bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of turbulent channel flow, with application to Malkus's theory
Authors: Reynolds W. C. , Tiederman W. G.
Published: March 2006

The Orr-Sommerfeld stability problem has been studied for velocity profiles appropriate to turbulent channel flow. The intent was to provide an evaluation of Malkus's theory that the flow assumes a state of maximum dissipation, subject to certain constraints, one of which is that the mean velocity profile is marginally stable. Dissipation rates and neutral stability curves were obtained for a representative two-parameter family of velocity profiles. Those in agreement with experimental profiles were found to be stable; the marginally stable profile of greatest dissipation was not in good agreement with experiments. An explanation for the apparent success of Malkus's theory is offered.
The large-scale structure of homogeneous turbulence
Authors: Saffman P. G.
Published: March 2006

A field of homogeneous turbulence generated at an initial instant by a distribution of random impulsive forces is considered. The statistical properties of the forces are assumed to be such that the integral moments of the cumulants of the force system all exist. The motion generated has the property that at the initial instant\[ E(\kappa) = C\kappa^2 + o(\kappa^2), \]whereE(k) is the energy spectrum function,kis the wave-number magnitude, andCis a positive number which is not in general zero. The corresponding forms of the velocity covariance spectral tensor and correlation tensor are determined. It is found that the terms in the velocity covarianceRij(r) areO(r−3) for large values of the separation magnituder.

An argument based on the conservation of momentum is used to show thatCis a dynamical invariant and that the forms of the velocity covariance at large separation and the spectral tensor at small wave number are likewise invariant. For isotropic turbulence, the Loitsianski integral diverges but the integral\[ \int_0^{\infty} r^2R(r)dr = \frac{1}{2}\pi C \]exists and is invariant.
The use of schlieren and shadowgraph techniques in the study of flow patterns in density stratified liquids
Authors: Mowbray D. E.
Published: March 2006

Various methods of establishing fluid media stably stratified in either temperature or the concentration of a foreign constituent are compared. A method used to construct a salt solution with precisely predetermined stratification is described. The Euler-Lagrange equations are solved exactly for a medium whose refractive index varies linearly in one dimension; the solution shows that a schlieren system may be used. For media with non-linear one-dimensional variation of refractive index, a modified shadowgraph technique may be employed. A device for the quantitative evaluation of a one-dimensional stratification is outlined and an example of its use is given.
The effects of curvature and buoyancy on the laminar dispersion of solute in a horizontal tube
Authors: Erdogan M. Emin , Chatwin P. C.
Published: March 2006

When a solute is injected into a straight circular tube through which a solvent is in steady laminar flow it spreads out longitudinally under the combined effect of molecular diffusion and advection with the flow. Taylor (1953) showed that, provided there is no density difference between the solute and the solvent, the distribution of mean concentration satisfies a diffusion equation with a certain longitudinal diffusivity for large times and with respect to axes moving with the discharge velocity. In the present paper it is shown by a general argument that this statement remains true if the pipe is uniformly curved. An expression is given for the diffusivity, valid when the radius of curvature is sufficiently large. For all common liquids and most gases the diffusivity is reduced by the curvature. The rest of the paper deals with the effects of buoyancy forces caused by a density difference between the solute and the solvent, when the tube is horizontal. It is shown that in general a longitudinal diffusivity does not exist and an equation is derived that replaces the diffusion equation if the effects of buoyancy are small. A prediction from this equation is that buoyancy should not have a noticeable effect on the longitudinal dispersion for Péclet numbers near a certain value at which the two opposing influences of horizontal spreading due to gravity and lateral mixing due to secondary flow are in balance. This prediction is consistent with some observations made by Reejhsinghani, Gill & Barduhn (1966).
Large-amplitude unsteady flow in liquid-filled elastic tubes
Authors: Olsen John H. , Shapiro Ascher H.
Published: March 2006

Unsteady, large-amplitude motion of a viscous liquid in a long elastic tube is investigated theoretically and experimentally, in the context of physiological problems of blood flow in the larger arteries. Based on the assumptions of long wavelength and longitudinal tethering, a quasi-one-dimensional model is adopted, in which the tube wall moves only radially, and in which only longitudinal pressure gradients and fluid accelerations are important. The effects of fluid viscosity are treated for both laminar and turbulent flow. The governing non-linear equations are solved analytically in closed form by a perturbation expansion in the amplitude parameter, and, for comparison, by numerical integration of the characteristic curves. The two types of solution are compared with each other and with experimental data. Non-linear effects due to large amplitude motion are found to be not as large as those found in similar problems in gasdynamics and water waves.
Peristaltic motion
Authors: Burns J. C. , Parkes T.
Published: March 2006

