bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion solvation dynamics in water–methanol and water– dimethylsulfoxide mixtures
Authors: Day Tyler J. F. , Patey G. N.
Published: July 2002

The dynamics of selective ion solvation in water–methanol and water–dimethylsulfoxide (DMSO) systems is investigated using molecular dynamics simulations. The results are compared with earlier work involving simple dipolar models. Solvation response functions, S(t), are used to follow the energetic relaxation, and particle solvation response functions, P(t), introduced in our earlier work, are used to examine the compositional relaxation of the first solvation shell. Despite the presence of hydrogen bonding, the dynamical behavior in water–methanol systems was found to be very similar to that observed for simple Stockmayer solvents. For the water–DMSO mixtures, it was found that the relative sizes and geometries of the solvent species can have a substantial influence on the preferential solvation process. For positively charged solutes in water–DMSO the physical picture does not differ greatly from the water–methanol case. However, for negative solutes the DMSO component shows only a weak response to the charge, and the solvation process consists largely of water molecules moving slowly to the solute through an essentially static DMSO medium. Our results also illustrate that the usual solvation response function, which depends on the total solute–solvent energy, is not a very sensitive probe of selective solvation dynamics. The contribution to the total solute–solvent energy from preferential solvation is very small compared to the contributions from the relatively rapid solvent reorientation and electrostriction processes. The dynamics of selective solvation is evident in species-specific functions but these are not obtained experimentally. The insensitivity of the usual solvation response function has been noticed in recent experimental studies, and an alternative function that appears to be more sensitive to the solvent composition near the solute has been suggested.
Monte Carlo study of the phase structure of compact polymer chains
Authors: Irbäck Anders , Sandelin Erik
Published: July 2002

We study the phase behavior of single homopolymers in a simple hydrophobic/hydrophilic off-lattice model with sequence independent local interactions. The specific heat is, not unexpectedly, found to exhibit a pronounced peak well below the collapse temperature, signalling a possible low-temperature phase transition. The system size dependence at this maximum is investigated both with and without the local interactions, using chains with up to 50 monomers. The size dependence is found to be weak. The specific heat itself seems not to diverge. The homopolymer results are compared with those for two nonuniform sequences. Our calculations are performed using the methods of simulated and parallel tempering. The performances of these algorithms are discussed, based on careful tests for a small system.
Statistical thermodynamics of lattice models in zeolites: Implications of local versus global mean field interactions
Authors: Ayappa K. G.
Published: July 2002

The statistical thermodynamics of adsorption in caged zeolites is developed by treating the zeolite as an ensemble of M identical cages or subsystems. Within each cage adsorption is assumed to occur onto a lattice of n identical sites. Expressions for the average occupancy per cage are obtained by minimizing the Helmholtz free energy in the canonical ensemble subject to the constraints of constant M and constant number of adsorbates N. Adsorbate–adsorbate interactions in the Brag–Williams or mean field approximation are treated in two ways. The local mean field approximation (LMFA) is based on the local cage occupancy and the global mean field approximation (GMFA) is based on the average coverage of the ensemble. The GMFA is shown to be equivalent in formulation to treating the zeolite as a collection of interacting single site subsystems. In contrast, the treatment in the LMFA retains the description of the zeolite as an ensemble of identical cages, whose thermodynamic properties are conveniently derived in the grand canonical ensemble. For a z coordinated lattice within the zeolite cage, with εaa as the adsorbate–adsorbate interaction parameter, the comparisons for different values of εaa*=εaaz/2kT, and number of sites per cage, n, illustrate that for −1<εaa*<0 and n⩾10, the adsorption isotherms and heats of adsorption predicted with the two approaches are similar. In general, the deviation between the LMFA and GMFA is greater for smaller n and less sensitive to n for εaa*>0. We compare the isotherms predicted with the LMFA with previous GMFA predictions [K. G. Ayappa, C. R. Kamala, and T. A. Abinandanan, J. Chem. Phys. 110, 8714 (1999)] (which incorporates both the site volume reduction and a coverage-dependent εaa) for xenon and methane in zeolite NaA. In all cases the predicted isotherms are very similar, with the exception of a small steplike feature present in the LMFA for xenon at higher coverages.
Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy
Authors: Nienhuys Han-Kwang , Woutersen Sander , van Santen Rutger A. , Bakker Huib J.
Published: July 2002

We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0→1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1→2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74±0.01 ps at 298 K to 0.90±0.02 ps at 363 K. These results indicate that the O–H⋯O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration.
Photodissociation dynamics of the propargyl radical
Authors: Deyerl Hans-Jürgen , Fischer Ingo , Chen Peter
Published: July 2002

The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C3H2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 106 s−1, are in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization.
Multivalent ion–DNA interaction: Neutron scattering estimates of polyamine distribution
Authors: Zakharova S. S. , Egelhaaf S. U. , Bhuiyan L. B. , Outhwaite C. W. , Bratko D. , van der Maarel J. R. C.
Published: July 2002

