bluuebunny/crossref_metadata_2025 huggingface dataset:
Microclimate Control of Growth Rates and Habitats of the Boreal Forest Mosses, Tomenthypnum nitens and Hylocomium splendens
Authors: Busby John R. , Bliss L. C. , Hamilton Catherine D.
Published: April 2006

Tomenthypnum nitens forms tall turfs in fens where the tree and shrub canopy is sparse and the water table is close to the surface. The feather moss species (Hylocomium splendens, Pleurozium schreberi, and Ptilium crista—castrensis) form wefts in shaded and better drained habitats. Growth of T. nitens was measured using small wire stakes, from which net production averaged 190 g/m2 in 1975. Growth rates were correlated with color of the apical region, the result of variation in apical water contents. Seasonal variation in growth rate was correlated with depth from the canopy surface to the ground water table, controlled by the balance between rate of transport of water through the canopy and loss by evaporation. The most important environmental factor appears to be total precipitation, followed by evaporation stress. Growth of the feather mosses was estimated by measuring dry weight changes of segments of Hylocomium splendens. Net production averaged 79 g/m2. Growth was correlated with the length of time that the moss was wet, the result of precipitation frequency. Because growth rates were measurably reduced by removal of a sparse shrub canopy, evaporation stress is probably another important factor. Survival and growth of Tomenthypnum nitens is limited by evaporation stress in habitats where ground water is not available. Feather mosses are limited by radiation damage and evaporation stress in open habitats, by depression of net assimilation and other deleterious effects of saturation in wet habitats, and by insufficient rainfall under dense tree canopies.
Nutritional Ecology of Heliconia Herbivores: Experiments with Plant Fertilization and Alternative Hosts
Authors: Auerbach Michael J. , Strong Donald R.
Published: April 2006

Efficiencies of food use, growth rates, and consumption rates were measured for larvae of eight tropical insect species, reared upon normal and fertilized Heliconia species (Zingiberales: Heliconiaceae) and Musa sp. (Zingiberales: Musaceae) in Costa Rica. Cephaloleia consanguinea and Chelobasis perplexa (Coleptera: Chrysomelidae, Hispinae) feed only on Heliconia. Caligo memnon and Opsiphanes tamarindi (Lepidoptera: Brassolidae) are oligophagous, with known hosts in two families of Zingiberales. Sibine apicalis, Sibine sp., and Metraga sp. (Lepidoptera: Limacodidae) and Megalopyge sp. (Lepidoptera: Megalopygidae) are polyphagous, with dicot and monocot hosts. Foliar nitrogen content of natural Heliconia imbricata (° = 1.76%) was lower than that of H. latispatha (° = 3.01%) and Musa sp. (° = .30%). The oligophagous brassolids generally had lower growth rates, lower efficiencies of food use, and lower nitrogen accumulation rates on H. imbricata than on Musa sp. The hispines had lower relative consumption and growth rates than any of the lepidopterans examined on any host or treatment. Growth rates and nitrogen accumulation rates for the hispine species were not affected by host plant fertilization. Efficiencies of nitrogen use declined with increasing foliar nitrogen content for the hispines. Several parameters of food use efficiency were correlated with larval feeding specialization. The specialized hispines and oligophagous brassolids had higher efficiencies of food assimilation and nitrogen use on H. imbricata than did the polyphagous limacodids and megalopygid. Other efficiency indices, such as net growth efficiency, were not correlated with feeding specialization. The hispine Ch. perplexa has one of the longest larval developmental times known for a nondiapausing chrysomelid (°200 d). Its slow growth is correlated with a low metabolic rate, which we interpret as a physiological adaptation for starvation resistance. Larvae feed only on rolled Heliconia leaves, which are produced infrequently by host plants, and long fasts are a predictable feature of development. Ch. perplexa's eight larval instars are about equal in duration, and closely match the duration of a rolled leaf's suitability as food for these larvae. After a rolled leaf unfurls, the larvae must either wait for another on the same plant, or search haphazardly for one on nearby plants. The feeding of Ce. consanguinea larvae is apparently moisture limited in this tropical rain forest, and these larvae also have a high resistance to starvation. The oligophagous brassolids have become pests of banana, which is introduced into the Americas. These species increase growth rates with increased nitrogen content of host tissue. They are infrequent on H. imbricata, which has few chemical defenses against herbivores. Thus, the low nitrogen content of H. imbricata may protect it from some herbivory.
Twenty Years of Ecosystem Reorganization Following Experimental Deforestation and Regrowth Suppression
Authors: Reiners William A.
Published: April 2006

