bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spatially amplifying modes of the Charney baroclinic-instability problem
Authors: Pierrehumbert R. T. 
Published: April 2006

We determine the circumstances under which baroclinic instability in the Charney model subjected to localized time-periodic forcing manifests itself as a wavetrain that oscillates at the source frequency and amplifies in space with distance from the source; analytical and numerical results describing the salient characteristics of such waves are presented. The spatially amplifying disturbance is a hitherto unsuspected part of the response to a pulsating source, and coexists with the more familiar neutral Rossby wavetrains; it is likely to play a role in a wide range of atmospheric and oceanic phenomena.The central results rely on a careful application of a causality criterion due to Briggs. These results illustrate a practical means of attacking spatial instability problems, which can be applied to a broad class of systems besides the one at hand. We have found that the Charney problem with positive vertical shear is not absolutely unstable, so long as the wind at the ground is non-negative. This implies that spatial instability and forced stationary-wave problems are well posed in an open domain under typical atmospheric circumstances.The amplifying waves appear on the downstream side of the source, have eastward (downstream) phase propagation and have wavelengths that increase monotonically with decreasing frequency, becoming infinite at zero frequency. When the surface wind is not too large, the spatial amplification rate has a single maximum near the frequency ωm= (f/N)Uz, wherefis the Coriolis parameter,Nis the stability frequency andUzis the vertical shear; the rate approaches zero at zero frequency and asymptotes algebraically to zero at large frequency for any positive surface wind. Distinct Charney and Green modes do not appear until the surface wind is made very large. The amplification rate at ωmbecomes infinite as surface wind approaches zero, suggesting a mechanism for the concentration of eddy activity.We also discuss the relationship of these results to the structure of low- and high-frequency atmospheric variability.
The motion of a viscous drop sliding down a Hele-Shaw cell
Authors: Jansons Kalvis M. 
Published: April 2006

The motion of a viscous drop in a vertical Hele-Shaw cell is studied in a limit where the effect of surface tension through contact-angle hysteresis is significant. It is found that a rectangular drop shape is a possible steady solution of the governing equations, although this solution is unstable to perturbations on the leading edge. Even though the unstable edge is one where a viscous fluid is moving into a less viscous fluid, in this case air, this is shown to be a special case of the well-known Saffman—Taylor instability. An experiment is performed with an initially circular drop in which it is observed that the drop shape becomes approximately rectangular except at the leading edge, where it becomes rounded and sometimes has a ragged appearance.A drop sliding down a vertical Hele-Shaw cell is an example of a system where the action of surface tension is not always one of smoothing, since in this case it leads to the formation of right-angle corners at the back of the drop (rounded only slightly on the lengthscale of the gap thickness of the cell).
Maximal two-layer exchange over a sill and through the combination of a sill and contraction with barotropic flow
Authors: Farmer D. M. , Armi L. 
Published: April 2006

The analysis of two-layer exchange flow through contractions with a barotropic component treated by Armi & Farmer (1986) is extended to include exchange flows over sills and through a combination of a sill and contraction. It is shown that exchange over a sill is fundamentally different from exchange through a contraction. Control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer. This asymmetry in the control results in asymmetrical flows. The interface depth above the crest is not one half the total depth, as assumed in other studies by analogy with flow through contractions, but is somewhat deeper; the maximal exchange rate is less than for flow through a contraction of equal depth. When both a sill and a contraction are present, the contraction influences control at the sill crest only if it lies between the sill and the source of denser water. The response to barotropic flow is also asymmetrical: the transition to single-layer flow occurs at much lower speeds for a barotropic component in one direction than the other.Results of the analysis are applied to exchange flow through the Strait of Gibraltar, which includes both a sill and a contraction. It is shown that maximal exchange conditions apply throughout part of the tidal cycle, and observations illustrate several of the analytical predictions for barotropic flows, including the formation of fronts, single-layer flow, submaximal exchange and reverse flow.
On the role of subharmonic perturbations in the far wake
Authors: Meiburg E. 
Published: April 2006

