bluuebunny/crossref_metadata_2025 huggingface dataset:
The vibrational spectrum of H2O2+⋅ radical cation: An illustration of symmetry breaking
Authors: Hrušák Jan , Iwata Suehiro
Published: July 2002

The H2O2+⋅ radical cation has been investigated with ab initio MO methods using both the single reference and multiconfiguration based wave functions. For the electronic states of two low lying isomers of the trans-(2Bg) and cis-(2A2) forms, the effect of the choice of the reference wave function on the shape of the potential energy surfaces near the equilibrium geometry and also at the dissociation limit (OH⋅ and OH+) was studied. In addition to the standard ab initio methods such as MP(n), CISD, CCSD(T), BD(T), and CASSCF, two different density functional methods were also employed. The force constants for the asymmetrical vibrations evaluated with the MP2 method are very large and consequently the corresponding ν5 (bu or b2) calculated frequencies approach unphysical numbers, which are orders of magnitude larger than the expected. In addition the ν2 (bu) frequency, which corresponds to the trans-conformer in the proper range around 1350 cm−1, possesses a huge IR intensity. For the cis-structure even the first asymmetrical frequency ν4 (b2), is too high (2216–2444 cm−1), and has an unreasonable IR intensity. It is shown that the symmetry breaking of the wave function is fully responsible for this unphysical behavior which leads to unreliable predictions of the whole vibrational spectrum. Detailed analyses of the symmetry breaking in the calculations of harmonic vibrational frequencies are presented. Finally, reliable, vibrational spectra for both the H2O2+⋅ isomers based on the full valence CASSCF and BD(T) methods are presented.
Theoretical investigation of the Ar+H2+(0⩽v⩽4, j=0)→ArH++H nonadiabatic reaction dynamics
Authors: Aguillon F. , Sizun M.
Published: July 2002

The title reaction is investigated using a semiclassical coupled wave packet method where the hyperspherical radius ρ is treated classically and the other coordinates quantally. Dynamical calculations are performed in a coplanarlike approximation using eight coupled electronic states. State-to-state reaction cross sections are obtained in the energy range 0.3 eV⩽Ecoll⩽5 eV for five different initial rovibrational states. The internal energy of the ArH+ product is found to be very high, especially at low collision energy. A comprehensive analysis of the reaction mechanisms is presented.
Polarons, localization, and excitonic coherence in superradiance of biological antenna complexes
Authors: Meier T. , Zhao Y. , Chernyak V. , Mukamel S.
Published: July 2002

A real-space formulation of time-resolved fluorescence of molecular aggregates is developed using the one-exciton density matrix ρ(t) of the optically driven system. A direct relationship is established between the superradiance enhancement factor Ls and the exciton coherence size Lρ associated with the off-diagonal density matrix elements in the molecular representation. Various factors which affect the latter, including finite temperature, energetic disorder, coupling with phonons, and polaron formation are explored. The theory is applied for the interpretation of recent measurements in the B850 system of the LH2 photosynthetic complexes.
Decoherent histories and nonadiabatic quantum molecular dynamics simulations
Authors: Bittner Eric R. , Rossky Peter J.
Published: July 2002

The role of quantum coherence loss in mixed quantum-classical dynamical systems is explored in the context of the theory of quantum decoherence introduced recently by Bittner and Rossky [J. Chem. Phys. 103, 8130 (1995)]. This theory, which is based upon the consistent histories interpretation of quantum mechanics, introduces decoherence in the quantum subsystem by carefully considering the relevant time and length scales over which one must consider the effects of phase interference between alternative histories of the classical subsystem. Such alternative histories are an integral part of any quantum-classical computational scheme which employs transitions between discrete quantum states; consequently, the coherences between alternative histories have a profound effect on the transition probability between quantum states. In this paper, we review the Bittner–Rossky theory and detail a computational algorithm suitable for large-scale quantum molecular dynamics simulations which implements this theory. Application of the algorithm towards the relaxation of a photoexcited aqueous electron compare well to previous estimates of the excited state survival time as well as to the experimental measurements.
Vibrational predissociation spectra of size selected methanol clusters: New experimental results
Authors: Buck Udo , Ettischer Ingo
Published: July 2002

Vibrational predissociation spectra of methanol (CH3OH)n clusters have been measured in the frequency range of the CO stretch mode for n=7 and n=8 and in the range of the OH stretch mode from n=4 to n=9. The clusters are completely size selected in a scattering experiment with helium atoms. For the IR excitation a line-tunable CO2-laser (1020 to 1080 cm−1) and an Optical Parametric Oscillator (3000 to 3800 cm−1) were used, respectively. The spectra of the CO stretch mode are slightly blue shifted by about 12 cm−1 and show a two peak structure for n=8 and a broad unstructured behavior for n=7 as is predicted in the theoretical calculations based on symmetric and asymmetric cyclic structures. In contrast, the spectra of the OH stretch mode exhibit large red shifts up to 500 cm−1, but also show the change of structured and unstructured behavior for even and odd sizes, respectively. The calculations based on the new potential model of the preceding paper are able to reproduce the general structures and the gross features of the spectra but fail for the absolute magnitude of the shifts.
The asymmetric cage structure of (H2O)7 from a combined spectroscopic and computational study
Authors: Brudermann Jesko , Melzer Mario , Buck Udo , Kazimirski Jan K. , Sadlej Joanna , Bush Victoria
Published: July 2002

