bluuebunny/crossref_metadata_2025 huggingface dataset:
A weakly nonlinear theory of continental shelf waves
Authors: Pierini Stefano
Published: April 2006

In this paper three systems of evolution equations are presented which describe the free propagation of long continental shelf waves in the linear and weakly nonlinear regime. Two different degrees of nonlinearity are considered: for the first the Korteweg-de Vries equation is found to govern the dynamics of the system in the case of a single energy-containing mode (theories by Smith 1972; Grimshaw 1977), whereas, for the second nonlinear range, a nonlinear hyperbolic equation is derived. The nonlinear interactions among shelf-wave modes are also considered: they are modelled through nonlinear coupling terms in the evolution equations. This theory allows the timescale for the development of dispersive and nonlinear effects to be determined for each parameter range. The amplitude ranges corresponding to linear and nonlinear shelf waves are evaluated for the Oregon and the East Australian shelves, and some qualitative conclusions on the importance of nonlinear effects are derived. Finally the case of a shelf with longshore variation in topography is analysed and coupling terms in the evolution equations appear. They account for the scattering of energy between the various modes due to the linear and nonlinear interactions of the wave with the topographic changes.
Sound generated by instability waves of supersonic flows. Part 1. Two-dimensional mixing layers
Authors: Tam Christopher K. W. , Burton Dale E.
Published: April 2006

The problem of acoustic radiation generated by a spatially growing instability wave of a supersonic two-dimensional mixing layer is studied. It is shown that at high supersonic Mach numbers the classical locally parallel-flow hydrodynamic stability theory as well as the more recent theories based on the method of multiple scales (e.g. Saric & Nayfeh 1975; Crighton & Gaster 1976; Plaschko 1979; Tam & Morris 1980) would fail to give even a first-order instability wave solution. Physically, at these high flow speeds the radiated sound field is no longer an insignificant part of the total phenomenon. The disturbances associated with the flow-instability process now extend from the mixing layer all the way to the far field. The problem is therefore global in nature. Methods of solution which are predicated on local approximations such as the classical locally parallel-flow hydrodynamic-stability theory or the method of multiple scales are hence inappropriate and inapplicable. A global solution based on the method of matched asymptotic expansions is constructed. The outer solution is valid outside the mixing layer. It provides a mathematical description of the radiated acoustic field and the pressure near field. The near field in this case consists of both the acoustic and the hydrodynamic (non-propagating) fluctuation components. The inner solution is valid inside and in the immediate vicinity of the mixing layer. Physically it represents the instability wave of the flow. Matching is carried out according to the intermediate matching principle of Van Dyke (1975) and Cole (1968). Matching terms to order unity gives the basic instability-wave solution. Matching terms to the next order gives the instability- and acoustic-wave amplitude equation. For low-Mach-number flows it is found that the present results agree with the multiple-scales solution of Tam & Morris (1980).
The mixing layer: deterministic models of a turbulent flow. Part 1. Introduction and the two-dimensional flow
Authors: Corcos G. M. , Sherman F. S.
Published: April 2006

The prevalence in a turbulent mixing layer of dynamical events with a coherent history over substantial times suggests that it is profitable to study in detail entirely deterministic versions of this flow and to attempt to use a simplified synthesis of these solutions as the fundamental representation in a stochastic treatment of the layer. It is proposed that the deterministic representation of the flow be achieved by the embedding of a short hierarchy of motions which are studied in detail, though not exhaustively, in Parts 1, 2 and 3. Part 1 deals with the fundamental or first-order motion, which is the evolution of a layer constrained to be purely two-dimensional.
Calculation of the resistance and mobility functions for two unequal rigid spheres in low-Reynolds-number flow
Authors: Jeffrey D. J. , Onishi Y.
Published: April 2006

Two unequal rigid spheres are immersed in unbounded fluid and are acted on by externally applied forces and couples. The Reynolds number of the flow around them is assumed to be small, with the consequence that the hydrodynamic interactions between the spheres can be described by a set of linear relations between, on the one hand, the forces and couples exerted by the spheres on the fluid and, on the other, the translational and rotational velocities of the spheres. These relations may be represented completely by either a set of 10 resistance functions or a set of 10 mobility functions. When non-dimensionalized, each function depends on two variables, the non-dimensionalized centre-to-centre separation s and the ratio of the spheres’ radii λ. Two expressions are given for each function, one a power series in s−1 and the other an asymptotic expression valid when the spheres are close to touching.
A turbulent mixing layer constrained by a solid surface. Part 2. Measurements in the wall-bounded flow
Authors: Wood D. H. , Bradshaw P.
Published: April 2006