The partial structure factors pertaining to DNA–DNA, DNA–polyamine, and polyamine–polyamine density correlations in DNA fragment (contour length 54 nm) solutions have been measured with small angle neutron scattering and contrast matching in water. The effect of the polyamines putrescine and spermidine on the DNA molecular structure is gauged from the limiting behavior of the DNA–DNA partial structure factor at high values of momentum transfer. The double layer structure and the extent to which the polyamines can approach the DNA are derived from the DNA–polyamine and polyamine–polyamine partial structure factors. For this purpose, the structure factors are interpreted with the correlation functions derived from the classical Poisson–Boltzmann and the modified Poisson–Boltzmann equations and/or Monte Carlo simulation. For simple salt free DNA with tetramethylammonium or putrescine counterions, spatial fluctuations in the charge density are discussed in terms of the charge structure factor. The structural arrangement of putrescine and spermidine can be fully rationalized in terms of their valence. In the case of spermidine, it is necessary to include ionic correlation effects, but this could be accomplished by modeling the ligands as hard spheres. The polyamines have no detectable effect on the DNA molecular structure and are too large to penetrate the grooves to any significant extent. These results imply that DNA condensation in the presence of polyamines is largely governed by electrostatic interactions, rather than by the binding of the multivalent cation per se.
Photochromism of salicylideneaniline (SA). How the photochromic transient is created: A theoretical approach
Authors: Zgierski Marek Z. , Grabowska Anna
Published: July 2002

The theoretical ab initio studies of the singlet states of salicylideneaniline (SA) are presented. The enol, cis-keto and trans-keto tautomers were treated by the HF/6-31G* (geometries and force fields of the ground states), and the CIS (excited states), methods. For the dynamic calculations of the rates of proton transfer (PT) in S1 states, the instanton approach was applied. It was found that the SA molecule in S0 and S1 states of both tautomers needs nonplanarity to stabilize. In the ground state the corresponding angle was calculated as 44° vs the experimental value, 49°. Upon twist of the excited system, the conical intersection of (π,π*) and (n,π*) potential surfaces takes place. In enol form the absolute minimum on the S1 potential energy surface belongs to a strongly twisted (n,π*) state. In keto-form this minimum corresponds to a planar (π,π*) state, while the twisted (n,π*) has the energy ≈1055 cm−1 higher. The angles of distortion are equal 93° and 80°, for the enol and keto form, respectively. Both (n,π*) excited tautomers are practically unable to undergo the PT reaction. This, according to the calculations, goes via the planar unstable (π,π*) state. The calculated structures and force-fields lead to the (calculated) proton transfer rate of the order observed experimentally. The calculation of the transfer rate includes multimode nature of the transfer process. The PT cycle of the SA molecule involving the creation of photochromic transient has been proposed.
Quantum generalized Langevin equation: Explicit inclusion of nonlinear system dynamics
Authors: McDowell H. Keith
Published: July 2002

We derive a quantum nonlinear generalized Langevin equation (GLE) which includes system anharmonic and nonlinear effects explicitly through either gas phase potentials or potentials of mean force. The GLE is applicable to a broad class of nonlinear Hamiltonians with time reversal invariance being the principal restriction. The constraint of linear coupling of the system to the bath is removed. Molecular time scale generalized Langevin equation theory (MTGLE) emerges as the limit case when the nonlinearities are removed explicitly from the system. Specifically, the usual harmonic approach to the dynamics of the MTGLE primary zone (or system in a system/bath partitioning) is replaced by a more general approach which allows for anharmonic and nonlinear effects. Appropriate statistical averages are developed which permit averaging over the bath and a reduction of the number of degrees of freedom to those present in the system. The final form of the quantum nonlinear GLE with attendant statistical relations is similar to the form usually assumed, particularly in the theory of chemical reactions in liquids, and differs principally in the inclusion of a frequency renormalization term, the inclusion of a shift operator which determines the system nonlinear force operator relative to its value at time zero, and the manner in which the friction kernel appears in the second fluctuation-dissipation relation.
Thermodynamic stability of Sn4, Sn5, Sn6, and Sn7 clusters by Knudsen cell mass spectrometry
Authors: Meloni G. , Schmude R. W. , Kingcade J. E. , Gingerich K. A.
Published: July 2002

The Knudsen cell mass spectrometric method has been employed to measure the partial pressures of Snn (n=1–7) under equilibrium conditions above liquid tin or a tin–gold alloy, contained in a graphite Knudsen cell. From the all-gas analyzed equilibria the following atomization enthalpies ΔaH0∘(Snn), and enthalpies of formation, ΔfH298.15∘(Snn), in kJ mol−1, have been obtained: Sn4, 750.2±14 and 450.6±14; Sn5, 990.2±22 and 512.3±22; Sn6, 1349.7±28 and 452.8±28; Sn7, 1644.2±37 and 460.0±37. The atomization energies are compared with available theoretical values.
Observation of pressure-induced direct-to-indirect band gap transition in InP nanocrystals
Authors: Lee Chieh-Ju , Mizel Ari , Banin Uri , Cohen Marvin L. , Alivisatos A. Paul
Published: July 2002