The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule.
Vibrational spectroscopy and intramolecular energy transfer in isocyanic acid (HNCO)
Authors: Coffey M. J. , Berghout H. L. , Woods E. , Crim F. F.
Published: July 2002

Room temperature photoacoustic spectra in the region of the first through the fourth overtones (2ν1 to 5ν1) and free-jet action spectra of the second through the fourth overtones (3ν1 to 5ν1) of the N–H stretching vibration permit analysis of the vibrational and rotational structure of HNCO. The analysis identifies the strong intramolecular couplings that control the early stages of intramolecular vibrational energy redistribution (IVR) and gives the interaction matrix elements between the zero-order N–H stretching states and the other zero-order states with which they interact. The experimentally determined couplings and zero-order state separations are consistent with ab initio calculations of East, Johnson, and Allen [J. Chem. Phys. 98, 1299 (1993)], and comparison with the calculation identifies the coupled states and likely interactions. The states most strongly coupled to the pure N–H stretching zero-order states are ones with a quantum of N–H stretching excitation (ν1) replaced by different combinations of N–C–O asymmetric or symmetric stretching excitation (ν2 or ν3) and trans-bending excitation (ν4). The two strongest couplings of the nν1 state are to the states (n−1)ν1+ν2+ν4 and (n−1)ν1+ν3+2ν4, and sequential couplings through a series of low order resonances potentially play a role. The analysis shows that if the pure N–H stretch zero-order state were excited, energy would initially flow out of that mode into the strongly coupled mode in 100 fs to 700 fs, depending on the level of initial excitation.
Exponential intermolecular dynamics in optical Kerr effect spectroscopy of small-molecule liquids
Authors: Loughnane Brian J. , Scodinu Alessandra , Farrer Richard A. , Fourkas John T. , Mohanty Udayan
Published: July 2002

Optical Kerr effect spectroscopy has been employed to study the behavior of six symmetric-top liquids (acetonitrile, acetonitrile-d3, benzene, carbon disulfide, chloroform, and methyl iodide) over a broad range of temperatures. In all of the liquids, an exponential intermolecular response is observed on a time scale of a few hundreds of femtoseconds. Comparison of the temperature dependence of the time scale of this relaxation with the viscosity and single-molecule and collective orientational times in the liquids suggests that the exponential relaxation arises from motional narrowing.
High resolution spectroscopy of MgOH (X 2Σ+)in its V2mode: Further evidence for quasilinearity
Authors: Apponi A. J. , Anderson M. A. , Ziurys L. M.
Published: July 2002

Pure rotational spectraof the MgOH and MgOD radicals have been recorded in the v2bending vibration of their X 2Σ+ground electronic statesusing millimeter-wave direct absorption spectroscopy. Multiple rotational transitions arising from the v2l=11,22,20,31,33,42,and 44substates have been measured in the frequency range 240–520 GHz for these species. Both the spin-rotation and l-type doubling interactions have been resolved in the spectra. The complete data sets for MgOH and MgOD have been analyzed using a linear model for the Hamiltonian which takes into account higher order (l=±4)l-type interactions. The global analyses were adequate, but anomalous behavior was apparent in both molecules. In particular, the Bvvs v2relation was found to be highly nonlinear, large variations in the l-type doubling constant q were observed with vibrational level, and r0,rs,and r̃estructures determined differed substantially. Such findings suggest that MgOH is highly quasilinear,comparable to HNCO. The competition between ionic and covalent bonding is therefore becoming apparent in the lighter alkaline earth hydroxide species.
Closely approximating second-order Mo/ller–Plesset perturbation theory with a local triatomics in molecules model
Authors: Lee Michael S. , Maslen Paul E. , Head-Gordon Martin
Published: July 2002

A new ansatz for local electron correlation is introduced, which truncates double substitutions subject to a triatomics in molecules (TRIM) criterion. TRIM includes all double substitutions in which one occupied-virtual substitution is atomic while the other substitution can be nonlocal (a cubic number, before cutoffs). With an additional approximation, the TRIM second-order Mo/ller–Plesset perturbation theory (MP2) model can be noniteratively solved; this is the model that is implemented. Results are shown for absolute energies of alkane and polyene chains, rotational barriers of substituted ethylenes and benzenes, and association energies of the water and neon dimers. Over 99.7% of the untruncated MP2 energy is recovered for the test cases, and the relative energies of small systems are in error by less than 0.1 kcal/mol. By contrast, a diatomics in molecules (DIM) truncation recovers about 95% of the full MP2 energy, and yields errors several times larger for relative energies.
Impurity quenching of fluorescence in intense light. Violation of the Stern–Volmer law
Authors: Igoshin O. A. , Burshtein A. I.
Published: July 2002