The single- and two-point measurements made in a high-Reynolds-number single-stream mixing layer growing to encounter a wind-tunnel floor on its high-velocity side that were described by Wood & Bradshaw (1982) have been extended to the wall-bounded flow. It is shown that the expected large amplification of the normal-stress components in the plane of the wall does not occur until after the mixing layer reaches the surface. There is some evidence that the double-roller component of the large-eddy structure of the original free shear layer is being re-established in the wall-bounded flow after having been stretched and weakened by the initial effect of the wall. The triple-product terms appearing in the turbulent-energy and shear-stress equations are altered in a way that cannot be reproduced by models used in current calculation methods. It appears that all the pressure-fluctuation terms in the individual normal-stress and shear-stress transport equations respond in a non-monotonic manner to the imposition of the wall. The implications for calculation methods suitable for predicting the change from an initially unaffected free shear layer to a wall-bounded flow are discussed.
Note on the paper of Kochar & Jain on Howard's semicircle theorem
Authors: Makov Yu. N. , Stepanyants Yu. A.
Published: April 2006

A further refinement of the Howard-Kochar-Jain theorem is given which allows the estimation of the range of complex wave velocity for growing perturbations in a stratified shear flow. According to the results obtained, the boundary of this region depends both on the minimum Richardson number and on the wavenumber of the perturbations. The effect of external boundaries on the stability of parallel flows is defined. An estimate of the maximum rate of growth versus dimensionless wave-number is found. The theoretical results are compared with numerical computations and laboratory experiments of other authors.
Flow in a small annulus between concentric cylinders
Authors: Lücke M. , Mihelcic M. , Wingerath K. , Pfister G.
Published: April 2006

Rotationally symmetric vortex flows between concentric cylinders with the inner one rotating and the outer one at rest have been investigated by numerical simulation and by laser-Doppler velocimetry for an annulus of aspect ratio Γ = 1.05 with a radius ratio η = 0.5066. Stationary states and relaxation towards them were explored close to the transition from the primary flow, which is mirror symmetric with respect to the midplane of the annulus, to a flow which gradually loses the symmetry. Detailed comparison of numerically simulated and measured velocity fields is made.
Progress towards a theory of jet-flap thrust recovery
Authors: Bevilaqua P. M. , Schum E. F. , Woan C. J.
Published: April 2006

A combination of analysis and testing has been utilized to develop a theory of jet-flap thrust recovery at the low speeds and high deflection angles characteristic of V/STOL lift systems. The contribution of jet mixing to the loss of thrust recovery has been computed with a viscid/inviscid interaction analysis. The results of this computation are compared to surface pressure and wake survey measurements made with a two-dimensional jet-flapped airfoil model. It is concluded that the jet-mixing drag causes a small loss of recovery at small values of the jet-thrust coefficient and deflect, an angle. However, at larger values of either jet parameter, the mainstream separates from the airfoil, producing a large loss of recovery. The loss increases suddenly, since it is due to bursting of the leading-edge separation bubble.
Effective viscosity of a periodic suspension
Authors: Nunan Kevin C. , Keller Joseph B.
Published: April 2006

The effective viscosity of a suspension is defined to be the four-tensor that relates the average deviatoric stress to the average rate of strain. We determine the effective viscosity of an array of spheres centred on the points of a periodic lattice in an incompressible Newtonian fluid. The formulation involves the traction exerted on a single sphere by the fluid, and an integral equation for this traction is derived. For lattices with cubic symmetry the effective viscosity tensor involves just two parameters. They are computed numerically for simple, body-centred and face-centred cubic lattices of spheres with solute concentrations up to 90% of the close-packing concentration. Asymptotic results for high concentrations are obtained for arbitrary lattice geometries, and found to be in good agreement with the numerical results for cubic lattices. The low-concentration asymptotic expansions of Zuzovsky also agree well with the numerical results.
Monte Carlo simulation of coagulation in discrete particle-size distributions. Part 2. Interparticle forces and the quasi-stationary equilibrium hypothesis
Authors: Valioulis I. A. , List E. J. , Pearson H. J.
Published: April 2006

Hunt (1982) and Friedlander (1960a, b) used dimensional analysis to derive expressions for the steady-state particle-size distribution in aerosols and hydrosols. Their results were supported by the Monte Carlo simulation of a non-interacting coagulating population of suspended spherical particles developed by Pearson, Valioulis & List (1984). Here the realism of the Monte Carlo simulation is improved by accounting for the modification to the coagulation rate caused by van der Waals', electrostatic and hydrodynamic forces acting between particles. The results indicate that the major hypothesis underlying the dimensional reasoning, that is, collisions between particles of similar size are most important in determining the shape of the particle size distribution, is valid only for shear-induced coagulation. It is shown that dimensional analysis cannot, in general, be used to predict equilibrium particle-size distributions, mainly because of the strong dependence of the interparticle force on the absolute and relative size of the interacting particles.
Flow in a differentially rotated cylindrical drop at moderate Reynolds number
Authors: Harriott George M. , Brown Robert A.
Published: April 2006

Galerkin finite-element approximations are combined with computer-implemented perturbation methods for tracking families of solutions to calculate the steady axisymmetric flows in a differentially rotated cylindrical drop as a function of Reynolds number Re, drop aspect ratio and the rotation ratio between the two end disks. The flows for Reynolds numbers below 100 are primarily viscous and reasonably described by an asymptotic analysis. When the disks are exactly counter-rotated, multiple steady flows are calculated that bifurcate to higher values of Re from the expected solution with two identical secondary cells stacked symmetrically about the axial midplane. The new flows have two cells of different size and are stable beyond the critical value Rec. The slope of the locus of Rec for drops with aspect ratio up to 3 disagrees with the result for two disks of infinite radius computed assuming the similarity form of the velocity field. Changing the rotation ratio from exact counter-rotation ruptures the junction of the multiple flow fields into two separated flow families.
A potential-flow theory for the dynamics of cylinder arrays in cross-flow
Authors: Paidoussis M. P. , Mavriplis D. , Price S. J.
Published: April 2006

