bluuebunny/crossref_metadata_2025 huggingface dataset:
Freezing/melting phenomena for Lennard-Jones methane in slit pores: A Monte Carlo study
Authors: Miyahara Minoru , Gubbins Keith E.
Published: July 2002

We report grand canonical Monte Carlo simulations for a Lennard-Jones (LJ) fluid modeled on methane in slit-shaped pores of several materials and pore widths. Three types of pore wall were considered: graphitic carbon (strongly attractive walls), “methane’’ walls (wall attractions equal to those in the adsorbate phase), and hard walls. For each system the change from a fluidlike to a solidlike adsorbed phase was observed, and the shift in freezing or melting temperature from that of the bulk adsorbate material was determined. As well as changes in the overall properties of the adsorbate phase, corresponding changes in the individual adsorbate layers in the pore were studied. In addition hysteresis on heating and cooling was examined. For the graphitic carbon walls the freezing temperature was raised relative to that of the bulk material, the elevation being greater for smaller pore widths; however, no freezing transition was observed for pore widths below about 5.3σ. In addition, the contact layer of adsorbate froze at a temperature higher than the inner layers. For pores with methane walls (walls of LJ molecules having the same density and intermolecular interactions as the adsorbate phase) no shift in freezing temperature occurred, while pores with hard walls showed a decrease in freezing temperature relative to the bulk; in the case of hard walls, the contact layer of adsorbate froze at a lower temperature than the inner layers. Considerable hysteresis was observed in some cases, and the width of the hysteresis loop was sensitive to pore size, being wider for pores in which the adsorbed layers are tightly packed. The results indicate that the direction and magnitude of the shift in freezing temperature in the pore is strongly dependent on the strength of the attractive forces between the adsorbate molecules and the wall, and particularly on the magnitude of this relative to such forces between the adsorbate and a wall composed of the same adsorbate molecules. A simple thermodynamic model based on this idea is proposed, and showed to give a good account of the simulation results for methane in carbons. In the simple systems studied here the confinement causes little change in the solid lattice structure of the bulk material. This is unlikely to be the case for more complex pore geometries, and the analysis of such cases is likely to involve additional structural effects.
Mass-resolved two-photon spectra of Xe2 in the region of Xe*(5d). II. Dominant ion-core assignments by dispersive photoelectron spectroscopy
Authors: Hu X. K. , Mao D. M. , Dimov S. S. , Lipson R. H.
Published: July 2002

New dispersive photoelectron spectra have been used to identify the dominant ion cores associated with the XeXe*(5d) Rydberg states analyzed in the preceding paper. These results, for the most part, confirm predictions made from angular momentum arguments. Evidence for excited state predissociation and perturbations are also presented.
Distributed approximating functional approach to the Fokker–Planck equation: Time propagation
Authors: Wei G. W. , Zhang D. S. , Kouri D. J. , Hoffman D. K.
Published: July 2002

The Fokker–Planck equation is solved by the method of distributed approximating functionals via forward time propagation. Numerical schemes involving higher-order terms in Δt are discussed for the time discretization. Three typical examples (a Wiener process, an Ornstein–Uhlenbeck process, and a bistable diffusion model) are used to test the accuracy and reliability of the present approach, which provides solutions that are accurate up to ten significant figures while using a small number of grid points and a reasonably large time increment. Two sets of solutions for the bistable system, one computed using the eigenfunction expansion of a preceding paper and the other using the present time-dependent treatment, agree to no fewer than five significant figures. It is found that the distributed approximating functional method, while simple in its implementation, yields the most accurate numerical solutions yet available for the Fokker–Planck equation.
Correlations between boson peak strength and characteristics of local segmental relaxation in polymers
Authors: Ngai K. L. , Sokolov A. , Steffen W.
Published: July 2002

A correlation between the strength of the boson peak, R1(Tg), and the “fragility” of glass-forming liquids proposed earlier is reanalyzed more exactly for polymers. New experimental data are included, making the correlation more evident for the family of polymers. An additional correlation between R1(Tg) and the shape parameter β of the Kohlrausch correlation function of the α relaxation is shown to hold as well. With these results, the important connection of the boson peak to the glass transition dynamics has become more apparent.
Near threshold photodissociation of acetylene
Authors: Mordaunt David H. , Ashfold Michael N. R. , Dixon Richard N. , Löffler Peter , Schnieder Ludger , Welge Karl H.
Published: July 2002

