bluuebunny/crossref_metadata_2025 huggingface dataset:
The alanine dipeptide free energy surface in solution
Authors: Smith Paul E.
Published: July 2002

Molecular dynamics (MD) simulations have been used to determine the two dimensional free energy surface of the alanine dipeptide in solution. The intramolecular dipeptide interactions were described by the CHARMM22 force field. Three models of solvation were considered; (i) a simple scaling of electrostatic interactions, (ii) solvation energies determined by finite difference Poisson–Boltzmann calculations, and (iii) inclusion of explicit TIP3P solvent molecules. All three solvation models produced qualitatively similar effects, but differed significantly in their quantitative values. In particular, the continuum based models predicted the β (extended) conformation to be most stable, whereas the explicit solvent model favored the αR (folded) conformation. Comparison with experimental data for the H–N–Cα-Hα coupling constant, helix–coil transition thermodynamics, and polypeptide infinite chain length characteristic ratios was performed. The explicit solvent surface produced the strongest agreement with experiment, although there is no unambiguous data to support any of the solvent models.
Geometric and electronic structures of metal-substituted fullerenes C59M (M=Fe, Co, Ni, and Rh)
Authors: Changgeng Ding , Jinlong Yang , Xiangyuan Cui , Chan C. T.
Published: July 2002

Geometric and electronic structures of metal-substituted fullerenes C59M (M=Fe, Co, Ni, and Rh) have been studied using the local spin density formalism. The clusters are found to be stable in the cage structure, while the deformation of the fullerene network due to the substituted metal atom is larger than that due to the substituted boron or nitrogen atom. The electronic structure of C59M varies with the M, and can be described in terms of defect levels in the host fullerene. Deep defect states due to the metal atom appear in the carbon-derived energy gap, and high chemical reactivity is predicted for the clusters. The magnetic moment of the M atom in C59M is found to be almost completely quenched due to the strong hybridization between the orbitals of the M atom and of their neighboring carbons. The vertical ionization potentials and electron affinities are predicted for all the clusters.
Monte Carlo and numerical self-consistent field study of end-tethered polymers in good solvent
Authors: Pépin M. P. , Whitmore M. D.
Published: July 2002

We present a Monte Carlo (MC) and numerical self-consistent field (NSCF) study of end-tethered polymers in good solvent, for surface coverages found in most experiments. Properties of the system, including the layer thickness, are calculated as functions of the degree of polymerization and surface density of the chains. A lower limit on the range of surface coverage above which the NSCF approach agrees well with the MC simulations is identified. Both approaches indicate that chain stretching begins at very low coverage, but that it remains modest throughout the regime of interest. The layer thickness does not scale linearly with degree of polymerization, and the radius of gyration of a free polymer in solution remains a relevant length scale. The MC and NSCF results are in good agreement with experiment.
Formation and equilibrium properties of living polymer brushes
Authors: Milchev A. , Wittmer J. P. , Landau D. P.
Published: July 2002

Polydisperse brushes obtained by reversible radical chain polymerization reaction onto a solid substrate with surface-attached initiators are studied by means of an off-lattice Monte Carlo algorithm of living polymers (LP). Various properties of such brushes, like the average chain length and the conformational orientation of the polymers, or the force exerted by the brush on the opposite container wall, reveal power-law dependence on the relevant parameters. The observed molecular weight distribution (MWD) of the grafted LP decays much more slowly than the corresponding LP bulk system due to the gradient of the monomer density within the dense pseudobrush which favors longer chains. Both MWD and the density profiles of grafted polymers and chain ends are well fitted by effective power laws whereby the different exponents turn out to be mutually self-consistent for a pseudobrush in the strong-stretching regime. The specific values are, however, inconsistent with a standard self-consistent field theory of pseudobrushes which predicts a much softer mushroomlike layer.
Near edge X-ray absorption fine structure measurements (XANES) and extended x-ray absorption fine structure measurements (EXAFS) of the valence state and coordination of antimony in doped nanocrystalline SnO2
Authors: Rockenberger J. , zum Felde U. , Tischer M. , Tröger L. , Haase M. , Weller H.
Published: July 2002

Colloids of nanocrystalline tin dioxide containing 9.1 at. % and 16.7 at. % antimony have been prepared by the coprecipitation method. High-resolution transmission electron microscopy (TEM) images show crystalline particles in the 2–6 nm size regime. X-ray powder diffraction patterns of nanocrystalline powders obtained by drying the colloids and heating to 100 °C indicate the same rutile lattice structure known from bulk SnO2. On heating to 500 °C in air, the nanocrystalline powder shows a slight increase in particle size but especially a change in color from yellowish to bluish which is accompanied by the development of n-type conductivity. The coordination of antimony in the SnO2 nanocrystallites has been investigated by extended x-ray absorption fine structure measurements (EXAFS) at the Sb K-edge at 5 K while its valence state was determined by near edge x-ray absorption fine structure measurements (XANES) at the Sb L1 edge. The Sb higher neighbor shell distances in the doped material differ from the corresponding distances in Sb2O3 or Sb2O5 but are identical to those in tin dioxide, indicating that antimony is almost completely incorporated into the tin dioxide lattice despite the high doping level. XANES measurements reveal that a large fraction of SbIII employed during the synthesis is already oxidized to SbV at low temperatures. On the basis of these observations, a two-step model for the formation of n-conductive Sb-doped SnO2 nanocrystals is given and quantitatively discussed with respect to the data.
A complete basis set model chemistry. VII. Use of the minimum population localization method
Authors: Montgomery J. A. , Frisch M. J. , Ochterski J. W. , Petersson G. A.
Published: July 2002

