bluuebunny/crossref_metadata_2025 huggingface dataset:
Segmental dynamics of miscible polymer blends: Comparison of the predictions of a concentration fluctuation model to experiment
Authors: Kamath Sudesh , Colby Ralph H. , Kumar Sanat K. , Karatasos Kostas , Floudas George , Fytas George , Roovers Jacques E. L.
Published: July 2002

We recently proposed a concentration fluctuation model to describe the segmental dynamics of miscible polymer blends [Kumar et al., J. Chem. Phys. 105, 3777 (1996)]. This model assumes the existence of a cooperative volume, similar to that in the Adam-Gibbs picture of the glass transition, over which segments have to reorganize in a concerted fashion to facilitate stress relaxation. No molecular theory exists for the cooperative volume. Consequently, here we critically compare two alternative functional dependences for this quantity in the context of the segmental dynamics of the most extensively studied miscible polymer blend, 1,4-polyisoprene (PI) and polyvinylethylene (PVE): (a) The Donth model, which assumes the Vogel form for the temperature dependence of relaxation processes, with a relaxation time that diverges at the Vogel temperature, roughly 50 K below the glass transition, and (b) a more recent dynamic scaling model that predicts the relaxation time diverges algebraically, only about 10 K below the glass transition. We find that the dynamic scaling model provides a near-quantitative description of the segmental relaxation in PI/PVE blends. In contrast, the Donth model predicts that the relaxation time spectrum for PI, the faster relaxing component, is bimodal, in qualitative disagreement with NMR experiments and our dielectric measurements reported here. Our results therefore emphasize two findings. First, our model can describe the segmental relaxations of the components of a polymer blend in a near-quantitative manner. Second, and more fundamentally, it appears that the dynamic scaling model describes segmental dynamics of polymers near their glass transition.
Rotationally resolved pulsed field ionization photoelectron study of CO+(X 2Σ+,v+=0–42) in the energy range of 13.98–21.92 eV
Authors: Evans M. , Ng C. Y.
Published: July 2002

We have obtained rotationally resolved pulsed field ionization–photoelectron (PFI-PE) spectra of CO in the energy range of 13.98–21.92 eV, covering the ionization transitions CO+(X 2Σ+,v+=0–42,N+)←CO(X 1Σ+,v″=0,N″). The PFI-PE bands for CO+ (X 2Σ+, v+=8–22, 24, and 28–39) obtained here represent the first rotationally resolved spectroscopic data for these states. The high-resolution features observed in the PFI-PE spectra allow the identification of vibrational bands for the CO+ (X 2Σ+, v+=10, 14, 15, 17, 18, 21, 24, 25, 29–31, 33, 35–37, and 39) states, which strongly overlap with prominent vibrational bands of the CO+(A 2Π3/2,1/2,B 2Σ+) states. The simulation using the Buckingham–Orr–Sichel model has provided accurate molecular constants for CO+(X 2Σ+,v+=0–42), including ionization energies, vibrational constants (ωe+=2218.8±3.5 cm−1, ωe+xe+=16.20±0.32 cm−1, ωe+ye+=0.074±0.011 cm−1, and ωe+ze+=−0.001 83±0.000 13 cm−1), and rotational constants [Be+=1.9797±0.0051 cm−1, αe+=0.0201±0.0011 cm−1, γe+=0.000 122±0.000 067 cm−1, ze+=−(5.2±1.1)×10−6 cm−1]. Enhancement of ΔN<0 rotational branches, attributable to field-induced rotational autoionization, was clearly discernible in PFI-PE bands for CO+ (X 2Σ+, v+=0–5, 11, and 12). Significant local enhancements due to near-resonance autoionization were observed for low v+ (<10) PFI-PE bands of CO+(X 2Σ+), where the density of interloper Rydberg states converging to higher ionic levels is high as manifested in the photoion spectrum. The observation of a long vibrational progression in the Franck–Condon gap region, where strong autoionization states are absent, is consistent with the suggestion that high-n Rydberg states converging to highly excited vibrational levels of CO+(X 2Σ+) are partially populated via direct excitation to a repulsive neutral state. The relatively minor band intensity variation observed for high v+ PFI-PE bands is also in accord with the direct excitation model. Since ΔN=0, ±1, ±2, and ±3 rotational branches are observed in the PFI-PE spectra, we conclude that the ejected photoelectrons are restricted to angular momentum continuum states l=0–4.
On the role of conformational geometry in protein folding
Authors: Du Rose , Pande Vijay S. , Grosberg Alexander Yu. , Tanaka Toyoichi , Shakhnovich Eugene
Published: July 2002

Using a lattice model of protein folding, we find that once certain native contacts have been formed, folding to the native state is inevitable, even if the only energetic bias in the system is nonspecific, homopolymeric attraction to a collapsed state. These conformations can be quite geometrically unrelated to the native state (with as low as only 53% of the native contacts formed). We demonstrate these results by examining the Monte Carlo kinetics of both heteropolymers under Go interactions and homopolymers, with the folding of both types of polymers to the native state of the heteropolymer. Although we only consider a 48-mer lattice model, our findings shed light on the effects of geometrical restrictions, including those of chain connectivity and steric excluded volume, on protein folding. These effects play a complementary role to that of the rugged energy landscape. In addition, the results of this work can aid in the interpretation of experiments and computer simulations of protein folding performed at elevated temperatures.
A direct-dynamics study of proton transfer through water bridges in guanine and 7-azaindole
Authors: Smedarchina Zorka , Siebrand Willem , Fernández-Ramos Antonio , Gorb Leonid , Leszczynski Jerzy
Published: July 2002

