bluuebunny/crossref_metadata_2025_split huggingface dataset:
Marginal deformations of tree‐level 𝒩 = 4 SYM from twistor string theory
Authors: Kulaxizi M. , Zoubos K. 
Published: April 2006

AbstractThe topological B–model with target the supertwistor space ℂP3|4 is known to describe perturbative amplitudes of 𝒩 = 4 Super Yang–Mills theory. We review the extension of this correspondence to the superconformal gauge theories that arise as marginal deformations of 𝒩 = 4 by considering the effects of turning on a certain closed string background, which results in non–anticommutativity in the fermionic directions of ℂP3|4. We generalise the twistor string prescription for amplitudes to this case and illustrate it with some simple examples.
Fibrous long spacing type collagen fibrils have a hierarchical internal structure
Authors: Wen Chuck K. , Goh M. Cynthia 
Published: April 2006

AbstractNanodissection of single fibrous long spacing (FLS) type collagen fibrils by atomic force microscopy (AFM) reveals hierarchical internal structure: Fibrillar subcomponents with diameters of ∼ 10 to 20 nm were observed to be running parallel to the long axis of the fibril in which they are found. The fibrillar subcomponent displayed protrusions with characteristic ∼ 270 nm periodicity, such that protrusions on neighboring subfibrils were aligned in register. Hence, the banding pattern of mature FLS‐type collagen fibrils arises from the in‐register alignment of these fibrillar subcomponents. This hierarchical organization observed in FLS‐type collagen fibrils is different from that previously reported for native‐type collagen fibrils, displaying no supercoiling at the level of organization observed. Proteins 2006. © 2006 Wiley‐Liss, Inc.
Diffusion‐collision of foldons elucidates the kinetic effects of point mutations and suggests control strategies of the folding process of helical proteins
Authors: Capriotti Emidio , Compiani Mario 
Published: April 2006

AbstractIn this article we use mutation studies as a benchmark for a minimal model of the folding process of helical proteins. The model ascribes a pivotal role to the collisional dynamics of a few crucial residues (foldons) and predicts the folding rates by exploiting information drawn from the protein sequence. We show that our model rationalizes the effects of point mutations on the kinetics of folding. The folding times of two proteins and their mutants are predicted. Stability and location of foldons have a critical role as the determinants of protein folding. This allows us to elucidate two main mechanisms for the kinetic effects of mutations. First, it turns out that the mutations eliciting the most notable effects alter protein stability through stabilization or destabilization of the foldons. Secondly, the folding rate is affected via a modification of the foldon topology by those mutations that lead to the birth or death of foldons. The few mispredicted folding rates of some mutants hint at the limits of the current version of the folding model proposed in the present article. The performance of our folding model declines in case the mutated residues are subject to strong long‐range forces. That foldons are the critical targets of mutation studies has notable implications for design strategies and is of particular interest to address the issue of the kinetic regulation of single proteins in the general context of the overall dynamics of the interactome. Proteins 2006. © 2006 Wiley‐Liss, Inc.
Determination of bank erodibility for natural and anthropogenic bank materials using a model of lateral migration and observed erosion along the Willamette River, Oregon, USA
Authors: Wallick Jennifer Rose , Lancaster Stephen T. , Bolte John P. 
Published: April 2006

