bluuebunny/crossref_metadata_2025 huggingface dataset:
Vorticity and turbulence production in pattern recognized turbulent flow structures
Authors: Eckelmann Helmut , Nychas Stavros G. , Brodkey Robert S. , Wallace James M.
Published: March 2003

A pattern recognition technique has been applied to data obtained in a turbulent channel flow. The u signal patterns are recognized using several simple criteria, but simultaneously, other signals are processed. A newly designed probe provides, in addition to simultaneous u, v, and w signals, vorticity signals, ωz and ωy, and a signal which can be interpreted as the instantaneous turbulence production, uv (∂U/∂y). The ensemble averaged results provide insight into the vorticity dynamics and turbulence production characteristics of organized structures. The instantaneous turbulent production results show that ejection-type motions are the principal turbulent energy producers and that the interesting dynamics occur during the acceleration phase of the recognized pattern. A partial model of the flow which explains the results is offered.
Mechanical models of dilute polymer solutions in strong flows
Authors: Hinch E. J.
Published: March 2003

In strong flows, such as turbulence, it is suggested that randomly coiled macromolecules might become nearly fully extended. To investigate this extended state, four models are considered which show the importance of the inextensibility of the polymer chain and the variation of the friction coefficient with the extension. The rheological consequences of these processes are then explored in an appropriately modified dumb-bell model. A dilute polymer solution is predicted to have a high extensional viscosity but a low shear viscosity, a stress hysteresis, and a slow stress relaxation. The action of the novel rheology is shown in two simple flows. Finally, speculations are made on the possible way dilute polymer solutions can modify turbulence.
Large structure in a turbulent boundary layer
Authors: Brown Garry L. , Thomas Andrew S. W.
Published: March 2003

Using an array of hot wires and wall shear stress probes results are presented which support the hypothesis that an organized structure exists in the boundary layer. From the correlation between wall shear stress and velocity it is shown that this structure is at an oblique angle to the free stream. The inferred angle from this correlation is found to agree remarkably with recent flow visualization photographs. Evidence is found that the large scale motion in the organized structure produces a slowly varying component in the wall shear stress and, importantly, a high frequency large amplitude fluctuation occurring near the maximum in the slowly varying wall shear. It is suggested that this latter fluctuation is associated with the bursting phenomenon. A model which accounts for these results is described. It leads to a description of the bursting process in terms of a rotational instability and while too approximate to give more than good order of magnitude estimates, it gives values of the wall streak spacing which are not inconsistent with previous measurements.
Dependence of effective slip on wall roughness in dilute polymer solutions
Authors: Saffman P. G.
Published: March 2003

A hypothesis is made for the modification of a complete turbulence model to take account of a dilute polymer concentration. The equations are integrated in the wall layer and give the effective slip u*ΔB as functions of wall roughness and empirical polymer properties, where u* is the friction velocity and ΔB is the change of the constant in the logarithmic law of the wall. Results in qualitative agreement with experiment are found.
Onset and saturation limit of polymer effects in porous media flows
Authors: Naudascher E. , Killen J. M.
Published: March 2003

The non-Newtonian features of onset, saturation, and degradation exhibited by the flow of dilute water solution of linear polymers through a porous medium are described. A physical model for the non-Newtonian behavior is presented as arising from the gradual extension of the polymer molecules, acting as elongated particles, and their alignment in the direction of flow. Comparisons are made with data from measurements in idealized porous-media configurations.
Burst cycle and drag reduction
Authors: Gyr A.
Published: March 2003

In two visualization experiments in water, Gyr and Müller1 showed that the addition of dilute polymer solutions changes the pattern of the flow structures near the wall. They observed a thickening of the viscous sublayer and of the buffer zone, as well as a stabilization of the shear waves in the direction of flow connected with an increase in the formation of transverse vortices. This observation is related to the burst cycle mechanism as described by Offen and Kline.2 Based upon their burst mechanism a hypothesis is proposed that the drag reduction is the result of a higher alignment of the wall near separation vortices. Such a stabilization of the local pattern would produce a higher number of enrolled vortices, which means more elements of larger coherent motion. At the same time it would reduce the chaotic redistribution in the velocity field. The hypothesis is in good agreement with the observation of higher turbulent intensity u′ in the direction of flow in the buffer zone. To describe this higher stabilization it is proposed to use a constitutive relation of the form, σ=2μ[E+ϑx̂x̂ (x̂Ex̂)] which is based on the concept that the polymer molecules are elongated in the direction of flow (x̂) and can support strain forces in this direction.3 To get an idea of the stabilization process the stability of a plane Poiseuille flow is demonstrated for a fluid with such an anisotropic viscosity, although for a complete investigation at least a two-dimensional flow system should have been taken into consideration. (The full paper is published in Ref. 4.)
Drag reducing polymer in helicoidal flow
Authors: Kovasznay Leslie S. G. , Kuo Jing Tzong
Published: March 2003