Watershed 2 of the Hubbard Brook Experimental Forest was experimentally deforested in the winter of 1965—1966, and subsequent plant regeneration was suppressed by herbicide application until the 1969 growing season. Changes in species structure, plant abundance, aboveground primary productivity and biomass, and aboveground nutrient pools were recorded on 70 permanent plots distributed across the watershed during 1, 2, 3, 5, 11, and 20 yr of subsequent succession. Species richness increased rapidly over time but equitability and the Shannon—Wiener diversity index declined with succession. Plant densities of herb, shrub, sapling, and tree strata increased, then decreased, in successive waves as the vegetation increased in height. Basal area (stems >5.1 cm dbh) initially increased exponentially, then linearly after the 5th yr to attain a total of 18.7 m2/ha by the 20th yr. Primary productivity also increased exponentially at first, but then increased linearly through the 20th yr at which time annual aboveground primary productivity was 20 Mg/ha. Aboveground biomass increased linearly after the 5th yr until by the 20th yr it was 52 Mg/ha, 38% of the reference forest biomass. Aboveground nutrient pools of some elements accumulated faster than biomass by the 20th yr when calculated as percentages of the 55—yr—old reference forest pools and biomass. These included: potassium–52%, phosphorus–44%, magnesium–42%. Other accumulated more slowly than biomass: sulfur–32%, and nitrogen and calcium each–29%. While recovery rate on Watershed 2 was delayed in the first decade of regrowth, and composition was somewhat altered because of the 3—yr suppression, this forest nevertheless displayed a vigorous capacity for regeneration after suppression ceased.
Episodic Hypoxia in Chesapeake Bay: Interacting Effects of Recruitment, Behavior, and Physical Disturbance
Authors: Breitburg Denise L.
Published: April 2006

Physical disturbance can be an important force at the individual, population, and community levels of organization. The effects of disturbance may differ for mobile and sessile organisms, however, because of differences in the potential for escape and postdisturbance recolonization by survivors. I used field sampling and laboratory experiments to examine how episodic movement of severely oxygen—depleted (hypoxic) bottom water into nearshore habitat in the Chesapeake Bay affects population density, recruitment, and reproduction of a mobile species–the naked goby (Gobiosoma bosc), a benthic oyster bed fish. Oxygen depletion is a common physical disturbance in freshwater, estuarine, and coastal aquatic systems. In this study, episodic hypoxia influenced mortality, size structure of the population, reproductive behavior, and spatial distribution. Intrusion of severely hypoxic water occurred in late July and early August during the 2—yr study. These intrusions coincided temporally with peak periods of recruitment, and caused the most severe drops in dissolved oxygen concentrations in deep and mid—depth areas of the oyster reef, where recruitment was highest. Laboratory experiments suggested that newly settled recruits require higher oxygen concentrations for survival than do older individuals. Field samples also indicated that these new recruits are less able to escape to more highly oxygenated shallow water refuges when an intrusion occurs. Thus, the spatial and temporal patterns of recruitment and disturbance, and physiological requirements, combine to result in extremely high mortality of new recruits during severe intrusions. In contrast to effects on new recruits, some large juveniles and adults successfully migrate inshore when oxygen levels decline. In both field samples and laboratory experiments, adult males continued to guard eggs and shelters until dissolved oxygen closely approached lethal levels. Calculations based on size—specific physiological tolerances and swimming speeds suggest that the occurrence of lethal conditions in the fluctuating environment may be more predictable to larger individuals than to new recruits. This predictability may increase the possibility of an appropriate response to low oxygen disturbances by large juveniles and adults. After the disturbance abates, surviving individuals recolonize abandoned areas. This ability of mobile animals to recolonize a disturbed area as adults or juveniles, rather than solely through reproduction, may lead to differences in postdisturbance ecological interactions and differences in selection for colonizing ability between mobile and sessile species.
A Late‐Quaternary Vegetation History from Hanging Lake, Northern Yukon
Authors: Cwynar Les C.
Published: April 2006