To evaluate the efficiency of bridges of water molecules as proton conduits, multidimensional ab initio proton transfer rate constants are reported for complexes of guanine and 7-azaindole with one and two water molecules. These water molecules form hydrogen-bonded bridges between functional groups involved in tautomerization via proton transfer and catalyze this transfer. Structures and energies of the relevant stationary configurations are optimized at the second-order Møller–Plesset level and vibrational force fields are evaluated at the Hartree–Fock level. The proton transfer rate constants, calculated with the instanton method, show the effect of the structure and strength of the hydrogen bonds, reflected in couplings between the tunneling mode and the other vibrations of the complexes. The results indicate that strongly hydrogen-bonded, strain-free water bridges can serve as very efficient proton conduits.
Four-component relativistic density functional calculations of heavy diatomic molecules
Authors: Varga S. , Fricke B. , Nakamatsu H. , Mukoyama T. , Anton J. , Geschke D. , Heitmann A. , Engel E. , Baştuǧ T.
Published: July 2002

We perform accurate four-component calculations for heavy closed-shell diatomic molecules in the framework of relativistic density functional theory using local and gradient corrected density functional schemes. As examples we have chosen Cu2, Ag2, Au2, Tl2, Pb2, Bi2, and Pt2. The potential energy curves show the quality, and the discrepancies of the density functionals unscreened from any approximation of the relativistic effects.
Theoretical study of multidimensional proton tunneling in the excited state of tropolone
Authors: Wójcik Marek J. , Nakamura Hiroki , Iwata Suehiro , Tatara Wiktor
Published: July 2002

Ab initio CIS/6-31G** and CIS/6-31++G** calculations have been carried out in the Ã state of tropolone for the stable and saddle point structures. The calculated energies and geometries have been compared with the previous results of Vener et al. The energy barrier for the proton tunneling amounts to 12.0 kcal/mol from the CIS/6-31G** calculations, and 13.0 kcal/mol from the CIS/6-31++G** calculations. The normal mode frequencies have been computed including modes coupled to the proton tunneling mode. Two-dimensional and three-dimensional model potentials, formed from symmetric mode coupling potential and squeezed double well potential, have been fitted to the calculated energy barrier, geometries, and frequencies, and used to analyze proton dynamics. The calculated energy splittings for different vibrationally excited states have been compared with the available experimental data. The CIS/6-31++G** calculation gives good estimation of the tunneling energy splitting in the vibrationally ground state of tropolone. The model PES explain monotonic decrease in tunneling splittings with the excitation of the out-of-plane modes, however underestimate the actual splittings. The increase of the tunneling splitting with the excitation of the v13 and v14 modes is qualitatively correct.
Hydrogen adsorption in the NaA zeolite: A comparison between numerical simulations and experiments
Authors: Darkrim Farida , Aoufi Asdin , Malbrunot Pierre , Levesque Dominique
Published: July 2002

At room temperature and high pressures between 10 MPa and 140 MPa, hydrogen adsorption in the NaA zeolite was studied by grand canonical Monte Carlo simulation. The computed values of the average number of hydrogen molecules adsorbed in a crystal unit were compared to those measured at the same temperature and pressures between 10 MPa and 70 MPa. A quantitative agreement between the two sets of values was obtained by using, in the simulations, a model of zeolite crystal where the Al, Si, O, and Na atoms, disposed in accordance with the crystallographic structure of the NaA zeolite determined by x-ray diffraction, had effective electric charges and were sources of a van der Waals interaction. The adsorption of hydrogen molecules in the NaA zeolite resulted from the combined effects of van der Waals interactions and polarization of hydrogen molecules induced by the electric field of the effective charges.
Boson realization of sp(4). I. The matrix formulation
Authors: Castaños O. , Chacón E. , Moshinsky M. , Quesne C.
Published: July 2002

Holstein and Primakoff derived long ago the boson realization of a su(2) Lie algebra for an arbitrary irreducible representation (irrep) of the SU(2) group. The corresponding result for su(1,1)≅sp(2) is also well known. This raises the question of whether it is possible to obtain in an explicit, analytic, and closed form, and for any integer d, the boson realization of a sp(2d) Lie algebra for an arbitrary irrep of the Sp(2d) group, which is a problem of considerable physical interest. The case d=2 already illustrates the problem in its full generality and thus in this paper we concentrate on sp(4). The Dyson realization is well known, and the passage to bosons satisfying the appropriate Hermiticity conditions can be done by a similarity transformation through an operator K. What we want, though, is an explicit boson realization for sp(2d) similar to the one that exists for sp(2). In Sec. VI we show how we can get it for sp(4) if the operator K is known. Unfortunately while the matrix form of K2 can be explicitly derived from definite recursion relations, the same cannot be said of K as it involves, in general, the solution of algebraic equations of high degree. Thus the conclusion, corroborated also by a classical analysis where K does not appear, is that an explicit, analytic, and closed boson realization of sp(4), and thus also of sp(2d), is only possible for particular irreps of the corresponding groups.
Vector fields near caustics
Authors: Gorman Arthur D.
Published: July 2002