The possibility of an excitation of individual subharmonic perturbations in each of the shear layers forming the far wake is investigated numerically. Principal considerations allow for the existence of two equivalent subharmonic modes which by opposite routes can lead to a doubling of the wavelength in the wake. Since vortical disturbances in the far wake are amplified only convectively, the simultaneous existence of both modes in the flow field is possible, which could provide an explanation for the group structure observed experimentally in the far wake. These considerations also provide a logical explanation of the finding of a very regular vortex pairing process in forced wakes.Two-dimensional numerical simulations assuming incompressible flow and almost inviscid dynamics illustrate the opposite developments of regions dominated by the two different modes and also confirm the possibility of a resulting group structure. As an important result it is demonstrated that, if vortex pairing plays an important role in the growth of the far-wake structure, this does not have to be related to the excitation of the subharmonic peak in the frequency spectrum. Quite the contrary, it is to be expected that the subharmonic itself is of minor importance and that instead a small frequency and its multiples related to the group structure of the flow dominate the spectrum. In the light of these considerations measurements by Cimbala (1984) are discussed and frequency spectra recorded by him are analysed more closely. Various properties of these spectra seem to indicate that vortex pairing might be significant with respect to the evolution of the far-wake structure.
The structure of a turbulent shear layer bounding a separation region
Authors: Castro I. P. , Haque A. 
Published: April 2006

Detailed measurements within the separated shear layer behind a flat plate normal to an airflow are reported. A long, central splitter plate in the wake prevented vortex shedding and led to an extensive region of separated flow with mean reattachment some ten plate heights downstream. The Reynolds number based on plate height was in excess of 2 × 1044.Extensive use of pulsed-wire anemometry allowed measurements of all the Reynolds stresses throughout the flow, along with some velocity autocorrelations and integral timescale data. The latter help to substantiate the results of other workers obtained in separated flows of related geometry, particularly in the identification of a very low-frequency motion with a timescale much longer than that associated with the large eddies in the shear layer. Wall-skin-friction measurements are consistent with the few similar data previously published and indicate that the thin boundary layer developing beneath the separated region has some ‘laminar-like’ features.The Reynolds-stress measurements demonstrate that the turbulence structure of the separated shear layer differs from that of a plane mixing layer between two streams in a number of ways. In particular, the normal stresses all rise monotonically as reattachment is approached, are always considerably higher than the plane layer values and develop in quite different ways. Flow similarity is not a useful concept. A major conclusion is that any effects of stabilizing streamline curvature are weak compared with the effects of the re-entrainment at the low-velocity edge of the shear layer of turbulent fluid returned around reattachment. It is argued that the general features of the flow are likely to be similar to those that occur in a wide range of complex turbulent flows dominated by a shear layer bounding a large-scale recirculating region.
Swirling recirculating flow an a liquid-metal column generated by a rotating magnetic field
Authors: Davidson P. A. , Hunt J. C. R. 
Published: April 2006

In this paper we consider theoretical and experimental aspects of axisymmetric, swirling flow which is generated in a column of liquid metal by a rotating magnetic field. Two cases are discussed, one in which there is no axial variation in the stirring force, and one where the body force is restricted to a relatively short length of the column. The latter case is of considerable practical interest in continuous casting.One-dimensional stirring, where the swirl is independent of z and θ, is well understood. The magnetic body force is balanced by shear, all inertial forces being zero (except for the centripetal acceleration). However, in two-dimensional axisymmetric stirring, the axial variation in swirl drives a strong secondary poloidal flow. The principal local force balance is between the magnetic torque and inertia. The body force spins up the fluid as it passes through the forced region and the secondary flow sweeps this angular momentum into the unforced region. Consequently, the size and distribution of the swirl is controlled by the secondary flow.The role of wall friction is considered and shown to control the length of the recirculating eddy. An approximate solution of the inviscid equations of motion, based on the angular momentum integral, is derived for the flow in the forced region. This is compared with the results of numerical experiments.The analysis predicts that the swirl velocity scales on {B(σ/ρω)½}ωR, has a maximum at the bottom of the driven region, and penetrates an axial distance of the order ℝR away from the forced region. (For turbulent flow the Reynolds number ℝ must be based on an effective eddy viscosity.) All these features were reproduced experimentally.
Developing fluid flow in a curved duct of square cross-section and its fully developed dual solutions
Authors: Soh W. Y. 
Published: April 2006

Developing fluid flow in a curved duct of square cross-section is studied numerically by a factored ADI finite-difference method on a staggered grid. A central-difference scheme with primitive variables is used inside the computational domain to reduce numerical diffusion. Two Reynolds numbers, 574 and 790, based upon a bulk velocity and hydraulic diameter are chosen for curvature ratios of 1/6.45 and 1/2.3, respectively. It is found that the secondary flow is far more complicated than expected, with the appearance of at least two pairs of vortices. Main-flow separation is also observed for the higher curvature ratio. Furthermore, it is observed that the flow develops into two quite different states downstream, depending upon the inlet conditions.Solution of the fully developed Navier-Stokes equations is shown to be not unique beyond a certain critical Reynolds number. Developing flow seems to evolve into the fully developed state along a particular branch, into which the fully developed solution bifurcates.
Large structure in the far wakes of two-dimensional bluff bodies
Authors: Cimbala John M. , Nagib Hassan M. , Roshko Anatol 
Published: April 2006

Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Kármán-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased.The far-wake structure does not depend directly on the scale or frequency of Kármán vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re ≈ 6000), which do not initially shed Kármán-type vortices into the wake.Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.
Similarity solutions for two-dimensional steady laminar gravity currents
Authors: Brighton P. W. M. 
Published: April 2006

Similarity solutions have been found for steady two-dimensional laminar flow in which dense fluid is emitted upwards from a horizontal plane into a laminar shear flow or into a uniform flow. The solutions also apply to a light fluid released at an upper horizontal surface. The Navier-Stokes equations and the diffusion-advection equation are simplified by making the Boussinesq approximation and the boundary-layer approximation, which here also implies that pressure is hydrostatic.For an oncoming linear shear flow representing flow near a solid surface, a similarity solution is obtained with depth proportional to $x^{\frac{1}{3}}$ where x is the horizontal coordinate. Horizontal velocity and concentration of dense fluid both increase as $x^{\frac{1}{3}}$, so that the solution represents fluid propagating upstream along the surface, and diffusing vertically to be swept downstream again. Numerical solutions for vertical profiles of velocity and concentration are presented for a Schmidt or Prandtl number σ between 0.71 and infinity. Two alternative sets of boundary conditions are possible. In one set, the pressure above the boundary layer is unchanged but the velocity profile is displaced upwards. In the second, this displacement is forced to be zero with the result that a pressure gradient is generated in the outer flow. These two boundary conditions are known to apply to disturbances in a laminar boundary-layer on horizontal lengthscales respectively greater or smaller than the triple-deck scale.With a uniform velocity upstream and a stress-free boundary, representing flow at a free surface, similarity solutions exist only for a plume growing downstream from the source of a buoyancy flux B, with depth increasing as x½ and concentration decreasing as x−½. When gravity has negligible effects, so that B = 0, the solution is a Gaussian plume. With finite B, there is an adverse gradient of hydrostatic pressure and the plume is decelerated so that it is deeper than in neutral flow. Numerical solutions for σ = 0.71 reveal that there is a maximum buoyancy flux Bcrit above which no similarity solution exists. This occurs with a non-zero value of the surface velocity. For B < Bcrit it is found that there are in fact two possible solutions. One has surface velocity greater than at the critical flux and tends to the passive Gaussian plume as B → 0. In the other, surface velocity decreases from Bcrit, reaching zero at a non-zero value of B. Similar behaviour is found in an asymptotic solution for very large σ.
The mixing layer and its coherence examined from the point of view of two-dimensional turbulence
Authors: Lesieur Marcel , Staquet Chantal , Roy Pascal Le , Comte Pierre 
Published: April 2006

A two-dimensional numerical large-eddy simulation of a temporal mixing layer submitted to a white-noise perturbation is performed. It is shown that the first pairing of vortices having the same sign is responsible for the formation of a continuous spatial longitudinal energy spectrum of slope between k−4 and k−3. After two successive pairings this spectral range extends to more than 1 decade. The vorticity thickness, averaged over several calculations differing by the initial white-noise realization, is shown to grow linearly, and eventually saturates. This saturation is associated with the finite size of the computational domain.We then examine the predictability of the mixing layer, considering the growth of decorrelation between pairs of flows differing slightly at the first roll-up. The inverse cascade of error through the kinetic energy spectrum is displayed. The error rate is shown to grow exponentially, and saturates together with the levelling-off of the vorticity thickness growth. Extrapolation of these results leads to the conclusion that, in an infinite domain, the two fields would become completely decorrelated. It turns out that the two-dimensional mixing layer is an example of flow that is unpredictable and possesses a broadband kinetic energy spectrum, though composed mainly of spatially coherent structures.It is finally shown how this two-dimensional predictability analysis can be associated with the growth of a particular spanwise perturbation developing on a Kelvin-Helmholtz billow: this is done in the framework of a one-mode spectral truncation in the spanwise direction. Within this analogy, the loss of two-dimensional predictability would correspond to a return to three-dimensionality and a loss of coherence. We indicate also how a new coherent structure could then be recreated, using an eddy-viscosity assumption and the linear instability of the mean inflexional shear.
On direct methods in water-wave theory
Authors: Shields Jonathan J. , Webster William C. 
Published: April 2006