A 403—cm core was recovered from Hanging Lake in unglaciated northern Yukon. Twenty—one radiocarbon dates indicate that the section is at least 25 000 and possibly 33 000 yr old; they permit the calculation of pollen influxes for the full—glacial in eastern Beringia. Numerical methods were used to divide the pollen stratigraphy into five zones. From prior to 33 000 to 18 450 BP, a herb zone was dominant (zone HL 1) with high percentages of Gramineae, Artemisia, and Cruciferae. However, the low pollen influx, ranging from 5—100 grains°cm—2°yr—1, the low organic content of the sediment, and the occurrence of open—ground taxa all indicate that the vegetation cover was sparser than it is today. The arctic—alpine affinities of the herb pollen show that generically the vegetation was akin to modern arctic plant communities. Modern fellfield communities in the northern Yukon and Siberia have a rich and endemic Artemisia flora and they can produce pollen spectra comparable to that of the herb zone. Percent and influx values for spruce, alder, and birch increased slightly during subzone H 1B (21 680—18 450 BP); this subzone probably represents an interstadial. From 18 450 to 14 600 BP, a Salix—Cyperaceae zone (HL 2) occurred, suggesting the development of snowbed and willow scrub communities in sheltered areas. Between 14 600 and 11 100 BP Betula pollen dominated (zone HL 3) indicating the spread of dwarf birches, but the influx data show that this initial increase was modest compared with the subsequent zone and thus dwarf birches were probably restricted at this time to more favorable habitats. The spread of birch together with the increased total pollen influx, the higher organic content of the sediment, and the increased richness of herb pollen indicates that the local flora was more diverse and that the vegetative cover increased. The climate must have warmed. Zone HL 4 (Ericales zone) spanned the period from 11 100 to 8900 BP. Wet heath communities became locally abundant, poplar was more abundant at the beginning of this zone than at any subsequent time, the ranges of Typha latifolia and Myrica gale were greater than today, and pollen influx and sedimentation rate both increased greatly. Spruce became regionally abundant. These varied changes are probably in response to a warmer and wetter climate; they provide evidence for a late Pleistocene to early Holocene warm interval initially recognized by McCulloch and Hopkins (1966). Zone HL 5 (Alnus crispa zone) has been dated from 8900 BP to the present; it reflects the regional expansion of Alnus crispa on organic soils.
Distribution Ecology: Variation in Plant Recruitment over a Gradient in Relation to Insect Seed Predation
Authors: Louda Svata M.
Published: April 2006

Although predispersal seed predation by insects is common, no test exists of its effect on plant recruitment. This study examines seed predation in the population dynamics of a native, temperate shrub, Haplopappus squarrosus H. and A. (Asteraceae), over an elevational gradient in the coastal sage scrub vegetation of San Diego County, California USA. Frequency and abundance of H. squarrosus increase from coast to mountains. Expected abundance, based on flowers initiated, was highest at the coast and lowest in the interior, the opposite of the observed adult plant distribution. Overall flower and seed predation by insects was high (44—73%) and was greatest at the coast. Insect exclusion experiments at sites along the gradient had three main results. (1) Seedling recruitment was proportional to the number of viable seeds after seed predation. The exclusion plots had significantly higher numbers of seedlings established at all sites than did control plots; the greatest increase was at the coast. (2) Seedling survivorship was independent of both seedling and adult densities. (3) Juvenile recruitment was proportional to seedling establishment. Predation by insects prior to release of seeds played a critical role in the population recruitment of H. squarrosus within and among sites along the gradient. Survivorship of established plants, in addition, was as high at the coast as it was in the interior. These results suggest that indigenous insect seed predators can be a major force controlling the dynamics of a native plant species over its natural distributional range.
Temporal Succession in a Desert Stream Ecosystem Following Flash Flooding
Authors: Fisher Stuart G. , Gray Lawrence J. , Grimm Nancy B. , Busch David E.
Published: April 2006

Recovery of a desert stream after an intense flash flooding event is described as a model of temporal succession in lotic ecosystems. A late summer flood in Sycamore Creek, Arizona, virtually eliminated algae and reduced invertebrate standing crop by 98%. Physical and morphometric conditions typical of the preflood period were restored in 2 d and the biota recovered in 2—3 wk. Algal communities responded rapidly and achieved a standing crop of nearly 100 g/m2 in 2 wk. Community composition was dominated by diatoms early in succession and by filamentous greens and blue—greens later. Macroinvertebrates also recolonized denuded substrates rapidly, largely by immigration of aerial adults and subsequent oviposition. Growth and development were rapid and several generations of the dominant mayfly and dipteran taxa were completed during the 1st mo of recovery. Invertebrate dry biomass reached 7.3 g/m2 in 1 mo. Gross primary production (Pg) measured as O2 increased in a similar asymptotic fashion and reached 6.6 g°m—2°d—1 in 30 d. Pg exceeded community respiration (R) after day 5 and Pg/R averaged 1.46 for the remainder of the 2—mo sequence. This ecosystem is thus autotrophic and exports organic matter downstream and by drying, laterally. Uptake of nitrate and phosphorus were proportional to net primary production and exhibited a marked downstream decline in concentration during both light and dark periods. Temporal trajectories of various community and ecosystem attributes are compared with those suggested by Odum (1969) to be diagnostic of successional status. Agreement was poor in attributes which are especially modified in open, frequently disturbed ecosystems such as streams.
Structured Population Models of Herbivorous Zooplankton
Authors: McCauley Edward , Nisbet Roger M. , De Roos Andre M. , Murdoch William W. , Gurney William S. C.
Published: April 2006

