bluuebunny/crossref_metadata_2025 huggingface dataset:
PMMA/Zinc Oxide Nanocomposites Prepared by In‐Situ Bulk Polymerization
Authors: Demir Mustafa M. , Memesa Mine , Castignolles Patrice , Wegner Gerhard
Published: April 2006

Summary: Dispersing surface‐modified zinc oxide nanoparticles (ZnO) in methyl methacrylate (MMA) improves the free radical bulk polymerization process as well as the thermal stability of the formed polymer. Hydroxy groups available on the ZnO surface may induce a degenerative transfer. This suppresses the gel effect, which leads to a better control of the heat evolution during the late stages of polymerization. The formation of chains having vinylidene end groups and head‐to‐head links is suppressed, which shifts the onset of thermal decomposition to the regime where decomposition occurs by random chain scission.

Thermal degradation profiles of PMMA and its composite with ZnO at 11 wt.‐%.

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Thermal degradation profiles of PMMA and its composite with ZnO at 11 wt.‐%.
Preparation of poly(lactic acid)/poly(ethylene glycol)/organoclay nanocomposites by melt compounding
Authors: Tanoue Shuichi , Hasook Aniwat , Iemoto Yoshiyuki , Unryu Tsunemune
Published: April 2006

Poly(lactic acid) (PLA)/organoclay nanocomposites were prepared by melt compounding in a co‐rotating twin screw extruder. Two types of commercialized organoclay (dimethyl benzyl stearyl ammonium ion and dimethyl distearyl ammonium ion intercalated between clay platelets named as Clay A and Clay B, respectively) and two grades of poly(ethylene glycol) (PEG) with different molecular weight (Mw = 2,000 and 300,000–500,000 named as PEG2k and PEG500k, respectively) were used in this study. The Young's modulus improved by the addition of organoclay to PLA matrix. The Young's modulus decreased with the addition of PEG to PLA/organoclay nanocomposites. The tensile strength and elongation of PLA/Clay B nanocomposites increased with the addition of PEG2k. The effect of the addition of PEG on d‐spacing of PLA/organoclay nanocomposites is dependent upon the kind of organoclay. The sizes of clay agglomerations in PLA/PEG/organoclay nanocomposites are larger than those of PLA/organoclay ones in the same organoclay. Addition of PEG to PLA/organoclay nanocomposites during melt compounding will not be useful for the preparation of PLA/organoclay having fully exfoliated clay platelets. The shear thinning properties of the nanocomposites are independent of the addition of PEG. On the whole, PEG2k is good plasticizer for PLA/organoclay nanocomposites. POLYM. COMPOS. 27:256–263, 2006. © 2006 Society of Plastics Engineers
Synthesis and characterization of nanosilica/polyacrylate composite latex
Authors: Wang Yaqiang , Li Yuping , Zhang Runyang , Huang Li , He Weiwei
Published: April 2006

The nanosilica/polyacrylate organic–inorganic composite latex was synthesized by in‐situ emulsion polymerization of methyl methacrylate (MMA) and butyl acrylate (BA) in the presence of silica nanoparticles, which were modified by silane coupling agent. The surface properties and dispersibility of silica nanoparticles modification, chemical structure, Zeta potential, diameter distribution of the composite latex prepared, surface roughness, and thermal stability of the hybrid film formed by the composite latex were investigated by fourier transform infrared spectrometer (FTIR), transmission electron microscopy (TEM), Zeta meter, ZetaPlus apparatus (dynamic light scattering method), atomic force microscopy (AFM), and thermogravimetric analysis (TGA), respectively. After modification with silane coupling agent, silane was grafted onto the surface of silica nanoparticles to form the organic layers, which was able to efficiently prevent the silica nanoparticles from aggregating to individually homogeneous disperse in the in‐situ emulsion polymerization system and improve the compatibility of silica nanoparticles with the acrylate monomers. The nanosilica/polyacrylate organic–inorganic composite latex prepared had the properties of silica nanoparticles and pure polyacrylate latex but was not simply a combination. Strong chemical bonding tethered the silica and acrylate chains to form the core/shell structural composite latex. Consequently, the hybrid film formed by nanosilica/polyacrylate composite latex exhibited a smooth surface and better thermal properties than the pure polyacrylate film. POLYM. COMPOS. 27:282–288, 2006. © 2006 Society of Plastics Engineers
Preparation and characterization of nano‐CaCO3 encapsulated by copolymerization of styrene and maleic anhydride
Authors: Li Gu , Mai Kan‐Cheng , Feng Kai‐Cai , Huang Yong‐Pin
Published: April 2006

