bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical potentials extracted from protein structures: Are these meaningful potentials?
Authors: Ben-Naim A.
Published: July 2002

We present an inventory of factors that determine the stability of proteins. It is shown that the (true) potential of mean force between pairs of amino acid residues does not belong to this inventory. Therefore, such quantities are not needed for either estimating the stability of a protein or for predicting its structure. It is also shown that the so-called “statistical potential” as derived from the data bank of protein structures is neither the potential nor the potential of mean force for pairs of amino-acid residues. A critical examination of the nonadditivity of the many body potential of the mean force is also presented.
Phase separation kinetics in a binary mixture of polyethylene glycol and polypropylene glycol studied by light scattering after a pressure jump: Pinning of domain growth by hydrogen bond structures
Authors: Steinhoff B. , Rüllmann M. , Kühne L. , Alig I.
Published: July 2002

The phase separation kinetics of fluid mixtures of polyethylene glycol/polypropylene glycol (a system with an upper critical mixing point) is studied after a pressure jump from the homogeneous one-phase region into the two-phase region of the phase diagram. The growth of the emerging domains of the coexisting phases is observed by small angle laser light scattering. In additional measurements the pressure dependence of the phase separation temperature is analyzed. In the kinetic experiments the time-dependent structure function is detected for a mixture with near-critical as well as for a mixture with off-critical composition. For the near-critical mixture an increase of the maximum of the scattering intensity with time has been found, which qualitatively is typical for the intermediate to late stages of spinodal decomposition. A closer analysis of the late stages reveals two maxima in the structure factor with their own set of growth exponents for the scattering vector and for the intensity. The data of the low q maximum are compatible with a two-dimensional growth process which is interpreted as a demixing process in a wetting layer. The data of the high q maximum are according to a three-dimensional process. It is assumed that this maximum reflects the demixing process in the bulk phase. The values of the three-dimensional growth exponents, which are considered to be late stage values, are not compatible with observations on other fluid systems but are close to those for solid systems or, in general, for systems with suppressed hydrodynamic interactions. For the mixture with an off-critical composition the structure function remains constant for larger times (pinning effect). The occurrence of a pinning effect in samples of relatively low molecular weight Mw (Mw⩽1019 g/mol) and the apparently suppressed hydrodynamic interactions in a fluid sample are explained with specific interactions caused by hydrogen bonding (i.e., transient entanglement or a dynamic network).
Microfibrillar buckling within fibers under compression
Authors: Odijk Theo
Published: July 2002

A tentative theory is presented of microfibrillar buckling within compressed fibers. A quantitative harmonic analysis is given of the semiclassical buckling of a clamped stiff chain; the influence of thermal undulations is incorporated in Euler buckling. A scaling analysis including entropy allows one to understand semiclassical buckling. The buckling of a microfibril within a fibrous environment is analyzed in two limits: (a) when the fiber is incompressible; (b) when the matrix is assumed to be a fixed harmonic potential. In the latter case, a network of microfibrils may melt at high enough compression before the usual buckling occurs. We also study the renormalization of the confining potential by long-range elastic fields. A provisional comparison with experimental studies on macroscopic failure is given.
Highly excited electronic states of molecular clusters and their decay
Authors: Zobeley J. , Cederbaum L. S. , Tarantelli F.
Published: July 2002

Highly excited electronic states of molecular clusters with intermediate-shell vacancies are calculated and analyzed using large scale ab initio Green’s function calculations. In sharp contrast to molecules, an intermolecular Coulombic mechanism has been found to lead to an ultrafast decay of singly and doubly ionized states with vacancies in the inner-valence region. Small hydrogen-bonded (HF)n clusters (n=2–4) have been selected as explicit examples to illustrate the proposed decay process, which does not occur in the HF monomer. The decay mechanism and the main factors that exercise an influence on it are discussed. The corresponding decay widths are estimated in selected cases, showing that the lifetimes of the states are of the order of few femtoseconds.
Ejection of CN(A) from CN(B)–Arn clusters and the radiative lifetime of CN(A,7⩽υ⩽9)
Authors: Chen Yaling , Heaven Michael C.
Published: July 2002

Electronic predissociation of CN(B)–Arn (n≈100) clusters has been examined using optical–optical double resonance techniques. Rotationally cold (Trot≈130 K) CN(A,υ=9) fragments were detected following excitation of CN(B,υ=0)–Arn. An upper bound for the predissociation rate of Γp<2×106 s−1 was determined. These results are consistent with a surface-bound structure for the CN–Arn clusters, or a sequential dissociation process that involves a surface-bound intermediate. Demonstration that free CN(A) is produced by predissociation of CN(B)–Arn validates a previous determination of the radiative lifetime for CN(A)7⩽υ⩽9 levels.
Excited states and solvatochromic shifts within a nonequilibrium solvation approach: A new formulation of the integral equation formalism method at the self-consistent field, configuration interaction, and multiconfiguration self-consistent field level
Authors: Mennucci Benedetta , Cammi Roberto , Tomasi Jacopo
Published: July 2002