The full linear unsteady potential-flow solution for fluid flowing across a bank of cylinders has been obtained. The potential function is expanded into a Fourier series and the boundary condition of impermeability is applied at the moving cylinder surfaces. Mutual contradictions among the various potential-flow solutions available in the literature are exposed, and it is shown that the present solution is consistent with certain basic physical checks, which some of the previous solutions could not meet. The effect of fluid viscosity is incorporated solely as a phase lag between the steady-state lift and drag coefficients on each cylinder and its respective motions. By incorporating the fluid-dynamic forces obtained from this modified potential-flow theory in a stability analysis, the threshold for fluid-elastic instability is predicted. Comparison with experimentally observed thresholds is encouraging, given the high level of idealization of the theory and the accuracy of present-day semi-empirical prediction methods.
High-Reynolds-number viscous flow in collapsible tubes
Authors: Tutty O. R.
Published: April 2006

This study is concerned with steady laminar high-Reynolds-number flow in collapsible tubes, where the position of the tube wall is a function (the tube law) only of the pressure exerted by the fluid on the wall. The system is controlled by two main parameters: the Reynolds number of the incoming flow, and the ‘compliance’, which characterizes the response of the wall to a change in fluid pressure. Restrictions are placed on these parameters so that the streamwise lengthscale is large, and, to the order worked, the pressure is uniform across the tube. Attention is restricted to (axi)symmetric systems.

Channels are considered in most detail, the results for axisymmetric pipes being largely similar.

For a model tube law the flow in the converging section of a channel is investigated in detail. Solutions are presented for certain of the parameter values. For some channels a singularity is found in the solution such that the channel width tends to zero at a finite distance downstream. No way was found to integrate past this singularity.

For particular channels and pipes, solutions are found only for flows in which the mean fluid velocity is less than the propagation speed of frictionless waves. This is consistent with experimental results.
Nonlinear fluid motions in a container due to the discharge of an electric current
Authors: Ajayi O. O. , Sozou C. , Pickering W. M.
Published: April 2006

The nonlinear electromagnetic stirring induced in a hemispheroidal container by the axisymmetric discharge of an electric current is investigated. The electric current is discharged into the fluid from a circular electrode which is at the centre of the equatorial plane of the container, the remaining part of the equatorial plane being a free surface. The equations of the problem are solved semi-analytically and results are presented for several sets of data. In the case of a point electrode when the current exceeds a critical value we have velocity breakdown. Here it is shown that, as the size of the area through which the current is discharged increases, the intensity of the flow field decreases, and thus for a larger electrode a larger amount of current can be discharged without velocity breakdown. When, however, the current is sufficiently large the solution becomes unstable, and this indicates velocity breakdown. Finally in an Appendix the solution for the case of a point discharge in a semi-infinite fluid is expressed in analytic (series) form.
An experimental investigation of coherent substructures associated with turbulent spots in a laminar boundary layer
Authors: Itsweire Eric C. , Atta Charles W. Van
Published: April 2006

Longitudinal and transverse components of the velocity were simultaneously measured for various vertical locations at 20 off-centreline positions in turbulent spots artificially generated in a zero-pressure-gradient laminar boundary layer. Global ensemble-averaged velocity diagrams and vertical vorticity contours were computed in similarity coordinates ξ = (x−x0)/(U∞(t−t0)) and ζ =z/(U∞(t−t0)) for different heights η =y/δ* above the plate. This global averaging technique inadequately describes the spot, which is not a single large vortex structure. A discriminative averaging technique was developed to construct a ‘statistically most-probable’ spot with sufficient resolution to include some of the largest substructures detected in visual studies. Four eddies were identified in vertical slices of the central region of the spot, while several rows appeared in the plan view. These features of the velocity and vorticity contours of the statistically most-probable spot exhibit similarities with structures observed in flow visualizations.
Final stages of transition to turbulence in plane channel flow
Authors: Biringen S.
Published: April 2006

This paper involves a numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 1500. Three-dimensional incompressible Navier–Stokes equations are numerically integrated to obtain the time evolution of two- and three-dimensional finite-amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 × 51 × 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and characteristic of the various stages of transition that lead to final breakdown. In particular, evidence points to the formation of a A-shaped vortex and the subsequent system of horsehoe vortices inclined to the main flow direction as the primary elements of transition. Details of the resulting flow field after breakdown indicate the evolution of streaklike formations found in turbulent flows. Although the flow field does approach a steady state (turbulent channel flow), the introduction of subgrid-scale terms seems necessary to obtain fully developed turbulence statistics.
Cell-pattern sensitivity to box configuration in a saturated porous medium
Authors: Borkowska-Pawlak B. , Kordylewski W.
Published: April 2006

Steady small-amplitude thermal convection in a fluid-saturated. infinitely extended porous layer is investigated theoretically in the wavenumber range 1/√2−1. It was shown that the point of multiple bifurcation Ra0 = (3/√2 + 2)π2, α0 = 2−0.25 leads to secondary bifurcation when the wavenumber decreases.