We investigate the quantum size effects in the pressure-induced direct-to-indirect band gap transition in InP nanocrystals. Hydrostatic pressures of up to 13 GPa are applied to two different sizes of InP nanocrystal samples in a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra show that the nature of the emitting states at pressures up to 9 GPa is similar to that at ambient pressure, suggesting that no direct-to-indirect transition happens below 9 GPa. For both sizes, the PL peak energy exhibits a strong blueshift with rising pressure until approximately 9 to 10 GPa. Above this pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission dominates the PL spectra. As the pressure is released, both the luminescence intensity and the peak position recover in a fully reversible manner. The change in the sign of the band gap energy pressure dependence and the disappearance of the band edge luminescence indicate the pressure-induced direct-to-indirect band gap transition. Contrary to theoretical calculations, no substantial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure.
Generalizations of the quaternion algebra and Lie algebras
Authors: Plebański J. F. , Przanowski M.
Published: July 2002

Natural generalizations of the quaternion algebra called quaternionlike algebras (ql algebras) are considered. The notion of a Lie algebra induced by a ql algebra is introduced and a classification of such Lie algebras is presented. It is shown that local Lie groups defined by Lie algebras induced by ql algebras exhaust, with accuracy to local isomorphisms, all local Lie groups endowed with some simple composition laws of their local parameters.
Canonoid transformations in generalized mechanics
Authors: Teixeira Julio M. , Negri Luiz J. , De Oliveira Luimar C.
Published: July 2002

The problem of obtaining two Hamiltonian functions linked by a canonoid transformation is solved in the realm of generalized mechanics. By pairing the canonically conjugated pairs of variable a technique that also appears to be useful in solving many other problems in generalized mechanics is introduced.
Conformally symmetric static fluid spheres
Authors: Maartens R. , Maharaj M. S.
Published: July 2002

Solutions of the Einstein–Maxwell equations for static spheres of charged imperfect fluids are investigated, where the space-time geometry is assumed to admit a conformal symmetry. Previous work is generalized by considering a nonstatic conformal symmetry. This allows the possibility of solutions that are nonsingular at the center, unlike the previous solutions based on a static conformal symmetry. Two such regular solutions are presented for charged spheres. The further generalization necessary to find stable exact stellar models with a conformal symmetry is indicated.
Canonical Poisson brackets of different gradings and strange superalgebras
Authors: Khudaverdian O. M. , Nersessian A. P.
Published: July 2002

The superalgebra of the transformations that preserve simultaneously the even and the odd canonical Poisson brackets on superspace are studied.
Bound states and inverse scattering for the Schrödinger equation in one dimension
Authors: Aktosun Tuncay
Published: July 2002

The one-dimensional Schrödinger equation is considered when the potential and its first moment are absolutely integrable. When the potential has support contained on the left (right) half-line, it is uniquely constructed by using only the reflection coefficient from the right (left). The bound state norming constants determine whether the potential has support contained on a half-line or on the full-line. The bound state energies and the unique set of norming constants yielding the potential with support contained on the left (right) half-line are completely determined by the reflection coefficient from the right (left). An explicit example is provided.
Five-dimensional cosmological models in induced matter theory
Authors: McManus Des J.
Published: July 2002

A class of five-dimensional space–times that contain four-dimensional hypersurfaces whose intrinsic metrics are Friedman–Robertson–Walker (FRW) metrics is examined (the four-dimensional intrinsic Lorentzian metrics are both spatially homogeneous and isotropic). The five-dimensional vacuum field equations, 5Gij=0, are shown to induce four-dimensional perfect-fluid FRW field equations, 4Gij=4Tij, and thus these FRW cosmological models may be interpreted as being purely geometrical in origin.
Wavelets on the n-sphere and related manifolds
Authors: Antoine J.-P. , Vandergheynst P.
Published: July 2002

We present a purely group-theoretical derivation of the continuous wavelet transform (CWT) on the (n−1)-sphere Sn−1, based on the construction of general coherent states associated to square integrable group representations. The parameter space of the CWT, X∼SO(n)×R*+, is embedded into the generalized Lorentz group SO0(n,1) via the Iwasawa decomposition, so that X≃SO0(n,1)/N, where N≃Rn−1. Then the CWT on Sn−1 is derived from a suitable unitary representation of SO0(n,1) acting in the space L2(Sn−1,dμ) of finite energy signals on Sn−1, which turns out to be square integrable over X. We find a necessary condition for the admissibility of a wavelet, in the form of a zero mean condition, which entails all the usual filtering properties of the CWT. Next the Euclidean limit of this CWT on Sn−1 is obtained by redoing the construction on a sphere of radius R and performing a group contraction for R→∞, from which one recovers the usual CWT on flat Euclidean space. Finally, we discuss the extension of this construction to the two-sheeted hyperboloid Hn−1∼SO0(n−1,1)/SO(n−1) and some other Riemannian symmetric spaces.
Algebraic L2 decay for the solution to a class system of non-Newtonian fluid in Rn
Authors: Guo Boling , Zhu Peicheng
Published: July 2002