AbstractMany large rivers flow through a variety of geologic materials. Within the span of several kilometres, bends may alternately flow against recently reworked sediments, older, more indurated sediments or highly resistant materials. As sediment size, cementation, and other properties strongly influence the erodibility of river banks, erosion rates and channel planform are likely to vary significantly along the length of large rivers. In order to assess the role of bank materials on bank erosion rates, we develop a method for detecting relative differences in erodibility between bank materials along large floodplains. By coupling historic patterns of channel change with a simple model of bank erodibility we are able to track relative changes in bank erodibility among time intervals and bank materials. We apply our analysis to the upper Willamette River, in northwestern Oregon for three time periods: 1850–1895, 1895–1932 and 1972–1995 and compute relative differences in bank erodibility for Holocene alluvium, partially cemented Pleistocene gravels, and revetments constructed in the 20th century. Although the Willamette is fundamentally an anastomosing river, we apply the model to single‐thread portions of the channel that evolved through lateral migration. Our simple model of bank erodibility reveals that for all three‐time periods, banks composed of Holocene alluvium are at least 2–5 times more erodible than banks composed of Pleistocene gravels. Revetment installed in the 20th century is highly resistant to erosion and is at least 10 times less erodible than Pleistocene gravels. Copyright © 2006 John Wiley & Sons, Ltd.
Stream-Based Specification of Mobile Systems
Authors: Grosu Radu , Stølen Ketil 
Published: April 2006

Abstract.
This paper presents a formal specification technique for mobile systems based on input/output relations on streams. We consider networks of components communicating asynchronously via unbounded directed channels. Mobility is achieved by allowing the components to communicate channel ports. We distinguish between many-to-many and two variants of point-to-point communication. The communication paradigms are semantically underpinned by denotational models. The models are formulated in the context of timed non-deterministic data ow networks and presented in a stepwise fashion. The emphasis is on capturing the special kind of dynamic hiding characterising mobile systems. We demonstrate the proposed approach in a number of small examples.
Couette flow for a gas with a discrete velocity distribution
Authors: Cabannes Henri 
Published: April 2006

We consider a kinetic theory model of a gas, whose molecular velocities are restricted to a set of fourteen given vectors. For this model we study the Couette flow problem, the boundary conditions on the walls being the conditions of pure diffuse reflexion. The kinetic equations can be integrated by quadrature under the assumption that the walls have opposite velocities and equal temperatures. The presence on the walls of tangential velocities leads to the consequence that the velocity slip coefficient does not in general vanish when the Knudsen number goes to zero.Considering the same problem again after the suppression of tangential velocities, we obtain formulae for the velocity and temperature slip coefficients which generalize results of Broadwell (1964b), and which agree qualitatively with experiments.
The impingement of underexpanded axisymmetric jets on wedges
Authors: Lamont P. J. , Hunt B. L. 
Published: April 2006

This paper reports an experimental investigation into the impingement of underexpanded axisymmetric jets on each of three wedges arranged symmetrically in the jets. Three jets were used; two were produced by a convergent-divergent nozzle of exit Mach number 2.2 which was operated at underexpansion ratios of 1·2 and 2, while the third jet was produced by a convergent nozzle operated at an underexpansion ratio of 4. The base widths of the wedges equalled the exit diameter of the convergent-divergent nozzle, their apex angles were 90°, 60° and 45° and they were situated within or just downstream of the first cell of each jet.Detailed front-face pressure distributions were obtained for 15 different configurations. Shadowgraph photographs were taken of these and other cases. The results show a variety of possible flow patterns. The major factors determining which flow pattern occurs are the combination of the centre-line Mach number and wedge apex angle and the jet shock strength and position. It was found that observed shock intersections can be reconstructed by means of shock polars, but that a four-shock confluence will not always occur when it is possible according to the shock polars. In one case there is evidence of the existence of a β triple-shock confluence. An interesting and unusual flow involving a stagnation bubble was obtained when the wedges were situated downstream of the free-jet Mach disk.
A moving-wall boundary layer with reverse flow
Authors: Klemp J. B. , Acrivos Andreas 
Published: April 2006