In this paper, we investigate whether a stage—structured population model can explain major features of dynamics of the herbivores Daphnia galeata and Bosmina longirostris reared under controlled laboratory conditions. Model parameters are determined from independent individual—based information gleaned from the literature on feeding, growth, reproduction, and survivorship of these herbivores. We tested predictions of our model against published observations on the dynamics of laboratory populations. The feeding protocols used in these experiments present a highly dynamic food environment that rigorously challenges the ability of stage—structured models to predict the dynamics of populations as they approach equilibrium. For both herbivore species, the models correctly predict feasible equilibria and some features of their dynamics (e.g., periodicity, cycle amplitude, demography, and fecundity) for experiments in which the species were raised in isolation and food transfers were relatively frequent (at least one transfer per instar). With frequent food transfers, the model also correctly predicts coexistence of the herbivores during competition experiments and suggests a novel mechanism for coexistence. The model fails to predict correctly single—species dynamics and the outcome of competition in experiments where food transfers were infrequent and utilization of internal reserves by individuals in the populations must have been high.
Temperature and Plant Species Control Over Litter Decomposition in Alaskan Tundra
Authors: Hobbie Sarah E.
Published: April 2006

I compared effects of increased temperature and litter from different Alaskan tundra plant species on cycling of carbon and nitrogen through litter and soil in microcosms. Warming between 4° and 10°C significantly increased rates of soil and litter respiration, litter decomposition, litter nitrogen release, and soil net nitrogen mineralization. Thus, future warming will directly increase rates of carbon and nitrogen cycling through litter and soil in tundra. In addition, differences among species' litter in rates of decomposition, N release, and effects on soil net nitrogen mineralization were sometimes larger than differences between the two temperature treatments within a species. Thus, changes in plant community structure and composition associated with future warming will have important consequences for how elements cycle through litter and soil in tundra. In general, species within a growth form (graminoids, evergreen shrubs, deciduous shrubs, and mosses) were more similar in their effects on decomposition than were species belonging to different growth forms, with graminoid litter having the fastest rate and litter of deciduous shrubs and mosses having the slowest rates. Differences in rates of litter decomposition were more related to carbon quality than to nitrogen concentration. Increased abundance of deciduous shrubs with future climate warming will promote carbon storage, because of their relatively large allocation to woody stems that decompose slowly. Changes in moss abundance will also have important consequences for future carbon and nitrogen cycling, since moss litter is extremely recalcitrant and has a low potential to immobilize nitrogen.
Larval Transport and Population Dynamics of Intertidal Barnacles: A Coupled Benthic/Oceanic Model
Authors: Alexander Susan E. , Roughgarden Jonathan
Published: April 2006

The dynamics of barnacle species inhabiting the rocky intertidal zone of central California are analyzed using an advection—diffusion model for barnacle larvae coupled to a space—limited model for barnacle adults. The model explores the link between physical transport processes and the population dynamics of coastal barnacle species and inspects the hypothesis that recruitment events result from the collision of upwelling fronts with the intertidal zone. Initially, a one—dimensional model is presented that incorporates eddy diffusion, offshore advection, and an offshore reflecting boundary representing an upwelling front. Physical movement of the front, in response to fluctuating upwelling, is modeled by varying the position of the reflecting boundary. Results show analytically that populations can sustain only very low levels of offshore flow when not bounded by an offshore front, and that increased advection can cause a population to go extinct. The criterion for population viability is dependent upon the strength of advection and position of the offshore front. The one—dimensional model is subsequently extended to two dimensions to allow for the spatial distribution of both larvae and adults in a more realistic context. Recruitment patterns, adult dynamics, and larval distribution patterns are loosely based on parameters from the study area bounded between the Monterey Peninsula and Point Sur. Upwelling/relaxation events are phenomenologically incorporated into the model by varying the position of the offshore boundary and the magnitude of cross—shelf advection. The analysis shows how the interaction of advective and diffusive processes and the location of the offshore front determines the dynamics of coastal barnacle populations. The two—dimensional model represents the next step toward assimilation of time—dependent remote sensing data and/or linking to numerical circulation models.
Increased expression of glycosyl‐phosphatidylinositol anchor attachment protein 1 (GPAA1) is associated with gene amplification in hepatocellular carcinoma
Authors: Ho Jenny C. , Cheung Siu Tim , Patil Mohini , Chen Xin , Fan Sheung Tat
Published: April 2006

