bluuebunny/crossref_metadata_2025 huggingface dataset:
Constructions of 2‐cover‐free families and related separating hash families
Authors: Li P. C. , van Rees G. H. J. , Wei R.
Published: April 2006

Cover‐free families (CFFs) were considered from different subjects by numerous researchers. In this article, we mainly consider explicit constructions of (2; d)‐cover‐free families. We also determine the size of optimal 2‐cover‐free‐families on 9, 10, and 11 points. Related separating hash families, which can be used to construct CFFs, are also discussed. © 2006 Wiley Periodicals, Inc. J Combin Designs 14: 423–440, 2006
Commuting decompositions of complete graphs
Authors: Akbari Saieed , Herman Allen
Published: April 2006

We say that two graphs G and H with the same vertex set commute if their adjacency matrices commute. In this article, we show that for any natural number r, the complete multigraph K is decomposable into commuting perfect matchings if and only if n is a 2‐power. Also, it is shown that the complete graph Kn is decomposable into commuting Hamilton cycles if and only if n is a prime number. © 2006 Wiley Periodicals, Inc. J Combin Designs
Comparative study of microglia activation induced by amyloid‐beta and prion peptides: Role in neurodegeneration
Authors: Garção Pedro , Oliveira Catarina R. , Agostinho Paula
Published: April 2006

The inflammatory responses in Alzheimer's disease (AD) and prion‐related encephalopathies (PRE) are dominated by microglia activation. Several studies have reported that the amyloid‐beta (Aβ) peptides, which are associated with AD, and the pathogenic isoform of prion protein (PrPSc) have a crucial role in neuronal death and gliosis that occur in both of these disorders. In this study, we investigate whether Aβ and PrPSc cause microglia activation per se and whether these amyloidogenic peptides differentially affect these immunoeffector cells. In addition, we also determined whether substances released by Aβ‐ and PrP‐activated microglia induce neuronal death. Cultures of rat brain microglia cells were treated with the synthetic peptides Aβ1–40, Aβ1–42 and PrP106–126 for different time periods. The lipopolysaccharide was used as a positive control of microglia activation. Our results show that Aβ1–40 and PrP106–126 caused similar morphological changes in microglia and increased the production of nitric oxide and hydroperoxides. An increase on inducible nitric oxide synthase expression was also observed in microglia treated with Aβ1–40 or PrP106. However, these peptides affected in a different manner the secretion of interleukin‐1β (IL‐1β) and interleukin‐6 (IL‐6) secretion. In cocultures of microglia‐neurons, it was observed that microglia treated with Aβ1–40 or PrP106–126 induced a comparable extent of neuronal death. The neutralizing antibody for IL‐6 significantly reduced the neuronal death induced by Aβ‐ or PrP‐activated microglia. Taken together, the data indicate that Aβ and PrP peptides caused microglia activation and differentially affected cytokine secretion. The IL‐6 released by reactive microglia caused neuronal injury. © 2006 Wiley‐Liss, Inc.
Maximum intraoperative elongation of the rat sciatic nerve with tissue expander: Functional, neurophysiological, and histological assessment
Authors: Arnaoutoglou Christina M. , Sakellariou Aikaterini , Vekris Marios , Mitsionis Grigoris I. , Korompilias Anastasios , Ioakim Elli , Harhantis Antonia , Beris Alexandros
Published: April 2006

The purpose of this study was to assess the maximum rapid intraoperative elongation of the rat sciatic nerve with the use of tissue expander, and its possible functional recovery. One hundred and eight rats were divided into five groups, and their right sciatic nerves were expanded with a 10‐cc, 12‐cc, 14‐cc, 16‐cc, and 18‐cc expander, respectively, for 1 h. The functional recovery of the nerve was assessed at intervals up to 3 months, using the sciatic function index (SFI), neurophysiological indices, and histology. The maximum intraoperative elongation was observed in group IV (16‐cc volume of tissue expander), at about 23.83%. SFI decreased between the first and seventh postoperative days, but gradually recovered, reaching preoperative values in all groups according to the formulas of De Medinaceli et al. (Exp. Neurol. 77:634–643, 1982) and Bain et al. (Plast. Reconstr. Surg. 83:129–136, 1989). Latency and motor conduction velocity demonstrated deterioration after expansion, which peaked after surgery. Recovery was gradually completed by the end of the experiment. The histological findings indicated minor aberrations immediately after expansion and maximal demyelination with axonal disruption on day 15. The reparative process started by day 30 and continued until day 90, when almost no histological changes were observed. In conclusion, intraoperative nerve expansion successfully elongates the rat sciatic nerve up to 23.83%. But it causes functional and morphological abnormalities, which are of moderate to severe degree, are of short duration, and are reversible. Intraoperative nerve expansion might be a valuable solution in the treatment of short nerve gaps, but its clinical application still needs to be evaluated. © 2006 Wiley‐Liss, Inc. Microsurgery, 2006
A recursive factorization method for the metric affine projection model
Authors: Fujiki Jun , Kurata Takeshi
Published: April 2006

