bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational spectra and structures of three hydrogen-bonded complexes between formic acid and water
Authors: Priem Dominique , Ha Tae-Kyu , Bauder Alfred
Published: July 2002

Rotational transitions of several hydrogen-bonded complexes between formic acid and water have been observed with a pulsed nozzle Fourier transform microwave spectrometer between 8 and 26 GHz. Three sets of rotational transitions have been assigned with the help of their Stark effects and of microwave–microwave double resonance experiments to formic acid–water, formic acid–(water)2 and (formic acid)2–water. Rotational constants and some centrifugal distortion constants have been fitted for each complex, and the components of the permanent electric dipole moments have been determined from Stark splittings. Structures and binding energies from ab initio calculations have been determined to the observed formic acid–water complexes.
Spacelike conformal Killing vectors and spacelike congruences
Authors: Mason D. P. , Tsamparlis M.
Published: July 2002

Necessary and sufficient conditions are derived for space-time to admit a spacelike conformal motion with symmetry vector parallel to a unit spacelike vector field na. These conditions are expressed in terms of the shear and expansion of the spacelike congruence generated by na and in terms of the four-velocity of the observer employed at any given point of the congruence. It is shown that either the expansion or the rotation of this spacelike congruence must vanish if Dna/dp =0, where  p denotes arc length measured along the integral curves of na, and also that there exist no proper spacelike homothetic motions with constant expansion. Propagation equations for the projection tensor and the rotation tensor are derived and it is proved that every isometric spacelike congruence is rigid. Fluid space-times are studied in detail. A relation is established between spacelike conformal motions and material curves in the fluid: if a fluid space-time admits a spacelike conformal Killing vector parallel to na and naua =0, where ua is the fluid four-velocity, then the integral curves of na are material curves in an irrotational fluid, while if the fluid vorticity is nonzero, then the integral curves of na are material curves if and only if they are vortex lines. An alternative derivation, based on the theory of spacelike congruences, of some of the results of Collins [J. Math. Phys. 25, 995 (1984)] on conformal Killing vectors parallel to the local vorticity vector in shear-free perfect fluids with zero magnetic Weyl tensor is given. The necessary and sufficient condition for vortex lines to be material lines is derived and the restriction this places on the flow of a thermodynamical perfect fluid is determined. As an application, a pure magnetic field in a rotational fluid is considered and results similar in nature to Ferraro’s law of isorotation are obtained. Throughout, corresponding results for a timelike conformal motion and for Newtonian theory are given for comparison.
Operator Racah algebra and Laplace-type expansions of irreducible spherical tensors
Authors: Niukkanen A. W.
Published: July 2002

Differential formulas for coefficients in the Laplace-type series of an arbitrary spherical tensor fLM (r+R) are given in terms of an operator N applied to the radial part φ(r) of fLM(r). Very compact and convenient expressions for N in terms of operator Pochhammer symbols are established. A special representation of the coefficients of the Laplace-type series, in terms of the operator Gauss function 2F1, is given, which, in turn, provides a remarkably short proof of two earlier Sack expansions. More general gradient formulas are introduced and numerous particular cases of the Laplace-type expansions are considered in detail.
The inverse scattering problem for the soft ellipsoid
Authors: Dassios George
Published: July 2002

A soft triaxial ellipsoid, of unknown semiaxes and orientation, is excited into secondary radiation by a plane acoustic wave of a fixed low frequency. It is proved that one measurement of the leading low-frequency coefficient and exactly six measurements of the second low-frequency coefficient of the real part of the forward or the backward scattering amplitude are enough to specify completely both the semiaxes, as well as the orientation of the ellipsoid. Therefore, only the first two low-frequency coefficients of the real part of the scattering amplitude are needed in order to solve the inverse scattering problem for the soft ellipsoid. For the case of spheroids, the number of measurements is restricted to one for the first and three for the second coefficient. Finally, the sphere is specified by a single measurement of the leading coefficient. The special cases where the orientation or the semiaxes are known are also discussed.
Relativistic fluids with shear and timelike conformal collineations
Authors: Duggal K. L.
Published: July 2002

This paper deals with perfect fluid matter plus nonsingular aligned electromagnetic fields admitting a timelike conformal collineation. It is proved that nonvanishing shear is itself a symmetry condition that partially underlines timelike conformal collineations with symmetry vector parallel to the fluid velocity vector. The results provide a natural counterpart to the study on shear-free space-times. It is also shown that the present study is applicable to a variety of physical problems.
Dissipation in nonlinear response
Authors: Lévy Laurent P. , Ogielski Andrew T.
Published: July 2002

It is shown that the measurable nonlinear susceptibilities of third and higher orders probe only a fraction of equilibrium fluctuations, and in general can give only lower bounds on the equilibrium correlation functions. This contrasts with the linear and quadratic response, which completely determines the corresponding correlations by general fluctuation–dissipation theorems. The exactly solvable one-dimensional kinetic Ising model illustrates which fluctuations give rise to dissipation; only two-magnon processes of short wavelength and opposite momenta contribute to the third-order nonlinear susceptibilities, while two- and three-magnon processes contribute to the corresponding equilibrium correlation functions.
Analytical solutions to the generalized spheroidal wave equation and the Green’s function of one-electron diatomic molecules
Authors: Liu J. W.
Published: July 2002