Glycosyl‐phosphatidylinositol (GPI) anchor attachment protein 1 (GPAA1) transcript level was frequently up‐regulated in our earlier study on gene expression profile. We therefore analyzed the potential involvement of GPAA1 in hepatocellular carcinoma (HCC) as GPAA1 gene locates at chromosome 8q24.3 which chromosome region is frequently amplified in HCCs. In this study, we observed that GPAA1 transcript in the HCCs (n = 93) showed a significantly higher expression level compared with their paralleled adjacent nontumor liver tissues, cirrhosis (n = 15) and normal (n = 16) liver tissues using real‐time quantitative RT‐PCR (p < 0.005). We also demonstrated that GPAA1 protein up‐regulation was common in HCCs (90%, 9/10), and GPAA1 gene was frequently amplified (73%, 11/15) using quantitative microsatellite analysis. Increased GPAA1 expression was significantly associated with HCCs poor cellular differentiation (p = 0.011) and poor prognosis (p = 0.010). We then modulated the GPAA1 expression level in HCC cells (Hep3B) by transfection experiments, which was shown to positively regulate cell adhesion ability (p = 0.004) and proliferation rate (p = 0.037). Our data revealed GPAA1 gene amplification with overexpression of RNA and protein in HCC. GPAA1 is a potential amplification target of chromosome 8q and responsible to regulate tumor cells behavior. © 2006 Wiley‐Liss, Inc.
Multiple-contour-dynamic simulation of eddy scales in the plane shear layer
Authors: Jacobs P. A. , Pullin D. I.
Published: April 2006

The method of contour dynamics (CD) is applied to several inviscid prototype flows typical of the motions found in the transition region of the free shear layer. Examples of the interaction between the fundamental streamwise-layer perturbation and its first subharmonic are presented that illustrate the events of pairing and tearing of two rolled-up cores and also the coalescence of three rolled-up cores. The present simulations of the temporally unstable two-dimensional layer, at effectively infinite Reynolds number, support the hypothesis that the dynamics of the large-scale roll-up is only weakly dependent on Reynolds number. However, we find fine-scale structure that is not apparent in previous simulations at moderate Reynolds number. Spiral filaments of rotational fluid wrap around the rolled-up vortex cores producing ‘spiky’ vorticity distributions together with the entanglement of large quantities of irrotational fluid into the layer. Simulations proceeded only until the first such event because we were unable to resolve the fine detail generated subsequently. The inclusion of prescribed vortex stretching parallel to the vortex lines is found to accelerate the initial roll-up and to enhance the production of spiral vortex filaments. In the fundamental-subharmonic interaction, vortex stretching slows but does not prevent pairing.
Selective withdrawal and spin-up of a rotating stratified fluid
Authors: Monismith S. G. , Maxworthy T.
Published: April 2006

We present an experimental study demonstrating that rotation has a pronounced effect on currents induced by selective withdrawal of fluid from a density-stratified reservoir. Our observations show that initiating outflow from the reservoir generates Kelvin shear waves. These waves propagate cyclonically around the perimeter of the reservoir, establishing an anticyclonic withdrawal-layer flow. This flow accelerates owing to the production of relative vorticity by compression of ‘planetary’ vorticity. The withdrawal-layer thickness as shown by vertical profiles of the horizontal velocity grows with time. Separation of the side-wall boundary layers in the corners of the tank causes the spun-up flow to eventually break up into a series of counterrotating gyres. We also present a model that describes many features of the spin-up process observed before onset of separation and subsequent gyre formation. The model shows that vertical diffusion of vorticity plays an important role in the spin up process, leading us to conclude that apparent thickening of the withdrawal layer over time, as seen in changes in the velocity profile, is associated with the vertical diffusion of the spun-up vorticity.
A theoretical and experimental study of double-layer convection
Authors: Rasenat S. , Busse F. H. , Rehberg I.
Published: April 2006

The onset of thermal convection in a double layer of two superimposed immiscible fluids heated from below is investigated. The linearized equations of the problem are analysed in a much wider region of the parameter space than has been studied before. It is shown that the onset of steady convection in the two layers may occur in the form of either viscously or thermally coupled motions. In addition to the oscillatory interfacial instability, which depends on a non-vanishing distortion of the interface, there exists another oscillatory instability which corresponds to a cyclic variation between viscous and thermal coupling. Conditions for the onset of this instability are outlined and its connections with the other modes of the system are demonstrated in bifurcation diagrams. In the experiments the shadowgraph method is used for the visualization of the onset of convection and for the measurement of its wavelength. Changeovers between viscous and thermal coupling can be identified, but the experimental realization of an oscillatory onset has been elusive so far.
Topographic waves in open domains. Part 1. Boundary conditions and frequency estimates
Authors: Johnson E. R.
Published: April 2006

The problem is considered of topographic waves propagating on depth gradients in rotating domains. A variational principle is derived for the eigenfunctions and eigenfrequencies of normal modes on a domain, and applied to subdomains of the whole domain. Considering suitable boundary conditions on the open boundary between the subdomain and the remainder of the whole domain gives upper and lower bounds and estimates for the frequencies of normal modes localized in the subdomain without the complication of solving over the whole domain. It is shown that applying a zero-mass-flux condition at the open boundary leads to a lower bound on the frequency whereas requiring a particular form of the energy flux to vanish identically at each point of the boundary provides an upper bound.
Cross-sectional convection induced by an insulated boundary in a cylinder
Authors: Quon Charles
Published: April 2006