The task of reconstructing camera motion and the shapes of objects from multiple images is a fundamental and important one in computer vision. Although among the reconstruction methods, the batch processing factorization method proposed by Tomasi and Kanade gives mathematically stable and good reconstruction results, it requires computational time that increases proportionately with the number of images handled and thus cannot be easily applied to real‐time processing. This paper proposes a recursive factorization method for a metric affine projection model, which allows a factorization method to be applied to real‐time processing. The proposed method makes it possible to minimize the sizes of the matrices handled and to shorten the computational time significantly while realizing a reconstruction accuracy comparable to that of motion matrices, by lossless compression of the motion information of the motion matrices by means of principal component analyses. In addition, the proposed method facilitates the processing of occlusion points and the addition of feature points. © 2006 Wiley Periodicals, Inc. Syst Comp Jpn, 37(7): 83–93, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10134
Dynamic texture: A physically based 2D animation system
Authors: Aoki Masakatsu , Shinya Mikio , Tsutsuguchi Ken
Published: April 2006

The authors developed a method (the Dynamic Texture method) to create realistic texture images for the purpose of creating CG animations. In their method, a skeleton representing a dynamic framework is specified for the given image. Dynamic simulation is then applied to this, and realistic movement of the skeleton can be created. By deforming the original image in concert with the skeleton, realistic animations can be created. The way in which this method is most different from conventional approaches is that a 3D model is not required. In other words, a characteristic of this method is that realistic animations can be created without the involved effort of 3D modeling. The authors used trees and grass swaying in the wind and a person walking as example objects and performed experiments that yielded good results. © 2006 Wiley Periodicals, Inc. Syst Comp Jpn, 37(6): 73–81, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10200
A genetic algorithm for the optimization of cable routing
Authors: Ma Xuan , Iida Kazuhiro , Xie Mengchun , Nishino Junji , Odaka Tomohiro , Ogura Hisakazu
Published: April 2006

In this paper the authors propose a structural method for a genetic algorithm (GA) for the optimization problem of cable routing in which cables have to be laid optimally. When there are no limits on the layout routes, the optimal routes for the individual cables can be found using the Dijkstra method in a finite discrete graph. However, when there are limits on the capacity in the layout routes, optimization cannot be performed using the Dijkstra method. The proposed GA is a two‐level hierarchical GA that uses chromosome coding involving two levels. The routes for each cable and the combinations of cable routes are defined respectively, and layout route selection is optimized overall through genetic operations involving these levels. Block crossover and block mutation are used as genetic operations in the first level. In addition, a method to take advantage of lethal genes, which do not satisfy the constraints and are generated in the second level is used. Computer simulations confirm that the two‐level hierarchical GA with these genetic operations functions effectively for the optimization problem of cable routing. © 2006 Wiley Periodicals, Inc. Syst Comp Jpn, 37(7): 61–71, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10250
3D flow estimation using perspective and parallel projection images
Authors: Shimamura Jun , Fujii Kensaku , Arakawa Kenichi , Arikawa Tomohiko
Published: April 2006

In this paper, the authors discuss a new method for using perspective projection image sequences and approximated parallel projection images that were generated from multiple perspective projection images to estimate the 3D flow representing the motion of a target in space. This technique uses the light ray information of multiple perspective projection images that were obtained by capturing the target at different positions to generate the approximated parallel projection images, which are projected in multiple ray directions. It then uses the optical flows that were obtained from both the perspective projection image sequence and approximated parallel projection image sequence to estimate the 3D flow of the measurement points. A feature of this technique is that using images that were projected in the ray directions of the measurement points that were established on a perspective projection image as the approximated parallel projection image sequence corresponding to the perspective projection image sequence makes it unnecessary to map pixel values between stereo images, which is required in the conventional technique. The authors verified through experiments that this technique enabled approximated parallel projection images to be generated and the 3D flow to be estimated satisfactorily by using them. © 2006 Wiley Periodicals, Inc. Syst Comp Jpn, 37(7): 47–60, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.20258
Bubble-driven liquid flows
Authors: Durst F. , Schönung B. , Selanger K. , Winter M.
Published: April 2006

Detailed information is provided in this paper on the physics of momentum transfer in bubble-driven liquid flows. Experimental information is obtained on the flow around bubbles and on the axisymmetric bubble-driven liquid flow inside liquid-filled cylinders located with their axes in the vertical direction. A laser-Doppler anemometer extended for particulate two-phase flows is employed for these measurements to yield local fluid velocity information as well as the rise velocity of bubbles. The bubble top radius and the bubble shape were also found from these measurements.