The flow of a viscous fluid through axially symmetric pipes and symmetrical channels is investigated under the assumption that the Reynolds number is small enough for the Stokes flow approximations to be made. It is assumed that the cross-section of the pipe or channel varies sinusoidally along the length. The flow is produced by a prescribed pressure gradient and by the variation in cross-section that occurs during the passage of a prescribed sinusoidal peristaltic wave along the walls. The theory is applied in particular to two extreme cases, peristaltic motion with no pressure gradient and flow under pressure along a pipe or channel with fixed walls and sinusoidally varying cross-section. Perturbation solutions are found for the stream function in powers of the ratio of the amplitude of the variation in the pipe radius or channel breadth to the mean radius or breadth respectively. These solutions are used to calculate, in particular, the flux through the pipe or channel for a given wave velocity in the first case and for a given pressure gradient in the second case. With a suitable notation it is possible to combine the analysis required for the two cases of pipe and channel flow.
Forced oscillations of an enclosed rotating fluid
Authors: Baines P. G.
Published: March 2006

The initial value problem related to axisymmetric forced oscillations of a rigidly rotating inviscid fluid enclosed in a finite circular cylinder is examined in linear approximation with the aid of the Laplace transform technique. An impulsive starting motion is considered. The solution consists of a ‘periodic’ motion which oscillates with the forcing frequency, together with a doubly infinite set of inertial modes whose presence is determined by the initial conditions and whose frequencies form a dense set in the range (−2ω, 2ω), where ω is the angular velocity. The nature of the periodic or ‘steady-state’ part of the solution is strongly dependent on the precise value of the forcing frequency α (α > 0) when α ≤ 2ω. In particular the system will resonate if α equals any one value of the dense set of resonant frequencies. It is shown that no internal sets of discontinuities in velocity or velocity gradient are present in the inviscid flow for finite times. Effects of viscosity on the inviscid solution are also discussed, and it is argued that when the inertial modes decay the steady-state flow will contain pseudo-random patterns of internal shear layers for some values of α < 3ω. It seems possible that these shear layers may be interpreted as owing their existence indirectly to viscosity.
Measurements of drag in a conducting fluid with an aligned field and large interaction parameter
Authors: Yonas G.
Published: March 2006

The drag of spheres and disks has been measured in a flow of liquid sodium within an aligned magnetic field. A slightly viscous, strong field limit is discussed and explored experimentally. In this limit the drag coefficient was found to have an asymptotic dependence proportional to the square root of the interaction parameter, the ratio of magnetic to inertial force, independent of body shape. A physical model is presented along with preliminary verification of its basic characteristics.
A theoretical and experimental investigation of the phase configuration of internal waves of small amplitude in a density stratified liquid
Authors: Mowbray D. E. , Rarity B. S. H.
Published: March 2006

Experiments were conducted to test the linear theory of internal gravity waves produced in a stably stratified liquid by the forced oscillations and the initial impulsive motion of a two-dimensional stationary disturbance. The measurements of the wave configuration in a medium whose density increased linearly with depth were made by means of a Toepler-schlieren system. The agreement between observation and prediction was found to be good.
Three-dimensional flow near a two-dimensional stagnation point
Authors: Davey A. , Schofield D.
Published: March 2006

This paper shows the existence of a three-dimensional solution of the boundary-layer equations of viscous incompressible flow in the immediate neighbourhood of a two-dimensional stagnation point of attachment. The numerical solution has been obtained.
On vortex street wakes
Authors: Bearman P. W.
Published: March 2006

The flow in the wake of a two-dimensional blunt-trailing-edge body was investigated in the Reynolds number range, Reynolds number being referred to base height, 1·3 × 104 to 4·1 × 104. The effects of splitter plates and base bleed on the vortex street were examined. Measurements were made of the longitudinal spacing between vortices and the velocity of the vortices, and compared with values predicted by von Kármán's potential vortex street model. The lateral spacing was estimated by using both the von Kármán and Kronauer stability criteria. A new universal wake Strouhal number is devised, using the value of lateral spacing predicted by the Kronauer stability condition as the length dimension. A correlation of bluff-body data was found when pressure drag coefficient times Strouhal number was plotted against base pressure.
The slow motion of a flat plate in a viscous stratified fluid
Authors: Martin Seelye , Long Robert R.
Published: March 2006

The existence of a ‘wake’ upstream of an obstacle moving slowly through a stratified fluid has been known for some time. The present study shows that a thin, flat plate moving slowly and horizontally through a linearly stratified salt-water mixture has, in addition, a boundary layer over the plate whose thickness increases upstream from thebackof the plate.

The theory assumes that the ratio of diffusivity to viscosity is small, and that the plate moves so slowly that inertia forces are negligible; under these conditions, a similarity solution is derived describing the boundary layer over the plate. The study also shows that salt diffusion is important in a second, thinner boundary layer whose thickness increases from the front of the plate.

In the experiment, a plate was towed through a tank of linearly stratified salt water. From streak photographs of the boundary layer over the plate, it was possible to confirm quantitatively the similarity solution and to infer at very slow velocities the presence of the thin diffusion boundary layer.
The unstable thermal interface
Authors: Elder J. W.
Published: March 2006

The motion which develops in a deep layer of a viscous, thermally conducting fluid initially hot below and cold above some horizontal plane, so that the system is gravitationally unstable, is studied by laboratory and numerical experiments. Three cases are considered: (i) the flow which occurs in a porous medium when the interface is the lower boundary of the system; (ii) a similar study in a viscous fluid; (iii) an interface distant from the confining horizontal boundaries, in a viscous fluid. In all cases the initial development of the flow—assuming an initial source of noise, for example as temperature fluctuations—occurs within the thermal interface between the hot and cold fluid. The scale of the motion is set by the thickness of the interface.