It is shown that localization is necessary to preserve size consistency in nonlinear extrapolations of molecular energies. We demonstrate that the unphysical behavior of Mulliken populations obtained from extended basis set wave functions can lead to incomplete localization of orbitals by the Pipek–Mezey population localization method, and introduce a modification to correct this problem. The new localization procedure, called minimum population localization, is incorporated into the CBS-QB3 and the new CBS-4M model chemistries, and their performance is assessed on the G2/97 test set. The errors found for CBS-QB3 are comparable with those for the G3 and G3(MP2) (mean absolute deviation of 1.10, 0.94, and 1.21 kcal/mol, respectively, on the G2/97 test set). The CBS-4M is less accurate than the other models (mean absolute deviation of 3.26 kcal/mol on the G2/97 test set), but can be applied to much larger systems. The modified localization method resolves several problem cases found with CBS-4 and improves the reliability of CBS-QB3.
End-to-end distribution function for dilute polymers
Authors: Caracciolo Sergio , Causo Maria Serena , Pelissetto Andrea
Published: July 2002

We study the end-to-end distribution function for dilute polymers. We present a computation to order O(ε2), ε=4−d, and discuss in detail its asymptotic behavior for small and large distances. The theoretical predictions are compared with Monte Carlo results, finding good agreement. We show that the McKenzie–Moore–des Cloizeaux phenomelogical ansatz provides a very precise approximation to the exact end-to-end distribution function.
Singlet and triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers
Authors: Peeters Emiel , Ramos Alicia Marcos , Meskers Stefan C. J. , Janssen René A. J.
Published: July 2002

The excited state properties of a series of α,ω-dimethyl-oligo{2,5-bis[2-(S)-methylbutoxy]-p-phenylene vinylene}s (OPVns, with n the number phenyl rings) are investigated for n=2–7 in solution at ambient temperature, under matrix-isolated conditions at low temperature, and as nanoaggregates using absorption (time-resolved), photoluminescence, photoinduced absorption, circular dichroism, and circular polarized luminescence spectroscopies. The singlet (S1←S0) and triplet (Tn←T1) transition energies decrease with conjugation length. For the S1 state of OPVn the lifetime strongly decreases with chain length due to enhanced nonradiative decay and radiative decay. The increase in the nonradiative decay rate constant is much more pronounced, and as a result the photoluminescence quantum yield is less for longer oligomers. Studies at low temperature afforded spectra with well-resolved vibronic fine structure. Under these conditions the Stokes’ shift is very small (⩽0.04 eV). The Huang–Rhys parameter and relaxation energies in the excited state associated with S1←S0 and Tn←T1 transitions and their evolution with conjugation length have been determined via a Franck–Condon analysis of the vibronic fine structure. For OPV7, the relaxation energy of S1 with respect to S0 (0.165±0.02 eV) is considerably larger than for Tn with respect to T1 (0.03 eV), but the latter increases for the shorter oligomers, in contrast to the relaxation energy of S1. The OPVns (n⩾5) form nanoaggregates in polar solvents such as ethanol. The changes in absorption and emission together with a strong circular dichroism and circularly polarized emission indicate the formation of chiral H-type aggregates under these conditions.
π
*
–σ
*
hyperconjugation mechanism on the rotational barrier of the methyl group (I): Substituted toluenes in the ground, excited, and anionic states
Authors: Nakai H. , Kawai M.
Published: July 2002

We theoretically investigate the internal rotations of the methyl group in substituted toluenes such as fluorotoluene (-F), toluidine (-NH2), cresol (-OH), and tolunitrile (-CN) in the ground, excited, and anionic states. The calculated rotational barriers reproduce well the experimental data. Orbital pictures are given for the barrier variations by excitation and electron attachment. An idea of π*–σ* hyperconjugation is introduced for a comprehensive interpretation of the barrier variations. The π*–σ* hyperconjugation mechanism clarifies the differences among ortho-, meta-, and para-systems, between π-electron donating and accepting substituents, and between first and second excited (anionic) states.
Bäcklund transformation and the Painlevé property
Authors: Weiss John
Published: July 2002

