bluuebunny/crossref_metadata_2025 huggingface dataset:
Global multiple‐sequence alignment with repeats
Authors: Sammeth Michael , Heringa Jaap
Published: April 2006

Repeating fragments in biological sequences are often essential for structure and function. Over the years, many methods have been developed to recognize repeats or to multiply align protein sequences. However, the integration of these two methodologies has been largely unexplored to date. Here, we present a new method capable of globally aligning multiple input sequences under the constraints of a given repeat analysis. The method supports different stringency modes to adapt to various levels of detail and reliability of the repeat information available. Proteins 2006. © 2006 Wiley‐Liss, Inc.
Fish assemblages and stream hydraulics: consistent relations across spatial scales and regions
Authors: Lamouroux N. , Cattanéo F.
Published: April 2006

General relationships between organisms and their habitat, consistent across spatial scales and regions, suggest the existence of repeatable ecological processes and are useful for the management of stream networks. From published data, we defined four guilds of European fish species with contrasting preferences for microhabitat hydraulics within stream reaches. At the scale of stream reaches and across 139 French sites (590 460 fishes sampled), we analysed how fish guild proportions were related to reach hydraulics (proportion of pools vs. riffles %POOL; median discharge by unit width Q50/W). The strongest correlations were observed between two fish guilds and %POOL (p < 0.001, r2 ≥ 0.41) and between one fish guild proportion and Q50/W (p < 0.001, r2 = 0.10). These reach–scale relationships were consistent across six large French basins, and consistent with the analyses made at the microhabitat scale. Therefore, microhabitat preferences for hydraulics are strong enough to generate consistent reach‐scale community responses to hydraulics across regions, despite the influence of other filters such as temperature, nutrient levels or history. The distribution of basic geomorphic features (pools, riffles) in streams and their modification (by dams, weirs and dikes) can modify the proportion of fish guilds by up to 80%, probably contributing to the long‐term decline of riffle‐dwelling species in Europe. Copyright © 2006 John Wiley & Sons, Ltd.
Neurodevelopmental outcome of preterm infants with ventricular dilatation with and without associated haemorrhage
Authors: Vollmer Brigitte , Roth Simon , Riley Katharine , Sellwood Mark W , Baudin Jenny , Neville Brian GR , Wyatt John S
Published: April 2006

This study investigated whether in preterm children who had ventricular dilatation (VD) on neonatal cranial ultrasound outcome at age 8 years was influenced by the additional presence of germinal matrix haemorrhage — intraventricular haemorrhage (GMH–IVH). Six‐hundred and ninety‐nine preterm infants (<33wks' gestation, mean 29.6wks [SD 2.1]) with either normal cranial ultrasound (n=616; 286 females, 330 males), or with VD with (n=66; 32 females, 34 males) or without (n=17; 4 females, 13 males) GMH–IVH were enrolled in the study. At age 8 years outcome was assessed in 567 (81%) of the 699 children by neurological examination, the Test of Motor Impairment (TOMI), the test of Visuo‐Motor Integration (VMI), and the Wechsler Intelligence Scales for Children. Results showed that the proportion of children with disabling impairments was higher in the group with VD and GMH–IVH. Performance on TOMI and VMI (even in those without disabling impairments) was poorer in those with VD and GMH–IVH than in children with normal scans or those with VD only. Children with VD and GMH–IVH had significantly lower performance IQ than children with normal ultrasound, whereas those with VD only were not different from those with normal scans. Results suggest the presence of subtle white matter injury that has not been identified by neonatal cranial ultrasound. Although this study did not investigate biochemical markers of haemorrhage, we hypothesize that non‐proteinbound iron is likely to be a contributing factor to white matter damage in preterm infants.
Early motor development and later language and reading skills in children at risk of familial dyslexia
Authors: Viholainen Helena , Ahonen Timo , Lyytinen Paula , Cantell Marja , LicSSc Asko Tolvanen , Lyytinen Heikki
Published: April 2006

Relationships between early motor development and language and reading skills were studied in 154 children, of whom 75 had familial risk of dyslexia (37 females, 38 males; at‐risk group) and 79 constituted a control group (32 females, 47 males). Motor development was assessed by a structured parental questionnaire during the child's first year of life. Vocabulary and inflectional morphology skills were used as early indicators of language skills at 3 years 6 months and 5 years or 5 years 6 months of age, and reading speed was used as a later indicator of reading skills at 7 years of age. The same subgroups as in our earlier study (in which the cluster analysis was described) were used in this study. The three subgroups of the control group were ‘fast motor development’, ‘slow fine motor development’, and ‘slow gross motor development’, and the two subgroups of the at‐risk group were ‘slow motor development’ and ‘fast motor development’. A significant difference was found between the development of expressive language skills. Children with familial risk of dyslexia and slow motor development had a smaller vocabulary with poorer inflectional skills than the other children. They were also slower in their reading speed at the end of the first grade at the age of 7 years. Two different associations are discussed, namely the connection between early motor development and language development, and the connection between early motor development and reading speed.
Aggravation of epilepsy by anti‐epileptic drugs
Authors: Gayatri NA , Livingston JH
Published: April 2006