The photodissociation of jet-cooled HCCH molecules following excitation to their S1 state has been investigated further, at a number of wavelengths in the range 205–220 nm, using the H atom photofragment translational spectroscopy (PTS) technique. Analysis of the rovibrational structure evident in the total kinetic energy release (TKER) spectra so obtained confirms previous reports that the resulting C2H(X̃) fragments are formed in most (if not all) of the v2 bending vibrational levels permitted by energy conservation, and that there is a clear preference for populating those states in which the axial projection of this vibrational angular momentum is maximized (i.e., states with l=v2). The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths (e.g., λphot=205.54 nm) shows bimodal rotational distributions, and a marked anisotropy—especially in the case of those H atoms that are formed in association with C2H(X̃) fragments carrying little rotational excitation. Two competing dissociations mechanisms have been identified. Our discussion of these observations is guided by the recent ab initio calculations of Cui and Morokuma [Chem. Phys. Lett. 272, 319 (1997)]. Channel I conforms to their proposal that the S1 molecules reach the H+C2H(X̃) asymptote as a result of sequential nonadiabatic couplings via the T3, T2, and T1 potential energy surfaces. The product energy disposal at the longest excitation wavelengths is rationalized in terms of the forces acting as the dissociating molecule traverses a late barrier in the C–H exit channel on the T1 surface, while the propensity for populating states with l=v2 reflects the importance of parent torsional motion in promoting the S1→T3, T3→T2, and T2→T1 surface couplings. The population of low rotational states with high recoil anisotropy at shorter excitation wavelengths is ascribed to channel II, involving a direct nonadiabatic transition from S1 to T1 for a structure with one near linear CCH angle. In contrast to channel I, there is no extensive torsional motion and the anisotropy of the initial excitation is retained through to fragmentation. Excitation of the ν1′ mode of HCCH enhances the branching to channel II.
CO dissociation characteristics on size-distributed rhodium islands on alumina model substrates
Authors: Andersson S. , Frank M. , Sandell A. , Giertz A. , Brena B. , Brühwiler P. A. , Mårtensson N. , Libuda J. , Baümer M. , Freund H.-J.
Published: July 2002

The dissociation of CO on size-distributed Rh particles supported on a thin alumina film has been studied with high resolution X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). Adsorbed CO dissociates upon heating to temperatures above 300 K. The dissociation activity is dependent on the island size, exhibiting a maximum for islands with around 1000 atoms. We have identified size-dependent changes in the C 1s photoelectron spectra for these CO–Rh systems occurring at temperatures lower than the onset of both the dissociation and desorption processes. These changes are interpreted as being due to adsorbed CO shifting into more highly coordinated sites. The dissociation activity is directly correlated to the availability of these sites, where the observed dissociation is proposed to occur. These results can be interpreted primarily in terms of the size and shape of the deposited Rh particles.
Experimental study of CO oxidation by an atomic oxygen beam on Pt(111), Ir(111), and Ru(001)
Authors: Wheeler M. C. , Reeves C. T. , Seets D. C. , Mullins C. B.
Published: July 2002

Impinging O-atoms react with adsorbed CO on Pt(111), Ir(111), and Ru(001), to form CO2 at surface temperatures as low as 77 K. The initial reaction probability is measured on these three surfaces using reflectivity techniques and is much lower on Pt(111) than previously supposed. The reaction probability is measured as a function of surface temperature, incident O-atom flux, kinetic energy, and angle. Interestingly, a significant dependence on incident angle is observed on all surfaces (the reaction probability is ∼2.5 times greater at normal incidence than at glancing angles), and a kinetic energy effect is noted at the higher incident angles studied. Also, surface temperature is shown to have an effect on the reaction probability in measurements performed on Pt(111) and Ir(111) at normal incidence.
Simple liquids confined to molecularly thin layers. I. Confinement-induced liquid-to-solid phase transitions
Authors: Klein Jacob , Kumacheva Eugenia
Published: July 2002

A surface force balance with extremely high resolution in measuring shear forces has been used to study the properties of films of the simple organic solvents cyclohexane, octamethylcyclotetrasiloxane, and toluene, confined in a gap between smooth solid surfaces. We were able to probe in detail the transition between liquidlike and solidlike behavior of the films as the gap thickness decreased. Our results reveal that in such confined layers the liquids are fluid down to a film thickness of few molecular layers (typically seven, depending on the particular liquid examined). On further decreasing the gap thickness by a single molecular layer, the films undergo an abrupt transition to become solidlike in the sense that they are able to sustain a finite shear stress for macroscopic times. At the transition, the effective rigidity of the films, quantified in terms of an effective creep viscosity, increases by at least seven orders of magnitude. This sharp transition is reversible and occurs as a function of the confinement alone: it does not require external applied pressure. Following the transition the confined films behave under shear in a manner resembling ductile solids.
First spectroscopic evidence for molecular HCl on a liquid surface with sum frequency generation
Authors: Baldelli Steve , Schnitzer Cheryl , Shultz Mary Jane
Published: July 2002

Sum frequency generation spectroscopy has been used to obtain the vibrational spectrum of HCl on the surface of a liquid. HCl was studied on the surface of 96 wt % H2SO4, 12 M HCl solution, liquid HCl and glass, of which only liquid HCl produces a resonant signal. Implications for the form of HCl on surfaces and the reactions in the atmosphere are discussed.
Comparison of thermal and photochemical behavior of O2 chemisorbed on Pt(335)
Authors: Heyd D. V. , Scharff R. J. , Yates J. T.
Published: July 2002