The effects of the solvation on excited states are studied in the framework of a nonequilibrium regime between solute and solvent charge distributions. The approach, which exploits a separation of the polarization into slow and fast components, is inserted in a new formulation of the recently developed continuum solvation model known as integral equation formalism. This new version, implying a large computational gain both in time consuming and memory occupation, is here implemented at the Hartree–Fock level as well as at the multiconfiguration self-consistent field and configuration interaction levels. Examples of application of the method to solvatochromic shifts for low-lying excitation energies of formaldehyde, acetaldehyde, and acetone in water are shown.
Phase behavior of the restricted primitive model and square-well fluids from Monte Carlo simulations in the grand canonical ensemble
Authors: Orkoulas Gerassimos , Panagiotopoulos Athanassios Z.
Published: July 2002

Coexistence curves of square-well fluids with variable interaction width and of the restricted primitive model for ionic solutions have been investigated by means of grand canonical Monte Carlo simulations aided by histogram reweighting and multicanonical sampling techniques. It is demonstrated that this approach results in efficient data collection. The shape of the coexistence curve of the square-well fluid with short potential range is nearly cubic. In contrast, for a system with a longer potential range, the coexistence curve closely resembles a parabola, except near the critical point. The critical compressibility factor for the square-well fluids increases with increasing range. The critical behavior of the restricted primitive model was found to be consistent with the Ising universality class. The critical temperature was obtained as Tc=0.0490±0.0003 and the critical density ρc=0.070±0.005, both in reduced units. The critical temperature estimate is consistent with the recent calculation of Caillol et al. [J. Chem. Phys. 107, 1565 (1997)] on a hypersphere, while the critical density is slightly lower. Other previous simulations have overestimated the critical temperature of this ionic fluid due to their failure to account for finite-size effects in the critical region. The critical compressibility factor (Zc=Pc/ρcTc) for the ionic fluid was obtained as Zc=0.024±0.004, an order of magnitude lower than for nonionic fluids.
Vapor-liquid equilibria of square-well spheres
Authors: Elliott J. Richard , Hu Liegi
Published: July 2002

Results of molecular dynamics (MD) simulations on square-well fluids with λ=1.25, 1.375, 1.5, 1.75, and 2.0 are presented. The calculation of vapor-liquid equilibrium was performed by isochoric integration of the liquid NVE data to obtain the free energy of the liquid and equating this to the vapor free energy from a modified virial equation. The saturation pressure was investigated and compared with that from Monte Carlo simulation and second-order analytical perturbation theory. The vapor pressures from the isochoric integration technique are shown to be smoother than previous results, permitting accurate estimation of the effect of the square-well width on acentric factor. With the saturated properties from molecular dynamics, the f value used in Kofke’s Gibbs-Duhem integration was calculated and was found to be nearly constant. The related integration of the Clapeyron equation was implemented as a check on thermodynamic consistency. Vapor pressures presented here are consistent to within 2%.
Quantum cluster equilibrium theory of liquids: Freezing of QCE/3-21G water to tetrakaidecahedral “Bucky-ice”
Authors: Ludwig R. , Weinhold F.
Published: July 2002

We extend the Quantum Cluster Equilibrium (QCE/3-21G-level) theory of liquid water to include larger icelike clusters, including tetrahedral and fullerenelike clusters up to 26-mers. A low-energy tetrakaidecahedral 24-mer (labeled w24_0, with two hexagonal apical faces bridged by 12 pentagons in a cooperatively charge-balanced H-bond pattern) is found to lead to a new low-T phase that bounds both liquid and vapor regions in first-order transition lines, giving rise to a true QCE triple point. We characterize the microstructural composition and macroscopic properties of this “Bucky-ice” phase and examine its dependence on calculated energetics and vibrational properties of constituent clusters. Although the Bucky-ice phase differs in significant respects from physical ice-I (e.g., melting point ∼20 K too high, molar volume ∼5% too low), it manifests qualitatively correct thermodynamic features of true ice polymorphs, suggesting an important role of clathratelike clusters in the liquid/solid transition region.
Effect of the cluster size in modeling the H2 desorption and dissociative adsorption on Si(001)
Authors: Penev E. , Kratzer P. , Scheffler M.
Published: July 2002

Three different clusters, Si9H12, Si15H16, and Si21H20, are used in density-functional theory calculations in conjunction with ab initio pseudopotentials to study how the energetics of H2 dissociative adsorption on and associative desorption from Si(001) depends on the cluster size. The results are compared to five-layer slab calculations using the same pseudopotentials and high quality plane-wave basis set. Several exchange-correlation functionals are employed. Our analysis suggests that the smaller clusters generally overestimate the activation barriers and reaction energy. The Si21H20 cluster, however, is found to predict reaction energetics, with Eades=56±3kcal/mol (2.4±0.1eV), reasonably close (though still different) to that obtained from the slab calculations. Differences in the calculated activation energies are discussed in relation to the efficiency of clusters to describe the properties of the clean Si(001)-2×1 surface.
Forces between surfaces immersed in polyelectrolyte solutions
Authors: Yethiraj Arun
Published: July 2002