The original integral encounter theory (IET) was used to describe the kinetics and quantum yield of fluorescence in the presence of intense pumping light. We compare the energy quenching through an impurity induced interconversion, with energy quenching produced by bimolecular charge transfer from the excited donor to the acceptor of the electron. In the former case, the convolution recipe which expresses the quantum yield via the system response to δ-pulse excitation is confirmed under special conditions, while in the latter case it was shown to not be applicable at all. By means of IET we found the stationary concentrations of excitations and ions and demonstrated the qualitative violation of the classical Stern–Volmer law at high intensity of pumping light. The modified form of this law was proposed instead and the light dependence of its constant was determined in the contact approximation.
A geometric setting for internal motions of the quantum three-body system
Authors: Iwai Toshihiro
Published: July 2002

Quantum mechanics for internal motions of the three-body system is set up on the basis of the complex vector bundle theory. The three-body system is called a triatomic molecule in the Born–Oppenheimer approximation. The internal states of the molecule are described as cross sections in the complex vector bundle assigned by an eigenvalue of the square of the total angular momentum operator. This bundle is equipped with a linear connection, which is a natural consequence of a geometric interpretation of the so-called Eckart condition. The coupling of the internal motion with the rotation is understood naturally in terms of this connection. The internal Hamiltonian operator is obtained which includes the internal motion–rotation coupling and a centrifugal potential. The complex vector bundle for the triatomic molecule proves to be a trivial bundle, though the geometric setting for the internal motion is independent of whether the bundle is trivial or not.
Polynomials and operator orderings
Authors: Bender Carl M. , Dunne Gerald V.
Published: July 2002

It is shown that there is an exact one-to-one correspondence between all possible sets of polynomials (both orthogonal and nonorthogonal) and rules for operator orderings. Operator orderings that are Hermitian give polynomials with definite parity. Most of the standard classical orthogonal polynomials are associated with operator orderings that are not particularly simple. However, there is a special one-parameter class of Hermitian operator orderings that corresponds to a class of elegant but little-known orthogonal polynomials called continuous Hahn polynomials.
SL(3,R) as the group of symmetry transformations for all one-dimensional linear systems. II. Realizations of the Lie algebra
Authors: Aguirre M. , Krause J.
Published: July 2002

The two-dimensional space-time realizations of the Lie algebra of SL(3,R) are obtained, when the group acts as the maximal point symmetry group of any given one-dimensional Newtonian linear system. It is shown that these realizations are isomorphic with the realization of the Lie algebra of the projective group in the plane. Next an active point of view is introduced, and SL(3,R) is interpreted as a group of mappings that transform one admissible world line of the system into another. Thus a new mechanical realization of the sl(3,R) algebra comes to the fore. Some miscellaneous examples are included.
Maximum entropy formalism and analytic extrapolation
Authors: Antolín J.
Published: July 2002

The maximum entropy principle is used to obtain a new analytic extrapolation method just complementary to the Padé-type method which leads to rigorous upper and lower bounds on the extrapolated function on the cut complex plane. Among the large class of functions that could equally well represent an analytical function in the experimental region a choice is made of the unique function that maximizes the entropy functional associated to this set of functions. The result is the least biased function compatible with the actual experimental data. This extrapolation method is applied to kaon–nucleon experimental data in order to obtain the most reasonable values for the KNY coupling constants compatible with the available experimental data and analytical constraints.
Curvature inheritance symmetry in Riemannian spaces with applications to fluid space times
Authors: Duggal K. L.
Published: July 2002

Katzin et al. [G. H. Katzin, J. Levine, and W. R. Davis, J. Math. Phys. 10, 617 (1969)] introduced curvature collineations (CC), defined by a vector ξ, satisfying LξRbcda=0, where Rbcda is the Riemann curvature tensor of a Riemannian space Vn and Lξ denotes the Lie derivative. They proved that a CC is related to a special conformal motion which implies the existence of a covariant constant vector field. Unfortunately, recent study indicates that the existence of a covariant constant vector restricts Vn to a very rare special case with limited physical use. In particular, for a fluid space time with special conformal motion, either stiff or unphysical equations of state are singled out. Moreover, perfect fluid space times do not admit special conformal motions. This information was not available, in 1969, when CC symmetry was introduced. In this paper, CC is generalized to another symmetry called ‘‘curvature inheritance’’ (CI) satisfying LξRbcda=2αRbcda, where α is a scalar function. We prove that a proper CI (i.e., α≠0) has direct interplay with the physically significant proper conformal motions. As an application, we show that a proper CI, which is also a conformal Killing vector (CKV), can generate new and physically relevant solutions for a variety of fluid spacetimes. In particular, it is shown, that, for CI with CKV, the known stiff or unphysical equations of state are not singled out.
Quaternions and special relativity
Authors: De Leo Stefano
Published: July 2002

