bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
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.
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.
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.
Instability of swirl in low-Prandtl-number thermal convection
Authors: Massaguer Josep M. , Mercader Isabel 
Published: April 2006

In the present paper we examine low-Prandtl-number thermal convection using a highly truncated modal approach. For the horizontal structure we assume a hexagonal planform as in Toomre Gough & Spiegel (1977) but including a vertical vorticity mode. The system develops a non-zero vertical vorticity component through a finite-amplitude instability. Following this, the system displays a Hopf bifurcation giving rise to periodic oscillations. The mechanism for this instability is associated with the growth of swirl in the azimuthal direction. We have found three different types of periodic solutions, possibly associated with subharmonic bifurcations, and their structure has been examined.A large part of the present work is devoted to exploring the cases of mercury and liquid helium - or air - as the best-known examples of low and intermediate-Prandtl-number fluids. Results for mercury are quite satisfactory as far as frequencies and fluxes are concerned and they show reasonable agreement with experimental measurements at mildly supercritical Rayleigh values. On the other hand, for liquid helium or air agreement is poor.
Vitamin A supplementation increases ratios of proinflammatory to anti-inflammatory cytokine responses in pregnancy and lactation
Authors: Cox S E , Arthur P , Kirkwood B R , Yeboah-Antwi K , Riley E M 
Published: April 2006

SummaryVitamin A supplementation reduces child mortality in populations at risk of vitamin A deficiency and may also reduce maternal mortality. One possible explanation for this is that vitamin A deficiency is associated with altered immune function and cytokine dysregulation. Vitamin A deficiency in pregnancy may thus compound the pregnancy-associated bias of cellular immune responses towards Th-2-like responses and exacerbate susceptibility to intracellular pathogens. We assessed mitogen and antigen-induced cytokine responses during pregnancy and lactation in Ghanaian primigravidae receiving either vitamin A supplementation or placebo. This was a double-blind, randomized, placebo-controlled trial of weekly vitamin A supplementation in pregnant and lactating women. Pregnancy compared to postpartum was associated with a suppression of cytokine responses, in particular of the proinflammatory cytokines interferon (IFN)-γ and tumour necrosis factor (TNF)-α. Mitogen-induced TNF-α responses were associated with a decreased risk of peripheral parasitaemia during pregnancy. Furthermore, vitamin A supplementation was significantly associated with an increased ratio of mitogen-induced proinflammatory cytokine (IFN-γ) to anti-inflammatory cytokine (IL-10) during pregnancy and in the postpartum period. The results of this study indicate that suppression of proinflammatory type 1 immune responses and hence immunity to intracellular infections, resulting from the combined effects of pregnancy and vitamin A deficiency, might be ameliorated by vitamin A supplementation.
Class‐3c compression bandaging for venous ulcers: comparison of spiral and figure‐of‐eight techniques
Authors: Coull Alison , Tolson Debbie , McIntosh Jean 
Published: April 2006

Aim.  This paper reports a study to compare the differences in compression produced on a limb using a spiral and a figure‐of‐eight bandaging technique.Background.  Chronic leg ulcers are a major health problem in Western countries. Compression bandaging is the accepted treatment for venous leg ulcers. The spiral and the figure‐of‐eight techniques are traditional methods of applying compression bandages. but their performance in terms of pressure profiles is unknown. Despite a lack of evidence about technique, class‐3c compression bandages are not currently recommended for application in a figure‐of‐eight as this is thought to produce dangerously high pressure.Methods.  A cross‐over experimental design was used with a sample of 26 nurse bandagers. Consistency of performance was tested. Bandage stretch and overlap were measured and analysed by standard deviation to ensure that the techniques were consistent and repeatable. Pressure probes were then applied to the medial, posterior and lateral aspects of the lower limb of a healthy volunteer. The class‐3c compression bandage was applied using both techniques and the pressure profiles around the leg were measured under the bandage. The results of each technique were compared using independent sample t‐tests. The data were collected in 1999.Results.  The figure‐of‐eight technique provided statistically significantly higher compression at lower areas of the leg than the spiral technique. Mean pressure difference was 20·3 mmHg at the ankle, 22·2 mmHg at the gaiter, 9·4 mmHg at the calf. The pressure differences decrease towards the knee with both techniques and decrease more steeply with the figure‐of‐eight technique.Conclusion.  The spiral technique was a safe and comfortable method of bandage application. The figure‐of‐eight also provided graduated compression but with some high areas of pressure, notably overlying the Achilles tendon. However, before firm practice recommendations are made, replication of the study using a padding layer and a larger sample is needed.
MreB of Streptomyces coelicolor is not essential for vegetative growth but is required for the integrity of aerial hyphae and spores
Authors: Mazza Paola , Noens Elke E. , Schirner Kathrin , Grantcharova Nina , Mommaas A. Mieke , Koerten Henk K. , Muth Günther , Flärdh Klas , Van Wezel Gilles P. , Wohlleben Wolfgang 
Published: April 2006

SummaryMreB forms a cytoskeleton in many rod‐shaped bacteria which is involved in cell shape determination and chromosome segregation. PCR‐based and Southern analysis of various actinomycetes, supported by analysis of genome sequences, revealed mreB homologues only in genera that form an aerial mycelium and sporulate. We analysed MreB in one such organism, Streptomyces coelicolor. Ectopic overexpression of mreB impaired growth, and caused swellings and lysis of hyphae. A null mutant with apparently normal vegetative growth was generated. However, aerial hyphae of this mutant were swelling and lysing; spores doubled their volume and lost their characteristic resistance to stress conditions. Loss of cell wall consistency was observed in MreB‐depleted spores by transmission electron microscopy. An MreB–EGFP fusion was constructed to localize MreB in the mycelium. No clearly localized signal was seen in vegetative mycelium. However, strong fluorescence was observed at the septa of sporulating aerial hyphae, then as bipolar foci in young spores, and finally in a ring‐ or shell‐like pattern inside the spores. Immunogold electron microscopy using MreB‐specific antibodies revealed that MreB is located immediately underneath the internal spore wall. Thus, MreB is not essential for vegetative growth of S. coelicolor, but exerts its function in the formation of environmentally stable spores, and appears to primarily influence the assembly of the spore cell wall.
Fine‐scale time‐lapse analysis of the biphasic, dynamic behaviour of the two Vibrio cholerae chromosomes
Authors: Fiebig Aretha , Keren Kinneret , Theriot Julie A. 
Published: April 2006

SummaryUsing fluorescent repressor‐operator systems in live cells, we investigated the dynamic behaviour of chromosomal origins in Vibrio cholerae, whose genome is divided between two chromosomes. We have developed a method of analysing fine‐scale motion in the curved co‐ordinate system of vibrioid bacteria. Using this method, we characterized two different modes of chromosome behaviour corresponding to periods between segregation events and periods of segregation. Between segregation events, the origin positions are not fixed but rather maintained within ellipsoidal caged domains, similar to eukaryotic interphase chromosome territories. These domains are approximately 0.4 µm wide and 0.6 µm long, reflecting greater restriction in the short axis of the cell. During segregation, movement is directionally biased, speed is comparable between origins, and cell growth can account for nearly 20% of the motion observed. Furthermore, the home domain of each origin is positioned by a different mechanism. Specifically, the oriCI domain is maintained at a constant actual distance from the pole regardless of cell length, while the oriCII domain is maintained at a constant relative position. Thus the actual position of oriCII varies with cell length. While the gross behaviours of the two origins are distinct, their fine‐scale dynamics are remarkably similar, indicating that both experience similar microenvironments.