In this paper, we investigate the L2 decay rates for the solution to a class system of non-Newtonian fluid in Rn(n⩾2). The decay rates are optimal in the sense that they coincide with the decay rates of the solution to the heat system.
The curvature of material surfaces in isotropic turbulence
Authors: Pope S. B. , Yeung P. K. , Girimaji S. S.
Published: July 2002

Direct numerical simulation is used to study the curvature of material surfaces in isotropic turbulence. The Navier–Stokes equation is solved by a 643 pseudospectral code for constant-density homogeneous isotropic turbulence, which is made statistically stationary by low-wavenumber forcing. The Taylor-scale Reynolds number is 39. An ensemble of 8192 infinitesimal material surface elements is tracked through the turbulence. For each element, a set of exact ordinary differential equations is integrated in time to determine, primarily, the two principal curvatures k1 and k2. Statistics are then deduced of the mean-square curvature M= (1)/(2) (k21+k22), and of the mean radius of curvature R=(k21+k22)−1/2. Curvature statistics attain an essentially stationary state after about 15 Kolmogorov time scales. Then the area-weighted expectation of R is found to be 12η, where η is the Kolmogorov length scale. For moderate and small radii (less than 10η) the probability density function (pdf) of R is approximately uniform, there being about 5% probability of R being less than η. The uniformity of the pdf of R, for small R, implies that the expectation of M is infinite. It is found that the surface elements with large curvatures are nearly cylindrical in shape (i.e., ‖k1‖≫‖k2‖ or ‖k2‖≫‖k1‖), consistent with the folding of the surface along nearly straight lines. Nevertheless the variance of the Gauss curvature K=k1k2 is infinite.
Computer simulations of rapid granular shear flows between parallel bumpy boundaries
Authors: Louge M. Y. , Jenkins J. T. , Hopkins M. A.
Published: July 2002

In this Brief Communication the results of numerical simulations for the rapid, steady shear flow of identical disks are compared with the recent predictions of Hanes et al. [J. Appl. Mech. 55, 969 (1988)] for the shear stress, normal stress, and gap width. The smooth and nearly elastic disks are driven by identical, parallel, bumpy boundaries. The variations of the dynamic friction coefficient are confirmed by the simulations. The coefficient may be made large or small by suitably preparing the boundaries of the cell. The simulations also indicate that the predictions of kinetic theory apply to gaps as small as three particle diameters in width.
Turbulence spectra in free convection flow
Authors: Kotsovinos Nikolas E.
Published: July 2002

The results of a laboratory experiment to study the spectral density of temperature fluctuations in a fully developed round plume are presented. It is found that there is a range of low wave numbers K (the next to inertial subrange) where the spectra of temperature are proportional to K−3. The modification of turbulent spectrum at small wave numbers due to buoyancy forces in free convection flow is discussed.
Secondary instability of high-speed flows and the influence of wall cooling and suction
Authors: El-Hady Nabil M.
Published: July 2002

The periodic streamwise modulation of the supersonic and hypersonic boundary layers by a two-dimensional first-mode or second-mode wave makes the resulting base flow susceptible to a broadband spanwise-periodic three-dimensional type of instability. The principal parametric resonance of this instability (subharmonic) has been analyzed using Floquet theory. The effect of Mach number and the effectiveness of wall cooling or wall suction in controlling the onset, the growth rate, and the vortical structure of the subharmonic secondary instability are assessed for both a first-mode and a second-mode primary wave. Results indicate that the secondary subharmonic instability of an insulated wall boundary layer is weakened as Mach number increases. Cooling of the wall destabilizes the secondary subharmonic of a second-mode primary wave, but stabilizes it when the primary wave is a first mode. Suction stabilizes the secondary subharmonic at all Mach numbers.
Miscible viscous fingering: Experiments versus continuum approach
Authors: Bacri J.-C. , Rakotomalala N. , Salin D. , Wouméni R.
Published: July 2002

The growth of viscous fingers inside three-dimensional (3-D) porous media is studied using an acoustic technique to determine the concentration profile. Three different porous media and a wide range of viscosity ratios and flow rates have been considered. The experimental data support the definition of an instability parameter that characterizes the essential features of the viscous fingering phenomenon. The dependence of this parameter on viscosity ratio, flow rate, and the nature of the medium is compared to theoretical predictions made using a continuum approach. The data demonstrate the predicted crossover between diffusive and linear growth, and the increase of the instability with heterogeneity. The enhancement of the growth rate due to the coupling between large viscosity ratio and velocity-dependent hydrodynamic dispersion is also observed.
A nonlinear bounce-kinetic equation for trapped electrons
Authors: Gang F. Y. , Diamond P. H.
Published: July 2002