A microscopic theory is presented for the behavior of polyelectrolyte solutions between surfaces. The theory predicts oscillatory forces between hydrophobic surfaces immersed in dilute salt-free polyelectrolyte solutions. The period of the oscillation, p, scales with concentration, ρ, as p∼ρ−1/3 and p∼ρ−1/2, in the dilute and semidilute solutions, respectively. With a small amount of excess salt, the oscillatory forces are replaced by a short-ranged, predominantly attractive force. Further addition of salt causes the force to become purely repulsive. These predictions can, in principle, be tested experimentally.
Photoionization of methanol dimer using a tunable vacuum ultraviolet laser
Authors: Tsai Shang-Ting , Jiang Jyh-Chiang , Lee Yuan T. , Kung A. H. , Lin S. H. , Ni Chi-Kung
Published: July 2002

Methanol dimer (CD3OH)2 ion–molecule reaction is initialized by VUV (vacuum ultraviolet) laser photoionization. The proton and deuteron transfers are the dominant reactions. The relative probabilities of deuteron transfer from the methyl group and proton from the hydroxyl group were measured as a function of VUV photon energy between 10.91 to 10.49 eV. According to those results, the probability of proton transfer from the hydroxyl group increases with the VUV photon energy. Isotopic scrambling is not complete before dissociation of the ion complex in the photon energy used. In addition, ab initio calculations are performed and four stable structures of the methanol dimer ion are found. One of these structures is an unreported complex, CD3OHD+⋯CD2OH, which has a very unusual type of hydrogen bond. This complex plays a significant role in the deuteron transfer reaction in the range of excitation energies used in this study.
Transient resonance structures in electron tunneling through water
Authors: Peskin Uri , Edlund Åke , Bar-On Ilan , Galperin Misha , Nitzan Abraham
Published: July 2002

The mechanism of electrons tunneling through a narrow water barrier between two Pt(100) metal surfaces is studied. Assuming an adiabatic picture in which the water configuration is static on the time scale of the electron motion, the tunneling probabilities are found to increase nonmonotonically as a function of incident electron energy. A numerical investigation of single electron scattering wave functions suggests that the tunneling is enhanced by resonances, associated with molecular cavities in which the electron is trapped between repulsive oxygen cores. The lifetimes of these resonances are calculated using a novel filter diagonalization scheme, based on a converging high-order perturbative expansion of the single-electron Green’s function, and are found to be of order ⩽10 fs. The possibility that transient resonance supporting structures contribute to the enhancement of tunneling through water is discussed.
Geometrical reconstructions and electronic relaxations of silicon surfaces. I. An electron density topological study of H-covered and clean Si(111)(1×1) surfaces
Authors: Cargnoni Fausto , Gatti Carlo , May Emanuela , Narducci Dario
Published: July 2002

The relaxations of the first three interlayer distances in the H-covered Si(111)(1×1) surface were calculated using a fully periodic Hartree–Fock approach and a finely tuned slab model. All computed relaxations fall well within the error bounds of the experiment, provided the relevant geometrical parameters and the basis set of the first layer Si atoms (Si1) are both optimized. The quantum theory of atoms in molecules is applied on the wave functions of Si bulk and of H-covered or clean Si(111)(1×1) slabs so as to shed light on how the electronic perturbation caused by H adsorption and surface formation propagates and dampens through the first Si atoms layers. In the H-covered surface, the large charge transfer from Si1 to H induces a noticeable asymmetry in and strengthening of the surface Si1–Si2 back bonds, whereas in the clean slab the same bonds are found to be weakened compared to the bulk in agreement with the well-known tendency of this system to evolve in favor of other reconstructions. The negatively charged hydrogen layer in the Si(111)(1×1)–H slab is almost entirely counterbalanced by the first two silicon layers with the Si1 atoms bearing more than 94 percent of the compensating positive charge. The hydrogen and Si1 atoms in the H-covered surface polarize in such a way as to oppose the electric field created by charge transfer into the surface double layer. The effect of H-coverage is to reverse the outwards polarization of Si1 atoms present in the clean system and to enhance its magnitude. Due to the surface electric field, the atomic energies in both slabs are not found to converge towards bulk values even for the atoms of the innermost layers, although the other calculated local and integrated properties exhibit an almost perfect convergence beyond the first two or three atomic layers. In the H-covered slab, the Si1 atoms have their interatomic surface completely isolated from the outside through their interaction with H atoms, while Si2 are found to be the only surface silicon atoms in agreement with the experimental observation that passivant substitution or oxidation are mediated by Si2 and never occur directly at Si1 atoms.
Calculation of the lifetime of positronium in polymers via molecular dynamics simulations
Authors: Schmitz Heiko , Müller-Plathe Florian
Published: July 2002

A method for the study of positronium annihilation in polymers is presented. The polymer configurations are generated by atomistic molecular dynamics simulations; their geometry is fully known, so that the size and the shape of the holes can be analyzed. The positronium lifetime in the free volume sites is calculated at finite temperature with the path integral Monte Carlo method in the framework of an extended model, which comprises the positronium–polymer interaction and the electron density in the polymer. The spectra and free volume properties are compared to results from positronium annihilation spectroscopy experiments. As an example, the method is applied to polystyrene at ambient conditions.
Quantized field description of rotor frequency-driven dipolar recoupling
Authors: Boender G. J. , Vega S. , de Groot H. J. M.
Published: July 2002

