bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonclassical nucleation theory for the gas–liquid transition
Authors: Oxtoby David W. , Evans R.
Published: July 2002

We use density functional methods to develop a new nonclassical theory for the homogeneous nucleation of the gas to liquid phase transition. The extent of agreement between our results and the classical prediction of Becker, Döring, and Zeldovich is strongly dependent on the range of the attractive potential which we employ. We show that our predictions are consistent with experimental data using cloud chambers, and we suggest several directions in which experimentalists might look in order to find nonclassical effects. In particular, we suggest that cavitation (gas bubble formation in a liquid subjected to tensile stress) should nucleate at a significantly greater rate than that predicted by classical theory.
Rotational level dependent quenching of the A 3Πi, v′=0 state of NH
Authors: Garland Nancy L. , Crosley David R.
Published: July 2002

Quenching cross sections σQ have been measured for several rotational levels N′ in the A 3∏i, v′=0 state of NH, for a variety of collision partners. Ground state NH was generated in a room temperature discharge flow and excited with a pulsed laser, and the time decay of fluorescence was measured. The radiative lifetime for the levels N′=1 to 5 is 418 ± 8 ns. The σQ’s are generally large, up to 90 Å2, and decrease with increasing N′. This indicates the influence of an anisotropic, attractive interaction in most but not all cases. The present values of σQ are compared with those of other experiments; because σQ depends both on N′ and collision energy, experimental conditions must be carefully specified to yield results which are readily comparable.
Gas phase Raman scattering cross sections of benzene and perdeuterated benzene
Authors: Fernández-Sánchez J. M. , Montero S.
Published: July 2002

Depolarization ratios and Raman scattering differential cross sections of the Raman active fundamental bands of gas phase C6H6 and C6D6 are reported. These quantities are first reduced to derivatives of the molecular polarizability with respect to symmetry coordinates, and then these derivatives are interpreted in terms of bond properties, proving that the bond polarizability concept is as useful for aromatic systems as for other smaller nonaromatic molecules.
Water adsorption on clean and caesium covered Cu{110}
Authors: Lackey Damian , Schott Joachim , Straehler Bernd , Sass Jürgen K.
Published: July 2002

The interaction of water on clean and caesium dosed Cu{110} has been investigated as a model for the simulation of electrochemical double layers in ultrahigh vacuum. We present new data from high resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption ion angular distributions which provide a detailed picture of the growth and structure of water on clean Cu{110}. Water adsorbs molecularly on this surface and forms hydrogen bonded clusters at all coverages. The presence of caesium has a strong influence on the adsorption of water. For ΘCs=0.06 water is molecularly adsorbed with an increase in the heat of adsorption. The strongly bound water is associated with an increase in the work function and with striking changes in the HREELS spectra: the libration mode at 750 cm−1, which dominates the clean surface water spectrum, is absent in the presence of caesium. These effects are attributed to a reorientation of water in the vicinity of the caesium adatom.
The equilibrium carbon–carbon single-bond length in ethane
Authors: Harmony Marlin D.
Published: July 2002

The recently developed scaled moment-of-inertia procedure for obtaining near-equilibrium bond distances has been applied to ethane. The resulting prototype carbon–carbon equilibrium bond length is found to be 1.522±0.002 Å, in good agreement with an earlier estimate based on electron diffraction results.
Effective coupling in bridged electron transfer molecules: Computational formulation and examples
Authors: Evenson Jeffrey W. , Karplus Martin
Published: July 2002

The effective electronic donor/acceptor coupling (HDA) for a bridged electron transfer system is defined. An expression for HDA in terms of the bridge Green’s function is developed for systems represented by a tight-binding Hamiltonian. HDA is computed exactly for two systems and the existence of a dimensionless parameter which determines whether the effective coupling oscillates or decays with increasing donor/acceptor distance is demonstrated. A numerical technique for computing HDA is developed and shown to be significantly more powerful than conventional techniques.
A perturbation approach to predict infrared spectra of small molecular clusters applied to methanol
Authors: Buck Udo , Schmidt Burkhard
Published: July 2002

A method for predicting splittings and shifts of bands in infrared spectra of small clusters of polyatomic molecules is presented. Based on an approach of early publications of Buckingham, the influence of the intermolecular forces on the vibrational energy levels of the constituent molecules is calculated using perturbation theory to second order. In order to describe the interaction of identical molecules, this ansatz is extended to also cover degenerate systems. In first order, a coupling of the vibrational modes of the interacting molecules occurs which leads to delocalized vibrations of all the molecules in the cluster. The second order correction of the vibrational excitation frequencies are found to be dominated by the intramolecular couplings of the normal modes due to the cubic anharmonicity of the force field. The procedures developed here are applied for the interpretation of vibrational photodissociation spectra of small methanol clusters in the region of the fundamental excitation frequency of the OH stretching mode (ν1, 3681.5 cm−1), the CH3 rocking mode (ν7, 1074.5 cm−1), and the CO stretching mode (ν8, 1033.5 cm−1). Using semiempirical models for the intermolecular potential functions, splittings and positions of the experimental bands can well be explained. The nonequivalent positions of the two molecules in the linear dimer structure give rise to two different absorption frequencies for each of the three modes of the donor and the acceptor molecule, respectively. The trimer and tetramer spectrum with only one absorption band are in agreement with the existence of symmetric planar ring structures (C3h and C4h) for these species. The pentamer spectrum which also consists of one band is explained by the occurrence of three closely spaced frequencies of an asymmetric ring. The double peak structure in the hexamer spectra can be attributed to a distorted ring structure of S6 symmetry, while the occurrence of other energetically near-degenerate isomers can be ruled out by means of their spectra.
A method for dynamic simulation of rigid and flexible fibers in a flow field
Authors: Yamamoto Satoru , Matsuoka Takaaki
Published: July 2002