The thermal and photochemical behavior of O2 chemisorbed on a stepped Pt(335) surface have been compared using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Molecular O2 adsorbs preferentially at 85 K on step sites, followed by adsorption on terrace sites. On heating below 250 K, depopulation of terrace sites occurs first followed by depopulation of step sites. Both desorption and dissociation of O2(a) occur below 250 K. Photolysis of O2 occurs preferentially on step sites, with a cross section ≈3×10−21 cm2. The chemisorbed O(a) produced by either thermal dissociation or photodissociation of O2(a) is identical, exhibiting a Pt–O stretching mode at 480 cm−1 and a frustrated translation mode at 330–350 cm−1. Two regimes of photodepletion are observed, dominated by photolysis of step bound-O2 (regime I) and of terrace bound-O2 (regime II). Regime II is characterized by the presence of interactions of photoproduced electronegative O(a) with remaining O2(a), causing an increase of the O–O mode frequency due to electron withdrawal from the O2(a)π* orbital.
Theoretical study on quantum control of photodissociation and photodesorption dynamics by femtosecond chirped laser pulses
Authors: Mishima Kenji , Yamashita Koichi
Published: July 2002

We have theoretically studied the effect of chirping one-photon incident laser pulses on (I) the branching ratio of the HOD molecule in the photochemical reaction D+OH←HOD→H+OD and (II) the UV photodesorption dynamics of NH3 and ND3 from Cu(111). As was predicted in our previous 1D model, wave packet calculations have demonstrated that it is possible, in practice, to control the branching ratio of reaction (I) and to greatly enhance the desorption probability of the photodesorption reaction (II) by negatively chirped laser pulses. It was found that two characteristics of (negatively) chirped laser pulses contribute to this remarkable effect; the mechanism of adiabatic rapid passage for the population transfer between the ground and excited states, and the intrapulse pump-dump process for determining the branching ratio and photodesorption yield.
Ab initio analysis of the transition states on the lowest triplet H2O2 potential surface
Authors: Karkach Sergei P. , Osherov Vladimir I.
Published: July 2002

The lowest triplet H2O2 potential surface was analyzed for the transition and minimum-energy structures in the range from −0.2 to +5.4 eV with respect to the H2+O2 energy. All the transition structures, the reaction pathways, and the local minima were found to have planar configurations for the atoms. We focus on the transition structures responsible for the main bimolecular chemical reactions formally possible on this surface: H2+O2↔2HO, H+HO2, and H2O+O; H+HO2↔2HO and H2O+O; and 2HO↔H2O+O. For these reactions, activation energies and rate constants in the transition state approximation were evaluated. Our computed rate constants confirm the recommended values for the H+HO2→H2+O2 and HO+HO→H2O+O reactions. The results obtained refute the elementary character of the H+HO2→H2O+O process and call into question the possibility of chain initiation in the H2/O2 system by means of a bimolecular reaction. Most likely, the chain initiation in the gas phase is owing to trimolecular reactions H2+2O2→2HO2, 2HO+O2. Special attention was paid to accurate prediction of electronic energies in the transition structures. A new procedure developed, “extrapolation to zero high-level correction,” results in very realistic activation energies. Predictions of molecular energies are coincident with those from the widely used G2 scheme but have smaller uncertainty.
Cross section and asymmetry parameter calculation for sulfur 1s photoionization of SF6
Authors: Natalense Alexandra P. P. , Lucchese Robert R.
Published: July 2002

We use the polyatomic Schwinger variational method with Padé corrections to calculate the cross section and asymmetry parameter for the sulfur 1s core level photoionization of SF6, for photon energies from threshold up to 2600 eV. Our results show very good agreement with experimental cross sections. Our asymmetry parameter show good qualitative agreement with experiment. A resonant feature at a photon energy of ≈2550 eV is found to be due to a nonvalence type resonant state which is trapped by an l=9 angular momentum barrier.
Nuclear spin–spin coupling in the acetylene isotopomers calculated from ab initio correlated surfaces for 1J(C,H), 1J(C,C), 2J(C,H), and 3J(H,H)
Authors: Wigglesworth Richard D. , Raynes William T. , Kirpekar Sheela , Oddershede J. , Sauer Stephan P. A.
Published: July 2002

Ab initio calculated coordinate and internal valence coordinate coefficients for each of the four spin–spin coupling surfaces of the acetylene molecule—1J(C,H), 1J(C,C), 2J(C,H), and 3J(H,H) are presented. Calculations were carried out at the SOPPA(CCSD) level using a large basis set. Couplings were calculated at 35 geometries (including equilibrium) giving 35 distinct sites on the 1J(C,C) and 3J(H,H) surfaces and 53 distinct sites on the 1J(C,H) and 2J(C,H) surfaces. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. All couplings are sensitive to geometry with the principal features being (a) an even steeper increase of J(C1, H1) with CC bond stretching than with CH bond stretching—an example of “unexpected differential sensitivity” (or UDS), (b) very opposite variations of 2J(C1,H2) with variations of the CC and C2H2 bond lengths, (c) very opposite variations of 1J(C,C) with a CC stretch and a CH stretch and (d) very opposite variations of 1J(C1,H1) with variations of the H1C1C2 and C1C2H2 angles with the latter variation being three times more effective (another example of UDS). The surfaces are averaged over a very accurate force field to give values of all couplings in the ten isotopomers containing all possible combinations of 12C, 13C, 1H, and 2H nuclei at 0 K and at a number of selected temperatures in the range accessible to experiment. The dominant nuclear motion effect comes from bending at the carbon atoms with stretching being of greater importance only for 1J(C,H). Agreement with recent experimental data both for the absolute values of the couplings and for isotope effects on them is generally very good although there are some disappointments for 1J(C,H). Values of the reduced coupling constants and their derivatives for the carbon–carbon and the one-bond carbon–proton coupling in acetylene are compared with recent results for some other molecules.
Stoichiometric and sodium-doped titanium silicate molecular sieve containing atomically defined –OTiOTiO– chains: Quantumab initiocalculations, spectroscopic properties, and reactivity
Authors: Bordiga S. , Turnes Palomino G. , Zecchina A. , Ranghino G. , Giamello E. , Lamberti C.
Published: July 2002