A nonlinear bounce-kinetic equation that is suitable for studies of nonlinear trapped-electron dynamics in the presence of short wavelength electrostatic fluctuations is systematically derived. The equation is used to analyze the nonlinear response of trapped electrons to the short wavelength collisionless trapped-electron mode in a sheared magnetic field. It is shown that nonlinear trapped-electron–wave interaction (trapped-electron Compton scattering) transfers wave energy from long to short wavelengths. However, the transfer rate is significantly restricted by magnetic shear, which limits the interaction to a narrow region near the rational surface that is typically smaller than the linear mode width. Therefore, it is expected that, in a sheared magnetic field, ion Compton scattering rather than nonlinear trapped-electron–wave interaction will be the dominant nonlinear process in short wavelength trapped-electron mode turbulence.
Experiments on vortex dynamics in pure electron plasmas
Authors: Driscoll C. F. , Fine K. S.
Published: July 2002

Magnetically confined columns of electrons are excellent experimental manifestations of two-dimensional (2-D) vortices in an inviscid fluid. Surface charge perturbations on the electron column (diocotron modes) are equivalent to surface ripples on extended vortices; and unstable diocotron modes on hollow electron columns are examples of the Kelvin–Helmholtz instability. Experiments demonstrate that the stable and unstable modes are distinct and may coexist, having different frequencies and radial eigenfunctions. For azimuthal mode number l=1, an exponentially unstable mode is observed on hollow columns, in apparent contradiction to 2-D fluid theory. For l=2, a similar unstable mode is observed, consistent with fluid theory. These diocotron instabilities on hollow columns saturate with the formation of smaller vortex structures, and radial transport is determined by the nonlinear interaction of these secondary vortices. The vortex pairing instability has been observed for isolated, well-controlled vortices, and the instability is found to depend critically on the vortex separation distance.
Electrostatic ion-cyclotron waves in a nonuniform magnetic field
Authors: Cartier S. L. , D’Angelo N. , Merlino R. L.
Published: July 2002

The properties of electrostatic ion-cyclotron waves excited in a single-ended cesium Q machine with a nonuniform magnetic field are described. The electrostatic ion-cyclotron waves are generated in the usual manner by drawing an electron current to a small exciter disk immersed in the plasma column. The parallel and perpendicular (to B) wavelengths and phase velocities are determined by mapping out two-dimensional wave phase contours. The wave frequency  f  depends on the location of the exciter disk in the nonuniform magnetic field, and propagating waves are only observed in the region where  f≳fci, where  fci is the local ion-cyclotron frequency. The parallel phase velocity is in the direction of the electron drift. From measurements of the plasma properties along the axis, it is inferred that the electron drift velocity is not uniform along the entire current channel. The evidence suggests that the waves begin being excited at that axial position where the critical drift velocity is first exceeded, consistent with a current-driven excitation mechanism.
Flow characteristics of orifice and tapered jets
Authors: Gutmark Ephraim , Schadow Klaus C.
Published: July 2002

Two types of small-aspect-ratio slot jets were investigated. An orifice slot jet with a 3:1 aspect ratio was shown to have features similar to an elliptic jet with a well-designed contraction. A larger spreading rate and enhanced turbulent mixing, relative to a circular jet, can be obtained using this orifice nozzle, which is advantageous for many practical applications. By adding a conical contraction to the slot jet (tapered slot jet) the characteristics were totally altered. The axes switching, typical to an elliptic jet, did not occur and the spreading rate at the major axis plane was larger than at the minor axis plane. The turbulence amplification in the initial core region was higher than both in the circular and elliptic jets. The transition from the conical contraction of the nozzle to the slot shape of the outlet section produced three-dimensional vortical structures. These structures interacted with the asymmetric vortices shed from this noncircular configuration to enhance the small scale turbulence in the core region as well as in the jet circumference. The large spreading rate of the two configurations of slot jets is beneficial to increase the bulk mixing with the surrounding fluid through entrainment. The augmented turbulence level of the tapered slot jet is important for reacting flows where small scale mixing is required.
On the bifurcation structure of axisymmetric vortex breakdown in a constricted pipe
Authors: Lopez J. M.
Published: July 2002

The bifurcation structure is presented for an axisymmetric swirling flow in a constricted pipe, using the pipe geometry of Beran and Culick [J. Fluid Mech. 242, 491 (1992)]. The flow considered has been restricted to a two-dimensional parameter space comprising the Reynolds number Re and the relative swirl V0 of the incoming swirling flow. The bifurcation diagram is constructed by solving the time-dependent axisymmetric Navier–Stokes equations. The stability of the steady results presented by Beran and Culick, obtained from a steady axisymmetric Navier–Stokes code, has been confirmed. Further, the steady solution branch has also been extended to much larger V0 values. At larger V0, a stable unsteady solution branch has been identified. This unsteady branch coexists with the previously found stable steady solution branch and originates via a turning point bifurcation. The bifurcation diagram is of the type described by Benjamin [Proc. R. Soc. London Ser. A 359, 1 (1978)] as the canonical unfolding of a pitchfork bifurcation. This type of bifurcation structure in the two-dimensional parameter space (Re,V0), suggests the possibility of hysteresis behavior over some part of parameter space, and this is observed in the present study. The implications of this on the theoretical description of vortex breakdown and the search for a criterion for its onset are discussed.
Hydrodynamic interactions in deep bed filtration
Authors: Ghidaglia Claude , de Arcangelis Lucilla , Hinch John , Guazzelli Elisabeth
Published: July 2002