A method is proposed for simulating the dynamic behavior of rigid and flexible fibers in a flow field. The fiber is regarded as made up of spheres that are lined up and bonded to each neighbor. Each pair of bonded spheres can stretch, bend, and twist, by changing bond distance, bond angle, and torsion angle between spheres, respectively. The strength of bonding, or flexibility of the fiber model, is defined by three parameters of stretching, bending, and twisting constants. By altering these parameters, the property of the fiber model can be changed to be rigid to flexible. The motion of the fiber model in a flow field is determined by solving the translational and rotational equations for individual spheres under the hydrodynamic force and torque exerting on. This method was applied to simulate rotational motions with and without bending deformation of the fiber in a simple shear flow under the conditions of infinitely dilute system, no hydrodynamic interaction and low Reynolds number of a particle. For the rigid fiber, the computed period of rotation and the computed distribution of orientation angle agree with those calculated by Jeffery’s equation with an equivalent ellipsoidal aspect ratio. For the flexible fiber, the period of rotation decreases rapidly with the growth of bending deformation of the fiber and rotation orbits deviate from a circular one of the rigid fiber. These tendencies are similar to experimental ones described by Forgacs and Mason. These results show that the proposed method using bonded spheres’ model can reproduce the dynamic behavior of rigid and flexible fibers in a flow field successfully.
Hyperfine constants of bromine and iodine monofluoride
Authors: Müller Holger S. P. , Gerry Michael C. L.
Published: July 2002

The J′−J″=1−0 rotational transitions of 79BrF, 81BrF, and IF have been reinvestigated in the ground and first excited vibrational states using microwave Fourier transform spectroscopy. For IF, lines were also detected in the second excited vibrational state. Bromine and iodine nuclear quadrupole coupling constants, spin–rotation constants for all nuclei, and tensor and scalar spin–spin coupling constants (S and J), have been determined along with the rotational constants. Antishielding occurs at the fluorine nucleus for both BrF and IF, as has been found earlier for ClF. The antishielding has been rationalized following a model developed for ClF. The ratio 1.197 0514 (32) has been found for the equilibrium bromine quadrupole coupling constants of 79/81BrF.
Vibronic coupling mechanism in the Ã 2A2–B̃ 2B2 excited states of benzyl radical
Authors: Eiden Gregory C. , Weisshaar James C.
Published: July 2002

We report two-color resonant two-photon ionization spectra of internally cold benzyl-h7, benzyl-αd2, and benzyl-d7 radicals in the region of the vibronically mixed Ã 2A2–B̃ 2B2 excited states near 450 nm. Spectra of the corresponding 1:1 van der Waals complexes benzyl⋅Ar are reported as well. Band intensities of threshold photoionization spectra using a variety of mixed Ã 2A2–B̃ 2B2 vibronic states as intermediates provide additional new information about the mechanism of vibronic coupling. A semiquantitative coupling model based on crude adiabatic states attempts to interpret all available data from absorption, dispersed fluorescence, and pulsed field ionization (ZEKE-PFI) spectra. The two b1-symmetry modes ν28 (an in-plane skeletal deformation) and ν21 (an in-plane skeletal plus CCH bending motion) couple the Ã and B̃ states most strongly. In contrast to earlier interpretations, we find that the b1 combination ν17+ν36 plays a prominent role, while the b1 in-plane–CH2 rock ν29 is unimportant. The dispersed fluorescence work of Selco and Carrick and of Fukushima and Obi shows clear evidence of substantial coupling of the Ã and C̃ states through the a1 mode ν13, in accord with the semiempirical vibronic coupling calculations of Negri et al. In contrast with those calculations, our model seemingly demands no Ã–B̃ vibronic coupling matrix elements larger than 100–200 cm−1. Thus the dramatic effects of Ã–B̃ vibronic coupling result primarily from the near-degeneracy of the two excited states rather than unusually strong vibronic coupling matrix elements. Some fluorescence and PFl band intensities involving ν28 and ν21 deviate substantially from simple predictions based on products of squared mixing coefficients times Franck–Condon factors. A complete understanding of the spectra will require a quantitative account of Duschinsky mixing, which in turn requires accurate excited state vibrational modes.
Curvature elasticity and thermodynamic stability of electrically charged membranes
Authors: May Sylvio
Published: July 2002