The eigenvalue problem of the angular part of generalized spheroidal wave equations (SWE) for describing the scattering of a charged particle on two Coulomb centers with different charges is cast in a matrix form which is then solved by a standard matrix procedure. This simple mathematical method is shown to be the most efficient and accurate among the others given for obtaining the solution to the ordinary SWE and the generalized SWE. An application to a complete solution for describing the scattering of a charged particle by a dipole is presented. The mathematical method for the analytical solution to the radial part of SWE obtained by series expansion in Coulomb wave functions for the two-center Coulomb scattering is found to be more efficient and accurate than the numerical methods. In the conclusion this work provides an analytical solution to the generalized spheroidal wave equation and the Green’s function of one-electron diatomic molecules.
A completely integrable system and parametric representation of solutions of the Wadati–Konno–Ichikawa hierarchy
Authors: Qiao Zhijun
Published: July 2002

A finite-dimensional involutive system is presented, and the Wadati–Konno–Ichikawa (WKI) hierarchy of nonlinear evolution equations and their commutator representations are discussed in this article. By this finite-dimensional involutive system, it is proven that under the so-called Bargmann constraint between the potentials and the eigenfunctions, the eigenvalue problem (called the WKI eigenvalue problem) studied by Wadati, Konno, and Ichikawa [J. Phys. Soc. Jpn. 47, 1698 (1979)] is nonlinearized as a completely integrable Hamiltonian system in the Liouville sense. Moreover, the parametric representation of the solution of each equation in the WKI hierarchy is obtained by making use of the solution of two compatible systems.
Commutator expansion. II. Relativistic reduced Green’s functions and the Lamb shift calculation
Authors: Hostler Levere
Published: July 2002

This is a continuation of an earlier paper [J. Math. Phys. 34, 5509 (1993)]. A projection operator technique is introduced in the Lamb shift calculation in order to manage spurious infrared divergences. The method provides an efficient tool to exhibit factors of the field in the commutator expansion of the Lamb shift. Mass eigenfunction expansion concepts developed elsewhere provide the setting for the new technique. As a by-product of our method, relativistic reduced Green’s functions are found to appear in a natural way. The method may prove useful, since reduced Green’s functions are simpler than the full Green’s functions, and behave like constant operators as regards the parameter integrals.
Quantum scattering theory in light of an exactly solvable model with rearrangement collisions
Authors: Varma S. , Sudarshan E. C. G.
Published: July 2002

We present an exactly solvable quantum field theory which allows rearrangement collisions. We solve the model in the relevant sectors and demonstrate the orthonormality and completeness of the solutions, and construct the S-matrix. In light of the exact solutions constructed, we discuss various issues and assumptions in quantum scattering theory, including the isometry of the Möller wave matrix, the normalization and completeness of asymptotic states, and the nonorthogonality of basis states. We show that these common assertions are not obtained in this model. We suggest a general formalism for scattering theory which overcomes these and other shortcomings and limitations of the existing formalisms in the literature.
An Itzykson–Zuber-like integral and diffusion for complex ordinary and supermatrices
Authors: Guhr Thomas , Wettig Tilo
Published: July 2002

We compute an analogue of the Itzykson–Zuber integral for the case of arbitrary complex matrices. The calculation is done for both ordinary and supermatrices by transferring the Itzykson–Zuber diffusion equation method to the space of arbitrary complex matrices. The integral is of interest for applications in quantum chromodynamics and the theory of two-dimensional quantum gravity.
Heat kernel expansion for operators containing a root of the Laplace operator
Authors: Gorbar E. V.
Published: July 2002

We suggest a method for the calculation of the DeWitt–Seeley–Gilkey (DWSG) coefficients for the operator −∇2+V(x) basing on a generalization of the pseudodifferential operator technique. The lowest DWSG coefficients for this operator are calculated by using the method proposed. We show that the problem of the calculation of the DWSG coefficients for operators of the type (−∇2+V(x))1/q, where q is an arbitrary natural number, is solvable. Namely, we deduce an explicit formula expressing the DWSG coefficients for operators with the root through the DWSG coefficients for operators without the root.
On periodical trajectories of the billiard systems within an ellipsoid in Rd and generalized Cayley’s condition
Authors: Dragović Vladimir , Radnović Milena
Published: July 2002

The analytical condition for a trajectory of the billiard system within an ellipsoid of any dimension is given. An interesting property is obtained—every periodical trajectory with the period less than the dimension of the space is placed in a coordinate hyperplane.
The theory of caustics and wave front singularities with physical applications
Authors: Ehlers Jürgen , Newman Ezra T.
Published: July 2002