The modified Lagrange manifold technique enables determination of the asymptotic series solution of linear scalar wave equations near caustics of simple geometries. Here the technique is extended to include vector field considerations and more complicated caustic geometries.
Generation of new solutions of the stationary axisymmetric Einstein equations by a double complex function method
Authors: Zhong Zai-Zhe
Published: July 2002

A new approach to the solution of certain differential equations, the double complex function method, is developed, combining ordinary complex numbers and hyperbolic complex numbers. This method is applied to the theory of stationary axisymmetric Einstein equations in general relativity. A family of exact double solutions, double transformation groups, and n-soliton double solutions are obtained.
Ovsiannikov’s method and the construction of partially invariant solutions
Authors: Sastri C. C. A. , Dunn K. A. , Rao D. R. K. S.
Published: July 2002

A plausibility argument presented by the first two authors in an earlier paper [J. Math. Phys. 26, 3042 (1985)] concerning the existence of partially invariant solutions for some equations of the Fokker–Planck type is made precise by the explicit construction of one such solution. In the process a substantial simplification of Ovsiannikov’s method for finding partially invariant solutions is achieved. In addition, the class of partially invariant solutions obtained by Ovsiannikov for the equations of transonic flow of a gas is enlarged.
Algebraic calculation of the Green’s function for the Hartmann potential
Authors: Vaidya Arvind Narayan , Filho Henrique Boschi
Published: July 2002

Using the so(2,1) Lie algebra and the Baker–Campbell–Hausdorff formulas, the Green’s function for the Hartmann potential is constructed and its bound state energy spectrum is found. Also, this Green’s function is constructed in a coherent state basis and the equivalence of the two descriptions is shown.
Extended BRST quantization of gauge theories in the generalized canonical formalism
Authors: Batalin I. A. , Lavrov P. M. , Tyutin I. V.
Published: July 2002

The rules of canonical quantization of gauge theories are formulated on the basis of the extended BRST symmetry principle. The existence of solutions of the generating equations of the gauge algebra is proved. Equivalence between the extended BRST quantization and the standard method of generalized canonical quantization is established. Ward identities corresponding to invariance of a theory under the extended BRST symmetry are obtained.
Continuous quantum jumps and infinite-dimensional stochastic equations
Authors: Gatarek Dariusz , Gisin Nicolas
Published: July 2002

From a mathematical point of view, a class of infinite-dimensional stochastic differential equations describing continuous spontaneous localization in quantum dynamics will be studied. Existence and uniqueness of weak and strong solutions of respective equations are proven via Cameron–Martin–Girsanov transformation. The case of Gaussian initial states is explicitly solved.
An Sp(2)-covariant quantization of gauge theories with linearly dependent generators
Authors: Batalin I. A. , Lavrov P. M. , Tyutin I. V.
Published: July 2002

The Sp(2)-symmetric version of covariant quantization is formulated for the general case of any stage-reducible gauge theories.
Duality for the matrix quantum group GLp,q(2,C)
Authors: Dobrev V. K.
Published: July 2002

It is shown that the algebra 𝒰p,q dual to GLp,q(2,C) is isomorphic to U(pq)1/2(sl(2,C)) ⊗ 𝒵 as a commutation algebra, where 𝒵 is a subalgebra central in 𝒰p,q. The subalgebra 𝒵 is a Hopf subalgebra of 𝒰p,q, while the commutation subalgebra U(pq)1/2(sl(2,C)) is not a Hopf subalgebra.
Regularized long wave equation and inverse scattering transform
Authors: Yan Chuntao
Published: July 2002

The sech2 solitary wave solution of the regularized long wave equation is reobtained via the inverse scattering transform. The wave function of the eigenvalue problem of the relevant Schrödinger equation is proved reflectionless with sech2 potential of arbitrary amplitude. Moreover, the nonexistence of N-solitary wave solution (sech2 form and N≥2) is confirmed for the regularized long wave (RLW) equation and the method provides the possibility of solving some incompletely integrable equations via the inverse scattering transform.
Zeta-regularization of the O(N) nonlinear sigma model in D dimensions
Authors: Elizalde Emilio , Odintsov Sergei D. , Romeo August
Published: July 2002

The O(N) nonlinear sigma model in a D-dimensional space of the form RD−M×TM, RD−M×SM, or TM×SP is studied, where RM, TM, and SM correspond to flat space, a torus, and a sphere, respectively. Using zeta-regularization and the 1/N expansion, the corresponding partition functions—for deriving the free energy—and the gap equations are obtained. In particular, the free energy at the critical point on R2q+1×S2p+2 vanishes in accordance with the conformal equivalence to the flat space RD. Numerical solutions of the gap equations at the critical coupling constants are given for several values of D. The properties of the partition function and its asymptotic behavior for large D are discussed. In a similar way, a higher-derivative nonlinear sigma model is investigated, too. The physical relevance of our results is discussed.
Null surfaces, initial values, and evolution operators for spinor fields
Authors: Adler Ronald J. , Jacob Ovid C.
Published: July 2002