We reformulate Special Relativity by a quaternionic algebra on reals. Using real linear quaternions, we show that previous difficulties, concerning the appropriate transformations on the 3+1 space–time, may be overcome. This implies that a complexified quaternionic version of Special Relativity is a choice and not a necessity.
On the definition of entropy production, via examples
Authors: Maes Christian , Redig Frank , Moffaert Annelies Van
Published: July 2002

We present a definition of entropy production rate for classes of deterministic and stochastic dynamics. The point of departure is a Gibbsian representation of the steady state path space measure for which “the density” is determined with respect to the time-reversed process. The Gibbs formalism is used as a unifying algorithm capable of incorporating basic properties of entropy production in nonequilibrium systems. Our definition is motivated by recent work on the Gallavotti–Cohen (local) fluctuation theorem and it is illustrated via a number of examples.
Experimental and theoretical study of the optical properties of crystalline and amorphous TiSi2 films
Authors: Antonov V. N. , Kudryavtsev Yu. V. , Makogon Yu. N. , Nemoshkalenko V. V. , Perlov A. Ya. , Silakova T. T.
Published: July 2002

Optical properties of amorphous and polycrystalline TiSi2 films have been studied experimentally in a wide spectral range (0.5–5.0 eV). The structures in the optical conductivity spectrum of polycrystalline TiSi2 sample are explained on the basis of comparison of the experimental data and the calculated data. Theoretical spectra were obtained within the local-density approximation using the semirelativistic linear-muffin-tin-orbital method (LMTO). It is shown that the transition of crystalline TiSi2 to the amorphous state leads to significant changes in the optical properties.
Peculiarities of nonlinear electrical conductivity of two-dimensional ballistic contacts
Authors: Moskalets M. V.
Published: July 2002

Peculiarities of the conductivity of two-dimensional ballistic contacts that are sensitive to the nature of the confining potential as well as the existence of electrostatic potential inside the microconstriction are considered. It is shown that the position, amplitude, and shape of the peculiarities carry direct information about the position of quantization levels, the magnitude of potential inside the microconstriction, and the probability of passage of an electron through the contact.
Vortex splitting and its consequences in the vortex street wake of cylinders at low Reynolds number
Authors: Eisenlohr Holger , Eckelmann Helmut
Published: July 2002

Based on the observation of vortex splitting in the laminar wake of thin flat plates placed parallel to the flow, an investigation on the consequences of such events for the von Kármán vortex street in the wake of circular cylinders was carried out. It was found that a ‘‘design break line’’ of vortex axes can lead to the decoupling of a wake flow from the always present disturbances deriving from the ends. The decoupling gives rise to parallel vortex shedding of a slightly higher frequency, instead of the oblique or slanted vortex shedding at a lower frequency usually observed.
Simulation of the Kolmogorov inertial subrange using an improved subgrid model
Authors: Chasnov Jeffrey R.
Published: July 2002

A subgrid model is developed and applied to a large-eddy simulation of the Kolmogorov inertial subrange. Currently popular subgrid models are derived from models of the turbulent energy equation, resulting in a significant loss of information as a consequence of the statistical averaging performed in going from the Navier–Stokes equation to the energy equation. The subgrid model developed here is based directly on a model of the Navier–Stokes equation. The improved subgrid model contains two terms: an eddy viscosity and a stochastic force. These terms are computed from the EDQNM stochastic model representation of the momentum equation, and from a fully resolved direct numerical simulation. Use of the subgrid model in a forced large-eddy simulation results in an energy spectrum that exhibits a clear k−5/3 power-law subrange with an approximate value Ko=2.1 of the Kolmogorov constant.
Nonlinear growth of the quasi-interchange instability
Authors: Waelbroeck F. L.
Published: July 2002

The nonlinear growth of the low-shear quasi-interchange instability is investigated using a low-beta expansion of the reduced magnetohydrodynamic equations. These equations are shown to be identical to the full magnetohydrodynamic equations in the linear regime, except for the neglect of parallel kinetic energy and the effects of compressibility. The nonlinear forces are found to be stabilizing for the profiles investigated. These forces lead to the appearance of stable, finite amplitude bifurcated equilibria above marginal stability.
Ionospheric plasma flow over large high-voltage space platforms. II: The formation and structure of plasma wake
Authors: Wang J. , Hastings D. E.
Published: July 2002

Theory and particle simulation results are presented for ionospheric plasma flow over large high-voltage space platforms at a large angle of attack. Both the transient formation of the space-charge wake and its steady-state structure are studied. The wake-side ion impact and current collection are obtained. It is found that the wake behind a high-voltage plate is characterized by two ion-rich sheaths embedded in a quasineutral background wake. The embedded sheath is formed by the ions passing through the sheath around the plate edge and serves as their trajectory path. Depending on the surface potential, the plate dimension, and the angle of attack, the embedded sheath may either extend downstream or curve back to form a ‘‘hook’’ structure. As a result, the plate’s wake-side surface may receive a high, localized ion flux at the location the embedded sheath strikes.
On the relation between ‘‘mixing length’’ and ‘‘direct interaction approximation’’ theories of turbulence
Authors: Sudan R. N. , Pfirsch D.
Published: July 2002