The elastic behavior and stability of electrically charged amphiphilic membranes is investigated. In the present study, we address the question whether the electrostatic contribution to the curvature elastic moduli of a uniformly charged symmetric membrane leads to a curvature instability. To this end we consider a membrane in which the overall number of molecules is conserved during any deformation. In order to estimate both the molecular equilibrium area and the position of the neutral surface of each monolayer during bending, we include in the expression of the bilayer free energy beside an electrostatic, also a nonelectrostatic contribution. The former is described within the Gouy–Chapman theory of the diffuse double layer. The latter is a sum of a chain, an interfacial, and a nonelectrostatic head group contribution. The chain part is described using a detailed mean-field conformational free energy which is based on a molecular chain model. For the interfacial and nonelectrostatic head group contribution we use simple but general phenomenological expressions. It is shown that for medium and high membrane surface charge densities the electrostatic contribution to the bending moduli is not negligible. For highly charged membranes, the model predicts an instability with respect to a spherical deformation. This is discussed referring to the experimentally observed process of spontaneous vesiculation upon jump in pH of certain ionizable amphiphilic molecules.
Space-time correlated reaction field: A stochastic dynamical approach to the dielectric continuum
Authors: Tironi Ilario G. , Luty Brock A. , van Gunsteren Wilfred F.
Published: July 2002

In molecular dynamics (MD) simulations, the continuum description of electrostatic interactions has been based in most cases on the static response of the dielectric continuum. In this work, a time-dependent reaction field expression including stochastic and friction terms is derived in analogy to the generalized Langevin equation involving hydrodynamic interactions. Simulations of water using the extended simple-point charge model have been performed under different boundary conditions. The effects of a time-dependent treatment of the reaction field are demonstrated by comparing the results to those of MD simulations using a static, instantaneous reaction field and of MD simulations employing the Ewald sum. It is shown for the first time that the straight cutoff approach not only leads to a disruption of the time, but also of the spatial correlation of the dipole moment.
Phase transition behavior of a linear macromolecule threading a membrane
Authors: Di Marzio Edmund A. , Mandell Arnold J.
Published: July 2002

The problem of a polymer molecule whose two ends reside on opposite sides of a membrane or partition separating two solutions is solved exactly in the limit of no self-excluded volume. The monomers can go from one side of the membrane to the other only by threading serially through one hole in the membrane. The ends can be free, confined to run freely on the membrane surfaces, or be fixed to specific points on the membrane. It is found that the equilibrium thermodynamic phase transition is first order in all cases so that slight changes in pH, ionic strength, or temperature can move the polymer from being completely on one side of the membrane to being completely on the other side. Application to two biological problems are suggested: (1) the breaching of cell walls by the nuclear material of T2 bacteriophages, and (2) the transport of drugs that are affixed to these translocating polymers. The relation of this newly discovered transition to four other phase transitions that occur in isolated macromolecules (helix–random coil; equilibrium polymerization; polymer collapse; surface adsorption) is briefly discussed.
Thermal diffusion in (LiCl, RH2O)
Authors: Colombani J. , Bert J. , Dupuy-Philon J.
Published: July 2002

By means of an accurate investigation of the hydrodynamical behavior of the studied liquid, we have been able to perform interdiffusion and pure Soret effect measurements in (LiCl, RH2O) with R ranging from 6 to 100 and temperature from −60 to 0 °C. The interdiffusion coefficient corrected by a thermodynamic factor shows a characteristic evolution with concentration, which is a straightforward consequence of the role of the ions in this direct transport phenomenon. The Soret coefficient has revealed itself to be strongly negative, to increase with temperature, and to exhibit a sharp minimum with concentration. By comparison with measurements in other aqueous alkali chlorides, these results have contradicted the existing theory: negative Soret coefficients do not seem to be associated with poorly organized water. Furthermore, the extremum in the medium concentration range is likely to be related to the polarizing influence of the alkali ions on the outer hydration shells.
Phase behavior of casein micelles/exocellular polysaccharide mixtures: Experiment and theory
Authors: Tuinier R. , de Kruif C. G.
Published: July 2002

Dispersions of casein micelles and an exocellular polysaccharide (EPS), obtained from Lactococcus lactis subsp. cremoris NIZO B40 EPS, show a phase separation. The phase separation is of the colloidal gas–liquid type. We have determined a phase diagram that describes the separation of skim milk with EPS into a casein-micelle rich phase and an EPS rich phase. We compare the phase diagram with those calculated from theories developed by Vrij, and by Lekkerkerker and co-workers, showing that the experimental phase boundary can be predicted quite well. From dynamic light scattering measurements of the self-diffusion of the casein micelles in the presence of EPS the spinodal could be located and it corresponds with the experimental phase boundary.
Unidirectional diffusion of methane in AlPO4-5
Authors: Tepper H. L. , Hoogenboom J. P. , van der Vegt N. F. A. , Briels W. J.
Published: July 2002