We analyze the initial value problem for spinor fields obeying the Dirac equation, with particular attention to the characteristic surfaces. The standard Cauchy initial value problem for first-order differential equations is to construct a solution function in a neighborhood of space and time from the values of the function on a selected initial value surface. On the characteristic surfaces the solution function may be discontinuous, so the standard Cauchy construction breaks down. For the Dirac equation the characteristic surfaces are null surfaces. An alternative version of the initial value problem may be formulated using null surfaces; the initial value data needed differs from that of the standard Cauchy problem. We study, in particular, the intersecting pair of characteristics t=x and t=−x (and supress the y and z dependence). In this case the values of separate components of the spinor function on the intersecting pair of null surfaces comprise the necessary initial value data. We present an expression for the construction of a solution from the null surface data; two analogs of the quantum mechanical Hamiltonian operator determine the evolution of the system.
Asymptotics of oscillatory Riemann–Hilbert problems
Authors: Varzugin G. G.
Published: July 2002

A classical method of stationary phase for oscillatory integrals is generalized to oscillatory Riemann–Hilbert problems of the kind arising in the theory of integrable nonlinear equations. The proposed approach is developed for the phase with N first-order stationary points, and the final formulas can immediately be applied to the problem of long-time behavior of the solutions of equations such as the NLS, KdV, mKdV sine-Gordon, and others.
Unitary transformation approach to the exact solution for a class of time-dependent nonlinear Hamiltonian systems
Authors: Maamache M.
Published: July 2002

By performing unitary transformations on the time-dependent Schrödinger equation, the exact solution for a class of nonlinear Hamiltonian systems is obtained and it is shown that these time-dependent problems are related to the associated time- independent problems. In addition, the evolution operator is derived. Nonadiabatic Berry’s phase is calculated on the basis of the exact solution. The theory is applied to some illustrative examples.
Cauchy noise and affiliated stochastic processes
Authors: Garbaczewski Piotr , Olkiewicz Robert
Published: July 2002

By departing from the previous attempt [Phys. Rev. E 51, 4114 (1995)] we give a detailed construction of conditional and perturbed Markov processes, under the assumption that the Cauchy law of probability replaces the Gaussian law (appropriate for the Wiener process) as the model of primordial noise. All considered processes are regarded as probabilistic solutions of the so-called Schrödinger interpolation problem, whose validity is thus extended to the jump-type processes and their step process approximants.
Collisionless mechanism of zero-point sound attenuation in a normal Fermi liquid
Authors: Slyusarenko Yu. V.
Published: July 2002

The possibility of collisionless (Landau) mechanism of attenuation of long-lived zero-point oscillations in a normal Fermi liquid is studied. Expressions are obtained for the coefficients of zero-point sound attenuation in a simple model. Limiting cases of large and small amplitudes of particle interaction are considered.
Lognormality of the scalar dissipation pdf in turbulent flows
Authors: Dahm Werner J. A. , Buch Kenneth A.
Published: July 2002

It is shown that measurements of lower-dimensional projections of the scalar gradient in an isotropic conserved scalar field ζ(x,t) are sufficient to construct the probability density function (pdf) for the true scalar dissipation χ. With this general method, the true scalar dissipation pdf is obtained from the result reported recently for the approximate pdf based on ‖∇ζ‖2≈3⋅(∂ζ/∂x)2. Results show that (i) the pdf of the true scalar dissipation is virtually lognormal and (ii) the most probable value of χ increases and the rms fluctuations of χ decrease in comparison with their one-dimensional estimates.
Chaotic Lagrangian trajectories around an elliptical vortex patch embedded in a constant and uniform background shear flow
Authors: Polvani L. M. , Wisdom J.
Published: July 2002

The Lagrangian flow around a Kida vortex [J. Phys. Soc. Jpn. 50, 3517 (1981)], an elliptical two-dimensional vortex patch embedded in a uniform and constant background shear, is described by a nonintegrable two-degree-of-freedom Hamiltonian. For small values of shear, there exist large chaotic zones surrounding the vortex, often much larger than the vortex itself and extremely close to its boundary. Motion within the vortex is integrable. Implications for two-dimensional turbulence are discussed.
A mapping closure for turbulent scalar mixing using a time-evolving reference field
Authors: Girimaji Sharath S.
Published: July 2002

A methodology for modeling passive scalar mixing in homogeneous turbulence using a new mapping closure approach is developed. The physical scalar field φ(y,t) (y-space coordinate, t time) is mapped using a mapping function, X(t), onto a reference scalar field θ(y,t), whose evolution is known. There are two main differences between the present approach and that of Chen et al. [Phys. Rev. Lett. 63, 2657 (1989)]. In the present approach, the reference field is time evolving and not restricted to a Gaussian. The evolution equation of the mapping function is derived for a general reference field. Gaussian and β-pdf reference fields are investigated. Calculations from both reference fields are compared with the heat conduction simulation (HCS) data of Girimaji (NASA Contract. Rep. 4446, 1992). The limits of validity of the mapping closure approach are discussed.
Shear stabilization of miscible displacement processes in porous media
Authors: Rogerson A. , Meiburg E.
Published: July 2002