As a result a new branch of a stable, complicated, three-dimensional flow in the square cell was discovered for α close to α0. This branch joins two adjacent branches of three-dimensional flows emanating from the trivial solution and causes their stability transition at the branching points.
Asynchronous whirl in a rotating cylinder partially filled with liquid
Authors: Berman A. S. , Lundgren T. S. , Cheng A.
Published: April 2006

Experimental and analytical results are presented for the self-excited oscillations that occur in a partially filled centrifuge when centrifugal forces interact with shallow-water waves. Periodic and aperiodic modulations of the basic whirl phenomena are both observed and calculated. The surface waves are found to be hydraulic jumps, undular bores or solitary waves.
Square-pattern convection in fluids with strongly temperature-dependent viscosity
Authors: Busse F. H. , Frick H.
Published: April 2006

Three-dimensional numerical solutions are obtained describing convection with a square lattice in a layer heated from below with no-slip top and bottom boundaries. The limit of infinite Prandtl number and a linear dependence of the viscosity on temperature are assumed. The stability of the three-dimensional solutions with respect to disturbances fitting the square lattice is analysed. It is shown that convection in the form of two-dimensional rolls is stable for low variations of viscosity, while square-pattern convection becomes stable when the viscosity contrast between upper and lower parts of the fluid layer is sufficiently strong. The theoretical results are in qualitative agreement with experimental observations.
Further experimental evidence of vortex splitting
Authors: Freymuth P. , Bank W. , Palmer M.
Published: April 2006

Further experimental evidence of vortex splitting in wake flows is presented. Streakline visualizations reveal the range of conditions and the phenomenology of this novel process.
Turbulent-boundary-layer development in an adverse pressure gradient after an interaction with a normal shock wave
Authors: Schofield W. H.
Published: April 2006

An experimental study has been made of the development of a turbulent boundary layer in an adverse pressure gradient after an interaction with a normal shock wave that was strong enough to separate the boundary layer locally. The pressure gradient applied to the layer was additional to the pressure gradients induced by the shock wave. Measurements were taken for several hundreds of layer thicknesses downstream of the interaction. To separate the effects of shock wave and pressure gradient a second set of observations were made in a reference layer that developed in the same adverse pressure gradient without first interacting with a normal shock wave. It is shown that the adverse pressure gradient impressed on the flow downstream of the shock has a major effect on the structure of the interaction region and the growth of the layer through it. Consequently, existing results for interactions without a postshock pressure gradient should not be used as a model for predicting practical flows, which typically have strong pressure gradients applied downstream of the shock wave. It is also shown that the shock wave produces a pronounced stabilizing effect on the downstream flow, which can be attributed to the streamwise vortices shed into the flow from the separated region formed by the shock wave. The implications of this result for nominally two-dimensional flow situations and to flows involving weak interactions without local separations are discussed.
Simulation of blunt-fin-induced shock-wave and turbulent boundary-layer interaction
Authors: Hung Ching-Mao , Buning Pieter G.
Published: April 2006

The Reynolds-averaged Navier–Stokes equations are solved numerically for supersonic flow over a blunt fin mounted on a flat plate. The fin shock causes the boundary layer to separate, which results in a complicated, three-dimensional shock-wave and boundary-layer interaction. The computed results are in good agreement with the mean static pressure measured on the fin and the flat plate. The main features, such as peak pressure on the fin leading edge and a double peak on the plate, are predicted well. The role of the horseshoe vortex is discussed. This vortex leads to the development of high-speed flow and, hence, low-pressure regions on the fin and the plate. Different thicknesses of the incoming boundary layer have been studied. Varying the thicknesses by an order of magnitude shows that the size of the horseshoe vortex and, therefore, the spatial extent of the interaction are dominated by inviscid flow and only weakly dependent on the Reynolds number. Coloured graphics are used to show details of the interaction flow field.
The hydrodynamic interactions between two spheres in a Brinkman medium
Authors: Kim Sangtae , Russel William B.
Published: April 2006

The hydrodynamic interaction between two spheres in a Brinkman medium has been calculated using both the method of reflections and the boundary collocation technique. In particular, calculation of the forces and dipoles for two spheres in a uniform stream and linear field show that the method of reflections converges more rapidly than in the Stokes case, owing to screening of interactions, and that the boundary collocation technique produces accurate solutions at almost all separations (except touching) with relatively few collocation points.
The stability and energetics of corotating uniform vortices
Authors: Dritschel David G.
Published: April 2006