When a long horizontal cylinder filled with fluid is differentially heated at the end walls at high Rayleigh number, A, the axial flow in the midsection consists of boundary layers at the top and bottom of the cylinder flowing in reverse directions, and the temperature is stably and linearly distributed in the vertical. Both the temperature is stably and linearly distributed in the vertical. Both the temperature and the flows are almost independent of the axial dimension. The adiabatic boundary condition on the cylinder requires temperature corrections which can induce cross-section boundary layers on the cylindrical wall and vertical internal boundary layers in the middle. Both types of boundary layers are O(A−¼) in width. Matching different boundary layers at the poles is achieved through additional A−⅙ and A−⅛ layers. The maximum boundary-layer velocity is calculated to be almost one-quarter of the axial velocity observed in experiments for A = 108.
Modulated Taylor–Couette flow
Authors: Barenghi C. F. , Jones C. A.
Published: April 2006

The onset of instability in temporally modulated Taylor-Couette flow is considered. Critical Reynolds numbers have been found by computing Floquet exponents. We find that frequency modulation of the inner cylinder introduces a small destabilization, in agreement with the narrow-gap theory of Hall and some recent experiments of Ahlers. We review the previous computational literature on this problem, and find a number of contradictory results: the source of these discrepancies is examined, and a satisfactory resolution is achieved. Nonlinear axisymmetric calculations on the modulated problem have been done with an initial value code using a spectral method with collocation. The results are compared satisfactorily with Ahlers' measurements.

At low modulation frequency, a large destabilization has been observed in past experiments. We show that this cannot be explained on the basis of perfect bifurcation theory: an analysis of the modulated amplitude equation shows that very small imperfections can substantially affect the behaviour at low frequency by giving rise to ‘transient’ vortices at subcritical Reynolds number. We argue that these ‘transient’ vortices are the source of the large destabilization seen in some experiments. Modelling the imperfections in the initial-value code provides additional confirmation of this effect.
Experiments on the forced wake of an airfoil
Authors: Gharib M. , Williams-Stuber K.
Published: April 2006

The effect of initial flow conditions on the wake of an airfoil is examined in an experiment which uses the ‘strip heater’ technique to externally force the airfoil wake. The strip heaters are used to introduce waves into the top and bottom boundary layers of a thin symmetric airfoil which are subsequently amplified and introduced to the wake. The evolution and interaction of the waves in the wake is the primary interest of this study. A linear stability analysis is applied to the mean velocity profiles in order to understand the frequency selection process in the wake. It is seen that the mean velocity profile adjusts itself in order to become more receptive to the forced frequency of oscillation, resulting in the suppression of previously existing frequencies. The amplitude of oscillations in the wake can be controlled by varying the phase relation between two input signals. In this respect, cancellation and enhancement of the oscillations is possible. The linear stability analysis is applied to the cancellation/enhancement flow to verify the level of cancellation achieved. The receptivity of the system to external forcing is established. A substantial reduction in drag is achieved for forcing frequencies near the centre of the receptivity range.
Growth of bioconvection patterns in a suspension of gyrotactic micro-organisms in a layer of finite depth
Authors: Hill N. A. , Pedley T. J. , Kessler J. O.
Published: April 2006

The effect of gyrotaxis on the linear stability of a suspension of swimming, negatively buoyant micro-organisms is examined for a layer of finite depth. In the steady basic state there is no bulk fluid motion, and the upwards swimming of the cells is balanced by diffusion resulting from randomness in their shape, orientation and swimming behaviour. This leads to a bulk density stratification with denser fluid on top. The theory is based on the continuum model of Pedley, Hill & Kessler (1988), and employs both asymptotic and numerical analysis. The suspension is characterized by five dimensionless parameters: a Rayleigh number, a Schmidt number, a layer-depth parameter, a gyrotaxis number G, and a geometrical parameter measuring the ellipticity of the micro-organisms. For small values of G, the most unstable mode has a vanishing wavenumber, but for sufficiently large values of G, the predicted initial wavelength is finite, in agreement with experiments. The suspension becomes less stable as the layer depth is increased. Indeed, if the layer is sufficiently deep an initially homogeneous suspension is unstable, and the equilibrium state does not form. The theory of Pedley, Hill & Kessler (1988) for infinite depth is shown to be appropriate in that case. An unusual feature of the model is the existence of overstable or oscillatory modes which are driven by the gyrotactic response of the micro-organisms to the shear at the rigid boundaries of the layer. These modes occur at parameter values which could be realized in experiments.
Buoyancy transfer across a diffusive interface
Authors: Fernando Harindra J. S.
Published: April 2006