Ab initio calculations on a linear –OTiOTiO– chain embedded in an envelope of (SiO4) tetrahedra, mimicking the structure of Na2TiSi5O13 molecular sieve (ETS-10), confirm that the peculiar optical properties of the solid are associated with the presence of –OTiOTiO– linear chains behaving as quantum wires. The optical [in the UV-Vis (ultraviolet-range)] and the magnetic [(ESR) electron spin resonance] properties of these chains can be modified by adsorbing Na vapors. The sodium atoms diffusing into the channels undergo a ionization process with formation of Na+ (localized in the main channels) and Ti3+ (in the –OTiOTiO– chain, which so becomes a nonstoichiometric wire) characterized by Ti/Na ratios in the 2–4 range. Successive adsorption of oxygen at room temperature leads to the partial (Ti/Na∼2) or total (Ti/Na∼4) restoration of the chain stoichiometry and to the predominant formation of sodium oxide. The formation of a minor fraction of superoxide negative ions whose magnetic properties are revealed by ESR spectroscopy is also observed. Total restoration of the optical properties of the original samples is always obtained when the oxygen adsorption is made at 473 K. The sample keeps its structural integrity during the reduction and successive oxidation process.
Energy-resolved collision-induced dissociation of Cun+ (n=2–9): Stability and fragmentation pathways
Authors: Ingólfsson Oddur , Busolt Ulrike , Sugawara Ko-ichi
Published: July 2002

Collision induced dissociation of Cun+ clusters (n=2–9) in collision with Xe is presented in the center-of-mass energy range from about 100 meV to above 15 eV. The collision energy dependence is measured for the total and the partial dissociation cross sections, and the dissociation thresholds for the dominating processes are derived. The threshold energies show pronounced odd–even alternations, reflecting a higher stability of the odd-numbered, Cu2n+1+, clusters. Further, the evaporation of a single neutral atom is found to be the energetically favorable process for the even-numbered clusters, while the loss of the neutral dimer is favorable in the case of the odd-numbered clusters. An exception is Cu9+, where the formation of Cun−1+ is energetically favorable, and the energetics of the Cun−2+ formation are in good agreement with sequential evaporation of two neutral monomers. Here we discuss the energy dependency of the total and partial dissociation cross sections, and try to give a consistent picture of the dissociation dynamics. We present binding energies for the cationic clusters from their dissociation thresholds, and use those, in combination with the literature values for the ionization potentials of Cun, to estimate the binding energies for neutral copper clusters. Finally, we compare this work to earlier theoretical calculations, as well as experimental estimations of the binding energies.
Relativistic coupled cluster calculations for neutral and singly charged Au3 clusters
Authors: Wesendrup Ralf , Hunt Tricia , Schwerdtfeger Peter
Published: July 2002

Relativistic coupled cluster studies are performed for the structures, dissociation energies, ionization potentials and electron affinities for Au, Au2 and Au3. The calculations show that the upward shifts of the ionization potentials and electron affinities of Aun clusters by approximately 2 eV compared to Cun or Agn base on relativistic effects. Au3+ is predicted to adopt a trigonal planar structure (D3h, A11), Au3 a E⊗ε Jahn–Teller distorted structure (C2v,2A1) 0.1 eV below the linear Σu+2 arrangement, and Au3− adopts a linear structure (1Σg+).
Effect of the fluid-wall interaction on freezing of confined fluids: Toward the development of a global phase diagram
Authors: Radhakrishnan Ravi , Gubbins Keith E. , Sliwinska-Bartkowiak Malgorzata
Published: July 2002

We report molecular simulation studies of the freezing behavior of fluids in nano-porous media. The effect of confinement is to induce spatial constraints as well as energetic heterogeneity on the confined fluid, thereby altering the bulk phase behavior drastically. We consider the effect of the fluid-wall interaction energy on the shift of the freezing temperature and on the fluid structure, using a novel approach to calculate the free energy surface based on Landau theory and order parameter formulation. Corresponding states theory is then used to map out the global freezing behavior of a Lennard-Jones (LJ) fluid in model slit-shaped pores of varying fluid-wall interaction strengths. Using LJ parameters fitted to thermophysical property behavior, we predict the qualitative freezing behavior for a variety of fluids and nano-porous materials, based on a global freezing diagram. We have attempted to verify these predictions by comparing with experimental data for several systems, and show that in these cases, the experimental observations and the predictions are in agreement.
On the spectrum of a two-level system
Authors: Kuś Marek
Published: July 2002