A systematic procedure is outlined for deriving a spectral equation from the renormalized theory of turbulence based on the so-called direct interaction approximation. This spectral equation should be valid for a large class of plasma and fluid systems.
Drift-cyclotron instability with trapped particle effects in a linear multiple mirror
Authors: Makowski M. A. , Emmert G. A.
Published: July 2002

A drift-cyclotron instability is observed at low magnetic field strengths in a linear multiple mirror. It is characterized by a frequency which decreases with increasing magnetic field strength, propagation in the direction of the electron diamagnetic current, and an azimuthal mode number of 2. The mode is observed when the ion-cyclotron frequency is less than the ion diamagnetic frequency. The frequency is Doppler-shifted by an E×B drift. Effects of particles trapped in the mirror cells alter the characteristics of the instability. The frequency decreases slightly and the fluctuation level increases with increasing mirror ratio for mirror ratios less than 1.1, but then levels off at higher mirror ratios. The drift wave is analyzed by a linear theory which treats finite ion gyroradius, an electron temperature gradient, and trapped electrons. The theory predicts three unstable modes, only one of which is consistent with all of the above measurements. This instability is a Doppler-shifted drift-cyclotron instability modified by trapped particle effects.
New self-similar solutions for the unsteady one-dimensional expansion of a gas into a vacuum
Authors: Schmalz R. F.
Published: July 2002

New simple self-similar solutions for the unsteady expansion of a gas into a vacuum are found. They describe supersonic rarefaction in cylindrical and spherical symmetry for an arbitrary adiabatic index. An inner boundary exists at a constant self-similar coordinate. In the asymptotic region a universal isothermal density law is given which depends only on geometry. Important applications lie in the field of laser-generated plasmas.
Görtler instability of boundary layers over concave and convex walls
Authors: Floryan J. M.
Published: July 2002

The stability analysis of boundary layers over curved surfaces is presented. A simple inviscid stability criterion is developed and used to identify shear layers which are potentially unstable with respect to the Görtler-type disturbances. A flow is stable if the velocity magnitude increases with distance away from the wall in the case of convex surfaces and decreases in the case of concave surfaces. The flows with nonmonotonic velocity distributions are therefore always potentially unstable regardless of the type of surface being considered. When viscous stability theory is applied to selected boundary layers the results confirm conclusions based on the inviscid theory. Detailed calculations are carried out for the Blasius boundary layer (monotonic velocity distribution) and a wall jet (nonmonotonic velocity distribution). Results suggest that flows with monotonic velocity distributions become unstable earlier than flows with nonmonotonic velocity distributions.
Viscous resistive magnetic reconnection
Authors: Porcelli Francesco
Published: July 2002

A unified linear analysis of viscous effects on resistive internal kink and tearing modes in current carrying magnetized plasmas is presented. The plasma is modeled by single-fluid magnetohydrodynamic (MHD) equations extended to include the dissipative processes caused by finite cross-field viscosity (μ⊥) and parallel electrical resistivity (η∥). Two parameters suffice to characterize the relevant instability regimes: the ideal stability parameter ΛH, which is proportional to the minimum value of the energy functional δW, and the viscosity parameter V∝μ⊥/η∥. Both positive and negative values of ΛH are considered to simulate the effect of varying the geometry of the plasma. Analytic expressions are found using generalized Fourier techniques for the mode structure and growth rate for arbitrary values of ΛH and V. Within the scope of the present model, the transverse viscosity slows down the growth of the linear mode, but it does not remove the instability. Novel instability regimes are investigated. For moderate values of the plasma viscosity parameter (V<1), the most important effect is on tearing modes, where the usual ordering for the mode growth rate γ∼η3/5∥ is modified into γ∼η5/6∥μ−1/6⊥, while the width of the viscous-resistive layer is found to scale as δ∼η1/6∥μ1/6⊥. The same scaling for δ is obtained near the ideal-MHD marginal stability threshold (‖ΛH‖<1); however, this requires a larger viscosity (V>1). The width of the ‘‘viscous-inertial’’ layer is instead independent of viscosity on the ideal-MHD unstable side of the spectrum (ΛH>1).
Relativistic magnetosonic solitons with reflected particles in electron–positron plasmas
Authors: Alsop David , Arons Jonathan
Published: July 2002