High polymer additives in low concentration have a rather important effect in turbulent flows, but when using the same concentration in a variety of laminar flows, in general, they have only very small effects, only of the order of the viscosity change. In order to see a flow configuration where a small amount of additive (e.g., 10 to 100 ppm) would cause a large effect comparable to that in the turbulent case, a novel configuration was explored. A screw pump, consisting of a smooth hollow cylinder and inside it a concentrically placed smooth screw with a gap comparable to the dimensions of the screw thread was constructed and the flow variables were measured. The pressure head developed by the screw pump was found to be strongly dependent on the presence of additives (by a factor of 2–3). This work was supported by the Office of Naval Research contract N00014−67−A−0163−0002.
Triggered transition in the pipe flow of dilute solutions of random-coiling macromolecules
Authors: Virk P. S. , Ohara M.
Published: March 2003

Laminar-to-turbulent transition was studied experimentally in the flow of dilute, distilled water solutions of two polethyleneoxide polymers through three pipes, each 200 diam long and equipped with a ’’trigger’’ orifice, of i.d. half the pipe i.d., to provide a strong inlet disturbance. Polymer molecular weights, 0.7×106 and 5.0×106, and pipe sizes, 0.292, 0.457, 0.945 cm i.d., were chosen to cause transition from the laminar regime (L) into each of three turbulent regimes associated with drag reduction,1 namely, Newtonian (N), polymeric (P), and maximum drag reduction (M). The corresponding three types of laminar-to-turbulent transitions observed, designated L→N, L→P, L→M, had the following characteristics: (1) L→N. In this case the transition occurred from laminar flow into a turbulent regime wherein the polymer solutions did not cause drag reduction, and the transition process appeared to be the same as in the usual Newtonian case.2,3 (2) L→P. In this case the polymer solutions exhibited drag reduction at the lowest fully turbulent flow rates. The critical Reynolds number, below which turbulent slugs were not observed, was essentially the same as Newtonian. At a given Re in the transitional regime, the intermittency factor at the pipe axis was distinctly greater than Newtonian, the more so with increasing drag reducing ability of the polymer solution. The greater intermittency factor seemed to result mainly from an increased turbulent slug formation frequency relative to Newtonian; turbulent slug lengths had mean values essentially the same as Newtonian, although the dispersion about the mean was somewhat greater in the polymer solutions. (3) L→M. In this case the transition occurred from laminar flow into the turbulent regime of maximum drag reduction. Velocity fluctuations appeared at Re≃1500, appreciably lower than the Newtonian critical Re≃2000, and the fluctuation amplitude increased by an order of magnitude over the transitional range, 1500<Re<4000. Intermittency could not be discerned during this type of transition.
Influence of polymer solutions on the structure of turbulence in a pipe
Authors: Scrivener O. , Kopp Ch.
Published: March 2003

A study of the alterations in the structure of turbulence due to the presence of PEO 301 dilute solutions was made using two experimental techniques. A photographic method was used to explore the wall region from 0.05 to 1 mm of the wall1 and laser Doppler anemometry in the core and the buffer layer.2 It was shown that, even if the velocities are reduced in the vicinity of the wall (in correlation with the fluid characteristics), the reduced coordinate profile remains unchanged in the laminar sublayer (y+<5). With very low concentrations (∼5 ppm) the mean velocity profile is not strongly affected even if the drag reduction is more than 40%. In the core, the shift of the profile observed by several authors was confirmed. The most important effect of the macromolecules was observed in the buffer layer where both the influence of the Reynolds number and the characteristics of the fluid were noted. The change in the structure of turbulence was observed3 in measuring the streamwise turbulent intensities. It was shown that the maximum is shifted far from the wall in relation to the increase in the thickness of the viscous sublayer. A spectral analysis of the longitudinal velocity fluctuations by calculation of the probability density functions, the autocorrelation functions, and the power spectrum confirmed the idea of a transfer of energy from high frequency to low frequency turbulence structures. Therefore, it appears that the mechanism of drag reduction has to be found in the change in both the turbulent energy production (increase in the great structures) and dissipation (decrease in the importance of small eddies).
Effect of compliant wall motion on turbulent boundary layers
Authors: Bushnell Dennis M. , Hefner Jerry N. , Ash Robert L.
Published: March 2003

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.
Dynamics of boundary layer turbulence and the mechanism of drag reduction
Authors: Landahl M. T.
Published: March 2003

The dynamics of the fluctuating velocity field in a turbulent boundary layer is discussed on the basis of a simplified analysis of localized unsteady perturbations in a parallel shear flow. Three classes of such disturbances may be distinguished, namely: (i) large-scale propagating disturbances which may be represented by a superposition of shear waves; (ii) convected disturbances which decay only slowly through the action of viscosity; and (iii) small-scale disturbances resulting from the secondary instability of the large-scale motion. Coupling between large and small scales is incorporated in a two-scale model in which the large scales are considered driven by the Reynolds stresses produced by the small scales. Shearing by the mean flow is shown to cause intensification of internal shear layers in the flow and could lead to local inflection in the instantaneous velocity profile, thus making secondary instability possible. Drag reducing additives, as modeled by some simple constitutive relations are found to be able to inhibit secondary instability, and would hence lower turbulent stress production according to the model employed.
Survey and new measurements of turbulent structure near the wall
Authors: Willmarth William W. , Bogar Thomas J.
Published: March 2003