This is intended as an introduction to and review of the theory of Lagrangian and Legendrian submanifolds and their associated maps developed by Arnold and his collaborators. The theory is illustrated by applications to Hamilton–Jacobi theory and the eikonal equation, with an emphasis on null surfaces and wave fronts and their associated caustics and singularities.
Hitchin systems at low genera
Authors: Gawȩdzki Krzysztof , Tran-Ngoc-Bich Pascal
Published: July 2002

The paper gives a quick account of the simplest cases of the Hitchin integrable systems and of the Knizhnik–Zamolodchikov–Bernard connection at genus 0, 1, and 2. In particular, we construct the action-angle variables of the genus 2 Hitchin system with group SL2 by exploiting its relation to the classical Neumann integrable systems.
Mobility and localization of charge carriers in a quasi-one-dimensional electron system over liquid helium
Authors: Nikolaenko V. A. , Kovdrya Yu. Z. , Yayama H. , Tomokiyo A.
Published: July 2002

The conductivity and mobility of charge carriers in a quasi-one-dimensional electron system over liquid helium is measured in the temperature range 0.5–1.8 K in confining electric fields up to 2.5 kV/cm. The system of quasi-one-dimensional channels is constructed by using high-quality optical diffraction gratings arranged at a certain height h over liquid helium which fill the grooves of the gratings, thus creating one-dimensional liquid channels. It is shown that the electron mobility decreases with increasing h, the value of the mobility being smaller than the corresponding value for bulk helium. As the temperature decreases, the mobility increases, passes through a peak, and then decreases. The observed effects can be explained by localization of charge carriers in quasi-one-dimensional electron systems.
Magnetic and electrostatic Aharonov–Bohm effects in a pure mesoscopic ring
Authors: Moskalets M. V.
Published: July 2002

The effect of weak scalar potential on a persistent current in a one-dimensional ballistic ring is considered. It is shown that the current amplitude oscillates with a change in the potential.
Formation of long-range ferrimagnetic order in dilute spinels Li0.5Fe2.5−xGaxO4 near the multicritical point
Authors: Efimova N. N.
Published: July 2002

The influence of spatial inhomogeneity of short-range exchange on the formation of long- range ferrimagnetic order in dilute spinels is investigated. The objects of investigation are two types of polycrystalline samples of Li0.5Fe1.1Ga1.4O4 differing in the extent of heterogeneity in the distribution of magnetic (Fe3+) and nonmagnetic (Ga3+) ions in the lattice. The heterogeneity was created by using two regimes of thermal treatment, i.e., slow cooling (S) and quenching (Q) of samples from 1550 to 300 K. The results of analysis of magnetization isotherms σT(H) at T=4.2 K, (77–300) K, H⩽16 kOe and temperature dependences of initial susceptibility χ0(T) show that the long-range ferrimagnetic order near the multicritical point (x0=1.5) of the x–T phase diagram is stable to variations of the degree of spatial heterogeneity of the exchange.
Crystal phase formation and growth in rare-gas clusters
Authors: Kovalenko S. I. , Solnyshkin D. D. , Bondarenko E. A. , Verkhovtseva È. T.
Published: July 2002

The structure of clusters formed in supersonic jets of heavy rare gases is studied by the methods of electron diffractometry. It is found that a crystalline fcc structure with “deformation-type” stacking faults (SF) is formed in aggregates consisting of N⩾2⋅103 atoms/cluster. The SF density is a linear function of N−1/3. The number of “defective” planes does not depend on the cluster size and is equal to four in all cases. Such a number of intersecting SF leads to the formation of nonvanishing atomic steps ensuring a rapid and subsequently defectless growth of the cluster on all densely packed planes facing the cubooctahedron. The obtained results confirm experimentally the important role of the kinetic factor in the formation of the atomic structure of a cluster.
Detection of absolute rotation using superfluid He4
Authors: Schwab K. , Bruckner N. , Packard R.
Published: July 2002

We have developed the superfluid analog of the superconducting rf SQUID. Such a device is a quantum mechanically based, absolute gyroscope and has been used to sense the rotation of the Earth. Our device is fabricated using silicon processing techniques and forms a planer sensing loop of superfluid helium which couples to the applied rotation. A much more sensitive superfluid gyroscope based on the principle’s demonstrated with this device, might ultimately be used to detect the precession of our local inertial frame with respect to the fixed stars by the gravitomagnetic field of the rotating Earth. We compare the superfluid gyroscope against two other experiments aimed at detecting this general relativistic effect.
Low-temperature resistivity of Zener ferromagnets
Authors: Krivoruchko V. N. , Yakovenko A. M.
Published: July 2002

Low-temperature resistivity ρ(T) of Zener ferromagnets due to the interaction between the conducting and magnetic subsystem is studied. The effective Hamiltonian is obtained by using the representation of Hubbard’s operators in terms of local pseudospins and spinless fermions. The contribution to ρ(T) from scattering of charge carriers by spin waves and magnetic inhomogeneities is considered. The latter are simulated by a vector field with random orientation and location of scattering center. The obtained dependence ρ(T)=ρ0+ρ1T3/2+ρ2T5/2 is compared with the available experimental and theoretical results.
Long-wave fluctuation kinetics and quasi-linear relaxation for zero-point sound in a normal Fermi liquid
Authors: Slyusarenko Yu. V.
Published: July 2002

