bluuebunny/crossref_metadata_2025 huggingface dataset:
Collective coordinate treatment of mode coupling in a plasma
Authors: Lamborn B. N. A.
Published: March 2003

The coupled equations of motion describing electrostatic oscillations in a plasma are derived in a collective coordinate treatment. The stability of these equations is discussed using Liapunov’s direct method, periodic solutions found using Poincaré theory, and the asymptotic solution of the equations developed, following the method of Krylov, Bogoliubov, and Mitropolsky. Explicit results are presented for two and three interacting modes. For two interacting modes all solutions are periodic. For a three-mode system, the mode amplitudes approach equilibrium values in time under certain initial conditions on a time scale τ≈ (5Q2k0ωp)−1. Qk0 is the Fourier coefficient of the density fluctuation of the lowest mode and is proportional to δN/N.
Anharmonic oscillator description of plasma oscillations
Authors: Mahaffey R. A.
Published: March 2003

Three recently investigated plasma oscillations have been found to be described by a nonlinear differential equation of the form d2x/dt2+ω20x+bx2+ax3=0. This equation is similar to the Duffing equation, but has many additional features. These features include additional shifts in the frequency of oscillation, asymmetry of the amplitude of oscillation, effects on the phase plane singularities, and changes in the resonance response curve. A discussion of each of these features is presented and a comparison of these features with experiment is made.
Excitation of fast waves by slow waves near the lower-hybrid frequency
Authors: Berger R. L. , Chen Liu
Published: March 2003

Resonant and nonresonant decay of short wavelength lower-hybrid waves into long wavelength whistler waves and ion acoustic waves are considered. It is shown that the dominant coupling to the ion acoustic mode arises from the magnetic force producing a pressure along the magnetic field lines. This decay instability provides a mechanism for dissipating the wave energy that is observed to accumulate in the lowest frequency lower-hybrid waves.
Straight l=1 equilibrium and stability
Authors: Marder B. M.
Published: March 2003

The equilibrium and stability of a straight, two-dimensional, l=1 plasma configuration in which no net longitudinal current is allowed to flow on any flux surface has been examined numerically. It is conjectured that equilibria exist and are unique for parameter ranges far in excess of those of physical interest. The δW stability analysis is done in the slow z variation limit and predicts gross modes which are qualitatively and quantitatively similar to the sharp boundary theory. In addition, other modes with higher nodal structure are found but appear to be stable.
Pressure anisotropy effects on long-wavelength guiding center plasma stability
Authors: Schmidt Michael J. , Vahala George
Published: March 2003

The guiding center plasma model is used for a normal mode analysis of the bumpy ϑ pinch. Pressure anisotropies of any order in ε (measures the period of the bumpiness of the field lines, compared with the radius of the plasma) and δ (measures the amplitude of the bumpiness) are considered. It is shown that O (ε0, δ0) pressure anisotropies are needed to alter the ideal magnetohydrodynamic results. The magnetohydrodynamic and guiding center plasma growth rates are compared for various pressure anisotropies.
Response of a turbulent boundary layer to random fluctuations in the external stream
Authors: Charnay G. , Mathieu J. , Comte-Bellot G.
Published: March 2003

The growth of the turbulent boundary layer is investigated primarily by means of conditional averages in the outer region (i.e., averages inside and outside the bulges limiting the free edge). The main quantities analyzed are: mean velocities, mean velocity gradients, turbulence intensities, Reynolds stresses, kinetic energy balances, skewness and flatness factors of the velocity and Reynolds stress fluctuations, intermittency factor, and indentations of the free edge. Considerable attention is paid to the cumulative straining process of the turbulence by the mean velocity gradient.
A ramp model for turbulent temperature fluctuations
Authors: Antonia R. A. , Atkinson J. D.
Published: March 2003

Temperature fluctuations in a turbulent shear flow are modeled by a ramp-like signal on which relatively high frequency Gaussian fluctuations are superimposed. Both linear ramps with fixed height and length and exponential ramps of variable length are considered. Parameters which characterize the exponential ramps are obtained from experimental data in a heated jet. High order moments of temperature calculated with the use of the ramp model are in reasonable agreement with available experimental measurements.
Clumping instability of a falling horizontal lattice
Authors: Crowley Joseph M.
Published: March 2003

Particle falling through a viscous liquid tend to form clumps under the influence of viscous forces. An analysis of this effect for a falling horizontal lattice indicates that the clumps which form are much larger than expected on the basis of nearest neighbor approximations. This difference in behavior can be traced to the increasing influence of distant neighbors in two dimensions, and suggests that the clumping instability in three dimensions may not be described as a simple extension of the one-dimensional instability.
Distribution of pure vapor between two parallel plates under the influence of strong evaporation and condensation
Authors: Fischer Johann
Published: March 2003

The distribution of pure vapor between two plane parallel surfaces under the influence of strong evaporation and recondensation is treated by the nonlinear BGK kinetic equation. The boundary conditions allow for evaporation, condensation, and diffuse reflection. The corresponding system of integral equations is brought into a form that allows numerical treatment down to a Knudsen number of 0.1. For two special cases the flux rates as well as the density and temperature profiles are given. In the case of evaporation into an infinite half-space at the maximum attainable flow rate about 15% of the particles leaving the surface are backscattered.
Knudsen flow under an external potential field
Authors: Rhim Hannong , Hwang Sun-Tak
Published: March 2003