A survey of recent measurements of turbulent structure near the wall which are applicable to the phenomenon of drag reduction by polymer additives is presented. Knowledge of the vorticity in the wall region provides a framework for understanding turbulent structure and the effects of polymer additives. New measurements using a small, calibrated streamwise vorticity probe are described. Near the wall where the turbulent production and dissipation are a maximum the streamwise vorticity is a maximum and is highly intermittent. New measurements of the Reynolds stress have also been made with a very small, computer calibrated hot-wire array. Intermittent small scale structures producing rapid oscillations of the hot-wire signals are detected near the wall which are too small to be resolved by the array. Their spatial scale is much less than the Kolmogoroff length. It is suggested that they are associated with streamwise vorticity.
Resonances cone and interference pattern in the field of a point charge in a flowing warm magnetoplasma
Authors: Singh Nagendra
Published: March 2003

The group velocity resonance cone in a drifting warm magnetoplasma is studied. Potential of a point charge is calculated in such a plasma. It is found that for a drift velocity Vd<Vt/tan ϑc, where Vt is the electron thermal velocity and ϑc is the cold cone angle, the downstream interference structure is useful for diagnostics of electron temperature using the formulae of a stationary plasma. The downstream interference pattern is appreciably modified for Vd≳Vt/tan ϑc. The upstream interference pattern is appreciably modified even for small drifts. For Vd≳Vt/tan ϑc, a new interference pattern appears for angles given by ϑc<ϑ<180°−ϑc. This new interference pattern is useful from the view point of measuring field-aligned drifts in laboratory and space plasmas.
Synchrotron emissivity near the electron cyclotron and upper hybrid frequencies
Authors: Freund H. P. , Wu C. S.
Published: March 2003

The spontaneous synchrotron emissivity from a high density, weakly relativistic plasma in thermal equilibrium is discussed. Thermal effects on the dielectric polarization of the plasma are included in the analysis, and the result is compared with computations of the emissivity based upon the cold plasma approximation. A numerical analysis is performed for frequencies in the vicinity of both the electron cyclotron frequency and the upper hybrid resonance. Significant modifications to the synchrotron emissivity in the cold plasma approximation are found in both of these frequency regimes.
Parametric excitation of kinetic waves—Ion-Bernstein waves by a lower hybrid pump wave
Authors: Watson Duncan C. , Bers Abraham
Published: March 2003

A new formulation of the nonlinear coupling of coherent waves described by the Vlasov equation is presented. This is used to derive the parametric growth rate for the down conversion from a lower hybrid pump wave to ion-Bernstein waves. It is found that the maximum growth rate for this interaction is larger than that for any other resonant interaction. The relevance of this to the current interest in lower hybrid heating of plasmas is discussed.
Cross-field current-driven ion acoustic instability in a bounded plasma with collisions
Authors: Hayzen A. J. , Barrett P. J.
Published: March 2003

The spatial growth rate of the cross-field ion acoustic instability is measured in an ion-beam-plasma system with a transverse magnetic field. By taking into account the finite width of the plasma, the ion beam temperature, and ion-neutral collisions, good agreement with the theoretical growth rate is obtained when (i) the normalized wavenumber kνρe is varied and (ii) the magnetic field direction is varied through a small angle about the direction perpendicular to the beam.
Solution of the three-wave resonant equations with one wave heavily damped
Authors: Chu Flora Y. F. , Karney Charles F. F.
Published: March 2003

The three wave equations in the limit where the waves at the upper two frequencies are undamped and the lowest frequency mode is heavily damped so that its dynamic equation becomes γ3a3=K*a1a2,are considered.These equations are solved (by quadrature) in two dimensions and time subject to arbitrary initial and boundary conditions. Illustative examples arising in tokamak heating by lower hybrid waves are presented.
Soliton in the damped nonlinear Schrödinger equation
Authors: Pereira N. R.
Published: March 2003

A numerical solution of the damped nonlinear Schrödinger equation is compared to analytical predictions that assume invariance of the soliton shape. The agreement is fair for the damping laws of the form γk∞‖k‖b. Good agreement is found for γk∞k2, and this case is studied analytically including second order effects of the damping.
Coupling of tearing modes in tokamaks
Authors: Finn J. M.
Published: March 2003

The simultaneous presence of tearing modes of different helical pitches leads to the destruction of magnetic surfaces, which has been suggested as the mechanism leading to the onset of the disruptive instability in tokamaks. For current profiles in which the m=2 mode is unstable, but the m=3 is stable, the coupling of the m=3 to the m=2 through the poloidal variation of the toroidal field can drive the m=3 amplitude ψ3 to order ψ2 times the inverse aspect ratio. Detailed calculations, including nonlinear effects, have been performed for two models for the equilibrium and m=2 mode structure. A slab model and incompressible m=3 perturbations are assumed. The m=3 amplitude increases with shear, up to a point, showing that as the current channel shrinks, overlap of resonances becomes more likely. Our results also apply qualitatively to other m, m±1 interactions.
Neoclassical transport in an elliptic tokamak
Authors: Tsang K. T.
Published: March 2003

