most abundant product ions identified in the present work, three fragmentation pathways have been proposed to occur most frequently in PBAd: β‐hydrogen‐transfer rearrangement, leading to the selective cleavage of the OCH2 bonds; CH2CH2 (ββ) bond cleavage in the adipate moiety; and ester bond scission. Copyright © 2006 John Wiley & Sons, Ltd.
Induced smoothing for rank regression with censored survival times
Authors: Brown B. M. , Wang You‐Gan
Published: April 2006

Adaptions of weighted rank regression to the accelerated failure time model for censored survival data have been successful in yielding asymptotically normal estimates and flexible weighting schemes to increase statistical efficiencies. However, for only one simple weighting scheme, Gehan or Wilcoxon weights, are estimating equations guaranteed to be monotone in parameter components, and even in this case are step functions, requiring the equivalent of linear programming for computation. The lack of smoothness makes standard error or covariance matrix estimation even more difficult. An induced smoothing technique overcame these difficulties in various problems involving monotone but pure jump estimating equations, including conventional rank regression. The present paper applies induced smoothing to the Gehan–Wilcoxon weighted rank regression for the accelerated failure time model, for the more difficult case of survival time data subject to censoring, where the inapplicability of permutation arguments necessitates a new method of estimating null variance of estimating functions. Smooth monotone parameter estimation and rapid, reliable standard error or covariance matrix estimation is obtained. Copyright © 2006 John Wiley & Sons, Ltd.
Cellular coexistence of two high molecular subsets of eEF1B complex
Authors: Le Sourd Frédéric , Cormier Patrick , Bach Stéphane , Boulben Sandrine , Bellé Robert , Mulner-Lorillon Odile
Published: April 2006

The elongation factor eEF1B involved in protein translation was found to contain two isoforms of the eEF1Bδ subunit in sea urchin eggs. The eEF1Bδ2 isoform differs from eEF1Bδ1 by a specific insert of 26 amino acids. Both isoforms are co‐expressed in the cell and likely originate from a unique gene. The feature appears universal in metazoans as judged from in silico analysis in EST‐databanks. The eEF1B components were co‐immunoprecipitated by specific eEF1Bδ2 antibodies. Quantification of the proteins in immunoprecipitates and on immunoblots demonstrates that eEF1Bδ1 and eEF1Bδ2 proteins are present in two subsets of eEF1B complex. We discuss and propose a model for the different subsets of eEF1B complex concomitantly present in the cell.
Quality audit roles and skills: Perceptions of non‐financial auditors and their clients
Authors: Power Damien , Terziovski Milé
Published: April 2006

A major development in Operations Management over the past two decades has been the use of quality audits to gauge the effectiveness of quality management systems in manufacturing and service organizations. Achieving optimal benefits from non‐financial audits will, however, require that auditors and their clients obtain a greater understanding of and appreciation for the appropriate roles, responsibilities and skills of the non‐financial auditor. This research uses data obtained from surveys of 126 practicing non‐financial auditors and 400 non‐financial audit clients in Australasia to test two propositions about the roles and requirements of non‐financial auditors. The results indicate that non‐financial auditors believe they are providing a style of auditing that is strongly focused on developing continuous improvement of client's quality systems. Non‐financial auditors also appear to have a desire to meet what they perceive to be the requirements of certified organizations by emphasizing the importance of continuous improvement focused auditing. However, their clients have the opposite perception believing that they are getting less continuous improvement focused auditing than they would like, and more compliance auditing than they need. The results show that client organizations are, in general, looking for a more balanced approach to non‐financial auditing in terms of compliance and continuous improvement. The evidence also suggests that a major constraint identified relates to some fundamental issues of auditor independence. This dilemma created is apparent both in the understanding of what the appropriate role of the auditor is (in both the mind of the client and the auditor), and in the nature of the different roles each plays as a stakeholder in a quality system.
Measurements of the concentration field in nonlinear travelling-wave convection
Authors: Winkler Bernhard L. , Kolodner Paul
Published: April 2006