Utilizing experimentally gained information and employing the basic equations for particulate two-phase flows, permits finite difference equations to be formulated that allow bubble-driven liquid flows to be computed. Results are presented for boundary conditions corresponding to those of the experimental studies. Comparisons of numerical and experimental results are shown to be in good agreement. This is taken as a justification to employ the developed computer programs to carry out parameter studies for bubble-driven liquid flow inside circular cylinders. Results of these studies are presented and discussed.
Mixing and chemical reactions in a turbulent liquid mixing layer
Authors: Koochesfahani M. M. , Dimotakis P. E.
Published: April 2006

An experimental investigation of entrainment and mixing in reacting and non-reacting turbulent mixing layers at large Schmidt number is presented. In non-reacting cases, a passive scalar is used to measure the probability density function (p.d.f.) of the composition field. Chemically reacting experiments employ a diffusion-limited acid–base reaction to directly measure the extent of molecular mixing. The measurements make use of laser-induced fluorescence diagnostics and high-speed, real-time digital image-acquisition techniques.

Our results show that the vortical structures in the mixing layer initially roll-up with a large excess of fluid from the high-speed stream entrapped in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but its composition also changes. It is found that the range of compositions of the mixed fluid, above the mixing transition and also throughout the transition region, is essentially uniform across the entire transverse extent of the layer. Our measurements indicate that the probability of finding unmixed fluid in the centre of the layer, above the mixing transition, can be as high as 0.45. In addition, the mean concentration of mixed fluid across the layer is found to be approximately constant at a value corresponding to the entrainment ratio. Comparisons with gas-phase data show that the normalized amount of chemical product formed in the liquid layer, at high Reynolds number, is 50% less than the corresponding quantity measured in the gas-phase case. We therefore conclude that Schmidt number plays a role in turbulent mixing of high-Reynolds-number flows.
A hybrid method for transonic flow past multi-element aerofoils
Authors: Hill M. G. , Riley N.
Published: April 2006

A method for calculating transonic potential flow past a multi-element aerofoil configuration is presented. The method is a hybrid method that is based upon a compressible-flow panel method, valid for subcritical flow, and a finite-difference method that is suitable for supercritical flow calculations. The effectiveness of the proposed method is demonstrated, first by application to a single aerofoil and then to a three-element aerofoil.
The limiting behaviour of turbulence near a wall
Authors: Chapman Dean R. , Kuhn Gary D.
Published: April 2006

Three different Navier-Stokes computational models of incompressible viscoussublayer turbulence have been developed. Comparison of computed turbulence quantities with experiment is made for the mean streamwise velocity, Reynolds stress, correlation coefficient and dissipation; for the r.m.s. fluctuation intensities of streamwise vorticity, Reynolds stress and three velocity components; and for the skewness and flatness of fluctuating streamwise velocity and Reynolds stress. The comparison is good for the first three of these quantities, and reasonably good for most of the remainder.

Special computer runs with a very fine mesh and small Courant number were made to define the limiting power-law behaviour of turbulence near a wall. Such behaviour was found to be confined to about 0.3 wall units from the wall, and to be: linear for streamwise turbulence, spanwise turbulence, vorticity normal to the wall, and for the departures from their respective wall values of dissipation, streamwise vorticity and spanwise vorticity; second power for turbulence normal to the wall; third power for Reynolds stress; and a constant value of the correlation coefficient for Reynolds stress. A simple physical explanation is given for the third-power variation of Reynolds stress and for the broad generality of this limiting variation.

Applications are made to Reynolds-average turbulence modelling: damping functions for Reynolds stress in eddy-viscosity models are derived that are compatible with the near-wall limiting behaviour; and new wall boundary conditions for dissipation ink-ε models are developed that are similarly compatible.
Secondary bifurcations of Taylor vortices into wavy inflow or outflow boundaries
Authors: Iooss G.
Published: April 2006

Experiments of Andereck et al. (1986) with corotating cylinders, show that Taylor-vortex flow (TVF) can bifurcate into one of the following cellular flows: wavy vortices (WV), twisted vortices (TW), wavy inflow boundaries (WIB), wavy outflow boundaries (WOB). We describe here the structure of these different flows, showing how they result from simple symmetry breaking. Moreover we consider the codimension-two situation where WIB and WOB interact, since this is an observed physical situation.

The method used in this paper is based on symmetry arguments. It differs notably from the Liapunov-Schmidt reduction used in particular by Golubitsky & Stewart (1986) on the same problem with counter-rotating cylinders. Here we take into account all the dynamics, instead of restricting the study to oscillating solutions. In addition to the standard oscillatory modes, we have a translational mode due to the indeterminacy of TVF under the shifts along the axis. We derive an amplitude-expansion procedure which allows the translational mode to depend on time. Our amplitude equations have nevertheless a simple structure because the oscillatory modes have a precise symmetry. They break, in general, the rotational invariance and they are either symmetric or antisymmetric with respect to the plane z = 0. Moreover, the most typical cases are when either of these modes has the same axial period as TVF or when their axial period is double this. This leads to four different cases which are shown to give WV, TW, WIB or WOB, all these flows being ‘rotating waves’, i.e. they are steady in a suitable rotating frame.