The linearized Boltzmann equation with the relaxation model is solved numerically for the gas flow through a parallel micro-slit under an external potential field. It was found that the mass-averaged velocity is slightly reduced in the bulk phase, but considerably increased near the solid wall by the presence of the external potential field. As a net result, the total flow is increased considerably compared with the case in which there is no potential field. This extra flow can be interpreted as the so-called surface diffusion.
Detonation reaction time in nitromethane
Authors: Mallory H. Dean
Published: March 2003

In recent years reaction zone measurements for nitromethane have been made and interpreted to show failure of the Chapman–Jouguet theory. Others later showed calculations which seemed to predict the experimental results obtained while retaining the theory unchanged. The reaction time in nitromethane has been measured by a new and independent method. The measured reaction time, 22±3 nsec, supports the above-mentioned calculated results and the Chapman–Jouguet theory as historically stated.
Thermodynamics of two-dimensional plasmas or discrete line vortex fluids
Authors: Seyler C. E.
Published: March 2003

The equilibrium properties of a two-dimensional plasma are examined theoretically within the framework of the random phase approximation. The guiding-center case is of most interest because of the formal equivalence to two-dimensional discrete line vortex fluids and because of the formal admissibility of negative temperatures. Using a judicious treatment of the periodic boundary conditions, extensive thermodynamics for nonnegative temperatures and a new value for the energy at which the temperature becomes negative are found. An explicit form for the structure function is derived and compared with the result of a Monte Carlo calculation. Good agreement is found for the negative temperature threshold prediction and for the general shape of the structure function. An equilibrium energy spectrum is calculated, which for positive temperatures, is the Debye–Hückel result; for negative temperatures it predicts an enhanced excitation of the lowest mode, corresponding to macroscopic charge separation.
Equatorial propagation of axisymmetric magnetohydrodynamic shocks. II
Authors: Rosenau Philip
Published: March 2003

Self-similar solutions are presented for the equatorial propagation of axisymmetric, piston-driven magnetohydrodynamic shocks into an inhomogeneous ideal gas (∼r−ω, 0⩽ω<3) permeated by an azimuthal magnetic field (∼r−m, 0<m<1.5). Several regimes of magnetically dominated flow near the piston are possible (depending on the ambient density and field distribution of the unshocked gas) along with a new quasi-hydrodynamic flow where the magnetic field is depressed near the piston. Here, the hydrodynamic pressure replaces the magnetic pressure as the means of transmitting forces to the gas adjacent to the piston. Since in each regime gradients of different quantities diverge, the results indicate that in each case different effects of dissipation and diffusion may thus be needed. The theory is applied to describe shock waves in a gravitational field due to a heavy nucleus at the origin.
Self-trapping instability of plasma cyclotron waves
Authors: Sethuraman Amarnath , Seshadri S. R.
Published: March 2003

The self-trapping instability of cyclotron waves propagating along the magnetostatic field in a cold, uniform plasma of infinite extent is analyzed.
Absolute two plasmon parametric decay of obliquely incident electromagnetic waves in an inhomogeneous plasma
Authors: Schuss J. J.
Published: March 2003

The parametric decay of an electromagnetic wave into two electron plasma waves in an inhomogeneous plasma is considered. The electromagnetic wave is obliquely incident on the density gradient. The resulting analysis shows that the threshold for absolute instability decreases as the angle of incidence of the electromagnetic wave relative to the density gradient aproaches 90°. The limitations of this analysis and its importance to laser fusion are discussed.
Generation of density cavities and localized electric fields in a nonuniform plasma
Authors: Morales G. J. , Lee Y. C.
Published: March 2003

A simple model is presented which is capable of describing the detailed space-time evolution of the interaction of long wavelength electromagnetic radiation with an unmagnetized plasma having a nonuniform density profile. The model consists of describing the electric field via the nonlinear Schrödinger equation and the density changes through the ion-acoustic wave equation with the ponderomotive force effects included self-consistently. In the linear regime, this formulation explains the time evolution of the mode-conversion process that leads to the excitation of a short wavelength Langmuir wave in the neighborhood of the resonance layer where the frequency of the external radiation matches the local value of the electron plasma frequency. In the nonlinear regime, the model predicts the generation of density cavities and the associated spatial localization of the electric field. These features are in good agreement with the experimental results of Kim, Wong, and Stenzel. In addition, the model predicts a variety of new interesting phenomena such as the excitation of ion-acoustic oscillations and nonlinear relaxation oscillations which are amenable to experimental observation.
Theory of foil-less diode generation of intense relativistic electron beams
Authors: Ott Edward , Antonsen Thomas M. , Lovelace R. V.
Published: March 2003