When a differential equation possesses the Painlevé property it is possible (for specific equations) to define a Bäcklund transformation (by truncating an expansion about the ‘‘singular’’ manifold at the constant level term). From the Bäcklund transformation, it is then possible to derive the Lax pair, modified equations and Miura transformations associated with the ‘‘completely integrable’’ system under consideration. In this paper completely integrable systems are considered for which Bäcklund transformations (as defined above) may not be directly defined. These systems are of two classes. The first class consists of equations of Toda lattice type (e.g., sine–Gordon, Bullough–Dodd equations). We find that these equations can be realized as the ‘‘minus-one’’ equation of sequences of integrable systems. Although the ‘‘Bäcklund transformation’’ may or may not exist for the ‘‘minus-one’’ equation, it is shown, for specific sequences, that the Bäcklund transformation does exist for the ‘‘positive’’ equations of the sequence. This, in turn, allows the derivation of Lax pairs and the recursion operation for the entire sequence. The second class of equations consists of sequences of ‘‘Harry Dym’’ type. These equations have branch point singularities, and, thus, do not directly possess the Painlevé property. Yet, by a process similar to the ‘‘uniformization’’ of algebraic curves, their solutions may be parametrically’’ represented by ‘‘meromorphic’’ functions. For specific systems, this is shown to provide a natural extension of the Painlevé property.
Classical Liouville completely integrable systems associated with the solutions of Boussinesq–Burgers’ hierarchy
Authors: Gu Zhuquan
Published: July 2002

Two new finite-dimensional completely integrable systems in the Liouville sense are obtained. The solutions of Boussinesq–Burgers’ hierarchy are generated by using involutive solutions of the commutable flows in the completely integrable systems.
Coherent states for n-dimensional Euclidean groups E(n) and their application
Authors: Isham C. J. , Klauder J. R.
Published: July 2002

The n-dimensional Euclidean group—the group of rotations and translations of Rn —is the natural canonical group for quantizing a system whose configuration space is the sphere Sn−1, n≥2. Since every unitary irreducible representation of E(n) is nonsquare integrable it follows that coherent states involving the whole group do not exist for such representations. Instead, coherent states are defined based on unitary, reducible representations of the group, and it will be shown that, with such states, quantum dynamics on the sphere admits a path-integral representation involving novel first-order classical actions defined on E(n) as well as integrations over the entire group manifold at each time slice. Irreducibility of the quantization is achieved through a final limiting process.
A simple method for group analysis and its application to a model of detonation
Authors: Ibragimov N. H. , Torrisi M.
Published: July 2002

A simple procedure is suggested to obtain extensions of the principal Lie algebra, for invariance transformations of a given family of equations, by using an equivalence algebra. An application to a qualitative model of detonation is performed.
On the self-linking of knots
Authors: Bott Raoul , Taubes Clifford
Published: July 2002

This note describes a subcomplex F of the de Rham complex of parametrized knot space, which is combinatorial over a number of universal ‘‘Anomaly Integrals.’’ The self-linking integrals of Guadaguini, Martellini, and Mintchev [‘‘Perturbative aspects of Chern–Simons field theory,’’ Phys. Lett. B 227, 111 (1989)] and Bar-Natan [‘‘Perturbative aspects of the Chern–Simons topological quantum field theory,’’ Ph.D. thesis, Princeton University, 1991; also ‘‘On the Vassiliev Knot Invariants’’ (to appear in Topology)] are seen to represent the first nontrivial element in H0(F)—occurring at level 4, and are anomaly free. However, already at the next level an anomalous term is possible.
Inelastic microstructure in rapid granular flows of smooth disks
Authors: Hopkins Mark A. , Louge Michel Y.
Published: July 2002

Computer simulations of two-dimensional rapid granular flows of uniform smooth inelastic disks under simple shear reveal a dynamic microstructure characterized by the local, spatially anisotropic agglomeration of disks. A spectral analysis of the concentration field suggests that the formation of this inelastic microstructure is correlated with the magnitude of the total stresses in the flow. The simulations confirm the theoretical results of Jenkins and Richman [J. Fluid Mech. 192, 313 (1988)] for the kinetic stresses in the dilute limit and for the collisional stresses in the dense limit, when the size of the periodic domain used in the simulations is a small multiple of the disk diameter. However, the kinetic and, to a lesser extent, collisional stresses both increase significantly with the size of the periodic domain, thus departing from the predictions of the theory that assumes spatial homogeneity and isotropy.
Long-wavelength instabilities in three-layer flow down an incline
Authors: Weinstein Steven J. , Kurz Mark R.
Published: July 2002

This paper reports a long-wavelength instability which has not previously been identified for three-layer free-surface flow down an inclined plane. The instability is identified in the zeroth-order asymptotic solution in wave number, indicating that neither inertial nor finite wavelength effects are necessary to induce instabilities in three-layer systems. Various neutral stability boundaries are presented which demonstrate the effect of viscosity stratification, density stratification, and layer thicknesses. It is found that destabilization occurs in cases where the middle-layer viscosity (for equal densities in each layer) or density (for equal viscosities in each layer) is smaller than those of the adjacent layers. The regions of instability afford a smooth transition between neutrally stable regions of the parameter space where the in-phase and out-of-phase characteristics of the interfaces differ.
Wave formation in the gravity-driven low-Reynolds number flow of two liquid films down an inclined plane
Authors: Chen KangPing
Published: July 2002