Deep bed filtration has been studied experimentally and numerically for small non-Brownian particles flowing into a random packing of monosize glass spheres at low Reynolds number. It was discovered that packets of particles penetrated further than the same number of particles released one at a time. These collective effects are attributed to hydrodynamic phenomena, one plausible explanation being the existence of relaunchable ‘‘hydrodynamic captures’’ in addition to ‘‘geometric captures.’’
Buoyant-thermocapillary instabilities of differentially heated liquid layers
Authors: Mercier J. F. , Normand C.
Published: July 2002

The stability of buoyant-thermocapillary-driven flows in a fluid layer subjected to a horizontal temperature gradient is analyzed. Our purpose is the modelization of recent experimental results obtained for a fluid of Prandtl number Pr=7, by Daviaud and Vince [Phys. Rev. E 48, 4432 (1993)], who observed a transition between traveling waves and stationary rolls when the height of fluid is increased. Our model takes into account several effects that were examined separately in previous studies. The relative importance of buoyancy and thermocapillarity is controlled by the ratio W of Marangoni number to Rayleigh number. The fluid layer is bounded below by a rigid plane whose temperature varies linearly along the direction of the thermal gradient. A Biot number is introduced to describe heat transfer at the top free surface. Our stability analysis establishes the existence of a transition between stationary and oscillatory modes when W exceeds a value W0 which is a function of the Biot number. Moreover, two types of oscillatory modes have been identified which differ by the range of variation of their critical parameters (wave number, frequency, angle of propagation) versus W.
Scalar imaging velocimetry measurements of the velocity gradient tensor field in turbulent flows. I. Assessment of errors
Authors: Su Lester K. , Dahm Werner J. A.
Published: July 2002

The concept of flow field velocimetry based on scalar imaging measurements [Phys. Fluids A 4, 2191 (1992)] is here formulated in terms of an integral minimization implementation, where the velocity field u(x,t) is found by minimizing weighted residuals of the conserved scalar transport equation, along with the continuity condition and a smoothness condition. We apply this technique to direct numerical simulation (DNS) data for the limiting case of turbulent mixing of a Sc=1 passive scalar field. The spatial velocity fields u(x,t) thus obtained demonstrate good correlation with the exact DNS fields, as do the statistics of the velocity and the velocity gradient fields. The results from this integral minimization implementation also show significant improvement over those from the direct inversion technique reported earlier. These results are shown to be largely insensitive to noise at levels characteristic of current fully resolved scalar field measurements.
Elliptic instability in two-dimensional flattened Taylor–Green vortices
Authors: Sipp D. , Jacquin L.
Published: July 2002

The aim of the present paper is to study three-dimensional elliptic instability in two-dimensional flattened Taylor–Green vortices, which constitutes a model problem for the topics of wake vortex dynamics. Shortwave asymptotics and classical linear stability theory are developed. Both approaches show that the flow is unstable. In particular, the structure of the most amplified growing mode is the same as that obtained in unbounded elliptical flows. The limits of the linear regime and the effects of the nonlinear interactions are characterized by means of a spectral Direct Numerical Simulation (DNS).
An experimental and numerical study of parasitic capillary waves
Authors: Fedorov Alexey V. , Melville W. Kendall , Rozenberg Anatol
Published: July 2002

We report laboratory measurements of nonlinear parasitic capillary waves generated by longer waves in a channel. The experiments are conducted for three frequencies of longer waves (4, 5, and 6 Hz), corresponding to wavelengths of approximately 11, 7, and 5 cm. For these wavelengths we apply a model developed recently by Fedorov and Melville [J. Fluid Mech. 354, 1 (1998)] to predict the wave profile. Based on a viscous boundary layer approximation near the surface, the model enables us to efficiently calculate gravity-capillary waves. We present direct comparisons that show good agreement between the measurements and numerical predictions over a range of parameters. Finally, we give some simple estimates for a sharp cutoff in the wave number spectra observed in both the numerical solutions and the laboratory measurements of short gravity-capillary waves.
Some numerical studies of exotic shock wave behavior
Authors: Bates Jason W. , Montgomery David C.
Published: July 2002

