bluuebunny/crossref_metadata_2025 huggingface dataset:
Tensile deformation of electrospun nylon‐6,6 nanofibers
Authors: Zussman E. , Burman M. , Yarin A. L. , Khalfin R. , Cohen Y.
Published: April 2006

Nylon‐6,6 nanofibers were electrospun at an elongation rate of the order of 1000 s−1 and a cross‐sectional area reduction of the order of 0.33 × 105. The influence of these process peculiarities on the intrinsic structure and mechanical properties of the electrospun nanofibers is studied in the present work. Individual electrospun nanofibers with an average diameter of 550 nm were collected at take‐up velocities of 5 and 20 m/s and subsequently tested to assess their overall stress–strain characteristics; the testing included an evaluation of Young's modulus and the nanofibers' mechanical strength. The results for the as‐spun nanofibers were compared to the stress–strain characteristics of the melt‐extruded microfibers, which underwent postprocessing. For the nanofibers that were collected at 5 m/s the average elongation‐at‐break was 66%, the mechanical strength was 110 MPa, and Young's modulus was 453 MPa, for take‐up velocity of 20 m/s—61%, 150 and 950 MPa, respectively. The nanofibers displayed α‐crystalline phase (with triclinic cell structure). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1482–1489, 2006
On Killing tensors and cubic vertices in higher‐spin gauge theories
Authors: Bekaert X. , Boulanger N. , Cnockaert S. , Leclercq S.
Published: April 2006

The problem of determining all consistent non‐Abelian local interactions is reviewed in flat space‐time. The antifield‐BRST formulation of the free theory is an efficient tool to address this problem. Firstly, it allows to compute all on‐shell local Killing tensor fields, which are important because of their deep relationship with higher‐spin algebras. Secondly, under the sole assumptions of locality and Poincaré invariance, all non‐trivial consistent deformations of a sum of spin‐three quadratic actions deforming the Abelian gauge algebra were determined. They are compared with lower‐spin cases.
Black holes, entropy and string theory
Authors: Lopes Cardoso G.
Published: April 2006

I review insights from supergravity and string theory on the macroscopic and the microscopic description of black hole entropy.
Unitarity in the presence of closed timelike curves
Authors: Costa M.S.
Published: April 2006

We conjecture that, in certain cases, quantum dynamics is consistent in the presence of closed timelike curves. We consider time dependent orbifolds of three dimensional Minkowski space describing, in the limit of large AdS radius, BTZ black holes inside the horizon. Although perturbative unitarity fails, we show that, for discrete values of the gravitational coupling, particle propagation is consistent with unitarity. This quantization corresponds to the quantization of the black hole angular momentum. We perform the computation at very low energies, where string effects are irrelevant and interactions are dominated by graviton exchange in the eikonal regime.
Electric tachyon inflation
Authors: Cremades D.
Published: April 2006

We propose that under certain conditions the universal open string tachyon can drive topological inflation in moduli stabilised frameworks. Namely, the presence of electric field in the world volume of the D‐brane can slow down its decay leading to a phenomenological model of inflation. The conditions for inflation to take place are difficult to satisfy in the standard warped deformed conifold but easier to realise in other geometries.
Non‐commutative moduli spaces of topological D‐branes
Authors: Lazaroiu C.I.
Published: April 2006

We give a general construction of extended moduli spaces of topological D‐branes as non‐commutative algebraic varieties. This shows that noncommutative symplectic geometry in the sense of Kontsevich arises naturally in String Theory.
Non‐critical superstrings and supersymmetric QCD
Authors: Niarchos V.
Published: April 2006

In the present note we review the salient features of D‐brane physics in a specific four‐dimensional non‐critical superstring theory which involves the 𝒩 = 2 SL(2)/U(1) Kazama‐Suzuki model at level 1. We pay special attention to a configuration of D3‐ and D5‐ branes that realizes 𝒩 = 1 supersymmetric QCD and briefly discuss some implications of this construction for the (non)AdS/(non)CFT correspondence in the context of non‐critical superstring theory.
Moduli stabilization in toroidal type IIB orientifolds
Authors: Reffert S. , Scheidegger E.
Published: April 2006

We discuss the first step in the moduli stabilization program a la KKLT for a general class of resolved toroidal type IIB orientifolds. In particular, we discuss their geometry, the topology of the divisors relevant for the D3–brane instantons which can contribute to the superpotential, and some non–trivial aspects of the orientifold action.
Non‐Abelian tensor gauge fields and higher‐spin extension of standard model
Authors: Savvidy G.
Published: April 2006

We suggest an extension of the gauge principle which includes non‐Abelian tensor gauge fields. The invariant Lagrangian is quadratic in the field strength tensors and describes interaction of charged tensor gauge bosons of arbitrary large integer spin 1,2,⋖. Non‐Abelian tensor gauge fields can be viewed as a unique gauge field with values in the infinite‐dimensional current algebra associated with compact Lie group. The full Lagrangian exhibits also enhanced local gauge invariance with double number of gauge parameters which allows to eliminate all negative norm states of the nonsymmetric second‐rank tensor gauge field, which describes therefore two polarizations of helicity‐two massless charged tensor gauge boson and the helicity‐zero “axion” The geometrical interpretation of the enhanced gauge symmetry with double number of gauge parameters is not yet known.
On supersymmetry and other properties of a class of marginally deformed backgrounds
Authors: Hernández R. , Sfetsos K. , Zoakos D.
Published: April 2006