A formalized many-particle nonrelativistic classical quantized field interpretation of magic angle spinning (MAS) nuclear magnetic resonance (NMR) radio frequency-driven dipolar recoupling (RFDR) is presented. A distinction is made between the MAS spin Hamiltonian and the associated quantized field Hamiltonian. The interactions for a multispin system under MAS conditions are described in the rotor angle frame using quantum rotor dynamics. In this quasiclassical theoretical framework, the chemical shift, the dipolar interaction, and radio frequency terms of the Hamiltonian are derived. The effect of a generalized RFDR train of π pulses on a coupled spin system is evaluated by creating a quantized field average dipolar-Hamiltonian formalism in the interaction frame of the chemical shift and the sample spinning. This derivation shows the analogy between the Hamiltonian in the quantized field and the normal rotating frame representation. The magnitude of this Hamiltonian peaks around the rotational resonance conditions and has a width depending on the number of rotor periods between the π pulses. Its interaction strength can be very significant at the n=0 condition, when the chemical shift anisotropies of the interacting spins are of the order of their isotropic chemical shift differences.
Equivalence between the Lagrangian and Hamiltonian formalism for constrained systems
Authors: Batlle C. , Gomis J. , Pons J. M. , Roman-Roy N.
Published: July 2002

The equivalence between the Lagrangian and Hamiltonian formalism is studied for constraint systems. A procedure to construct the Lagrangian constraints from the Hamiltonian constraints is given. Those Hamiltonian constraints that are first class with respect to the Hamiltonian constraints produce Lagrangian constraints that are FL-projectable.
Linear transport theory in a random medium
Authors: Levermore C. D. , Pomraning G. C. , Sanzo D. L. , Wong J.
Published: July 2002

The time-independent linear transport problem in a purely absorbing (no scattering) random medium is considered. A formally exact equation for the ensemble averaged distribution function 〈Ψ〉 is derived. Under the assumption of a two-fluid statistical mixture, with the transition from one fluid to the other assumed to be determined by a Markov process, an exact solution to this equation for 〈Ψ〉 is obtained. In the source-free case, this solution is shown to agree with the result obtained by ensemble averaging simple exponential attenuation. Several approximations to the exact equation for 〈Ψ〉 are considered, and numerical results given to assess the accuracy of these approximations.
Renewal theory for transport processes in binary statistical mixtures
Authors: Levermore C. D. , Wong J. , Pomraning G. C.
Published: July 2002

Renewal theory is used to analyze linear particle transport without scattering in a random mixture of two immiscible fluids, with the statistics described by arbitrary (non-Markovian) fluid chord length distributions. One conclusion (for unimodal distributions) that is drawn is that the mean and variance of the chord length distributions through each fluid is sufficient knowledge of the statistics to give a reasonably accurate description of the ensemble averaged intensity. Expressions for effective cross sections and an effective source to be used in the usual deterministic transport equation are also obtained. The use of these effective quantities allows statistical information to be introduced very simply into a standard transport equation. An analysis is given which shows how the transport description, including scattering, in a Markovian mixture can be modified to yield an approximate description of transport in a non-Markovian mixture. Numerical results are given to assess the accuracy of this model, as well as the simpler model involving the effective cross sections and source.
Some isoscalar factors for SON⊇SON−1 and state expansion coefficients for SON⊇SON−1 in terms of SUN⊇SUN−1⊇SON−1
Authors: Pan Feng , Cao Yu-Fang
Published: July 2002

General analytical expressions of state expansion coefficients for SON⊇SON−1 in terms of SUN⊇SUN−1⊇SON−1 are derived. Analytical expressions of isoscalar factors for SON⊇SON−1 for coupling (w1w2)×(w30) to (w′1w2) for the SON (N>4) irrep and (v1v2)×(v30) to (v′1v2) for the SON−1 irrep with w1+w2+w3=w′1+w2 and v1+v2+v3=v′1+v2 are obtained by using these coefficients and isoscalar factors for SUN⊇SUN−1.
Monopoles and instantons from Berry’s phase
Authors: Benedict M. G. , Fehér L. Gy. , Horváth Z.
Published: July 2002

The topological phase picked up by an arbitrary spin undergoing a cyclic series of quantum jumps is deduced by geometric considerations. Generalizing the Hamiltonian of a spin in a magnetic field, higher-dimensional Dirac monopoles are derived from the non-Abelian adiabatic phase. These monopoles appear in the momentum space description of zero mass particles in higher dimensions.
Three generation superstring models with maximal discrete symmetries
Authors: Lazarides G. , Shafi Q.
Published: July 2002

Discrete symmetries typically arise when the ten-dimensional heterotic E8×E8 superstring theory is compactified on Calabi–Yau (CY) manifolds and they are expected to play an essential role in phenomenological applications. Yau has shown how a CY manifold that leads to three generation models can be constructed. The discrete symmetries associated with several of these models are found. In particular, the models that have the largest (maximal) discrete symmetries are identified.
Geometry of superspace with even and odd brackets
Authors: Khudaverdian O. M.
Published: July 2002

The geometry of the superspace provided by two arbitrary Poisson brackets of different gradings is studied. It will be shown that the group of transformations that preserve these two brackets is finite-dimensional.
Sectional curvature, symmetries, and conformally flat plane waves
Authors: Hall G. S. , Hossack A. D. , Pulham J. R.
Published: July 2002