A study is made of the generation of intense hollow relativistic electron beams in a foil-less diode. A strong magnetic field is assumed so that the electron motion is one dimensional. Also, the electron motion is considered to be ultra-relativistic. The problem of the self-consistent space charge in the diode is reduced to a singular integral equation. This integral equation is solved and the nature of the solutions is discussed. The beam density increases at the beam edges and has a minimum in the beam interior. The diode impedance as a function of the beam thickness and geometry is discussed.
Relativisitic electron beam pinch formation processes in low impedance diodes
Authors: Blaugrund A. E. , Cooperstein G. , Goldstein Shyke A.
Published: March 2003

The process of pinch formation in large aspect ratio diodes has been studied by means of streak photography and time-resolved x-ray detectors. A tight pinch is formed at the anode center by a collapsing thin hollow electron beam emitted from a hollow cathode. The collapse velocity depends, amongst other things, on the type of material in the top 1 μm layer of the anode. In the suggested model it is assumed that the anode plasma is created from gases released from the surface layer of the anode by the heating action of the beam. These gases are ionized by avalanche breakdown aided by primary, backscattered, and secondary electrons. Ions emitted from this plasma modify the electron trajectories in the diode leading to a radial collapse of the hollow electron beam. The observed monotonic dependence of the collapse velocity on the atomic number of the anode material can be explained by the smooth dependence on Z of both the specific heat and the electron backscatter coefficient. In the case of high-Z anodes the ion expansion time is calculated and shown to be the factor limiting the collapse velocity. The use of thin foils to speed up the collapse rate is suggested. Detailed experimental data are presented.
Lower bounds for the onset time of instability in heated layers
Authors: Wankat P. C. , Homsy G. M.
Published: March 2003

The method of energy is used to treat the stability of a fluid layer subject to a linear increase in bottom temperature with time. The results yield bounds on the onset time which are conservative, but agree with experiment in the sense of lower bounds.
Blast waves with heat transfer
Authors: Ahlborn Boye , Ariga Seiichi
Published: March 2003

A model is given for blast waves which are dominated by heat transfer rather than by shock heating and adiabatic expansion. The wave motion is attributed to a net heat flow W (watts/cm2) from the expanding hot core to the surrounding cold gas (density ρ1). The surface of the hot core is initially a supersonic heat wave without a shock. It changes into a subsonic heat wave with a leading shock after quickly passing through the detonation mode. A quantitative analysis of the space time trace near the detonation point yields the enthalpy hf and other parameters of the hot core, the thermal response function hf=f (W) of the background medium, and the blast wave energy E0=const ρ1V2sr3H (where Vs is the shock front velocity and rH is the radius of the hot core). The analysis shows the steady transition of such a blast wave from the early stage of a ’’thermal’’ wave with negligible mass motion to the late stage (which approaches ’’classical’’ blast waves) with negligible heat transfer. Applications to laser spark plasmas are discussed.
Calculation of thermal ionization rate coefficients of alkali atoms in molecular gases
Authors: Shui Ven H.
Published: March 2003

The Wigner–Keck theory of dissociation and recombination is applied to the thermal ionization reaction A+N2→A++e−+N2, where A is an alkali atom. The calculated upper-bound ionization rate coefficient (ki) is in good agreement with flame measurements and indicates that high ionization rates are theoretically possible. The very large ionization cross sections (σi∼10−12 cm2) are due to (a) ionization from excited states lying within 2 kT of the ionization limit, and (b) inelastic energy transfers between electrons and the collision partner. In contradistinction to conventional collision theories, the present work shows that ki is independent of the mass of A.
Plasma impurity study using far-infrared emission analysis
Authors: Scaturro Louis S. , Marshall T. C.
Published: March 2003

Measurements of the effective Z of a plasma produced in a coaxial electromagnetic shock tube (ne≈2×1016 cm−3, Te ≈10 eV, Ti≈300 eV) were made in both deuterium and helium gas. A fast, Ge: In photodetector (4°K) was used to measure the total infrared emission spectrum (20–100 μm) and a laser interferometer was used to measure electron density. In helium, electron temperatures were measured by line emission intensity ratios; Thomson scattering was used to measure Te in deuterium. After extensive discharge cleaning (approximately 100 shots) the emission from the plasma was found to agree with the standard theory of bremsstrahlung [jω dω ∝ (ne2〈Z〉/T1/2e) dω] with 〈Z〉 =2.2±0.9 for deuterium and 〈Z〉=4.7±2.2 for helium. The infrared detector was calibrated absolutely using a CO2 laser at 10.6 μm and a relative response curve. Specific types of impurities present were identified by spectral line emission in the visible region and analysis using mass spectrometry.
Laser light absorption by short wavelength ion turbulence
Authors: Faehl R. J. , Kruer W. L.
Published: March 2003

The absorption of laser light due to short wavelength ion fluctuations (λ∼10 electron Debye lengths) is considered. The effective collision frequencies and heated electron distributions are computed as a function of plasma density and compared with theory. A simple upper bound to the level of ion turbulence is given, and several sources for the turbulence are discussed briefly. Finally, the absorption due to this mechanism in an inhomogeneous, expanding plasma is calculated by putting the estimate into a fluid code including wave propagation and density profile steepening by laser light pressure. The results are related to some recent experiments with short-pulse-length Nd laser light.
Kinetic description of the initial stages of shock formation in the piston problem
Authors: Sentman L. H. , Grimm A. D.
Published: March 2003

