bluuebunny/crossref_metadata_2025 huggingface dataset:
Preferential concentration of particles by turbulence
Authors: Squires Kyle D. , Eaton John K.
Published: July 2002

Direct numerical simulation of isotropic turbulence was used to investigate the effect of turbulence on the concentration fields of heavy particles. The hydrodynamic field was computed using 643 points and a statistically stationary flow was obtained by forcing the low-wave-number components of the velocity field. The particles used in the simulations were time advanced according to Stokes drag law and were also assumed to be much more dense than the fluid. Properties of the particle cloud were obtained by following the trajectories of 1 000 000 particles through the simulated flow fields. Three values of the ratio of the particle time constant to large-scale turbulence time scale were used in the simulations: 0.075, 0.15, and 0.52. The simulations show that the particles collect preferentially in regions of low vorticity and high strain rate. This preferential collection was most pronounced for the intermediate particle time constant (0.15) and it was also found that the instantaneous number density was as much as 25 times the mean value for these simulations. The fact that dense particles collect in regions of low vorticity and high strain in turn implies that turbulence may actually inhibit rather than enhance mixing of particles.
Subgrid-scale modeling of compressible turbulent flows
Authors: Yoshizawa Akira
Published: July 2002

A subgrid-scale (SGS) model for compressible turbulent flows is presented in the combination of the mass-weighted filtering procedure with the theoretical results from a two-scale DIA. This model consists of the filtered equations for mass, momentum, and internal energy as well as the equation for the intensity of SGS density fluctuation. In this model, the effects of fluid compressibility appear through the intensity of SGS density fluctuation and the divergence of the grid-scale velocity. The model with the SGS density fluctuation neglected leads to the simplest compressible version of the Smagorinsky model.
Axisymmetric jet falling from a vertical nozzle and bubble rising in a tube of circular cross section
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

The motion of a bubble rising at a constant velocity U in a vertical tube of circular cross section is considered. The problem is formulated by using the Stokes’ streamfunction as one of the independent variables. The resulting equations are solved by finite differences. It is found that there is a flow for each value of the Froude number F =U/√gD. Here D is the diameter of the tube and g is the acceleration of gravity. For F≳Fd∼0.49, there is a cusp at the apex of the bubble. For F<Fd the slope of the free surface profile is continuous at the apex. For F=Fd, the solution is a pointed bubble with a 130° angle at the apex. The results for F<Fd agree with the solutions previously computed by Levine and Yang. In addition, the problem of an axisymmetric jet falling from a vertical nozzle is considered. It is shown that there is a flow for each value of the Froude number.
A technique for direct measurement of time-dependent complex eigenfrequencies of waves in fluids
Authors: Aldridge K. D. , Stergiopoulos S.
Published: July 2002

A method is developed for obtaining best estimates of slowly varying time-dependent complex eigenfrequencies of waves in fluids. The procedure is to excite the wave motion of interest in a fluid by a mechanical forcing and to subsequently switch off this forcing when resonance has been reached so that the fluid is free to oscillate at its natural complex eigenfrequency. The pressure response of the freely decaying waves is modeled with frequencies and decay rates represented as power series expansions in time up to the linear term. Estimates are obtained using a least-squares method after linearization about initial estimates of the parameters. The performance of the method was compared with the theoretical bounds (i.e., Cramer–Rao lower bounds) and was verified by inversion of simulated records that included pseudorandom noise. Applications of the method using real data were made by recovering slowly varying time-dependent complex eigenfrequencies of inertial waves in a rotating fluid during spin-up of the fluid from rest, confirming the merits of this technique. Methods developed in this work will find application in the recovery of frequencies and decay rates for fluid oscillations of the Earth’s fluid core.
Soliton turbulence and strange attractor
Authors: Moon Hie Tae
Published: July 2002

This study finds that there exists a set of three basic evolution patterns including a dissipative soliton in the non-Hamiltonian media governed by the Ginzburg–Landau equation. A global analysis in the introduced subspace shows that the soliton is a spiral sink enclosed by a doubly connected homoclinic orbit. The soliton, prior to a turbulent state, breaks up into recurring pulses through a Hopf bifurcation. The strange attractor underlying the turbulence is found and presented with discussion. The Lyapunov number, found from a one-dimensional reduction of the attractor, is given by L≊0.34.
Polymer drag reduction of a zero-pressure-gradient boundary layer
Authors: Koskie John E. , Tiederman William G.
Published: July 2002

The wall shear stress of a zero-pressure-gradient boundary layer was reduced by slot injection of a 1000 ppm solution of Separan AP-273 at 5.1 times the flow rate in the linear sublayer. Velocity and concentration data were acquired at four streamwise locations that correspond to a Reθ range of 2478–3935 in the Newtonian boundary layer. The slope of the logarithmic region of the mean velocity profile in the drag-reduced flow increased over the Newtonian slope. The sum of the viscous and Reynolds shear stresses was less than the total shear stress for 6≲y+≲100.
On the nature of the instability in buoyancy-driven flows in inclined settlers
Authors: Zhang K. , Acrivos A. , Triantafyllou G. S.
Published: July 2002