Encapsulated nanometer calcium carbonate (nano‐CaCO3) was prepared using styrene and maleic anhydride (MAH) copolymer in 2‐propanol or methanol–water mixture in the presence of different initiator systems. The particle morphology and physical properties of the encapsulated nano‐CaCO3 particles, such as the interaction between the encapsulating polymer and the nano‐CaCO3, and the thermal stability of encapsulated nano‐CaCO3 were studied by Fourier‐transform infrared spectroscopy (FTIR), Soxhlet extraction experiments, thermogravimetric analysis banded with FTIR (TGA‐FTIR) and transmission electron microscopy (TEM). The encapsulating ratio and the stable encapsulating ratio of encapsulated nano‐CaCO3 were characterized. The results showed that a strong interfacial interaction was obtained due to the formation of a chemical bond or ion‐dipole between the CO group of MAH and Ca2+ ion of nano‐CaCO3. The encapsulating ratio and stable encapsulating ratio of nano‐CaCO3 initiated by AIBN was higher than that initiated by BPO. Addition of maleic anhydride increased the encapsulating ratio and the stable encapsulating ratio of encapsulated nano‐CaCO3. For the encapsulated nano‐CaCO3 prepared in methanol–water, the diameter of the encapsulated nano‐CaCO3 particle increased from 60–70 nm to about 100 nm and the morphology changed from a cube with a sharp edge to spherical with a rough surface. Copyright © 2006 Society of Chemical Industry
Carbon nanotubes as electron sources
Authors: Milne W. I. , Teo K. B. K. , Mann M. , Bu I. Y. Y. , Amaratunga G. A. J. , De Jonge N. , Allioux M. , Oostveen J. T. , Legagneux P. , Minoux E. , Gangloff L. , Hudanski L. , Schnell J.‐P. , Dieumegard L. D. , Peauger F. , Wells T. , El‐Gomati M.
Published: April 2006

Carbon nanotubes (CNTs) are a unique form of carbon filament/fiber in which the graphene walls roll up to form tubes. They can exhibit either metallic‐like or semiconductor‐like properties. With the graphene walls parallel to the filament axis, nanotubes (single wall metallic‐type or multi‐wall) exhibit high electrical conductivity at room temperature. This high electrical conductivity allied to their remarkable thermal stability has made CNTs one of the most intensely studied material systems for field emission (FE) applications. In this paper we will describe the growth of multiwall CNTs and their application in a range of field emission based systems including their use in SEM sources, emitters for use in microwave amplifiers and as emitters in field emission based displays (FEDs). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Buckling in boron sheets and nanotubes
Authors: Cabria Iván , Alonso Julio A. , López María J.
Published: April 2006

Density functional calculations have been carried out to obtain the geometric and electronic structure of boron sheets and nanotubes. The buckled boron sheet formed by alternating up and down rows of B atoms, with a buckling height of 0.85 Å, is about 0.20 eV/B atom more stable than the corresponding atomically flat sheet. Two different B–B bond‐lengths characterize the buckled geometry: (i) 1.63 Å between the B atoms in a row and (ii) 1.81 Å between the B atoms in adjacent rows. Boron nanotubes (BNTs) formed by rolling up a B sheet exhibit also a buckled surface when their helicity allows for the formation of alternating up and down B rows in the surface. In all other cases, BNTs exhibit only flat surfaces. Hence, all the (n , 0) BNTs have a buckled geometry, while not all the (n , n ) BNTs have the same geometry. The buckled and flat nanostructures have different geometric and bonding characteristics, but both are metallic. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
European research on emerging nanoelectronic information processing components
Authors: Van Hove P.
Published: April 2006

This contribution highlights advanced research challenges and opportunities in the area of devices and components responding to projected information processing requirements in a 10–20 year time frame. While CMOS technology is expected to be the major information technology vehicle in this time frame, the possibility of a transition to a complementary technology within 10–15 years opens up opportunities and requirements for research in new phenomena, new structures, new materials and new implementations of information processing components. In the presence of growing worldwide awareness of these needs, early European efforts in nanoelectronics should be pursued and well targeted. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Equiatomic FePt nanoparticles synthesized via pyrolysis of iron(III) ethoxide and platinum(II) acetylacetonate
Authors: Saita S. , Maenosono S.
Published: April 2006

FePt nanoparticles (NPs) of 4.5 nm mean diameter were synthesized using iron(III) ethoxide and platinum(II) acetylacetonate as precursors. The reaction proceeded without any reducing agent, providing a facile way of synthesis of FePt NPs. FePt NPs were characterized by transmission electron microscopy (TEM), energy‐dispersive X‐ray fluorescence spectrometry (EDX), and inductively coupled plasma atomic emission spectrometry (ICP‐AES). The average composition of NPs was found to be equiatomic and the composition distribution of each NP was considerably narrow. The spatio‐temporal homogeneity and thermal decomposition mode of iron source are considered to be the reason for the improvement of composition distribution. Phase transfer of FePt NPs from oil phase to water phase was also investigated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Waferscale assembly of Field‐Aligned nanotube Networks (FANs)
Authors: Dimaki Maria , Bøggild Peter
Published: April 2006