The first iterate solution of the Boltzmann equation is given for an impulsively started, specularly reflecting piston. An approximation for each collision operator that preserves the significant contribution from each of the fourteen collision classes which are generated by the single discontinuity in momentum space is obtained. Five of the sevenfold integrals appearing in the expressions for the first iterate moments are evaluated in closed form and the remaining two integrals are evaluated numerically. This iterative solution is valid for large departures from equilibrium, and, to the order to which the iteration has been carried, to times of the order of one or two mean free times between collisions.
Impurity transport in tokamaks in the plateau to Pfirsch–Schlüter regimes
Authors: Chu Ming Sheng , Rawls John M. , Wong Seung Kai , Hinton F. L.
Published: March 2003

Neoclassical transport coefficients for ion and impurity species are computed in a large aspect ratio toroidal magnetic confinement system for intermediate-to-high collision frequencies. A large range of values of the relevant density and charge parameters is explored and terms of order (mi/mI)1/2 are retained. The behavior of the transport coefficients is found to depend on the collisionality of both the ions and the impurities. A simple approximate analytic representation of these results which incorporates the correct asymptotic plateau and Pfirsch–Schlüter behavior is presented. In the plateau regime, when (nI/ni)(mI/mi)1/2≪1, the inward impurity flux is found to be suppressed for experimentally obtainable plasma parameters, implying that the time scale for impurity concentration could be too long to be observable.
Transition from classical to vortex diffusion in the Wisconsin levitated octupole
Authors: Navratil G. A. , Post R. S. , Ehrhardt A. Butcher
Published: March 2003

The transition between the anomalous diffusion previously reported in pulsed multipoles and classical diffusion has been observed at low poloidal magnetic fields, B≲100 G. The helium plasma parameters were ne =1010–1011 cm−3, Ti∼Te∼0.2 eV. The anomalous diffusion seen at higher magnetic fields scaled like vortex diffusion, but the transition point corresponds to a nonthermal vortex spectrum with effective temperature, T*∼106 eV. Transition from anomalous to classical regime is observed on a classical viscous damping time scale.
Effect of turbulence on theta pinch modeling by hybrid numerical models
Authors: Hamasaki S. , Krall N. A. , Wagner C. E. , Byrne R. N.
Published: March 2003

A newly developed numerical model is described which calculates plasma implosion on the Alfvén wave time scale, appropriate for modeling cylindrical pinch experiments. The simulation includes fluid electrons, particle ions, cylindrical geometry, and self-consistent plasma turbulence effects on the resistivity and the ion and electron heating. A series of trial runs is presented to document the effect of including or neglecting turbulent ion heating and cylindrical geometry in the model.
Single-mode saturation of the bump-on-tail instabilty: II. Mobile ions
Authors: Simon Albert
Published: March 2003

A previous calculation of the single-mode nonlinear saturation of the bump-on-tail instability assumed immobile ions. Here, the ions are also allowed to respond. The usual van Kampen continuum of real eigenmodes is shown to change to a double continuum. The analog of the Case class 1c mode is now singular and has an undetermined coefficient. The corresponding charge density is however nonsingular. The nonlinear calculation carries through as before, and the result is an obvious generalization of the one-component case. Total energy and momentum are conserved, but the partition of momentum and energy between ions and electrons is not determined by asymptotic considerations alone. The result also applies to the threshold region for the ion-sound instability.
Effect of helical instability on the internal magnetic field in a magnetized positive column
Authors: Imazu Shingo , Maruyama Takeo , Nagai Yoji , Maruyama Takesuke
Published: March 2003

The effect of the helical instability on the internal magnetic field in the positive column, is investigated theoretically under the assumptions that the densities of electrons and ions are perturbed independently and that the perturbed densities have finite amplitudes. The internal magnetic field is found as a function of the amplitude of the superimposed helix for the helical instability (m=1) and the phase shift δm between the perturbed densities of electrons and ions. The theoretical results are compared with experimental data and found to be in fair agreement with experimental values. It is concluded that the direction of the internal magnetic field produced by the helical instability is parallel to that of the applied magnetic field.
Observation of velocity-shear flute instability in the region of bad curvature
Authors: Sugai H. , Komori A. , Sawafuzi T. , Sato N. , Hatta Y.
Published: March 2003

A low-frequency flute instability in a collisionless plasma flowing along a curved magnetic field has been observed to localize in regions of bad curvature (∇B2∇≳0). The experiment was performed in a single-ended Q machine. A low density plasma was injected along the lines of force which were bent into the arc of a circle. As the flow velocity of the plasma was increased, the flute type oscillations were found to be stabilized in the region of good curvature and to be destabilized in the region of bad curvature. The experimental results are well explained by simplified calculations. For slow flow velocity the instability is mainly driven by a transverse Kelvin–Helmholtz effect due to the velocity shear of the E×B rotation, so that it is almost independent of the magnetic field curvature. For large velocities, however, the curvature effect stems from a Rayleigh–Taylor ’’gravitational’’ effect due to the centrifugal force, which makes the oscillation localize in the region of bad curvature.
Microscopic theory of impurity pumping
Authors: Chu Ming-Sheng , Rawls John M.
Published: March 2003