The development of the disturbances in the interface involves: a period of local thickening and induced, damped motions in which the diffusion of heat and vorticity dominate; a period of gestation, involving rapid amplification, with the disturbance imbedded in the interface and diminishing importance of the role of diffusion of heat; a period of emergence of the disturbances from the interface, during which the accelerations are sufficiently rapid for molecular processes to be unimportant, entrainment being the dominant process, and the gravitational energy accumulated locally in the interface is largely removed; and finally a period of adjustment of the large eddies. The amplification process is adequately described by the linearized equations of motion.
The propagation upward of the shock wave from a strong explosion in the atmosphere
Authors: Hayes Wallace D.
Published: March 2006

A method is established for the calculation of the trajectories of shocks moving upward in the atmosphere, on the basis of the assumption that they are of the self-propagating type. The results of calculations for self-similar motions are given, and these are used to establish a propagation law based upon the concepts of the Chisnell, Chester and Whitham (CCW) approximation. This propagation law enters a characteristics law based upon that proposed by Whitham, but reformulated for the computation of axisymmetric shocks with varying density.

An asymptotic self-preserving shock shape is investigated, and is computed for the case γ = 1·4. A parabolic approximation scheme suggested by the self-preserving solution is developed, in which the solution near the axis is reduced to the solution of a system of ordinary differential equations. Finally, the governing equation for the general case without axial symmetry (but without winds) is presented.
Vorticity expulsion by turbulence: astrophysical implications of an Alka-Seltzer experiment
Authors: Gough D. O. , Lynden-Bell D.
Published: March 2006

Arguments are given which suggest that the mean flow of a turbulent fluid is irrotational far from the boundaries. Mathematical difficulties precluded a detailed theory so a simple experiment was performed. This experiment qualitatively confirmed the suggestion. Other well-known experiments on the mean flow of turbulent fluid may also be interpreted in terms of vorticity expulsion, an idea closely related to Scorer's hypothesis on turbulent mixing.

Theoretical papers on stellar rotation usually assume that convective regions rotate uniformly because of a high turbulent viscosity. However, if vorticity expulsion occurs, convective stellar cores will not rotate. The concept of a turbulent viscosity is criticized.
A linear model of the Antarctic circumpolar current
Authors: Gill A. E.
Published: March 2006

A simple frictional wind-driven model of the Antarctic circumpolar current is examined. The geometry includes what are thought to be the main features effecting the current, namely a gap corresponding to Drake Passage and a partial barrier corresponding to the South American peninsula. Solutions obtained by both numerical and analytical methods are presented. The analytic solution, valid for small values of a friction parameter, enables the total rate of transport of water by the current to be calculated as a function of the friction parameter and the wind stress distribution. The width which controls the rate of transport by the current tends to be the narrowest encountered rather than the average width as assumed in earlier zonal models. However, the current spreads out to several times this width due to frictional effects. The values of the eddy viscosity required to give a rate of transport of the observed order are about 103cm2/s for a bottom-friction model and of the order of 108cm2/s for a lateral friction model.
Eigenvalue bounds for the Orr-Sommerfeld equation
Authors: Joseph Daniel D.
Published: March 2006

Estimates of the eigenvalues C belonging to the manifold of solutions of the Orr-Sommerfeld equation are constructed by application of elementary isoperimetric inequalities. The inequalities also lead to a considerable improvement on the estimate of (αR) regions of linear stability given by Synge.
Effect of a stabilizing gradient of solute on thermal convection
Authors: Veronis George
Published: March 2006

A stabilizing gradient of solute inhibits the onset of convection in a fluid which is subjected to an adverse temperature gradient. Furthermore, the onset of instability may occur as an oscillatory motion because of the stabilizing effect of the solute. These results are obtained from linear stability theory which is reviewed briefly in the following paper before finite-amplitude results for two-dimensional flows are considered. It is found that a finite-amplitude instability may occur first for fluids with a Prandtl number somewhat smaller than unity. When the Prandtl number is equal to unity or greater, instability first sets in as an oscillatory motion which subsequently becomes unstable to disturbances which lead to steady, convecting cellular motions with larger heat flux. A solute Rayleigh number, Rs, is defined with the stabilizing solute gradient replacing the destabilizing temperature gradient in the thermal Rayleigh number. When Rs is large compared with the critical Rayleigh number of ordinary Bénard convection, the value of the Rayleigh number at which instability to finite-amplitude steady modes can set in approaches the value of Rs. Hence, asymptotically this type of instability is established when the fluid is marginally stratified. Also, as Rs → ∞ an effective diffusion coefficient, Kρ, is defined as the ratio of the vertical density flux to the density gradient evaluated at the boundary and it is found that κρ = √(κκs) where κ, κs are the diffusion coefficients for temperature and solute respectively. A study is made of the oscillatory behaviour of the fluid when the oscillations have finite amplitudes; the periods of the oscillations are found to increase with amplitude. The horizontally averaged density gradients change sign with height in the oscillating flows. Stably stratified fluid exists near the boundaries and unstably stratified fluid occupies the mid-regions for most of the oscillatory cycle. Thus the step-like behaviour of the density field which has been observed experimentally for time-dependent flows is encountered here numerically.
The interaction trapping of internal gravity waves
Authors: Phillips O. M.
Published: March 2006