We summarize our recent work on supergravity backgrounds dual to part of the Coulomb branch of 𝒩 = 1 theories constructed as marginal deformations of 𝒩 = 4 Yang–Mills. In particular, we present a summary of the behaviour of the heavy quark‐antiquark potential which shows confining behaviour in the IR as well as of the spectrum of the wave equation. The reduced supersymmetry is due to the implementation of T‐duality in the construction of the deformed supergravity solutions. As a new result we analyze and explicitly solve the Killing spinor equations of the 𝒩 = 1 background in the superconformal limit.
Supergravity gaugings and moduli superpotentials
Authors: Derendinger J.‐P.
Published: April 2006

These lecture notes describe the method of N = 4 supergravity gaugings used as a four‐dimensional effective Lagrangian description of the moduli superpotentials generated by superstring vacua with fluxes.
Interdomain mobility in di‐ubiquitin revealed by NMR
Authors: Ryabov Yaroslav , Fushman David
Published: April 2006

Domain orientation and dynamics can play an essential role in the function of multidomain proteins. Lys48‐linked polyubiquitin chains, the principal signal for proteasomal protein degradation, adopt a closed conformation at physiological conditions, in which the functionally important residues Leu8, Ile44, and Val70 are sequestered at the interdomain interface. This interface must open in order for these groups to become available for interactions with various chain‐recognition factors. Knowledge of the mechanism of domain motion leading to the opening of the interdomain interface in polyubiqutin is, therefore, essential for the understanding of the processes controlling molecular recognition events in polyubiquitin signaling. Here we use NMR to characterize the interdomain dynamics that open the interface in a di‐ubiquitin chain. This process occurs via domain reorientations on a 10‐ns time scale and with the amplitudes that are sufficient for making functionally important hydrophobic residues in polyubiquitin available for direct interactions with various ubiquitin‐binding factors. The analysis revealed the structures of the interconverting conformational states of di‐ubiquitin and the rates and amplitudes of this process at near‐physiological and acidic pH. The proposed mechanism of domain reorientation is quite general and could serve as a paradigm of interdomain mobility in other multidomain systems. Proteins 2006. © 2006 Wiley‐Liss, Inc.
Fabrication of a transparent p–n heterojunction thin film diode composed of p‐CuAlO2/n‐ZnO
Authors: Kim Dae‐Sung , Park Tae‐Jin , Kim Dae‐Hyun , Choi Se‐Young
Published: April 2006

An all‐oxide transparent p–n heterojunction on a glass substrate was fabricated. The structure of the diode was ITO electrode/p‐CuAlO2/n‐ZnO/In electrode on the glass substrate. The p‐CuAlO2 thin film was deposited by e‐beam evaporation, which was annealed by the wet‐oxidation method. The p–n heterojunction thin film diode showed rectifying current–voltage characteristics, dominated in forward bias by the flow of space‐charge‐limited current. The ratio of forward current to the reverse current exceeded 40 within the range of applied voltages of –4.0 to +4.0 V and the turn‐on voltage was 0.3 V. Optical transmission of the diode was about 40% in the visible range. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ecology of an estuarine mysid shrimp in the Columbia River (USA)
Authors: Haskell Craig A. , Stanford Jack A.
Published: April 2006

The estuarine mysid, Neomysis mercedis, has colonized John Day and other run‐of‐the‐river Reservoirs of the Columbia River, over 400 km from the estuary. In John Day Reservoir N. mercedis numbers peaked (2 m−3) in August in areas near the dam in association with lower water velocity and softer bottom than at the upstream sampling sites. Neomysis broods were primarily released in late spring and early fall. Gut content analysis showed that Neomysis feeds mostly on cladoceran zooplankton and rotifers in John Day Reservoir. Diel vertical migration was documented, with daytime distribution restricted to the bottom and preferentially to the soft‐textured sediments in the deepest areas. Common pelagic fishes in the reservoir, especially juvenile American shad (Alosa sapidissima) and chinook salmon (Oncorhynchus tshawytscha), are daytime zooplankton feeders that cannot prey on Neomysis owing to mysid diel vertical migration. Thus, Neomysis has become an important food web component in John Day Reservoir. We also collected N. mercedis further upstream in Lower Granite Reservoir, where another estuarine crustacean, Corophium salmonis, also is reported, underscoring the need to better understand the role of these estuarine invertebrates in the trophic ecology of the Columbia River. Copyright © 2006 John Wiley & Sons, Ltd.
Social adjustment of children with cerebral palsy in mainstream classes: peer perception
Authors: Nadeau Line , Tessier Réjean
Published: April 2006