Neoclassical transport coefficients for ion and impurity species in the plateau to Pfirsch–Schlüter regime are calculated using sources of particles and heat as driving forces. In the Pfirsch–Schlüter regime, such sources can lead to a reversal of the usual inward flux of impurities.
Reduction of eddy diffusion for momentum and heat in viscoelastic fluid flow in a circular tube
Authors: Mizushina Tokuro , Usui Hiromoto
Published: March 2003

The velocity profile, longitudinal turbulent intensity profile, and bursting period in drag reducing pipe flow were measured using a laser Doppler technique. The temperature distribution and heat transfer coefficient were measured with thermocouples in the same system. The results of the decrease in eddy diffusivities of momentum and heat near the wall, the gross flow measurements, and the velocity distribution were shown to be in a good agreement with the predictions of the eddy diffusivity model for viscoelastic fluids previously proposed. A simple method based on the eddy diffusivity model to calculate the rate of heat transfer in the fully developed pipe flow of viscoelastic fluids is proposed, and is ascertained to be effective by the previous experiments.
Additive effects on free turbulent flows
Authors: Tulin Marshall P. , Wu Jin
Published: March 2003

A theory is presented which predicts the spreading in time of a cylindrical turbulent cloud, allowing an estimation of the interface entrainment coefficient. In light of the theory, previously published experiments are analyzed and the effect of polymer concentration on entrainment is deduced; the entrainment is substantially reduced by concentrations as low as 25 ppm. It is suggested that this reduction accompanies a thickening of the interfacial superlayer of the same magnitude as the sublayer thickening which accompanies substantial drag reduction.
Coherent motions in the outer region of turbulent boundary layers
Authors: Falco R. E.
Published: March 2003

The technique of simultaneous hot-wire anemometry and flow visualization has been used to determine the average characteristics of two important scales of motion in the outer region of turbulent boundary layers: large scale motions (average length 1.6δ), and ’’typical eddy’’ motions (average streamwise length approximately 200 ν/uτ). Results showed that the Reynolds number dependent ’’typical eddies’’ produced most of the Reynolds stress in the outer half of the layer at Rϑ≈1200, and that they are formed on the upstream side of large scale motions at all Reynolds numbers investigated. This phase relationship explains the scaling of the frequency of occurrence of outer layer bursts (which are identified with ’’typical eddies’’) on the free stream velocity and overall boundary layer thickness, although it is found that the lengths of the ’’typical eddies’’ scale on inner layer variables. In the log region, roughly one-half of the large scale motions sampled had zone-averaged streamwise velocity defects. Ensemble averaged results showed that they were associated with significant Reynolds stress contributions. A structural model showing the phase relationship of ’’typical eddies’’ and large scale motions is presented.
Effects of roughness on the intermittent maintenance of Reynolds shear stress in pipe flow
Authors: Sabot J. , Saleh I. , Comte-Bellot G.
Published: March 2003

The presence of a rough wall does not fundamentally alter the intermittent maintenance of Reynolds shear stress as found for turbulent flows over a smooth wall, be it boundary layers or pipe flows. Ejections, and to a lesser extent sweeps, are still the dominant events contributing to the shear stress. The mean period of occurrence of the violent ejections (those whose amplitude exceeds a characteristic threshold) is, however, larger for the rough pipe than it is for the smooth pipe at the same Reynolds number. A scaling based on specific parameters of the largest scale motions is proposed. The mean length scale of the violent ejections is larger than those encountered for a smooth pipe, but still seems to be of the order of the integral length scale of the transverse component of the velocity fluctuation.
Flow time scales and drag reduction
Authors: Berman Neil S.
Published: March 2003

The effects of the flow time scales on the onset and magnitude of drag reduction in turbulent pipe flow were studied using comparatively low molecular weight samples of water soluble polymers in glycerine-water mixtures and in two different pipe sizes. The results show that onset depends only on the time scale νuτ2 with no Reynolds number dependence. The magnitude of drag reduction after the molecules are extended was found to increase with pipe diameter. A correlation with the lifetime of large eddies or the mean period of turbulence production is suggested.
Rotation of the Reynolds’ stress tensor in a decaying grid-generated turbulent flow
Authors: Hinze J. O. , Builtjes P. J. H.
Published: March 2003

The fully-developed turbulent flow generated by a grid is axisymmetric, the intensity of the axial turbulence component being greater than that of the lateral component. Since the mean-velocity distribution is uniform and consequently, the gradient of the mean velocity is zero everywhere, a local stress-strain relationship is not possible. Measurements of the turbulence structure in a region up to 50 grid mesh-sizes downstream of the grid, have revealed that long-distance memory effects are responsible for a nonlocal relationship. In the local shear layers just downstream of the rods, the principal axes of the stress tensor make an angle of roughly 40 deg with the direction of the main flow. This angle decreases in the developing region, and becomes zero in the fully-developed region. The plane of maximum shear stress then makes an angle of 45 deg with the direction of the main flow. The transport equations for turbulence are transformed to nonhomogeneous linear relaxation equations, which clearly show the long-distance memory behavior of the maximum shear stress.
Mean velocity and turbulent energy closures for flows with drag reduction
Authors: Poreh M. , Hassid S.
Published: March 2003