Model equations for three-dimensional, inviscid flow between two arbitrary, time-varying material surfaces are derived using a ‘direct’ or variational approach due to Kantorovich. This approach results in a hierarchy of approximate theories, each of a higher level of spatial approximation and complexity. It can be shown that the equations are equivalent in substance to ‘the theory of directed fluid sheets’ of Green & Naghdi (1974, 1976).The theory can be used to study the propagation of long waves in water of finite depth and, as such, competes with theories derived using the classical Rayleigh–Boussinesq perturbation methods. In order to demonstrate that there is an advantage to the present approach, we compare predictions for steady, two-dimensional waves over a horizontal bottom. Numerical solutions indicate that the direct theory converges more rapidly than the perturbation theories. Also, the equations of the higher-order direct theories contain singularities related to waves of limiting height, and indeed such waves can be predicted with relative accuracy. Finally, the range of applicability of the direct theory is far greater: waves as short as three times the water depth can be modelled. This is essentially a deep-water condition, well beyond the range of convergence of the Rayleigh–Boussinesq approach.
Endogenous markers of hypoxia/anaerobic metabolism and anemia in primary colorectal cancer
Authors: Koukourakis Michael I. , Giatromanolaki Alexandra , Polychronidis Alexandros , Simopoulos Costantinos , Gatter Kevin C. , Harris Adrian L. , Sivridis Efthimios 
Published: April 2006

Anemia has been implicated in the decreased oxygen tension noted within the tumor environment. In a series of 79 colorectal adenocarcinomas we investigated the role of anemia in activating molecular pathways regulated by hypoxia. Preoperative Hb levels were correlated with the immunohistochemical expression of HIF1α and HIF2α, LDH5, GLUT1, VEGF, DEC1 and BNIP3, and with angiogenesis and the cancer cell proliferation index. Upregulation of HIF1α and HIF2α proteins, found in 43% and 44.3% of cases, respectively, was not related to anemia (Hb < 10 g%). This is in agreement with other studies suggesting that HIF activation occurs for various reasons, such as poor or irregular vascularity, or oncogene activation. Nevertheless, low Hb levels (<10 g%) were linked to activated anaerobic metabolism (LDH5 overexpression) in a subset of tumors not expressing HIF1α (P < 0.01). Overexpression of HIFs, whether linked to anemia or not, was associated with a number of factors related to tumor aggressiveness (assessed as local invasion and nodal metastasis), anaerobic metabolism and intratumoral acidosis (LDH5, GLUT1; increased glucose metabolism to lactate), activation of genes related to necrosis (BNIP3) and angiogenesis (VEGF). Expression of BNIP3 emerged as the strongest independent factor related to transmural invasion and metastasis to lymph nodes. Identification of specific patterns of the hypoxia molecular cascade activated in cancer cells might help in developing specific therapeutic policies. (Cancer Sci2006; 97: 582–588)
Chemotherapeutic efficacy of paclitaxel in combination with Withania somnifera on benzo(a)pyrene‐induced experimental lung cancer
Authors: Senthilnathan Palaniyandi , Padmavathi Radhakrishnan , Magesh Venkatraman , Sakthisekaran Dhanapal 
Published: April 2006

Lung cancer is one of the leading causes of cancer death in the world and is notoriously difficult to treat effectively. In the present study, male Swiss albino mice were divided into five groups of six animals each: group I animals received corn oil orally and served as a control; group II cancer‐induced animals received benzo(a)pyrene (50 mg/kg bodyweight dissolved in corn oil, orally) twice weekly for four successive weeks; group III cancer‐bearing animals (after 12 weeks of induction) were treated with paclitaxel (33 mg/kg bodyweight, i.p.) once weekly for 4 weeks; group IV cancer‐bearing animals were treated with paclitaxel along with Withania somnifera (400 mg/kg bodyweight) orally once weekly for 4 weeks; and group V animals constituted the drug control treated with paclitaxel along with W. somnifera. The serum, lung and liver were investigated biochemically for aryl hydrocarbon hydroxylase, γ‐glutamyl transpeptidase, 5′‐nucleotidase, lactate dehydrogenase and protein‐bound carbohydrate components (hexose, hexosamine and sialic acid). These enzyme activities were increased significantly in cancer‐bearing animals compared with control animals. The elevation of these in cancer‐bearing animals was indicative of the persistent deteriorating effect of benzo(a)pyrene in cancer‐bearing animals. Our data suggest that paclitaxel, administered with W. somnifera, may extend its chemotherapeutic effect through modulating protein‐bound carbohydrate levels and marker enzymes, as they are indicators of cancer. The combination of paclitaxel with W. somnifera could effectively treat the benzo(a)pyrene‐induced lung cancer in mice by offering protection from reactive oxygen species damage and also by suppressing cell proliferation. (Cancer Sci 2006; 97: 658–664)