The aim of this study was to describe the social experience of children with cerebral palsy (CP) in mainstream classes in Canada and compare it with that of their classmates without disability. The CP group included 25 females and 35 males (mean age 10y 5mo [SD 0.95], range 10y 4mo–10y 10mo) diagnosed as having hemiplegia (n=44) or diplegia (n=16) and classified as Level I on the Gross Motor Function Classification System (GMFCS). Fifty‐seven comparison children, born at term and without any motor and/or sensory impairment, were recruited from the classes of the children with CP during a school visit (mean age 10y 3mo, [SD 1.0], range 10y–10y 6mo). They were matched to children with CP for sex, age, parents' education level, and family income. Social adjustment measures (social status, reciprocated friendships, social isolation, aggression, sociability/‐leadership, and verbal and/or physical victimization) were obtained by conducting a class‐wide sociometric interview (n=943) in the classes of the children with CP. Findings showed that children with CP (specifically females with CP and irrespective of their type of disability) had fewer reciprocated friendships, exhibited fewer sociable/leadership behaviours, and were more isolated and victimized by their peers than their classmates without a disability. This seems to suggest that females and males with CP are perceived differently from their peers in a mainstreaming context. The discussion addresses the issue of age‐ and sex‐related differences and provides avenues of intervention relating to personal and environmental factors that could facilitate or interfere with the social experience of children with CP in a mainstream environment.
Medical, surgical, and health outcomes of gastrostomy feeding
Authors: Craig GM , Carr LJ , Cass H , Hastings RP , Lawson M , Reilly S , Ryan M , Townsend J , Spitz L
Published: April 2006

A prospective controlled study with repeated measures before and after surgery examined the medical, surgical, and health outcomes of gastrostomy for children with disabilities at a tertiary paediatric referral centre in the North Thames area, UK. Anthropometric measures included weight, mid‐upperarm and head circumference. Five‐day prospective food diaries were completed and data on physical health and surgical outcomes recorded. Seventy‐six children participated and underwent gastrostomy (44 males, 32 females; median age 3y 4mo, range 4mo–17y 5mo), and 35/76 required an anti‐reflux procedure. Categories of disability were: cerebral palsy (32/76), syndrome of chromosomal or other genetic origin (25/76), slowly progressive degenerative disease (11/76), and unconfirmed diagnosis (8/76). Most children had gross motor difficulties (99%) and were non‐ambulant (83%). Oromotor problems were identified in 78% of children, 69% aspirated, and 65% were fed nasogastrically before surgery. The mean weight before surgery was –2.84 standard deviation score (SDS; SD 2.21, range –9.8 to 3.4). Two‐thirds of children achieved catchup growth postoperatively: weight‐for‐age (mean difference 0.51 SDS, 95% CI 0.23–0.79,p=0.001) and mid‐upper arm circumference (mean difference 1.12cm, 95% confidence interval 0.50–1.75,p=0.001). Health gains included a reduction in drooling, secretions, vomiting, and constipation. Major surgical complications were found in 13/74 children. The study provides evidence that catch‐up growth and health gains are possible following gastrostomy.
Neuropsychiatric and neurodevelopmental outcome of children at age 6 and 7 years who screened positive for language problems at 30 months
Authors: Miniscalco Carmela , Nygren Gudrun , Hagberg Bibbi , Kadesjö Björn , Gillberg Christopher
Published: April 2006

We present a prospective study at school age of neuropsychiatric and neurodevelopmental outcome of language delay suspected at child health screening around 30 months of age. In a community sample, 25 children (21 males, 4 females) screening positive and 80 children (38 males, 42 females) screening negative for speech and language problems at age 30 months were examined in detail for language disorders at age 6 years. The screen‐positive children were then followed for another year and underwent in‐depth neuropsychiatric examination by assessors blind to the results of previous testing. Detailed follow‐up results at age 7 years were available for 21 children. Thirteen of these 21 children (62%) had a major neuropsychiatric diagnosis (autism, atypical autism, Asperger's syndrome, attention‐deficit‐hyperactivity disorder [ADHD]), or combinations of these. Two further children (10%) had borderline IQ with no other major diagnosis. We conclude that children in the general population who screen positive for speech and language problems before age 3 years appear to be at very high risk of autism spectrum disorders or ADHD, or both, at 7 years of age. Remaining language problems at age 6 years strongly predict the presence of neuropsychiatric or neurodevelopmental disorders at age 7 years.
Specific memory impairment in a multiple disabled male with fragile X syndrome and temporal lobe epilepsy
Authors: Wouters Heidi , Fonteyne Annick , Stiers Peter
Published: April 2006

Evaluation of the cognitive repercussions of an epileptic disorder and its treatment are important issues in clinical follow‐up. This especially holds true for temporal lobe epilepsy (TLE) where resective surgery can be a valid treatment option. However, in patients where TLE coexists with another neurocognitive disorder, questions can arise about the precise nature of the neuropsychological deficits. The aim of the present study was to evaluate memory impairments, found in a male aged 12 years who had the dual pathology of fragile X syndrome and refractory TLE. Memory functions of this child were compared with those of a male aged 11 years 7 months with fragile X syndrome matched for intellectual functioning as indicated by highly comparable verbal (5y 5mo vs 5y 9mo) and non‐verbal (7y 2mo vs 6y 1mo) cognitive age equivalents. Performance on each neuropsychological measure was evaluated twice, separately with normative data based on chronological age and on verbal or non‐verbal cognitive level. A specific, distinguishable profile of task performance could be found only when controlling for general level of cognitive functioning. This made it possible to accurately evaluate neuropsychological abilities before and 6 months after anterior temporal lobe resection even in this male with a complex neurological pathology.
On the calculation of wave patterns
Authors: Hunter C.
Published: April 2006