Finally we consider the interaction between WIB and WOB that occurs when, at the onset of instability, the two critical modes arise simultaneously. In this case we show in particular that there may exist a stable quasi-periodic flow bifurcating from WIB or WOB. The two main frequencies are those of underlying WIB and WOB, while there may exist a third frequency corresponding to a slow superposed travelling wave in the axial direction.

The method was used in the counter-rotating case for interacting non-axisymmetric modes (see Chossat et al. 1986). One of the original contributions here is not only to clarify the origin of all observed bifurcations from TVF, but also to handle the translational mode which may not stay small. This technique combined with centre-manifold and equivariance techniques may be helpful for many problems starting with orbits of solutions, such as the TVF considered here.
Vortex shedding from square prisms in smooth and turbulent flows
Authors: Nakamura Yasuharu , Ohya Yuji
Published: April 2006

Visualization and measurements of velocity and pressure were made for the flow past prisms of variable length with square cross-section, placed normal to smooth and turbulent approaching flows. Square prisms shed vortices in one of the two fixed wake planes. The plane of shedding is switched irregularly from one to the other. Flow visualization confirms the two main effects of small– and large-scale turbulence on the flow past square prisms that had previously been suggested. In particular, large-scale turbulence intensifies vortex shedding from square prisms through resonant interaction, thereby reducing the base pressure considerably.
The three-dimensional hydrodynamic interaction of a finite sphere with a circular orifice at low Reynolds number
Authors: Yan Zong-Yi , Weinbaum Sheldon , Ganatos Peter , Pfeffer Robert
Published: April 2006

This paper proposes a combined multipole-series representation and integral-equation method for solving the low-Reynolds-number hydrodynamic interaction of a finite sphere at the entrance of a circular orifice. This method combines the flexibility of the intergral-equation method in treating complicated geometries and the accuracy and computational efficiency of the multipole-series-representation technique. For the axisymmetric case, the hydrodynamic force has been solved for the difficult case where the sphere intersects the plane of the orifice opening, which could not be treated by previous methods. For the three-dimensional case, the first numerical solutions have been obtained for the spatial variation of the twelve force and torque correction factors describing the translation or rotation of the sphere in a quiescent fluid at a pore entrance or the Sampson flow past a fixed sphere. Restricted by excessive computation time, accurate three-dimensional solutions are presented only for a sphere which is one-half the orifice diameter. However, based on an analysis of the behaviour of the force and torque correction factors for this case, approximate interpolation formulas utilizing the results on or near the orifice axis and in the far field are proposed for other diameter ratios, thus greatly extending the usefulness of the present solution.
An asymptotic theory for the turbulent flow over a progressive water wave
Authors: Jacobs S. J.
Published: April 2006

The turbulent flow over a progressive water wave is studied using an eddy viscosity model. The governing equations are treated asymptotically for the case ε [Lt ] 1, where ε is the square root of a characteristic drag coefficient. A calculation of the phase shift between the wave-induced pressure perturbation and the surface elevation shows that the phase shift is induced by a term in the gradient of the Reynolds stress. Growth rates are determined, and are shown to agree well with observations for the most rapidly amplifying waves. However, the present model and previous turbulence calculations are found to provide significantly lower growth rates than those measured by Snyder et al. (1981) for waves with phase velocities comparable to the wind speed.
Nonlinear transition in three-dimensional convection
Authors: Kessler R.
Published: April 2006

Steady and oscillatory convection in a rectangular box heated from below are studied by means of a numerical solution of the three-dimensional, time-dependent Boussinesq equations. The effect of the rigid sidewalls of the box on the spatial structure and the dynamical behaviour of the flow is analysed. Both conducting and adiabatic sidewalls are considered. Calculated streamlines illustrate the three-dimensional structure of the steady flow with Prandtl numbers 0.71 and 7. The onset and the frequency of the oscillatory instability are calculated and compared with available experimental and theoretical data. With increasing Rayleigh number a subharmonic bifurcation and the onset of a quasi-periodic flow can be observed. A comparison of the different time-dependent solutions shows some interesting relations between the spatial structure and the dynamical behaviour of the confined flow.
Surface pressure fluctuations in a separating turbulent boundary layer
Authors: Simpson Roger L. , Ghodbane M. , Mcgrath B. E.
Published: April 2006

Measurements of surface pressure-fluctuation spectra and wave speeds are reported for a well-documented separating turbulent boundary layer. Two sensitive instrumentation microphones were used in a new technique to measure pressure fluctuations through pinhole apertures in the flow surface. Because a portion of the acoustic pressure fluctuations is the same across the nominally two-dimensional turbulent flow, it is possible to decompose the two microphone signals and obtain the turbulent flow contributions to the surface pressure spectra. In addition, data from several earlier attached-flow surface-pressure-fluctuation studies are re-examined and compared with the present measurements.