A universal phenomenological approach to studying macroscopic fluctuations is proposed and general kinetic equations for long-wave fluctuations in a normal Fermi liquid are derived. These kinetic equations form the basis of the theory of quasi-linear relaxation for zero-point sound in a normal Fermi liquid in the paired fluctuation approximation. The conditions for the existence of the quasi-linear approximation are considered.
On the existence of long-range magnetic order in two-dimensional easy-plane magnets
Authors: Ivanov B. A. , Tartakovskaya E. V.
Published: July 2002

A consistent phenomenological approach is used to show that a true long-range order can exist in two-sublattice two-dimensional antiferromagnets (AFM) and ferrites closed to the compensation point. The effect is due to the long-range component of dipole forces. A similar result was obtained earlier for ferromagnets by Maleev [Sov. Phys. JETP 43, 1240 (1976)], who suggested that the Mermin–Wagner theorem may not be valid for interactions decreasing in proportion to 1/R3 or more slowly. It is found that the effect exists in the case of magnets with completely identical sublattices (AFM) only due to some types of the Dzyaloshinskii–Moriya interaction. For example, it is observed for AFM with an even (in Turov’s sense) principal axis and is absent otherwise. For a magnet with nonidentical sublattices, the effect can take place only for ferrites, i.e., for sublattices that are not compensated in the exchange approximation. The effect of stabilization of long-range order disappears at the point of compensation of magnetic moment. If this point does not coincide with the point of compensation of spin angular momentum, the intensities of fluctuations are nonmonotonic functions of temperature. The obtained estimates for the phase transition temperature are compared with experimental results.
On the spectrum of the Hubbard model with infinite repulsion on anisotropic triangular ladder-type lattice
Authors: Cheranovskii V. O. , Ezerskaya E. V. , Krikunov M. V.
Published: July 2002

Low-energy states of the Hubbard model with infinite electron repulsion on an anisotropic triangular strip-type lattice formed by weakly interacting linear segments are studied. The estimates of the stability region boundaries for the ferromagnetic ground state of the lattice are obtained in first order perturbation theory in the interaction between the segments. It is shown that a magnetic transition accompanied by a jumpwise variation of the total spin of the ground state from the minimum to the maximum value is possible for lattices with cyclic boundary conditions and the number of electrons greater than the number of segments by unity.
A comparison of shear stress fluctuation statistics between microbubble modified and polymer modified turbulent boundary layers
Authors: Pal S. , Deutsch S. , Merkle C. L.
Published: July 2002

Shear stress fluctuation statistics, as measured by flush mounted hot film probes, are presented for both microbubble and polymer injection into the same zero pressure gradient turbulent boundary layer. At equivalent values of drag reduction, the moments of the probability distribution are remarkably similar. For drag reductions larger than about 40%, the deviation of the skewness and kurtosis values from their pure water values argue against simple scaling explanations of the drag reduction phenomenon.
A collision-selection rule for a particle simulation method suited to vector computers
Authors: Baganoff D. , McDonald J. D.
Published: July 2002

A theory is developed for a selection rule governing collisions in a particle simulation of rarefied gas-dynamic flows. The selection rule leads to an algorithmic form that is highly compatible with fine grain parallel decomposition, allowing for very efficient utilization of supercomputers having vector or massively parallel single instruction multiple data (SIMD) architectures. A comparison of shock-wave profiles obtained using both the selection rule and Bird’s direct simulation Monte Carlo (DSMC) method show excellent agreement. This serves to establish the validity of the method, as the DSMC method is known to compare well with experimentally determined shock-wave profiles. In addition, the equation on which the selection rule is based is shown to be directly related to the time-counter procedure in the DSMC method, further establishing their equivalence. The results of several example simulations of representative rarefied flows are presented, for which the number of particles used ranged from 106 to 107, demonstrating the greatly improved computational efficiency of the method.
The effect of small particles on fluid turbulence in a flat-plate, turbulent boundary layer in air
Authors: Rogers C. B. , Eaton J. K.
Published: July 2002

This paper describes the result of both an experimental and an analytical investigation of the response of a two-dimensional, turbulent boundary layer in air to the presence of particles. Copper shot, 70 μm in diameter, were uniformly introduced into a vertical boundary layer, at a momentum thickness Reynolds number of about 1000. The particle mass flux was set at 20% of the fluid mass flux, and all measurements were made using a single-component, forward-scatter laser Doppler anemometer. The measurements clearly demonstrated that the particles damped fluid turbulence, apparently affecting all scales equally. The measurements further showed a strong correlation between the degree of damping and the particle concentration in the log region of the boundary layer.
Transport and mixing in traveling waves
Authors: Weiss Jeffrey B.
Published: July 2002