The calculation of the pattern of waves produced by a point disturbance in a steady field which may be non-uniform can be performed straightforwardly by a single process of integration along the characteristic rays. For illustration, the method is applied to gravity waves produced by a source moving either in a straight line or in a circular path, to the symmetrical waves produced by a source in an expanding sheet and to the waves resulting from an instantaneous disturbance in :I stratified fluid.
The flow caused by the differential rotation of a right circular cylindrical depression in one of two rapidly rotating parallel planes
Authors: Foster M. R.
Published: April 2006

The flow induced by the differential rotation of a cylindrical depression of radius a in one of two parallel rigid planes rapidly rotating about their common normal at speed Q is studied. A Taylor column bounded by the usual Stewartson layers arises, but the shear-layer structure is rather different from any previously studied. The Ei-layers (E = v/ωa2) smooth the discontinuity in the geostrophic flow, but the way in which this is accomplished is related to the possible singu-larities of the E1/3-layer solutions. The fact that the 1/4-layer is partially free and partially attached to a vertical boundary accounts for the new joining conditions for the 1/4-layer. The drag on a right circular cylindrical bump in uniform flow is given in addition to some general comments on the applicability of these joining conditions to the motion of an axisymmetric object of quite general shape.
Mass transport mechanisms in partially stratified estuaries
Authors: Fischer Hugob.
Published: April 2006

A number of mechanisms which transport dissolved constituents and maintain the salt balance in estuaries are described. Although some of these have already been analysed, two which appear to be the most important in many real estuaries have received little previous attention. The Mersey Estuary is used as an example in which to estimate the amount of mass transported by each mechanism. The computations show in part how little is known and how much hypothesis is still required in spit?e of the number of previous estuarine studies; one may conclude, however, that in many real estuaries the most important mass transport mechanism is the net (non-tidal) transverse circulation, which is induced in part by the boundary geometry and in part by the longitudinal density gradient.
Thermocapillary flow near a hemispherical bubble on a heated wall
Authors: Kao Y. S. , Kenning D. B. R.
Published: April 2006

The flow driven by variations in surface tension round a hemispherical gas or vapour bubble on a heated wall has been investigated numerically for steady-state conditions over a wide range of values of dimensionless parameters, and experimentally for one set of conditions. Although six parameters are needed to specify the flow conditions, the magnitude of the liquid flow normal to the heated wall is determined primarily by tihe Marangoni number, Prandtl number and the Biot number based on the effective heat-transfer coefficient at the liquid-gas interface. The interior temperature of the bubble depends in addition on the thermal conductivity ratio of the liquid and the wall material. The flow is very sensitive t o the presence of surface-active contaminants. For water, calculations and experimental observations both indicate that contamination which lowers the static surface tension by only 0.1% may suppress the thermocapillary motion.
The propagation of a weak nonlinear wave
Authors: Davey A.
Published: April 2006

We consider the propagation of a weak nonlinear wave whose energy is concentrated in a narrow band of wavenumbers in a fluid which is both dispersive and dissipative. We use the small amplitude equations of Whitham's theory of slowly varying wave trains, modified slightly to include dissipation, to show that the modulation of the wave may be described by a nonlinear Schrödinger equation. For long waves which are purely dispersive we obtain the Kortewegde Vries equation, and for long waves which are dissipative we obtain Burgers’ equation by suitable transformations of the nonlinear Schrödinger equation. We mention the problem of Stokes waves in deep water and comment briefly upon invariant far-field theory.
Quasi-steady flow of a rotating stratified fluid in a sphere
Authors: Friedlander Susan
Published: April 2006

The steady and quasi-steady motion achieved in a rotating stratified sphere of fluid is studied in the context of a linearized Boussinesq model. In certain parameter ranges an explicit expression is obtained for the flow field as a functional of the surface stress. The non-singular interior solution is used to examine the behaviour of the boundary layer close to the equator. The results agree with previous conclusions about the behaviour of a rotating stratified fluid in simpler geometries. Viewing the problem as a simple model for the interior core of the sun, this work indicates a solar spin-down time that is within the lifetime of the sun.
The growth of gravity-capillary waves in a coupled shear flow
Authors: Valenzuela G. R.
Published: April 2006

The growth rates and phase speeds of gravity-capillary wind waves are investigated through numerical solution of a linear, viscous, coupled, shear-flow perturbation model. Numerical results are obtained by transforming the boundary-value problem of a perturbed mean laminar shear flow into a matrix-eigenvalue problem using standard finite-difference methods.

Detailed calculations are performed for a basic state composed of a logarithmic-linear mean flow profile in the air and a linear-logarithmic mean flow profile in the water. We exclude turbulent Reynolds stresses. Calculated growth rates show excellent agreement with corresponding experimental growth rates. This implies that the initial growth of gravity-capillary wind waves is almost certainly due to the instability of the coupled laminar shear flow in the air and water.