The r.m.s. of the surface pressure fluctuation p′ increases monotonically through the adverse-pressure-gradient attached-flow region and the detached-flow zone. Apparently p′ is proportional to the ratio α of streamwise lengthscale to lengthscales in other directions. For non-equilibrium separating turbulent boundary layers, α is as much as 2.5, causing p′ to be higher than equilibrium layers with lower values of α.

The maximum turbulent shearing stress τM appears to be the proper stress on which to scale p′; p′/τM from available data shows much less variation than when p′ is scaled on the wall shear stress. In the present measurements p′/τM increases to the detachment location and decreases downstream. This decrease is apparently due to the rapid movement of the pressure-fluctuation-producing motions away from the wall after the beginning of intermittent backflow. A correlation of the detached-flow data is given that is derived from velocity- and lengthscales of the separated flow.

Spectra Φ (ω) for ωδ*/U∞ > 0.001 are presented and correlate well when normalized on the maximum shearing stress τM. At lower frequencies, for the attached flow Φ (ω) ∼ ω−0.7 while Φ(ω) ∼ (ω)−3 at higher frequencies in the strong adverse-pressuregradient region. After the beginning of intermittent backflow, Φ(ω) varies with ω at low frequencies and ω−3 at high frequencies; farther downstream the lower-frequency range varies with ω1.4.

The celerity of the surface pressure fluctuations for the attached flow increases with frequency to a maximum; at higher frequencies it decreases and agrees with the semi-logarithmic overlap equation of Panton & Linebarger. After the beginning of the separation process, the wave speed decreases because of the oscillation of the instantaneous wave speed direction. The streamwise coherence decreases drastically after the beginning of flow reversal.
An experimental study of strongly nonlinear waves in a rotating system
Authors: Renouard Dominique P. , D'Hières Gabriel Chabert , Zhang Xuizhang
Published: April 2006

The influence of rotation upon internal solitary waves is studied in a (10 m × 2 m × 0.6 m) channel located on the large rotating platform at Grenoble University. We observe an intumescence which moves along the right-hand side of the channel with respect to its direction of propagation. Along the side, once the intumescence reaches its equilibrium shape, the height variation of the interface with time is correctly described by the sech2 function, and the characteristic KdV scaling law linking the maximum amplitude and the wavelength along the side is fulfilled. The intumescence is a stable phenomenon which moves as a whole without deformation apart from the viscous damping. For identical experimental conditions, the amplitude of the intumescence along the side increases with increasing Coriolis parameter, and at a given period of rotation of the platform, the celerity along the side increases with increasing amplitude. But for identical conditions, we found that the celerity along the side is equal to the celerity that the wave would have for such conditions without rotation. The amplitude of the intumescence in a plane perpendicular to the wall decreases exponentially with increasing distance from the side, but the crest of the wave is curved backward.
Nonlinear mechanics of fluidization of beds of spherical particles
Authors: Fortes A. F. , Joseph D. D. , Lundgren T. S.
Published: April 2006

Experiments on fluidization with water of spherical particles falling against gravity in columns of rectangular cross-section are described. All of them are dominated by inertial effects associated with wakes. Two local mechanisms are involved: drafting and kissing and tumbling into stable cross-stream arrays. Drafting, kissing and tumbling are rearrangement mechanisms in which one sphere is captured in the wake of the other. The kissing spheres are aligned with the stream. The streamwise alignment is massively unstable and the kissing spheres tumble into more stable cross-stream pairs of doublets which can aggregate into larger relatively stable horizontal arrays. Cross-stream arrays in beds of spheres constrained to move in two dimensions are remarkable. These arrays may even coalesce into aggregations of close-packed spheres separated by regions of clear water. A somewhat weaker form of cooperative motion of cross-stream arrays of rising spheres is found in beds of square cross-section where the spheres may move freely in three dimensions. Horizontal arrays rise where drafting spheres fall because of greater drag. Aggregation of spheres seems to be associated with relatively stable cooperative motions of horizontal arrays of spheres rising in their own wakes.
Reflection of a shallow-water soliton. Part 2. Numerical evaluation.
Authors: Sugimoto N. , Kusaka Y. , Kakutani T.
Published: April 2006

Reflection of a shallow-water soliton at a plane beach is investigated numerically based on the edge-layer theory developed in Part 1 of this series. The offshore behaviour in the shallow-water region is first obtained by solving the Boussinesq equation under the ‘reduced’ boundary condition. The spatial and temporal variations of the surface elevation are displayed for two typical values of the inclination angle of the beach. Using these solutions, the nearshore behaviour is then evaluated to obtain the surface elevation and the velocity distribution in the edge layer. Both offshore and nearshore behaviours furnish a full knowledge of the reflection problem of a shallow-water soliton. To check the applicability of the edge-layer theory, a ‘computational experiment’ is carried out based on the boundary-element method, in which the Laplace equation is solved numerically under the full nonlinear boundary conditions at the free surface without introducing the edge-layer concept. Both results show a fairly good agreement for the overall reflection behaviour of a shallow-water soliton except for the surging movement at the shoreline.
Three-dimensional steady streaming in a uniform tube with an oscillating elliptical cross-section
Authors: Padmanabhan N. , Pedley T. J.
Published: April 2006