We demonstrate the integration of nanotube networks on 512 individual devices on a full 4‐inch wafer in less than 60 seconds with a roughly 80% yield using dielectrophoresis. We present here investigations of the morphology and electrical resistance of such field aligned networks for different frequencies of the electrical field used to attract the nanotubes to the electrodes. Preliminary data of response to visible light irradiation as well as changes in the humidity indicate that the field aligned networks could be used as sensor components that may well integrate with CMOS due to mild assembly conditions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rf glow discharge optical emission spectrometry for the analysis of arrays of Ni nanowires in nanoporous alumina and titania membranes
Authors: Prida V. M. , Navas D. , Pirota K. R. , Hernandez‐Velez M. , Menéndez A. , Bordel N. , Pereiro R. , Sanz‐Medel A. , Hernando B. , Vazquez M.
Published: April 2006

Anodic alumina (Al2O3) and titania (TiO2) nanoporous oxide membranes are among the most widely studied self‐organized nanopore templates, formed by uniform and well aligned arrays of synthetized nanometric pores or tubes. Here, we perform a comparative study of the depth profiling analysis in self‐ordered alumina and titania nanoporous membrane templates by means of the radiofrequency glow discharge coupled to optical emission spectrometry (rf‐GD‐OES) technique. The densely packed columnar arrays of hexagonally self‐ordered nanoporous alumina membranes investigated, with an average inner pore diameter of 35 nm and 105 nm interspacing, give an uniform thickness pore length about more than 5 µm, depending on the anodization time. Meanwhile, the analysis of the anodized titania nanotubes, with an average inner pore diameter of 100 nm and 40 nm wall thickness, shown to be about 300 nm in length. Each type of membranes were also studied in both cases, when the nanopores were empty and after filling with electrodeposited Ni. The direct analysis by rf‐GD‐OES reveals the ability of this technique to control the quality of these so synthesized nanocomposites formed by electrodeposited Ni nanowires into the alumina and titania nanoporous templates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth and characterization of Sn doped ZnO thin films by pulsed laser deposition
Authors: López‐Ponce E. , Costa‐Krämer J. L. , Martín‐González M. S. , Briones F. , Fernández J. F. , Caballero A. C. , Villegas M. , de Frutos J.
Published: April 2006

Sn:ZnO thin films with different Sn concentrations were grown by pulsed laser deposition (PLD) onto single‐crystal Si(001) substrates at an oxygen pressure of 2 × 10–2 mbar and substrate temperature of 600 °C. The targets used were high density Sn:ZnO pellets with different Sn concentrations produced by mixing ZnO and SnO2 by conventional ceramic routes. A deep structural and electrical characterization was carried out in order to determine the role of an increasing Sn nominal concentration on the ZnO film transport properties. Only films with a nominal 0.1 at% Sn show an improvement of the transport properties, lower resistivity and higher donor concentration, with respect to pure ZnO thin films. For films with larger Sn nominal concentrations segregated SnZnO phases appear that lead to larger film resistivities and no increase in donor concentration. The 0.1 at% Sn film is accordingly a good candidate to study the possible room temperature ferromagnetism when co doping with Mn. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Synthesis and perspectives of complex crystalline nano‐structures
Authors: Kudera Stefan , Carbone Luigi , Zanella Marco , Cingolani Roberto , Parak Wolfgang J. , Manna Liberato
Published: April 2006

Research on inorganic colloidal nanocrystals has moved from the synthesis of simple structures, such as spherical nanoparticles, to more elaborate particles with shapes such as rods, stars, discs, and branched nanocrystals, and recently to nanoparticles that are composed out of sections of different materials. Nanocrystal heterostructures represent a convenient approach to the development of nanoscale building blocks, as they merge sections with different functionality in the same particle, without the need of inorganic cross‐linkers. The present article gives an overview of synthesis strategies to complex nanocrystals and will highlight their structural properties, as well as discuss some envisaged applications. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Carbon nanotube based composites for electronic applications: CNT–conducting polymers, CNT–Cu
Authors: Ferrer‐Anglada N. , Gomis V. , El‐Hachemi Z. , Weglikovska U. Dettlaff , Kaempgen M. , Roth S.
Published: April 2006

We performed two different types of composites using single wall carbon nanotubes: 1) transparent electrodes, using a thin network of carbon nanotubes as electrode on which we can grow a conducting polymer electrochemically on it, polyaniline or polypirrole, CNT–CP; the obtained thin films are both transparent (transmission coefficient T from 70% to 95%) and electrically conducting. The samples were observed by AFM, optical and Raman spectroscopy, room temperature electrical conductivity were performed. 2) CNT with a metal, copper in our case (CNT–Cu), as a material with high electrical and thermal conductivity and low coefficient of thermal expansion (CTE), to be used in electronic packaging. We observed the samples by AFM and SEM. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Free‐electron spin surfaces in 3D and 2D zinc‐blende semiconductors
Authors: Dargys A.
Published: April 2006

Using the Dresselhaus Hamiltonian and parametrized form of the wave functions it is shown that 2D electron spin surfaces are the ellipsoids of rotation in zinc‐blende‐type semiconductors, when the spin‐splitting is included. However, in bulk semiconductors the exact solution shows that the spin surfaces are of higher, spherical symmetry. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coherent structure of the convective boundary layer derived from large-eddy simulations
Authors: Schmidt Helmut , Schumann Ulrich
Published: April 2006