Tracer diffusion of methane molecules in the unidirectional channels of AlPO4-5 has been studied by means of molecular dynamics simulations. A one-dimensional hop-and-cross model is introduced and is shown to be able to reproduce the molecular dynamics results accurately and we profit by extensive speedup in computational time. After elimination of system size effects by using the new model, two regimes can be recognized: a short-time regime where the mean square displacement is proportional to t0.6, and a long-time regime where the proportionality is linear.
Host–guest interactions in fluorinated polymer electrolytes: A 7Li-13C NMR study
Authors: Mustarelli P. , Quartarone E. , Capiglia C. , Tomasi C. , Ferloni P. , Magistris A.
Published: July 2002

Gel-type electrolytes based on fluorinated polymers are of interest for electrochemical devices. We present a 7Li-13C solid-state NMR and modulated differential scanning calorimetry (MDSC) study of gel electrolytes based on a copolymer poly(vinylidene fluoride) (PVdF)–hexafluoropropylene (HFP) activated with a nonaqueous solution ethylene carbonate (EC)–propylene carbonate (PC)–LiN(CF3SO2)2. We show that the narrowing of the Li lineshape is decoupled from the glass transition. The behavior of the longitudinal relaxation times, T1, confirms that the host polymer matrix simply behaves like a quasiinert cage for the solution. These results are confirmed by 13C NMR at the magic angle (MAS) data, which show that the presence of the polymer does not significantly affect the chemical shift changes induced in the EC/PC carbons by the imide salt.
Liquid crystal polymorphism in F-actin: Optical microscopic and rotatory dispersion studies
Authors: Das P. , Xu J. , Roy J. , Chakrabarti N.
Published: July 2002

Filamentous actin (F-actin), which occurs abundantly and ubiquitously in living mechanisms such as muscle, nonmuscle cells, and cytoskeleton, exhibits a liquid crystal polymorphism as its concentration is increased. The double-stranded, helical F-actin filaments are 1–70 μm long in our samples. The chirality of the filaments is consistent with the experimental observation of the characteristic cholesteric fingerprint and uniform textures. In addition to the chiral nematic phase, our optical microscopic studies show the occurrence of tilted chiral smectic phases, e.g., smectic C*, I*, or F*, and a smectic B phase. Measurement of optical rotatory dispersion in the cholesteric phase confirms the prediction of the de Vries equation describing the optical rotatory dispersion behavior for cholesterics. The observed lyotropic polymorphism is consistent with the Flory lattice model and a semiflexible polymer mesogen model of F-actin.
Elastic molecular dynamics with self-consistent flexible constraints
Authors: Zhou Jin , Reich Sebastian , Brooks Bernard R.
Published: July 2002

A new algorithm for constrained molecular dynamics is proposed. In contrast to the standard approach, the constrained bond-length/bond-angle value is adjusted at each time step so that total energy is minimized with respect to the constrained distances. This can be viewed as modifying the equilibrium bond-length/bond-angle according to external and centripetal forces. Two approaches are constructed to implement the algorithm. Method I includes all energy terms, but it is neither holonomic nor symplectic. Method II neglects a rotational kinetic energy term, resulting in a more expensive symplectic integrator. Both integrators are reversible and well conserve total energy. Due to the iterative nature of the optimization, which requires an energy evaluation at each iteration, these methods are intended for use in conjunction with other integration techniques that require an optimization at each time step iteration, such as the self-consistent electronic polarization methods or iterative combined quantum mechanics/molecular mechanics (QM/MM) hybrid simulations. Simulation results for collisions between two diatomic molecules, two water molecules, and for a periodic water box are compared and contrasted with fully constrained and free dynamics. These results demonstrate the utility and efficacy of these new methods.
Unusual bridged site for adsorbed oxygen adatoms: Theory and experiment for Ir{100}–(1×2)–O
Authors: Johnson K. , Ge Q. , Titmuss S. , King D. A.
Published: July 2002

The structure of oxygen adsorbed on Ir{100} has been determined by density functional theory slab calculations and quantitative low energy electron diffraction. The two studies produce essentially identical results, experiment providing a benchmark for the first principles calculation, while the latter provides a more complete understanding of the structure. Oxygen forms a (1×2) overlayer with adsorption on the bridge site: an unusual result for oxygen. There is a significant row pairing of the iridium atoms in the surface layer which strongly stabilizes the structure, and is one of the main factors favoring the bridge site occupation and the (1×2) configuration.
Barrier for the H2CO→H2+CO reaction: A discrepancy between high-level electronic structure calculations and experiment
Authors: Feller David , Dupuis Michel , Garrett Bruce C.
Published: July 2002