Neoclassical transport for an elliptic tokamak in all collisional regimes is investigated by the technique of partitioning the velocity space. It is found that in a tokamak of moderate elongation, particle and ion heat confinement times are increased by a factor of σ2, where σ is the ratio of the vertical minor radius to the horizontal minor radius. Ripple diffusion in an elliptic tokamak is also studied. Ion heat conductivity due to ripples is reduced by a factor of approximately σ2.
Nonlinear saturation of two unstable modes; survival competition
Authors: Simon Albert , Migliuolo Stefano
Published: March 2003

A bounded uniform one-dimensional collisionless plasma is considered. The velocity distribution is of the ’’bump-on-tail’’ type and two linear eigenmodes with closely spaced wave vectors have phase velocities located on the lowest part of the positive slope of the bump. All other eigenmodes are assumed reasonably separated in phase velocity and located on negative (stable) slope regions. The time-asymptotic saturated nonlinear solutions are evaluated by a recently developed variation of the Bogoliubov method. In the ’’quasi-linear’’ approximation (i.e., zero-frequency mode coupling terms, alone, are retained) the only stable solution is one in which the initially fastest growing mode attains its ’’single-mode’’ amplitude and the second mode has zero amplitude. In the full nonlinear calculation, the result is the same unless the two modes are above a critical distance from the bottom of the slope. Above this critical value, there are two stable solutions. One as just described. The other in which the lower mode reaches its full single-mode amplitude and the first has zero amplitude. No stable solution exists for which both modes have finite amplitude. Similar mode survival competition may obtain in multi-mode situations.
Self-similar magnetohydrodynamic diffuser flows with induced magnetic fields
Authors: Choi S. H. , Wilhelm H. E.
Published: March 2003

The steady diffuser flow of a viscous incompressible fluid with a finite electrical conductivity in the presence of an external magnetic field is analyzed by means of similarity theory. Power series expansions for the velocity and induced magnetic fields are presented, and discussed as dependent on the Reynolds, Hartmann, magnetic Reynolds numbers, and duct angle. Short-circuit current and the critical values of magnetic Reynolds number and duct angle for flow separation are calculated. It is shown that the induced magnetic field changes the velocity profile and considerably reduces the critical duct angle at which flow separation occurs. The critical magnetic Reynolds number for flow separation becomes smaller as the Reynolds number increases.
Coupled relaxations in a plasma with strong thermodynamic nonequilibrium
Authors: Rowe B.
Published: March 2003

The problem of coupled thermal and chemical relaxation in a nitrogen-argon plasma mixture is studied for the case of thermodynamic nonequilibrium. Firstly, the equations which describe the time evolution of macroscopic quantities such as temperature and concentration are studied; then, the computation of some plasma evolutions is performed. As a result, the coupled thermal effect on the reaction and the chemical effect on thermal relaxation are shown.
Collisional damping of Langmuir waves in the collisionless limit
Authors: Auerbach S. P.
Published: March 2003

Linear Langmuir wave damping by collisions is studied in the limit of collision frequency ν approaching zero. In this limit, collisions are negligible, except in a region in velocity space, the boundary layer, centered about the phase velocity. If κ, the ratio of the collisional equilibration time in the boundary layer to the Landau damping time, is small, the boundary layer width scales as ν1/3, and the perturbed distribution function scales as ν−1/3. The damping rate is thus independent of ν, although essentially all the damping occurs in the collision-dominated boundary layer. Solution of the Fokker–Planck equation shows that the damping rate is precisely the Landau (collisionless) rate. The damping rate is independent of κ, although the boundary layer thickness is not.
Dynamics of coupled solitons
Authors: Appert K. , Vaclavik J.
Published: March 2003

The stability of coupled Langmuir and ion-acoustic solitons has been investigated by means of numerical computations. Using the Zakharov equation to describe the envelope of the oscillating electric field, and the Korteweg–de Vries equation with the ponderomotive driving term, to describe the low-frequency electron density variation, it was found that (1) Langmuir waves and short scale sound waves do not affect the soliton; (2) two solitons destroy each other when colliding; and (3) a long scale sound wave or ion-acoustic soliton break up a coupled soliton in the interaction. Moreover, no initial condition far from a soliton state was found which would create a coupled soliton.
Lower hybrid parametric instabilities—Nonuniform pump waves and tokamak applications
Authors: Berger R. L. , Chen Liu , Kaw P. K. , Perkins F. W.
Published: March 2003