We analyse the steady streaming generated in an infinite elliptical tube containing a viscous, incompressible fluid when the boundary oscillates in such a way that the area and ellipticity of the cross-section vary with time but remain independent of the longitudinal coordinate. The parameters α−1 = (ν/Ωa02)½ and ε = U0/a0Ω, where ν is the kinematic viscosity, Ω is the oscillation frequency, a0 is the undisturbed semi-major axis and U0 is a typical wall velocity, are taken to be small, so that the Stokes layer is thin and the interaction which leads to the steady streaming can be analysed as a small perturbation. Coupled axial and transverse velocities, both oscillatory and steady, are generated. A complication is the need to specify the tangential as well as the normal velocity component on the tube wall, which requires an assumption concerning its elastic properties. We have considered two cases: (i) constant major axis, in which all boundary points move parallel to the minor axis, and (ii) an inextensible wall. The three-dimensional steady streaming in the core of the tube is computed only in the limit of small steady-streaming Reynolds number, Rs = ε2α2.
Diffusion of matter by a non-buoyant plume in grid-generated turbulence
Authors: Nakamura Ikuo , Sakai Yasuhiko , Miyata Masafumi
Published: April 2006

The turbulent diffusion process is investigated for a continuous point source of a non-buoyant plume in grid-generated water turbulence. Two kinds of biplanar grids with a mesh length of 10 mm and 20 mm were used. The mesh Reynolds numbers were 1480 and 2970, respectively. The mean and fluctuating concentration fields of aqueous dye solution were measured by the light absorption method. Experimental results for both grids were compared.

For both grids, the mean concentration radial profiles proved to have a similar Gaussian shape, and the mean concentration on the plume axis obeys the hyperbolic decay law well. These mean concentration profiles and their decay show an excellent agreement with the results deduced from the similarity analysis for the mean concentration field.

Radial profiles of the fluctuation r.m.s. value and relative intensity (i.e. the ratio of the r.m.s. value to the mean concentration) were found also to be nearly similar, and the centreline r.m.s. value decays downstream as a hyperbola. The relative intensity on the centreline tends to increase slightly downstream. All experimental results obtained were much less scattered and more reliable than those reported earlier.

The similarity for the concentration fluctuation intensity has been analysed using a thin-layer approximation. Also, an approximate analysis of the fluctuating concentration field is given by replacing the fluctuating concentration signal by a randomly spaced sequence of rectangular waves with various heights and widths.
The effects of turbulence on a separated and reattaching flow
Authors: Nakamura Yasuharu , Ozono Shigehira
Published: April 2006

The effect of free-stream turbulence on the mean pressure distribution along the separation bubble formed on a flat plate with rectangular leading-edge geometry is investigated experimentally in a wind tunnel using turbulence-producing grids. Emphasis is placed on finding the effect of turbulence scale. The ratio of turbulence scale to plate thickness investigated was about 0.5 to 24 for two values of turbulence intensity of about 7 and 11%. The Reynolds number based on plate thickness was approximately (1.4–4.2) × 104.

It is found that the main effect of free-stream turbulence is to shorten the separation bubble. It is progressively shortened with increasing turbulence intensity. The mean pressure distribution along the shortened separation bubble is insensitive to changing turbulence scale up to a scale ratio of about 2. With further increase in the scale ratio it asymptotes towards the smooth-flow distribution. There is no trace of interaction between turbulence and vortex shedding (the impinging-shear-layer instability) in the mean pressure distribution.
The general solution for an ellipsoid in low-Reynolds-number flow
Authors: Kim Sangtae , Arunachalam P. V.
Published: April 2006

The general solution for low-Reynolds-number flow about an ellipsoid is derived by the singularity method and by representation in ellipsoidal harmonics. It is shown that, as in potential flow, the focal ellipse is the image system for the ellipsoid. A simple transformation which resembles a step in the derivation of the Dirichlet potential is introduced and its implications are explored. This transformation converts the velocity representation for an nth-order ambient field into that for the (n + 1)th-order field. The method furnishes an explanation for the invariance of the domain of the singularity distribution (the focal ellipse) with respect to the ambient field. Faxén relations for all multipole moments for arbitrary Stokes flow are derived in both integral and symbolic operator forms.
Large deformations of a cylindrical liquid-filled membrane by a viscous shear flow
Authors: Zahalak George I. , Rao Peddada R. , Sutera Salvatore P.
Published: April 2006

This paper treats the steady flow fields generated inside and outside an initially circular, inextensible, cylindrical membrane filled with an incompressible viscous fluid when the membrane is placed in a two-dimensional shear flow of another viscous fluid. The Reynolds numbers of both the interior and exterior flows were assumed to be zero (‘creeping flow’), but no further approximations were made in the formulation. A series solution of the resulting free boundary-value problem in powers of a dimensionless shear rate parameter was constructed through fifth order. When combined with a conformal coordinate transformation this series gave accurate results for large deformations of the membrane (up to an aspect ratio of 2.5). The rather tedious algebraic manipulations required to obtain the series solution were done by computer with a symbol-manipulation program (reduce), which both formulated the boundary-value problems for each successive order and solved them. Results are presented which show how the shear rate and fluid viscosities influence the internal and external velocity and pressure fields, the membrane deformation and its ‘tank-treading’ frequency, and the membrane tension.