The unimolecular dissociation of formaldehyde to H2+CO was studied using extended basis set calculations and a variety of medium-to-high accuracy correlation recovery techniques. These included second and fourth order perturbation theory, multireference configuration interaction wave functions, coupled cluster theory with perturbative triples and full iterative triples, and estimated full configuration interaction wave functions. The intrinsic error of the electronic structure methods was assessed by extrapolating total energies to the complete basis set limit. Our best estimate of the barrier height, including zero point vibrational effects, is 81.9±0.3 kcal/mol, almost 3 kcal/mol larger than the experimental value of 79.2±0.8 kcal/mol. This estimate includes corrections for the effects of finite basis set truncation (which is negligible at the quintuple zeta level), higher order correlation recovery, core/valence correlation, and scalar relativistic effects. Using the same theoretical approach, we estimate the exothermicity of the dissociation reaction to be −1.6 kcal/mol, compared to experimental values in the −0.4 to −2.2 kcal/mol range. New calculations of the unimolecular dissociation rate constants using a variety of techniques failed to reconcile theory and experiment.
The effect of Coulomb interactions on the ac mobility of charges in quasi-one-dimensional systems. Example: discotic liquid crystals
Authors: Siebbeles Laurens D. A. , Movaghar Bijan
Published: July 2002

Using Monte Carlo simulations we calculate the frequency dependence of the diffusive mobility of a group of carriers on a short one-dimensional chain. We allow the carriers to interact with each other through weakly screened long-range Coulomb potentials. We consider both doped systems with discrete immobile counterions and gated systems with constant neutralizing background. In doped systems the counterions can act as traps for the mobile charge, which results in a strong frequency-dependent conductivity. In the gated charged system, the mobility per particle decreases with particle density. The calculations show that the electron–electron interaction, in the diffusion limit, acts very much like a source of random disorder. Because the mobility depends on the number of carriers, arrays of coupled wires can exhibit current “noise” which is directly related to charge exchange between the wires. This noise is in principle observable in signal processing. Basing ourselves on our numerical results, we have been able to conclude that the low-frequency ac mobility measured in doped triphenylenes must be due to spatial charge inhomogeneities, and cannot be due to intrinsic charge dynamics.
Generalization of the splitting theorem to arbitrary one- and two-particle operators
Authors: Živković Tomislav P.
Published: July 2002

The splitting theorem, previously derived for symmetric operators, is generalized to arbitrary one- and two-particle operators. The notion of alternant systems, as defined in various semiempirical theories of alternant hydrocarbons, is accordingly generalized to arbitrary Hamiltonians. A simple test to decide whether or not a given Hamiltonian is alternant is obtained. A configuration interaction space Xn generated by n electrons moving over 2n orthonormalized orbitals is considered. Eigenstates Ψ ∈ Xn of alternant Hamiltonians are contained in complementary spaces X+n and X−n, subspaces of the space Xn. Each state Ψ± ∈ X±n is called alternantlike, and it has characteristic properties of eigenstates associated with neutral alternant hydrocarbons, such as uniform charge density distribution, vanishing bond orders between vertices of the same parity, etc. The complete set of linear properties associated with one- and two-particle operators and common to all alternantlike states Ψ± ∈ X±n is obtained.
Inverse scattering via low-frequency moments
Authors: Charalambopoulos Antonios , Dassios George
Published: July 2002

An acoustically soft scatterer defined by a closed and star shape polynomial surface of any degree disturbs the propagation of a time harmonic plane incident wave. It is demonstrated that, under the hypotheses of Schiffer’s uniqueness theorem, all the generalized low-frequency moments corresponding to the capacity potential can be obtained from the scattering amplitude. An analytic algorithm is proposed that recovers the geometry of the body whenever a finite number of generalized moments generated by the leading low-frequency approximation are given. What is striking here is the fact that a surface measure generated by a potential problem is enough to recover the geometry of the scatterer. The idea here is to relate the given moments to a set of particular combined spherical moments that appear as coefficients of an algebraic linear system, whose solution provides the coefficients of the scattering surface in spherical harmonics. This is done with the help of an inner product defined over the surface of the unit sphere with respect to an unknown positive surface measure. In contrast to other existing techniques of shape reconstruction, the one proposed here does not involve the solution of any optimization problem. Instead, only some finite expansions in spherical harmonics and the solution of a linear algebraic system is involved. Tikhonov regularization is used to treat the case of inexact data. The proposed method is illustrated in the case of second degree surfaces where exact analytical data are available.
The mechanism of heat transport in thermal convection at high Rayleigh numbers
Authors: Asaeda Takashi , Watanabe Kunio
Published: July 2002

The small scale structure of free convection at high Rayleigh number was investigated by a flow visualization technique and temperature measurements. The dominant feature of the flow field was many sheetlike plumes consisting of several thermals, which transport heat from the conduction layer to the interior region. It was found that the boundary heat flux and the fluid’s properties determine the average characteristics of the thermals, namely, (a) the distance from the boundary where the thermals are generated, (b) the temperature excess transported by thermals, (c) the upward velocity of the fluid in a thermal, and (d) the horizontal dimension of the thermal. Velocity fluctuations and the heat transport rate can be explained through these characteristic values.
Mode switching in supersonic circular jets
Authors: Gutmark E. , Schadow K. C. , Bicker C. J.
Published: July 2002