The instability in a buoyancy-driven convective flow, which develops when solid particles in a solid–liquid suspension settle under gravity in vessels having inclined walls, is investigated on the basis of a small disturbance linear theory [A. Bers, in Handbook of Plasma Physics, edited by M. N. Rosenbluth and R. Z. Sagdeev (North-Holland, New York, 1983), p. 451], which properly takes into account the distinction between a convective and an absolute instability, as well as the physical significance of the growing spatial modes. It is shown for the case of dilute monodisperse suspensions that, in general, the flow is convectively unstable, in that it develops spatially growing waves at the interface between the suspension and the clarified fluid above it. On the other hand, due to the action of a newly discovered instability mode, the flow becomes absolutely unstable if the viscosity of the suspending fluid is reduced to a value close to that of water. Moreover, within the convectively unstable range, the growth rate of the most highly amplified two-dimensional spatial disturbances is found to increase monotonically with a decrease in the suspending-fluid viscosity, while the corresponding wavelengths are found to monotonically decrease. These results provide an excellent qualitative description of the experimentally observed interfacial instabilities in such flows.
Dyson equation analysis of inertial-range turbulence
Authors: Woodruff S. L.
Published: July 2002

The Dyson equations of Martin et al. [Phys. Rev. A 8, 423 (1973)] are truncated after the first non-Gaussian correction, yielding Kraichnan’s direct interaction approximation [J. Fluid Mech. 5, 497 (1959)], and the self-similarity of the inertial range is used to reduce the number of independent variables. Yakhot and Orszag’s result [J. Sci. Comput. 1, 3 (1986)] is recovered by expanding these equations in the parameter ε, which measures the departure of the energy-spectrum exponent from one. In making this expansion, only certain nonlocal, eddy-viscosity-like interactions are retained at first order; all other interactions are relegated to higher orders. As a result, the expansion fails to notice that the direct interaction approximation gives the incorrect energy-spectrum exponent.
Penetrative convection: Parametrized expression for the growth rates
Authors: Savolainen V. , Canuto V. M. , Schilling O.
Published: July 2002

The equations determining the linear growth rate ω characterizing a convectively unstable fluid with Rayleigh number Ru bounded below by an impenetrable free boundary and above by a convectively stable fluid with Rayleigh number Rs, are solved numerically. Using the analytical Rayleigh–Bénard growth rate ωRB as a convenient functional form, it is possible to fit the numerical values for ω if the vertical wave number kz = nπ and the Rayleigh number RRB are taken to be functions of Rs, Ru, and the horizontal wave number k⊥, rather than n=integer as in the Rayleigh–Bénard case. In addition, contrary to Rayleigh–Bénard convection, in which the critical Rayleigh number is fixed (RcrRB = 657.5), it is found that Rcru is variable in the presence of a stable layer, i.e., it depends on Rs. Specifically, for Rs < 6300, Rcru < RcrRB so that the stable fluid hastens the onset of convection; on the other hand, when Rs ≳ 6300, Rcru ≳ RcrRB so that the system becomes more stable.
Massive vortex motion in the presence of solid boundaries
Authors: Tucker Bryan , Conlisk A. T.
Published: July 2002

In the present paper, the motion of finite-core vortices in the presence of both infinite and semi-infinite bodies is considered. The vortices are massive in the sense that the cores are assumed to be inviscid with constant vorticity after Batchelor [J. Fluid Mech. 1, 177 (1956)]; this requires that the core radius is much greater than the inverse square root of the appropriate Reynolds number. Four geometries are considered: the plane wall, a corner, a thin semi-infinite plate, and a semi-infinite wedge. The flow field is assumed to be inviscid, two-dimensional, and irrotational outside the core of the vortex and the shapes of the vortices are computed as solutions to an initial value problem. Results for the plane wall and the corner flow suggest that the vortices do not rebound due to the effect of the finite core; moreover, the motion of the centroid of the vortex is virtually identical to the motion of the associated point vortex of the same strength. The computed shapes of the vortices are, in general, not elliptical. For the semi-infinite plate and wedge, violent collisions with the leading edge may occur depending on the initial position of the vortex. The results for the wedge are compared with the flow visualization studies of Ziada and Rockwell [J. Fluid Mech. 118, 79 (1982)].
The linear stability of a spherical drop migrating in a vertical temperature gradient
Authors: Ascoli E. P. , Lagnado R. R.
Published: July 2002

The linear stability of a spherical drop migrating due to thermocapillarity and buoyancy in an unbounded fluid is investigated. The scope of the analysis is confined to axisymmetric perturbations of drop shape and conditions of negligible Reynolds number and Peclet number for which the basic flow is given by the solution of Young et al. [J. Fluid Mech. 6, 350 (1959)]. The spherical drop is found to be unstable when the capillary number exceeds a critical value that depends on the dynamic Bond number, the viscosity ratio, and the thermal conductivity ratio.
The effect of bottom friction on the cyclone–anticyclone asymmetry in rotating shallow-water flows with small deformation radius
Authors: Bazdenkov S. V.
Published: July 2002

The cyclone–anticyclone asymmetry, i.e., the predominant generation of anticyclones in rotating shallow water, is considered from the viewpoint of flow relaxation under the action of bottom friction toward vortices with minimal energy and fixed enstrophy (‘‘selective decay’’ process). Three invariants of the set of equations for rotating shallow water are taken into account: total energy, enstrophy, and ‘‘mass.’’ A nonlinear second-order differential equation is obtained that describes the relaxed flows in the limit of small deformation radius. It is shown that anticyclonelike solution corresponds to a minimal energy value, in comparison with the cyclonelike solution for the same generalized enstrophy and ‘‘mass.’’
Rising bubble in a two-dimensional tube: Asymptotic behavior for small values of the surface tension
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