An iterated map is constructed that captures the essential features of particle trajectories in a class of quasiperiodic traveling waves: large-amplitude single-frequency traveling waves with two-dimensional structure perturbed by a wave with a second frequency. The map provides an efficient method for numerical calculation of the transport and mixing properties of such waves, and is used here to study the properties of a chaotic separatrix layer. It is found that the average position increases linearly with time indicating the existence of a well-defined transport velocity. The transport velocity grows faster than linearly as the perturbation parameter k increases. The mixing takes the form of anomalous diffusion where the mean-square deviation of position grows as tν, with ν>1. The data is consistent with the diffusion exponent ν growing linearly with k.
Reducing turbulence by phase juggling
Authors: Murakami Youichi , Shtilman Leonid , Levich Evgeny
Published: July 2002

The dynamics of decaying turbulence disturbed at small and intermediate scales by energy-conserving disturbances is investigated in direct numerical simulations at relatively low Reynolds number (Rλ≊20–30). Three types of disturbances are introduced. The reduction of the energy dissipation and related enstrophy growth is achieved by partial destruction of phase coherence ostensibly presented at almost all scales of turbulence. The disturbance preserving both energy and helicity spectrum has been found a most destructive one. Possible implications of the results for anisotropic turbulence is briefly discussed.
Drag reduction in turbulent channel flow by phase randomization
Authors: Handler R. A. , Levich E. , Sirovich L.
Published: July 2002

Results of numerical simulations of plane turbulent channel flow are presented in which a forcing is introduced which derives from the randomization of selected Fourier modes. In all cases, the randomization is introduced uniformly throughout the channel. The properties of the resulting turbulence are strongly dependent on both the wave numbers whose phases are randomized and the forcing frequency. Two principal wave-number bands have been selected. The first includes a selected subset of the largest length scales of the turbulence. Forcing in this band results in a fully sustained maximum mass flux increase above that of normal turbulence of 30%, which translates into a drag reduction of 58%. Many of the statistical properties of the simulated drag-reduced turbulence generated in this manner are in good qualitative agreement with the statistical properties of turbulence observed in experiments in which drag reduction is achieved through the introduction of small concentrations of long-chained polymers into the flow. In a second set of simulations, the phases of the intermediate and smallest wavelengths were randomized. Forcing at these scales of motion results in a drag increase. Speculations on the mechanism of the drag reduction by phase randomization are offered.
Stokes resistance, added mass, and Basset force for arbitrarily oriented, finite-length cylinders
Authors: Loewenberg M.
Published: July 2002

The Stokes resistance, added mass, and Basset force are numerically computed for finite-length, circular cross-section cylinders using a boundary integral formulation. Numerical results are presented for cylinders translating parallel and perpendicular to their symmetry axes with length-to-width ratios in the range: 0.01≤a/b≤100. There is good agreement with the limited experimental and numerical results of previous studies and slender-body theory. Simple, analytical formulas for the Stokes force, added mass, and Basset force of spheroids are contrasted with the numerical results for cylinders of the same aspect ratio. For some of these parameters, significant differences persist for disk- and rod-shaped particles.
Measurement of the translational and rotational velocities of a noncolloidal sphere rolling down a smooth inclined plane at low Reynolds number
Authors: Smart Jeffrey R. , Beimfohr Sean , Leighton David T.
Published: July 2002

The translational and rotational velocities of a sphere rolling down an inclined plane at low Reynolds number were measured as a function of the angle of inclination of the plane. Both 140 μm glass and 6350 μm diam acrylic particles were used. A theoretical model including the effects of particle surface roughness was developed, and is in quantitative agreement with the measurements. In addition, statistically significant velocity fluctuations were measured and are explained by variation in the coefficient of friction of the sphere. The disagreement between the measurements of Carty (B.S. thesis, Massachusetts Institute of Technology, 1957) and the theory of Goldman et al. [Chem. Eng. Sci. 22, 637 (1967)] is explained.
Drift-Alfvén kinetic stability theory in the ballooning mode approximation
Authors: Hong Bong-Guen , Horton W. , Choi Duk-In
Published: July 2002

The coupled drift-shear Alfvén mode including the complete Bessel function gyroradius effect and the ∇⊥B -curvature guiding center drift resonance of kinetic theory is solved for the toroidal ballooning mode eigenvalues and eigenfunctions. Comparisons between nonlocal (ballooning) and local kinetic theory and between nonlocal fluid and kinetic theory are made. The critical plasma pressure for kinetic ballooning mode instability is only the same as the magnetohydrodynamic (MHD) theory critical pressure βMHD for ηi=0. The critical kinetic theory plasma pressure βK(ηi) is well below βMHD and the kinetic theory growth rate is unstable for all k. The MHD second stability region is also unstable in the kinetic theory. The kinetic theory growth rate is a maximum around k≤0.3–0.5 for finite aspect ratio εn=rn/R. The effects of trapped electrons are found to be weakly stabilizing both analytically and numerically, and the instability is still significant outside the ideal MHD stable window from the ion magnetic drift resonances when ηi≳1. The kinetic growth rate is a function of the six dimensionless parameters k, q2β, εn, s, ηi, and τ=Te/Ti.
Imploding cylindrical shock in a perfectly conducting and radiating gas
Authors: Radha Ch. , Sharma V. D.
Published: July 2002