Equilibrium shapes of two-dimensional rotating configurations of uniform vortices are numerically calculated for two to eight corotating vortices. Additionally, a perturbation series is developed which approximately describes the vortex shapes. The equilibrium configurations are subjected to a linear stability analysis. This analysis both confirms existing results regarding point vortices and shows that finite vortices may destabilize via a new form of instability derived from boundary deformations. Finally, we examine the energetics of the equilibrium configurations. We introduce a new energy quantity called ‘excess energy’, which is particularly useful in understanding the constraints on the evolution of unstable near-equilibrium configurations. This theory offers a first glance at nonlinear stability. As an example, the theory explains some features of the merger of two vortices.
On a submerged sphere in a viscous fluid excited by small-amplitude periodic motions
Authors: Jenkins S. A. , Inman D. L.
Published: April 2006

Higher-order nonlinear corrections to the Stokes pendulum problem are calculated in perturbation schemes for small values of Reynolds numbers R(2) = umd0/2ν. Here the controlling lengthscale ½d0 is the displacement amplitude of the undisturbed periodic motion, um is the velocity amplitude and ν is the kinematic viscosity. Solutions for two general types of periodic motion are found; namely orbital motion as under deep water waves and oscillatory motion as under shallow water or acoustic waves. These solutions are found by matched asymptotic expansions using the fundamental irrotational oscillation to drive a thin Stokes a.c. boundary layer over the surface of the sphere. From the boundary layer several secondary motions are excited which die away in the neighbouring fluid. Among these are an orthogonal system of steady, rotational Eulerian streaming currents, and two outwardly radiating non-dispersive waves, one having the frequency of the fundamental but with a phase shift, the other appearing at the second harmonic.

With these solutions the forces and torques on a fixed sphere were computed. One of the orthogonal components of the rotational streaming field was found to produce a rotary lift force which opposed virtual-mass forces and diminished the resultant force component in quadrature to the fundamental oscillation. The other streaming component contributed damping terms which, unlike leading-order Stokes drag, vary nonlinearly with the displacement amplitude. Steady and second-harmonic torques were found to act on the sphere about the horizontal axis transverse to the fundamental oscillation.
Marginal separation of a three-dimensional boundary layer on a line of symmetry
Authors: Brown S. N.
Published: April 2006

The marginal separation of a laminar incompressible boundary layer on the line of symmetry of a three-dimensional body is discussed. The interaction itself is taken to be quasi-two-dimensional but the results differ from those for a two-dimensional boundary layer in that the effect of the gradient of the crossflow is included. Solutions of the resulting integral equation are computed for two values of the additional parameter, and comparisons made with an analytical prediction of the asymptotic form as the length of the separation bubble tends to infinity. The occurrence of the phenomenon is confirmed by an examination of the results of an existing numerical integration of the boundary-layer equations for the line of symmetry of a paraboloid.
Characteristics of pressure-wave propagation in a compliant tube with a fully collapsed segment
Authors: Shimizu Masashi
Published: April 2006

In order to model the fluid dynamics of Korotkoff sound generation when the artery under the cuff is fully collapsed during most of the heart cycle, the characteristics of pressure-wave propagation in a long silicone-rubber tube were studied experimentally. The central portion of this tube was designed to collapse to zero cross-sectional area as a result of high negative transmural pressure, thus simulating a collapsed artery. Propagation of a single half-sinusoidal pressure wave in and around this segment was studied in detail by pressure, velocity and tube-longitudinal-shape measurements.

A very steep wave front (shock wave) capable of producing a short tapping sound was formed by an overtaking phenomenon in the fully collapsed tube segment and it propagated into the inflated tube distal to the collapsed segment. An empirical equation relating the flow rate penetrating into the collapsed segment, the incident-wave pressure and the external pressure Pc over the collapsed segment was obtained. This equation predicts that the pressure-wave propagation in a fully collapsed segment depends only on the flow rate into the collapsed segment.

The initial internal pressure of the tube distal to the collapsed segment Pd is one independent variable in the high-cuff-pressure condition. The amplitude of the steep wave front and the shape of the pressure wave in the inflated tube distal to the collapsed segment are governed by Pc–Pd and the flow rate penetrating the collapsed segment. For the same flow rate, if Pc–Pd is lower than a critical value, the amplitude of the pressure in the distal tube decreases with increasing Pd because of positive pressure-wave reflection at the exit of the collapsed segment. On the other had, if Pc–Pd is higher than that value, no wave reflection occurs and the amplitude of the pressure wave is independent of Pd. In the latter case a severe constriction exists near the distal end of the collapsed segment, and flow occurs as two thin high-speed jets.
Time-dependent convective instabilities in a closed vertical cylinder heated from below
Authors: Abernathey J. R. , Rosenberger F.
Published: April 2006

The convective behaviour of xenon gas in a vertical thermally conducting cylinder (height/radius = 6) heated from below was investigated. Convectively induced temperature fluctuations in the gas were analysed with digital signal-processing techniques over a range of Rayleigh number 0 ≤ Ra [lsim ] 2300. Quiescent, steady-state, periodic and weakly turbulent convective regimes were characterized. Bistability of steady states (mode switching) was observed in the range 400 [lsim ] Ra [lsim ] 700. At Ra = 1550 a strictly periodic flow developed. With increasing Ra two additional incommensurate frequencies appeared, leading to ‘turbulence’ at Ra ≈ 2000. This turbulence, characterized by a broadband power spectrum, intermittently showed periodic flow. A periodic window with a period-doubling sequence appeared between 2100 [lsim ] Ra [lsim ] 2200. The spectral features of this sequence can be followed into the broad band noise at higher Ra. Although these experiments were conducted quasistatically, a strong hysteresis was observed with decreasing Ra. Furthermore, it was demonstrated that the sequence of convective regimes can be fundamentally altered by minor perturbations (self-heating) from the flow sensors.
The history of the industrial and commercial area of ‘Abbāsid Al-Raqqa, called Al-Raqqa Al-Muhtariqa
Authors: HEIDEMANN STEFAN
Published: April 2006