Earlier models for flows with drag reduction are reviewed and compared with a new energy-dissipation closure which describes the observed effects of drag reduction on the mean velocity, turbulent energy, and turbulent length-scale distributions, and suggests a new maximum drag reduction law related to the onset of drag reduction.
Influence of molecular aggregates on drag reduction
Authors: Dunlop E. H. , Cox L.R.
Published: March 2003

Methods of detecting and characterizing molecular aggregates in solution are discussed and it is shown that several polymer/solvent systems of interest in drag reduction also form aggregates. Drag reduction results obtained with a spinning disk apparatus can most readily be explained by an aggregation hypothesis, as can a number of other results reported in the literature. The relevance of molecular aggregates to the development of drag reduction theory and investigations is considered.
On the role of phase information in conditional sampling
Authors: Blackwelder Ron
Published: March 2003

Conditional sampling techniques have been studied in order to better understand the phase relationship between the signal used to detect the events and the sampled data signal. It is shown that whenever there is a mean time delay between these two signals or whenever they are obtained from two different spatial locations, a random phase will exist between them if the signals are obtained from a random environment. This randomness is not accounted for in the usual definitions of conditional sampling, and it can cause a serious degradation of the conditionally averaged results. A corrective method is discussed and illustrated with examples from a turbulent boundary layer.
Turbulence structure in a water jet discharging in air
Authors: Hoyt J. W. , Taylor J. J.
Published: March 2003

Special purpose high-speed cameras have been developed which enable the detailed visualization of turbulent water jets discharging in air. The laminar-turbulent transition and initial turbulent formation on the surface of the jet is followed by amplified disturbances which result in spray detachment. A very large modification to this process is observed when polymers are added to the water.
Eigenmode description of the hot ion cyclotron beam-whistler mode instability in parallel shock waves
Authors: Golden K. I. , Chen Liu , Cipolla J. W. , Silevitch M. B.
Published: March 2003

The local linear dispersion relation for the hot ion cyclotron beam-whistler mode instability near the leading edge of parallel shock waves is generalized into a WKB eigenvalue problem. Its solution reveals the existence of stationary eigen growth modes capable of generating turbulence in the leading edge region.
Polymer additive mixing and turbulent drag reduction
Authors: Stenberg Lars-Göran , Lagerstedt Torgny , Lindgren E. Rune
Published: March 2003

The turbulent drag reducing abilities of polyethylenoxide polymer additive is shown to depend not only upon the rate of mixing, but upon premixing procedures as well, of parent polymer solution being injected into the water flow at the inlet end of a rectangular experiment pipe via a rotating impeller mixer. Pressure drop measurements over a downstream measuring section along the pipe, correlated with visual observations on the presence of polymer strands in the flow by means of a schlieren arrangement indicate that forced mixing, so that all visible polymer strands dissappear, does not impair the drag reducing abilities of the additive until quite high rates of mixing.
Temperature dissipation fluctuations in a turbulent boundary layer
Authors: Sreenivasan K. R. , Danh H. Q. , Antonia R. A.
Published: March 2003

Simultaneous measurements have been made of all three components of the fluctuating temperature dissipation in the inner region of a fully developed turbulent boundary layer at a moderate Reynolds number. Measurements are made with a four-wire arrangement which consists of two parallel vertical wires mounted a small distance upstream of two parallel horizontal wires. Each of the four wires is operated at very low current by a contant current anemometer and is sensitive to only the temperature fluctuation Θ. The separation between wires in each parallel pair is kept small, so that the differences between the outputs of each pair are reasonable approximations to ∂Θ/∂z and ∂Θ/Λy, the temperature derivatives in the transverse and vertical directions, respectively. The streamwise derivative ∂Θ/Λx was obtained from the time derivative, through use of Taylor’s hypothesis. Mean square and spectral density measurements show that in the inner region local isotropy is not closely approximated [(∂Θ/∂z)2≳ (∂Θ/∂y)2≳ (∂Θ/∂x)2] and (∂Θ/∂x)2 is richer in high frequency content than the other two components or the sum. The probability density of the sum χ[= (∂Θ/∂x)2+(∂Θ/∂y)2+(∂Θ/∂z)2] has a lower skewness and flatness factor and is more closely log-normal than the probability densities of the individual components. This is true regardless of whether χ and its components are unaveraged or locally averaged over a linear dimension r, in the Obukhov–Kolmogoroff sense. The variance σ2 of the logarithm of the locally averaged χ is proportional to log r over a wide range of r (rmax/rmin≃30), in contrast to the individual components where this ratio may be as small as 3. The value of the Kolmogoroff constant μ determined from the slope of σ2 vs log r is about 0.35. This is consistent with the slope of the spectral density of χ and is also in agreement with previous best estimates of μ obtained at high Reynolds numbers. Using only one component of χ, the evaluation of μ either from the slope of the spectral density or from the slope of σ2 vs log r seems to be highly ambiguous and can lead to erroneous results.
Influence of drag reducing polymers on the turbulent boundary layer of a body of revolution
Authors: Lang B.
Published: March 2003