Doubly degenerate energy levels of the two level atom interacting with a single mode of the electromagnetic field are exactly calculated. The dependence of the number of such levels on the values of the level separation energy and a coupling constant is determined. Some general conclusions about the spectrum are drawn.
Thermodynamics of dimers on a rectangularL×M×Nlattice
Authors: Phares Alain J. , Wunderlich Francis J.
Published: July 2002

The exact closed-form analytic solution of the problem of dimers on infinite two-dimensional and three-dimensional lattices is obtained. Entropy, isothermal compressibility, and constant pressure heat capacity of the system are given in terms of the normalized number density of dimers. The absolute activity of dimers is also given in terms of the normalized number density; it exhibits a behavior near close packing with a critical exponent exactly equal to 2, and with an amplitude 1/(4φ), where φ is the molecular freedom per dimer at close packing.
Relativistic brachistochrone
Authors: Goldstein Harris F. , Bender Carl M.
Published: July 2002

The trajectory joining two points a1 and a2, which minimizes the transit time for a particle, initially at rest, to fall in a uniform gravitational field from a1 to a2, is called the brachistochrone. Johann Bernoulli was the first to find an analytical form for the brachistochrone; in 1696, he discovered that the trajectory is a cycloid. In this paper the relativistic generalization of this classic problem is presented. Four separate curves are actually identified: a particle falling in both a uniform electric and uniform gravitational field is considered. The curves that minimize the times of flight measured by an observer in a laboratory in which a1 and a2 are fixed and also the curves that minimize the proper times of flight are found.
Finite-dimensional irreducible representations of the SU(3/1) superalgebra
Authors: Mitra Bani
Published: July 2002

The SU(3/1) superalgebra is constructed using Schwinger’s harmonic oscillator technique. Finite-dimensional irreducible representations of the superalgebra are obtained choosing a suitable weight Ψm, which will be called the maximal weight, using the commutation relations of the superalgebra only and without any knowledge of the matrix elements of the generators of the superalgebra. Atypical representations of the superalgebra are obtained and a comparison of these representations is made with finite-dimensional irreducible representations of SU(4).
Covariant algebraic method for calculation of the low-energy heat kernel
Authors: Avramidi I. G.
Published: July 2002

Using our recently proposed covariant algebraic approach, the heat kernel for a Laplace-like differential operator in a low-energy approximation is studied. Neglecting all the covariant derivatives of the gauge field strength (Yang–Mills curvature) and the covariant derivatives of the potential term of third order and higher, a closed formula for the heat kernel as well as its diagonal is obtained. Explicit formulas for the coefficients of the asymptotic expansion of the heat kernel diagonal in terms of the Yang–Mills curvature, the potential term and its first two covariant derivatives are obtained.
Application of the discrete Wentzel–Kramers–Brillouin method to spin tunneling
Authors: Garg Anupam
Published: July 2002

A discrete version of the Wentzel–Kramers–Brillouin (WKB) method is developed and applied to calculate the tunnel splittings between classically degenerate states of spin Hamiltonians. The results for particular model problems are in complete accord with those previously found using instanton methods. The discrete WKB method is more elementary and also yields wave functions.
Self-testing algorithms for self-avoiding walks
Authors: Randall Dana , Sinclair Alistair
Published: July 2002

We present a Markov chain Monte Carlo algorithm for almost uniformly generating and approximately counting self-avoiding walks in rectangular lattices Zd. These are classical problems that arise, for example, in the study of long polymer chains. While there are a number of Monte Carlo algorithms used to solve these problems in practice, these are heuristic and their correctness relies on unproven conjectures. In contrast, our algorithm is shown rigorously to produce answers with specified accuracy and confidence. Only the efficiency of the algorithm relies on a widely believed conjecture, and a novel feature is that this conjecture can be tested as the algorithm proceeds. With this self-testing feature incorporated, the algorithm has polynomially bounded running time and is completely reliable, in the sense that it either outputs answers that are guaranteed to be within the specified accuracy and confidence bounds, or finds a counter-example to the conjecture.
Streaming flows generated by high-frequency small-amplitude oscillations of arbitrarily shaped cylinders
Authors: Kim Sung Kyun , Troesch Armin W.
Published: July 2002

Small-amplitude harmonic oscillations of arbitrarily shaped cylinders are considered both experimentally and theoretically. For the theoretical model, the flow regime is separated into inner and outer regions. In the inner region, the flow is governed by the classical Stokes boundary layer equation. In the outer region, the full Navier–Stokes equation for the steady streaming flow is solved numerically by using a finite difference method coupled with conformal mapping techniques. Numerical results of streaming, a nonlinear response to harmonic motion, show complicated flow schemes. Experimental results confirm the existence of such flows. Streaming flow around a sharp corner of a square cylinder is investigated through numerical calculation and experimental flow visualization. The absence of any vortex shedding on the time scale of the streaming flow is noted. These results suggest that in the limit of a small amplitude of oscillation, or equivalently large Strouhal numbers, sharp-edged bodies experience attached flow in the mean sense.
On the temperature distribution in liquid–vapor phase change between plane liquid surfaces
Authors: Shankar P. N. , Deshpande M. D.
Published: July 2002