The history of the industrial and commercial district between al-Raqqa and al-Rāfiqa is reconstructed on the basis of literary sources, numismatic finds and aerial views from the early twentieth century. It probably came into being during the 160s/780s when the ‘old market of the caliph Hishām’ was transferred from within al-Raqqa to the free land between the two cities. The decision of Hārūn al-Rashīd to reside in al-Raqqa created a new demand, and consequently glass furnaces and pottery kilns were set up for mass production. A road running from the east gate of al-Rāfiqa connected this area. After 198/213 the governor of the west, Tārhir ibn al-Husain, erected a wall north of the area in order to protect it from Bedouin raids. During the third/ninth century at the latest the area developed into a third urban entity. Al-Muqaddasī mentions an al-Raqqa al-Muhtariqa. The identification with the commercial and industrial area is proposed. The decline of al-Raqqa al-Muhtariqa began in the 270s/880s and 280s/890s. The devastating rule of the Hamdânids probably marks its end.
Rotation sets for networks of circle maps
Authors: Parwani Kamlesh , Josić Krešimir
Published: April 2006

We consider continuous maps of the torus, homotopic to the identity, that arise from systems of coupled circle maps and discuss the relationship between network architecture and rotation sets. Our main result is that when the map on the torus is invertible, network architecture can force the set of rotation vectors to lie in a low-dimensional subspace. In particular, the rotation set for an all-to-all coupled system of identical cells must be a subset of a line.
Dynamical network interactions in distributed control of robots
Authors: Buscarino Arturo , Fortuna Luigi , Frasca Mattia , Rizzo Alessandro
Published: April 2006

In this paper the dynamical network model of the interactions within a group of mobile robots is investigated and proposed as a possible strategy for controlling the robots without central coordination. Motivated by the results of the analysis of our simple model, we show that the system performance in the presence of noise can be improved by including long-range connections between the robots. Finally, a suitable strategy based on this model to control exploration and transport is introduced.
Vibrational structure and methyl C–H dynamics in propyne
Authors: Portnov Alexander , Blockstein Lior , Bar Ilana
Published: April 2006

Our previous study [J. Chem. Phys. 122, 224316 (2005)] presented the photoacoustic and action spectra of the V=2, 3, 4, and 5 manifolds of the C–H methyl stretching vibrations of propyne and their analysis in terms of a simplified joint local mode/normal mode model. In the current paper the C–H transition intensities were calculated using B3LYP∕6-311++G(d,p) level of theory to obtain the dipole moment functions. The diagonalization of the vibrational Hamiltonian revealed new model parameters obtained by least square fitting of the eigenvalues to the action spectra band origins, while examining the correspondence between the calculated intensities and simulated band areas. The newly derived parameters predict well the band positions and the observed intensities, allowing new assignment of the features. The derived Hamiltonian was also used to obtain the overall temporal behavior of the C–H stretches as a result of the Fermi couplings and interactions with the bath states. These results indicate that any specificity attained by suitable excitation of the methyl C–H stretches is lost on picosecond time scale, primarily due to strong interactions with doorway states in the lower overtone and coupling with bath states in the region of the higher ones.
Quaternionic root systems and subgroups of the Aut(F4)
Authors: Koca Mehmet , Al-Barwani Muataz , Koç Ramazan
Published: April 2006

Cayley-Dickson doubling procedure is used to construct the root systems of some celebrated Lie algebras in terms of the integer elements of the division algebras of real numbers, complex numbers, quaternions, and octonions. Starting with the roots and weights of SU(2) expressed as the real numbers one can construct the root systems of the Lie algebras of SO(4),SP(2)≈SO(5),SO(8),SO(9),F4 and E8 in terms of the discrete elements of the division algebras. The roots themselves display the groups structures besides the octonionic roots of E8 which form a closed octonion algebra. The automorphism group Aut(F4) of the Dynkin diagram of F4 of order 2304, the largest crystallographic group in four-dimensional Euclidean space, is realized as the direct product of two binary octahedral group of quaternions preserving the quaternionic root system of F4. The Weyl groups of many Lie algebras, such as, G2,SO(7),SO(8),SO(9),SU(3)XSU(3), and SP(3)×SU(2) have been constructed as the subgroups of Aut(F4). We have also classified the other non-parabolic subgroups of Aut(F4) which are not Weyl groups. Two subgroups of orders 192 with different conjugacy classes occur as maximal subgroups in the finite subgroups of the Lie group G2 of orders 12096 and 1344 and proves to be useful in their constructions. The triality of SO(8) manifesting itself as the cyclic symmetry of the quaternionic imaginary units e1,e2,e3 is used to show that SO(7) and SO(9) can be embedded, triply symmetric way in SO(8) and F4 in respectively.
High-pressure phases of InP: An ab initio and molecular-dynamics study
Authors: Branicio Paulo S. , Rino José Pedro , Shimojo Fuyuki
Published: April 2006