We present observations of travelling-wave convection in ethanol/water mixtures in a rectangular container using shadowgraphic flow visualization along the axes of the convective rolls. The entrainment of solute into the centre of the fluid layer from the boundary layers at the top and bottom of the cell is clearly observed in the shadowgraph images as sharp features at the roll boundaries. The shadowgraph images are integrated to yield a map of the two-dimensional refractive-index field. The contributions to this field from the convective temperature and the concentration fields have sufficiently different spatial character to be reliably separated using image-processing techniques. This has allowed us to produce a map of these two flow fields that is in good quantitative agreement with numerical integrations of the full Navier–Stokes equations in the Boussinesq approximation. We also present qualitative visualizations of transients caused by abruptly triggering and quenching travelling-wave convection. These experiments illustrate the formation of concentration boundary layers and reveal the existence of large-scale currents which are an inherent part of travelling-wave convection.
On transition due to three-dimensional disturbances in plane Poiseuille flow
Authors: Klingmann Barbro G. B.
Published: April 2006

The purpose of the present study is to characterize the process of laminar–turbulent transition at Reynolds numbers which are subcritical from the two-dimensional linear point of view. The development of a point-like disturbance was studied in an air flow channel with hot-wire anemometry at a Reynolds number of 1600. Localized disturbances were triggered at one of the walls and their development followed downstream by traversing the hot-wire probe in the streamwise direction over a distance of 90 half channel heights, as well as in the normal and spanwise directions. The disturbance evolved into elongated streaky structures with strong spanwise shear (i.e. normal vorticity) which grew in amplitude and streamwise extension and thereafter either decayed or gave rise to a turbulent spot. The results indicate that the mechanism underlying the initial growth is a linear one resulting from the coupling between the normal velocity and the normal vorticity, as described by the three-dimensional linear equations. The nonlinear development of the structure leads to the formation of intense normal shear layers and the appearance of oscillations and ‘spikes’, which multiply and form the rear or a turbulent spot.
The wavenumbers of supercritical surface-tension-driven Bénard convection
Authors: Koschmieder E. L. , Switzer D. W.
Published: April 2006

The cell size or the wavenumbers of supercritical hexagonal convection cells in primarily surface-tension-driven convection on a uniformly heated plate has been studied experimentally in thermal equilibrium in thin layers of silicone oil of large aspect ratio. It has been found that the cell size decreases with increased temperature difference in the slightly supercritical range, and that the cell size is unique within the experimental error. It has also been observed that the cell size reaches a minimum and begins to increase at larger temperature differences. This reversal of the rate of change of the wavenumber with temperature difference is attributed to influences of buoyancy on the fluid motion. The consequences of buoyancy have been tested with three fluid layers of different depth.
Complete adiabatic evaporation of highly superheated liquid jets
Authors: Kurschat Th. , Chaves H. , Meier G. E. A.
Published: April 2006

A nozzle expansion into a vacuum chamber was used to investigate the evaporation of highly superheated liquid jets. The large molar specific heat of fluids with high molecular complexity — in this case C6F14 — is responsible for the new phenomena reported here. A model was developed to describe the basic physical effects. A cubic equation of state was used to describe the thermodynamic properties of the fluid. The evaporation was modelled as a sonic deflagration followed by an axisymmetric supersonic expansion. As in the case of hypersonic gas jets the final state is reached by a normal shock. For sufficiently high temperatures and expansion ratios a complete adiabatic evaporation of the liquid was found. At even higher temperatures the liquid evaporates completely within a rarefaction discontinuity. The predictions of the model are in good agreement with the experimental results.
The concentration distribution near a continuous point source in steady homogeneous shear
Authors: Bowen James D. , Stolzenbach Keith D.
Published: April 2006