An experimental investigation of various aspects of buoyancy transfer across a diffusive density interface that separates stably stratified, turbulently convecting layers of relatively fresh cold water overlying hot salty water is described. It is argued that the interfacial layer should possess a double boundary-layer structure, in which the thicknesses of the salt and heat interfacial layers are determined by a balance between the opposing effects of diffusion and entrainment. Based on this argument, a simple theory, that predicts the interfacial-layer thickness, the diffusive heat and salt fluxes across the density interface, and the time variation of the temperature and salt concentrations in the convecting layers, is proposed for the case in which the convection is driven by a constant heat flux supplied to the lower layer. During a certain time interval, the theory and experiment agree well, but thereafter distinct differences can be seen. Measurements suggest that these differences may be due to the distortion of the density interface at low interfacial stabilities by turbulent eddies, which leads to a change in the buoyancy transfer mechanism. When the Richardson number falls below a critical value Riv, the interface was found to migrate slowly upwards and the mechanism of entrainment was the detachment of thin sheets of fluid by eddies scouring the interface.
Instability and filamentation of finite-amplitude waves on vortex layers of finite thickness
Authors: Pullin D. I. , Jacobs P. A. , Grimshaw R. H. J. , Saffman P. G.
Published: April 2006

We study the instability of finite-amplitude waves on uniform vortex layers of finite thickness bounded by a plane rigid surface. A weakly nonlinear analysis of vorticity interface perturbations, and spectral stability calculations using the full equations of motion, together show that steady progressive waves are unstable to general subharmonic pertubations in the range 0.094 < d/λ < 1.7, where d is the mean layer thickness and λ is the primary wavelength. The relevance of this instability to ultimate interface filamentation is tested by performing several numerical contourdynamical simulations of the nonlinear interface evolution for initial disturbances consisting of the finite amplitude wave plus eigenfunctions obtained from the spectral calculations. The results indicate that within the band of unstable wavelengths, small perturbations to the steady non-uniform flow given by the finite amplitude wave motion (vortex equilibrium) are able to grow in magnitude, until at a time tf, the wave extremum encounters a hyperbolic critical point of the velocity field after which filamentation occurs. Arguments are put forward based on the unsteady simulations with the purpose of identifying the preferred frame of reference for viewing the kinematical events controlling the filamentation process. An estimate for tf is then made, and the mechanism of filamentation found is discussed in relation to the recently proposed nonlinear-cascade mechanism of Dritschel (1988a).
Solidification of an alloy cooled from above Part 1. Equilibrium growth
Authors: Kerr Ross C. , Woods Andrew W. , Worster M. Grae , Huppert Herbert E.
Published: April 2006

The interaction between the solidification and convection that occurs when a melt is cooled from above is investigated in a series of three papers. In these papers we consider a two-component melt that partially solidifies to leave a buoyant residual fluid. The solid forms a mushy layer of dendritic crystals, the interstices of which accommodate the residual fluid. The heat extraction through the upper boundary, necessary to promote solidification, drives convection at high Rayleigh numbers in the melt below the mushy layer. The convection enhances the heat transfer from the melt and alters the rate of solidification. In this paper the various phenomena are studied in a series of laboratory experiments in which ice is frozen from aqueous solutions of isopropanol. The experiments are complemented by the development of a general theoretical model in which the mush is treated as a continuum phase with thermodynamic properties that are functions of the local solid fraction. The model, which is based upon principles of equilibrium thermodynamics and local conservation of heat and solute, produces results in good agreement with the experimental data. Careful comparisons between this theory and experiments suggest the need to explore non-equilibrium effects, which are investigated in Parts 2 and 3.
Wave–current interactions: an experimental and numerical study. Part 2. Nonlinear waves
Authors: Thomas G. P.
Published: April 2006

The interaction between a regular wavetrain and a current possessing an arbitrary distribution of vorticity, in two dimensions, is considered for waves of finite amplitude. A numerical model is constructed, primarily for use in the finite depth regime, extending the work of Dalrymple (1973, 1977) and this is used to predict the wavelength and the particle velocities under the waves. These predictions agree very well with experimentally obtained data and the importance of the vorticity in the wave–current interaction is clarified. Amplitude and wavelength modulations are considered for finite amplitude waves on a slowly varying irrotational current; moderate agreement is found between theory and experiment.
The wide-spacing approximation applied to multiple scattering and sloshing problems
Authors: Evans D. V.
Published: April 2006

Linear water-wave theor is used in conjuctin with a wide-spacing approximation to develop closed-form expressions for the reflection and transmission coeffcients appropriate to a plane wave incident upon any number of identical equally spaced obstacles in two dimensins, and also to derive a real expressin from which the sloshing requencies, which occur when the bodies are bounded by rigid walls, can be determined. In each case the solutin is in terms of known properties of radiation problems associated with any one of the bodies in isolation.
A jet in crossflow. Part 2
Authors: Needham D. J. , Riley N. , Lytton C. C. , Smith J. H. B.
Published: April 2006