Similarity solutions to a problem in radiative magnetogasdynamics with strong shocks are presented. Group theoretic methods are used to obtain a condition on the shock Cowling number, and to characterize the state dependent form of the gas absorption coefficient for which the self-similar solutions are admitted. Numerical calculations have been performed to determine the value of the self-similar exponent and the profiles of the flow variables, which enable one to assess the influence of thermal radiation, the magnetic field strength, and the adiabatic heat exponent on the flow pattern.
Shear-induced particle migration in Couette and parallel-plate viscometers: NMR imaging and stress measurements
Authors: Chow Andrea W. , Sinton Steven W. , Iwamiya Joseph H. , Stephens Thomas S.
Published: July 2002

Couette and parallel plate viscometers are two commonly used flow geometries to characterize shear viscosity of concentrated suspensions. In Couette flow, it is well documented that prolonged shearing causes a decrease in the apparent viscosity of concentrated suspensions due to shear-induced particle migration from the annulus region to the stagnant region under the bob. In this study, the technique of nuclear magnetic resonance imaging (NMRI) was used to measure the evolution of suspension concentration profiles in Couette and parallel-plate flow devices upon shearing. Neutrally buoyant suspensions of nearly monodisperse, non-Brownian spherical particles at a volume fraction of 0.5 in a Newtonian fluid were used. The same flow cells and suspensions were also used in a rheometer to measure the changes in shear stress under identical experimental conditions such that a direct comparison can be made between the stress and concentration data. For Couette flow, the NMRI data correlated very well with the stress measurements and directly confirmed the Leighton–Acrivos [J. Fluid. Mech. 181, 415 (1987)] shear-induced migration theory. In torsional flow between parallel plates, no detectable change was found in particle concentration in the radial direction, but some decrease in the apparent viscosity was observed. These results provide some important clues for developing and evaluating more general descriptions of particle migration for nonrectilinear shear flows.
Asymmetric, oscillatory motion of a finite-length cylinder: The macroscopic effect of particle edges
Authors: Loewenberg Michael
Published: July 2002

The oscillatory motion of a finite-length, circular cylinder perpendicular to its symmetry axis in an incompressible, viscous fluid is described by the unsteady Stokes equations. Numerical calculations are performed using a first-kind, boundary-integral formulation for particle oscillation periods comparable to the viscous relaxation time. For high-frequency oscillations, a two-term, boundary layer solution is implemented that involves two, sequentially solved, second-kind integral equations. Good agreement is obtained between the boundary layer solution and fully numerical solutions at moderate oscillation frequencies. At the edges, where the base joins the side of the cylinder, the pressure and both components of tangential stress exhibit distinct, singular behaviors that are characteristic of steady, two-dimensional, viscous flow. Numerical calculations accurately capture the theoretically predicted singular behavior. The unsteady flow reversal process is initiated by a complex near-field flow reversal process that is inferred from the tangential stress distribution. A qualitative picture is constructed that involves the formation of three viscous eddies during the decelerating portion of the oscillation cycle: two attached to the ends of a finite-length cylinder, and a third that wraps around the cylinder centerline; the picture is similar to the results for axisymmetric flow. As deceleration proceeds, the eddies grow and coalesce at the cylinder edges to form a single eddy that encloses the entire particle. The remainder of the oscillatory flow cycle is insensitive to particle geometry and orientation. The macroscopic effect of the sharp edges is illustrated by considering ultrasonic, viscous dissipation in a dilute suspension. For a fixed particle-to-fluid density ratio, four different frequency regimes are identified. Four distinct viscous dissipation spectra are shown for different particle-to-fluid density ratios. The results indicate that particle geometry is important only for particles considerably less dense than the suspending fluid. The effect of edges is most apparent for disk- and rod-shaped particles.
Phase-locked eduction of vortex shedding in flow past an inclined flat plate
Authors: Lam K. M.
Published: July 2002

Flow past an inclined flat plate at an angle of attack of 30° and a Reynolds number of 30 000 is investigated experimentally. The velocity field in the wake is measured with a laser doppler anemometer (LDA) in the region from one plate breadth downstream to three and a half-plate breadths downstream. Controlled forcing is applied to the wake by vibrating the plate in the across-wind direction at a frequency in the middle of the lock-in range. The forcing serves to enhance the regularity and two-dimensionality of vortex shedding from the plate. It also facilitates phase-locked averaging of the LDA data. The LDA bursts are sorted according to their arrival instants relative to a particular phase of the vortex shedding cycles. The phase-averaged velocity results reveal large-scale vortical structures in the wake. Dynamical properties of these structures such as coherent vorticity and Reynolds stress production are discussed. The wake is found to be strongly asymmetric. The flow dynamics in the wake are dominated by a train of counterclockwise vortices shed from the trailing edge of the plate. The development, shedding and subsequent convection of these vortices are studied by following the consecutive phases of the shedding cycle.
Numerical study on blast flowfields induced by supersonic projectiles discharged from shock tubes
Authors: Jiang Z. , Takayama K. , Skews B. W.
Published: July 2002