It is shown that as a result of their non-linear interactions, internal gravity waves in an unbounded fluid can be trapped to a layer of finite depth by periodic small variations in either the density gradient or in a weak horizontal steady current. This trapping occurs when the vertical component of the wave-number is half that of the density gradient or of the current variations. The energy density of the wave motion trapped near the ocean surface decreases exponentially with depth over a distance that is inversely proportional to the magnitude of the variations in density gradient or in horizontal current speed.
Note on the stability of a visco-elastic liquid film flowing down an inclined plane
Authors: Gupta A. S. , Rai Lajpat
Published: March 2006

An analysis is presented for the stability of a viscoelastic liquid film flowing down an inclined plane with respect to three-dimensional disturbances. It is shown that under certain circumstances, these disturbances are more unstable than the two-dimensional ones, contrary to Squire's theorem.
The degree of knottedness of tangled vortex lines
Authors: Moffatt H. K.
Published: March 2006

Letu(x)be the velocity field in a fluid of infinite extent due to a vorticity distributionw(x)which is zero except in two closed vortex filaments of strengthsK1,K2. It is first shown that the integral\[ I=\int{\bf u}.{\boldmath \omega}\,dV \]is equal to αK1K2where α is an integer representing the degree of linkage of the two filaments; α = 0 if they are unlinked, ± 1 if they are singly linked. The invariance ofIfor a continuous localized vorticity distribution is then established for barotropic inviscid flow under conservative body forces. The result is interpreted in terms of the conservation of linkages of vortex lines which move with the fluid.

Some examples of steady flows for whichI± 0 are briefly described; in particular, attention is drawn to a family of spherical vortices with swirl (which is closely analogous to a known family of solutions of the equations of magnetostatics); the vortex lines of these flows are both knotted and linked.

Two related magnetohydrodynamic invariants discovered by Woltjer (1958a, b) are discussed in ±5.
Collective instability of salt fingers
Authors: Stern Melvin E.
Published: March 2006

We first consider a steady laminar model of salt fingers and show that the latter become unstable with respect to internal gravity waves when the finger Reynolds number exceeds a critical value. The criterion is then used in speculations about the statistically steady state in a fully developed similarity model where horizontally averaged temperature and salinity gradients are constant at all depths. Dimensional reasoning is used to obtain the asymptotic dependence of the turbulent flux on the molecular salt diffusivity. From this and other relationships order-of-magnitude estimates are obtained and compared with laboratory experiments and ocean observations.
Rossby wave packet interactions
Authors: Newell A. C.
Published: March 2006

A mechanism is proposed whereby planetary zonal flows can be generated by the resonant interaction of Rossby wave packets whose amplitudes are slowly varying functions of both space and time. Equations are derived describing the long-time behaviour of a resonantly interacting triad. At the first closure certain properties analogous to those already known for discrete waves are deduced. At the second closure, in the particular case when one of the members of the triad is a zonal flow, it is shown that the sideband resonance mechanism can cause energy to be gained or lost by this zonal flow. It is also shown that a single Rossby wave packet can exchange energy with a zonal flow with weak shear. In the final section a resonant quarter mechanism for producing zonal flows is discussed. A numerical estimate of the acceleration of a zonal current from a zero initial state gives values of a few km/day per day.
A note on reverse transition
Authors: Bradshaw P.
Published: March 2006

It is shown that the criteria of Preston and of Patel & Head are special cases of a general Reynolds-number criterion which can be deduced as a requirement that there shall be some part of the boundary layer (in practice, the inner layer) in which the energy-containing and dissipating ranges of eddy size do not quite overlap.
Management of retrosternal goitres: results of early surgical intervention to prevent airway morbidity, and a review of the english literature
Authors: Hashmi S M , Premachandra D J , Bennett A M D , Parry W
Published: March 2006

We present our experience and also review the world literature on the management of retrosternal goitres (RSGs). There is now irrefutable evidence that almost all RSGs will continue to grow and eventually cause airway compression. We describe the diagnosis, investigation and surgical approach to the management of this condition.
Itelmen reduplication: Edge-In association and lexical stratification
Authors: BOBALJIK JONATHAN DAVID
Published: March 2006