The concentration distribution resulting from a continuous point source in a fluid with a steady linear variation in velocity is distorted by the flow at distances greater than (K/Eb)½, whereKis the molecular diffusion coefficient andEbis a characteristic shear rate. The distribution has two distinct shapes depending on the number of principal axes of fluid strain that are expansive and the relative magnitude of irrotational and rotational shears. For irrotational flows a single expansive principal axis of strain results in a tube-like distribution, while two expansive axes results in a disk-like distribution. Approximate analytical solutions, derived by neglecting diffusion along the expansive axes, agree well with concentrations calculated by numerically convolving the exact instantaneous source solution. The effect of fluid vorticity is generally to reorient the distribution away from the principal axes of strain and to reduce the asymmetry of the concentration distribution. Aside from reorientation, the concentration distribution varies little until the vorticity approaches a critical value defined by a kinematic condition for equilibrium orientation in the presence of rotation. For vorticity greater than the critical value, the concentration distribution becomes axisymmetric around the axis of rotation. Application of these results to numerical simulations of isotropic turbulence suggests that tubes are more common than disks and that vorticity exceeds the critical value in at least 25% of the fluid.
Interfacial instabilities in aluminium reduction cells
Authors: Sneyd A. D.
Published: April 2006

This paper extends the analysis of Sneyd (1985) on interfacial instabilities in aluminium reduction cells. The cell model consists of a plane fluid layer of relatively low electrical conductivity, sandwiched between an upper rigid wall and lower fluid layer, both of high conductivity. A steady current passes through the layers, and the magnetic field is assumed to be a linear function of position. The principal new effects introduced are (i) a horizontal current component in the aluminium; (ii) vertical magnetic field components, and vertical field gradients; (iii) an aluminium pool of finite depth; and (iv) uniform zeroth-order flow in the fluid layers, and mechanical dissipation. A dispersion relation for small-amplitude waves is derived and discussed. The destabilizing Kelvin—Helmholtz mechanism and electromagnetic forces compete with gravity, surface tension and mechanical dissipation. Electromagnetic destabilization is likely to occur in practice at wavelengths of 1 m or more, and becomes more intense with decreasing layer depths. The most dangerous mechanism appears to be driven by vertical gradients of the horizontal field.
The dynamics of intrusions into a thermohaline stratification
Authors: Schladow S. Geoffrey , Thomas Ellen , Koseff Jeffrey R.
Published: April 2006

Physical and numerical experiments were performed for a linearly stratified heat—salt system, uniformly heated at one endwall. The initial stratification was in the diffusive sense. Intrusions formed at the heated endwall and propagated out into the interior fluid. Three classes of flow were identified, based upon the gravitational stability ratio, Rp, and a lateral stability parameter, R1, For R1 > 1, a vertical lengthscale for the initial intrusion thickness was developed which agreed well with that observed in the physical experiments. In all cases, a region of salt fingering developed due to gradient reversal at the heated endwall. Two very distinct merging processes were observed depending on the specific flow class. The first process occurred under conditions of high gravitational and lateral stability, and appeared to be controlled by horizontal motions induced by the intrusions. The second process was observed under less stable conditions and was a result of vertical motions at the heated endwall within the intrusions themselves. In the least stable class of flow (low gravitational and lateral stability), the intrusions were found to be self-perpetuating in the sense that they continued to propagate following removal of the endwall heat flux.
Experiments on mixing in continuous chaotic flows
Authors: Kusch H. A. , Ottino J. M.
Published: April 2006

We present the design and operation of a flow apparatus for investigations of mixing in time-periodic and spatially periodic chaotic flows. Uses are illustrated in terms of two devices operating in the Stokes regime: thepartitioned-pipe mixer, a spatially periodic system consisting of sequences of flows in semicircular ducts, and theeccentric helical annular mixer, a time-periodic velocity field between eccentric cylinders with a superposed Poiseuille flow; other mixing flows can be implemented with relative ease. Fundamental differences between spatially periodic and time-periodic duct flows are readily apparent. Steady spatially periodic systems show segregated KAM-tubes coexisting with chaotic advection; such tubes are remarkably stable under a variety of experimental conditions. Time-periodic duct flows lead to complex streakline structures; since regular regions in the cross-sectional flow move through space, a streakline can find itself injected in a regular domain for some time then be trapped in a chaotic region, and so on, leading to ‘intermittent’ behaviour.
A numerical study of vortex shedding from flat plates with square leading and trailing edges
Authors: Ohya Yuji , Nakamura Yasuharu , Ozono Shigehira , Tsuruta Hideki , Nakayama Ryuzo
Published: April 2006