Turbulence in the convective boundary layer (CBL) uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for zero mean horizontal flow using large-eddy simulations (LES). The Rayleigh number is effectively infinite, the Froude number of the stable layer is 0.09 and the surface roughness height relative to the height of the convective layer is varied between 10−6 and 10−2. The LES uses a finite-difference method to integrate the three-dimensional grid-volume-averaged Navier–Stokes equations for a Boussinesq fluid. Subgrid-scale (SGS) fluxes are determined from algebraically approximated second-order closure (SOC) transport equations for which all essential coefficients are determined from the inertial-range theory. The surface boundary condition uses the Monin–Obukhov relationships. A radiation boundary condition at the top of the computational domain prevents spurious reflections of gravity waves. The simulation uses 160 × 160 × 48 grid cells. In the asymptotic state, the results in terms of vertical mean profiles of turbulence statistics generally agree very well with results available from laboratory and atmospheric field experiments. We found less agreement with respect to horizontal velocity fluctuations, pressure fluctuations and dissipation rates, which previous investigations tend to overestimate. Horizontal spectra exhibit an inertial subrange. The entrainment heat flux at the top of the CBL is carried by cold updraughts and warm downdraughts in the form of wisps at scales comparable with the height of the boundary layer. Plots of instantaneous flow fields show a spoke pattern in the lower quarter of the CBL which feeds large-scale updraughts penetrating into the stable layer aloft. The spoke pattern has also been found in a few previous investigations. Small-scale plumes near the surface and remote from strong updraughts do not merge together but decay while rising through large-scale downdraughts. The structure of updraughts and downdraughts is identified by three-dimensional correlation functions and conditionally averaged fields. The mean circulation extends vertically over the whole boundary layer. We find that updraughts are composed of quasi-steady large-scale plumes together with transient rising thermals which grow in size by lateral entrainment. The skewness of the vertical velocity fluctuations is generally positive but becomes negative in the lowest mesh cells when the dissipation rate exceeds the production rate due to buoyancy near the surface, as is the case for very rough surfaces. The LES results are used to determine the root-mean-square value of the surface friction velocity and the mean temperature difference between the surface and the mixed layer as a function of the roughness height. The results corroborate a simple model of the heat transfer in the surface layer.
Minimum induced power requirements for flapping flight
Authors: Hall Kenneth C. , Hall Steven R.
Published: April 2006

The Betz criterion for minimum induced loss is used to compute the optimal circulation distribution along the span of flapping wings in fast forward flight. In particular, we consider the case where flapping motion is used to generate both lift (weight support) and thrust. The Betz criterion is used to develop two different numerical models of flapping. In the first model, which applies to small-amplitude harmonic flapping motions, the optimality condition is reduced to a one-dimensional integral equation which we solve numerically. In the second model, which applies to large-amplitude periodic flapping motions, the optimal circulation problem is reduced to solving for the flow over an infinitely long wavy sheet translating through an inviscid fluid at rest at infinity. This three-dimensional flow problem is solved using a vortex-lattice technique. Both methods predict that the induced power required to produce thrust decreases with increasing flapping amplitude and frequency. Using the large-amplitude theory, we find that the induced power required to produce lift increases with flapping amplitude and frequency. Therefore, an optimum flapping amplitude exists when the flapping motion of wings must simultaneously produce lift and thrust.
Three-dimensional Floquet stability analysis of the wake of a circular cylinder
Authors: Barkley Dwight , Henderson Ronald D.
Published: April 2006

Results are reported from a highly accurate, global numerical stability analysis of the periodic wake of a circular cylinder for Reynolds numbers between 140 and 300. The analysis shows that the two-dimensional wake becomes (absolutely) linearly unstable to three-dimensional perturbations at a critical Reynolds number of 188.5±1.0. The critical spanwise wavelength is 3.96 ± 0.02 diameters and the critical Floquet mode corresponds to a ‘Mode A’ instability. At Reynolds number 259 the two-dimensional wake becomes linearly unstable to a second branch of modes with wavelength 0.822 diameters at onset. Stability spectra and corresponding neutral stability curves are presented for Reynolds numbers up to 300.
Turbulence measurements around a mild separation bubble and downstream of reattachment
Authors: Alving Amy E. , Fernholz H. H.
Published: April 2006

This paper describes the behaviour of a turbulent boundary layer on a smooth, axisymmetric body exposed to an adverse pressure gradient of sufficient strength to cause a short region of mean reverse flow ('separation’). The pressure distribution is tailored such that the boundary layer reattaches and then develops in a nominally zero pressure gradient. Hot-wire and pulsed-wire measurements are presented over the separated region and downstream of reattachment. The response of the turbulence quantities to separation and to reattachment is discussed, with emphasis on the relaxation behaviour after reattachment. Over the separation bubble, the response is characteristic of that seen by other workers: the Reynolds stresses in the inner region are reduced and stress peaks develop away from the wall. At reattachment, the skewness of the fluctuating wall shear stress vanishes, as it is known to do at separation. After reattachment, the outer-layer stresses decay towards levels typical of unperturbed boundary layers. But the inner-layer relaxation is unusual. As the viscous wall stress increases downstream of reattachment, the recovery does not start at the wall and travel outward via the formation of an ‘internal’ layer, the process observed in many other relaxing flows. In fact, the inner layer responds markedly more slowly than the outer layer, even though response times are shortest near the wall. It is concluded that the large-scale, outer structures in the turbulent boundary layer survive the separation process and interfere with the regeneration of Reynolds stresses in the inner region after reattachment. This behaviour continues for at least six bubble lengths (20 boundary-layer thicknesses) after reattachment and is believed to have profound implications for our understanding of the interaction between inner and outer layers in turbulent boundary layers.
Higher-order resonant instabilities of internal gravity waves
Authors: Sonmor L. J. , Klaassen G. P.
Published: April 2006