For shock waves propagating in materials with nonideal equations of state, a variety of nonstandard phenomena can occur. Here, we present numerical studies of two such exotic shock effects: (i) “anomalous” behavior, in the terminology of Zel’dovich and Raizer; and (ii) a search for “acoustic emission instabilities.” The motivation is in part the possibility of such phenomena in the implosion of inertial confinement fusion (ICF) pellet materials, whose equations of state are currently far from well known. In shock wave theory, anomalous materials are those whose isentropes have regions of negative curvature (in the plane of pressure versus specific volume) through which the shock adiabatic passes. The existence of such regions is significant because they can interfere with the steepening of compressive pulses into shocks, lead to the formation of rarefactive shock waves, and even cause shocks to “split.” A van der Waals fluid with a large heat capacity is one example of a material possessing such anomalous properties. Acoustic emission instability—the second exotic shock mechanism considered—may occur when the slope of the shock adiabatic lies below a critical value. In this phenomenon, perturbations of a two-dimensional planar shock front can render it unstable, and lead to the downstream emission of acoustic waves. In addition to the van der Waals fluid, an equilibrium dissociation model for strong shocks in diatomic hydrogen is shown to fulfill the theoretical criteria for this instability, but its numerical verification has been hard to achieve, suggesting that further study is needed. Both classes of phenomena may be expected to play a role in ICF compression scenarios.
Inertial particle motion in a Taylor Couette rotating filter
Authors: Wereley Steven T. , Lueptow Richard M.
Published: July 2002

In rotating filtration, which is based on supercritical cylindrical Couette flow with a rotating porous inner cylinder, the motion of particles in the suspension depends on both centrifugal sedimentation and transport due to the vortical motion of Taylor vortices. We have simulated the motion of dilute, rigid, spherical particles in Taylor Couette flow using computational particle tracking in an analytic velocity field for flow just above the transition to supercritical Taylor vortex flow. Neutrally buoyant particles follow fluid streamlines closely, but not exactly due to the curvature of the velocity field very near the particle. The motion of particles with a density greater than the fluid is primarily determined by the competition between the centrifugal sedimentation related to the primary cylindrical Couette flow and the secondary radial and axial transport of the Taylor vortex flow. As a result, particles that start near the outer edge of a vortex spiral inward toward a limit cycle orbit. Likewise, particles initially near the center of a vortex spiral outward toward the same limit cycle orbit. Even when a small radially inward throughflow is imposed, particles can remain trapped in retention zones that are away from the wall of the annulus. Consequently, the dynamics of the flow field result in particles tending to be transported away from the porous inner cylinder, thus contributing to the antiplugging character of rotating filter devices.
Effect of wall slip on the stability of viscoelastic plane shear flow
Authors: Black William B. , Graham Michael D.
Published: July 2002

We show that for shear flow of an upper convected Maxwell fluid with small but nonzero slip velocity, an increasing dependence of the slip velocity on the elastic normal stress in the flow direction leads to short wavelength flow instability at sufficiently high Weissenberg number (≳10). Pressure-dependent slip can also lead to instability, but only at unrealistically large Weissenberg number. If the slip velocity depends only on shear stress, then the flow is always stable. These analytical results are valid in a specific asymptotic limit, but are independent of the specific form of the model for slip. Numerical results for specific, phenomenological slip models and the Phan-Thien–Tanner bulk constitutive model show that the results are robust in the presence of nonlinear viscoelasticity. The scaling of the critical shear stress for instability with modulus and molecular weight and of the distortion period with polymer relaxation time are qualitatively consistent with experimental observations of the sharkskin instability in linear polyethylenes. The results may also have some relationship to the recent experimental observation of short wavelength instability in plane Couette flow of an entangled solution with wall slip.
Steady streaming of fluid in the entrance region of a tube during oscillatory flow
Authors: Goldberg Irwin S. , Zhang Zongqin , Tran Minhtan
Published: July 2002

To gain understanding and insight of entrance-flow phenomena, we solve a fluid dynamics problem involving an oscillating flat velocity profile (with zero mean flow) at the end (entrance) of a semi-infinite, rigid tube. A successive-approximation method is used. Analytic solutions are given for the first-order (linear) approximation. Time averages of products of these first-order solutions for velocity components and their spatial derivatives are used to approximate the nonlinear terms in the second-order equations, which are solved numerically. The time-averaged second-order results demonstrate steady bidirectional streaming in the entrance region. Solutions are given to an illustrative problem with a Reynolds number of 20 and a Womersley unsteadiness parameter of (200)1/2. At this low Reynolds number, the streaming effects are small, with the magnitudes of the calculated streaming velocities less than three percent of the amplitude of the velocity specified at the entrance. The calculated magnitudes of streaming velocities would be expected to increase at higher Reynolds numbers; however, the convergence of the second-order solution would be less certain at higher Reynolds numbers. The steady streaming is related to first-order velocity variations that exist near the entrance of the tube.
Two-dimensional nuclear magnetic resonance measurements and numerical simulations of fluid transport in porous rocks
Authors: Stapf S. , Packer K. J. , Békri S. , Adler P. M.
Published: July 2002