The possibility that so‐called anti‐epileptic drugs (AEDs) may aggravate epilepsy must always be borne in mind by the clinician. Many reports of such aggravation of seizures have been published. Most such reports are anecdotal and speculative, and suggest that many such reactions are idiosyncratic. However, for some there is a sufficient body of evidence to suggest that some AEDs used in certain epilepsies may consistently cause worsening of seizures. Seizure aggravation may include increase in the frequency or severity of existing seizures, emergence of new types of seizure, or the occurrence of status epilepticus. The pathophysiology of seizure aggravation is poorly understood including nonspecific effects such as those associated with sedation, druginduced encephalopathy, and paradoxical or inverse pharmacodynamic effects. For some epilepsies the choice of AEDs may be inappropriate, and although the mechanism of seizure aggravation is not clear, its occurrence may be fairly predictable. This is best documented for the use of carbamazepine in idiopathic generalized and myoclonic epilepsies. Most other AEDs have been reported occasionally to cause seizure aggravation. The lowest risk of seizure aggravation appears to be with valproate. Risk factors for worsening of seizures are epileptic encephalopathy, polytherapy, high frequency of seizures, and cognitive impairment. Advances in pharmacogenomics may in the future enable such adverse effects to be predicted for individual patients. Meanwhile, a systematic approach to reporting AED‐induced seizure aggravation should be developed.
An experimental investigation of an asymmetrical turbulent wake
Authors: Palmer M. D. , Keffer J. F.
Published: April 2006

Experiments on the two-dimensional turbulent wake generated by pairs of cylinders of unequal diameter have revealed some interesting flow characteristics. The wake width grew asymmetrically in the downstream direction, spread rate and entrainment coefficients proving larger on the small diameter cylinder side. Mean velocity profiles were also skewed to this side while maximum values of Reynolds stresses were larger on the other. Close to the cylinder, a region or turbulent ‘energy reversal’ was measured. The level of turbulence and the diffusion mechanism were high at this point and some comments are made concerning the structure of the flow under these conditions.
Eigenvalue bounds in linear inviscid stability theory
Authors: Craik Alex D. D.
Published: April 2006

New eigenvalue bounds are derived for the linear stability of inviscid parallel flows, both for homogeneous and for stratified fluids. The usefulness of these bounds, as compared with that of previous results, is assessed for several examples. For homogeneous fluids the new upper bounds for the imaginary part ci of the complex phase velocity are sometimes better than previous criteria. For both homogeneous and stratified flows, the new upper bounds for the wave-number α of neutrally stable disturbances improve on previous results, giving values within 10% of the known exact solution in several cases.
Analysis of the swimming of elastic slender bodies excited by an external force
Authors: Lavie A. M.
Published: April 2006

The essential difference, from the theoretical point of view, between an externally excitedlciody and a fish is that the latter can apply lateral vibratory movements at any part of its surface, whereas in the ‘artificial fish’ lateral vibrations are applied only at the point where the external force acts on the body. A good example which illustrates how the artificial fish swims is the ‘Pod’. The Pod is a medical device consisting of a small magnet attached to a plastic ‘tail’. If the Pod is placed in a patient's blood vessel, and an alternating magnetic field is applied, the magnet oscillates angularly and the plastic tail causes it to swim. The purpose of the device is to deliver medicaments at any desired location in the circulatory system.

In this paper the theory of swimming of elastic slender bodies excited by an external force is presented. Special reference is made to the hydrodynamic forces acting on a swimming cylinder in viscous fluctuating flow. The results obtained are used in the analysis of the propulsion mechanism of the Pod.
Thermoconvective instabilities in a porous medium bounded by two concentric horizontal cylinders
Authors: Caltagirone Jean-Paul
Published: April 2006

The study of natural convection in a saturated porous medium bounded by two concentric, horizontal, isothermal cylinders reveals different types of evolution according to the experimental conditions and the geometrical configuration of the model. At small Rayleigh numbers the state of the system corresponds to a regime of pseudo-conduction. The isotherms are coaxial with the cylinders. At larger Rayleigh numbers a regime of steady two-dimensional convection sets in between the two cylinders. Finally, for Rayleigh numbers above the critical Rayleigh number Ra*c the phenomena become three-dimensional and fluctuating. The appearance of these different regimes depends, moreover, on the geometry considered and, in particular, on two numbers: R, the ratio of the radii of the cylinders, and A, the ratio of the length of the cylinders to the radius of the inner one. In order to approach these experimental observations and to obtain realistic theoretical models, several methods of solving the equations have been used.

The perturbation method yields information about the thermal field and the heat transfer between the cylinders under conditions close to the equilibrium state.

A numerical two-dimensional model enables us to extend the range of investigation and to represent properly the phenomena when steady convection appreciably modifies the temperature distribution and the velocities within the porous layer.