This work demonstrates that classical perturbation techniques combined with computer algebra offer a useful alternative to purely numerical methods for problems of this type.
An extension of the ‘Malkus hypothesis’ to the turbulent base flow of blunt sections
Authors: Vorus William S. , Chen Liyong
Published: April 2006

This work develops and demonstrates an approximate theory for the mean turbulent near-wake of cylindrical bodies with blunt after edges. A closed free-streamline model is used to relate base pressure to the separation-streamline length. The additional relationship between base pressure and streamline length required for closure is an extension of the maximum dissipation hypothesis of Malkus, which was originally proposed for the turbulent-channel-flow problem. In the current application, the Malkus hypothesis leads to maximization of the rate of change of mean kinetic energy along the separation-cavity streamline.

The theory is implemented in terms of a linearized closed free-streamline theory of thin blunt-based symmetric sections, which was actually developed as an application to supercavitation, rather than separation. The calculations performed compare quite well with experimental measurements of mean base pressures and section drag. However, the linearizing assumptions on section-cavity slenderness and base-pressure magnitude are not so well preserved in the calculated results. Numerical analysis in terms of a nonlinear closed free-streamline model is a possible recourse. However, an important class of problems where the assumptions required in applying the linear theory may be better represented by the prevailing physics is the superseparation of thin lifting foils. This would appear to be the immediately most fruitful direction for the work, as a very mature theory of linearized supercavitation of thin lifting foils is available for providing the required free-streamline model.

Even though the formulation contains the effects of turbulent dissipation, the intricacies of the wake turbulence are avoided in achieving the solution. The lumped mean turbulent transfer rate per unit length of the mean dividing streamline is quantified, and defined in terms of the turbulence variables. Because of this, it is suggested that the theory may prove useful as a tool for studying wake turbulence characteristics.
On the genesis of quasi-steady vortices in a rotating turbulent flow
Authors: Mory Mathieu , Caperan Philippe
Published: April 2006

Turbulent flows subjected to rotation display vortices parallel to the rotation axis and exhibiting a long timescale compared to the turbulent turnover time and the rotation period. A similar flow pattern is observed arising from the thermal instability in a rotating fluid. We demonstrate the analogy between turbulence and thermal convection in a rotating fluid. A basic quasi-geostrophic turbulent flow is considered which is forced at the bottom of the layer by a stochastic component of velocity parallel to the rotation axis. The turbulent basic state has no mean flow and the gradient along the rotation axis of the turbulent kinetic energy −∂z〈ω2〉 is analogous to the mean temperature profile in thermal convection. The linear perturbation equations of this basic turbulent state are given, where the thermal diffusion equation is replaced by the turbulent kinetic energy equation. Using a simple closure of this equation the model demonstrates the occurrence of an instability when the Reynolds number exceeds a critical value. Marginal stability curves are deduced by numerical integration of the perturbation equations. The results show order-of-magnitude agreement with laboratory experiments.
Instability and breaking of a solitary wave
Authors: Tanaka M. , Dold J. W. , Lewy M. , Peregrine D. H.
Published: April 2006

The result of a linear stability calculation of solitary waves which propagate steadily along the free surface of a liquid layer of constant depth is examined numerically by employing a time-stepping scheme based on a boundary-integral method. The initial’ growth rate that is found for sufficiently small perturbations agrees well with the growth rate expected from the linear stability calculation. In calculating the later ‘nonlinear’ stage of the instability, it is found that two distinct types of long-time evolution are possible. These depend only on the sign of the unstable normal-mode perturbation that is superimposed initially on the steady wave. The growth of the perturbation ultimately leads to breaking for one sign. Unexpectedly, for the opposite sign, there is a monotonic decrease in the total height of the wave. In this latter case there is a smooth evolution to a stable solitary wave of lesser amplitude but very nearly the same energy.
On the secondary instability of Taylor-Görtler vortices to Tollmien-Schlichting waves in fully developed flows
Authors: Bennett James , Hall Philip
Published: April 2006