An underexpanded supersonic circular jet was studied experimentally in a pressure ratio range of 2–15, yielding an equivalent Mach number range of 1.1–2.4 downstream of the nozzle. Spark Schlieren photography and near-field pressure measurements showed large changes in the jet structure for different Mach numbers. For nearly sonic exit velocity, the jet had a dominant symmetric structure and spreading rate similar to subsonic conditions. For an exit velocity equivalent to M>1.2, the jet switched mode to a helical structure with an accompanying substantial increase in the spreading rate. A transitional region was determined for 1.12<M<1.18, in which the jet switched modes intermittently from axisymmetric to helical. For Mach numbers higher than 1.6, another structural change was observed, related to the emergence of a normal shock in the first shock cell. The structural changes affected the near-field pressure fluctuations of the jet. The jet with the helical structure had a strong pressure field component in the upstream direction that could be related to the higher spreading rate of the jet.
Oscillatory flow with heat transfer in a square cavity
Authors: Biringen S. , Danabasoglu G.
Published: July 2002

A computational study is presented for the flow inside an oscillatory cavity. The numerical scheme employs a semi-implicit, time-splitting method to integrate the two-dimensional full Navier–Stokes equations satisfying continuity to machine accuracy. The efficient use of direct solvers for the uncoupled momentum and pressure equations is demonstrated. The oscillatory cavity flow is studied considering the effects of heat transfer, Reynolds number, and oscillatory Stokes number.
Evolution equations for stratified dilute suspensions
Authors: Rubinstein Jacob
Published: July 2002

Evolution equations are derived for the density function of a suspension sedimenting above a plane. The equations are in the form of a nonlinear conservation law and the solutions might develop jump discontinuities in a finite time. The analysis starts on a local microscopic level, and the averaged equations are obtained via explicit calculations of particle–particle and particle–wall interactions.
Approximate equations for the slow spreading of a thin sheet of Bingham plastic fluid
Authors: Liu K. F. , Mei C. C.
Published: July 2002

A model that can approximately describe a non-Newtonian fluid such as paint and fluid mud is a Bingham plastic with a yield stress. To facilitate the study of slow but transient spreading of a thin sheet of fluid mud, we need the approximate equations governing the nonlinear motion. Beginning with a modified Bingham model with two viscosities, the approximate equations are derived by a systematic perturbation analysis. The controlling parameters are found to be the Reynolds number R, the shallowness ratio h̄/L̄=δ1/2, and the ratio of viscosities β=ν/ν1 [as defined in (45)]. Results valid for R,β,δ≪1 but δ/β2≤O(1) are obtained. In the special case when δ/β2≪1, the limits can also be obtained by heuristic arguments similar to those in the lubrication theory. Two examples are discussed.
Brownian particle deposition in a directly simulated turbulent channel flow
Authors: Ounis Hadj , Ahmadi Goodarz , McLaughlin John B.
Published: July 2002

Diffusion of submicron particles in a simulated turbulent channel flow field is studied. The turbulent flow velocity field is generated by a direct numerical integration of the Navier–Stokes equation with the pseudospectral method. The equation of motion of particles including the Brownian effects is used and ensembles of particle trajectories are numerically evaluated and statistically analyzed. A uniform initial concentration of particles is used in the analysis. Effects of size on particle dispersion and wall deposition processes are studied. The results are compared with the existing models for the submicron particle deposition rate.
The multiplicity of steady flows in confined double-diffusive convection with lateral heating
Authors: Tsitverblit N. , Kit E.
Published: July 2002

In a number of studies concerning the lateral heating of a vertical rectangular enclosure containing a stably stratified brine solution, diverse flow regimes have been observed for different values of the buoyancy ratio. In this Brief Communication, certain intervals of the buoyancy ratio have been revealed to possess multiple steady flows. The presence of the multiple solutions was demonstrated to be underlain by complex bifurcation phenomena including numerous limit points and the symmetry-breaking bifurcation. The results appear to be insightful for the understanding of various findings of previous works.
Stimulated Raman forward scattering and the relativistic modulational instability of light waves in rarefied plasma
Authors: McKinstrie C. J. , Bingham R.
Published: July 2002

The modulational instability of copropagating light waves was studied recently by McKinstrie and Bingham [Phys. Fluids B 1, 230 (1989)], who applied their general theory to the study of the relativistic modulational instability (RMI) of light waves in the beat-wave accelerator. However, in rarefied plasma, the RMI merges with stimulated Raman forward scattering. The longitudinal RMI is suppressed over most of its expected range and the study of secondary instabilities in the beat-wave accelerator must be amended accordingly. A preliminary analysis indicates that stimulated Raman backward scattering is likely to be important in current beat-wave experiments, while near-resonant stimulated Raman forward scattering and near-forward stimulated Raman scattering could be important in proposed beat-wave experiments.
Electron–ion hybrid instability in laser-produced plasma expansions across magnetic fields
Authors: Peyser T. A. , Manka C. K. , Ripin B. H. , Ganguli G.
Published: July 2002