The motion of a two-dimensional bubble rising at a constant velocity in a tube is considered. Numerical results [Vanden-Broeck, Phys. Fluids 27, 2604 (1984)] have shown that the physically relevant solution can be selected by introducing an arbitrary small amount of surface tension. It is shown that the selection mechanism is associated with exponentially small terms in the surface tension.
Real gas effects on the normal shock behavior near curved walls
Authors: Schnerr G. H. , Leidner P.
Published: July 2002

The influence of dense gases on the curvature and the strength of a normal shock near curved walls is discussed. In classical gasdynamics there is a postshock expansion and the shock is curved upstream if the wall is convex. The transonic small disturbance theory for the problem has been derived, including the fundamental gasdynamic derivative Γ and the second nonlinearity parameter Λ. Special attention is drawn on flows where Γ becomes either zero or negative. According to the similarity laws, change of the sign of Γ changes the sign of the velocity disturbances φx and φy and of the wall curvature. Thus an expansion shock is followed by a compression if Γ<0 and the wall is concave. Again the shock is curved upstream. A variation of the second nonlinearity parameter Λ influences the strength of the shock, its curvature, and of the postshock expansion. Considering a Γ≳0 flow at a convex wall, the postshock expansion and the shock curvature are weakened if Λ≳0. Comparing the real gas flow with the perfect gas flow (e.g., Γ=1.2 and Λ=0), both the expansion behind the shock and the shock curvature increase if Λ becomes negative.
Quasiequilibrium meniscus formation with hysteresis effects
Authors: Shyy W. , Udaykumar H. S. , Liang S.-J.
Published: July 2002

For many materials processing techniques, the meniscus of liquid bridging the crystal to the melt is critical in determining the properties of the solidified crystal. It is standard practice for existing theoretical models to use equilibrium meniscus shapes with specified contact angle to represent the behavior of the meniscus. It is shown here that with such boundary conditions, multiple solutions exist to the axisymmetric form of the Laplace–Young equation. Furthermore, these possible meniscus profiles may, depending on the interaction of Bond number, pressurization, aspect ratio and contact angle, correspond to minima, maxima or nonextrema points, as far as energy is concerned. The implications of this observation on meniscus stability are explored. The effect of direction of pulling in relation to gravity is also investigated. It appears that for tall menisci, commonly adopted equilibrium shapes may be unstable and the consequent dynamic behavior must be considered. Quasiequilibrium dynamics of the meniscus is simulated using a simplified hysteresis model for the contact angle at the top of the meniscus. A variety of behavior is found to arise, which is not fully captured by relations governing meniscus behavior used hitherto in many theoretical simulations.
Erosion of the basin of stability of a floating body as caused by dam breaking
Authors: Infeld E. , Lenkowska T. , Thompson J. M. T.
Published: July 2002

The dam breaking problem in terms of changing water levels behind and ahead of the dam, has been solved in the shallow water model. Here the effect of dam breaking on a symmetric downstream floating body is considered. A forced Duffing equation model, proposed by one of the authors, is used. The analysis has practical implications in terms of how far a marina should be situated from a large dam. Comparison with basin erosion, as caused by a water surface soliton, is instructive.
Radiative properties of a plasma moving across a magnetic field. I: Theoretical analysis
Authors: Roussel-Dupré Robert , Miller Ronald H.
Published: July 2002

The early-time evolution of plasmas moving across a background magnetic field is addressed with a two-dimensional model in which a plasma cloud is assumed to have formed instantaneously with a velocity across a uniform background magnetic field and with a Gaussian density profile in the two dimensions perpendicular to the direction of motion. This model treats both the dynamics associated with the formation of a polarization field and the generation and propagation of electromagnetic waves. In general, the results indicate that, to zeroth order, the plasma cloud behaves like a large dipole antenna oriented in the direction of the polarization field which oscillates at frequencies defined by the normal mode of the system. The magnitude of the radiation field and the amount of plasma momentum and energy carried away by and stored instantaneously in the fields are discussed only qualitatively in this paper, quantitative results for specific cloud parameters and scaling laws for the magnitude of the fields and the slowing down of the plasma cloud are presented in a companion manuscript.
Radiative properties of a plasma moving across a magnetic field. II: Numerical results
Authors: Roussel-Dupré Robert , Miller Ronald H.
Published: July 2002

A theoretical analysis developed in a companion paper to treat the early-time evolution of plasmas moving across a background magnetic field is applied to the modeling of low-beta, barium chemical releases in the magnetosphere. The results indicate that radiation damping plays an important role in defining the plasma cloud evolution, causing a rapid decay of the polarization field and a loss of plasma kinetic energy and momentum on time scales comparable to several ion gyroperiods. The radiation spectrum consists of a burst of chirped, high-frequency (in the range of the cloud plasma frequencies) waves, followed by a pulse of whistler waves, and subsequently by ion cyclotron emission. Scaling laws are derived for the plasma momentum and energy loss rates and predictions for the braking time, the amplitude and spectrum of the radiation field, and the total radiated power are presented for conditions relevant to the recent Combined Release and Radiation Effects Satellite (CRRES) experiments [Phys. Fluids B 4, 2249 (1992)].
Confinement and acceleration of pellet material by one-sided laser irradiation
Authors: Jarboe Thomas R.
Published: July 2002