Neither of these models allows account to be taken of the instabilities observed experimentally above a critical Rayleigh number Ra*c. For this reason, a study of stability has been carried out using a Galerkin method based on equations corresponding to an initial state of steady convection. The results obtained show the importance of three-dimensional effects for the onset of fluctuating convection. The critical transition Rayleigh number Ra*c is thus determined in terms of the ratio of the radii R by solving an eigenvalue problem.

A numerical three-dimensional model based on the method of finite elements has thus been developed in order to point out the different types of evolution with time. Steady two-dimensional convection and fluctuating three-dimensional convection have been actually found by calculation. The solution of the system of equations by the method of finite elements is briefly described.

The experimental and theoretical results are then compared and a general physical interpretation is given.
Shock attenuation by a single transverse slit
Authors: Lee J. H. S. , Ostrowski P. P. , Wu J. H. T.
Published: April 2006

Attenuation of a plane shock due to the interaction with a single slit is examined via spark schlieren photography for shock Mach numbers between 1·17 and 2·44 and slit widths between 0·173 and 3·175 cm. Wave speed measurements by piezoelectric transducers indicate that the attenuation is weak and adequately predicted by Whitham's ray-shock theory. The slit width is observed to produce only a secondary effect on the shock attenuation rate. Stability of the attenuating shock is demonstrated from a wave diagram constructed by the ray-shock technique.
The influence of vortex shedding on the generation of sound by convected turbulence
Authors: Howe M. S.
Published: April 2006

This paper discusses the theory of the generation of sound which occurs when a frozen turbulent eddy is convected in a mean flow past an airfoil or a semi-infinite plate, with and without the application of a Kutta condition and with and without the presence of a mean vortex sheet in the wake. A sequence of two-dimensional mathematical problems involving a prototype eddy in the form of a line vortex is examined, it being argued that this constitutes the simplest realistic model. Important effects of convection are deduced which hitherto have not been revealed by analyses which assume quadrupole sources to be at rest relative to the plate or airfoil. It is concluded that, to the order of approximation to which the sound from convected turbulence near a scattering body is usually estimated, the imposition of a Kutta condition at the trailing edge leads to a complete cancellation of the sound generated when frozen turbulence convects past a semi-infinite plate, and to the cancellation of the diffraction field produced by the trailing edge in the case of an airfoil of compact chord.
Thermal convection with strongly temperature-dependent viscosity
Authors: Booker John R.
Published: April 2006

This paper experimentally investigates the heat transport and structure of convection in a high Prandtl number fluid layer whose viscosity varies by up to a factor of 300 between the boundary temperatures. An appropriate definition of the Rayleigh number R uses the viscosity at the average of the top and bottom boundary temperatures. With rigid boundaries and heating from below, the Nusselt number N normalized with the Nusselt number N0 of a constant-viscosity fluid decreases slightly as the viscosity ratio increases. The drop is 12% at a variation of 300. A slight dependence of N/N0 on R is consistent with a decrease in the exponent in the relation N ∝ Rβ from its constant-viscosity value of 0·281 to 0·25 for R [lsim ] 5 × 104. This may be correlated with a transition from three- to two-dimensional flow. At R ∼ 105 and viscosity variation of 150, the cell structure is still dominated by the horizontal wavelength of the marginally stable state. This is true with both free and rigid upper boundaries. The flow is strongly three-dimensional with a free upper boundary, while it is nearly two-dimensional with a rigid upper boundary.
Axisymmetric homogeneous turbulence: a comparison of direct spectral simulations with the direct-interaction approximation
Authors: Schumann U. , Herring J. R.
Published: April 2006

The return to isotropy of homogeneous axisymmetric turbulence at Reλ ≈ 28 is studied by means of the direct spectral simulation (DSS) technique of Orszag & Patterson (1972) and the direct-interaction approximation (DIA) of Kraichnan (1959) as implemented by Herring (1974). The results of the two methods are compared for different initial degrees of anisotropy. The general agreement between the methods is good. Most of the discrepancies can be attributed to present technical limitations in implementing both schemes. The DSS has been found to be superior for strong anisotropies, because the numerical method used for solving the DIA equations is limited in its angular resolution. For small anisotropies the angular anisotropy becomes less important and the DIA results are accurate; in this case the DIA seems to be superior as it is free from the statistical uncertainties inherent in the DSS method. With respect to a return-to-isotropy study these statistical errors are large, in particular for small anisotropies. The physical interpretation of the angular energy distribution is discussed also. The numerical results and theoretical considerations for the DIA equations show that one should retain angular moments at least up to the fourth in order to obtain accurate values of the Rotta constant at moderate anisotropies.
Migration of rigid spheres in a two-dimensional unidirectional shear flow of a second-order fluid
Authors: Ho B. P. , Leal L. G.
Published: April 2006