This paper describes a numerical study of the flow around flat plates with square leading and trailing edges on the basis of a finite-difference analysis of the two-dimensional Navier—Stokes equations. The chord-to-thickness ratio of a plate, d/h, ranges from 3 to 9 and the value of the Reynolds number based on the plate's thickness is constant and equal to 103. The numerical computation confirms the finding obtained in our previous experiments that vortex shedding from flat plates with square leading and trailing edges is caused by the impinging-shear-layer instability. In particular, the Strouhal number based on the plate's chord increases stepwise with increasing d/h in agreement with the experiment. Numerical analyses also provide some crucial information on the complicated vortical flow occurring near the trailing edge in conjunction with the vortex shedding mechanism. Finally, the mechanism of the impinging-shear-layer instability is discussed in the light of the experimental and numerical findings.
Hydrodynamic diffusion of a sphere sedimenting through a dilute suspension of neutrally buoyant spheres
Authors: Davis Robert H. , Hill N. A.
Published: April 2006

The motion of a heavy sphere sedimenting through a dilute background suspension of neutrally buoyant spheres is analysed for small Reynolds number and large Péclet number. For this particular problem, it is possible not only to calculate the mean velocity of the heavy particle, but also the variance of the velocity and the coefficient of hydrodynamic diffusivity. Pairwise, hydrodynamic interactions between the heavy sphere and the background sphere are considered exactly using volume integrals and a trajectory analysis. Explicit formulae are given for the two limiting cases when the radius of the heavy sphere is much greater and much less than that of the background spheres, and numerical results are given for moderate size ratios. The mean velocity is relatively insensitive to the ratio of the radius of the background spheres to that of the heavy sphere, unless this ratio is very large, whereas the hydrodynamic diffusivity increases rapidly as the radius ratio is increased. The predictions are in reasonable agreement with the results of falling-ball rheometry experiments.
Low-Reynolds-number effects in a fully developed turbulent channel flow
Authors: Antonia R. A. , Teitel M. , Kim J. , Browne L. W. B.
Published: April 2006

Low-Reynolds-number effects are observed in the inner region of a fully developed turbulent channel flow, using data obtained either from experiments or by direct numerical simulations. The Reynolds-number influence is observed on the turbulence intensities and to a lesser degree on the average production and dissipation of the turbulent energy. In the near-wall region, the data confirm Wei & Willmarth's (1989) conclusion that the Reynolds stresses do not scale on wall variables. One of the reasons proposed by these authors to account for this behaviour, namely the ‘geometry’ effect or direct interaction between inner regions on opposite walls, was investigated in some detail by introducing temperature at one of the walls, both in experiment and simulation. Although the extent of penetration of thermal excursions into the opposite side of the channel can be significant at low Reynolds numbers, the contribution these excursions make to the Reynolds shear stress and the spanwise vorticity in the opposite wall region is negligible. In the inner region, spectra and co-spectra of the velocity fluctuations u and v change rapidly with the Reynolds number, the variations being mainly confined to low wavenumbers in the u spectrum. These spectra, and the corresponding variances, are discussed in the context of the active/inactive motion concept and the possibility of increased vortex stretching at the wall. A comparison is made between the channel and the boundary layer at low Reynolds numbers.
Numerical investigation of the high-speed conical flow past a sharp fin
Authors: Panaras Argyris G.
Published: April 2006

The supersonic flow past a fin mounted on a flat plate is simulated numerically by solving the Reynolds averaged Navier—Stokes equations. The results agree well with the experimental data. Post-processing of the numerical solution provides the missing flow-field evidence for confirming the currently accepted flow model, whose conception was based mainly on surface data. It is found that the flow is dominated by a large vortical structure, which lies on the plate and whose core has a remarkably conical shape with flattened elliptical cross-section. Along the fin and close to the corner, a slowly growing smaller vortex develops. On top of the conical vortex and along it a λ-shock is formed. Quantitative data are presented, which show that the flow is not actually purely conical but a small deviation exists, especially at the part between the separation shock and the plate. This deviation is detected when the stream wise extent of the flow is more than 20–30 initial boundary-layer thicknesses. Owing to the rather quasi-conical nature of the flow, the various flow variables do not remain constant along rays that start at the origin of the conical flow field, but they vary slowly. Data are presented which support the view that this deviation from conical behaviour is mainly due to the effect of the smaller rate of development of the boundary later of the plate, compared to the conical vortex.
Two-dimensional vortex-dipole interactions in a stratified fluid
Authors: Voropayev S. I. , Afanasyev Ya. D.
Published: April 2006