Wave formation in the gravity-driven low-Reynolds number flow of two liquid films down an inclined plane is studied by a linear stability analysis. Wavy motion can appear due to an instability of either the fluid–fluid interface or the fluid-air free surface. It is shown that the flow is always unstable and wavy motion can occur when the less viscous layer is in the region next to the wall for any Reynolds number and any finite interface and surface tensions. Stability can be achieved for the configuration with the more viscous component adjacent to the wall in the presence of interfacial tension when Reynolds number is small enough.
Direct numerical simulation of three-dimensional open-channel flow with zero-shear gas–liquid interface
Authors: Komori Satoru , Nagaosa Ryuichi , Murakami Yasuhiro , Chiba Satoshi , Ishii Katsuya , Kuwahara Kunio
Published: July 2002

Turbulence structure in an open-channel flow with a zero-shear gas–liquid interface was numerically investigated by a three-dimensional direct numerical simulation (DNS) based on a fifth-order finite-difference formulation, and the relationship between scalar transfer across a zero-shear gas–liquid interface and organized motion near the interface was discussed. The numerical predictions of turbulence quantities were also compared with the measurements by means of a two-color laser Doppler velocimeter. The results by the DNS show that the vertical motion is restrained in the interfacial region and there the turbulence energy is redistributed from the vertical direction to the streamwise and spanwise directions through the pressure fluctuation. The large-scale eddies are generated by bursting phenomena in the wall region and they are lifted up toward the interfacial region. Then, the eddies renew the interface and promote the scalar transfer across the gas–liquid interface. Both the damping effect and the generation process of the surface-renewal motions predicted by the DNS explain well the experimental results deduced in previously published studies. Furthermore, the predicted bursting frequency and mass transfer coefficient are in good agreement with the measurements.
A review of free-electron lasers
Authors: Roberson C. W. , Sprangle P.
Published: July 2002

Free-electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high-gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof-of-principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.
Flow visualization of Dean vortices in a curved channel with 40 to 1 aspect ratio
Authors: Ligrani P. M. , Niver R. D.
Published: July 2002

Results from a flow visualization study of Dean vortex flow are presented. These were obtained over a range of Dean numbers from 40 to 220 using a transparent channel with mild curvature, an aspect ratio of 40 to 1, and an inner to outer radius ratio of 0.979. Observations and photographs show evidence of pairs of counter-rotating Dean vortices indicated by mushroom-shaped smoke patterns for Dean numbers greater than 64 and angular positions at least 85° from the start of curvature. Photographs showing nonsymmetric Dean vortices with rocking motion are presented and believed to be evidence of a twisting mode of oscillations. Dean vortices with oscillations mostly in the radial direction are also observed, which are believed to strongly depend on the small amplitude disturbances that trigger initial vortex development. Photographic evidence of small secondary vortex pairs, and vortices with simultaneous radial and spanwise oscillations are also given along with a domain map showing the experimental conditions for different types of vortex behavior.
Intermittency, local isotropy, and non-Gaussian statistics in atmospheric surface layer turbulence
Authors: Katul Gabriel G. , Parlange Marc B. , Chu Chia R.
Published: July 2002

Orthonormal wavelet expansions are applied to atmospheric surface layer velocity and temperature measurements above a uniform bare soil surface that exhibit a long inertial subrange energy spectrum. In order to investigate intermittency effects on Kolmogorov’s theory, a direct relation between the nth-order structure function and the wavelet coefficients is derived. This relation is used to examine deviations from the classical Kolmogorov theory for velocity and temperature in the inertial subrange. The local nature of the orthonormal wavelet transform in physical space aided the identification of events directly contributing to intermittency buildup at inertial subrange scales. These events occur at edges of large eddies and contaminate the Kolmogorov inertial subrange scaling. By suppressing these events, the statistical structure of the inertial subrange for the velocity and temperature, as described by Kolmogorov’s theory, is recovered. The suppression of intermittency on the nth-order structure function is carried out via a conditional wavelet sampling scheme. The conditioned wavelet statistics reproduced the Kolmogorov scaling (up to n=6) in the inertial subrange and result in a zero intermittency factor. The conditional wavelet statistics for the mixed velocity temperature structure functions are also presented. It was found that the conditional wavelet statistics for these mixed moments result in a thermal intermittency parameter consistent with other laboratory and field measurements. The relationship between Kolmogorov’s theory and near-Gaussian statistics for velocity and temperature gradients is also considered.
Solitary wave dynamics of film flows
Authors: Liu Jun , Gollub J. P.
Published: July 2002