The lateral migration of a neutrally buoyant rigid sphere suspended in a second-order fluid is studied theoretically for unidirectional two-dimensional flows. The results demonstrate the existence of migration induced by normal stresses whenever there is a lateral variation of the shear rate in the undisturbed flow. The migration occurs in the direction of decreasing absolute shear rate, which is towards the centre-line for a plane Poiseuille flow and towards the outer cylinder wall for Couette flow. The direction of migration agrees with existing experimental data for a viscoelastic suspending fluid, and qualitative agreement is found between the theoretically predicted and experimentally measured sphere trajectories.
Hydrodynamic interaction between gas bubbles in liquid
Authors: Wijngaarden L. Van , Jeffrey D. J.
Published: April 2006

A calculation is given of the velocity which a cloud of identical gas bubbles acquires when the liquid in which the cloud is immersed is impulsively accelerated. From the results an expression follows for the effective virtual mass of a bubble in a gas-bubble/liquid mixture. Further consideration is given to that part of the momentum flux in the mixture associated with relative motion between liquid and bubbles. An expression for this quantity is derived which appears to differ from the one used in practice. It is shown that qualitative support for the expression obtained here is provided by experimental observations reported in the literature.
Bispectral measurements in turbulence
Authors: Lii K. S. , Rosenblatt M. , Atta C. Van
Published: April 2006

A bispectral analysis of high Reynolds number turbulent velocity-derivative data is carried out. The computations suggest that contributions of wavenumber triplets to the rate of vorticity production and spectral transfer are non-local in wavenumber space and comparable over the whole range of wavenumbers studied. Statistical resolvability of the bispectral estimates is obtained. An appendix on the asymptotic behaviour of bispectral estimates is given.
On the variation and growth of wave-slope spectra in the capillary-gravity range with increasing wind
Authors: Long Steven R. , Huang Norden E.
Published: April 2006

A new laser device has been used to make direct wave-slope measurements in the capillary-gravity range. Owing to the design principles, the digital nature of the system and the use of a laser beam as a probe, the earlier problems of intensity variations and meniscus effects were avoided. Using this new technique, wave-slope spectra both down and across the channel were obtained for different wind conditions, along with corresponding mean-square slope values. Comparisons are made with existing data. The results indicate that a quasi-equilibrium state may exist for each wind speed and that it increases in intensity with increasing wind, which may imply an asymptotic nature for the equilibrium-range coefficientCaa. From the data, two significant frictional velocities, 17.5 and 31 cm/s respectively, are identified as critical values for different ranges of wave development.
Resonant oscillations in closed tubes: the solution of Chester's equation
Authors: Keller Jakob
Published: April 2006

A closed tube is considered in which the oscillations of a gas column are driven by the sinusoidal motion of a piston. The case where the frequency of the gas column in the tube lies near one of its resonant frequencies is of special interest. The aim of this paper is to extend the theory of Chester (1964), who has given solutions in the inviscid case and for very small boundary-layer friction, to cases of frictional effects of arbitrary strength. This is done by means of a combination of analytical and numerical methods. Different methods are applied for different strengths of the boundary-layer friction. The cases where the influence of the Stokes boundary layer is either very strong or very weak are not especially difficult to treat. The main part of this paper considers cases of intermediate friction, i.e. when the shock strength has grown rather small owing to the influence of the Stokes boundary layer. To obtain an overall view of the phenomena which occur in the Merent regions, a number of solutions have been calculated.
Stability of slowly diverging jet flow
Authors: Crighton D. G. , Gaster M.
Published: April 2006

Coherent axisymmetric structures in a turbulent jet are modelled as linear instability modes of the mean velocity profile, regarded as the profile of a, fictitious laminar inviscid flow. The usual multiple-scales expansion method is used in conjunction with a family of profiles consistent with similarity laws for the initial mixing region and approximating the profiles measured by Crow & Champagne (1971), Moore (1977) and other investigators, to deal with the effects of flow divergence. The downstream growth and approach to peak amplitude of axisymmetric wave modes with prescribed real frequency is calculated numerically, and comparisons are made with various sets of experimental data. Excellent agreement is found with the wavelength measurements of Crow & Champagne. Quantities such as the amplitude gain which depend on cumulative effects are less well predicted, though the agreement is still quite tolerable in view of the facts that this simple linear model of slowly diverging flow is being applied far outside its range of strict validity and that many of the published measurements are significantly contaminated by nonlinear effects. The predictions show that substantial variations are to be expected in such quantities as the phase speed and growth rate, according to the flow signal (velocity, pressure, etc.) measured, and that these variations depend not only on the axial measurement location but also on the cross-stream position. Trends of this kind help to explain differences in, for example, the preferred Strouhal number found by investigators using hot wires or pressure probes on the centre-line, in the mixing layer or in the near field.
On over-reflexion
Authors: Acheson D. J.
Published: April 2006