Planar motion produced when a viscous fluid is forced from an initial state of rest is studied. We consider a vortex dipole produced by the action of a point force (Cantwell 1986), and a vortex quadrupole produced by the action of two equal forces of opposite direction. We also present results from an experimental investigation into the dynamics of the interactions between vortex dipoles as well as between vortex dipoles and a vertical wall in a stratified fluid. Theoretical consideration reveals that the dynamics of two-dimensional vortex-dipole interactions are determined by two main governing parameters: the dipolar intensity of the vorticity distribution (momentum) and the quadrupolar intensity of the vorticity distribution of the flow. We document details of different basic types of interactions and present a physical interpretation of the results obtained in terms of vortex multipoles: dipoles, quadrupoles and their combinations.
Three-dimensional vortex formation from an oscillating, non-uniform cylinder
Authors: Nuzzi F. , Magness C. , Rockwell D.
Published: April 2006

A cylinder having mild variations in diameter along its span is subjected to controlled excitation at frequencies above and below the inherent shedding frequency from the corresponding two-dimensional cylinder. The response of the near wake is characterized in terms of timeline visualization and velocity traces, spectra, and phase plane representations. It is possible to generate several types of vortex formation, depending upon the excitation frequency. Globally locked-in, three-dimensional vortex formation can occur along the entire span of the flow. Regions of locally locked-in and period-doubled vortex formation can exist along different portions of the span provided the excitation frequency is properly tuned. Unlike the classical subharmonic instability in free shear flows, the occurrence of period-doubled vortex formation does not involve vortex coalescence; instead, the flow structure alternates between two different states.
The flow over a backward-facing step under controlled perturbation: laminar separation
Authors: Hasan M. A. Z.
Published: April 2006

The flow over a backward-facing step with laminar separation was investigated experimentally under controlled perturbation for a Reynolds number of 11000, based on a step height h and a free-stream velocity UO. The reattaching shear layer was found to have two distinct modes of instability: the ‘shear layer mode’ of instability at Stθ ≈ 0.012 (Stθ ≡ fθ/UO, θ being the momentum thickness at separation and f the natural roll-up frequency of the shear layer); and the ‘step mode’ of instability at Sth ≈ 0.185 (Sth ≡ fh/U0). The shear layer instability frequency reduced to the step mode one via one or more stages of a vortex merging process. The perturbation increased the shear layer growth rate and the turbulence intensity and decreased the reattachment length compared to the unperturbed flow. Cross-stream measurements of the amplitudes of the perturbed frequency and its harmonics suggested the splitting of the shear layer. Flow visualization confirmed the shear layer splitting and showed the existence of a low-frequency flapping of the shear layer.
Heat-transfer enhancement due to slender recirculation and chaotic transport between counter-rotating eccentric cylinders
Authors: Ghosh S. , Chang H.-C. , Sen M.
Published: April 2006