A simple model is developed to predict the shape of the laser–pellet interface for pellet material being accelerated by one-sided laser irradiation. The ablation pressure is assumed to be much greater than the yield strength of the pellet material which is treated as a gas. The model predicts a shape of the critical surface contour that is consistent with that inferred from experimental data. The key new concept used is that, while the laser light is on, the freely flowing pellet material is not only accelerated but also completely confined by the laser ablation. In addition, it is shown that a hollow laser intensity profile, with the scale length for intensity change about equal to the pellet size, not only keeps the pellet material centered on the beam but also stabilizes the Rayleigh–Taylor instability over most of the laser–material interface. Thus concerns about overstressing or shock heating initially very soft pellet material, such as solid hydrogen, with very high acceleration pressure are alleviated. A possible method for accelerating millimeter-sized hydrogen pellets to very high velocity (∼200 km/sec) by laser irradiation on one side is described.
Radio-frequency excitation of harmonic microwave radiation from a Penning reflex discharge
Authors: Tate J. P. , Wharton C. B.
Published: July 2002

Experimental results on multiple-harmonic emission at 8.8 GHz from a Penning reflex discharge (PRD) are reported. Observations of the frequency spectra of microwave emission showed copius harmonic generation of frequencies having two completely different origins: (1) spontaneously excited high harmonics of the electron cyclotron frequency and (2) high harmonics of the frequency of an injected signal independent of the magnetic field strength, a phenomenon reported here for the first time. For spontaneous harmonic emission there was a current threshold, whose magnitude depended on gas pressure and magnetic field strength. When a signal was injected, however, high harmonics (up to the 18th) could be seen at discharge currents well below this threshold value. Comparisons between the two types of radiation are made and discussion of possible mechanisms is provided. It is concluded that the coupling efficiency of the radio-frequency (rf)-excited emission is dependent on the relationship between the rf drive frequency and the electron cyclotron frequency. Finite Larmor radius effects may also influence this coupling. The plasma sheath size will also be a factor in the transfer of energy from the probe to the bulk plasma. Results which seek to elucidate these effects are presented.
Radiative preheat in laser produced aluminum plasma
Authors: Salzmann David , Szichman Henry , Krumbein Aaron D. , Capjack Clarence E.
Published: July 2002

The preheat of the cold substrate by the x rays generated in the coronal region of a laser produced aluminum plasma was calculated for a trapezoidal laser pulse in the intensity range I=1013–8×1014 W/cm2. The computer code incorporates nonlocal thermodynamic equilibrium (non-LTE) radiation emission rates in the hot region and photoionization as the main photoabsorbing process in the cold compressed region. The temperature and the density variations of the photoionization cross section were accounted for. Black-body radiation diffusion was also included. The results indicate the formation of a well-defined radiation heated zone in the back portion (close to the ablation surface) of the shock wave. Comparisons were made on the effects of the x-ray radiation and the black-body diffusion on the shock wave propagation. The similarities and differences between x-ray and fast electron preheat are also discussed. In a different set of computations an attempt was made to simulate the measurements of McLean et al. [Phys. Rev. Lett. 45, 1246 (1980)] of the backsurface temperature of laser irradiated aluminum foils. The computational results agree favorably with those of the experiments.
Mechanical injection of magnetic helicity
Authors: Barnes D. C.
Published: July 2002

A novel technique for the injection of magnetic helicity into a closed volume is described. In this new approach, mechanical energy is converted directly into the production of magnetic helicity. Several geometries illustrate the flexibility of the approach. The application of mechanical injection of magnetic helicity to a magnetically insulated impact fusion system is briefly described. In this application, a β≫1 spheromak plasma is produced by impact of electrically conducting shells. Magnetic helicity is injected by the relative motion of these shells and subsequent plasma relaxation produces the spheromak magnetic field.
A model of hose instabilities in rotating electron beams
Authors: Brandenburg J. E. , Lee E. P.
Published: July 2002

A model for the hose instabilities of rotating electron beams is developed using the dynamics of single particles as a guide. Three unstable modes are found, all of which are helical in character. This model, which uses the spread mass formalism to incorporate phase mixing, is shown to give the correct high and low frequency limits obtained from a Vlasov treatment. More generally, the predicted growth rates agree well with simulation results.
Oscillating field current drive in spheromaks
Authors: Boozer Allen H.
Published: July 2002

If a spheromak plasma with a flux hole relaxes sufficiently rapidly to the flat current profile of a Taylor state, then the plasma current can be maintained, or increased, by varying certain geometric ratios of the plasma. The theory of this type of current drive is developed in terms of the internal and external inductances of a spheromak.
Detonation structures behind oblique shocks
Authors: Li Chiping , Kailasanath K. , Oran Elaine S.
Published: July 2002