The interface region between two fluids of different densities and viscosities in a porous medium in which gravity is directed at various angles to the interface is analyzed. Under these conditions, base states exist that involve both tangential and normal velocity components. These base states support traveling waves. In the presence of a normal displacement velocity, the amplitude of these waves grows according to the viscous fingering instability. For the immiscible case, it can easily be shown that the growth rate is not affected by the tangential velocities, while surface tension results in the usual stabilization. For the case of two miscible fluids, the stability of the base states using the quasi-steady-state approximation is investigated. The resulting equations are solved analytically for time t=0 and a criterion for instability is formulated. The stability of the flow for times t≳0 is investigated numerically using a spectral collocation method. It is found that the interaction of pressure forces and viscous forces is modified by tangential shear as compared to the classical problem, resulting in a stabilizing effect of the tangential shear. The key to understanding the physical mechanism behind this stabilization lies in the vorticity equation. While the classical problem gives rise to a dipole structure of the vorticity field, tangential shear leads to a quadrupole structure of the perturbation vorticity field, which is less unstable. This quadrupole structure is due to the finite thickness of the tangential base state velocity profile, i.e., the finite thickness of the dispersively spreading front, and hence cannot emerge on the sharp front maintained in immiscible displacements.
Further results on multifractality in shell models
Authors: Pisarenko D. , Biferale L. , Courvoisier D. , Frisch U. , Vergassola M.
Published: July 2002

Very long integrations, involving hundreds of millions of time steps, have been performed for the Gledzer–Ohkitana–Yamada ‘‘shell model’’ of fully developed turbulence, thereby allowing the computation of essentially noise-free structure functions at all inertial- and dissipation-range scales. Previously reported results by Jensen et al. [Phys. Rev. A 43, 798 (1991)] on the multifractal behavior of this model are confirmed. Oscillations in the structure functions are found to be genuine. An exact relation for certain cubic moments, equivalent to Kolmogorov’s four-fifth law, is derived and tested. The third-order structure function, here defined in terms of the third moment of shell amplitudes, is not directly determined by this relation and need not have its exponent equal to one. Significant discrepancies are actually found when the ratio between successive shell wave numbers is less than two.
A kinetic theory of trapped-electron-driven drift wave turbulence in a sheared magnetic field
Authors: Gang F. Y. , Diamond P. H. , Rosenbluth M. N.
Published: July 2002

A kinetic theory of collisionless and dissipative trapped-electron-driven drift wave turbulence in a sheared magnetic field is presented. Weak turbulence theory is employed to calculate the nonlinear electron and ion responses and to derive a wave kinetic equation that determines the nonlinear evolution of trapped-electron mode turbulence. The saturated fluctuation spectrum is calculated using the condition of nonlinear saturation. The turbulent transport coefficients (D, χi, χe), are, in turn, calculated using the saturated fluctuation spectrum. Because of the disparity in the three different radial scale lengths of the slab-like eigenmode: Δ (trapped-electron layer width), xt (turning point width), and xi (Landau damping point), Δ<xt<xi, it is found that ion Compton scattering rather than trapped-electron Compton scattering is the dominant nonlinear saturation mechanism. Ion Compton scattering transfers wave energy from short to long wavelengths where the wave energy is shear damped. As a consequence, a saturated fluctuation spectrum ‖φ‖2(kθ)∼k−αθ (α=2 and 3 for the dissipative and collisionless regimes, respectively) occurs for kθ ρs<1 and is heavily damped for kθ ρs>1. The predicted fluctuation level and transport coefficients are well below the ‘‘mixing length’’ estimate. This is due to the contribution of radial wave numbers x−1t<kr≤ρ−1i to the nonlinear couplings, the effect of radial localization of the trapped-electron response to a layer of width Δ, and the weak turbulence factor 〈γle/ωk〉k<1, which enters the saturation level.
Free-surface flow over an obstruction in a channel
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

Two-dimensional steady potential flow over a semicircular obstacle at the bottom of a channel is considered. The problem is solved numerically by using an integrodifferential equation formulation derived by Forbes and Schwartz [J. Fluid Mech. 114, 299 (1982)]. This equation is reduced to a set of algebraic equations by a difference method and solved by Newton’s method together with parameter variation. The numerical results for subcritical flows agree with those of Forbes and Schwartz. However, it is found that supercritical solutions exist only for values of the Froude number greater than some particular value. Furthermore, for some values of the Froude number, there are two supercritical solutions. One is a perturbation of a uniform stream whereas the other is a perturbation of a solitary wave.
A numerical study of two-dimensional instability-wave control based on the Orr–Sommerfeld equation
Authors: Bower W. W. , Kegelman J. T. , Pal A. , Meyer G. H.
Published: July 2002

A numerical method based on solution of the Orr–Sommerfeld equation is formulated to demonstrate the suppression of a two-dimensional, Tollmien–Schlichting-type, time-harmonic instability wave in a developing channel flow by a downstream secondary wave created through alternating suction and blowing at the surface. An analytical expression is derived providing the amplitude and a second parameter (the width or the phase) of the downstream surface-velocity distribution required to create a wave that will suppress the downstream continuation of the original disturbance.
Magnetic helicity injection for configurations with field errors
Authors: Finn John M. , Antonsen Thomas M.
Published: July 2002

The relative magnetic helicity and its rate of injection are computed for several examples of configurations representing magnetic fusion experiments, specifically reversed field pinches (RFP’s) with field errors. Field errors, here defined to be normal components of magnetic field at the wall, are believed to have a significant effect on the loop voltage, and therefore the volt-second consumption, in RFP’s. The perturbations producing these errors can arise from imbalance of the vertical field, leakage of Ohmic transformer flux, or toroidal field ripple. In the examples given, the relative helicity is the difference between two terms, which are called, respectively, the plasma linkage and the vacuum linkage. The latter quantity, which is zero for the simplified configurations studied previously, represents the linkage between vacuum fluxes occurring within the region of interest. On the other hand, the plasma linkage is the part of the relative helicity directly related to the instantaneous state of the plasma. It is found that the rate of change of the plasma linkage in the presence of poloidal field errors contains a new term proportional to the time rate of change of toroidal flux.
Study of discrete test filters and finite difference approximations for the dynamic subgrid-scale stress model
Authors: Najjar F. M. , Tafti D. K.
Published: July 2002