Using Stokes flow between eccentric, counter-rotating cylinders as a prototype for bounded, nearly parallel lubrication flow, we investigate the effect of a slender recirculation region within the flow field on cross-stream heat or mass transport in the important limit of high Péclet number Pe where the enhancement over pure conduction heat transfer without recirculation is most pronounced. The steady enhancement is estimated with a matched asymptotic expansion to resolve the diffusive boundary layers at the separatrices which bound the recirculation region. The enhancement over pure conduction is shown to vary as ε½ at infinite Pe, where ε½ is the characteristic width of the recirculation region. The enhancement decays from this asymptote as Pe−½. If one perturbs the steady flow by a time-periodic forcing, fast relative to the convective and diffusive times, the separatrices undergo a homoclinic entanglement which allows fluid elements to cross the separatrices. We establish the existence of this homoclinic entanglement and show that the resulting chaotic particle transport further enhances the cross-stream flux. We estimate the penetration of the fluid elements across the separatrices and their effective diffusivity due to this chaotic transport by a Melnikov analysis for small-amplitude forcing. These and the steady results then provide quantitative estimates of the timeaveraged transport enhancement and allow optimization with respect to system parameters. An optimum forcing frequency which induces maximum heat-transfer enhancement is predicted and numerically verified. The predicted optimum frequency remains valid at strong forcing and large Pe where chaotic transport is as important as the recirculation mechanism. Since most heat and mass transport devices operate at high Pe, our analysis suggests that chaotic enhancement can improve their performance and that a small amplitude theory can be used to optimize its application.
Interfacial mode interactions in horizontal gas—liquid flows
Authors: Jurman L. A. , Deutsch S. E. , Mccready M. J.
Published: April 2006

The behaviour of shear-generated interfacial waves in a cocurrent gas-liquid flow in a small rectangular channel is studied experimentally at conditions close to neutral stability. It is found that the linearly most unstable mode, which typically has a frequency of 8–10 Hz and a wavelength 1–4 cm, grows initially — followed immediately by the first overtone. Measurements of the bicoherence spectrum indicate that the overtone and fundamental are coherent in phase, which suggests that energy is transferred from the fundamental to the linearly stable first overtone. This energy transfer mechanism can stabilize the system, as evidenced by data, which shows that the first mode can saturate with a wave slope small as as 0.005. Theory based on weakly nonlinear mode-interaction equations suggests that this steady state should be stable at conditions close to neutral stability where only overtone modes are present. However, under more severe conditions, where the amplitude of the fundamental mode becomes sufficiently large, a subharmonic mode may be excited. The generation of the subharmonic, when it is linearly stable with respect to the flat film base state, can be interpreted as a linear instability of the steady state containing the fundamental and overtones. Modes that are sidebands (with wavenumbers = k ± δk) to the main peak may also occur. These can participate in interactions with low-frequency modes (i.e. δk) and thereby transfer energy to frequencies much below the fundamental. It is expected that all of these interactions play important roles in determining the wave spectrum of conditions far away from neutral stability.
Observations of fibre orientation in simple shear flow of semi-dilute suspensions
Authors: Stover Carl A. , Koch Donald L. , Cohen Claude
Published: April 2006

The orientations of fibres in a semi-dilute, index-of-refraction-matched suspension in a Newtonian fluid were observed in a cylindrical Couette device. Even at the highest concentration (nL3 = 45), the particles rotated around the vorticity axis, spending most of their time nearly aligned in the flow direction as they would do in a Jeffery orbit. The measured orbit-constant distributions were quite different from the dilute orbit-constant distributions measured by Anczurowski & Mason (1967b) and were described well by an anisotropic, weak rotary diffusion. The measured ϕ-distributions were found to be similar to Jeffery's solution. Here, ϕ is the meridian angle in the flow-gradient plane. The shear viscosities measured by Bibbo (1987) compared well with the values predicted by Shaqfeh & Fredrickson's theory (1990) using moments of the orientation distribution measured here.
Turbulence: the filtering approach
Authors: Germano M.
Published: April 2006

Explicit or implicit filtered representations of chaotic fields like spectral cut-offs or numerical discretizations are commonly used in the study of turbulence and particularly in the so-called large-eddy simulations. Peculiar to these representations is that they are produced by different filtering operators at different levels of resolution, and they can be hierarchically organized in terms of a characteristic parameter like a grid length or a spectral truncation mode. Unfortunately, in the case of a general implicit or explicit filtering operator the Reynolds rules of the mean are no longer valid, and the classical analysis of the turbulence in terms of mean values and fluctuations is not so simple.