Electrostatic lower hybrid ’’pump’’ waves are often launched into tokamak plasmas by structures (e.g., waveguides) whose dimensions are considerably smaller than characteristic plasma sizes. Such waves propagate in well-defined resonance cones and give rise to parametric instabilities driven by electron E×B velocities. The finite size of the resonance cone region determines the threshold for both convective quasi-mode decay instabilities and absolute instabilities. The excitation of absolute instabilities depends on whether a traveling or standing wave pump model is used; traveling wave pumps require the daughter waves to have a definite frequency shift. Altogether, parametric instabilities driven by E×B velocities occur for threshold fields significantly below the threshold for filamentation instabilities driven by pondermotive forces. Applications to tokamak heating show that nonlinear effects set in when a certain power-per-wave-launching port is exceeded. For sufficiently high powers, these instabilities will occur in the low-density edge region of a tokamak. They are characterized by a daughter wave frequency 10% below the pump wave frequency, in agreement with experimental observations.
Pfirsch–Schlüter friction in present tokamak plasmas
Authors: Hazeltine R. D. , Ware A. A.
Published: March 2003

The collisional friction force between two plasma species determines several quantities, such as particle diffusion rates, of central importance in tokamak transport theory. For an impure plasma in a regime of mixed collisionality, as pertains to most present tokamak experiments, electrostatic trapping leads to an unusually strong friction force, associated with slowly moving particles, which cannot be evaluated using the standard techniques of neoclassical kinetic theory. The Pfirsch–Schlüter part of the friction force is computed, and shown to depend logarithmically on parameters characterizing the tokamak equilibrium.
Mode conversion and tunneling in an inhomogeneous plasma
Authors: Ngan Y. C. , Swanson D. G.
Published: March 2003

A theory is presented describing mode conversion and tunneling in a warm, collisionless, toroidal plasma near the first ion-cyclotron harmonic frequency when the waves are propagating perpendicular to the static magnetic field. The coupling region may be characterized by a fourth-order differential equation with one turning point. Employing the method of Laplace integrals and the method of matched asymptotic expansions, transmission, reflection, and conversion coefficients are calculated assuming a fast wave is incident from either side of the coupling region. For a fast wave incident from the low magnetic field side, transmission, reflection, and conversion occur, while for a fast wave incident from the high magnetic field side, only transmission and conversion occur with no reflection, independent of the thickness of the tunneling layer, which implies complete conversion of the fast wave to the slow wave as the tunneling layer becomes thick. The problem of cavity resonance is discussed.
Influence of strong self-electric fields on the ion resonance instability in a nonneutral plasma column
Authors: Davidson R. C. , Uhm Hwan-sup
Published: March 2003

The influence of strong self-electric fields on the ion resonance instability is examined for a cylindrical nonneutral plasma column immersed in a uniform axial magnetic field B0êz. The analysis is carried out within the framework of a macroscopic cold-fluid model, and electrostatic stability properties are investigated for the case of rectangular electron and ion density profiles. The parameter δ= (2ω2pe/ω2ce)   (1−f) is introduced as a convenient measure of the relative strengths of the equilibrium self-electric force and the magnetic force on an electron fluid element. (Here, ω̂pe is the electron plasma frequency, ωce is the electron cyclotron frequency, f=n0i/n0e is the fractional charge neutralization, and δ=1 corresponds to the maximum allowed charge density for radial confinement of the equilibrium configuration.) An important conclusion of this study is that the equilibrium self-electric field can have a large influence on stability behavior. In particular, stability properties for δ≲1 differ substantially from those obtained when δ≪1. Moreover, for nonneutral plasma column with significant charge neutralization, it is found that the fundamental mode (l=1) is not the most unstable mode. Rather, higher harmonic perturbations have larger growth rates. However, in the limiting case where f≪1 and δ≪1, the l=1 mode can have the largest growth rate, which is consistent with the result previously obtained by Levy, Daugherty, and Buneman.
Kinetic theory of a chemically reacting polyatomic plasma
Authors: Saraf J. R.
Published: March 2003

A kinetic description is given for a polyatomic plasma consisting of electrons, ions, and atoms, the latter two species possessing internal degrees of freedom. The elastic, internal, and inelastic impact collisions have been modeled such that the system satisfies the conservation of charge, nuclei, mass, momentum, and translational plus rotational energy.
Turbulent vortex pairs in neutral surroundings
Authors: Wu Jin
Published: March 2003

Experiments were conducted in neutral surroundings on turbulent vortex pairs produced with various initial conditions. A pair was observed to spread linearly along the direction of motion: This is expressed as b=0.26z, where b is one half its maximum width and z is the elevation of its leading extremity. The advance of the pair was found to follow Z/b0=2.3(Tw0/b0)1/3, where Z and T are elevation and time, respectively, with respect to the virtual origin; and b0 and w0 are the initial size and velocity of the pair. These results appear to indicate that the motion of the pair follows the similarity solution and relates only to the initial momentum and the entrainment coefficient, regardless of the conditions of its generation. An additional experiment was conducted with polymer solutions to illustrate the influence of turbulent mixing inside the pair on its spreading behavior.
Calculations of turbulent boundary layer flows with drag reducing polymer additives
Authors: Durst F. , Rastogi A. K.
Published: March 2003