Detonation structures generated by wedge-induced, oblique shocks in hydrogen–oxygen–nitrogen mixtures were investigated by time-dependent numerical simulations. The simulations show a multidimensional detonation structure consisting of the following elements: (1) a nonreactive, oblique shock, (2) an induction zone, (3) a set of deflagration waves, and (4) a ‘‘reactive shock,’’ in which the shock front is closely coupled with the energy release. In a wide range of flow and mixture conditions, this structure is stable and very resilient to disturbances in the flow. The entire detonation structure is steady on the wedge when the flow behind the structure is completely supersonic. If a part of the flow behind the structure is subsonic, the entire structure may become detached from the wedge and move upstream continuously.
Comparative measurements in the canonical boundary layer at Reδ2≤6×104 on the wall of the German–Dutch windtunnel
Authors: Fernholz H. H. , Krause E. , Nockemann M. , Schober M.
Published: July 2002

Mean velocity and Reynolds-stress profiles were measured in the incompressible turbulent boundary layer with zero pressure gradient on the aerodynamically smooth sidewall of the German–Dutch windtunnel. Data were taken at Reynolds numbers Reδ2, based on momentum thickness δ2 of 2×104, 4×104, and 6×104 by means of four different types of hot-wire probes (three-wire probes, X wire, and normal-wire probes). There are also measurements of skin friction and of spectra. The data compare well with the few available other measurements, and all profiles show independence of Reynolds number in the outer region of the boundary layer when plotted against y/Δ, where Δ is the Rotta–Clauser length.
Particle velocity fluctuations and hydrodynamic self-diffusion of sedimenting non-Brownian spheres
Authors: Nicolai H. , Herzhaft B. , Hinch E. J. , Oger L. , Guazzelli E.
Published: July 2002

The motion of non-Brownian spheres settling in the midst of a suspension of like spheres has been experimentally studied under creeping flow conditions. A few glass spheres, marked with a thin coating of silver, were tracked in a suspension of unmarked glass spheres, made optically transparent by matching the index of refraction of the suspending fluid to that of the glass spheres. Particles were tracked with a real time digital imaging processing system. Particle trajectories were examined in the bulk region of the suspension for particle volume fractions ranging from 0% to 40% in 5% steps. Statistical analyses of local particle velocities yield the mean settling velocity, the RMS of the fluctuations of the vertical and horizontal particle velocity and the particle velocity autocorrelation functions. The long time fluctuating particle motion is demonstrated to be diffusive in nature. Vertical and horizontal correlation times and self-diffusivities are found as a function of particle volume fraction, and a strongly anisotropic diffusion noted.
Control of laminar vortex shedding behind a circular cylinder using splitter plates
Authors: Kwon Kiyoung , Choi Haecheon
Published: July 2002

Laminar vortex shedding behind a circular cylinder and its control using splitter plates attached to the cylinder are simulated. The vortex shedding behind a circular cylinder completely disappears when the length of the splitter plate is larger than a critical length, and this critical length is found to be proportional to the Reynolds number. The Strouhal number of the vortex shedding is rapidly decreasing with the increased plate length until the plate length (l) is nearly the same as the cylinder diameter (d). On the other hand, at 1<l/d<2, the control shows two different behaviors for the Reynolds numbers investigated. The net drag is significantly reduced by the splitter plate, and there exists an optimum length of the plate for minimum drag at a given Reynolds number. From an examination of the instantaneous flow fields, it is found that the Strouhal number modification by the splitter plate is closely related to the size of the primary vortex behind the cylinder and the length of the plate.
Kinetic study of the effects of boundary geometry on rarefied vapor flow
Authors: Waichman Karol
Published: July 2002

The effect of boundary geometry on the properties of a rarefied vapor flow emerging from a narrow and infinitely long source was studied. The boundary consisted of walls, along both sides of the source which were parallel to the bulk velocity direction and of a collimator which plane was perpendicular to this direction. Two-dimensional numerical solutions for the flow field were obtained based on the Bhatnagar–Gross–Krook model. It was found that the rarefied flow is affected mainly by the size of the collimator aperture and by the walls separation. The streamwise length of the walls and the location of the collimator have a minor influence on the far field density.
Three-dimensional disturbances; considering starting profiles and optimal profiles in Couette and Poiseuille flow
Authors: Diedrichs Ben
Published: July 2002

The influence of the velocity profile on transient (algebraic) growth of three-dimensional disturbances in channel flow is investigated. Only streamwise-independent perturbations are considered and the effect of a single forcing Orr–Sommerfeld mode is studied. This restriction is motivated by previous investigations, which show that the largest possible (global optimal) energy growth of a disturbance is generally obtained for streamwise-independent perturbations. Analytic solutions for the energy growth on starting profiles for plane Couette and plane Poiseuille flow are deduced and the findings reveal that the most favorable profiles for transient growth are indeed the fully developed ones. An isoperimetric method to compute optimal profiles is then presented with the purpose to increase the energy growth compared to the fully developed profiles. These profiles are dependent on the spanwise wave number, as well as the perturbation growth time. A numerical investigation show that significant energy magnification can take place on such a profile, even at short growth times of the perturbation. It is further established that these profiles are stable only for low Reynolds number.
Helicity fluctuations and turbulent energy production in rotating and non-rotating pipes
Authors: Orlandi P.
Published: July 2002