Reflexion coefficients greater than unity have now been predicted for a variety of different systems involving waves propagating towards a shear layer, but almost invariably only in regions of parameter space for which the layer exhibits Kelvin-Helmholtz instability. This paper contains a study of two examples in which, for appropriate parameter values, there are no such instabilities to obscure (or even prevent) the ‘over-reflexion’ of an incident wave, namely(a)hydro-magnetic internal gravity waves meeting a vortex-current sheet in a stratified fluid and(b)magneto-acoustic waves meeting a vortex sheet in a compressible fluid. In the former case the energetic aspects of over-reflexion are examined in detail, thus displaying the way in which the excess reflected energy is extracted from the mean motion and the sense in which the transmitted wave may be viewed, by analogy with certain concepts employed in plasma physics, as a carrier of so-called ‘negative energy’.
The initial dispersion of contaminant in Poiseuille flow and the smoothing of the snout
Authors: Chatwin P. C.
Published: April 2006

In Poiseuille flow in a circular tube passive contaminant initially spread uniformly over the cross-section would be pulled out in a paraboloidal snout in the absence of any diffusive mechanism, and there would be a discontinuity in $\overline{C}$, the mean concentration over the cross-section, associated with the contaminant at the front of the snout. In reality molecular diffusion smooths out this snout in two ways: direct longitudinal diffusion and the interaction between lateral diffusion and advection. The effect of these two mechanisms is discussed, and determined for small values of κt/a2, where t is the time since injection, κ is the molecular diffusivity and a is the tube radius. For such values, important in many applications, the tube walls play no part in the smoothing process. It is shown that for $\kappa t/a^2 < 0.25(\overline{u}a/\kappa)^{-\frac{2}{3}}$, where $\overline{u}$ is the discharge velocity, the effect of longitudinal diffusion dominates over that of the interaction, which is, in turn, dominant for $\kappa t/a^2 > 2.5(\overline{u}a/\kappa)^{-\frac{2}{3}}$, when $\overline{C}$ is close to the form described by Lighthill (1966).
Unsteady evolution of a horizontal jet in a stratified fluid
Authors: Peyret R.
Published: April 2006

The unsteady laminar flow due to the penetration of a horizontal jet of constant density into a stratified fluid is considered. A numerical solution of the Navier–Stokes equations under the Boussinesq approximation is obtained by means of an implicit finite-difference method. Results for different values of the Reynolds and internal Froude numbers are given and discussed.
Two-dimensional turbulence above topography
Authors: Bretherton Francis P. , Haidvogel Dale B.
Published: April 2006

In a turbulent two-dimensional flow enstrophy systematically cascades to very small scales, at which it is dissipated. The kinetic energy, on the other hand, remains at large scales and the total kinetic energy is constant. Above random topography an initially turbulent flow tends to a steady state with streamlines parallel to contours of constant depth, anticyclonic around a bump. A numerical experiment verifies this prediction. In a closed basin on a beta-plane the solution with minimum enstrophy implies a westward flow in the interior, returning in narrow boundary layers to the north and south. This result is interpreted using a parameterization of the effects of the eddies on the large-scale flow. The numerical solution is in qualitative agreement, but corresponds to a minimum of a more complex measure of the total enstrophy than the usual quadratic integral.
Spatially decaying turbulence and its relation to mixing across density interfaces
Authors: Hopfinger E. J. , Toly J.-A.
Published: April 2006

The turbulence generated by a vertically oscillating grid in a water tank and the entrainment across a salinity interface caused by this turbulence have been investigated experimentally. Measurements were carried out in a homogeneous layer of fluid as well as a two-layered fluid, which permitted us to determine the decay law of this turbulence and the way in which the structure of the turbulence depends on the mesh size and on the frequency and amplitude of the grid oscillation. It was found that the turbulent kinetic energy decays with distance from the grid according to a power law$\overline{q^2}\propto z^{-n}$, withnclose to 2, and that the turbulent Reynolds number remains approximately constant during decay. The linear dependence of the r.m.s. turbulent velocity on the grid oscillation frequency found by Thompson & Turner (1975) in the case of a square-bar grid has been confirmed. It is shown here that this linear relation remains valid when an interface is present and consequently the dependence of the entrainment velocity on the local Richardson number is of the form$u_e/u \propto Ri^{-\frac{3}{2}}$, the Péclet number being high. While the bearing of these results on the problem of the thermocline or an inversion is clear we wish to emphasize that the spatial decay of turbulence is interesting in itself.
A derivation of equations for wave propagation in water of variable depth
Authors: Green A. E. , Naghdi P. M.
Published: April 2006

Within the scope of the three-dimensional theory of homogeneous incompressible inviscid fluids, this paper contains a derivation of a system of equations for propagation of waves in water of variable depth. The derivation is effected by means of the incompressibility condition, the energy equation, the invariance requirements under superposed rigid-body motions, together with a single approximation for the (three-dimensional) velocity field.
Turbulent flow in a rectangular duct
Authors: Melling A. , Whitelaw J. H.
Published: April 2006

A detailed experimental study of developing turbulent flow in a rectangular duct was made using a laser-Doppler anemometer. The purposes of the work were to obtain data of value to fluid mechanicists, particularly those interested in the development and testing of mathematical turbulence models, and to evaluate the performance of the anemometer. For the first purpose, contours of axial mean velocity and turbulence intensity were measured in the developing flow, and all three mean velocity components and five of the six Reynolds stresses were obtained in the nearly fully developed flow.