The problem of uniform flow past a flat plate whose surface has a constant velocity λU opposite in direction to that of the mainstream is considered for large values of the Reynolds number R. In a previous communication (Klemp & Acrivos 1972) it was shown that, if the region of reverse flow which is established next to the plate as a consequence of its motion is O(R−1/2) in thickness, the appropriate laminar boundary-layer equations have a solution provided λ ≤ 0·3541. Here the analysis is extended to the range λ > 0·3541, which cannot be treated using a conventional boundary-layer approach. Specifically, it is found that for λ > 0·3541 the flow consists of three overlapping domains: (a) the external uniform flow; (b) a conventional boundary layer with reverse flow for xs < x < 1, where xs, refers to the point of detachment of the ψ = 0 streamline and x = 1 is the trailing edge of the plate; and (c) an inviscid collision region in the neighbourhood of xs, having dimensions O(R−1/2) in both the streamwise and the normal direction, within which the reverse moving stream collides with the uniform flow, turns around and then proceeds downstream. It is established furthermore that xs = 0 for 0 ≤ λ ≤ 1 and that xs < 0 for λ > 1.Also, detailed streamline patterns were obtained numerically for various λ's in the range of 0 ≤ λ ≤ 2 using a novel computational scheme which was found to be more efficient than that previously reported. Interestingly enough, the drag first decreased with λ, reached a minimum at λ = 0.3541, and then increased monotonically until, at λ = 2, it was found to have attained essentially the value predicted from the asymptotic λ → ∞ similarity solution available in the literature. Thus it is felt that the present numerical results plus the two similarity solutions for λ = 0 and for λ → ∞ fully describe the high-R steady flow for all non-negative values of λ.
An approximate theory for incompressible viscous flow past two-dimensional bluff bodies in the intermediate Reynolds number regime O(1) < Re < O(102)
Authors: Weinbaum Sheldon , Kolansky Michael S. , Gluckman Michael J. , Pfeffer Robert 
Published: April 2006

A new approximate theory is proposed for treating the flow past smoothly contoured two-dimensional bluff bodies in the intermediate Reynolds number rangeO(1) <Re< 0(102), where the displacement effect of the thick viscous layer near the surface of the body is large and a steady laminar wake is present. The theory is based on a new pressure hypothesis which enables one to take account of the displacement interaction and centrifugal effects in thick viscous layers using conventional first-order boundary-layer equations. The basic question asked is how the wall pressure gradient in ordinary boundary -layer theory must be modified if the pressure gradient along the displacement surface using the Prandtl pressure hypothesis is to be equal to the pressure gradient along this surface using a higher-order approximation to the Navier-Stokes equation in which centrifugal forces are considered. The inclusion of the normal pressure field with displacement interaction is shown to be equivalent to stretching the streamwise body co-ordinate in first-order boundary-layer theory such that the streamwise pressure gradient as a function of distance along the original and displacement body surfaces are equal.While the new theory is of a non-rigorous nature, it yields results for the location of separation and detailed surface pressure and vorticity distribution which are in remarkably good agreement with the large body of available numerical Navier-Stokes solutions. A novel feature of the new boundary-value problem is the development of a simple but accurate approximate method for determining the inviscid flow past an arbitrary two-dimensional displacement body with its wake.
The mechanism of bubble detachment from a wall at zero and negative gravity
Authors: Malcotsis G. 
Published: April 2006

An experiment has been carried out in which air bubbles were caused to grow isothermally at a vertical wall between two closely spaced horizontal plates. The experiment gives an approximate representation in two dimensions for the growth of a vapour bubble at a wall under some conditions of subcooled boiling in zero gravity. Although the effect of gravity was virtually eliminated in the experiment, it was found that a bubble could still detach itself from the wall, apparently owing to the effects of surface tension and inertia.Also, bubbles were seen to detach from a wall despite the presence of a slight gravity force directed to oppose such detachment.
Sedimentological and fluid-dynamic implications of the turbulent bursting phenomenon in geophysical flows
Authors: Jackson Roscoe G. 
Published: April 2006