High-intensity laser irradiation of hollow glass cylinders immersed in a magnetic field results in plasma expansions strongly collimated in the direction transverse to both the initial flow and the magnetic field, but jetlike in the direction parallel to the initial flow. Magnetic fields from B=0 kG to B=10 kG produced plasmas with markedly different geometrical features. Fast framing camera photographs show the plasmas propagating across magnetic field lines and undergoing structuring indicative of transverse velocity shear-driven instabilities. Comparison is made between the observed instability characteristics and predictions of Rayleigh–Taylor, classical Kelvin–Helmholtz, and the electron–ion hybrid instabilities. Only the electron–ion hybrid instability is consistent with the experimental results.
Parametric excitation of high-frequency electromagnetic waves by the lower-frequency dipole pumping
Authors: Gamayunov K. V. , Khazanov G. V. , Krivorutsky E. N. , Veryaev A. A.
Published: July 2002

The possibility of parametric excitation of high-frequency electromagnetic waves by lower-frequency dipole pumping is studied. It is shown that the obtained general dispersive equation may be reduced to the Mathieu equation, provided the case of the flux instability is neglected. In the framework of the developed approach, the excitation of magnetohydrodynamic waves and whistler oscillations is examined.
Testing a closure for velocity/pressure-gradient correlations in nonuniform turbulent flow
Authors: Gallagher B.
Published: July 2002

Certain velocity/pressure-gradient correlations arise in the Reynolds stress equation through the interaction of the turbulent velocity field with the mean strain rate field. An existing, rational closure representation for these correlations is extended here and then tested. The closure permits good agreement to be obtained between observed and computed Reynolds stress components as they evolve under the influence of uniform shear and plane strain.
Stokes drag on hollow cylinders and conglomerates
Authors: Lasso I. A. , Weidman P. D.
Published: July 2002

An experimental study of the drag on hollow cylinders and conglomerates falling in a viscous fluid under Stokes flow conditions is described. The experiments were carried out in a tank of square cross section using silicone oil as the Newtonian fluid. Settling velocities of the free falling objects were measured and corrected to conditions of zero Reynolds number flow in an unbounded fluid. The results reveal that all objects tested have Stokes settling velocities smaller than that of a sphere of equal mass and volume. Measurements are reported in terms of the settling speed ratio defined as the ratio of the Stokes settling speed to that of a sphere of equal mass and volume. For the hollow cylinders two parameters are varied: the aspect ratio (length to outside diameter) and the radius ratio (inner to outer radius). Measurements show that the settling speed ratio decreases markedly as the hollowness of the cylinder increases. Each fixed radius ratio data set exhibits a maximum settling speed ratio near an aspect ratio of 1.65. For conglomerates composed of n spheres two trends appear: one for planar configurations and the other for globular clusters. Experimental data for two spheres and three spheres in point contact are in good agreement with recent theoretical results.
Oscillating grids as a source of nearly isotropic turbulence
Authors: De Silva I. P. D. , Fernando H. J. S.
Published: July 2002

Turbulence generated by vertical oscillations of a horizontal monoplanar grid in homogeneous water contained in a tank was used to study certain properties of nearly isotropic turbulence. The cases of sustained oscillations and the removal of forcing after a period of oscillations were considered. The former was used to evaluate the Eulerian-frequency spectra of nearly isotropic turbulence, and the velocity–time power law and spectral behavior of decaying turbulence were studied using the latter. The experimental observations are compared with available theoretical formulations as well as previous experimental observations.
Wave flow of rivulets on the outer surface of an inclined cylinder
Authors: Alekseenko S. V. , Markovich D. M. , Shtork S. I.
Published: July 2002

In this paper we present the results of an experimental study of wavy rivulet flow along the lower side of an inclined cylinder. The influence of the inclination angle of the cylinder, mode of irrigation and flow rate on the hydrodynamical characteristics is investigated. It is shown that the rivulet flow is unstable. As a consequence the nonlinear waves appear on a rivulet surface. The different types of surface waves are described. For the purpose of a detailed study of the wave characteristics, a method of superimposed oscillations is applied. New regularities of wave motion were found in comparison to the case of liquid film flow. The critical conditions for liquid ejection from the rivulet are determined.
Modulational and Raman instabilities in the relativistic regime
Authors: Guérin S. , Laval G. , Mora P. , Adam J. C. , Héron A. , Bendib A.
Published: July 2002