We investigate the three-dimensional characteristics of a general class of resonant temporal instabilities of internal gravity waves, in which the disturbance comprises two infinitesimal internal gravity waves satisfying the conditions $\omega_I + \omega_{II} = n\tilde{\omega}$, with growth rates of order $(\vert \tilde{u}\vert \vert \tilde{k}\vert /N)^n$, assuming small dimensional primary-wave velocity amplitude $\vert \tilde{u} \vert$. We derive simple equations for their wave-numbers, frequency, growth rate, and energy budget. Interactions involving more than two disturbance components are shown not to represent distinct families of solutions, but rather to comprise order transitions linking together two or more ‘two-disturbance-component’ solutions of different order n. Unlike n = 1 resonant instabilities, those with n [ges ] 3 can align with the wave shear flow. We calculate the peak growth rate, spanwise wavenumber, and energy budget of shear-aligned resonance, as functions of wave frequency; they extract energy from both the wave shear and buoyancy fields.
Free-surface flows with two stagnation points
Authors: Vanden-Broeck J.-M. , Dias F.
Published: April 2006

Symmetric suction flows are computed. The flows are free-surface flows with two stagnation points. The configuration is related to the modelling of wave breaking at the bow of a ship. It is shown that there is a countably infinite number of solutions and that the free-surface profiles are characterized by waves.
Large-eddy simulation of transition to turbulence in a boundary layer developing spatially over a flat plate
Authors: Comte FréDÉRic Ducros, Pierre , Lesieur Marcel
Published: April 2006

It is well known that subgrid models such as Smagorinsky's cannot be used for the spatially growing simulation of the transition to turbulence of flat-plate boundary layers, unless large-amplitude perturbations are introduced at the upstream boundary: they are over-dissipative, and the flow simulated remains laminar. This is also the case for the structure-function model (SF) of Métais & Lesieur (1992). In the present paper we present a sequel to this model, the filtered-structure-function (FSF) model. It consists of removing the large-scale fluctuations of the field before computing its second-order structure function. Analytical arguments confirm the superiority of the FSF model over the SF model for large-eddy simulations of weakly unstable transitional flows. The FSF model is therefore used for the simulation of a quasi-incompressible (M∞ = 0.5) boundary layer developing spatially over an adiabatic flat plate, with a low level of upstream forcing. With the minimal resolution 650 × 32 × 20 grid points covering a range of streamwise Reynolds numbers Rex1 ε [3.4 × 105, 1.1 × 106], transition is obtained for 80 hours of time-processing on a CRAY 2 (whereas DNS of the whole transition takes about ten times longer). Statistics of the LES are found to be in acceptable agreement with experiments and empirical laws, in the laminar, transitional and turbulent parts of the domain. The dynamics of low-pressure and high-vorticity distributions is examined during transition, with particular emphasis on the neighbourhood of the critical layer (defined here as the height of the fluid travelling at a speed equal to the phase speed of the incoming Tollmien–Schlichting waves). Evidence is given that a subharmonic-type secondary instability grows, followed by a purely spanwise (i.e. time-independent) mode which yields peak-and-valley splitting and transition to turbulence. In the turbulent region, flow visualizations and local instantaneous profiles are provided. They confirm the presence of low- and high-speed streaks at the wall, weak hairpins stretched by the flow and bursting events. It is found that most of the vorticity is produced in the spanwise direction, at the wall, below the high-speed streaks. Isosurfaces of eddy viscosity confirm that the FSF model does not perturb transition much, and acts mostly in the vicinity of the hairpins.
On the formation of longitudinal vortices in a turbulent boundary layer over wavy terrain
Authors: Phillips W. R. C. , Wu Z. , Lumley J. L.
Published: April 2006