There are many flows of practical importance where both Tollmien-Schlichting waves and Taylor-Görtler vortices are possible causes of transition to turbulence. In this paper, the effect of fully nonlinear Taylor-Görtler vortices on the growth of small-amplitude Tollmien-Schlichting waves is investigated. The basic state considered is the fully developed flow between concentric cylinders driven by an azimuthal pressure gradient. It is hoped that an investigation of this problem will shed light on the more complicated external-boundary-layer problem where again both modes of instability exist in the presence of concave curvature. The type of Tollmien-Schlichting waves considered have the asymptotic structure of lower-branch modes of plane Poiseuille flow. Whilst instabilities at lower Reynolds number are possible, the former modes are simpler to analyse and more relevant to the boundary-layer problem. The effect of fully nonlinear Taylor-Görtler vortices on both two-dimensional and three-dimensional waves is determined. It is shown that, whilst the maximum growth as a function of frequency is not greatly affected, there is a large destabilizing effect over a large range of frequencies.
Measurements of the mean force on a particle near a boundary in turbulent flow
Authors: Hall D.
Published: April 2006

The transport of particles through gaseous systems is controlled by three factors: their arrival to the surface; whether or not they bounce upon impact; and when (if ever) they are resuspended from the surface. One of the parameters required in determining whether or not a particle is suspended is the lift force acting on the particle. We demonstrate that the fluid lift forces acting on particles as small as 1 μm in diameter can be modelled by particles of several mm in diameter. However, the forces involved in modelling such small particles are around 10−8 N, which is several orders of magnitude smaller than reported in published measurements of fluid lift forces. A system to determine such lift forces has been developed and is described. Measurements of the mean force acting on particles on both rough and smooth surfaces are presented.

The data recorded here for the mean fluid lift force on a sphere on a smooth surface are in good agreement with the relationship
\[
F^{+} = (20.90\pm 1.57)(a^{+})^{2.31\pm 0.02},
\]
where F+ is the non-dimensional force and a+ the non-dimensional particle radius scaled on fluid-boundary-layer parameters. It was observed that surface roughness can change the force by up to a factor of six.
The dynamics of towed flexible cylinders Part 1. Neutrally buoyant elements
Authors: Dowling A. P.
Published: April 2006

The transverse vibrations of a thin, flexible cylinder under nominally constant towing conditions are investigated. The cylinder is neutrally buoyant, of radius aA with a free end and very small bending stiffness. As the cylinder is towed with velocity U, the tangential drag causes the tension in the cylinder to increase from zero at its free end to a maximum at the towing point. Transverse vibrations of the cylinder are opposed by a normal viscous drag force. Both the normal and tangential viscous forces can be described conveniently in terms of drag coefficients CN and CT. The ratio CN/CT has a crucial effect on the motion of the cylinder. The form of the transverse displacement is found to be greatly influenced by the existence of a critical point at which the effect of tension in the cylinder is cancelled by a fluid loading term. Matched asymptotic expansions are used to extend the solution across this critical point to apply the downstream boundary condition. Displacements well upstream of the critical point have a simple form, while nearer to the critical point the solution depends on whether the normal drag coefficient CN is greater or less than one-half CT.

The typical acoustic streamer geometry considered is found to be stable to transverse displacements at all towing speeds. Forced perturbations of frequency ω are investigated. At low frequencies they propagate effectively along the cylinder with speed U. At higher frequencies they are attenuated.

The effect of a rope drogue of length lR, radius aR is investigated. Provided ωlRaR/UaA is very small, the drogue has the same effect as a small increase in the length of the cylinder. However at higher frequencies and for small values of the ratio CN/CT attaching a drogue may be disadvantageous because it reduces the attenuation of high-frequency disturbances as they propagate down the cylinder.
Pressure drop and power dissipation in oscillatory wavy-walled-tube flows
Authors: Ralph M. E.
Published: April 2006

Pressure drops occurring in oscillatory viscous flows in wavy-walled tubes have been studied experimentally, for Reynolds numbers up to 1500 and Strouhal numbers in the range 0.005 to 0.02, and by numerical solution of the Navier-Stokes equations, for Reynolds numbers up to 200 and Strouhal numbers between 0.005 and 0.1. Agreement was good for values of the mean modulus of the pressure drop at lower Strouhal numbers and for values of the mean power dissipation at all Strouhal numbers.

Numerical solutions have shown that the pressure drop may vary non-sinusoidally, even though the imposed variation in flow rate is sinusoidal. This cannot be explained by the nonlinearity of the steady pressure drop-flow rate relationship, and arises because the velocity field is not quasi-steady. In particular energy may be stored in strong vortices formed during the acceleration phase of the flow cycle, and partially returned to the main flow later. The peak pressure drops in such flows, which are associated with the formation of these vortices, can be almost twice as large as values predicted by adding the appropriate quasi-steady and unsteady inertial contributions. This finding is important in the wider context of unsteady conduit flow.

The dependences of the mean modulus of the pressure drop and the mean power dissipation on the Strouhal number and frequency parameter were investigated in detail numerically for two geometries. It was not possible to reduce either dependence to a function of a single parameter. The ‘equivalent’ straight-walled tube for power dissipation was found to have a smaller bore than that for pressure drop, leading to smaller ‘phase angles’ than might have been expected at large values of the frequency parameter. This is because as the pressure drop becomes increasingly dominated by unsteady inertia, there remain relatively large recirculations in which energy is dissipated.