In this paper finite-difference second-order accurate direct simulations have been used to investigate how the helicity density fluctuations change when a turbulent pipe rotates about its axis. In this case the rotation axis is parallel to the near wall vortical structures which play a fundamental role on the wall friction and turbulence production. The helicity density is the trace of the tensor γij′=〈vi′ωj′〉 whose elements form the components of v′×ω′. When the momentum equations are written in rotational form the turbulence energy production term splits into two parts, one related to the convection of the large scales and the other related to the energy cascade to the small scales. From data of direct simulations the changes of the turbulent production term profile with the rotation have been explained by the pdf of the v′×ω′ components. The links between the changes on the pdf of the v′×ω′ and the modifications of the vortical structures have been also investigated. The joint pdf of the dissipation with the helicity density has shown that the dissipation is highly correlated with regions of very low helicity density in the non-rotating pipe. When the pipe rotates the helicity density increases and the dissipation decreases. Since a drag reduction is one of the results of the background rotation, in this paper, it has been speculated that the alignment between velocity and vorticity could be a common feature in drag reducing flows.
Drop formation in viscous flows at a vertical capillary tube
Authors: Zhang D. F. , Stone H. A.
Published: July 2002

Drop formation at the tip of a vertical, circular capillary tube immersed in a second immiscible fluid is studied numerically for low-Reynolds-number flows using the boundary integral method. The evolution and breakup of the drop fluid is considered to assess the influences of the viscosity ratio λ, the Bond number ℬ, and the capillary number 𝒞 for 10−2⩽λ⩽10, 10−2⩽C⩽1, and 0.1⩽B⩽5. For very small λ, breakup occurs at shorter times, there is no detectable thread between the detaching drop and the remaining pendant fluid column, and thus no large satellite drops are formed. The distance to detachment increases monotonically with λ and changes substantially for λ>1, but the volume of the primary drop varies only slightly with λ. An additional application of the numerical investigation is to consider the effect of imposing a uniform flow in the ambient fluid [e.g., Oguz and Prosperetti, J. Fluid Mech. 257, 111 (1993)], which is shown to lead to a smaller primary drop volume and a longer detachment length, as has been previously demonstrated primarily for high-Reynolds-number flows.
Experiments on two-dimensional vortex patterns
Authors: Durkin D. , Fajans J.
Published: July 2002

The evolution of a strongly magnetized electron system is identical to that of an ideal two-dimensional (2-D) fluid; an electron column is equivalent to a fluid vortex. We have studied the stability of 2-D vortex patterns with electron columns confined in a Malmberg–Penning trap. The following cases are presented: the stability of N vortices arranged in a ring; the stability of N vortices arranged in a ring with a central vortex; the stability of more complicated vortex patterns.
Ion-acoustic solitons in electron–positron–ion plasmas
Authors: Popel S. I. , Vladimirov S. V. , Shukla P. K.
Published: July 2002

The ion-acoustic solitons are investigated in three-component plasmas, whose constituents are electrons, positrons, and singly charged ions. It is found that the presence of the positron component in such a multispecies plasma can result in reduction of the ion-acoustic soliton amplitudes.
Current and vorticity dynamics in three-dimensional magnetohydrodynamic turbulence
Authors: Politano H. , Pouquet A. , Sulem P. L.
Published: July 2002

Spectral numerical simulations of homogeneous incompressible magnetohydrodynamic turbulence at Reynolds mumbers up to about 500, are performed using a uniform grid of 1803 collocation points. Strong vorticity and current sheets obtain both in the presence and in the absence of magnetic nulls. Contrary to vortex sheets in hydrodynamics, these structures do not destabilize into filaments, but are locally disrupted. They are the main loci of kinetic and magnetic dissipations.
Nonlinear modification of laser–plasma interaction processes under crossed laser beams
Authors: Labaune C. , Baldis H. A. , Cohen B. , Rozmus W. , Depierreux S. , Schifano E. , Bauer B. S. , Michard A.
Published: July 2002

Effects of multiple crossed laser beams on stimulated Brillouin and Raman scattering (SBS and SRS) growth rates, as well as on the SBS and SRS competition, have been investigated in a preformed plasma using Thomson scattering of a short-wavelength probe beam. Reduction of the amplitude of ion acoustic waves (IAW) associated with SBS and amplification of the amplitude of electron plasma waves associated with SRS in the case of irradiation by two or three beams compared to a single beam are reported. Mechanisms by which crossed laser beams can reduce SBS by detuning the interaction or by nonlinearly enhancing the damping rate of the IAWs will be discussed. The same mechanisms could explain the enhancement of SRS backscatter through modified secondary decay processes of SRS. In a second experiment, nonlinear enhancement of extreme forward scattering of one laser beam going through a preformed plasma has been observed when a second interaction beam was present. The role of ion wave instabilities on forward scattering will be discussed, based on the spectral analysis of the forward scattered light.
Enhanced ‘‘tail’’ diffusion of phosphorus and boron in silicon: Self-interstitial phenomena
Authors: Morehead F. F. , Lever R. F.
Published: July 2002