The presence of magnetically reflected particles is shown to allow the existence of large amplitude magnetosonic solitary waves in relativistic electron–positron plasmas. If the flow is assumed to contain a single loop of gyrating particles, self-consistent structures are found with peak field amplitudes (B/B∞)max<(11)1/2, where B∞ is the magnitude of the upstream magnetic field. In contrast, without reflected particles, the amplitude of a relativistic magnetosonic soliton is restricted to (B/B∞) −1<2/γ∞, where γ∞ is the upstream Lorentz factor. Therefore, if γ∞≫1, reflected particles greatly increase the allowable amplitudes of these nonlinear waves. It is also shown that when γ∞≫1, the wave properties are independent of γ∞, and are completely parametrized by the ratio of the Poynting flux to the kinetic energy flux in the upstream flow. Some new features of solitary waves without reflected particles are also derived, and a heuristic model is presented which gives a simple physical interpretation of many of these results.
Excitation of kinetic Alfvén waves in the ion-Bernstein wave heating of a plasma
Authors: Sharma Anamika , Tripathi V. K.
Published: July 2002

A kinetic theory of nonlinear coupling of ion-Bernstein waves with the kinetic Alfvén wave in a plasma is developed using a guiding center formalism. The nonlinearity arises through the motion of ions. The coupling coefficient is simplified at the limit where the Alfvén wave’s wavelength is larger than an ion Larmor radius, whereas the ion-Bernstein waves may have arbitrary wavelengths. At power levels employed on large devices in the slow wave ion-cyclotron heating scheme, the excitation of magnetic fluctuations appears a genuine possibility.
Negative viscosity of ferrofluid under alternating magnetic field
Authors: Shliomis Mark I. , Morozov Konstantin I.
Published: July 2002

A stationary magnetic field induces an increase in the ferrofluid viscosity. An additional resistance to the flow occurs due to the field oriented magnetic particles impeded by free rotation in a vortex flow. It is shown that in an alternating, linearly polarized magnetic field the additional viscosity is positive at low frequencies of the field and negative at high frequencies. The point is that an alternating field induces rotatory oscillations of the particles, but does not single out any direction of their rotation. One can say that half of the particles rotate clockwise and the other half counterclockwise. Hence, the macroscopic angular velocity of the particles equals zero. However, this corresponds only to fluid at rest. Any shear (i.e., any vorticity) is sufficient to break down the degeneracy of the direction of rotation, which results in the nonzero angular velocity of the particles. The occurring ‘‘spin up’’ of the flow by the rotating particles leads to the decrease of the effective viscosity, which means the additional viscosity appears to be negative.
The accumulation and dispersion of heavy particles in forced two-dimensional mixing layers. Part 2: The effect of gravity
Authors: Raju N. , Meiburg E.
Published: July 2002

The dispersion and settling of small, heavy, spherical particles in a temporally evolving two-dimensional mixing layer under gravity is investigated. The dilute limit is assumed, in which both the effect of the particles on the fluid flow and the interaction among the particles is negligible. The particle dynamics is quantified as a function of the dimensionless Stokes and Froude numbers, St and Fr, which express the ratios of the three time scales related to (i) the fluid flow, (ii) the particles’ inertia, and (iii) their settling velocity, respectively. For horizontal flow in which the upper stream is the seeded one, the mixing layer accelerates the settling of particles with small St, whereas particles with large St are slowed down in their settling motion. At intermediate St and for moderate settling velocities, root-mean-square (RMS) data for the particle concentration field demonstrate the generation of strong inhomogeneities by the mixing layer. These regions of high particle concentration have the form of bands in the initially unseeded stream. Scaling laws for their angles and the distance between them are given. Furthermore, analytical results for linearized flow fields are derived that demonstrate the optimal efficiency of the dispersion and settling process at intermediate St. The numerical simulations show the existence of different parameter regimes, in which the particle motion is dominated by the coherent vortices and by gravity, respectively. Scaling laws are derived for the particle dispersion and settling for both of these regimes, which show reasonable quantitative agreement with the simulation data. Flows that exhibit a vortex pairing process show a reduced tendency of the particles toward suspension. For vertically upward flow in which the faster stream is seeded, is observed a sharp maximum in the particle dispersion measures for intermediate St and settling velocities equal to one-half the difference between the free-stream velocities. Under these conditions, the cross-stream fluid velocity components become optimally efficient in ejecting particles into the unseeded stream.
The effect of streamwise braid vortices on the particle dispersion in a plane mixing layer. II. Nonlinear particle dynamics
Authors: Marcu B. , Meiburg E. , Raju N.
Published: July 2002

A computational investigation of the nonlinear dynamics of heavy particles in a row of counterrotating strained vortices is presented. By tracking the particles numerically in the quasi-two-dimensional fluid velocity field, information is obtained on the nature of their trajectories, as well as on probability distribution functions and potential accumulation regions. The particle behavior is discussed as a function of the dimensionless strain rate, the particle Stokes number St, and the dimensionless gravity parameter Fr. Only for very low values of the St can the particles accumulate at the vortex centers. For moderate values of St, they remain trapped on closed trajectories around the vortex centers. Increasing St leads to periodic open trajectories that allow for spanwise transport of the particles. Further bifurcations lead to the generation of multiple trajectories, as well as to subharmonic solutions. Eventually, intermittent and chaotic particle dynamics are observed. In the chaotic regime, a simplified flow model is employed in order to derive various scaling laws for the particle concentration field. For strong levels of gravity, the accumulation of large numbers of particles is observed in the upwelling regions predicted in part I of the present investigation [B. Marcu and E. Meiburg, Phys. Fluids 8, 715 (1996)].
Vertical water entry of disks at low Froude numbers
Authors: Glasheen J. W. , McMahon T. A.
Published: July 2002