Reduplication patterns in Itelmen (Chukotko-Kamchatkan) present a prima facie challenge to the view that association in partial reduplication is always Edge-In (Yip 1988, McCarthy & Prince 1996). Closer investigation suggests that Itelmen reduplication may in fact be total copying, masked by an apocope rule. This solution is not obvious, however, as the apocope rule must be limited to ‘core’ or ‘native’ vocabulary. While Russian loans are reasonably transparent, the analysis requires making a distinction between cognates and loans from related Koryak, a distinction speakers are not consciously aware of. Positing that this distinction is part of the synchronic phonology provides a solution to other apparently unrelated phonological puzzles in the language. In addition to removing an apparent counter-example to universal Edge-In association, the proposals made here may also provide a small argument for the lexical stratification model of loanword phonology over a purely representational alternative.
Ambiguity resolution in sentence processing: the role of lexical and contextual information
Authors: PAPADOPOULOU DESPINA , CLAHSEN HARALD
Published: March 2006

This study investigates how the parser employs thematic and contextual information in resolving temporary ambiguities during sentence processing. We report results from a sentence-completion task and from a self-paced reading experiment with native speakers of Greek examining two constructions under different referential context conditions: relative clauses (RCs) preceded by complex noun phrases with genitives, [NP1+NP2Gen], and RCs preceded by complex noun phrases containing prepositional phrases, [NP1+PP[P NP2]]. We found different attachment preferences for these two constructions, a high (NP1) preference for RCs with genitive antecedents and a low (NP2) preference for RCs with PP antecedents. Moreover, referential context information was found to modulate RC attachment differently in the two experimental tasks. We interpret these findings from the perspective of modular theories of sentence processing and argue that on-line ambiguity resolution relies primarily on grammatical and lexical-thematic information, and makes use of referential context information only as a secondary resource.
NP-internal agreement and the structure of the noun phrase
Authors: VAN EYNDE FRANK
Published: March 2006

For the analysis of the noun phrase, the treatment which currently prevails in generative grammar is the one in which the head of the noun phrase is identified with the determiner, rather than with the noun. This D(et)P treatment has the advantage of providing a uniform account of all syntactic categories, both the substantive and the functional ones, and it provides a natural way to capture the co-occurrence restrictions between nouns and determiners, but it also faces a number of empirical problems. To solve them I propose an analysis in which the head of the noun phrase is identified with the noun, but in which the advantages of the DP treatment are incorporated as much as possible. This is done in two steps. First, I argue that the requirement (or the desirability) of a uniform treatment of all syntactic categories does not by itself favour the DP treatment, since there is no empirical evidence for the postulation of a separate syntactic category for the determiners. The argumentation is mainly based on an analysis of NP-internal agreement data and leads to the conclusion that the class of determiners is syntactically heterogeneous: there are the adjectival determiners, which are subject to morpho-syntactic agreement, and (pro)nominal ones, which are exempt from this agreement. Second, I dissociate the roles of head and selector. All prenominals, both the specifying and the modifying ones, are treated as functors which select a nominal head, rather than as heads which select a nominal complement. This functor treatment accounts in a natural and straightforward way for both morpho-syntactic agreement and semantic types of agreement. The language which is used for exemplification is Dutch, but at various points comparisons are made with German and English.
Recovery within a population of the Critically Endangered citron-crested cockatoo Cacatua sulphurea citrinocristata in Indonesia after 10 years of international trade control
Authors: Cahill Alexis J. , Walker Jonathan S. , Marsden Stuart J.
Published: March 2006

Moratoria on international trade are frequently used to protect threatened species but few studies have examined their effectiveness in allowing populations to recover. We present population data collected before and after a moratorium on trade in the citron-crested cockatoo Cacatua sulphurea citrinocristata, a distinctive subspecies of the yellow-crested cockatoo endemic to Sumba, Indonesia. Before legal trade ceased in 1993 numbers of cockatoos leaving Sumba averaged c. 1,600 per year, and the 1992 population, estimated at 3,200, surely could not sustain such a level of trade. We surveyed cockatoos in four forest patches on Sumba in 1992, and then surveyed these same forest patches 10 years later, using the same field methods. Forest cover within the four patches was similar between years. We recorded a statistically significant increase in overall cockatoo density, from c. 2 birds per km2 in 1992 to >4 per km2 in 2002. Group sizes were also larger in 2002 than in 1992. Densities at two forest sites had increased considerably, at another the population was stable, but at one small forest patch a small population in 1992 had probably decreased. While the population has made a modest recovery, densities remain low compared to cockatoo populations elsewhere. Illegal trade is known to persist and its volume should be monitored closely.
The origin of vowel-length neutralisation in vocoid sequences: evidence from Finnish speakers
Authors: Myers Scott , Hansen Benjamin B.
Published: March 2006

A number of languages have independently developed a pattern of vowel-length neutralisation according to which a vowel after a vocoid must be long. It is proposed that this pattern arises from the inherent acoustic ambiguity of such sequences, which are realised with a diphthongal transition from one formant pattern to the next, with no clear boundary between the two. Neutralisation in vocoid sequences originates from listeners' difficulties in determining the duration of vowels in this context. Lengthening of the second vocoid arises when listeners attribute some of the transition duration to that segment. The phonetic bases of this account are supported by three experiments with Finnish speakers. A production study shows that speakers treat the transition as belonging in part to the realisation of the postvocoid vowel. Two perception studies show that increasing the duration of the transition increases the probability of such a vowel being identified as long.
Metallothionein isoform 2A expression is inducible and protects against ROS-mediated cell death in rotenone-treated HeLa cells
Authors: Reinecke Fimmie , Levanets Oksana , Olivier Yolanda , Louw Roan , Semete Boitumelo , Grobler Anne , Hidalgo Juan , Smeitink Jan , Olckers Antonel , Van Der Westhuizen Francois H.
Published: March 2006