Results of investigations into numerical predictions of turbulent flows in plane two-dimensional channels and circular pipes with and without polymer additives are presented. These predictions are based on an available computer program for two-dimensional boundary layer flows and on the k-ε turbulence model. The model takes into account the low Reynolds number effects in the immediate vicinity of the wall and is modified to take into account the influence of the polymer additives in smooth as well as rough conduits in accordance with experimental results available in the literature. Results of the predictions are presented and compared with experiments. Based on the reliability of the present predictions and the uncertainty in the available measurements, a practical approach is outlined to study turbulent flows with drag reducing polymer additives. Degradation of polymer molecules is predictable with the described theoretical model.
Skewness of temperature derivatives in turbulent shear flows
Authors: Sreenivasan K. R. , Antonia R. A.
Published: March 2003

The nonzero value of the measured skewness of the streamwise temperature derivative is not necessarily inconsistent with the concept of local isotropy of small scale turbulence, and may be estimated adequately from a ramp-like model for the large scale structure of temperature.
Pressure gradient-velocity correlations for flows with two-and three-dimensional turbulence
Authors: Raj Rishi
Published: March 2003

The pressure gradient-velocity correlations as applied to curved and rotating turbulent flows are developed. A problem is raised for modeling pressure gradient-velocity correlations for two-dimensional turbulence. It is interesting to note how the behavior of the pressure gradient-velocity correlations differs for the cases of two- and three-dimensional turbulence near and away from the boundary. An explicit dependence of pressure gradient-velocity correlations on rotation is shown to exist in the case of rotating flows.
Inverse problem in transport theory. III
Authors: Case K. M.
Published: March 2003

Two explicit solutions of the inverse problem of one velocity transport theory are given. The first involves quantities K (n, n) obtained from a Gelfand–Levitan equation. The second involves quantities A (n, n) obtained from a Marchenko equation. These solutions are complementary. The first is most convenient to find the coefficients of the low order Legendre polynomials in the expansion of the scattering function. The second is efficient for calculating high order coefficients. The explicit relations between the A (n, n) and K (n, n) are given. Since most of the special results needed to obtain these solutions have been published elsewhere, proofs are at most sketched. Appropriate references provide any further details desired.
Dynamics of a free electron gas interacting with neutral atoms. Two-dimensional model
Authors: Peyraud Nelly
Published: March 2003

The dynamics of a free electron gas interacting with neutral and excited atoms whose populations are assumed fixed is considered. An analytic form of the time dependent electronic distribution function for arbitrary initial conditions, in the case of a two-dimensional model where interactions are only inelastic and superelastic between two atomic levels, is obtained. It is shown that this electron gas reaches a stationary state. The equilibrium distributions when the elastic collisions between electron and neutral atoms are taken into account is also considered. It is shown that the stationary equilibrium solutions for a three-dimensional model have the same structure as in the two-dimensional model.
Quasi-linear and nonlinear theory of dissipative trapped particle instabilities
Authors: Ehst David A.
Published: March 2003

An examination of particle linear orbits reveals how charged particles nonresonantly exchange energy with trapped particle modes in the presence of collisions. Phase space diffusion coefficients are constructed which describe the changes in the equilibrium particle distributions and which allow computation of the linear growth rates of the dissipative trapped particle instabilities. Wave-induced detrapping and resonance broadening are considered as nonlinear modifications of the particle-wave interaction and found to be ineffective as mechanisms to saturate growth of the dissipative trapped ion mode. Mode coupling with plasma diffusion across the magnetic field is a stronger effect for this and the dissipative trapped electron mode. Final evaluation of plasma diffusion coefficients is complicated by an ignorance of the nonlinear spectral shape.
Accessibility of equilibria in an electron beam plasma system. II
Authors: Cavalli Alexander , Greenly John B. , Walsh J. E.
Published: March 2003

Equilibrium analysis of the long time evolution of an electron beam plasma system has demonstrated the accessibility of more than one stable equilibrium. Experimental observations indicate that the plasma finds each of these equilibria depending on the operating parameters and that therefore a knowledge of the overall equilibrium behavior is essential to a complete understanding of the evolution of the system.
Finite-gyroradius stabilization of diffuse high beta stellarators
Authors: Freidberg J. P. , Hewett D. W.
Published: March 2003

A method is described for estimating the finite gyroradius stabilization of m⩾2 magnetohydrodynamic modes in diffuse high-β stellarators. The procedure combines both analysis and numerical computation. In particular, the difficult trajectory integrals which arise in the formulation are evaluated by plasma simulation techniques. The method is successfully applied to the m=2 modes in the high-β, l=1 stellarator configuration. It is found that the stability criterion for diffuse profiles is somewhat more optimistic than earlier estimates based on the sharp boundary model.
Theory of the multiple potential well structure created by bipolar injection in spherical geometry
Authors: Cherrington B. E. , Swanson D. A.
Published: March 2003

A theoretical analysis has been performed on the potential structure formed when electrons and ions are injected into the interior of a sphere. The solution of Poisson’s equation including spreads in the total energy and the angular energy of the injected particles has predicted a multiple potential structure very similar to that experimentally observed. The inclusion of trapped secondary electrons in the analysis also predicts a ’’triple-well’’ structure similar to that assumed to have been formed in Hirsch’s original experiments.
Magnetosonic waves in the presence of a lower hybrid turbulence and plasma heating
Authors: Sinha Mukul , Goswami B. N.
Published: March 2003