Parallel inviscid O(1) shear interacting with O(ε) spanwise-independent neutral rotational Rayleigh waves are used to model turbulent boundary layer flow over small-amplitude rigid wavy terrain. Of specific interest is the instability of the flow to spanwise-periodic initially exponentially growing longitudinal vortex modes via the Craik–Leibovich CL2-O(1) instability mechanism and whether it is this instability mechanism that gives rise to longitudinal vortices evident in the recent experiments of Gong et al. (1996). In modelling the flow, wave and turbulence length scales are assumed sufficiently disparate to cause minimal interaction. This allows the primary mean velocity profile to be specified. Two profiles were chosen: a power law and the logarithmic law of the wall. Important in wave–mean interactions of this class are the effect of wave-induced fluctuations upon the mean state and the influence of the developing mean flow on the fluctuating part of the motion. The former is described by a generalized Lagrangian-mean formulation; the latter by a modified Rayleigh equation. Together they comprise an eigenvalue problem for the growth rate appropriate to the initial stages of the instability. Both primary mean flows are unstable to longitudinal vortex form in the presence of Rayleigh waves whose amplitudes diminish with altitude. Moreover the interaction is most unstable for streamwise wavenumbers α = O(1), the growth rate increasing with increased spanwise wavenumber. In comparing the results with experiment, it is first shown that spanwise-independent waves excited in Gong et al.'s experiment depict velocity fluctuations whose amplitudes diminish with altitude in accord with those for appropriate Rayleigh waves. Concordantly, the longitudinal vortices depict transverse velocity components that are weaker by a factor of ε than the axial perturbation and are observed to grow at a rate consistent with exponential growth. All are key features of CL2-O(1), although the observed growth rate is not in accord with the maximal suggested by inviscid instability theory. Rather it appears that the spanwise wavenumber takes a value at which energy is extracted from the mean motion in an optimal volume-averaged sense while minimizing energy loss to both viscous dissipation and small-scale turbulence. It is concluded that the CL2-O(1) instability mechanism is physically realizable and that the data of Gong et al. represent the first documented observations thereof.
A note on Kolmogorov's third-order structure-function law, the local isotropy hypothesis and the pressure–velocity correlation
Authors: Lindborg Erik
Published: April 2006

We show that Kolmogorov's (1941b) inertial-range law for the third-order structure function can be derived from a dynamical equation including pressure terms and mean flow gradient terms. A new inertial-range law, relating the two-point pressure–velocity correlation to the single-point pressure–strain tensor, is also derived. This law shows that the two-point pressure–velocity correlation, just like the third-order structure function, grows linearly with the separation distance in the inertial range. The physical meaning of both this law and Kolmogorov's law is illustrated by a Fourier analysis. An inertial-range law is also derived for the third-order velocity–enstrophy structure function of two-dimensional turbulence. It is suggested that the second-order vorticity structure function of two-dimensional turbulence is constant and scales with$\epsilon ^{2/3}_\omega$in the enstrophy inertial range, εωbeing the enstrophy dissipation. Owing to the constancy of this law, it does not imply a Fourier-space inertial-range law, and therefore it is not equivalent to thek−1law for the enstrophy spectrum, suggested by Kraichnan (1967) and Batchelor (1969).
Weakly nonlinear oscillations of nearly inviscid axisymmetric liquid bridges
Authors: NicoláS José A. , Vega José M.
Published: April 2006

A weakly nonlinear analysis is presented of the small oscillations of nearly inviscid liquid bridges subjected to almost resonant axial vibrations of the disks. An amplitude equation is derived for the evolution of the complex amplitude of the oscillations that exhibits hysteresis and period doublings. We also analyse the steady streaming in the bulk forced by the oscillatory boundary layers near the disks; the boundary layer near the free surface produces no forcing terms. In particular some experimentally observed patterns are explained, and some new, non-observed ones are predicted. We conclude that the structure of this steady flow is not the appropriate one to counterbalance steady thermocapillary convection, but our results indicate how to get the desired counterbalancing effect.
Simulations of the viscous flow normal to an impulsively started and uniformly accelerated flat plate
Authors: Koumoutsakos P. , Shiels D.
Published: April 2006

The development of a two-dimensional viscous incompressible flow generated from an infinitesimally thin flat plate, impulsively started or uniformly accelerated normal to the free stream is studied computationally. An adaptive numerical scheme, based on vortex methods, is used to integrate the vorticity–velocity formulation of the Navier–Stokes equations. The results of the computations complement relevant experimental works while providing us with quantities such as the vorticity field and the unsteady forces experienced by the body. For the uniformly accelerated plate the present simulations capture the development of a number of centers of vorticity along the primary separating shear layer. This phenomenon has been observed in experimental works but has not been predicted by inviscid models. The present simulations suggest that this Kelvin–Helmholtz-type instability is driven by the interaction of primary and secondary vorticity near the tips of the plate and depends on the acceleration of the plate.
The stability of two-phase flow over a swept wing
Authors: Coward Adrian V. , Hall Philip
Published: April 2006

We use numerical and asymptotic techniques to study the stability of a two-phase air/water flow above a flat porous plate. This flow is a model of the boundary layer which forms on a yawed cylinder and can be used as a useful approximation to the air flow over swept wings. The air and water form an immiscible interface which can destabilize the flow, leading to travelling wave disturbances which move along the attachment line. This instability occurs for lower Reynolds numbers than is the case in the absence of a water layer. The two-fluid flow can be used as a crude model of the effect of heavy rain on the leading edge of a swept wing.