Pulsed magnetic field gradient nuclear magnetic resonance (PFG-NMR) measurements have been performed for water flowing through porous Fontainebleau sandstones and are compared with flow through a packed bed of monodisperse glass beads. Pulsed gradients were applied both parallel (Z) and perpendicular (X) to the main flow axis simultaneously to obtain the two-dimensional displacement joint probability density PΔ(X,Z) of the moving spins. The evolution of PΔ(X,Z) as a function of encoding time Δ and flow rate Q is investigated. Good agreement is found between experimental PΔ(X,Z) and those obtained by numerical simulations of flow through computer-generated structures of equivalent statistical properties to those studied. The simulations are employed to compare a wider range of flow parameters than those accessible by experiment. In addition to averaged quantities, such as dispersion coefficients and moments of displacement distributions, the correlations between displacements in both directions are presented. The average transverse dispersion, 〈X2〉, for a subset of particles possessing a given axial displacement, Z, at any encoding time Δ is found to scale with Z; for flow rates and times discussed in this study, a power law relation 〈X2〉∝Zγ is observed with the spreading exponent γ being characteristic of the connectivity and statistical geometric features of the pore space. The correlation coefficient ρX2,Z is found to be positive in all cases and strongly influenced by the ratio of convective to diffusive contributions to the total particle displacements, expressed by the Péclet number. A maximum in the correlation coefficient occurs at a time scale dependent on the Péclet number and in the structures studied here, this corresponds to a characteristic lengthscale of the systems, approximated by their average pore size.
The generalized accessibility and spectral gap of lower hybrid waves in tokamaks
Authors: Takahashi Hironori
Published: July 2002

The generalized accessibility of lower hybrid waves, which may include shifting of the parallel refractive index (n∥), either upward or downward, is investigated, using the cold plasma dispersion relation and various geometrical constraint (GC) relations imposed on the behavior of n∥. It is shown that n∥ upshifting is bounded and can be insufficient to bridge a large spectral gap to cause wave damping, if the GC relation does not allow the oblique resonance to occur. The traditional n∥ upshift mechanism based upon the pitch angle of magnetic field lines is shown to lead to contradictions with experimental observations. An upshift mechanism brought about by the density gradient along field lines is proposed, which is not inconsistent with experimental observations, and provides plausible explanations to some unresolved issues of lower hybrid wave theory, including generation of ‘seed electrons.’
Compact dispersion relation for parametric instabilities of electromagnetic waves in dusty plasmas
Authors: Ma J. X. , Shukla P. K.
Published: July 2002

A compact dispersion relation for parametric instabilities of an electromagnetic pump wave is derived, taking into account the dust-particle dynamics and the dust-grain charge fluctuations caused by wave motions in unmagnetized dusty plasmas. Specific results for quasistationary modulational and filamentation instabilities are presented. It is found that thresholds of these instabilities are reduced in the presence of quasistationary plasma slow response involving charged dust grains. The present investigation should be useful in understanding the nonlinear propagation of intense electromagnetic waves through dusty space and laboratory plasmas.
Studies of instability and transport in sheared-slab plasmas with very weak magnetic shear
Authors: Dong J. Q. , Zhang Y. Z. , Mahajan S. M.
Published: July 2002

Ion temperature gradient (ITG or ηi) driven microinstabilities are studied, using kinetic theory, for tokamak plasmas with very weak (positive or negative) magnetic shear (VWS). The gradient of magnetic shear as well as the effects of parallel and perpendicular velocity shear (v∥′ and vE′) are included in the defining equations. Two eigenmodes: the double (D) and the global (G) are found to coexist. Parametric dependence of these instabilities, and of the corresponding quasilinear transport is systematically analyzed. It is shown that, in VWS plasmas, a parallel velocity shear (PVS) may stabilize or destabilize the modes, depending on the individual as well as the relative signs of PVS and of the gradient of magnetic shear. The quasilinear transport induced by the instabilities may be significantly reduced with PVS in VWS plasmas. The vE′ values required to completely suppress the instabilities are much lower in VWS plasmas than they are in normal plasmas. Possible correlations with tokamak experiments are discussed.
Electron beam instabilities in gyrotron beam tunnels
Authors: Pedrozzi M. , Alberti S. , Hogge J. P. , Tran M. Q. , Tran T. M.
Published: July 2002

Electron beam instabilities occurring in a gyrotron electron beam can induce an energy spread which might significantly deteriorate the gyrotron efficiency. Three types of instabilities are considered to explain the important discrepancy found between the theoretical and experimental efficiency in the case of quasi-optical gyrotrons (QOG): the electron cyclotron maser instability (ECMI), the electrostatic Bernstein instability (BI) and the Langmuir instability (LI). When the magnetic field gradient in drift tubes of QOG is low, the ECMI can develop in the drift tube at very low electron beam currents. Experimental measurements show that with a proper choice of absorbing structures in the beam tunnel, this instability can be suppressed. At high beam currents, the BI can induce a significant energy spread at the entrance of the interaction region. The induced energy spread scales approximately linearly with the electron beam density and for QOG one observes that the beam density is significantly higher than the beam density of an equivalent cylindrical cavity gyrotron.