The high-pressure phases of InP up to 100GPa are investigated using ab initio calculations and molecular-dynamics simulations. Simulation results show that the sequence of high-pressure phases is zinc blende (ZB)→rocksalt(RS)→rhombohedral(RH)→CsCl. The continuous RH distortion of the RS structure is consistent with the observed Cmcm-like distortion. Ab initio results indicate that a mixture of Cmcm and RH distortions is energetically possible and could explain the experimental spectra misfit. The calculated equation of state is in very good agreement with experiments.
Electric-field-induced charge noise in doped silicon: Ionization of phosphorus donors
Authors: Ferguson A. J. , Chan V. C. , Hamilton A. R. , Clark R. G.
Published: April 2006

We report low-frequency charge noise measurement on silicon substrates with different phosphorus doping densities. The measurements are performed with aluminum single electron transistors (SETs) at millikelvin temperatures where the substrates are in the insulating regime. By measuring the SET Coulomb oscillations, we find a gate-voltage-dependent charge noise on the more heavily doped substrate. This charge noise is attributed to the electric-field-induced tunneling of electrons from their phosphorus donor potentials.
Polarization anisotropy in GaN films for different nonpolar orientations studied by polarized photoreflectance spectroscopy
Authors: Misra Pranob , Behn Udo , Brandt Oliver , Grahn Holger T. , Imer Bilge , Nakamura Shuji , DenBaars Steven P. , Speck James S.
Published: April 2006

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain.
Birdsfoot trefoil (Lotus corniculatus) and greater lotus (Lotus uliginosus) in perennial pastures in eastern Australia. 1. Effects of grazing management on persistence
Authors: Ayres J. F. , Blumenthal M. J. , O'Connor J. W. , Lane L. A. , Nicol H. I.
Published: April 2006

A study was undertaken in the Perennial Pasture Zone in New South Wales, Australia, to investigate the potential of lotus-based pastures (greater lotus, Lotus uliginosus Schukr.; birdsfoot trefoil, Lotus corniculatus L.) to improve grazing production. The study was based on a methodology that combined a grazing experiment to determine the effects of grazing management on lotus persistence, with a co-learning phase to assess the adaptation and applications of lotus across the high rainfall zone. The present paper reports on the grazing experiment that was replicated in 4 regions (North Coast, South Coast, Northern Tablelands, Southern Tablelands) and that included combinations of grazing strategy (summer rest, autumn rest, 14-day spell or 28-day spell), grazing intensity (low or high herbage mass), Lotus species and cultivar (L. uliginosus cvv. Grasslands Maku, Sharnae; L. corniculatus cv. Grasslands Goldie or ‘Spanish’ breeding line) and companion grass (sown or volunteer) treatments. The experiment provided results for the establishment and botanical presence of both Lotus species, and the expression of their persistence mechanisms in these 4 environments. Greater lotus cv. Grasslands Maku established best under coastal conditions and birdsfoot trefoil cv. Grasslands Goldie established best under tablelands conditions. The degree of nodulation of greater lotus cv. Sharnae and birdsfoot trefoil cv. Grasslands Goldie was less than nodulation of greater lotus cv. Grasslands Maku, but nodulation had no apparent effect on seedling vigour. The population density of all Lotus cultivars declined substantially at the North Coast, South Coast and Southern Tablelands sites during severe drought conditions. However, both birdsfoot trefoil cv. Grasslands Goldie and greater lotus cv. Grasslands Maku remained relatively stable at the Northern Tablelands site with greater lotus cv. Grasslands Maku maintaining 20–40% presence and birdsfoot trefoil cv. Grasslands Goldie retaining 30–50% presence, depending on time of year. In general, there was little effect of grazing management practice on arresting the decline in lotus presence due to drought, or on enhancing the regeneration processes associated with lotus persistence.
Common attributes of native-state structures of proteins, disordered proteins, and amyloid
Authors: Hoang Trinh X. , Marsella Luca , Trovato Antonio , Seno Flavio , Banavar Jayanth R. , Maritan Amos
Published: April 2006

We show that a framework derived from the common character of globular proteins can be used to understand the design of protein sequences, the behavior of intrinsically unstructured proteins, and the formation of amyloid fibrils in a unified manner. Our studies provide compelling support for the idea that protein native-state structures, the structures adopted by intrinsically unstructured proteins on binding as well as those of amyloid aggregates, all reside in a physical state of matter in which the free energy landscape is sculpted not by the specific sequence of amino acids, but rather by considerations of geometry and symmetry. We elucidate the key role played by sequence design in selecting the structure of choice from the predetermined menu of putative native-state structures.
Description of Fabibacter halotolerans gen. nov., sp. nov. and Roseivirga spongicola sp. nov., and reclassification of [Marinicola] seohaensis as Roseivirga seohaensis comb. nov.
Authors: Lau Stanley C. K. , Tsoi Mandy M. Y. , Li Xiancui , Plakhotnikova Ioulia , Dobretsov Sergey , Wu Madeline , Wong Po-Keung , Pawlik Joseph R. , Qian Pei-Yuan
Published: April 2006