In this paper a new operatorial approach to the study of turbulence based on the general algebraic properties of the filtered representations of a turbulence field at different levels is presented. The main results of this analysis are the averaging invariance of the filtered Navier—Stokes equations in terms of the generalized central moments, and an algebraic identity that relates the turbulent stresses at different levels. The statistical approach uses the idea of a decomposition in mean values and fluctuations, and the original turbulent field is seen as the sum of different contributions. On the other hand this operatorial approach is based on the comparison of different representations of the turbulent field at different levels, and, in the opinion of the author, it is particularly fitted to study the similarity between the turbulence at different filtering levels. The best field of application of this approach is the numerical large-eddy simulation of turbulent flows where the large scale of the turbulent field is captured and the residual small scale is modelled. It is natural to define and to extract from the resolved field the resolved turbulence and to use the information that it contains to adapt the subgrid model to the real turbulent field. Following these ideas the application of this approach to the large-eddy simulation of the turbulent flow has been produced (Germano et al. 1991). It consists in a dynamic subgrid-scale eddy viscosity model that samples the resolved scale and uses this information to adjust locally the Smagorinsky constant to the local turbulence.
A hydroelastic model of macromechanics in the endolymphatic vestibular canal
Authors: Rabbitt R. D. , Damiano E. R.
Published: April 2006

A hydroelastic model describing the mechanics of the human semicircular canal is presented that extends previous work to address the influence of the shape of the labyrinth and the interaction between the endolymph and the cupula. The analysis is based extensively on the three-dimensional geometry and structure of the system and exploits the slender toroidal geometry to obtain an asymptotic solution describing the velocity distribution of the endolymph, the pressure distribution and the deflection of the cupula. All parameters appearing in the model are explicitly defined in terms of physical properties and the geometry. Results for the structure of an infant human endolymphatic canal agree well with experimental measurements of the end-organ velocity gain and phase over the entire physiological range of angular head frequencies. From 0.09 to 1.5 Hz the mechanical response relative to head velocity is essentially constant and the end-organ acts as an angular velocity transducer. Below 0.09 Hz the velocity gain is diminished and above 1.5 Hz the velocity gain is enhanced. For a 1 rad sinusoidal rotation of the head, the analysis predicts an average cupula displacement for the infant canal of approximately 8 × 10−5 cm at 0.09 Hz and 2 × 10−3 cm at 2.0 Hz.
Assessment of two-equation models of turbulent passive-scalar diffusion in channel flow
Authors: Horiuti Kiyosi
Published: April 2006

Models for the transport of passive scalar in turbulent flow were investigated using databases derived from numerical solutions of the Navier—Stokes equations for fully developed plane channel flow, these databases being generated using large-eddy and direct numerical simulation techniques. Their reliability has been established by comparison with the experimental measurements of Hishida. Nagano & Tagawa (1986). The present paper compares these simulations and calculations using the Nagano & Kim (1988) ‘two-equation’ model for the scalar variance (kθ) and scalar variance dissipation (εθ). This model accounts for the dependence of flow quantities on the Prandtl number by expressing eddy diffusivity in terms of the ratio of the timescales of velocity and scalar fluctuations. However, the statistical analysis by Yoshizawa (1988) showed that there was an inconsistency in the definition of the isotropic eddy diffusivity in the Nagano—Kim model, the implications of which are clearly demonstrated by the results of this paper where large-eddy simulation and direct numerical simulation (LES/DNS) databases are used to compute the quantities contained in both models. An extension of the Nagano-Kim model is proposed which resolves these inconsistencies, and a further development of this model is given in which the anisotropic scalar fluxes are calculated. Near a rigid surface, a third-order ‘anisotropic representation’ of scalar fluxes may be used as an alternative model for reducing the eddy diffusivity, instead of the conventional ‘damping functions’. This model is similar but distinct from the algebraic scalar flux model of Rogers, Mansour & Reynolds (1989). A third aspect of this paper is the use of the LES/DNS databases to evaluate certain coefficients (those for modelling the pressure-scalar gradient terms) of another model of a similar type, namely the algebraic scalar flux model of Launder (1975).
Velocity profile statistics in a turbulent boundary layer with slot-injected polymer
Authors: Fontaine A. A. , Petrie H. L. , Brungart T. A.
Published: April 2006