The symmetry of the present flow appears to be better than that of previous measurements and the range of measurements is more extensive. In addition, the laser-Doppler anemometer has the potential advantage, particularly in the measurement of secondary velocities, of avoiding probe interference.
The second approximation to mass transport in cnoidal waves
Authors: Isaacson Michael De St Q.
Published: April 2006

A second approximation is developed for the mass-transport velocity within the bottom boundary layer of cnoidal waves progressing over a smooth horizontal bed. Mass-transport profiles through the boundary layer are obtained by considering terms of up to third order in the perturbation parameter. A comparison with results based on a first approximation indicates that the effect of the third-order terms is to predict a smaller mass-transport velocity and that this difference is generally significant, particularly for waves extending to the intermediate depth range. The predicted correction to the first approximation is qualitatively supported by experimental evidence.
The mixing layer at high Reynolds number: large-structure dynamics and entrainment
Authors: Dimotakis Paul E. , Brown Garry L.
Published: April 2006

A turbulent mixing layer in a water channel was observed at Reynolds numbers up to 3 × 106. Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observation of the reaction of a base and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using a laser-Doppler velocimeter (LDV) revealed a fundamental periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing-layer dynamics at any point are coupled to the large structure further downstream, and some possible consequences regarding the effects of initial conditions and of the influence of apparatus geometry are discussed.
Spin-up from a rotating disk
Authors: Chawla S. S.
Published: April 2006

An analytical–numerical analysis of the fluid motion induced by an impulsive change in the angular velocity of a rotating disk is presented. Separate solutions are derived for the early development and for the approach to the ultimate state. The superposed flow is found to have variations on two length scales, and is characterized by the generation and propagation of shear-type wave trains. These decaying wave trains ultimately lead to the new von Kármán state corresponding to the changed angular velocity of the disk.
Separation from the surface of two equal spheres in Stokes flow
Authors: Davis A. M. J. , O'Neill M. E. , Dorrepaal J. M. , Ranger K. B.
Published: April 2006

In this paper, it is shown that if two spheres of equal radii are placed axisymmetrically in a steady Stokes stream, separation of the flow from the spheres occurs if the distance between their centres is less than approximately 3-67 times the sphere radius. For spheres whose spacing is less than this value, wakes form on both spheres and the fluid within the wakes moves in closed eddy type motion. When the distance between the centres of the spheres is less than approximately 3.22 times the sphere radius, a cylinder of fluid links both spheres, and within this cylinder the fluid rotates in one or more ring vortices, the number of vortices increasing as the distance between the spheres is decreased. When the spheres are in contact, the fluid rotates in an infinite set of nested ring vortices.
On the spin-up and spin-down of a rotating fluid. Part 2. Measurements and stability
Authors: Weidman Patrick D.
Published: April 2006

Measurements of the azimuthal velocity inside a cylinder which spins up or spins down at constant acceleration were obtained with a laser-Doppler velocimeter and compared with the theoretical results presented in part 1. Velocity profiles near the wave front in spin-up indicate that the velocity discontinuity given by the inviscid Wedemeyer model is smoothed out in a shear layer whose thickness varies with radius and time but scales with hE1/4Ω. The spin-down profiles are always in excellent agreement with theory when the flow is stable. Visualization studies with aluminium tracers have made possible the determination of the stability boundary for Ekman spiral waves (principally type II waves) observed on the cylinder end walls during spin-up. For spin-down to rest the flow always experienced a centrifugal instability which ultimately disrupted the interior fluid motion.
Flow around a thin body moving in shallow water.
Authors: Mei Chiang C.
Published: April 2006

Approximations for shallow-water ship waves are sought which are valid near the critical speed U = (gh)1/2. For sufficiently thin bodies (struts or ships) the governing equation is dispersive. Simple analytic solutions are given which are valid for all F2 ≤ O(1). As the thickness increases, nonlinearity also enters. A soliton solution is discussed which applies to a sharp-nosed half-body at slightly supercritical speed.
On the separation of air flow over water waves
Authors: Banner M. L. , Melville W. K.
Published: April 2006

Conditions leading to the onset of air-flow separation over a mobile air-water interface are discussed. It is argued that, in a frame of reference in which the interfacial boundary assumes a steady shape, the occurrence of separation requires a stagnation point on the interface. In the case of air blowing over water waves, this corresponds to the onset of wave breaking. These arguments are strongly supported by flow visualization and pressure measurements carried out in a laboratory wind-wave flume. Furthermore, the pressure measurements show a greatly enhanced interfacial shear stress for a breaking wave compared with that over an unbroken wave of the same wavelength. The implications of these findings for wind-wave generation are discussed.
Note on the long-wave limit of the virtual-mass coefficient for a half-immersed circular cylinder heaving on water of finite depth
Authors: Rhodes-Robinson P. F.
Published: April 2006

This note provides numerical values for the long-wave limit of the virtual-mass coefficient relating to the heaving motion of a half-immersed circular cylinder on water of finite depth, found analytically by Ursell in the preceding paper; some preliminary analysis is needed, however.
Rimming flow of liquid in a rotating cylinder
Authors: Ruschak K. J. , Scriven L. E.
Published: April 2006