The development and interaction of solitary wave pulses is critical to understanding wavy film flows on an inclined (or vertical) surface. Sufficiently far downstream, the wave structure consists of a generally irregular sequence of solitary waves independent of the conditions at the inlet. The velocity of periodic solitary waves is found to depend on their frequency and amplitude. Larger pulses travel faster; this property, plus a strong inelasticity, causes larger pulses to absorb others during interactions, leaving a nearly flat interface behind. These wave interactions lead to the production of solitary wave trains from periodic small amplitude waves. The spacings between solitary waves can be irregular for several different reasons, including the amplification of ambient noise, and the interaction process itself. On the other hand, this irregularity is suppressed by the addition of periodic forcing.
On the mutual interaction of two gas bubbles in a sound field
Authors: Doinikov A. A. , Zavtrak S. T.
Published: July 2002

The expression for the mutual interaction force between two pulsating gas bubbles immersed in an incompressible and inviscid fluid is derived assuming that the distance between the bubbles is comparable to their sizes. The results of numerical calculations for air bubbles in water are presented. They show that at small distances between the bubbles the interaction force is substantially different from that given by Bjerknes’ theory [Fields of Force (Columbia University Press, New York, 1906)] in which the separation distance between the bubbles is assumed to be large in comparison with the bubble sizes. The discrepancy seems to result from the effect of multiple scattering which is no longer negligible when the bubbles come close to each other. The present study enables one to understand the mechanism of the formation of the stable bubble clusters (‘‘bubble grapes’’) observed experimentally by Kobelev et al. [Pis’ma Zh. Eksp. Teor. Fiz. 30, 423 (1979)].
Some considerations on the instabilities of nonpolar liquids subjected to charge injection
Authors: Pontiga F. , Castellanos A. , Malraison B.
Published: July 2002

The electrohydrodynamic stability of a plane layer of nonpolar liquid is analyzed under certain basic assumptions. The liquid is supposed to be perfectly insulating, the space charge resulting from a unipolar injection of charge. The injection is considered to be nonautonomous, that is, following a field dependent law. The regions of linear stability under stationary conditions have been computed both in terms of nondimensional parameters and directly measurable variables, such as the applied voltage and the gap spacing between the electrodes. The results have shown that the onset of instability is strongly dependent on an injection strength parameter and can be approximated by the linear criterion corresponding to autonomous injection, provided that the injected charge is given by the injection law. Experiments have also been performed to determine the instability threshold under transient conditions. An interpretation of these experiments has been done by generalizing an approximated theoretical study for autonomous injection to the case of a nonautonomous injection.
A phase field model of capillarity
Authors: Antanovskii Leonid K.
Published: July 2002

The phenomenological derivation of a phase field model of capillarity that accounts for the structure of an interfacial layer formed by two immiscible incompressible liquids is addressed. A rheological expression for the reversible component of capillary stresses is obtained in terms of the free energy of a binary fluid, which depends on the absolute temperature, composition, and gradient of composition. This model can be applied to those flows that involve change of topology of a capillary interface, such as coalescence and breakup of drops. As an illustration, an equilibrium of a binary fluid with either a flat or spherical interfacial layer is analyzed, and a thermocapillary flow in an infinite gap is considered.
Unsteady laminar flow between a pair of disks corotating in a fixed cylindrical enclosure
Authors: Humphrey J. A. C. , Schuler C. A. , Webster D. R.
Published: July 2002

The unsteady streamlined motion of a constant property fluid in the unobstructed space between a pair of disks corotating at angular velocity Ω in a fixed cylindrical enclosure is investigated numerically. Two-dimensional (axisymmetric) and three-dimensional calculations are performed using a second-order accurate time-explicit algorithm. The flow configuration corresponds to that investigated experimentally by Schuler et al. [Phys. Fluids A 2, 1760 (1990)]. The steady flow solutions are characterized by a symmetrical pair of counter-rotating toroidal vortices in the cross-stream (r-z) plane. This secondary motion is driven by the radial imbalance between the outward-directed centrifugal force and the inward-directed pressure gradient force. Axisymmetric calculations predict a flow that is steady for Re<22 200, where Re is the Reynolds number based on the disk radius, the tip speed of the disks, and the kinematic viscosity of the fluid. Above this value the motion is unsteady periodic and, while the features of the cross-stream flow pattern are broadly preserved, the symmetry of the motion about the midplane is broken by alternating periodic crossings of the toroidal vortices.

This instability is maintained through an interaction that arises between outward-directed fluid in the disk Ekman layers and inward-directed fluid in the return core flow. Three-dimensional calculations at Re=22 200 and 44 400 show that the toroidal vortices acquire a time-varying sinuous shape in the circumferential direction. These calculations reveal circumferentially periodic reversals of the axial velocity component in the cross-stream plane, including the detached shear layer separating the region of motion in solid-body rotation near the hub from the potential core, in agreement with the flow visualization observations of Humphrey and Gor [Phys. Fluids A 5, 2438 (1993)]. The wavelength of this oscillation is shown to be twice that of the circumferential velocity component which is responsible for the nodal distribution of axial vorticity. When plotted on the interdisk midplane, the axial component of vorticity manifests itself as an even integer number, 2n (n=1,2,...), of circumferentially periodic foci. Experiments show that the number of foci decreases in a stepwise manner with increasing Reynolds number. For the conditions of this study, the calculated dimensionless angular velocity of the foci, ΩF/Ω, ranges from 0.55 at Re=22 200 to 0.44 at Re=44 400. These values are close to the present experimental estimate ΩF/Ω=0.5.
Coarse-grained description of thermo-capillary flow
Authors: Jasnow David , Viñals Jorge
Published: July 2002