It is well known that high surface concentration phosphorus diffusion leads to deeply penetrating ‘‘tails’’ in its concentration profile. At 700 °C the tail diffusivity exceeds that of low concentration phosphorus by a factor of 1000. Less spectacular, but very significant tailing also affects boron, making the conventional models contained in commonly available process simulation programs quite inaccurate for high concentrations of boron. We show that the observed tailing can be accounted for by a model whose central assumption is the local equality of dopant and oppositely directed defect fluxes.
Influence of oxygen implantation conditions on the properties of a high-temperature-annealed silicon-on-insulator material
Authors: Golanski A. , Perio A. , Grob J. J. , Stuck R. , Maillet S. , Clavelier E.
Published: July 2002

A silicon-on-insulator (SOI) structure was formed by implanting 150 keV O+ ions into a single-crystal, n-type Si. The substrate temperature during implantation, Ti, was regulated within the range of 450–750 °C. Implanted samples were subsequently annealed at 1300 °C for 2–5 h and analyzed using transmission electron microscopy, Rutherford backscattering, the spreading resistance probe, and secondary ion mass spectroscopy. It is shown that the substrate temperature during oxygen implantation strongly influences the properties of the SOI structure after the high-temperature annealing: the residual oxygen concentration within the Si surface layer strongly depends on Ti and correlates with the carrier density, indicating the presence of oxygen-related thermal donors.
Depth profiling of As at the SiO2/Si interface using secondary ion mass spectrometry
Authors: Morgan Alan E. , Maillot Philippe
Published: July 2002

Arsenic profiles across the SiO2/Si interface have been measured with secondary ion mass spectrometry under different O+2 bombardment conditions using As implanted into SiO2 and Si for calibration purposes. Radiation-enhanced segregation causes interfacial pile-up in profiles obtained under oblique incidence bombardment both with and without O2 backfill. Thus, implantation profiles through SiO2 are best measured using near normal incidence bombardment. However, if As is actually piled up at the interface, oblique incidence bombardment in vacuo is preferable since oxygen saturation conditions broaden the interfacial peak.
Optical studies of excitons in Ga0.47In0.53As/InP multiple quantum wells
Authors: Westland D. J. , Fox A. M. , Maciel A. C. , Ryan J. F. , Scott M. D. , Davies J. I. , Riffat J. R.
Published: July 2002

We report optical absorption and photoluminescence measurements of excitons in Ga0.47In0.53As/InP multiple quantum wells grown by metalorganic chemical vapor deposition. At 4 K the luminescence linewidth for n=1 heavy-hole excitons is measured to be 7 meV for a 30-period structure with wells of width 154 Å. The absorption spectrum at low temperature shows four peaks which we assign to confined heavy-hole excitons. A theoretical calculation of the energies of these states indicates that the ratio of the conduction-band to valence-band energy discontinuities is 45:55.
Photoluminescence and reflection high-energy electron diffraction dynamics study of the interfaces in molecular beam epitaxially grown GaAs/Al0.33Ga0.67As(100) single quantum wells
Authors: Kim J. Y. , Chen P. , Voillot F. , Madhukar A.
Published: July 2002

Comparative photoluminescence studies of GaAs/Al0.33Ga0.67As(100) single quantum wells grown via molecular beam epitaxy at growth conditions identified by the reflection high-energy electron diffraction intensity static and dynamic behavior are shown, for the first time, to establish a connection between the RHEED determined growth conditions and the structural and chemical quality of the interfaces. This connection is explained on the basis of the expected consequences of the nature of the surface kinetic processes controlling formation of the normal and inverted interfaces.
Crosshatch patterns in GaAs films on Si substrates due to thermal strain in annealing processes
Authors: Nishioka Takashi , Itoh Yoshio , Yamamoto Akio , Yamaguchi Masafumi
Published: July 2002

Crosshatch patterns were observed for the first time on the annealed GaAs film surfaces on Si substrates. The alignment of etch pits and the reduced electron beam induced current (EBIC) lines were also observed. They indicate that dislocations move and arrange themselves onto specified slip planes of {111} directions due to thermal strains in the annealing process. Flatter morphologies and more reduced EBIC intensity variations of the samples with crosshatch patterns indicate improved GaAs film quality.
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u observation on electron beam induced chemical vapor deposition by Auger electron spectroscopy
Authors: Matsui Shinji , Mori Katsumi
Published: July 2002

W deposition, using WF6 gas source by electron beam induced surface reaction, has been studied by Auger electron spectroscopy. W Auger electron signals have been observed for WF6 adlayer by Auger electron spectroscopy. Moreover, initial growth for W deposition has been observed in situ by Auger electron spectroscopy. As a result, it became clear that a growth rate for W deposition is proportional to WF6 gas pressure and can be ∼1 Å/min at 2×10−7 Torr.
High-sensitivity laser probe for photothermal measurements
Authors: Fanton J. T. , Kino G. S.
Published: July 2002

A high-sensitivity interferometric laser probe has been developed to detect thermally generated surface displacements. Shot-noise-limited detection has been achieved with the use of inexpensive semiconductor lasers.
Improvements in the heteroepitaxy of GaAs on Si
Authors: Lum R. M. , Klingert J. K. , Davidson B. A. , Lamont M. G.
Published: July 2002