Steady two-dimensional flow of Newtonian liquid in a layer around the inside of a rotating horizontal cylinder is analysed as a regular perturbation from rigid-body motion. The equations governing the first perturbation are solved in closed form. Parameter limits are taken in order to elucidate the flow structure and to provide simpler working formulae. The limiting cases are for small Reynolds numbers, which resembles viscous film flow down a curved wall; for large Reynolds numbers, which involves a periodic boundary layer; and for small ratios of average film thickness to cylinder radius. In every case the maximum film thickness occurs in the upper quadrant on the rising side of the cylinder and the minimum thickness is diametrically opposite.
A note on static and dynamic calibration of constant-temperature hot-wire probes
Authors: Bruun H. H.
Published: April 2006

This note describes a comparative investigation of static and dynamic calibration procedures for standard hot-wire probes. It is demonstrated that nearly the same sensitivity dE/dV can be obtained by both procedures. The discrepancy reported by Perry & Morrison (1971) is shown to be due mainly to a poor approximation of static calibration data over a large velocity range by a constant-exponent power-law function.
Experiments on collisions between solitary waves
Authors: Maxworthy T.
Published: April 2006

Experiments on ‘head-on’ collisions between two solitary waves show that the waves reach a maximum amplitude greater than twice the initial wave amplitude and that they suffer a time delay during their interaction. These results are compared with available theories and found to be in qualitative but not quantitative agreement.
Constitutive equations in suspension mechanics. Part 2. Approximate forms for a suspension of rigid particles affected by Brownian rotations
Authors: Hinch E. J. , Leal L. G.
Published: April 2006

Approximate constitutive equations are derived for a dilute suspension of rigid spheroidal particles with Brownian rotations, and the behaviour of the approximations is explored in various flows. Following the suggestion made in the general formulation in part 1, the approximations take the form of Hand's (1962) fluid model, in which the anisotropic microstructure is described by a single second-order tensor. Limiting forms of the exact constitutive equations are derived for weak flows and for a class of strong flows. In both limits the microstructure is shown to be entirely described by a second-order tensor. The proposed approximations are simple interpolations between the limiting forms of the exact equations. Predictions from the exact and approximate constitutive equations are compared for a variety of flows, including some which are not in the class of strong flows analysed.
Hydromechanics of lunate-tail swimming propulsion. Part 2
Authors: Chopra M. G. , Kambe T.
Published: April 2006

This paper investigates the propulsive performance of the lunate tails of aquatic animals achieving high propulsive efficiency (the hydromechanical efficiency being defined as the ratio of the work done by the mean forward thrust to the mean rate at which work is done by the tail movements on the surrounding fluid). Small amplitude heaving and pitching motions of a finite flat-plate wing of general planform with a rounded leading edge and a sharp trailing edge are considered. This is a generalization of Chopra's (1974) work on model rectangular tails. This motion characterizes vertical oscillations of the horizontal tail flukes of some cetacean mammals. The same oscillations, turned through a right angle to become horizontal motions of side-slip and yaw, characterize the caudal fins of certain fast-swimming fishes; viz. wahoo, tunny, wavyback skipjack, etc., from the Percomorphi and whale shark, porbeagle, etc., from the Selachii. Davies’ (1963, 1976) method of finding the loading distribution on the wing and generalized force coefficients, through approximate solution of an integral equation relating the loading and the upwash (lifting-surface theory), is used to find the total thrust and the rate of working of the tail, which in turn specify the hydromechanical swimming performance of the animals. The physical parameters concerned are the tail aspect ratio ((span)2/planform area), the reduced frequency (angular frequency x typical length/forward speed), the feathering parameter (the ratio of the tail slope to the slope of the path of the pitching axis), the position of the pitching axis, and the curved shapes of the leading and trailing edges. The variation of the thrust and the propulsive efficiency with these parameters has been discussed to indicate the optimum shape of the tail. It is found that, compared with a rectangular tail, a curved leading edge as in lunate tails gives a reduced thrust contribution from the leading-edge suction for the same total thrust; however, a sweep angle of the leading edge exceeding about 30° leads to a marked reduction of efficiency. Another implication of the present analysis is that no negative work is involved in the actual oscillation of the tail.

The present results are used to obtain an estimate of the drag coefficient for the motion of the animals, based on observed data and the computed thrust. The results show some evidence of differences between the CD's for cetacean mammals and scombroid fish respectively. Some discussion of this difference is also given.
Obliquely interacting solitary waves
Authors: Miles John W.
Published: April 2006

Nonlinear oblique interactions between two slightly dispersive gravity waves (in particular, solitary waves) of dimensionless amplitudes α1 and α2 (relative to depth) and relative inclination 2ϕ (between wave normals) are classified as weak if sin2ϕ α1,2 or strong if ϕ2 = O(α1,2). Weak interactions permit superposition of the individual solutions of the Korteweg-de Vries equation in first approximation; the interaction term, which is O(α1α2), then is determined from these basic solutions.