The investigation also demonstrates that the shear flow in the water cannot be ignored in wave growth studies, since the usual 3-4%, highly sheared, wind-induced surface drift produces a significant increase in the growth of wind-generated gravity-capillary waves.
Velocity profiles and fence drag for a turbulent boundary layer along smooth and rough flat plates
Authors: Raju K. G. Ranga , Loeser J. , Plate E. J.
Published: April 2006

The properties of a turbulent boundary layer were investigated as they relate to the form drag on a two-dimensional fence. Detailed measurements were performed at zero pressure gradient of velocity profiles along smooth, rough and transitional flat plates. Upon comparison with other published data, these measurements resulted in simple formulae for the displacement thickness and the local shear coefficient and in a modification to the universal velocity defect law for equilibrium boundary layers.

With these boundary layers, experiments were performed to determine the drag on a two-dimensional fence. These data were analysed along with data from previous investigations. It was found that after suitable blockage corrections all form-drag coefficients for two-dimensional fences collapsed on a single curve if they were calculated with the shear velocity as the reference velocity and plotted against the ratio of the fence height to the characteristic roughness parameter of the approaching flow.
The shape of free jets of water under gravity
Authors: Tuck E. O.
Published: April 2006

A study is made of the form taken by a slender jet of water whose only boundary is a free surface. The only forces acting are inertial and gravitational. Attention is paid to the cross-flow velocity components and to the development of the shape of the cross-section of the jet as it progresses. It is established that a jet with initially elliptic cross-sections can remain elliptical, and the variation in the aspect ratio along the jet is determined.
Rolling motion of a sphere on a plane boundary in oscillatory flow
Authors: Martin C. Samuel , Padmanabhan M. , Ponce-Campos C. D.
Published: April 2006

The rolling motion of a sphere on a smooth plane boundary in a simple-harmonic water motion has been analytically and experimentally investigated. For spheres having specific gravities ranging from 0·09 to 15·18 the sphere motion was found to be sinusoidal for both low and high values of the period parameter defined by Keulegan & Carpenter. The knowledge of the sphere motion, and hence the resultant force, allowed the determination of inertia and drag coefficients from Fourier-averaging techniques. Experiments in the inertial range yielded an added-mass coefficient of 1·2, compared with 0·67 from inviscid theory for translating spheres. For values of the period parameter greater than 30 the drag coefficient is reported to be approximately 0·74.
Some problems concerning the production of a linear shear flow using curved wire-gauze screens
Authors: Castro I. P.
Published: April 2006

The flow of an incompressible fluid through a curved wire-gauze screen of arbitrary shape is reconsidered. Some inconsistencies in previously published papers are indicated and the various approximations and linearizations (some of which are necessary for a complete analytic solution) are discussed and their inadequacies demonstrated. Attention is concentrated on the common practical problem of calculating the screen shape required to produce a linear shear flow and experimental work is presented which supports the contention that the theoretical solutions proposed by Elder (1959)–subsequently discussed by Turner (1969) and Livesey & Laws (1973)-and Lau & Baines (1968) are inadequate, although, for the case of small shear, Elder's theory appears to be satisfactory. Since, in addition, there are inevitable difficulties concerning both the value of the deflexion coefficient appropriate to any particular screen and inhomogeneities in the screen itself, it is concluded that the preparation of a curved screen to produce the commonly required moderate to large linear shear flow is bound to be somewhat empirical and should be attempted with caution.
A moving fluid interface on a rough surface
Authors: Hocking L. M.
Published: April 2006

When an interface between two fluids moves in contact with a solid boundary, the Navier-Stokes equations and the no-slip boundary condition provide an unsatisfactory theoretical model, because they predict an undefined velocity at the contact line and a non-integrable stress on the solid boundary. If the surface irregularities are included in the model, the flow on a length scale large compared with their size can be calculated, using a slip coefficient and treating the surface as smooth.

A simple type of corrugated surface is examined, and the effective slip coefficient calculated, for grooves of finite and infinite depth. The slip coefficient when the grooves are filled with one fluid and another fluid flows over them is also calculated. It is suggested that, when a fluid displaces another on a rough surface, the displaced fluid remains in the hollows on the surface, thus providing a partly fluid boundary for the displacing fluid and leading to a slip coefficient for the flow.

Fluid contained between two vertical plates and rising between them provides a simple example of a flow for which the solution can be found with and without a slip coefficient. With slip present, the force on the plates is finite and its value is calculated.
Experiments on the stability of liquid films adjacent to supersonic boundary layers
Authors: Saric William S. , Nayfeh Ali H. , Lekoudis Spyridon G.
Published: April 2006