The role of MT (metallothionein) gene expression was investigated in rotenone-treated HeLa cells to induce a deficiency of NADH:ubiquinone oxidoreductase (complex I). Complex I deficiency leads to a diversity of cellular consequences, including production of ROS (reactive oxygen species) and apoptosis. HeLa cells were titrated with rotenone, resulting in dose-dependent decrease in complex I activity and elevated ROS production at activities lower than 33%. Expression of MT2A (MT isoform 2A), but not MT1A or MT1B RNA, was significantly inducible by rotenone (up to 7-fold), t-BHP (t-butyl hydroperoxide; 5-fold) and CdCl2 (50-fold), but not ZnCl2. Myxothiazol treatment did not elevate either ROS or MT2A levels, which supports a ROS-related mechanism for rotenone-induced MT2A expression. To evaluate the role of MT2A expression, MT2A and MT1B were overexpressed in HeLa cells and treated with rotenone. Compared with control and MT1B-overexpressing cells, ROS production was significantly lower and cell viability higher in MT2A-overexpressing HeLa cells when ROS production was enhanced by treatment with t-BHP. Mitochondrial membrane potential was noticeably less reduced in both MT-overexpressing cell lines. MT2A overexpression in rotenone-treated cells also significantly reduced or delayed apoptosis induction, as measured by caspase 3/7 activity and cytosolic nucleosome enrichment. We conclude that MT2A offers significant protection against the main death-causing consequences of rotenone-induced complex I deficiency in HeLa cells. Our results are in support of the protective role against oxidative stress ascribed to MTs and provide evidence that MT2A expression may be a beneficial downstream adaptive response in complex I-deficient cells.
Characterization of Escherichia coli
D-arabinose 5-phosphate isomerase encoded by kpsF: implications for group 2 capsule biosynthesis
Authors: Meredith Timothy C. , Woodard Ronald W.
Published: March 2006

In Escherichia coli, there are multiple paralogous copies of the enzyme API [A5P (D-arabinose 5-phosphate) isomerase], which catalyses the conversion of the pentose pathway intermediate Ru5P (D-ribulose 5-phosphate) into A5P. A5P is a precursor of Kdo (3-deoxy-D-manno-octulosonate), an integral carbohydrate component of various glycolipids coating the surface of the OM (outer membrane) of Gram-negative bacteria, including LPS (lipopolysaccharide) and many group 2 K-antigen capsules. The K-antigen-specific API KpsF has been cloned from the uropathogenic E. coli strain CFT073 and its biochemical properties characterized. Purified recombinant KpsF [K-API (K-antigen API)] is tetrameric and has optimal activity at pH 7.8. The enzyme is specific for A5P and Ru5P, with Km (app) values of 0.57 mM for A5P and 0.3 mM for Ru5P. The apparent kcat in the A5P to Ru5P direction is 15 and 19 s−1 in the Ru5P to A5P direction. While most of the properties are quite similar to its LPS API counterpart KdsD, the catalytic constant is nearly 10-fold lower. K-API is now the second Kdo biosynthetic related gene that has been characterized from the kps group 2 capsule cluster.
Thermodynamics and rheology of cycloolefin copolymers
Authors: Blochowiak M. , Pakula T. , Butt H.-J. , Bruch M. , Floudas G.
Published: March 2006

Cycloolefin copolymers of ethylene and norbornene, with norbornene content in the range from 36to62mol%, were studied with respect to the thermal, thermodynamic, and rheological properties using differential scanning calorimetry, pressure-volume-temperature, and dynamic mechanical measurements. All copolymers obey the principle of time-temperature superposition, i.e., they can be considered as thermorheologically simple except for a temperature range in the vicinity of Tg. Despite this, the results on (i) the ratio of activation energies EV*∕H* used to quantify the origin of the liquid-to-glass transition, (ii) the pressure coefficient of the glass temperature Tg(P), and (iii) the dynamic fragility m suggest increasing dynamic heterogeneity with increasing norbornene content that is driven by the structural heterogeneity along the backbone.
Drift-Alfvén eigenmodes in inhomogeneous plasma
Authors: Vranjes J. , Poedts S.
Published: March 2006

A set of three nonlinear equations describing drift-Alfvén waves in a nonuniform magnetized plasma is derived and discussed both in linear and nonlinear limits. In the case of a cylindric radially bounded plasma with a Gaussian density distribution in the radial direction the linearized equations are solved exactly yielding general solutions for modes with quantized frequencies and with radially dependent amplitudes. The full set of nonlinear equations is also solved yielding particular solutions in the form of rotating radially limited structures. The results should be applicable to the description of electromagnetic perturbations in solar magnetic structures and in astrophysical column-like objects including cosmic tornados.
On collisionless energy absorption in plasmas: Theory and experiment in spherical geometry
Authors: Walker D. N. , Fernsler R. F. , Blackwell D. D. , Amatucci W. E. , Messer S. J.
Published: March 2006