The problem of temperature distribution in liquid-vapor phase change between plane liquid surfaces is considered both analytically and experimentally. Classical kinetic theory calculations indicate that for a monatomic vapor, large temperature jumps exist at the interfaces, large enough possibly to cause the temperature distribution to be anomalous. Surprisingly, no experimental results have been reported so far. A continuum analysis, with special consideration of the Knudsen layers, clearly establishes the adverse roles played by polyatomic molecules and the presence of a contaminant gas. The need to work with monatomic vapors under low levels of contamination is clearly indicated. An experimental setup was constructed to measure temperature distributions in the vapor between an evaporating liquid surface and a cooler condensing surface. Temperature distributions were measured using Chromel–Alumel thermocouples. Measurements were made in water, Freon 113, and mercury. The temperature profiles obtained in mercury, over a wide range of temperatures, clearly show large temperature jumps at the interfaces, as large as almost 50% of the applied temperature difference. These results appear to be the first to confirm this important phenomenon.
Conditional moment closure for turbulent reacting flow
Authors: Bilger R. W.
Published: July 2002

Equations governing the variation through the flow field of averages of quantities such as species mass fractions, conditional on mixture fraction, are derived and modeled. The conditioning variable adds to the independent dimensions of the problem, but it is found that this is offset in some cases by reduction in the spatial dimensionality needed. Predictions are made for the reacting scalar mixing layer, and these show good agreement with experiment. The methodology effectively decouples the kinetics from the large inhomogeneity or macromixing aspects of the flow while preserving the input from the scalar dissipation or micromixing. Arbitrarily complex kinetics may be used within reasonable computational cost.
Propagation of a Gaussian beam in a nonhomogeneous plasma
Authors: Mazzucato E.
Published: July 2002

The asymptotic theory of Choudhary and Felsen [IEEE Trans. Antennas Propag. AP-21, 827 (1973)] on the propagation of scalar inhomogeneous waves in two-dimensional isotropic media is extended to the case of three-dimensional vector fields. The theory is applied to the propagation of Gaussian beams in nonhomogeneous media. The wave trajectory equations are then reformulated for anisotropic media and used for tracking a Gaussian beam in a tokamak plasma.
Radio-frequency (rf) forces and wave helicity transport
Authors: Chan V. S. , Chiu S. C.
Published: July 2002

A kinetic theory is developed which unifies the concept of radio-frequency (rf) current drive and wave helicity injection. It is shown that wave helicity injection and transport can be incorporated into the more general concept of rf forces. Moreover, the part related to wave helicity has the same form as a mean field dynamo. Present result suggests that the evaluation of rf current drive by conventional methods may leave out some important effects. The theory may also simplify the study of rf influence on transport and stability.
Stability conditions anda prioriestimates for equivalent-barotropic modons
Authors: Swaters Gordon E.
Published: July 2002

Linear Liapunov stability conditions are obtained for all classes of equivalent-barotropic modons. The stability conditions are stated in terms of a parameter η called the ‘‘generalized disturbance wavenumber’’ related to the ratio of the initial values of disturbance enstrophy to energy. It is shown that c>0 modons (c is the drift speed, nondimensionalized by the long wave speed) are stable when η≤κ (κ is the modon wavenumber), and that c<−1 modons are stable when η≥κ. This dependency of the stability on the initial spectral structure of the disturbance is observed in numerical calculations. A priori L2-type estimates bounding the growth of perturbations are derived. The instability mechanism is interpreted in terms of Fj≂rtoft’s energy cascade theorem [Tellus 5, 225 (1953)].
Broadening of interfacial solitary waves
Authors: Turner R. E. L. , Vanden-Broeck J.-M.
Published: July 2002

Progressing interfacial gravity waves are considered for two fluids of differing densities confined in a channel of finite vertical extent and infinite horizontal extent. An integrodifferential equation for the unknown shape of the interface is derived. This equation is discretized and the resulting algebraic equations are solved using Newton’s method. It is found that, for a range of heights and densities of the two fluids, the system supports a branch of solitary waves. Progression along the branch produces a broadening of the wave. With increased broadening both the amplitude and the wave speed approach limiting values. The results are in good agreement with analytical studies and indicate the existence of internal surges.
Validation of the Sharp–Wheeler bubble merger model from experimental and computational data
Authors: Glimm J. , Li X. L.
Published: July 2002

A statistical model to describe the late time chaotic behavior of a Rayleigh–Taylor unstable interface was introduced by Sharp and Wheeler [Physica D 12, 3 (1984); (private communication)]. Here the focus is on effective computations using this model and the comparison of the model with experiment. The numerical solution of the model shows that the height of the mixing layer, in which the bubbles of the light fluid penetrate the heavy fluid, is in a constant acceleration. This result agrees with the experiments of Read [Physica D 12, 45 (1984)].
The dynamics of bubble growth for Rayleigh–Taylor unstable interfaces
Authors: Gardner C. L. , Glimm J. , McBryan O. , Menikoff R. , Sharp D. H. , Zhang Q.
Published: July 2002