A mesoscopic or coarse-grained approach is presented to study thermo-capillary induced flows. An order parameter representation of a two-phase binary fluid is used in which the interfacial region separating the phases naturally occupies a transition zone of small width. The order parameter satisfies the Cahn–Hilliard equation with advective transport. A modified Navier–Stokes equation that incorporates an explicit coupling to the order parameter field governs fluid flow. It reduces, in the limit of an infinitely thin interface, to the Navier–Stokes equation within the bulk phases and to two interfacial forces: a normal capillary force proportional to the surface tension and the mean curvature of the surface, and a tangential force proportional to the tangential derivative of the surface tension. The method is illustrated in two cases: thermo-capillary migration of drops and phase separation via spinodal decomposition, both in an externally imposed temperature gradient.
Velocity turbulence properties in the near-field region of axisymmetric variable density jets
Authors: Djeridane T. , Amielh M. , Anselmet F. , Fulachier L.
Published: July 2002

This study concerns turbulent jets with density variations. The local statistical properties, obtained for the velocity field and the mean mass fraction, are needed both for better understanding of the density effects and for modeling of such flows. Special attention is paid to the near-field region (x/Dj≤20), where there is a lack of experimental results. A vertical axisymmetric turbulent jet emitted from a fully developed pipe flow, weakly confined in an air coflow, is investigated. Apart from the constant density case (air/air), two situations with variable density are treated with a density ratio varying from 0.14 [He/air] to 1.5 [CO2/air], with the momentum flux Mj maintained constant. Even though there is no exact similarity for jets when density varies, some pseudosimilarity laws can be established, and they are experimentally well confirmed. It is also found that the values of the correlations 〈u2v〉, 〈uv2〉, and 〈uw2〉 are negative in the central region (r/Lu<0.5), and this is valid for the three gases considered here. On the other hand, the use of the results for calculating balances of some of the flow conservation equations (i.e., continuity, momentum, and kinetic energy) allows us to confirm some measurements and particularly to estimate some terms that are not, and cannot be, measured at present in variable density flows, such as the dissipation rate of turbulent kinetic energy.
The effect of vortex generators on a jet in a cross-flow
Authors: Zaman K. B. M. Q. , Foss J. K.
Published: July 2002

The effect of vortex generators in the form of tabs on the penetration and spreading of a jet in a cross-flow has been studied experimentally. It is found that the tab has very little effect when placed on the leeward side, i.e., on the downstream edge of the jet nozzle relative to the free-stream flow. A study of the static pressure distribution reveals significantly lower pressures on the leeward side. Thus, when placed on that side the tab does not produce a ‘‘pressure hill’’ of sufficient magnitude that is the primary source of streamwise vorticity in the flow field over the tab. This qualitatively explains the ineffectiveness. In comparison, there is a significant effect on the flow field when the tab is placed on the windward side. The sense of vorticity generated by the tab in the latter configuration is opposite to that of the bound vortex pair that otherwise characterizes the flow. Thus, the strength of the bound vortex pair is diminished and the jet penetration is reduced.
Determination of the Smagorinsky–Lilly constant CS
Authors: Canuto V. M. , Cheng Y.
Published: July 2002

The Smagorinsky–Lilly (SL) SGS model νt=(CSΔ)2S yields a constant CS=0.20–0.22 which is a factor of 2 larger than what is needed in LES calculations; in addition, Deardorff and Hunt et al. suggested empirical corrections to the SL model to account for the effects of stratification and shear. In this paper, we propose an SGS model that naturally includes stratification and shear (recovering the two previous models) and that gives rise to a value of CS∼0.11. The three basic assumptions underlying the SL model are (1) Fickian approximation, b=−2νtS, where b is the Reynolds stress tensor and S is the strain rate tensor, (2) SGS satisfy Kolmogorov law, and (3) local equilibrium, P=ε, where P and ε are the rates of production and dissipation. We avoid (1) by using the most general b=b(S,R) relationship, where R is the vorticity, and (3) by letting the ratio P/ε vary. The most critical ingredient is (2). We derive the energy spectrum E(k) in the presence of buoyancy N and shear S and show that the SGS scales are not Kolmogorov, which sets in only for wave numbers k≫π/Δ. Integrating over all SGS scales we obtain the turbulent kinetic energy and then construct a new dissipation length l=l(N,S), which we validate in three ways: (a) use of l(N,0) reproduces the empirical SGS model by Deardorff, (b) use of l(0,S) reproduces the empirical SGS model of Hunt et al., and (c) the complete l(N,S) reproduces recent LES data that no other has been able to explain. Results for CS are as follows. Homogeneous shear: the removal of each of the three approximations is responsible for an almost equal (∼30%) lowering of CS from 0.2 to 0.1. Plane strain: the lack of vorticity makes the Fickian approximation an acceptable one. The lowering of CS is due in equal measure to the removal of (2) and (3) above. The two examples show that even though the numerical value of ∼0.11 may look like a “universal” constant, it is actually the combination of physical processes that differ from flow to flow. That CS is actually a dynamical variable that adjusts itself to each flow has already been observed by previous authors.
Velocity, acceleration and vorticity under a breaking wave
Authors: Chang Kuang-An , Liu Philip L.-F.
Published: July 2002