This paper is concerned with the sectional curvature function on space-time and discusses vector fields that generate symmetries of this function. It is shown that such vector fields form a Lie algebra and its general structure and the possibilities for its dimension are given. A consequence of the paper is a complete discussion of the Lie algebra of conformal vector fields for conformally flat plane waves.
q-exponential and q-gamma functions. I. q-exponential functionsa)
Authors: McAnally D. S.
Published: July 2002

Some properties of the q-exponential functions, standard and symmetric, are investigated for general complex nonzero q. In particular, for ‖q‖ ≠ 1, the asymptotic behavior of the q-exponential functions of a complex argument z is studied as z approaches ∞ through an infinite geometric sequence {zn(ζ) = qnζ}n∈Z of complex numbers with common ratio q. A generalized q-exponential function is defined and some of its properties are also discussed for q ≠ 0 (including the singular case of q a root of unity, or, more generally, ‖q‖=1).
On separable Schrödinger equations
Authors: Zhdanov Renat , Zhalij Alexander
Published: July 2002

We classify (1+3)-dimensional Schrödinger equations for a particle interacting with the electromagnetic field that are solvable by the method of separation of variables. As a result, we get 11 classes of the vector potentials of the electromagnetic field A(t,x⃗)=(A0(t,x⃗),A⃗(t,x⃗)) providing separability of the corresponding Schrödinger equations. It is established, in particular, that the necessary condition for the Schrödinger equation to be separable is that the magnetic field must be independent of the spatial variables. Next, we prove that any Schrödinger equation admitting variable separation into second-order ordinary differential equations can be reduced to one of the 11 separable Schrödinger equations mentioned above and carry out variable separation in the latter. Furthermore, we apply the results obtained for separating variables in the Hamilton–Jacobi equation.
Molecular dynamics of fluid flow at solid surfaces
Authors: Koplik Joel , Banavar Jayanth R. , Willemsen Jorge F.
Published: July 2002

Molecular dynamics techniques are used to study the microscopic aspects of several slow viscous flows past a solid wall, where both fluid and wall have a molecular structure. Systems of several thousand molecules are found to exhibit reasonable continuum behavior, albeit with significant thermal fluctuations. In Couette and Poiseuille flow of liquids it is found that the no-slip boundary condition arises naturally as a consequence of molecular roughness, and that the velocity and stress fields agree with the solutions of the Stokes equations. At lower densities slip appears, which can be incorporated into a flow-independent slip-length boundary condition. The trajectories of individual molecules in Poiseuille flow are examined, and it is also found that their average behavior is given by Taylor–Aris hydrodynamic dispersion. An immiscible two-fluid system is simulated by a species-dependent intermolecular interaction. A static meniscus is observed whose contact angle agrees with simple estimates and, when motion occurs, velocity-dependent advancing and receding angles are observed. The local velocity field near a moving contact line shows a breakdown of the no-slip condition and, up to substantial statistical fluctuations, is consistent with earlier predictions of Dussan [AIChE J. 23, 131 (1977)].
The flow between shrouded corotating disks
Authors: Abrahamson Scott D. , Eaton John K. , Koga Dennis J.
Published: July 2002

An experimental investigation of the flow between shrouded corotating disks has been performed. Three distinct regions of flow are observed: a solid body inner region near the hub, an outer region dominated by large counter-rotating vortices, and a boundary layer region on the shroud. The inner and outer regions were very two dimensional, with little variation in the direction parallel to the spin axis. The shroud boundary layer region was more three dimensional and contained a pair of toroidal vortices whose vorticity was in the ±θ direction. The effects of varying the Ekman number (ν/R2Ω) and the axial separation of the disks were studied. A decrease in the Ekman number or an increase of the axial separation of the disks resulted in fewer large vortices in the outer region yielding an increase in the relative motion in the inner region with a consequent increase in overall mixing. Corotating vortical structures were observed in the inner region at the largest axial separations and smallest Ekman numbers.
The conditional dissipation rate of an initially binary scalar in homogeneous turbulence
Authors: O’Brien Edward E. , Jiang Tai-Lun
Published: July 2002

It is shown that a necessary and sufficient condition for the scalar dissipation rate, conditioned on scalar value φ, to be independent of φ is that the one-point scalar probability distribution function (pdf) is Gaussian. It is then shown that the amplitude mapping closure yields a closed-form, separable expression for the φ dependence of the conditional dissipation rate in the case of an initial double-delta scalar pdf. If the initial binary scalar field is located at φ=±1, the solution is exp{ − 2[erf−1(φ)]2}, a result that is strongly supported by earlier direct numerical simulations.
On flow through a porous annular pipe
Authors: Banks W. H. H. , Zaturska M. B.
Published: July 2002

In this paper the axisymmetric flow of a viscous fluid through a porous annular pipe with constant and equal fluxes through each pipe wall is studied theoretically. A nondimensional parameter, δ, based on the radii of the walls of the pipe is defined and the equations and boundary conditions are written so that at δ=0 they coincide with the analogous two-dimensional problem. By numerical integration, the significant properties found previously for δ=0 are continued into the region 0<δ<1 corresponding to flow in an annular pipe. Of the steady, unsteady, periodic, quasiperiodic, and chaotic solutions found when δ=0, only steady, unsteady, and periodic solutions have been found when δ≠0.
Optimal perturbations and streak spacing in wall-bounded turbulent shear flow
Authors: Butler Kathryn M. , Farrell Brian F.
Published: July 2002