A statistical model is analyzed for the growth of bubbles in a Rayleigh–Taylor unstable interface. The model is compared to solutions of the full Euler equations for compressible two phase flow, using numerical solutions based on the method of front tracking. The front tracking method has the distinguishing feature of being a predominantly Eulerian method in which sharp interfaces are preserved with zero numerical diffusion. Various regimes in the statistical model exhibiting qualitatively distinct behavior are explored.
The critical Weber number for vortex and jet formation for drops impinging on a liquid pool
Authors: Hsiao Mingying , Lichter Seth , Quintero Luis G.
Published: July 2002

When a liquid drop impacts on a pool, the drop will either coalesce into the host liquid, with the creation of a vortex ring below the surface and little splashing above, or will splash, producing a cavity in the host liquid that collapses inward, producing an upward jet of fluid. It is found that there is a critical Weber number, Wec=U(ρD/T)1/2≂8, below which vortex rings are formed and above which a jet is produced for drops falling into the identical fluid. Here, U is the drop speed at impact, D is the drop diameter, and ρ and T are density and surface tension, respectively. The Weber number criterion is compared with experiments using water and mercury.
Anisotropy of the dissipation tensor in a turbulent boundary layer
Authors: Antonia R. A. , Djenidi L. , Spalart P. R.
Published: July 2002

The second and third invariants of the Reynolds stress anisotropy tensor and the Reynolds stress dissipation rate anisotropy tensor have been evaluated from direct numerical simulation data for a boundary layer. The departure from isotropy increases in both inner and outer regions of the layer as the momentum thickness Reynolds number Rθ decreases. The relationship between the two anisotropy tensors has been examined in some detail. A linear relation, with a constant of proportionality depending on the local turbulence Reynolds number, is well supported by the outer region data for three values of Rθ.
Experimental investigation of turbulence properties in the interface region of variable density jets
Authors: Gharbi A. , Amielh M. , Anselmet F.
Published: July 2002

The global evolution of variable density turbulent jets is now quite well documented, showing that the entrainment of external fluid into these jets is considerably modified by density variations. But, to our knowledge, no specific study has so far been devoted to the intermittent region of such flows. For constant density flows, the radial evolutions of the velocity variances are known to follow the so-called Phillips’ relations in this interface region. The main objective of the present work is to investigate whether density variations affect properties of the interface. It is found that Phillips’ relations are also valid in the presence of large-density variations, albeit their extent is slightly different. In relation to this, the structure of turbulence is almost unchanged, even in the outer region where large-scale structures are dominating the flow mixing properties, and departure from isotropy for the Reynolds stresses is rather similar for all jets within the range of density ratios considered here. Therefore, except in the very near-field region, most of the influence of density variations can be taken into account by considering only the different evolutions of the flow characteristics on the jet axis. The implication of our results for various aspects associated with these flows, such as in modeling, is also discussed.
On a self-sustaining process in shear flows
Authors: Waleffe Fabian
Published: July 2002

A self-sustaining process conjectured to be generic for wall-bounded shear flows is investigated. The self-sustaining process consists of streamwise rolls that redistribute the mean shear to create streaks that wiggle to maintain the rolls. The process is analyzed and shown to be remarkably insensitive to whether there is no-slip or free-slip at the walls. A low-order model of the process is derived from the Navier–Stokes equations for a sinusoidal shear flow. The model has two unstable steady solutions above a critical Reynolds number, in addition to the stable laminar flow. For some parameter values, there is a second critical Reynolds number at which a homoclinic bifurcation gives rise to a stable periodic solution. This suggests a direct link between unstable steady solutions and almost periodic solutions that have been computed in plane Couette flow. It is argued that this self-sustaining process is responsible for the bifurcation of shear flows at low Reynolds numbers and perhaps also for controlling the near-wall region of turbulent shear flows at higher Reynolds numbers.
The stability of evaporating thin liquid films in the presence of surfactant. I. Lubrication approximation and linear analysis
Authors: Danov Krassimir D. , Alleborn Norbert , Raszillier Hans , Durst Franz
Published: July 2002

The dynamics of an evaporating wetting liquid film in the presence of dissolved surfactant is investigated. The solid substrate is planar and is subjected to heating. The liquid–vapor interface is a two-dimensional continuum characterized by specific adsorption, interfacial viscosity, and surface tension, which depend on the surfactant subsurface concentration and temperature. In the case of small density, viscosity, and thermal conductivity of the vapor phase (compared to the respective values for the liquid phase), at small Reynolds and large Peclet numbers and for thin films, the lubrication approximation model can be applied. The effect of the van der Waals disjoining pressure is taken into account. The appearing dimensionless groups, defined in terms of the real physical parameters, can vary by several orders of magnitude depending on the film initial thickness, temperature difference, and type of surfactants. The developed linear theory describes the competition among the various instabilities. The numerical solution of the evolution equation provides information about the critical film thickness, critical lateral wave number, and time for rupture. The influence of the interfacial mass loss due to evaporation, the van der Waals attraction, the Marangoni effects due to thermal and concentration gradients, and the interfacial viscous friction upon the critical film thickness is discussed.
Experimental evidence of streamwise vortices as finite amplitude solutions in transitional plane Couette flow
Authors: Bottin S. , Dauchot O. , Daviaud F. , Manneville P.
Published: July 2002