An investigation of the rf impedance characteristics of a small spherical probe immersed in a laboratory plasma is ongoing in the large Space Physics Simulation Chamber [D. N. Walker et al., Rev. Sci. Instrum. 65, 661 (1994)] at the Naval Research Laboratory. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level rf signal to the probe which is either near floating potential or negatively dc biased in a low pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes “resistive,” even though the plasma is effectively collisionless, at frequencies below the bulk plasma frequency, thus leading to collisionless energy absorption. This behavior comes directly from Maxwell's equations along with the cold fluid equations. The solutions obtained indicate that this form of plasma resistance is inversely proportional to the plasma density gradient evaluated at the location where the plasma frequency is equal to the applied frequency. The sphere for this work is typically near plasma potential or biased more negatively into the ion collection regime. Applying a supplemental, negative dc bias to the sphere thickens the sheath and tends to raise its resistance as the density gradient is softened. Much of the earlier work in the area of collisionless resistance concentrated primarily on planar probes as opposed to the present work which is concerned with spheres. Interpreting the results is simpler for a sphere and the results obtained agree well with theory as described. For comparison to the theory we use only the S11 parameter outputs (or reflection coefficients) of the network analyzer in the experimental series. Significant energy absorption is observed at frequencies generally near one-half the plasma frequency. One result of this work is that the most efficient transfer of power to the plasma occurs not unexpectedly when there is impedance matching between input impedance and output (collisionless) impedance. This paper is an exposition of these ideas along with data results and a comparison to theory for the spherical probe which has not been published in this form.
Laser method of heating monolayer dusty plasmas
Authors: Nosenko V. , Goree J. , Piel A.
Published: March 2006

A method has been developed to heat and control temperature in a two-dimensional monolayer dusty plasma. A monolayer of highly charged polymer microspheres was suspended in a plasma sheath. The microspheres interacted with a Yukawa potential and formed a triangular lattice. Laser manipulation was used to apply random kicks to the particles. Two focused laser beams were moved rapidly around drawing Lissajous figures in the monolayer. The kinetic temperature of the particles increased with the laser power applied, and above a threshold a melting transition was observed. Characteristics of a thermal equilibrium of the laser-heated dusty plasma in solid and liquid states are discussed.
Two-stage gas amplifier for ultrahigh resolution low vacuum scanning electron microscopy
Authors: Thiel B. L. , Toth M. , Schroemges R. P. M. , Scholtz J. J. , van Veen G. , Knowles W. R.
Published: March 2006

We describe a magnetic field assisted, two-stage secondary electron gas amplification process for low vacuum scanning electron microscopy. The field of an ultrahigh resolution magnetic immersion objective lens and the electric field of an annular electrode configuration partition the amplification volume into two regions in which the electric and magnetic fields are parallel and crossed, respectively. The fields confine secondary electrons to axial and radial oscillations within the detector volume, until all of the kinetic energy imparted by an anode is dissipated through inelastic collisions with gas molecules. The electron confinement yields high gas amplification efficiency at short working distances and low gas pressures, facilitating high resolution imaging at low electron beam energies. Charging of insulating specimens is stabilized by positive ions produced in the gas ionization cascade. Furthermore, the signal to background level and bandwidth of this detector are superior to those of earlier generations of environmental secondary electron detectors. The combination of low vacuum, short working distance, and low beam energy is attractive to the semiconductor metrology industry, in particular, for critical dimension measurements on photolithographic masks.
Optical control of recovery speed of photoinduced third-harmonic generation in azo-copolymer thin films
Authors: Lin Jian Hung , Lai Ngoc Diep , Hsu Chia Chen
Published: March 2006

Recovery speed of photoinduced third-harmonic (TH) generation in azo-copolymer thin films can be controlled by a nanosecond laser excitation. When the excitation is tuned on, the TH signal decreases because of angular hole burning and angular redistribution effects. After turning off the excitation, the TH signal can recover to its original level either within 1min (high intensity excitation) or longer than several days (low intensity excitation). The fast recovery of the TH signal is attributed to the increase of temperature in the sample that causes molecules to more easily reorient and return to the original trans form.
Equation of state of nitrogen (N2) at high pressures and high temperatures: Molecular dynamics simulation
Authors: Krukowski Stanisław , Strąk Paweł
Published: March 2006