A large amplitude electromagnetic wave propagating in a plasma is known to be subject to severe modulational and Raman instabilities. Previous works were devoted to the weakly relativistic limit and applied mainly to a cold underdense plasma. One extends these works to include the fully relativistic limit for a circularly polarized light for which one derives the dispersion relation in a one-dimensional plasma. The characteristics of the instabilities are also calculated in the case where the plasma is classically overdense, with 1<(ωp/ω0)2<γ, where ωp is the plasma frequency, ω0 is the laser frequency, and γ is the relativistic factor of an electron in the laser field. Particle-in-cell simulations confirm the results of the numerical solutions of the dispersion relation. For (ωp/ω0)2/γ=0.57 the growth rate can be as large as 0.52ω0. The nonlinear stage of the instability results in a strong heating of the electron distribution function. The theory is further extended to the case of an initially hot plasma, for which the dispersion relation of the instabilities is established. Its analytical solution is given in the case of a low density plasma. Particle-in-cell simulations are used to treat the general case. One observes a strong reduction of the growth rate of the instability, which tends to restore the possibility to propagate relativistic waves in plasmas.
Self-organized criticality, multi-fractal spectra, sporadic localized reconnections and intermittent turbulence in the magnetotail
Authors: Chang Tom
Published: July 2002

It has been suggested that the dynamics of the Earth’s magnetotail may be described by the stochastic behavior of a nonlinear dynamical system near forced and/or self-organized criticality (SOC). It has been further argued that multiscale intermittent turbulence of overlapping plasma resonances is the underlying physics that can lead to the onset and evolution of substorms. Such a description provides a convenient explanation of the localized and sporadic nature of the reconnection signatures and fractal spectra that are commonly observed in the magnetotail region. These concepts provide a new paradigm for the understanding of the ever-elusive phenomenon of magnetic substorms. In this review, we describe some of the basic physical concepts and mathematical techniques (such as the dynamic merging of coherent structures, nonclassical nonlinear instability, path integrals, the theory of the renormalization-group, low-dimensional chaos, self-similarity and scaling, fractals, coarse-grained helicity and symmetry breaking) that play a central role in the development of these new ideas.
Frequency modulation noise and linewidth reduction in a semiconductor laser by means of a negative frequency feedback technique
Authors: Saito S. , Nilsson O. , Yamamoto Y.
Published: July 2002

Electrical negative frequency feedback control has been shown to reduce frequency modulation (FM) noise linewidth in semiconductor lasers. The method is based on the direct frequency modulation capability of a semiconductor laser. An error signal is extracted through optical heterodyne frequency discrimination detection using a stable master laser. FM noise is reduced by more than 20 dB and linewidth is reduced by one order of magnitude.
Age and stage dependency of estrogen receptor expression by lymphocyte precursors
Authors: Igarashi Hideya , Kouro Taku , Yokota Takafumi , Comp Phillip C. , Kincade Paul W.
Published: July 2002

Sex steroids negatively regulate B lymphopoiesis in adult mice. Paradoxically, lymphocytes arise during fetal life, when estrogen levels are high and maternal lymphopoiesis is suppressed. Here we demonstrate that embryonic B lymphopoiesis was unaffected by estrogen, but sensitive to glucocorticoids. Both fetal and adult precursors contained glucocorticoid receptor transcripts, but only adult precursors expressed estrogen receptor α and β together with the androgen receptor. Fetal hematopoietic cells did not efficiently acquire functional estrogen receptors after transplantation to irradiated adult mice. Sex steroid receptors were also expressed in a stage- and developmental age-dependent fashion in human precursors. A developmental switch in responsiveness of hematopoietic cells to sex steroids may be essential for formation of the immune system.
Genomic DNA breakpoints in
AML1
/
RUNX1
and
ETO
cluster with topoisomerase II DNA cleavage and DNase I hypersensitive sites in t(8;21) leukemia
Authors: Zhang Yanming , Strissel Pamela , Strick Reiner , Chen Jianjun , Nucifora Giuseppina , Le Beau Michelle M. , Larson Richard A. , Rowley Janet D.
Published: July 2002

The translocation t(8;21)(q22;q22) is one of the most frequent chromosome translocations in acute myeloid leukemia (AML).
AML1
/
RUNX1
at 21q22 is involved in t(8;21), t(3;21), and t(16;21) in
de novo
and therapy-related AML and myelodysplastic syndrome as well as in t(12;21) in childhood B cell acute lymphoblastic leukemia. Although DNA breakpoints in
AML1
and
ETO
(at 8q22) cluster in a few introns, the mechanisms of DNA recombination resulting in t(8;21) are unknown. The correlation of specific chromatin structural elements, i.e., topoisomerase II (topo II) DNA cleavage sites, DNase I hypersensitive sites, and scaffold-associated regions, which have been implicated in chromosome recombination with genomic DNA breakpoints in
AML1
and
ETO
in t(8;21) is unknown. The breakpoints in
AML1
and
ETO
were clustered in the Kasumi 1 cell line and in 31 leukemia patients with t(8;21); all except one had
de novo
AML. Sequencing of the breakpoint junctions revealed no common DNA motif; however, deletions, duplications, microhomologies, and nontemplate DNA were found. Ten
in vivo
topo II DNA cleavage sites were mapped in
AML1
, including three in intron 5 and seven in intron 7a, and two were in intron 1b of
ETO
. All strong topo II sites colocalized with DNase I hypersensitive sites and thus represent open chromatin regions. These sites correlated with genomic DNA breakpoints in both
AML1
and
ETO
, thus implicating them in the
de novo
8;21 translocation.