As basilisk lizards (Basiliscus basiliscus) and shore birds run along the water surface they support their body weight by slapping and stroking into the water with their feet. The foot motions exploit the hydrodynamic forces of low-speed water entry. To determine the forces that are produced during water entry at low speeds, we measured directly the impact and drag forces for disks dropped into water at low Froude numbers (u2/gr=1–80). Also, we measured the period during which the air cavity behind the disk remains open to atmospheric air. We found that the force impulse produced during the impact phase is due to the acceleration of the virtual mass of fluid associated with a disk at the water surface. A dimensionless virtual mass M, defined as M=mvirtual/(4/3)πρr3, has a value near 1/π for disks. After impact, as penetration depth of the disk increases, the drag force can rise by as much as 76% even though the downward velocity is steady. However, a dimensionless force which includes the contribution from hydrostatic pressure [CD*=Drag(t)/(ρSgh(t)+0.5ρSu2)] takes a constant value near 0.7 regardless of disk size, speed, or cavity depth. Over the entire range of disk sizes and velocities, the period between impact and cavity closure, Tseal, can be described by a single value of dimensionless time, τ=Tseal(g/r)0.5, near 2.3. We con- clude that the fundamental phenomena associated with the low-speed water entry of a disk can be characterized by three dimensionless numbers (M, CD*, and τ).
Lattice Boltzmann modeling of interfacial gravity waves
Authors: Buick J. M. , Greated C. A.
Published: July 2002

A novel numerical technique for simulating interfacial gravity waves is considered. The model comprises an immiscible, binary fluid lattice Boltzmann model incorporating a gravitational interaction and simulates two immiscible, viscous fluids of different densities with a sharp interface between them. The model is described and the accuracy of the technique is considered. Simulation results are presented and the wave velocities, the oscillation frequency and the damping rate are found and compared to theory. A good comparison is found suggesting that the lattice Boltzmann approach is indeed accurately mimicking the wave dynamics.
Effect of chemical reactions on decaying isotropic turbulence
Authors: Martı́n M. Pino , Candler Graham V.
Published: July 2002

There have been many studies of turbulent combustion flows, however the interaction between turbulent motion and the chemical reactions that occur in hypersonic flows has not been studied. In these flows, the rate of product formation depends almost exclusively on the temperature, and small temperature fluctuations may produce large changes in the rate of product formation. To study this process, we perform direct numerical simulations of reacting isotropic turbulence decay under conditions typical of a hypersonic turbulent boundary layer flow. We find that there is a positive feedback between the turbulence and exothermic reactions. That is, positive temperature fluctuations increase the reaction rate, thereby increasing the heat released by the reaction, which further increases the temperature. Simultaneously, the pressure increases causing localized expansions and compressions that feed the turbulent kinetic energy. The Reynolds stress budget shows that the feedback occurs through the pressure-strain term. We also find that the strength of the feedback depends on how much heat is released, the rate at which it is released, and the turbulent Mach number. The feedback process is negative for endothermic reactions, and temperature fluctuations are damped.
Elastic membranes in viscous shear flow
Authors: Navot Yiftah
Published: July 2002

Liquid capsules with an elastic membrane surface in simple shear flow are considered beyond the regime of small deformations. A simple model is introduced to give a mathematical description of an elastic membrane in viscous flow. The question of whether there is a steady state of a system of a single membrane in external shear flow is studied. It is found by analytical considerations that the possible steady-state flows are restricted by symmetry. The evolution of the membrane in time is found by numerical calculations. For a single spherically symmetric membrane, I find by numerical simulations, a steady-state shape and its dependence on the shear strength. For nonspherically symmetric membranes we see that there is no steady-state shape in general, but by numerical simulations I find that the shape changes can become periodic. This leads to a new alternative explanation of previous experimental results. The stress tensor of the membranes and the effective viscosity of a dilute system of elastic membranes immersed in a liquid is calculated. I find an agreement with former analytical calculations for small shear, and obtain shear thinning behavior when the shear rate is increased.
Critical behavior of drop breakup in axisymmetric viscous flow
Authors: Navot Yiftah
Published: July 2002

The critical behavior of a liquid droplet immersed in a host fluid under external axisymmetric viscous flow is studied. It is well known that when the external extensional flow is weak the system approaches a steady-state flow, but when the shear rate is increased beyond some critical value a steady state is never attained and the droplet is stretched to infinity. This behavior is explained qualitatively by a simple semianalytic argument. The critical power law behavior of the droplet shape and its time dependence when the shear rate approaches the critical value is studied and is verified by numerical simulations for linear axisymmetric flows. For biaxial extensional flow (negative elongational flow) it is known that another critical point appears, and the droplet goes over into a toroidal shape. Similar critical behavior is predicted at that point also.
Generalized two-fluid theory of nonlinear magnetic structures
Authors: Schep T. J. , Pegoraro F. , Kuvshinov B. N.
Published: July 2002