A theoretical investigation has shown that energy could be fed into a microturbulence from an externally excited low frequency magneto-acoustic source. A magnetosonic wave, normally undamped in a quiescent plasma, is shown to be damped in the presence of a lower hybrid microturbulence which in turn effectively heats the plasma ions. This process does not require any threshold amplitude of the external pump wave and therefore, very small amplitude waves can deposit their energy into the plasma unlike the usual parametric heating schemes which demand a minimum threshold amplitude.
Log-normality of the vorticity at a fluid particle
Authors: Hill R. J.
Published: March 2003

It is shown that the magnitude of the vorticity, at a given fluid particle in inviscid turbulent flow, is a log-normal random function of time.
Entrance region heat transfer with axial conduction in a cylindrical tube: Constant temperature wall
Authors: Barr B. J. , Wiginton C. L.
Published: March 2003

An analysis is made of the temperature profile for fully developed laminar flow in a circular duct with constant wall temperature and axial conduction. A series solution is assumed and two different numerical techniques are used to determine the full series coefficients.
Amplification rates and torques for Taylor-vortex flows between rotating cylinders
Authors: DiPrima R. C. , Eagles P. M.
Published: March 2003

Values of the amplification rate and torque for Taylor-vortex flows are given for radii ratios of 0.95 and 0.5 for several Taylor numbers greater than critical and for a number of different axial wavenumbers. The relevance of the results to experimental observations is discussed.
Vortex structure in the wake of a sphere
Authors: Pao Hsien-Ping , Kao Timothy W.
Published: March 2003

Results showing the three-dimensional vortex shedding structure when a sphere is towed at a constant velocity through a stratified fluid are presented. It is found that for small Richardson numbers (weak stratification) and Reynolds numbers in the range from 4×103 to 2×104 the vortex is shed three-dimensionally. However, stratification quickly and effectively inhibits the vertical motion and the initially turbulent wake collapses and reveals the vertically oriented portion of the vortex structure, reminiscent of a two-dimensional vortex street behind a circular cylinder when viewed from above. The structure is, however, distinctly three-dimensional. It is also found that the estimated vortex shedding frequency is in reasonable agreement with previously published results for a sphere in a homogeneous fluid. It is suggested that a weak stratification is an excellent means for revealing the vortex structure of a three-dimensional body in a homogeneous fluid, and that the vortex tube in the wake of a sphere in a homogeneous fluid has a closed-end double helical structure. Two branches of the double helix are continuously unwinding in an opposite sense from the formation region. Moreover, the present double helical model satisfies Thompson’s circulation theorem in contrast to previously proposed helical models.
Measurements in the turbulent boundary layer with slot injection of helium
Authors: LaRue John C. , Libby Paul A.
Published: March 2003

Hot-wire anemometry and digital techniques are used to develop time series for the streamwise and transverse velocity components and for the mass fraction of helium in the low-speed turbulent boundary layer of air involving the slot injection of helium. Profiles are developed at several streamwise stations. In the present contribution the data are presented principally in terms of conventional, unconditioned averages. It is found that the mean velocity and concentration profiles develop in the downstream direction in the expected fashion but that the intensities of the velocity, concentration, and density fluctuations increase sharply downstream of the slot and peak in the region where the mixing region from the splitter plate impinges on the wall. Close to the wall where the helium concentration is relatively high counterfluxes of helium are found. The data at one streamwise station are reanalyzed in terms of Favre-averaging so that the two means of averaging can be compared in a flow involving significant density fluctuations.
Electrophoresis in rare gas discharges
Authors: Fischer Johann
Published: March 2003

The momentum transferred from the ions to the neutrals in the positive column of a discharge is derived from the kinetic equation of the ions. A weakly ionized plasma, charge exchange collisions between the atomic ions and the neutrals at constant collision frequency, and the thermal energy of the neutrals small relatively to the field energy of the ions are assumed. For the collision dominated case the expression for the force on the neutrals is that obtained by Leiby and Oskam for low field energy, multiplied by a factor of 2. The calculated pressure gradient in the neutral gas is compared with the experimental results of Bergman and Chanin for helium on the high pressure side of the maximum.
Energy flux limitation by ion acoustic turbulence in laser fusion schemes
Authors: Manheimer Wallace M.
Published: March 2003

A quasi-linear theory of energy flux limitation by ion acoustic turbulence is presented. This distribution function is modeled by a Maxwellian plus an additional piece which carries a heat flux Q. By taking the fourth moment of the Vlasov equation, one finds the anomalous thermal conductivity K≈3veλDe (eφ/Te)−2. Other effects treated self-consistently include anomalous ion heating, electron cooling, current (and associated magnetic field) generation, and enhanced inverse bremsstrahlung due to the nonthermal ion fluctuations.
Ion cyclotron beam mode-whistler mode plasma instabilities and their role in parallel shock wave structures
Authors: Cipolla J. W. , Golden K. I. , Silevitch M. B.
Published: March 2003