In the application of polymers for the drag reduction of ships, the study of outer boundary layer flows is a pre-requisite. The static pressures and the velocity profiles in the boundary layer of a body of revolution at 24 static pressure holes and six Pitot tube rakes with five tubes each are measured by means of differential pressure transducers in connection with electronic data acquisition and processing. A comparison is made between water and homogeneous solutions of Polyox Coagulant in eight concentrations from 0.5 to 100 ppmw at Reynolds numbers from 106 to 8×106. Comparing the static pressure coefficients, a shift to higher values is observed especially behind the front pressure minimum with increasing polymer concentrations. The calculated specific pressure resistance coefficients CPV show smaller values with higher concentrations indicating the influence of drag reducing polymers on boundary layer development. From the velocity profiles measured with the Pitot tube rakes the boundary layer thicknesses are calculated. They become smaller with higher polymer concentrations up to a limiting concentration and then they grow sharply by some hundred percent. The limiting value increases with flow velocity. Some of these effects may depend on the measuring method, i.e., Pitot tube errors in drag reducing liquids, degradation, and the turbulence in the circulating water tank, although these influences were checked. Therefore, interpretation of the measured values may not be quite conclusive.
Model of drag reduction by compliant walls
Authors: Orszag Steven A. , Bushnell Dennis M. , Hefner Jerry N.
Published: March 2003

A numerical model of the effect of compliant walls on turbulent boundary layer flows, has been investigated. The model is based on Burton’s1 observation that turbulent bursts produce large pressure fluctuations that tend to produce low speed ’’streaks’’ near the wall. These streaks undergo space-time retardation and a new burst appears when the velocity profile becomes highly inflectional. The idea of the model is that the compliant wall motion interrupts this feedback loop of burst formation and that short wavelength wall motions can possibly delay burst formation long enough for the favorable gradient part of the pressure pulse caused by previous bursts to effect a decrease in the burst frequency. The numerical model involves solution of the two-dimensional time-dependent Navier–Stokes equations in a rectangular geometry. A typical calculation is performed using 257 grid points in the downstream direction with inflow-outflow boundary conditions applied. In the boundary layer direction 33 Chebyshev polynomials are used; linearized boundary conditions are applied at the compliant boundary and a match is made to a logarithmic velocity profile in the outer region. A large moving pressure pulse is applied in the outer region and it is assumed that the background turbulent Reynolds stress between bursts is a small fraction of the mean stress. This code is used to determine mean velocity profiles and has been checked thoroughly by comparison with the velocity profiles measured by Blackwelder. A stability analysis is then made of the resulting mean velocity profiles. The results of the stability analysis are not yet complete and will be reported in detail elsewhere.2 The results of our calculations to date indicate that only very small wavelength wall motions have a significant effect upon the stability of the turbulent boundary layer. This result suggests that novel structural dynamics will be an essential component of successful drag reduction by compliant walls. This work was supported by the National Aeronautics and Space Administration Langley Research Center under contract No. NAS1-14275.
Theories of compliant coating drag reductions
Authors: Blick Edward F.
Published: March 2003

Approximately two decades ago, Kramer1–3 reported skin friction drag reductions of up to 60% for compliant-walled bodies towed in water. Since Kramer’s initial experiments, many other investigators have delved into this intriguing area. The results have been mixed. Some have reported drag reductions in water and air with compliant coatings while others found no decrease in drag. Most of those who found decreases have reported that these decreases were found when the compliant wall was immersed in a turbulent boundary layer. Kramer originally stated that he felt the drag reduction was perhaps due to retarding the transition from laminar to turbulent flow. Benjamin,4 Boggs and Tokita,5 Landahl,6 Kaplan,7 and Gyorgyfalvy8 were among the early theoreticians who examined the possibilities of retarding transition by compliant walls. Generally, their calculations showed that with the proper compliant wall properties it was theoretically possible to retard transition. A few years later most of the theoretical work was aimed at trying to explain how a compliant coating might respond to a turbulent boundary layer in such a manner so as to reduce the turbulent wall skin friction. Different theories were proposed by Ffowcs-Williams,9 Blick,10 Amfilokhiyev,11 Semenov,12 Grosskreutz,13 Ash,14 and Zimmerman.15 Most of the theories are somewhat incomplete, some had errors in them, and none have demonstrated that they adequately explain all, or even a sizeable portion of the body of experimental data. However, at the moment Semenov’s theory seems to hold the most promise.
Anatomy of a turbulent spot
Authors: Cantwell Brian , Coles Donald , Dimotakis Paul
Published: March 2003