Successful application of GaAs on Si heteroepitaxy to majority-carrier device fabrication has recently been demonstrated. However, the quality of the GaAs heteroepitaxial films is considerably below that routinely achieved for films grown on GaAs substrates. We have investigated the initial growth stages of GaAs on Si using ion channeling and double-crystal -ray diffraction, and report improvements in the growth technique leading to higher quality GaAs films. The crystalline perfection of the films was found to be critically dependent on the growth parameters of the initial GaAs buffer layer. Growth interruption after deposition of this layer, followed by an in situ annealing step (10 min at 750 °C) prior to final GaAs growth, improved both the structural and optical properties of the films.
Neutron diffraction study of artificial CoO-NiO superlattices
Authors: Takano M. , Terashima T. , Bando Y. , Ikeda H.
Published: July 2002

Neutron diffraction measurements were made on artificial superlattices of CoO-NiO, (CoO)m(NiO)n×l prepared on the (0001) surface of α-Al2O3. (111) layers of CoO and NiO, each less than 2 nm thick (2≤m,n≤8), were stacked alternatively to a total thickness of less than 70 nm (m+n≤15,l≤25). A well-defined magnetic Bragg peak indexed as (1/2 1/2 1/2) was observed for all the samples, and a magnetic structure of type FCC2 with the spin axis lying in the (111) planes grown was suggested. The temperature dependence of the magnetic peak intensity revealed a sharp, single magnetic transition. The Néel temperature was found to vary as a linear function of n/(m+n).
Plasmon-phonon-assisted electron-hole recombination in silicon at high laser fluence
Authors: Rasolt Mark , Malvezzi Andrea Marco , Kurz Heinz
Published: July 2002

We present both theoretical and experimental results in silicon which clearly demonstrate that at time scales of 20 to 40 ps, after the pump laser pulse and at fluences greater than 100 mJ/cm2, the carrier density of the electron-hole plasma drops for increasing fluence; this is not explained by Auger recombination. We show that this drop is specific to plasmon-phonon-assisted recombination, which naturally explains this behavior.
Highly resolved excitonic spectra in GaAs/AlGaAs superlattices grown by organometallic chemical vapor deposition
Authors: Skromme B. J. , Bhat R. , Koza M. A.
Published: July 2002

We report the first observation of highly resolved photoluminescence and excitation spectra in GaAs/AlGaAs superlattices grown by organometallic chemical vapor deposition. Luminescence linewidths as narrow as 0.5 meV, and negligible Stokes shifts between luminescence and excitation peaks are measured for samples with 80–280 Å GaAs wells and 20 Å Al0.3Ga0.7As barriers. Both free heavy and light hole exciton and neutral donor-bound exciton peaks are observed in luminescence; excitation and temperature dependence is studied over the ranges 0.28 mW/cm2–10 W/cm2 and 1.7–30 K, respectively. The bound exciton luminescence saturates with respect to that of free excitons at high excitation, and rapid dissociation into free excitons is observed as the temperature increases. The free exciton peaks broaden at high temperature, while the bound exciton peaks remain sharp.
GaAs surface emitting lasers with circular buried heterostructure grown by metalorganic chemical vapor deposition and two-dimensional laser array
Authors: Koyama Fumio , Tomomatsu Kazuhiko , Iga Kenichi
Published: July 2002

A metalorganic chemical vapor deposition (MOCVD) was used to grow both double-heterostructure wafers and circular buried heterostructures for GaAlAs/GaAs surface emitting lasers. A vertical microcavity was formed with a diameter of 10 μm and a cavity length of 6 μm by a two-step MOCVD growth and fully monolithic technology. Threshold currents under room-temperature pulsed conditions ranged from 50 to 100 mA with a minimum of 50 mA. cw operation up to 160 K was obtained. In addition, a densely packed 5×5 SE laser array with 20 μm separation was demonstrated with a minimum threshold of 600 mA.
Band bending and interface states for metals on GaAs
Authors: Viturro R. E. , Shaw J. L. , Mailhiot C. , Brillson L. J. , Tache N. , McKinley J. , Margaritondo G. , Woodall J. M. , Kirchner P. D. , Pettit G. D. , Wright S. L.
Published: July 2002

We have used soft x-ray photoemission and optical emission spectroscopies to observe a broad range of Fermi level stabilization energies at metal interfaces with GaAs(100) surfaces grown by molecular beam epitaxy (MBE). The observed metal- and As-related interface cathodoluminescence plus orders-of-magnitude differences in bulk-defect-related photoluminescence between melt- versus MBE-grown GaAs suggest a role of bulk crystal growth and processing in controlling Schottky barrier formation.
Target presputtering effects on stoichiometry and deposition rate of Y-Ba-Cu-O thin films grown by dc magnetron sputtering
Authors: Selinder T. I. , Larsson G. , Helmersson U. , Olsson P. , Sundgren J.-E. , Rudner S.
Published: July 2002

Y-Ba-Cu-O thin films have been grown from stoichiometric compound targets using dc magnetron sputtering. Initially, the film composition was strongly off-stoichiometric and the deposition rate very low. However, after extensive presputtering, stoichiometric films, with respect to the metal content, were deposited at a rate of 2–2.5 Å/s. The long presputtering time (20–30 h) needed is due to a slowly changing oxygen content on the target surface. This is caused not only by preferential sputtering effects, but also by bulk diffusion in the target. The presputtering time is significantly decreased if the target is annealed in an argon atmosphere prior to use. Oxygen adsorbed on the target surface during venting of the chamber will also affect the process significantly.