The stability of liquid films adjacent to supersonic streams was investigated experimentally and compared with analytical results. Linear theories predict that films adjacent ta supersonic streams are much more unstable than those adjacent to subsonic streams. However, our supersonic experimental observa-tions indicated that the liquid film was stable with no entrainment under all test conditions. These conditions included laminar and turbulent boundary layers, a variation in the liquid Reynolds number (based on flow rate) from 1 to 200 and a variation in the free-stream unit Reynolds number from 1.6 × 106 m−1 to 110.0 × 106 m−1. These experimental observations can be explained by nonlinear theories, which predict that linear unstable disturbances do not grow indefhitely but achieve steady-state amplitudes in the supersonic case. The different aspects of the observed wave behaviour such as frequency, wavelength and amplitude are discussed and compared with previous experimental observations and the nonlinear theories.
A numerical model of the air flow above water waves
Authors: Gent P. R. , Taylor P. A.
Published: April 2006

A numerical model is proposed for the flow in a deep turbulent boundary layer over water waves. The momentum equations are closed by the use of an isotropic eddy viscosity and the turbulent energy equation. For small amplitudes the results are similar to those of Townsend's (1972) linear model, but nonlinear effects become important as the ratio of wave height to wavelength increases. With uniform surface roughness zo, the predicted fractional rate of energy input per radian advance in phase, ζ, decreases slightly with increasing amplitude and is of the same order of magnitude as in Miles’ (1957, 1959) and Townsend's linear theories. If zo is allowed to vary with position along the wave, however, the fractional rate of energy input can be significantly increased for small amplitude waves. If the variation in zo is half the mean value and the maximum wave slope zak is 0.01, we find ζ ≈ 60 (ρair/ρwater) (uo/c)2, where uo is the friction velocity and c the wave phase speed. Comparison is also made with recent laboratory and field data.
Roll-cell instabilities in rotating laminar and trubulent channel flows
Authors: Lezius Dietrich K. , Johnston James P.
Published: April 2006

The stability of laminar and turbulent channel flow is examined for cases where Coriolis forces are introduced by steady rotation about an axis perpendicular to the plane of mean flow. Linearized equations of motion are derived for small disturbances of the Taylor type. Conditions for marginal stability in laminar Couette and Poiseuille flow correspond, in part, to the analogous solutions of buoyancy-driven convection instabilities in heated fluid layers, and to those of Taylor instabilities in the flow between rotating cylinders. In plane Poiseuille flow with rotation, the critical disturbance mode occurs at a Reynolds number of Rec= 88.53 and rotation number Ro= 0.5. At higher Reynolds numbers, unstable conditions canexist over the range of rotation numbers given by 0 < Ro< 3, provided the undisturbed flow remains laminar. A two-layer model is devised to investigate the onset of longitudinal instabilities in turbulent flow. The linear disturbance equations are solved essentially in their laminar form, whereby the velocity gradient of laminar flow is replaced by a numerically computed profile for the gradient of the turbulent mean velocity. The turbulent stress levels in the stable and unstable flow regions are represented by integrated averages of the eddy viscosity. Onset of instability for Reynolds numbers between 6000 and 35 000 is predicted to occur at Ro= 0.022, a value in remarkable agreement with the experimentally observed appearance of roll instabilities in rotating turbulent channel flow.
Mixed-region collapse in a stratified fluid
Authors: Manins P. C.
Published: April 2006

A simplified theoretical model of a collapsing mixed region in a stably stratified fluid, as studied experimentally by Wu (1969), is presented. Unlike previous work the model describes the mixed region itself in the ‘principal stage’ of collapse. An inertia-buoyancy balance assumption is invoked, resulting in good quantitative agreement with Wu's results.
Coupling of surface and internal gravity waves: a mode coupling model
Authors: Watson Kenneth M. , West Bruce J. , Cohen Bruce I.
Published: April 2006

A surface-wave/internal-wave mode coupled model is constructed to describe the energy transfer from a linear surface wave field on the ocean to a linear internal wave field. Expressed in terms of action-angle variables the dynamic equations have a particularly useful form and are solved both numerically and in some analytic approximations. The growth time for internal waves generated by the resonant interaction of surface waves is calculated for an equilibrium spectrum of surface waves and for both the Garrett-Munk and two-layer models of the undersea environment. We find energy transfer rates as a function of undersea parameters which are much faster than those based on the constant Brunt-ViiisSila model used by Kenyon (1968) and which are consistent with the experiments of Joyce (1974). The modulation of the surface-wave spectrum by internal waves is also calculated, yielding a ‘mottled’ appearance of the ocean surface similar to that observed in photographs taken from an ERTS1 satellite (Ape1 et al. 1975b).
Exact large amplitude capillary waves on sheets of fluid
Authors: Kinnersley William
Published: April 2006

We generalize Crapper's exact solution for capillary waves on fluid of infinite depth. We find two finite-depth solutions involving elliptic functions. We show they can also be interpreted as large amplitude symmetrical and antisymmetrical waves on a fluid sheet. Particularly interesting are the waves obtained from our solution in the limit when the fluid sheet is extremely thin.
Strong MHD helical turbulence and the nonlinear dynamo effect
Authors: Pouquet A. , Frisch U. , Léorat J.
Published: April 2006

To understand the turbulent generation of large-scale magnetic fields and to advance beyond purely kinematic approaches to the dynamo effect like that introduced by Steenbeck, Krause & Radler (1966)’ a new nonlinear theory is developed for three-dimensional, homogeneous, isotropic, incompressible MHD turbulence with helicity, i.e. not statistically invariant under plane reflexions. For this, techniques introduced for ordinary turbulence in recent years by Kraichnan (1971 a)’ Orszag (1970, 1976) and others are generalized to MHD; in particular we make use of the eddy-damped quasi-normal Markovian approximation. The resulting closed equations for the evolution of the kinetic and magnetic energy and helicity spectra are studied both theoretically and numerically in situations with high Reynolds number and unit magnetic Prandtl number.