We also investigate the instability of inviscid stationary modes. We calculate the effective wavenumber and orientation of the stationary disturbance when the fluids have identical physical properties. Using perturbation methods we obtain corrections due to a small stratification in viscosity, thus quantifying the interfacial effects. Our analytical results are in agreement with the numerical solution which we obtain for arbitrary fluid properties.
Moderate and steep Faraday waves: instabilities, modulation and temporal asymmetries
Authors: Jiang Lei , Ting Chao-Lung , Perlin Marc , Schultz William W.
Published: April 2006

Mild to steep standing waves of the fundamental mode are generated in a narrow rectangular cylinder undergoing vertical oscillation with forcing frequencies of 3.15 Hz to 3.34 Hz. A precise, non-intrusive optical wave profile measurement system is used along with a wave probe to accurately quantify the spatial and temporal surface elevations. These standing waves are also simulated by a two-dimensional spectral Cauchy integral code. Experiments show that contact-line effects increase the viscous natural frequency and alter the neutral stability curves. Hence, as expected, the addition of the wetting agent Photo Flo significantly changes the stability curve and the hysteresis in the response diagram. Experimentally, we find strong modulations in the wave amplitude for some forcing frequencies higher than 3.30 Hz. Reducing contact-line effects by Photo-Flo addition suppresses these modulations. Perturbation analysis predicts that some of this modulation is caused by noise in the forcing signal through ‘sideband resonance’, i.e. the introduction of small sideband forcing can generate large modulations of the Faraday waves. The analysis is verified by our numerical simulations and physical experiments. Finally, we observe experimentally a new form of steep standing wave with a large symmetric double-peaked crest, while simulation of the same forcing condition results in a sharper crest than seen previously. Both standing wave forms appear at a finite wave steepness far smaller than the maximum steepness for the classical standing wave and a surface tension far smaller than that for a Wilton ripple. In both physical and numerical experiments, a stronger second harmonic (in time) and temporal asymmetry in the wave forms suggest a 1:2 resonance due to a non-conventional quartet interaction. Increasing wave steepness leads to a new form of breaking standing waves in physical experiments.
Flow field of a diffusion flame attached to a thick-walled injector between two coflowing reactant streams
Authors: Higuera F. J. , Liñán A.
Published: April 2006

The flow field of a diffusion flame attached to a thick-rim injector between two coflowing streams of fuel and oxidiser is analysed in the Burke–Schumann limit of infinitely fast reaction rate. The length of the recirculation region immediately behind the injector and the velocity of the recirculating fluid are proportional to the shear stresses of the reactant streams on the wall of the injector for a range of rim thicknesses, and the structure of the flow in the wake depends then on three main non-dimensional parameters, measuring the gas thermal expansion due to the chemical heat release, the air-to-fuel stoichiometric ratio of the reaction, and the air-to-fuel ratio of wall shear stresses. The recirculation region shortens with increasing heat release, and the position of the flame in this region depends on the other two parameters. An asymptotic analysis is carried out for very exothermic reactions, showing that the region of high temperature around the flame is confined by neatly defined boundaries and the hot fluid moves like a high-velocity jet under a favourable self-induced pressure gradient. The immediate wake is surrounded by a triple-deck region where the interacting flow leads to an adverse pressure gradient and a reduced shear stress upstream of the injector rim for sufficiently exothermic reactions. Separation of the boundary layers on the wall of the injector, however, seems to be postponed to very large values of the gas thermal expansion.
Fluxes and sources of particulate organic carbon in the Ganga‐Brahmaputra river system
Authors: Aucour Anne‐Marie , France‐Lanord Christian , Pedoja Kevin , Pierson‐Wickmann Anne‐Catherine , Sheppard Simon M. F.
Published: April 2006

Sources and fluxes of particulate organic carbon (OC) in the Ganga‐Brahmaputra river system were estimated from 13C/12C ratios of bedrocks, soils, bank and suspended river sediments from a Himalayan watershed (Narayani, Nepal), and the Bangladesh floodplain. In the watershed of the Narayani, OC δ13C values for bank and suspended sediments have similar values and narrow ranges, with a mean of about −24.2‰ at the base of the Himalayas. On the Bangladesh floodplain, bank and suspended sediments are enriched in 13C (δ13C about −22.5‰) relative to those collected at the base of the Himalayas. The OC exported at the base of the Himalayan range is estimated to include about 15% from the 13C enriched C4 biomass while the C4 contribution is about 25% of the OC exported by the Ganga and Brahmaputra. A calculated total OC flux of 0.65 × 1012 molC yr−1 is exported to the ocean or trapped in the plain. In sediments of the Ganga and Brahmaputra, the clay‐size fraction is depleted in 13C (<2.6‰) relative to the bulk sediment. Possible explanations are that either the organic matter associated with clays is largely inherited from the Himalayan watersheds, or, and considered more likely, the clays and coarser sediments sample different OC pools, possibly with different ages (the growth of C4 crops has dramatically increased since the mid twentieth century), on the floodplain. The OC budget of the actual Ganga‐Brahmaputra system is broadly comparable to that derived from the Quaternary sediments of the Bengal fan that represents about 10% of the global OC contribution to the continental margins.
Turbulence observations of the nearshore wave bottom boundary layer
Authors: Foster D. L. , Beach R. A. , Holman R. A.
Published: April 2006