The bursting process in turbulent boundary layers provides new insight on turbulence phenomena, mechanics of sedimentation, and genesis of bedforms in natural geophysical flows. Recent visualization experiments suggest that the turbulent boundary layer can be divided into an inner zone, whose essential characteristics scale with inner (wall) variables, and an outer zone, whose properties scale with the fluid-dynamic variables of the entire flow. The inner zone is distinguished by (i) a viscous sublayer displaying spanwise alternations of high-and low-speed streaks and (ii) episodic disruption by lift-ups of low-speed streaks. Oscillatory growth and breakup stages of the Stanford model of bursting characterize the turbulent structure of the outer zone. The burst cycle exists in turbulent boundary layers of all natural flows except perhaps (i) open-channel flows in the upper part of the upper flow regime and (ii) wind-generated surface waves.Fluid motions described as kolks and boils in incompressible open-channel flows correspond to the oscillatory growth stage andthe late oscillatory growthand breakup stages, respectively, of the Stanford model of bursting. Supporting evidence includes (i) close similarity of gross fluid motions, (ii) equivalent scaling of boils and bursts, and (iii) intensification of boils and bursts in adverse pressure gradients and over rough beds. McQuivey's (1973) turbulence measurements show that the Eulerian integral time scaleTEscales with the same outer variables asboilperiodicity and burst periodicity. It is hypothesized thatTEequals the mean duration of bursts at a point in the flow.Bedforms governed by the turbulent structure of the inner zone (microforms) cannot form if the sublayer is disrupted by bed roughness. The conditions for the existence of two common microforms and their spacings scale with the inner variables. Grain roughness increases the vertical intensity of the turbulence (by enhancing lift-ups) within the inner zone, thereby explaining textural differences between the coarse ripple and fine ripple bed stages of Moss (1972).Mesoforms respond to the fluid-dynamical regime in the outer zone and scale with the outer variables. The mean spacing of dunelike large-scale ripples in equilibrium open-channel flows is proportional to the boundary-layer thickness and equals the length scale formed by the product of the free-stream velocity and the boil period.Strong upward flow in a burst provides the vertical anisotropy in the turbulence which is needed to suspend sediment. Bursting promotes the entrain-ment of more and coarser sediment than tractive forces alone can accomplish.
Heat diffusion as a source of aerodynamic sound
Authors: Kempton A. J. 
Published: April 2006

The paper examines the role of heat diffusion as an internal noise source in aeroengines and as a source of noise in the mixing of hot jets. We consider a number of model problems and find that the sound induced by unsteady heat transfer can show an unusually weak dependence on the mean flow velocityU, the intensity scaling asU3in three dimensions. At low enough velocities diffusion effects will overwhelm other noise sources, but we have failed in our search for a significant practical situation in which we can prove that sound generated by diffusion clearly dominates over that excited by unsteady aerodynamic forces; they are sometimes comparable.We examine the possibility that diffusive monopole sources feature in the noise of hot jets using model problems in the linear case and using dimensional analysis in the nonlinear case, and conclude that no significant monopole exists when the specific heats are constant. But they are not constant at low frequencies when, for example, heat flows into and out of vibrational energy modes; then an important monopole source is present. This source shows an unusually complicated scale effect.
The stress field of slender particles oriented by a non-Newtonian extensional flow
Authors: Goddard J. D. 
Published: April 2006

An analysis is presented of the creeping motion around a flow-oriented slender particle in a material medium subject to a uniaxial extension in the far field. A general quasi-steady rheological model is adopted, of a kind representing isotropic (Noll) fluids subject to time-independent velocity gradients, or isotropic solids subject to time-independent strain fields. The analysis is based on the premise of a shear-dominated motion in the near field, which is joined asymptotically to the extension-dominated motion in the far field. For axisymmetric particles, and to the order of terms in slenderness considered here, the far-field perturbation due to the particle can be represented as a distributed coaxial line force in a transversely isotropic medium whose strength is governed by the structure of the near-field rheology.On the basis of the results for a single particle, a formula is derived for the stress contribution due to the presence of oriented slender fibres in dilute suspension in a non-Newtonian fluid. For certain simple rheological models exhibiting a strong shear-thinning behaviour, the particle contribution to tensile stress is greatly diminished relative to the Newtonian case, as was predicted by an earlier rudimentary treatment (Goddard 1975). The present analysis is thought to be highly promising for applications to general composite materials.
A moving fluid interface. Part 2. The removal of the force singularity by a slip flow
Authors: Hocking L. M. 
Published: April 2006