The mean streak spacing of approximately 100 wall units that is observed in wall-bounded turbulent shear flow is shown to be consistent with near-wall streamwise vortices optimally configured to gain the most energy over an appropriate turbulent eddy turnover time. The streak spacing arising from the optimal perturbation increases with distance from the wall and is nearly independent of Reynolds number, in agreement with experiment.
A note on the calculation of Landau constants
Authors: Crouch J. D. , Herbert Th.
Published: July 2002

This Brief Communication presents an alternative approach for determining the Landau constants associated with the implementation of weakly nonlinear theories. Results from this approach are shown to be in agreement with the usual solvability condition for the case of exact resonance. Away from resonance, the results are in agreement with a direct solution of the nonhomogeneous system. This approach also provides accurate results for the case of near resonance, in contrast to the approach currently in widespread use. Following this alternative approach, the Landau constants can be calculated accurately in a straightforward manner without requiring the solvability condition in some arbitrary neighborhood of resonance.
The added mass coefficient for rows and arrays of spheres oscillating along the axes of tubes
Authors: Cai Xiaolong , Wallis Graham B.
Published: July 2002

Rows of single spheres were attached to springs and oscillated in water-filled tubes of several diameters and lengths with various boundary conditions at the ends of the tubes. Similar tests were performed using arrays of spheres in a closed-end tube. The added mass coefficient was deduced from the measured natural frequency. Results for finite systems were systematically extrapolated to predict the coefficient for an infinite system. The results are closely described by adapting the spheres-in-tube potential flow theory of Cai and Wallis [Phys. Fluids A 4, 904 (1992)] and the image method applied to arrays by Wallis et al. [Chem. Eng. Commun. 118, 141 (1992)]. The coefficient of added mass is shown to depend on the external impedance of a fluid circuit to which the system is connected. Wallis’ [Multiphase Sci. Technol. 5, 239 (1989)] and Zuber’s [Chem. Eng. Sci. 19, 897 (1964)] coefficients are recovered as limiting cases.
Recirculating induction accelerators as drivers for heavy ion fusion*
Authors: Barnard J. J. , Deadrick F. , Friedman A. , Grote D. P. , Griffith L. V. , Kirbie H. C. , Neil V. K. , Newton M. A. , Paul A. C. , Sharp W. M. , Shay H. D. , Bangerter R. O. , Faltens A. , Fong C. G. , Judd D. L. , Lee E. P. , Reginato L. L. , Yu S. S. , Godlove T. F.
Published: July 2002

A two-year study of recirculating induction heavy ion accelerators as low-cost driver for inertial-fusion energy applications was recently completed. The projected cost of a 4 MJ accelerator was estimated to be about $500 M (million) and the efficiency was estimated to be 35%. The principal technology issues include energy recovery of the ramped dipole magnets, which is achieved through use of ringing inductive/capacitive circuits, and high repetition rates of the induction cell pulsers, which is accomplished through arrays of field effect transistor (FET) switches. Principal physics issues identified include minimization of particle loss from interactions with the background gas, and more demanding emittance growth and centroid control requirements associated with the propagation of space-charge-dominated beams around bends and over large path lengths. In addition, instabilities such as the longitudinal resistive instability, beam-breakup instability and betatron-orbit instability were found to be controllable with careful design.
Extreme interfacial waves
Authors: Grimshaw R. H. J. , Pullin D. I.
Published: July 2002

Numerical solutions are presented for large-amplitude interfacial waves of extreme form on the interface between two fluids of different densities in the Boussinesq approximation. The flow in the lower fluid is irrotational, but the upper fluid may have constant, nonzero vorticity. Only symmetric waves are calculated. The results suggest limiting wave profiles for which separate portions of the interface touch, forming stagnant zones of one fluid imbedded in the other fluid.
Underwater detonation of pentolite cylinders
Authors: Sternberg H. M.
Published: July 2002

Flows following the underwater detonation of pentolite cylinders at the center of one end are calculated to times when the distances of the main shock from the center are about 15 times the radius of a sphere of the same mass. Length to diameter (L/D) ratios of 1, 2, 4, and 7 are treated. An axisymmetric Lagrangian artificial viscosity scheme is used with lateral rezoning to tracks that radiate outward from the gas–water boundary. The usable energy in the water, the internal plus the kinetic energies minus the energy dissipated by shock heating, is obtained both as a function of the angle with the charge axis and the distance from the center. The usable energy is found to be generally decreasing with increasing L/D, greatest off the detonator end of the charge, and lowest off the nondetonator end, where there is the most energy dissipation. Off the side of the charge, the peak shock pressure increases with increasing L/D and 45% to 55% of the usable energy is concentrated near the shock front. Along the charge axis the pressure waves in the water become flatter, with more of the usable energy near the gas–water boundary, as the L/D ratio increases.
Self-focusing of short intense pulses in plasmas
Authors: Sun Guo-Zheng , Ott Edward , Lee Y. C. , Guzdar Parvez
Published: July 2002