The instability generated by two ion beams of arbitrary relative strength counterstreaming parallel to an external magnetic field in the presence of a charge and current neutralizing electron background has been analyzed using the linear Vlasov equation. This analysis extends previous theories by including the effects of finite temperatures in one or both ion beams. Using the Nyquist method, marginal stability curves for arbitrary beam strengths are obtained. For the case of a weak, hot ion beam interacting with a cold plasma whistler mode, an explicit expression for the growth rate (proportional to beam strength) has been obtained. The parameters are then chosen to be applicable to parallel shock waves, modeled as two interpenetrating ion beams. It is found that at the leading edge of the shock wave unstable whistlers may stand in the shock frame for all shock Alfvén Mach numbers MA⩾2.77. Furthermore, it is found that such unstable modes may exist for a wide range of points in the shock interior. The turbulent spectrum that results from a shock of fixed strength has a narrow bandwidth about the wavenumber of the stationary leading edge mode. These results significantly extend the previous cold theory and further support the contention that this instability is a viable mechanism for generating large amplitude turbulent structure in a collisionless shock wave.
Effects of finite plasma beta on the lower-hybrid-drift instability
Authors: Davidson R. C. , Gladd N. T. , Wu C. S. , Huba J. D.
Published: March 2003

The local dispersion relation for the lower-hybrid-drift isntability is derived in a fully self-consistent manner including the finite-beta effects associated with (a) transverse electromagnetic perturbations (δB≠0), and (b) resonant and nonresonant h/B0 electron orbit modifications. Moreover, the analysis is carried out for arbitrary values of local β=8πn (Te+Ti)/B02, Te/Ti, ω2pe/ω2ce, and VE/vi. (Here, VE is the cross-field E×B velocity, and vi is the ion thermal speed.) For all parameter regimes studied, the net effect of finite plasma beta is to reduce the maximum growth rate γm of the lower-hybrid-drift instability. The details, however, vary, depending on plasma parameters. For example, if Te≪Ti and VE<vi, then the maximum growth rate is reduced by a factor (1+βi/2)−1/2, relative to the value obtained when βi=8πnTi/B20→0. On the other hand, for Te≈Ti, there exists a critical value of plasma beta (βcr) such that the lower-hybrid-drift instability is completely stabilized (γ<0) for β≳βcr.
Production of dense plasmas in a hypocycloidal pinch apparatus
Authors: Lee Ja H. , McFarland Donald R. , Hohl Frank
Published: March 2003

A new type of high-power pinch apparatus consisting of disk electrodes was developed and diagnostic measurements to study its mechanism of dense plasma production have been made. The collapse fronts of the current sheets are well organized and dense plasma foci are produced on the axis with radial stability in excess of 5 μsec. A plasma density greater than 1018 cm−3 is determined with Stark broadening and CO2 laser absorption. Essentially complete absorption of a high-energy CO2 laser beam has been observed. A plasma temperature of approximately 1 keV is measured with differential transmission of soft x rays through thin foils. The advantages of this apparatus over the coaxial plasma focus are improvements in (1) plasma volume, (2) stability, (3) containment time, (4) access to additional heating by laser or electron beams, and (5) the possibility of scaling up to a multiple array for high-power operation.
Irradiation of parylene disks with a 1.06 μm laser
Authors: Haas R. A. , Mead W. C. , Kruer W. L. , Phillion D. W. , Kornblum H. N. , Lindl J. D. , MacQuigg D. , Rupert V. C. , Tirsell K. G.
Published: March 2003

Parylene (C8H8) disks have been irradiated with Nd:YAG-glass laser pulses focused to flux levels in the 1015 to 1017 W/cm2 range. The flux level was varied by changing the pulse length (50–150 psec), the laser energy (5–15 J), and the axial position of the target with respect to the f/1.1 focusing lens. An extensive array of diagnostics was used to measure the temporal and energy distribution of the focused laser light at the target, the temporal and angular distribution of the scattered laser light, the x-ray spatial and spectral emission characteristics, and the emitted ion and electron energy distributions. The experimental results, together with two-dimensional numerical simulations imply absorption via collective processes, laser generation of suprathermal electrons, and transport inhibition consistent with the presence of mega-Gauss level thermoelectric magnetic fields.
Clumping instability which is not predicted by the nearest neighbor approximation
Authors: Crowley Joseph M.
Published: March 2003

A vertical sheet of particles which are initially arranged in a diagonal lattice will tend to form clumps as it falls. The prediction of this clumping requires consideration of all neighbors, thus limiting the usefulness of the simpler nearest neighbor approximation.
Second order effect of a toroidal equilibrium
Authors: Yoshikawa S.
Published: March 2003

Toroidal equilibrium calculation was carried to the second order in the inverse aspect ratio. The plasma-vacuum boundary was assumed circular and the shape of conducting shell was determined. The major axis of the conducting shell lies in the z=0 plane.