If the no-slip condition is used to determine the flow produced when a fluid interface moves along a solid boundary, a non-integrable stress is obtained. In part 1 of this study (Hocking 1976), it was argued that, when allowance was made for the presence of irregularities on the solid boundary, an effective slip coefficient could be found, which might remove the difficulty.This paper examines the effect of a slip coefficient on the flow in the neighbourhood of the contact line. Particular cases which are solved in detail are liquid–gas interfaces at an arbitrary angle, and normal contact of fluids of arbitrary viscosity. The contribution of the vicinity of the contact line to the force on the boundary is obtained.The inner region, near the contact line, must be matched with an outer flow, in which the no-slip condition can be applied, in order to obtain the total value of the force on the boundary. This force is determined for the flow of two fluids between parallel plates and in a pipe, with a plane interface. The enhanced resistance produced by the presence of the interface is calculated, and it is shown to be equivalent to an increase in the length of the column of fluid by a small multiple of the pipe radius.
Spectral energy transfer in high Reynolds number turbulence
Authors: Helland K. N. , Atta C. W. Van , Stegen G. R. 
Published: April 2006

The spectral energy transfer of turbulent velocity fields has been examined over a wide range of Reynolds numbers by experimental and empirical methods. Measurements in a high Reynolds number grid flow were used to calculate the energy transfer by the direct Fourier-transform method of Yeh & Van Atta. Measurements in a free jet were used to calculate energy transfer for a still higher Reynolds number. An empirical energy spectrum was used in conjunction with a local self-preservation approximation to estimate the energy transfer at Reynolds numbers beyond presently achievable experimental conditions.Second-order spectra of the grid measurements are in excellent agreement with local isotropy down to low wavenumbers. For the first time, one-dimensional third-order spectra were used to test for local isotropy, and modest agreement with the theoretical conditions was observed over the range of wavenumbers which appear isotropic according to second-order criteria. Three-dimensional forms of the measured spectra were calculated, and the directly measured energy transfer was compared with the indirectly measured transfer using a local self-preservation model for energy decay. The good agreement between the direct and indirect measurements of energy transfer provides additional support for both the assumption of local isotropy and the assumption of self-preservation in high Reynolds number grid turbulence.An empirical spectrum was constructed from analytical spectral forms of von Kármán and Pao and used to extrapolate energy transfer measurements at lower Reynolds number to Rλ = 105 with the assumption of local self preservation. The transfer spectrum at this Reynolds number has no wavenumber region of zero net spectral transfer despite three decades of $k^{-\frac{5}{3}}$. behaviour in the empirical energy spectrum. A criterion for the inertial subrange suggested by Lumley applied to the empirical transfer spectrum is in good agreement with the $k^{-\frac{5}{3}}$ range of the empirical energy spectrum.
The near field in the mixing of a round jet with a cross-stream
Authors: Moussa Z. M. , Trischka John W. , Eskinazi S. 
Published: April 2006

In the mixing of a jet with a cross-stream, it is found that in the near field, defined as the region of the flow from the jet exit to a distance of a few diameters downstream of this exit, a considerable amount of dynamical adjustment takes place. This near-field region characterizes the subsequent behaviour and development of the jet, its wake and the cross-stream in the vicinity of this mixing region. The rapid evolution of the flow gives rise to a pair of bound vortices attached to the lee side of the jet boundary, to fast development of the turbulent and mean vorticity, to a vortex-shedding system, and to the largest rates of entrainment of cross-stream flow into the jet. Furthermore, it is found that the geometrical configuration of the boundaries at the jet exit plays an important role in the mixing and development processes.An intrinsic method is proposed for the delineation of the flow boundaries between the jet and the cross-stream. Calculations of mass, momentum and vorticity fluxes have been made. The vorticity flux gives evidence of the rapid stretching and tilting of the vorticity vector field in the near-field region.