The fluid particle velocities in the overturning jet of a breaking wave have been measured by the Particle Image Velocimetry (PIV) technique. Monochromatic waves with wave height of 14.5 cm and wavelength of 121 cm were generated in the water depth of 20 cm. The measured fluid particle velocity at the tip of the overturning jet reached 1.68 times of the phase velocity calculated from the linear wave theory. Fluid particle accelerations were estimated from the velocity data with the following results: The overturning jet enters the horizontal water surface with an acceleration of 1.1 g at an angle of 88° downward. The PIV technique was also used to measure the instantaneous vertical vorticities generated by breaking waves. The number and locations of the vortices on the horizontal plane appear to be random. The maximum instantaneous vorticity was in the order of magnitude of 20–30 s−1, whereas the ensemble-averaged vorticity was quite small.
Large deformation of red blood cell ghosts in a simple shear flow
Authors: Eggleton C. D. , Popel A. S.
Published: July 2002

Red blood cells are known to change shape in response to local flow conditions. Deformability affects red blood cell physiological function and the hydrodynamic properties of blood. The immersed boundary method is used to simulate three-dimensional membrane–fluid flow interactions for cells with the same internal and external fluid viscosities. The method has been validated for small deformations of an initially spherical capsule in simple shear flow for both neo-Hookean and the Evans-Skalak membrane models. Initially oblate spheroidal capsules are simulated and it is shown that the red blood cell membrane exhibits asymptotic behavior as the ratio of the dilation modulus to the extensional modulus is increased and a good approximation of local area conservation is obtained. Tank treading behavior is observed and its period calculated.
Eulerian acceleration statistics as a discriminator between Lagrangian stochastic models in uniform shear flow
Authors: Sawford B. L. , Yeung P. K.
Published: July 2002

Direct numerical simulation (DNS) calculations of Eulerian acceleration statistics for homogeneous turbulence in uniform shear flow are used to test the closures implied by two different Lagrangian stochastic models for turbulent dispersion. These different models, due to Thomson [J. Fluid Mech. 180, 529 (1987)] and Borgas [Preprints of the Eighth Symposium on Turbulence and Diffusion (American Meteorological Society, Boston, 1988), p. 96], are representative of the range of a class of models which are quadratic in the velocity and are both consistent with the Eulerian velocity statistics which characterize the flow. This is the so-called nonuniqueness problem; Eulerian velocity statistics are not sufficient to uniquely define a Lagrangian stochastic model. We show here that these two models give different Eulerian acceleration statistics, which thus serve to discriminate between the models. The drift term in these stochastic models is related to the mean of the acceleration conditioned on the velocity, enabling related joint statistics of the velocity and the acceleration, such as their covariance and their cross product, to be determined. The model closures represent these joint statistics in terms of the mean shear, the Reynolds stress tensor, and its rate of change. Differences between the two models show up in the direct contribution of the mean shear to off-diagonal components of the conditional mean acceleration and the acceleration–velocity covariance in the shear plane, and in the mean rate of rotation of the velocity vector in the shear plane. In particular, Thomson’s model allocates the direct shear contribution to the correct component of the acceleration–velocity covariance in the shear plane, whereas Borgas’s model does not. Other components are identical in the two models. Overall, Thomson’s model represents the DNS results very well. However, the relatively small deviations from Thomson’s model are real and these are reflected in the fact that the mean rotation of the velocity vector has a nonzero contribution from terms that are not closed in terms of the mean flow, the Reynolds stress, and its rate of change.
Modeling of plume dynamics in laser ablation processes for thin film deposition of materials
Authors: Leboeuf J. N. , Chen K. R. , Donato J. M. , Geohegan D. B. , Liu C. L. , Puretzky A. A. , Wood R. F.
Published: July 2002

The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for film growth by pulsed-laser deposition of materials, since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters. Dynamical calculations of plume propagation in vacuum and in background gas have been performed using particle-in-cell hydrodynamics, continuum gasdynamics, and scattering models. Results from these calculations are presented and compared with experimental observations.
Development of plasma cathode electron guns
Authors: Oks Efim M. , Schanin Peter M.
Published: July 2002