This paper investigates the effects of discrete test filters and finite-difference approximations for large-eddy simulations using the dynamic subgrid-scale stress model. Discrete explicit test filters based on finite-difference formulations have been constructed and the characteristics of their transfer function are studied. Several definitions of the scaling factor are investigated in the context of the discrete test filters. Two test filters, one based on a discrete representation of the top-hat filter (A), and another based on a high-order filtering operation (C) are evaluated in simulations of the turbulent channel flow at Reτ=180. It is found that filter A calculates a higher turbulent viscosity than filter C, which behaves more like a cutoff filter. For the same test filtering operation, the results are found to be sensitive to the ratio of the characteristic lengths of the test and grid filters. By testing two approximations to the convection terms based on second-order central difference and a nonconservative fifth-order upwind biased scheme, it is found that the dynamic procedure is receptive to the difference between the two approximations and adjusts the dynamic constant accordingly. It is found that the dynamic subgrid-scale stress model is more compatible with the Harlow–Welch scheme than dissipative schemes such as the fifth-order upwind biased approximation.
Anomaly of excess pressure drops of the flow through very small orifices
Authors: Hasegawa Tomiichi , Suganuma Masaki , Watanabe Hiroshi
Published: July 2002

Excess pressure drops are measured for the flow through very small orifices whose diameter ranges from the order of 1 mm to 10 μm using water, silicon oils, and solutions of glycerin in water: For larger orifices it is almost the same as, but for smaller orifices several times higher than, that of the numerical analysis of Newtonian fluid. Water gives the highest among the liquids used. Velocities are also measured along the centerline of a small orifice and found inconsistent with the result of the numerical analysis. A different mechanism may be dominant in the flow through very small orifices.
Centerline velocity decay of a circular jet in a counterflowing stream
Authors: Chan C. H. C. , Lam K. M.
Published: July 2002

We use an advection hypothesis to analyze the decay of centerline velocity of a circular jet issuing into a counterflowing stream. Working in the Lagrangian frame, we follow the locations and velocity gradients of jet fluid particles along the jet central axis while the particles are being advected backwards by the counterflow. The spatial velocity gradient along the jet centerline is thus obtained and subsequently integrated to describe the spatial decay of axial velocities. Laser-doppler velocity measurements are performed in the laboratory and the data are well predicted by our analytical expression of centerline velocity decay. Looking from another view, our treatment supports that the effect of an external axial flow stream on the jet flow field can be represented by a certain degree of stretching or contracting of the jet in the axial direction.
A subgrid-scale model for the scalar dissipation rate in nonpremixed combustion
Authors: Cook Andrew W. , Bushe W. Kendal
Published: July 2002

A subgrid-scale model is discussed for the scalar dissipation rate in nonpremixed turbulent reacting flows. The model contains a coefficient which is determined by assuming a form for the scalar energy spectrum. The model is tested a priori using Direct Numerical Simulation (DNS) results for a temporal reacting mixing layer. The estimated dissipation rates are in good agreement with dissipation rates computed directly from the DNS data. Furthermore, the assumed spectrum methodology is found to accurately predict the model coefficient, including its spatial and temporal variations.
Newtonian glass fiber drawing: Chaotic variation of the cross-sectional radius
Authors: Yarin A. L. , Gospodinov P. , Gottlieb O. , Graham M. D.
Published: July 2002

A model of Newtonian glass fiber drawing at fixed temperature in the unsteady range (the draw ratio E>20.22) is considered. In this range under steady boundary conditions, as is well known, the draw resonance instability sets in, resulting in self-sustained oscillations. These oscillations lead to a periodic variation of the cross-sectional radius of the fiber. In the present work we consider the case where the spinline radius varies periodically. Such a variation may result from flow oscillations in the fiber forming channels in the direct-melt process, or from the variation of the preform cross-sectional radius in drawing of optical fibers. When this variation takes place in the range E>20.22, the self-sustained periodic oscillations of the draw resonance are replaced by quasiperiodic and periodic (mode-locked) subharmonic or (under the appropriate conditions) chaotic oscillations (strange attractors). The routes to chaos found in the present work include (1) smooth period doubling bifurcation of (any) mode-locked periodic solution, (2) abrupt explosions of quasiperiodic solutions. The predicted chaotic variation of the spinline radius at the winding mandrel may result in a similar variation of the cross-sectional radius of the solidified fibers.
More results on oscillating edge-flames
Authors: Buckmaster J. , Hegab A. , Jackson T. L.
Published: July 2002

We examine a simple model of a side-anchored non-premixed edge-flame in order to gain insights into the oscillations that are sometimes observed in microgravity candle burning, flame-spread over liquids, etc. Previous results describe the role played by the Lewis number of the fuel, and the Damköhler number, and here we examine both the effects of an on-edge and off-edge convective flow, and the effects of a heat sink. The on-edge flow and the heat sink tend to destabilize and the off-edge flow tends to stabilize, results consistent with our hypothesis regarding the genesis of the oscillations.
Saturation of Richtmyer’s impulsive model
Authors: Hurricane Omar A. , Burke Eugene , Maples Steve , Viswanathan Manoj
Published: July 2002