Measurements have been made in the plane of symmetry of a turbulent spot to study the issue of entrainment. The mean flow is assumed to be similar in coordinates (x/t, y/t), and laser Doppler velocimeter data at several stations are used to determine the effective origin in x and t and to establish particle trajectories with respect to the stationary interface. Entrainment occurs over most of the rear interface and also close to the wall at the front of the spot. Flow visualization has also been used to obtain additional information about entrainment and sublayer structure. Pictures of the underside of spots, wedges, and boundary layers were taken through a glass wall, using a very heavy concentration of aluminum flakes, so that only motions occurring for y+ less than about 30 were visible. Sublayer streaks have essentially the same streaky, knotted appearance in all three types of flow. Optical correlations confirm the value λ+∼100. An attempt has been made to estimate the strength of sublayer streaks as streamwise vortices, assuming that the streaks are a manifestation of local Taylor–Görtler instability. The results account for many of the phenomena actually observed in the sublayer.
Effects of elongational viscosity of polymer solution on Taylor–Görtler vortices
Authors: Tomita Y. , Jotaki T.
Published: March 2003

Based on an assumption that drag reduction occurs due to the interaction of an elongational type of secondary flow with an elongational viscosity of a polymer solution, the effect of a polymer additive on Taylor–Görtler vortices was calculated. The results indicate that the Görtler parameter for the neutral stability always increases by increasing elongational viscosity and that the critical wavenumber decreases.
Turbulence structure in drag reducing polymer solutions
Authors: Patterson Gary K. , Chosnek Jack , Zakin J. L.
Published: March 2003

Longitudinal turbulence intensities measured by split-film anemometry in degraded polymer solutions in pipe flow peaked much farther from the wall than longitudinal turbulence intensities in a Newtonian oil at the same Reynolds numbers. The peak values were at about the same level for the degraded solutions as for the oil, but were higher and nearer the wall for fresh polymer solutions. Radial turbulence intensities were lower for the polymer solutions at all locations under all conditions. Drag reduction for the polymer solutions ranged from 1 to 21% based on Newtonian fluid of equal viscosity. For the fresh solutions the Reynolds stresses dropped to zero or low values much farther from the wall than normal, indicating a region of almost complete turbulence shear strain recovery near the wall (positive uv excursions balanced negative uv excursions). Numerical computations with a turbulence model involving balance equations for turbulence energy and turbulence energy dissipation rate (the k-ε model) show that the location of the peak turbulence intensity (energy) is sensitive to the model for Reynolds stress production. It was possible to model the velocity and turbulence intensity profiles and predict the correct level of drag reduction using the k-ε model with an eddy viscosity as a function of shear relaxation time.
Nonlinear Schrödinger equation including growth and damping
Authors: Pereira N.R. , Stenflo Lennart
Published: March 2003

The nonlinear Schrödinger equation, with complex coefficients that describe growth and damping, is considered. An exact stationary soliton solution is found for arbitrary growth and damping strength.
Time evolution of thermal instabilities
Authors: Dobrott D. , Miller R. L. , Rawls John M.
Published: March 2003

The steady-state temperature profiles in a magnetically confined cylindrical plasma obtained by balancing Ohmic power input and thermal diffusion are shown to exhibit a bifurcation as a function of the applied electric field. A linear stability analysis predicts that the upper (high temperature) branch is unstable and that the lower branch is stable. These results agree with those of a transport code that uses the steady-state profiles as initial conditions. The transport calculation reveals that the upper solution decays to the lower solution with a growth rate that is in agreement with the linear stability computation.
Absorption of high frequency electromagnetic waves in a magnetoplasma
Authors: Ferrari L. A. , McQuade A. W. , LaHaye R. J.
Published: March 2003

Measurements of strong absorption of high frequency (ω≃ωc, ω/2π=2.45 GHz electromagnetic waves in a plasma (ωp2/ω2=2.7) are reported. The absorption is almost complete; the transmitted power is less than 0.1% and the reflected power is less than 5%. Coincident with the onset of strong absorption is the appearance of microwave energy in the plasma in two frequency bands, and an increase in the electron temperature (from 6 to 10 eV). One frequency band is close to the plasma frequency (ωp/2π≃4 GHz); the second is a low frequency band below 2 GHz. These modes are identified as being of the Gould–Trivelpiece type. Wave amplitude, combined with transmitted and reflected power measurements, suggest that 95% or more of the incident energy may be converted into the low frequency modes which are trapped in the plasma and thus can be dissipated via resistive absorption.
Kinetic theory of plasma in the limiter-scrape-off layer
Authors: Daybelge U. , Bein B.
Published: March 2003

An asymptotic solution is given for the ion-drift-kinetic equation with a full Fokker–Planck term for the limiter-scrape-off layer in a tokamak. In this layer, the plasma is assumed to consist of hot, collisionless ions, and cold, collisional electrons. From the solution of the boundary-layer problem, ion and electron particle and energy losses to the limiter are calculated. Limiter load profiles due to ions are explicitly given as functions of the poloidal angle.
Exact solutions of the Boltzmann equation
Authors: Krook Max , Wu Tai Tsun
Published: March 2003

The nonlinear Boltzmann equation for the relaxation to equilibrium of a homogeneous one-component gas, is considered for a class of collision models. The models are characterized by elastic cross sections inversely proportional to the relative speed, but with arbitrary dependence on center-of-mass scattering angle. The Boltzmann equation is solved exactly for a particular family of physically interesting initial distributions. The distribution functions are of the similarity form and consist of the product of a Maxwell function with ’’time-dependent temperature’’ and a linear function of v2.