The status of experimental research and ongoing development of plasma cathode electron guns in recent years is reviewed, including some novel upgrades and applications to various technological fields. The attractiveness of this kind of e-gun is due to its capability of creating high current, broad or focused beams, both in pulsed and steady-state modes of operation. An important characteristic of the plasma cathode electron gun is the absence of a thermionic cathode, a feature which leads to long lifetime and reliable operation even in the presence of aggressive background gas media and at fore-vacuum gas pressure ranges such as achieved by mechanical pumps. Depending on the required beam parameters, different kinds of plasma discharge systems can be used in plasma cathode electron guns, such as vacuum arcs, constricted gaseous arcs, hollow cathode glows, and two kinds of discharges in crossed E×B fields: Penning and magnetron. At the present time, plasma cathode electron guns provide beams with transverse dimension from fractional millimeter up to about one meter, beam current from microamperes to kiloamperes, beam current density up to about 100 A/cm2, pulse duration from nanoseconds to dc, and electron energy from several keV to hundreds of keV. Applications include electron beam melting and welding, surface treatment, plasma chemistry, radiation technologies, laser pumping, microwave generation, and more.
Helicon experiments and simulations in nonuniform magnetic field configurations
Authors: Guo X. M. , Scharer J. , Mouzouris Y. , Louis L.
Published: July 2002

Wave, antenna impedance, plasma density, and temperature anisotropy measurements are carried out for a helicon plasma source in nonuniform and uniform static magnetic fields. Strong axial density gradients associated with the nonuniform magnetic fields are observed to affect wave fields, absorption, and source efficiency. The wave field and antenna input impedance measurements are compared with a new simulation code which also calculates Poynting power flow and wave absorption profiles. Wave amplitude measurements are shown to decay more rapidly and the phase velocity varies over a wide range for the nonuniform static magnetic field case.
Annealing characteristics of Si-rich SiO2 films
Authors: Nesbit L. A.
Published: July 2002

Silicon-rich SiO2 films of various compositions were deposited by atmospheric or plasma-enhanced chemical vapor deposition (CVD) techniques. These films were annealed at various temperatures between 700 and 1100 °C. The growth and crystallinity of the silicon clusters were monitored by transmission electron microscopy. The growth of the silicon clusters was found to be diffusion controlled with an activation energy measured to a first approximation at 1.9 eV/atom for both atmospheric and plasma-enhanced CVD films. A minimum annealing temperature of between 800 and 950 °C and a minimum amount of excess Si above SiO2 are required to form crystalline silicon particles in the films. The minimum silicon crystal diameter was measured at 2.5 nm.
Macromolecular electronic device: Field-effect transistor with a polythiophene thin film
Authors: Tsumura A. , Koezuka H. , Ando T.
Published: July 2002

The first solid-state field-effect transistor has been fabricated utilizing a film of an organic macromolecule, polythiophene, as a semiconductor. The device characteristics have been optimized by controlling the doping levels of the polymer. The device is a normally off type and the source (drain) current can be modulated by a factor of 102–103 by varying the gate voltage. The carrier mobility and the transconductance have also been determined to be ∼10−5 cm2/V s and 3 nS, respectively, by means of electrical measurements.
Magnetic imaging by ‘‘force microscopy’’ with 1000 Å resolution
Authors: Martin Y. , Wickramasinghe H. K.
Published: July 2002

We describe a new method for imaging magnetic fields with 1000 Å resolution. The technique is based on using a force microscope to measure the magnetic force between a magnetized tip and the scanned surface. The method shows promise for the high-resolution mapping of both static and dynamic magnetic fields.
Ultrafast all-optical switching in a dual-core fiber nonlinear coupler
Authors: Friberg S. R. , Silberberg Y. , Oliver M. K. , Andrejco M. J. , Saifi M. A. , Smith P. W.
Published: July 2002

We report the first demonstration of a nonlinear coupler switch capable of substantially complete all-optical switching at subpicosecond rates with no light-induced thermal effects.
Polarity of transcriptional enhancement revealed by an insulator element
Authors: Wei Wei , Brennan Mark D.
Published: July 2002

Transcriptional enhancers for genes transcribed by RNA polymerase
II may be localized upstream or downstream of the stimulated promoter
in their normal chromosomal context. They stimulate transcription in an
orientation-independent manner when assayed on circular plasmids. We
describe a transient transformation system to evaluate the orientation
preference of transcriptional enhancers in
Drosophila
.
To accomplish this, the
gypsy
insulator element was used
to block bidirectional action of an enhancer on circular plasmids. In
this system, as in the chromosome, blocking of enhancer activity
requires wild-type levels of the su(Hw) protein. We evaluated the
orientation preference for the relatively large (4.4 kb)
Adh
larval enhancer from
Drosophila
melanogaster
, used in conjunction with a luciferase reporter
gene under the control of a minimal
Adh
promoter. An
orientation preference was revealed by insertion of a single copy of
the insulator between the enhancer and the promoter. This orientation
effect was greatly amplified when the promoter was weakened by removing
binding sites for critical transcription factors, consistent with a
mechanism of insulator action in which the insulator intercepts signals
from the enhancer by competing with the promoter. The orientation
preference, as much as 100-fold, is a property of the enhancer itself
because it is displayed by gene constructions introduced into the
chromosome regardless of the presence of the insulator in a distal
location. These findings are most easily reconciled with a facilitated
tracking mechanism for enhancer function in a native chromosomal
environment.