Field observations of turbulence and sediment suspension in the nearshore wave bottom boundary layer obtained during the Duck94 field experiment on the North Carolina coast showed the generation of near‐bed turbulence to be highly intermittent. The intermittent nature of the flow was examined by applying homogeneous isotropic turbulence laws over small windows of data. The time‐varying estimates of turbulent kinetic energy, shear stress, dissipation, and concentration were averaged over the phase of the free stream wave. The observations show that the magnitude of the turbulent kinetic energy is largest under the wave crest and decreases over the decelerating flow phase until reversal to offshore flow. The turbulent kinetic energy observations under the crests compare favorably to those of three laboratory studies. The sediment suspension observations were shown to have an intermittent structure and to be biased toward the onshore decelerating phase of the flow. The shear stress was estimated with the orbital velocities by assuming an eddy viscosity model and was shown to be highly sensitive to the free stream velocity and less sensitive to the free acceleration. The intermittent signal of the turbulent kinetic energy and the suspension was not evident in the shear stress estimates, providing further support that eddy viscosity models are not appropriate for predicting the suspension of sediment. Finally, dissipation rate estimates increased with proximity to the bed but were several orders of magnitude lower than those found in the active breaking region.
Controls on back‐arc basin formation
Authors: Sdrolias Maria , Müller R. Dietmar
Published: April 2006

The relationship between subduction and back‐arc spreading has been well known since the early days of plate tectonics. However, the reasons why back‐arc basins are associated with some subduction systems but not all has remained elusive. We examine the kinematic controls on subduction and back‐arc basins for both the present‐day and Cenozoic to differentiate between the major competing hypotheses for back‐arc basin formation and to explain their temporal and spatial distribution. Our new data set of subduction and back‐arc basin parameters uses a new set of paleo‐oceanic age grids (Müller et al., 2005) associated with a moving Atlantic‐Indian Ocean hot spot reference frame (O'Neill et al., 2005). The plate model includes detailed reconstructed spreading histories of back‐arc basins based on marine geophysical and satellite gravity data. Our combined rotation and oceanic paleo‐age model provides the age distribution of subducting lithosphere through space and time, convergence rates, and the absolute motion of the downgoing and overriding plates. We find that back‐arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years. Additionally, we establish an age‐dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30° with back‐arc spreading. Our results suggest that back‐arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge, thereby creating accommodation space between the overriding and subducting plates. Once back‐arc extension is established, it continues regardless of overriding plate motion, indicating back‐arc spreading is not a simple consequence of overriding plate behaviour. The landward migration of the overriding plate as a precursor to back‐arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle. However, once back‐arc extension is established, rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space. Our analysis indicates the driving mechanism for back‐arc extension is a combination of surface kinematics, properties of the downgoing slab, the effect of lateral mantle flow on the slab, and mantle wedge dynamics.
Fate of outflowing suprathermal oxygen ions that originate in the polar ionosphere
Authors: Ebihara Y. , Yamada M. , Watanabe S. , Ejiri M.
Published: April 2006

We investigate the transport and ultimate fate of outflowing suprathermal O+ ions by kinetic means using a full‐particle tracing scheme. We launched O+ ions in order to determine their final destination in terms of the magnetopause, the distant tail, the ring current (L‐shell of 5), and the atmosphere. The number of O+ ions that reached each of the destinations was calculated quantitatively by employing an empirical model of the outflowing ion distribution. This empirical model is based on long‐term data from the Suprathermal Ion Mass Spectrometer aboard the Akebono satellite and depends on Kp and the sunspot number. Major results are as follows: (1) Under an active‐time magnetic field and a midstrength convection electric field, the majority of outflowing ions with initial speed greater than the escaping speed reached the ring current, with lesser amounts reaching the magnetopause, the distant tail, and the atmosphere. (2) The presence of an active‐time magnetic field results in an increase in the number of ions that reach the ring current by a factor of ∼3, the magnetopause by a factor of ∼0.5, the distant tail by a factor of ∼200, and the atmosphere by a factor of ∼0.01 in comparison with those under a quiet time magnetic field. (3) An energy distribution of ions at L = 5 shows that in addition to a thermal/suprathermal component, an energetic component peaking at ∼20 keV appears for the active condition, which resembles an averaged spectrum of ring current O+ ions.
Dissipation at the core‐mantle boundary on a small‐scale topography
Authors: Le Mouël J. L. , Narteau C. , Greff‐Lefftz M. , Holschneider M.
Published: April 2006

The parameters of the nutations are now known with a good accuracy, and the theory accounts for most of their values. Dissipative friction at the core‐mantle boundary (CMB) and at the inner core boundary is an important ingredient of the theory. Up to now, viscous coupling at a smooth interface and electromagnetic coupling have been considered. In some cases they appear hardly strong enough to account for the observations. We advocate here that the CMB has a small‐scale roughness and estimate the dissipation resulting from the interaction of the fluid core motion with this topography. We conclude that it might be significant.