Nitrogen equation of state at pressures up to 30GPa (300kbars) and temperatures above 800K was studied by molecular dynamics (MD) simulations. The dynamics of the N2 molecules is treated in hard rotor approximation, i.e., it accounts both translational and rotational degrees of freedom. The rotational motion of the N2 molecule is treated assuming constant moment of inertia of the nitrogen molecule. The new MD program fully accounts anisotropic molecular nitrogen interaction. The N2–N2 interaction potential has been derived by van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)] using the results of high precision Hartree-Fock ab initio quantum mechanical calculations. The potential, fully accounts rotational symmetry of the N2–N2 system, by employing 6-j Wigner symbols, i.e., preserving full rotational symmetry of the system. Various numerical algorithms were tested, in order to achieve the energy preservation during the simulation. It has been demonstrated that the standard Verlet algorithm was not preserving the energy for the standard MD time step, equal to 5×10−16s. Runge-Kutta fourth order method was able to preserve the energy within 10−4 relative error, but it requires calculation of the force four times for each time step and therefore it is highly inefficient. A predictor-corrector method of the fifth order (PC5) was found to be efficient and precise and was therefore adopted for the simulation of the molecular nitrogen properties at high pressure. Singer and Fincham algorithms were tested and were found to be as precise as PC5 algorithm and they were also used in the simulation of the equation of state. Results of MD simulations are in very good agreement with the experimental data on nitrogen equation of state at pressures below 1GPa (10kbars). For higher pressures, up to 30GPa (300kbars), i.e., close to molecular nitrogen stability limit, determined by Nellis et al. [Phys. Rev. Lett. 85, 1262 (1984)], the obtained numerical results provide new data of the experimentally unexplored region. These data were formulated in the analytical form of pressure-density-temperature equation of state.
Conversion efficiency of extreme ultraviolet radiation in laser-produced plasmas
Authors: Murakami M. , Fujioka S. , Nishimura H. , Ando T. , Ueda N. , Shimada Y. , Yamaura M.
Published: March 2006

A simple analytical model is presented for the conversion of laser beam energy into extreme ultraviolet radiation. The model is compared with experimental results to show good agreements under different conditions of the laser wavelength λL, the laser intensity SL, the pulse duration tL, and the target atomic number Z0. It turns out that relatively high conversion efficiencies are obtained when the Planck optical thickness of the plasma is τ≈0.3–0.5, which is attained under an optimized combination of SL and tL once λL is fixed. The λL scaling on the conversion efficiency is derived.
Modeling the melting of supported clusters
Authors: Ding Feng , Rosén Arne , Curtarolo Stefano , Bolton Kim
Published: March 2006

Molecular dynamics simulations have been used to study the structural and dynamic changes during melting of free and supported iron clusters ranging from 150 to 10000atoms. The results reveal a method for determining effective diameters of supported metal clusters, so that the melting point dependence on cluster size can be predicted in a physically meaningful way by the same analytic model used for free clusters.
Bending modes in the hexagonal dust-plasma crystal
Authors: Vladimirov S. V. , Yaroshenko V. V. , Morfill G. E.
Published: March 2006

We study modes of particle oscillations associated with out-of-plane motions propagating in a two-dimensional hexagonal lattice of charged solid particles in a plasma. The analytical dispersion relation explicitly containing dependencies on the main plasma parameters and the direction of the mode propagation is obtained. The characteristics of the modes, stability of equilibria, and a critical dependence on the dust and plasma parameters for the realistic experimental conditions are investigated.
Photonic crystal laser lift-off GaN light-emitting diodes
Authors: David Aurélien , Fujii Tetsuo , Moran Brendan , Nakamura Shuji , DenBaars Steven P. , Weisbuch Claude , Benisty Henri
Published: March 2006

We report on the fabrication and study of laser lift-off GaN-based light-emitting diodes, thinned down to the microcavity regime, incorporating two-dimensional photonic crystal diffraction gratings. Angle-resolved measurements reveal the photonic behavior of the devices, which strongly depends on the GaN thickness. Data point out the detrimental role of metal absorption. We explore theoretically the possibility to limit this loss channel.
Long-range-ordered CdTe∕GaAs nanodot arrays grown as replicas of nanoporous alumina masks
Authors: Jung Mi , Mho Sun-il , Park Hong Lee
Published: March 2006

Long-range-ordered CdTe nanodot arrays with controlled size and density were grown on GaAs substrates by using molecular-beam epitaxy with ultrathin nanoporous alumina masks. The CdTe∕GaAs nanodot arrays were grown as replicas of the self-assembled porous alumina masks in spite of the large lattice mismatch between GaAs and CdTe. Using ultrathin alumina masks (ca. 200nm in thickness), we fabricated CdTe nanodot arrays with uniform dot sizes in the ranges of 35nm (with a density of ∼2.5×1010cm−2) and 80nm (with a density of ∼8.1×109cm−2). This is the report on controlling both the size and the density of II-VI/III–V heterostructure semiconductor nanodots.
Low-temperature lasing action in a metal-backed monosubstituted polyacetylene
Authors: Huang Yuan Ming , Zhou Fufang , Xu Kaitian
Published: March 2006

Within a metal-backed feedback structure, low-temperature laser action has been observed in a highly luminescent monosubstituted polyacetylene, poly(n-[(4′-hexylcarbonyl-biphenyl-4-oxy)]-1-propyne}(PBP), at a low pump intensity of about 4kW∕cm2. Sandwiched between a finely polished copper plate and a quartz plate, solid films of the polymer PBP begin to give off sharp line emission at 570nm when the temperature drops below 200K. Both the temperature and the pump intensity dependences of the sharp line emission are investigated. Our results suggest that low-temperature lowered threshold makes lasing feasible in PBP at a considerably low pumping intensity.