Strong interactions are intrinsically nonlinear. It is shown that these interactions are phase-conserving (the sum of the phases of the incoming waves is equal to the sum of the phases of the outgoing waves) if |α2-α1 > (2ϕ)2 but not if |α2-α1| (2ϕ)2 (e.g. the reflexion problem, for which the interacting waves are images and α2 = α1). It also is shown that the interactions are singular, in the sense that regular incoming waves with sech2 profiles yield singular outgoing waves with - csch2 profiles, if
\[
\psi_{-}< |\psi| < \psi_{+},\quad{\rm where}\quad\psi_{\pm}={\textstyle\frac{1}{2}}\left|(3\alpha_2)^{\frac{1}{2}}\pm(3\alpha_1)^{\frac{1}{2}}\right|.
\]

Regular interactions appear to be impossible within this singular regime, and its end points, |ϕ| = ϕ±, are associated with resonant interactions.
On the excitation of edge waves on beaches
Authors: Minzoni A. A. , Whitham G. B.
Published: April 2006

The excitation of standing edge waves of frequency ½ω by a normally incident wave train of frequency ω has been discussed previously (Guza & Davis 1974; Guza & Inman 1975; Guza & Bowen 1976) on the basis of shallow-water theory. Here the problem is formulated in the full water-wave theory without making the shallow-water approximation and solved for beach angles β = π/2N, where N is an integer. The work confirms the shallow-water results in the limit N [Gt ] 1, shows the effect of larger beach angles and allows a more complete discussion of some aspects of the problem.
On a theory of amplitude vacillation in baroclinic waves
Authors: Smith R. K.
Published: April 2006

This study contributes to the theory of amplitude vacillation for finite amplitude baroclinic waves in a two-layer, quasi-geostrophic, zonal flow as worked out by Pedlosky. In a recent paper the author has shown that Pedlosky omitted a certain side-wall boundary condition on the mean zonal flow. The neglect of this boundary condition results in an unspecified energy source at the side-wall boundaries and the physical problem is incorrectly posed.

In this paper, Pedlosky's analysis is repeated but with the side-wall boundary condition included. It is shown that the side-wall energy source is negligible only when the zonal wavenumber of the disturbance is large compared with the meridional wavenumber, and not otherwise. Moreover, the energy conversions to and from mean zonal kinetic energy corresponding to Pedlosky's calculations and those given here have essential differences, although for fixed meridional wavenumber, these differences become less pronounced as the zonal wavenumber increases.

It is also shown that, when the side-wall condition is included, the mean flow distortion associated with the wave is different in structure to that which occurs when the condition is omitted. However, as the total disturbance wavenumber a increases, the influence of the side wall on the mean flow structure is confined to a boundary layer of width comparable with the internal deformation radius 2½a−1.

Even when the deformation radius is comparable with the channel width, the conclusions of Pedlosky (1972) concerning the existence of stable periodic solutions are correct, providing viscous effects are vanishingly small, and the criteria for stability of the steady solutions obtained herein are not significantly different from those given by Pedlosky. In this viscous regime, we have also studied the evolution to limit-cycle solutions.

Evolution in the case where viscous effects are small on the time scale for the initial growth of an incipient wave, but not vanishingly small, will be discussed in a subsequent paper.
Hypersonic flow with attached shock waves over plane delta wings
Authors: Woods B. A. , Mcintosh C. B. G.
Published: April 2006

A new form is given for the general solution to the thin-shock-layer equations for the flow over a nearly plane delta wing. Using this, the solution described conjecturally by Hayes & Probstein for symmetrical flow with attached shock waves over a plane delta wing is realized numerically. The flow construction devised for this purpose is applied also to yawed flows. The solutions obtained are found to agree moderately well with the results of numerical calculations from the full equations, but contain a number of anomalous features characteristic of the thin-shock-layer approximation.
Thermal convection in a Hele-Shaw cell
Authors: Hartline Beverly K. , Lister C. R. B.
Published: April 2006

We derive the Rayleigh number RHS for thermal convection in a Hele-Shaw cell with gap width d and full width (gap plus walls) Y. For the state of marginal stability, the system of equations is found to be formally identical to that describing flow through a uniform porous medium, if d3/12Y is identified as the Hele-Shaw permeability. Thus Lapwood's (1948) thermal-instability analysis should apply, and the critical Rayleigh number should be 4π2 when the cell has impermeable isothermal boundaries.

Baker's (1966) pH-indicator method for visualizing fluid flow has been adapted for use in a Hele-Shaw cell. In addition to revealing the convection pattern clearly, this technique proves to be an especially sensitive detector of incipient flow, and a highly accurate means of verifying the onset of convection. Our experiments confirm that the critical Hele-Shaw Rayleigh number is 40 ± 2, thereby validating our theoretically derived expression for the Rayleigh number. We also measure the vertical flow velocity wm and find that wm∝ (R2HS−402)½ closely fits our data for 40 < RHS < 140.