In this Brief Communication, we use results from two-dimensional simulations to calculate for what product of perturbation wave number (k) and initial amplitude (a0) the Richtmyer impulsive model will fail in its prediction of shock wave driven instability growth as a function of initial shock Mach number. For “large” ka0, we suggest a formula which bounds the maximum value of the ratio of the amplitude growth rate over the interface speed by one minus the ratio of the interface speed and the transmitted shock speed.
Experimental investigations of the formation of a plasma mirror for high-frequency microwave beam steering
Authors: Meger R. A. , Mathew J. , Gregor J. A. , Pechacek R. E. , Fernsler R. F. , Manheimer W. , Robson A. E.
Published: July 2002

The Naval Research Laboratory (NRL) has been studying the use of a magnetically confined plasma sheet as a reflector for high-frequency (X-band) microwaves for broadband radar applications [IEEE Trans. Plasma Sci. PS-19, 1228 (1991)]. A planar sheet plasma (50 cm×60 cm×1 cm) is produced using a 2–10 kV fast rise time square wave voltage source and a linear hollow cathode. Reproducible plasma distributions with density ≥1.2×1012 cm−3 have been formed in a low-pressure (100–500 mTorr of air) chamber located inside of a 100–300 G uniform magnetic field. One to ten pulse bursts of 20–1000 μs duration plasma sheets have been produced with pulse repetition frequencies of up to 10 kHz. Turn on and off times of the plasma are less than 10 μs each. The far-field antenna pattern of microwaves reflected off the plasma sheet is similar to that from a metal plate at the same location [IEEE Trans. Plasma Sci PS-20, 1036 (1992)]. Interferometer measurements show the critical surface to remain nearly stationary during the current pulse. Plasma density measurements and optical emissions indicate that the plasma is produced by a flux of energetic electrons formed near the hollow cathode. The sheet appears to be stable to driver voltage and current fluctuations (NRL Memorandum Report No. 7461, 28 March 1994, NTIS Document No. AD-A278758).
Spherical versus nonspherical plasma-screening effects on semiclassical electron–ion collisional excitations in weakly coupled plasmas
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Spherical and nonspherical plasma-screening effects are investigated on electron-impact excitation of hydrogenic ions in weakly coupled plasmas. Semiclassical straight-line trajectory method is used to describe the behavior of the projectile electron. Scaled 1s→2p−1 semiclassical excitation probabilities are obtained by the spherical and nonspherical Debye–Hückel interaction potentials. The plasma-screening effect of the bound atomic wave functions in the transition probability obtained by the spherical Debye–Hückel model is found to be more effective than that in the transition probability obtained by the nonspherical Debye–Hückel model. The maximum position of the transition probability obtained by the nonspherical Debye–Hückel potential is more receded from the nucleus than that of transition probability obtained by the spherical Debye–Hückel potential.
Suppression of secondary electron emission from the material surfaces with grazing incident magnetic field in the plasma
Authors: Takamura S. , Mizoshita S. , Ohno N.
Published: July 2002

Suppression of secondary electron emission from the material surfaces with an obliquely incident magnetic field is demonstrated experimentally in a plasma containing hot electrons.
Power coupling to helicon and Trivelpiece–Gould modes in helicon sources
Authors: Borg G. G. , Boswell R. W.
Published: July 2002

The effects of finite electron mass on the helicon mode and the role of the Trivelpiece-Gould (TG) mode in helicon sources are considered. In an unbounded plasma these waves are commonly referred to as whistler waves. A simple cold plasma antenna-wave coupling code gives results which compare favorably with experimental observations of the whistler wave resonance cone. The connection between these cones and the Trivelpiece-Gould eigenmodes of bounded plasmas is discussed. The theory is applied to the study of antenna-wave coupling in helicon sources. It reveals that finite electron mass effects do not significantly improve antenna-wave coupling for ω/ωce<0.5. It is also suggested that, because of the resonance cone dispersion, eigenmode cavity resonances of the Trivelpiece-Gould mode could be difficult to excite in practice. This precludes geometric resonances of the antenna impedance due to the existence of the TG mode as an explanation for the high plasma production efficiency of helicon sources.
Kinetics of excitation and ionization in nonideal plasmas
Authors: Starostin A. N. , Aleksandrov N. L.
Published: July 2002

It is shown that quantum effects in dense gases and nonideal plasmas lead to nonexponential tails in the distribution function of electron momenta. This in turn is responsible for nonexponential temperature dependence of ionization and excitation rates in thermal equilibrium. These changes are predicted to be more pronounced than those due to lowering of the ionization potential. Consistent description of this phenomenon relies upon the evolution of the generalized electron distribution function, which depends both on energy and momentum. A kinetic equation for this function in a strong electric field is derived.
Potential of a dielectric particle in a flow of a collisionless plasma
Authors: Ivlev A. V. , Morfill G. , Fortov V. E.
Published: July 2002