In this paper we report on a numerical study of the blast flowfield generated by a supersonic projectile released from the open-end of a shock tube into ambient air. The Euler equations, assuming axisymmetric flows, were solved using a dispersion-controlled scheme implemented with moving boundary conditions. Two initial test cases were calculated. One of them is for validation of the numerical method and the other for verification of the moving boundary conditions. After good agreement was achieved, four further cases were calculated for examining effects of various projectile speeds and different release times of the projectile after the precursor shock wave was discharged. The present numerical study confirms that complicated transient phenomena exist in the initial stages shortly after projectile release, and that the blast flowfield is much more complex than that which can be inferred from muzzle blast studies where combustion products obscure the flow.
Axisymmetric motion of multiple composite spheres: Solid core with permeable shell, under creeping flow conditions
Authors: Chen Shing Bor
Published: July 2002

The axisymmetric creeping motion of multiple composite spheres is analyzed to investigate the hydrodynamic interactions among these particles. A composite particle referred to in this paper is a spherical solid core covered with a permeable shell, whose thickness can be arbitrary. The Stokes equation and the Brinkman equation are used to describe the flow fields outside and inside the particle, respectively. For two identical composite spheres with thin porous layers in near contact, a lubrication analysis is employed to examine their relative motion. Analytic expressions for the pressure and the drag force are obtained for the layers having high permeability. For general cases, a boundary collocation method is applied to numerically solve for the unknown coefficients in the series solutions for the flow behavior of the multiple particles. The resulting drag forces are in good agreement with the predictions from the lubrication analysis and the reflection method. In general, the strength of hydrodynamic interaction among composite particles lies between the values among permeable particles with the same permeabilities and among solid particles. The hydrodynamic behavior for composite spheres may be approximated by that for permeable spheres when the porous layer is sufficiently thick, depending on the permeability. When the particles undergo relative motion, the drag increases with decreasing distance between them. However, the drag on the particle with larger size or lower permeability may reach a minimum at a certain distance for a chain of dissimilar particles, rather than in contact, when they translate at the same velocity.
Stability of liquid crystalline bridges
Authors: Mahajan Milind P. , Tsige Mesfin , Taylor P. L. , Rosenblatt Charles
Published: July 2002

The stability of cylindrical bridges of the liquid crystal octylcyanobiphenyl in an immiscible liquid bath was investigated in the nematic and smectic A phases. In the nematic phase the bridge was found to destabilize at a length-to-diameter (slenderness) ratio R similar to that of ordinary Newtonian fluids. On the other hand, the Bingham behavior of the smectic A phase, i.e., an apparent yield stress, enabled the formation of stable columns with R well in excess of π.
On the decay rate of isotropic turbulence laden with microparticles
Authors: Druzhinin O. A. , Elghobashi S.
Published: July 2002

This paper is concerned with the two-way coupling effects on the decay rate of isotropic turbulence laden with solid spherical microparticles whose response time, τp, is much smaller than the Kolmogorov time scale (τp≪τk). The particles volumetric concentration, C0, is small enough (C0∼10−4) to neglect particle–particle interactions, and the material density of the particle is much larger than the fluid density (δ=ρp/ρf≫1). We obtain asymptotic analytical solutions for the instantaneous particle velocity and kinetic energy spectrum, in the limit τp≪τk, which indicate that the two-way coupling increases the fluid inertia term in the fluid momentum equation by the factor (1+C0δ). Consequently, the high-wave number components of the spectra of turbulence energy and dissipation develop in time as ∼exp[−2νk2t/(1+C0δ)]. The net result is a reduction of the decay rate of turbulence energy compared to that of particle-free turbulence (i.e., the one-way coupling case where C0δ≡0). We also perform direct numerical simulation (DNS) of isotropic turbulence laden with microparticles, using the two-fluid (TF or Eulerian–Eulerian) approach developed in an earlier study [Druzhinin and Elghobashi, Phys. Fluids 3 (1998)]. Excellent agreement is achieved between the DNS results and the analytical solution for the particle kinetic energy spectrum. The DNS results show that the two-way coupling reduces the decay rate of turbulence energy compared to that of one-way coupling. In addition, we compare the temporal developments of the turbulence kinetic energy and its dissipation rate, obtained from DNS using TF, with those from DNS using the trajectory (Eulerian–Lagrangian) approach. Satisfactory agreement is achieved between the two approaches, with TF requiring considerably less computational time.
Kinetic theory of a binary mixture of nearly elastic disks
Authors: Willits J. T. , Arnarson B. Ö.
Published: July 2002

Constitutive relations for a binary mixture of nearly elastic, circular disks are derived using the Revised Enskog Theory. Viscosity results from hard disk simulations are compared to the Enskog viscosity for a radii ratio of 1.25. The agreement is good up to an area fraction of 0.40. In the Appendix, we provide bracket integrals calculated to determine the perturbation coefficients of the expansion parameter used in the theory.
Note on forced Burgers turbulence
Authors: Kraichnan Robert H.
Published: July 2002