Interactions between widely separated scales are much more important than for non-magnetic turbulence. Large-scale magnetic energy brings to equipartition small-scale kinetic and magnetic excitation (energy or helicity) by the ‘Alfvén effect’; the small-scale ‘residual’ helicity, which is the difference between a purely kinetic and a purely magnetic helical term, induces growth of large-scale magnetic energy and helicity by the ‘helicity effect’. In the absence of helicity an inertial range occurs with a cascade of energy to small scales; to lowest order it is a −3/2 power law with equipartition of kinetic and magnetic energy spectra as in Kraichnan (1965) but there are −2 corrections (and possibly higher ones) leading to a slight excess of magnetic energy. When kinetic energy is continuously injected, an initial seed of magnetic field will grow to approximate equipartition, at least in the small scales. If in addition kinetic helicity is injected, an inverse cascade of magnetic helicity is obtained leading to the appearance of magnetic energy and helicity in ever-increasing scales (in fact, limited by the size of the system). This inverse cascade, predicted by Frischet al.(1975), results from a competition between the helicity and Alféh effects and yields an inertial range with approximately — 1 and — 2 power laws for magnetic energy and helicity. When kinetic helicity is injected at the scale linjand the rate$\tilde{\epsilon}^V$(per unit mass), the time of build-up of magnetic energy with scaleL[Gt ] linjis$t \approx L(|\tilde{\epsilon}^V|l^2_{\rm inj})^{-1/3}.$
An experimental study of turbulent convective heat transfer from a flat plate
Authors: Perry A. E. , Hoffmann P. H.
Published: April 2006

A turbulent boundary layer developing on a smooth heated uniform-temperature plate in a zero pressure gradient was set up. The origins of the layers were matched to remove the effect of an In heated starting length. Similarity proposals were tested. The mean flow field followed the usual law of the wall and defect law for both temperature and velocity. Broad-band measurements of stream wise velocity and temperature fluctuations were made, and wall similarity and Townsend's self-preserving flow similarity were found to be applicable, at least after a sufficient flow development.

Some initial attempts to arrive at a comparison between heat and momentum transport are presented. The results include conditionally sampled measurements of instantaneous heat and momentum fluxes and correlations between these two quantities. The fluxes were divided into quadrants. Conditional probabilities and weighted joint probability density functions were measured to determine whether there was a similarity in behaviour of these two fluxes. The concept of ‘hole size’ developed for momentum flux was extended to heat flux and events corresponding to bursts and sweeps in the momentum flux were found to be accompanied by corresponding events in the heat flux.
Characteristics of a turbulent boundary layer with an external turbulent uniform shear flow
Authors: Ahmad Q. A. , Luxton R. E. , Antonia R. A.
Published: April 2006

Measurements are presented of both mean and fluctuating velocity components in a turbulent boundary layer subjected to a nearly homogeneous external turbulent shear flow. The Reynolds shear stress in the external shear flow is small compared with the wall shear stress. Its transverse mean velocity gradient λ (≃ 6 s−l) is also small compared with typical gradients based on outer variables (say Uw/δ, where Uwis the value of the linear velocity profile extrapolated to the wall and δ is the boundary-layer thickness), but is of the same order as Ut/δ (Ur is the friction velocity). The influence of both positive and negative transverse velocity gradients on the turbulent wall layer is investigated over a streamwise region where the normal Reynolds stresses in the external flow are approximately equal and constant in the streamwise direction. In this region, the integral length scale of the external flow is of the same order of magnitude as that of the wall layer. Measurements in the boundary layer are also given for an un-sheared external turbulent flow (λ = 0) with a turbulence level Tu of 1.5%, approximately the same as that for h = ± 6 s−1. (Tu, is defined as the ratio of the r.m.s. longitudinal velocity fluctuation to Uw.) The measurements are in good agreement with those available in the literature for a similar free-stream turbulence level and show that the external turbulence level and length scale exert a large influence on the turbulence structure in the boundary layer. The additional effect of the external shear on the mean velocity and turbulent energy budget distributions in the inner region of the boundary layer is found to be small. In the outer region, the ‘wake’ component of the mean velocity defect is lowered by the presence of free-stream turbulence and one extra effect due to the external shear is an increase in the Reynolds shear stress when h is positive and a decrease when h is negative. Another interesting effect due to the shear is the appearance near the edge of the layer of a small but distinct region where the local mean velocity is constant and the Reynolds shear stress is negligible.
Giant waves
Authors: Smith Ronald
Published: April 2006

It is suggested that giant waves, as observed on the Agulhas Current, occur where the wave groups are reflected by the current. The local behaviour of the wave amplitude is modelled by the nonlinear Schrodinger equation

iar = aρρ-ρ+β|a|2a.