The self-focusing of relativistically intense laser light pulses is analyzed, where the pulse length is short enough that ion inertia prevents any significant motion of ions. Self-focusing occurs as a result of an increase of the wave refractive index arising from two effects: the mass increase of electrons caused by their relativistic quiver velocity in the light wave, and the reduction of the electron density as a result of ponderomotive force expulsion of the electrons. The latter effect is significant even for rather small values of (P−PL)/PL, where P is the laser beam power and PL is the critical power above which self-focusing occurs. In fact, for (P−PL)/PL≳0.1 the effect is so strong that all electrons are expelled within a core radial region of the self-focused laser light channel (this new phenomenon is called electron cavitation).
Laser Raman scattering measurements of differential molecular diffusion in nonreacting turbulent jets of H2/CO2 mixing with air
Authors: Smith L. L. , Dibble R. W. , Talbot L. , Barlow R. S. , Carter C. D.
Published: July 2002

In this paper we report measurements of differential diffusion in nonreacting jet flows. Pulsed laser Raman scattering spectroscopy is used to measure species concentrations in nonreacting jets of H2/CO2 into air over a range of Reynolds numbers from 1000 to 64 000. Measurements are also made in strained laminar opposed flows of H2/CO2 against air, where differential diffusion effects on relative species concentrations are observed to be independent of strain rate. In laminar opposed flows and in a laminar jet (Re=1000), measurements of average species concentrations show significant differential diffusion effects. In jet flows of higher Reynolds numbers, only instantaneous species concentrations are affected by differential diffusion; measured species concentrations are the same, on average, as the species concentrations that would have been observed if all species diffusivities were equal. Instantaneous differential diffusion effects in turbulent jets are quantified by the variance of a differential diffusion variable z, where z is the difference between the normalized mole fractions of H2 and CO2 (and normalization refers to dividing each species mole fraction by the mole fraction of that species in pure jet fluid). Measurements show that zRMS diminishes with increasing Reynolds number. Histograms of z show negative skewness in the low Reynolds number jet. At higher Reynolds numbers the histograms become more symmetric, although asymmetries do remain at higher Reynolds numbers in the data obtained near the jet edge, where there is an interface between jet fluid and coflowing air.
Effect of film elasticity on the drift velocity of capillary–gravity waves
Authors: Weber Jan Erik , Saetra O/yvind
Published: July 2002

The effect of an insoluble, elastic surface film on the drift velocity of capillary–gravity waves is studied theoretically on the basis of a Lagrangian description of motion. There is no forcing from the atmosphere, and the wave amplitude is taken to attenuate in time. Defining a nondimensional parameter α, which combines film elasticity, fluid viscosity, and wave frequency, maximum damping of the linear waves occurs when α=1 (the Marangoni effect). In this case the frequency of capillary–gravity waves nearly coincides with that of elastic film waves. The nonlinear drift velocity is obtained for general values of α. In particular, it is found that the absolute maximum of the transient drift current is located below the surface when α≳2/3. At the surface, maximum drift velocity (in time domain) occurs for values of α that are somewhat less than one.
Interaction of spanwise vortices with a boundary layer
Authors: Luton Alan , Ragab Saad , Telionis Demetri
Published: July 2002

The interaction of a spanwise vortex with a boundary layer has been numerically simulated using a fractional-step method. The incompressible Navier-Stokes equations are solved to accurately predict the strong viscous–inviscid interaction between a vortex either near or embedded within a boundary layer of comparable size. A strong vortex induces an eruption and the production of a secondary vortex. The secondary vortex causes the primary vortex to rebound, a response observed in many previous experiments and numerical simulations. However, weaker vortices as well do not follow the inviscid trajectory despite the absence of a secondary vortex. Rather than creating vorticity at the wall, a weaker vortex mainly redistributes the vorticity of the boundary layer, pulling it away from the wall. The redistributed vorticity alters the path of the vortex. In the laminar cases studied the decay of the vortex is not significantly altered by the boundary layer.
Observations of fiber orientation in suspensions subjected to planar extensional flows
Authors: Rahnama Mani , Koch Donald L. , Cohen Claude
Published: July 2002

Fibers in a planar extensional flow tend to align parallel to the axis of extension, with a small orientational dispersion resulting from hydrodynamic, fiber–fiber interactions. We have visualized the orientation of an opaque, tracer fiber in the midst of an index of refraction and density matched fiber suspension in a four-roll mill and determined the mean squares of the components of the fiber orientation in the compressional and neutral directions. The observed mean-square orientations in suspensions in Newtonian fluids with nL3=5–17 are qualitatively consistent with the dilute limit of the theory of Shaqfeh and Koch [Phys. Fluids A 31, 728 (1988)]. Here n is the number of fibers per unit volume and L is the fiber length. It was not possible to perform the experiment for higher fiber concentrations, because of difficulties in keeping the tracer fiber at the stagnation point, which were attributed to translational, hydrodynamic diffusion. Measurements of orientational dispersion in polyacrylamide solutions indicated that the non-Newtonian stresses increase fiber alignment, as predicted by Harlen and Koch [Phys Fluids A 4, 5 (1992)].
Capillary bridge modes driven with modulated ultrasonic radiation pressure
Authors: Morse Scot F. , Thiessen David B. , Marston Philip L.
Published: July 2002