Bacterial strains UST030701-097T and UST030701-084T were isolated from a marine sponge in the Bahamas. Both strains were pink-pigmented, Gram-negative, strictly aerobic and chemo-organotrophic. Cells of strain UST030701-097T were short, curved rods with fast-gliding motility, whereas those of strain UST030701-084T were straight rods with a less rapid gliding motion. The two strains had MK-7 as the major respiratory quinone and did not produce flexirubin-type pigments. The DNA G+C contents of strains UST030701-097T and UST030701-084T were 42.5 and 43.7 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the two strains belonged to the family ‘Flexibacteraceae’ of the phylum Bacteroidetes. 16S rRNA gene sequence similarity between strains UST030701-097T and UST030701-084T was 95.0 %; their closest relative was [Marinicola] seohaensis, with 93.3 % and 96.0 % sequence similarity, respectively. Phylogenetic tree topology indicated that the two strains belonged to the same lineage, but were on separate branches. Whilst strain UST030701-084T and [Marinicola] seohaensis were found on one branch, strain UST030701-097T was in another branch that had no species with validly published names. Based on the polyphasic taxonomic data obtained in the present study, we propose that strain UST030701-097T represents a novel genus and that strain UST030701-084T represents a novel species in the phylum Bacteroidetes. The genus Fabibacter gen. nov. is proposed, with strain UST030701-097T (=NRRL B-41220T=JCM 13334T) as the type strain of the type species, Fabibacter halotolerans sp. nov. Strain UST030701-084T (=NRRL B-41219T=JCM 13337T) is proposed as the type strain of Roseivirga spongicola sp. nov. In an earlier study, it was suggested that the genus Marinicola is a later heterotypic synonym of the genus Roseivirga. However, a formal proposal to reclassify [Marinicola] seohaensis, the only member of the genus Marinicola, has not yet been made. The results of phylogenetic analyses in this study support the reclassification of [Marinicola] seohaensis as Roseivirga seohaensis comb. nov.
Effects of rac‐albuterol on arterial blood gases in patients with stable hypercapnic chronic obstructive pulmonary disease
Authors: Whale Christopher I. , Sovani Milind P. , Mortimer Kevin , Oborne Janet , Cooper Sue , Harrison Timothy W. , Tattersfield Anne E.
Published: April 2006

Aims

Many patients with chronic obstructive pulmonary disease (COPD) are treated with high dose β2‐adrenoceptor agonists, which can increase ventilation/perfusion mismatching, and tremor and cardiac output, thereby increasing oxygen uptake and carbon dioxide output (VCO2). Patients with severe COPD and hypercapnia may be unable to increase ventilation in response to increased VCO2, in which case arterial carbon dioxide tension (PaCO2) may rise. Our aim was to determine whether high dose nebulized rac‐albuterol could increase PaCO2 in patients with COPD, limited bronchodilator reversibilty and hypercapnia.

Methods

We compared 10 mg and 400 µg rac‐albuterol, given in two doses 1 h apart on nonconsecutive days, in a double‐blind randomized crossover study in 14 patients with severe COPD. PaCO2, arterial oxygen tension (PaO2) and heart rate were measured over 120 min and change from baseline was plotted against time to obtain an area under the curve.

Results

Mean PaCO2 fell slightly over 120 min, with no difference between treatments (0.03 kPa h−1 (95% confidence interval 0.02, 0.04)) and only three subjects had an increase in PaCO2 after high dose rac‐albuterol. High dose rac‐albuterol caused a greater fall in PaO2[0.1 kPa h−1 (95% confidence interval 0, 0.2)] and increase in heart rate than the low dose, although the differences were small.

Conclusions

Under stable conditions most subjects with severe COPD and hypercapnia will have a fall in PaCO2 and PaO2 following 10 mg rac‐albuterol, suggesting that they maintain capacity to respond to any increase in VCO2 and prevent a rise in PaCO2.
Major determinant factors of the extent of interaction between grapefruit juice and calcium channel antagonists
Authors: Ohnishi Ayako , Ohtani Hisakazu , Sawada Yasufumi
Published: April 2006

Aim

To evaluate quantitatively the influence of bioavailability and plasma unbound fraction (fu) on the extent of interaction between grapefruit juice (GFJ) and calcium antagonists.

Methods

The extent of drug–GFJ interaction was evaluated from reported clinical interaction studies of 11 calcium antagonists. The relationship between extent of interaction and bioavailability or fu was analysed.

Results

The following significant hyperbolic relationships were observed: [Extent] = 12.2/[bioavailability] + 1 and [Extent] = 0.718/[fu] + 1.

Conclusions

Bioavailability and fu are major determinant factors of calcium antagonist–GFJ interaction: a drug with smaller bioavailability or lower fu is likely to exhibit a more potent interaction, and vice versa.