It is shown that the charge distribution on the surface of a dielectric particle immersed in a supersonic plasma flow is strongly inhomogeneous, which results in the appearance of a large dipole moment. The dependences of both the charge and dipole moment of the particle on the drift velocity of the ambient plasma are calculated. It was found that under the same conditions the charge of a dielectric particle considerably exceeds the charge of a metal one.
Molecular beam epitaxy of GaSb0.5As0.5 and AlxGa1−xSbyAs1−y lattice matched to InP
Authors: Chiu T. H. , Tsang W. T. , Chu S. N. G. , Shah J. , Ditzenberger J. A.
Published: July 2002

Single crystal films of GaSby As1−y and Alx Ga1−x Sby As1−y alloys having compositions within the miscibility gap can be prepared using nonequilibrium growth techniques. We have successfully grown these alloys lattice matched to InP by molecular beam epitaxy. Transmission electron microscopy studies of these alloys grown on (100) InP at 500–560 °C show evidence of compositional modulation due to spinodal decomposition similar to that of InGaAsP grown by vapor phase epitaxy on InP used for laser diodes emitting at ∼1.5 μm. From the studies of optical microscopy, x-ray diffraction, and photoluminescence measurements at both room temperature and liquid helium temperature, these epitaxial layers are considered of high quality for optoelectronic device applications in the wavelength range of 0.65–1.7 μm lattice matched to InP substrates.
Transport properties of an ion implanted polydiacetylene
Authors: Elman B. S. , Sandman D. J. , Newkirk M. A.
Published: July 2002

Using 150-keV 75As ion beam irradiation, the conductivity of poly [1.6-bis (N-carbazolyl)-2.4 hexadiyne] crystals is increased by more than 15 orders of magnitude with respect to the value for unimplanted material. dc resistivity measurements are performed in the temperature range 20 K<T<290 K and the results yield a functional form ρ(T)=ρ0 exp(T0/T)m with a temperature-dependent exponential factor 1/2<m<1, where m≂1 at high (T>190 K) temperatures and m≂1/2 at low (T<80 K) temperatures. Mechanisms of conduction are considered in terms of existing theoretical models.
Raman study on silica optical fibers subjected to high tensile stress
Authors: Hibino Yoshinori , Hanafusa Hiroaki , Ema Kazuhiro , Hyodo Shin-ichi
Published: July 2002

For silica (pure and synthetic) optical fibers, on which tensile stresses from 0 to 4 GPa were imposed, Raman studies were carried out. It has become evident by deconvoluting the silica Raman peaks that the main peak intensity at 440 cm−1 decreases with increasing stress while the peaks at 490, 604, and 800 cm−1 remain practically unchanged. The fact that the 490 cm−1 defect line was not affected by the stress applied to fibers is contrary to the conclusion by G. E. Walrafen, P. N. Krishnan, and S. W. Freiman [J. Appl. Phys. 52, 2832 (1981)]. The results obtained in the present study are rather extensively discussed from a microscopic view of silica glass networks.
Acoustic phonon generation in the picosecond dynamics of dense electron-hole plasmas in InGaAsP films
Authors: Wiesenfeld Jay M.
Published: July 2002

Oscillations superposed on transient transmission and reflection from submicron films of InGaAsP (Eg=0.96 eV) and GaAs excited by subpicosecond optical pulses have been observed. The oscillations are due to coherent acoustic phonon generation in the thin films. Values for the velocity of sound, and the band-gap variation due to hydrostatic pressure and the difference in acoustic deformation potentials between conduction and valence bands are determined by analyzing the period and magnitude of the oscillations.
Studies of atomic and molecular fluorine reactions on silicon surfaces
Authors: Stinespring C. D. , Freedman A.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and an ultrahigh vacuum compatible microwave discharge effusive beam source have been used to study the reactions of atomic and molecular fluorine on Si(111) surfaces. Fluorine uptake and changes in binding energy and peak shape for the Si 2p and F 1s XPS peaks have been measured as functions of fluorine exposure. The results indicate that molecular fluorine is dissociatively chemisorbed to form an SiF2-like surface species. This reaction saturates at approximately one monolayer surface coverage. In contrast, atomic fluorine uptake extends well beyond the monolayer regime to include several Si layers. Additionally, as the uptake increases, the reaction product becomes SiF4-like. These findings are compared with previously reported results for XeF2 adsorption.
Variational calculations of subbands in a quantum well with uniform electric field: Gram–Schmidt orthogonalization approach
Authors: Ahn Doyeol , Chuang S. L.
Published: July 2002

We present variational calculations of subband eigenstates in an infinite quantum well with an applied electric field using Gram–Schmidt orthogonalized trial wave functions. The results agree very well with the exact numerical solutions even up to 1200 kV/cm. We also show that for increasing electric fields the energy of the ground state decreases, while that of higher subband states increases slightly up to 1000 kV/cm and then decreases for a well size of 100 Å.
Pulsed laser oxidation and nitridation of metal surfaces immersed in liquid media
Authors: Ogale S. B. , Polman A. , Quentin F. O. P. , Roorda S. , Saris F. W.
Published: July 2002

Nitridation and oxidation of titanium and iron immersed in liquid N2, NH3, or H2O are achieved by pulsed excimer laser treatment. Rutherford backscattering spectrometry reveals that significant quantities of nitrogen and oxygen can be incorporated in the metal matrices over a depth scale of several thousand angstroms. X-ray diffraction gives evidence for compound formation and scanning electron microscopy for large stress in the surface layer. The process is viewed as chemical reactive solute incorporation in the metal surface layer in its laser induced liquid state, followed by compound formation.