Elongated streamwise structures are considered as a key element of the transition to turbulence in various wall flows. In pure plane Couette flow (pCf), longitudinal streaks originating from pairs of streamwise counter-rotating vortices are clearly identified surrounding growing turbulent spots or at late stages of spot relaxation. The same structures bifurcate subcritically from a slightly modified Couette flow where a thin spanwise wire has been introduced in the zero-velocity plane. The basic flow profile, as measured by laser Doppler velocimetry, is shown to approach continuously the original linear velocity profile as the radius of the wire is decreased. On the other hand, the vortices remain almost unchanged and the bifurcation threshold remains bounded from above by the global stability threshold below which turbulent spots relax spontaneously. This supports the conjecture that a related nontrivial nonlinear solution exists in the pure pCf limit. These observations are compared to numerical stability calculations of the modified flow and to finite amplitude solutions to pCf problems with a different tunable modification.
Very large-scale motion in the outer layer
Authors: Kim K. C. , Adrian R. J.
Published: July 2002

Very large-scale motions in the form of long regions of streamwise velocity fluctuation are observed in the outer layer of fully developed turbulent pipe flow over a range of Reynolds numbers. The premultiplied, one-dimensional spectrum of the streamwise velocity measured by hot-film anemometry has a bimodal distribution whose components are associated with large-scale motion and a range of smaller scales corresponding to the main turbulent motion. The characteristic wavelength of the large-scale mode increases through the logarithmic layer, and reaches a maximum value that is approximately 12–14 times the pipe radius, one order of magnitude longer than the largest reported integral length scale, and more than four to five times longer than the length of a turbulent bulge. The wavelength decreases to approximately two pipe radii at the pipe centerline. It is conjectured that the very large-scale motions result from the coherent alignment of large-scale motions in the form of turbulent bulges or packets of hairpin vortices.
Bragg scattering of surface waves over permeable rippled beds with current
Authors: Ting Chao-Lung , Lin Ming-Chung , Kuo Chwen-Ling
Published: July 2002

In this study we develop a time-dependent wave equation for waves propagating with a current over permeable rippled beds. As well known, Bragg resonance occurs when the incident wavelength is twice the wavelength of the bottom ripple undulation and no current is present. However, the current in the near-shore region changes the resonance condition. A one-dimensional wave field is solved numerically based on the derived equation to study the effect of current on the Bragg resonance condition. Nonlinear wave–wave resonant interaction theory provides an explanation of the effect on Bragg resonance. Numerical results also indicate that the maximum reflection coefficient increases as current velocity increases from a negative to a positive value. Furthermore, the velocity of the current affects the position of the maximum reflection coefficient.
Generation of the saturated electrostatic potential along a magnetic field line
Authors: Katanuma I. , Kiwamoto Y. , Tatematsu Y. , Ishii K. , Saito T. , Yatsu K. , Tamano T.
Published: July 2002

The formation of plug potential is usually observed in a tandem mirror experiment without neutral beam injection in the plug region [Phys. Plasmas 2, 2321 (1995)]. The formation requires only the electron cyclotron resonance heating experimentally. In this Letter the plug potential is analytically shown to be created in a tandem mirror with passing ions, mirror trapped ions in the thermal barrier, passing electrons and φ-trapped electrons in a plug potential but without high energy sloshing ions.
Plug potential formation in a tandem mirror
Authors: Katanuma I. , Kiwamoto Y. , Tatematsu Y. , Ishii K. , Saito T. , Yatsu K. , Tamano T.
Published: July 2002

It is suggested experimentally that a plug potential forms in a different mechanism from the originally expected scenario in the present tandem mirror [T. Tamano, Phys. Plasmas 2, 2321 (1995)]. That is, the formation requires only electron cyclotron resonance heating experimentally. In this manuscript, therefore, the plug potential with continuous axial profile is analytically shown to be created with the plausible ion and electron distribution functions in a present tandem mirror, i.e., passing ions, mirror trapped ions in the thermal barrier, passing electrons, φ-trapped electrons in a plug potential, and the small amount of ions that are born around the plug region.
Generalized theory of helicon waves. II. Excitation and absorption
Authors: Arnush Donald , Chen Francis F.
Published: July 2002

Helicon waves in a plasma confined by a cylinder are treated. The undamped normal modes of the helicon (H) and Trivelpiece–Gould (TG) waves have distinctly different wave patterns at high magnetic fields but at low fields have similar patterns and therefore interact strongly. Damping of these modes, their excitation by antennas, and the rf plasma absorption efficiency are considered. Nonuniform plasmas are treated by solving a fourth-order ordinary differential equation numerically. A significant difference between this and earlier codes which divide the plasma into uniform shells is made clear. Excitation of the weakly damped H wave, followed by conversion to the strongly damped TG wave which leads to high helicon discharge efficiency, is examined for realistic density profiles. A reason for the greater heating efficiency of the m=+1 vs the m=−1 mode for axially peaked profiles is provided.