A putative powerlaw range of the probability density of velocity gradient in high-Reynolds-number forced Burgers turbulence is studied. In the absence of information about shock locations, elementary conservation and stationarity relations imply that the exponent −α in this range satisfies α⩾3, if dissipation within the power-law range is due to isolated shocks. A generalized model of shock birth and growth implies α=7/2 if initial data and forcing are spatially homogeneous and obey Gaussian statistics. Arbitrary values α⩾3 can be realized by suitably constructed homogeneous, non-Gaussian initial data and forcing.
A note on the overlap region in turbulent boundary layers
Authors: Österlund Jens M. , Johansson Arne V. , Nagib Hassan M. , Hites Michael H.
Published: July 2002

Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters describing the overlap region using the classical logarithmic relation. However, the data analysis demonstrates that the viscous influence extends within the buffer region to y+≈200, compared to the previously assumed limit of y+≈50. Therefore, the lowest Reθ value where a significant logarithmic overlap region exists is about 6000. This probably explains why a Reynolds number dependence had been found from the data analysis of many previous experiments. The parameters of the logarithmic overlap region are found to be constant and are estimated to be κ=0.38, B=4.1 and B1=3.6 (δ=δ95).
Simulation of the electron runaway in a plasma by Langevin equation
Authors: Zhidkov A. G.
Published: July 2002

A rigorous form of Langevin equation for the particle-in-cell simulation of a collisional plasma is obtained. The method is checked by reproducing the known results on the simulation of the electron runaway in a fully ionized uniform plasma with Maxwellian collisions. The non-Maxwellian effect owing to the electron heat flow on the runaway is explored.
Reduced charge state equations that describe Pfirsch Schlüter impurity transport in tokamak plasma
Authors: Peeters A. G.
Published: July 2002

The paper discusses the impurity transport in a tokamak in the Pfirsch Schlüter regime. A reduced charge state method is developed for the set of coupled equations that describe the poloidal variation of the heat flux and the third Laguerre polynomial expansion of the distribution function. A comparison is made with analytic equations in the disparate mass limit, and the influence of the cross terms of the coupled equations, which have been neglected in earlier work, on the transport coefficients of a hydrogen helium plasma is discussed.
Oxygen in titanium nitride diffusion barriers
Authors: Sinke W. , Frijlink G. P. A. , Saris F. W.
Published: July 2002

Oxygen has been found to play a dominant role in TiN barrier performance between Si(100) and Al. When TiN is exposed to air prior to Al deposition, interdiffusion of the Si/TiN/Al structure is not observed until heating to a temperature of 560 °C, whereas the barrier fails already below 400 °C when Al is deposited in situ onto TiN.
Boron doping in Si molecular beam epitaxy by co-evaporation of B2O3 or doped silicon
Authors: Ostrom R. M. , Allen F. G.
Published: July 2002

We report on controlled boron doping of silicon molecular beam epitaxy (MBE) films by two techniques: sublimation of saturated boron-doped silicon from a boron nitride crucible and evaporation of B2O3 from a tungsten crucible. Both methods were performed with the simultaneous evaporation of undoped silicon from an electron beam source. Sharp doping profiles and constant evaporation rates were obtained for both methods. Doping levels between 2×1017 and 1.5×1020 cm−3 were realized. The evaporations require only relatively low temperatures: 700–1150 °C for B2O3 and 1000–1400 °C for subliming silicon. These results make it possible to replace gallium as the p-type evaporative dopant in silicon MBE with the more desirable boron without using ion imbedding or a very high crucible temperature.
Amorphous silicon p-i-n solar cells with graded interface
Authors: Arya R. R. , Catalano A. , Oswald R. S.
Published: July 2002

The influence of inserting a thin graded interface layer at the p/i interface on the short-wavelength response and on the overall performance of amorphous silicon p-i-n solar cells is discussed. This device structure has resulted in fill factor values as high as 0.771.
Spatial variation of the observed energy gap in granular superconducting NbN films
Authors: Kirtley J. R. , Raider S. I. , Feenstra R. M. , Fein A. P.
Published: July 2002

We have simultaneously measured the surface topography and the tunneling current-voltage (I-V) characteristics of thin films of NbN as a function of lateral position using a low-temperature scanning tunneling microscope. We have imaged the superconducting energy gap by fitting the I-V’s with an energy gap for each spatial position. Such images showed significant variation of the gap, including areas with reduced gap when a magnetic field (up to 6 T) was applied. The features in the gap image were often correlated with features in the surface topographic images.
High-quality InAlAs grown by organometallic vapor phase epitaxy
Authors: Aina Leye , Mattingly Mike
Published: July 2002

High-quality InAlAs with excellent photoluminescence and low electron concentrations has been grown by organometallic vapor phase epitaxy (OMVPE). For InAlAs lattice matched to InP, electron concentrations as low as 7×1015 cm−3 and mobilities as high as 4472 cm2/V s at 300 K have been achieved. These values are comparable to results measured on material grown by liquid phase epitaxy and molecular beam epitaxy and exceed results for previously published OMVPE grown material. The variation of the structural, optical, and electrical properties of the InAlAs with lattice mismatch is reported.