For waves of a given incident wave amplitude the steady solutions are stable.
Measurements with a pulsed-wire and a hot-wire anemometer in the highly turbulent wake of a normal flat plate
Authors: Bradbury L. J. S.
Published: April 2006

This paper describes an investigation into the response of both the pulsed-wire anemometer and the hot-wire anemometer in a highly turbulent flow. The first part of the paper is concerned with a theoretical study of some aspects of the response of these instruments in a highly turbulent flow. It is shown that, under normal operating conditions, the pulsed-wire anemometer should give mean velocity and longitudinal turbulent intensity estimates to an accuracy of better than 10% without any restriction on turbulence level. However, to attain this accuracy in measurements of turbulent intensities normal to the mean flow direction, there is a lower limit on the turbulent intensity of about 50%. An analysis is then carried out of the behaviour of the hot-wire anemometer in a highly turbulent flow. It is found that the large errors that are known to develop are very sensitive to the precise structure of the turbulence, so that even qualitative use of hot-wire data in such flows is not feasible. Some brief comments on the possibility of improving the accuracy of the hot-wire anemometer are then given.

The second half of the paper describes some comparative measurements in the highly turbulent flow immediately downstream of a normal flat plate. It is shown that, although it is not possible to interpret the hot-wire results on their own, it is possible to calculate the hot-wire response with a surprising degree of accuracy using the results from the pulsed-wire anemometer. This provides a rather indirect but none the less welcome check on the accuracy of the pulsed-wire results, which, in this very highly turbulent flow, have a certain interest in their own right.
The measurement of the spectra of highly turbulent flows by a randomly triggered pulsed-wire anemometer
Authors: Gaster M. , Bradbury L. J. S.
Published: April 2006

The pulsed-wire anemometer has enabled velocity measurements to be made in a variety of unsteady turbulent flows. So far the use of the instrument has been confined to the determination of mean velocity, turbulence intensity and probability density. Here we show how spectral information can also be obtained. The instrument provides estimates of the velocity by measuring the time of flight of a heated flow tracer. Periodic samples of velocity can be generated by driving the anemometer with a regular train of pulses, but unfortunately it is not possible to pulse the heater at a rate greater than about 50 or so times per second, without risk of burning out the wire. This limits the spectral information that can be obtained to situations where all the energy occurs at frequencies below 25 Hz. The random sampling scheme used here avoids the aliasing problem inherent in periodic sampling and enables estimates of power spectral density to be formed up to frequencies many times the average sampling rate. This technique is used to obtain spectral estimates of the velocity fluctuations that arise at various locations in the wake of a flat plate.
Some notes on unsteady lifting-line theory
Authors: Holten Th. Van
Published: April 2006

A higher-order lifting-line theory for a wing in unsteady motion is discussed. Apart from the addition of higher-order terms, it also differs from the theory derived by James (1975) in its emphasis on ‘physical’ interpretations. This emphasis has made it possible to shed some new light on Prandtl's classical lifting-line theory, as well as on Weissinger's 3/4-chord theory.
Adjustment under gravity in a rotating channel
Authors: Gill A. E.
Published: April 2006

What transient motions occur as a fluid responds to gravitational forces in a rotating channel, and what equilibrium does the fluid adjust to? This problem is studied to illustrate how boundaries affect the process of adjustment to a geostrophic equilibrium. In particular, linear solutions are found for an infinitely long channel of constant width and constant depth when there is an initial discontinuity in the level of the free surface. The results are summarized in the figures, and can be described in terms of Poinearé waves and Kelvin waves. When the channel is wide compared with the Rossby radius, the final state involves a current of that width which follows the left-hand boundary (for Northern-Hemisphere rotation) to the position of the initial discontinuity, then crosses the channel and continues downstream along the right-hand wall.
A conservation law for internal gravity waves
Authors: Milder Michael
Published: April 2006

The scaled vorticity Ω/N and strain ∇ ζ associated with internal waves in a weak density gradient of arbitrary depth dependence together comprise a quantity that is conserved in the usual linearized approximation. This quantity I is the volume integral of the dimensionless density DI = ½[Ω2/N2 + (∇ ζ)2]. For progressive waves the ‘kinetic’ and ‘potential’ parts are equal, and in the short-wavelength limit the density DI and flux FI are related by the ordinary group velocity: FI = DIcg. The properties of DI suggest that it may be a useful measure of local internal-wave saturation.
On spatially growing baroclinic waves in the ocean
Authors: Hogg Nelson G.
Published: April 2006

It is shown that spatially growing waves with complex wavenumber and real frequency can exist in a baroclinic flow and that these waves are substantially different from the more commonly studied temporally growing ones. They are bounded by a low wavenumber cut-off which separates them from the temporally growing waves. Their amplitude and phase change most rapidly near their steering level and are almost depth independent away from it. Most of the energy conversion from mean flow to the waves occurs at this level. It is suggested that these motions may be forced by steady disturbances such as bottom relief.

The theory is compared with recent observations of strong small-scale motions in a region of rough topography of MODE and in the vicinity of the Gulf Stream. The vertical structure can be well matched with the theory but the complex wavenumber appears to be a factor of 2–3 greater than that predicted.