The method of modulated ultrasonic radiation pressure, previously used to drive the capillary modes of drops and bubbles, is used to excite the capillary modes of a cylindrical oil bridge in a Plateau tank. Specific modes may be selected by adjusting the modulation frequency and the location or orientation of the bridge in the ultrasonic field. Mode frequencies were measured as a function of the slenderness for the lowest two axisymmetric modes and two nonaxisymmetric modes. The frequencies of the lowest modes agree with an approximate theory which neglects viscous corrections where the interfacial tension is a fitted parameter.
Analysis and modeling of subgrid scalar mixing using numerical data
Authors: Girimaji Sharath S. , Zhou Ye
Published: July 2002

Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze, and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attempt to model the dissipation of the large-scale (supergrid) scalar fluctuations caused by the subgrid scales by decomposing it into two parts: (i) the effect due to the interaction among the subgrid scales, ℰ≫φ; and, (ii) the effect due to interaction between the supergrid and the subgrid scales, ℰ≳<φ. Model comparison with DNS data shows good agreement.
On the theory for the arrest of an advancing molten contact line on a cold solid of the same material
Authors: Schiaffino Stefano , Sonin Ain A.
Published: July 2002

We show that a conventional continuum formulation of the equations and boundary conditions for the spreading of a pure molten material over a cold, solid substrate of its own kind has no meaningful solution for the angle of attack θs of the fusion front at the contact line, which is the quantity that determines contact-line arrest. θs is determined by the heat flux just behind the contact line, and the heat flux in the mathematical model is singular at the contact line. The scale of the physical mechanism which limits the heat flux at the contact line and removes the singularity is estimated by computing the point where the continuum model must be cut off in order to bring it into agreement with the experimental data for a microcrystalline wax. The cutoff scale is in the range 0.1–1 μm, that is, much larger than molecular dimensions, but of order 10−2–10−1 times the convective thermal length scale α/U.
Electrohydrodynamic instability in a thin fluid layer with an electrical conductivity gradient
Authors: Baygents J. C. , Baldessari F.
Published: July 2002

The onset of electrohydrodynamic motion associated with the imposition of an electric field across a thin layer of liquid has been investigated for the case in which the electrical conductivity varies linearly over the depth of the layer. The variation of the conductivity is due to concentration gradients in the charge-carrying solutes and its spatiotemporal evolution is represented by a convective-diffusion equation. When the viscous relaxation time is long compared to the time for charge relaxation, the analysis reveals that the neutral stability curves for the layer can be characterized by three dimensionless parameters: Rae≡dεE02Δσ/μKeffσ0, an electrical Rayleigh number; Δσ/σ0, the relative conductivity increment; and α, the transverse wave number of the disturbance. Here d is the thickness, ε is the dielectric constant, and μ is the viscosity of the layer, E0 is the applied field strength at the lower conductivity boundary, and Keff is an effective diffusivity associated with the Brownian motion of the charge-carrying solutes. With stress-free boundaries, at which the electrical conductivity and current are prescribed, the critical Rae is 1.416×104 at a critical transverse wave number of 1.90 when Δσ/σ0 is 8. As Δσ/σ0 increases, the critical Rae increases and shifts to slightly shorter wavelength disturbances; the critical imposed field strength, however, passes through a minimum because the lower-conductivity boundary exerts a considerable stabilizing influence in the presence of steep conductivity gradients. For Δσ/σ0≲8, the critical Rayleigh number increases as Δσ/σ0 decreases and the layer is only sensitive to long wavelength disturbances (α<0.1) for Δσ/σ0 below 4. Similar trends were obtained for liquid layers with other boundary conditions; e.g., rigid boundaries and constant potential boundaries.
Self-induced motion and asymptotic expansion of the velocity field in the vicinity of a helical vortex filament
Authors: Kuibin P. A. , Okulov V. L.
Published: July 2002

A derivation of an expression for the self-induced velocity of a helical vortex filament with moderate and small pitch is presented for the cases of an infinite space as well as a space bounded by a cylindrical tube. This result supplements the known Kelvin’s formula for helix with large pitch in an infinite space.
Generation and evolution of edge-wave packets
Authors: Liu P. L.-F. , Yeh H. , Lin P. , Chang K.-T. , Cho Y.-S.
Published: July 2002

In this paper the generation and evolution of an edge-wave packet are studied experimentally and numerically. In the laboratory an edge-wave packet is first generated on a sloping beach by a hinge-type wave-maker. Both the free surface displacement and velocity field are measured along several on-offshore cross sections. Numerical results are also obtained by solving the linear shallow-water wave equations and are compared with experimental data. Numerically predicted wave evolution characteristics are in good agreement with those shown by laboratory data. Analyses of the wave amplitude density spectra of both numerical solutions and experimental data show that wave packets are indeed trapped in the nearshore region and consist of a mixture of Stokes and higher-mode edge waves. Furthermore, the Stokes mode dominates in the low frequency range. Two additional wave-maker designs, i.e., the piston-type and the reverse hinge-type, are investigated numerically. Away from the wave-maker the wave forms (time histories) of the wave packets are insensitive to the details of wave-maker movements. The effects of beach slope on the evolution of wave packets are investigated. The behavior of the velocity field and the attenuation rates of runup amplitudes are also discussed.