A system of equations is introduced and discussed that describe the nonlinear dynamics of magnetic perturbations in a magnetized, high-temperature plasma. Diamagnetism, ion gyroradii effects, and finite electron mass are taken into account. These equations govern Alfvén as well as electrostatic waves and vortices and describe the nonlinear evolution of reconnecting modes. Electrons are treated in a fluid model. The equation for the ion response is new and is a nonlinear generalization to all orders in the thermal ion gyroradius of the nonlinear fluid model. This system of equations conserves two fluxes that are different from, but related to, the magnetic flux. Two-dimensional equilibrium solutions in the form of stationary propagating magnetic structures are obtained with the methods introduced in the theory of vector nonlinearities in electrostatic drift vortices. In the noncollisional regimes of interest the inertia of the electrons resolves the singularity in the current density that tends to develop at magnetic separatrices. The positions of the X points of the conserved fluxes are mirror symmetric and at a distance of the order of the electron skin depth from the resonant surface. The set of equations admits an energy integral and can be cast in noncanonical Hamiltonian form. The role of the Casimir invariants, that are functions of the conserved fluxes, is investigated and the connection with ‘‘reduced magnetohydrodynamics’’ is emphasized.
A self-consistent nonlinear theory of resistive-wall instability in a relativistic electron beam
Authors: Uhm Han S.
Published: July 2002

A self-consistent nonlinear theory of resistive-wall instability is developed for a relativistic electron beam propagating through a grounded cylindrical resistive tube. The theory is based on the assumption that the frequency of the resistive-wall instability is lower than the cutoff frequency of the waveguide. The theory is concentrated on study of the beam current modulation directly related to the resistive-wall klystron, in which a relativistic electron beam is modulated at the first cavity and propagates downstream through the resistive wall. Because of the self-excitation of the space charge waves by the resistive-wall instability, a highly nonlinear current modulation of the electron beam is accomplished as the beam propagates downstream. A partial integrodifferential equation is obtained in terms of the initial energy modulation (ε), the self-field effects (h), and the resistive-wall effects (κ). Analytically investigating the partial integrodifferential equation, a scaling law of the propagation distance zm at which the maximum current modulation occurs is obtained. It is found in general that the self-field effects dominate over the resistive-wall effects at the beginning of the propagation. As the beam propagates farther downstream, the resistive-wall effects dominate. Because of a relatively large growth rate of the instability, the required tube length of the klystron is short for most applications.
Complexity in plasma: From self-organization to geodynamo
Authors: Sato Tetsuya ,   the Complexity Simulation Group
Published: July 2002

A central theme of ‘‘Complexity’’ is the question of the creation of ordered structure in nature (self-organization). The assertion is made that self-organization is governed by three key processes, i.e., energy pumping, entropy expulsion and nonlinearity. Extensive efforts have been done to confirm this assertion through computer simulations of plasmas. A system exhibits markedly different features in self-organization, depending on whether the energy pumping is instantaneous or continuous, or whether the produced entropy is expulsed or reserved. The nonlinearity acts to bring a nonequilibrium state into a bifurcation, thus resulting in a new structure along with an anomalous entropy production. As a practical application of our grand view of self-organization a preferential generation of a dipole magnetic field is successfully demonstrated.
Self-consistent stability analysis of ablation fronts with large Froude numbers
Authors: Goncharov V. N. , Betti R. , McCrory R. L. , Sorotokin P. , Verdon C. P.
Published: July 2002

The linear stability analysis of accelerated ablation fronts is carried out self-consistently by retaining the effect of finite thermal conductivity. Its temperature dependence is included through a power law (κ∼Tν) with a power index ν≳1. The growth rate is derived for Fr≫1 (Fr is the Froude number) by using a boundary layer analysis. The self-consistent Atwood number and the ablative stabilization term depend on the mode wavelength, the density gradient scale length, and the power index ν. The analytic formula for the growth rate is shown to be in excellent agreement with the numerical fit of Takabe, Mima, Montierth, and Morse [Phys. Fluids 28, 3676 (1985)] for ν=2.5 and the numerical results of Kull [Phys. Fluids B 1, 170 (1989)] over a large range of ν’s.
Energy transfer between crossing laser beams
Authors: Kruer William L. , Wilks Scott C. , Afeyan Bedros B. , Kirkwood Robert K.
Published: July 2002

Induced scattering between crossing laser beams is considered, including the effects of long-wavelength modulations in the plasma. This fundamental process can impact the choice of beam-smoothing techniques for laser-driven hohlraums. Study of this process is an ideal way to quantify stimulated scattering instabilities, since one can independently vary the intensity, polarization, and frequency separation of the crossing beams.