The modification of a flat-plate turbulent boundary layer resulting from the injection of drag-reducing polymer solutions through a narrow inclined slot into the near-wall region of the flow has been studied. Two-component coincident laser-Doppler velocity profile measurements were taken with a free-stream velocity of 4.5 m/s with polymer injection, water injection, and no injection. Polyethylene oxide solutions at concentrations of 500 and 1025 w.p.p.m. were injected. These data are complemented by polymer concentration profile measurements that were taken using a laser-induced-fluorescence technique. Also, integrated skin friction measurements were made with a drag balance for a range of polymer injection conditions and free-stream velocities. The immediate effects of polymer injection are a deceleration of the flow near the wall, a dramatic decrease of the vertical r.m.s. velocit}’ fluctuation levels and the Reynolds shear stress levels, and a mean velocity profile approaching Virk's asymptotic condition. These effects relax substantially with increasing stream wise distance from the injection slot and become similar to the effects observed for dilute homogeneous polymer flows.
Similarity solutions for viscous vortex cores
Authors: Mayer Ernst W. , Powell Kenneth G.
Published: April 2006

Results are presented for a class of self-similar solutions of the steady, axisymmetric Navier–Stokes equations, representing the flows in slender (quasi-cylindrical) vortices. Effects of vortex strength, axial gradients and compressibility are studied. The presence of viscosity is shown to couple the parameters describing the core growth rate and the external flow field, and numerical solutions show that the presence of an axial pressure gradient has a strong effect on the axial flow in the core. For the viscous compressible vortex, near-zero densities and pressures and low temperatures are seen on the vortex axis as the strength of the vortex increases. Compressibility is also shown to have a significant influence upon the distribution of vorticity in the vortex core.
Transient acoustic processes in a low-Mach-number shear flow
Authors: Wang Meng , Kassoy D. R.
Published: April 2006

A systematic perturbation procedure, based on a small mean flow Mach number and large duct Reynolds number, is employed to formulate and solve an initial-boundary-value problem for acoustic processes in a shear flow contained within a rigid-walled parallel duct. The results describe the general transient evolution of acoustic waves driven by a plane source located at a given duct cross-section. Forced bulk oscillations near the source and oblique wave generation are shown to result from refraction of the basic planar axial disturbance by the shear flow. Refraction also causes the axial waves to exhibit higher-order amplitude variations in the transverse direction. As the source frequency approaches certain critical values, specific refraction-induced oblique waves evolve into amplifying purely transverse waves. As a result, the magnitude of the refraction effect increases with time, and quasi-steady solutions do not exist. The analysis is extended to the thin acoustic boundary layer adjacent to the solid walls to examine the shear-layer structure induced by the variety of acoustic waves in the core flow. Nonlinear effects and acoustic streaming are shown to be negligibly small on the scale of a few axial wavelengths.
The three-dimensional stability of boundary-layer flow over compliant walls
Authors: Yeo K. S.
Published: April 2006

This paper examines the linear stability of the Blasius boundary layer over compliant walls to three-dimensional (oblique) disturbance wave modes. The formulation of the eigenvalue problem is applicable to compliant walls possessing general material anisotropy. Isotropic-material walls and selected classes of anisotropic-material walls are studied. When the properties of the wall are identical with respect to all oblique wave directions, the stability eigenvalue problem for unstable three-dimensional wave modes may be reduced to an equivalent problem for two-dimensional modes. The results for isotropic-material walls show that three-dimensional Tollmien–Schlichting instability modes are more dominant than their two-dimensional counterparts when the walls are sufficiently compliant. The critical Reynolds number for Tollmien-Schlichting instability may be given by three-dimensional modes. Furthermore, for highly compliant walls, calculations based solely on two-dimensional modes are likely to underestimate the maximum disturbance growth factor needed for transition prediction and correlation. However, because the disturbance growth rates on highly compliant walls are much lower than those on a rigid wall, significant delay of transition may still be possible provided compliance-induced instabilities are properly suppressed. Walls featuring material anisotropy which have reduced stiffness to shear deformation in the transverse and oblique planes are also investigated. Such anisotropy is found to be effective in reducing the growth rates of the three-dimensional modes relative to those of the two-dimensional modes.