A recent paper by Needham, Riley & Smith (1988) considers the flow of a jet of inviscid, incompressible fluid emerging from a circular pipe into a weak crossflow. In this paper we extend their results. In particular we show how the Kutta condition at the pipe lip may be satisfied, and we present details of the flow field obtained from both the formal solution and a field calculation.
Reynolds stress and the physics of turbulent momentum transport
Authors: Bernard Peter S. , Handler Robert A.
Published: April 2006

The nature of the momentum transport processes responsible for the Reynolds shear stress is investigated using several ensembles of fluid particle paths obtained from a direct numerical simulation of turbulent channel flow. It is found that the Reynolds stress can be viewed as arising from two fundamentally different mechanisms. The more significant entails transport in the manner described by Prandtl in which momentum is carried unchanged from one point to another by the random displacement of fluid particles. One-point models, such as the gradient law are found to be inherently unsuitable for representing this process. However, a potentially useful non-local approximation to displacement transport, depending on the global distribution of the mean velocity gradient, may be developed as a natural consequence of its definition. A second important transport mechanism involves fluid particles experiencing systematic accelerations and decelerations. Close to the wall this results in a reduction in Reynolds stress due to the slowing of sweep-type motions. Further away Reynolds stress is produced in spiralling motions, where particles accelerate or decelerate while changing direction. Both transport mechanisms appear to be closely associated with the dynamics of vortical structures in the wall region.
The phase diffusion and mean drift equations for convection at finite Rayleigh numbers in large containers
Authors: Newell Alan C. , Passot Thierry , Souli Mohammad
Published: April 2006

We derive the phase diffusion and mean drift equations for the Oberbeck–Boussinesq equations in large-aspect-ratio containers. We are able to recover all the long-wave instability boundaries (Eckhaus, zigzag, skew-varicose) of straight parallel rolls found previously by Busse and his colleagues. Moreover, the development of the skew-varicose instability can be followed and it becomes clear how the mean drift field conspires to enhance the necking of phase contours necessary for the production of dislocation pairs. We can calculate the wavenumber selected by curved patterns and find very close agreement with the dominant wavenumbers observed by Heutmaker & Gollub at Prandtl number 2.5, and by Steinberg, Ahlers & Cannell at Prandtl number 6.1. We find a new instability, the focus instability, which causes circular target patterns to destabilize and which, at sufficiently large Rayleigh numbers, may play a major role in the onset of time dependence. Further, we predict the values of the Rayleigh number at which the time-dependent but spatially ordered patterns will become spatially disordered. The key difficulty in obtaining these equations is the fact that the phase diffusion equation appears as a solvability condition at order ε (the inverse aspect ratio) whereas the mean drift equation is the solvability condition at order ε2. Therefore, we had to use extremely robust inversion methods to solve the singular equations at order ε and the techniques we use should prove to be invaluable in a wide range of similar situations. Finally, we discuss the introduction of the amplitude as an active order parameter near pattern defects, such as dislocations and foci.
Buoyancy-induced Stokes flow in a wedge-shaped enclosure
Authors: Yu K. M. , Nansteel M. W.
Published: April 2006

The problem of buoyancy-induced Stokes flow in a sectorial region is addressed. Skew-symmetric flows are considered for wedge or opening angles of the sector in the range 0 < α [les ] π. The basic structure and character of the motion are found to depend critically upon the relative dominance, near the sector vertex, of the particular solution of the system with respect to the leading eigenfunction. A simple criterion is developed for the appearance of eddies, such as those observed by Moffatt (1964), in the neighbourhood of the sector vertex. A calculation is carried out for the specific case of motion induced by different temperatures on the radial boundaries of the enclosure. It is found that corner eddies may be present in this circumstance for wedge angles in the range 126° [lsim ] α [lsim ] 146°. The eddying motion near the vertex is examined, in some detail, for the wedge angle α = 135°. In the limiting case of α = π, corresponding to a semicircular-shaped sector, the particular solution is found to exhibit singular behaviour. However, this singular nature is found to be spurious, as a bounded particular solution can be constructed with the aid of one of the eigensolutions. Results are given for no-slip and shear-free conditions on the circular boundary of the sector for the purpose of comparison.
Bubbly flow and its relation to conduction in composites
Authors: Smereka Peter , Milton Graeme W.
Published: April 2006

Following Wallis, the relation between non-viscous bubbly flow and conduction in composites is examined. The bubbles are treated as incompressible and correspond to non-conducting inclusions. A simple relation is found between the effective conductivity and the energy coefficient which is agreement with previous calculations. It is shown that the energy coefficient is frame dependent and, in the frame of zero volumetric flux, is equal to the virtual mass density. Zuber's virtual mass density corresponds to the conductivity of the Hashin–Shtrikman coated-sphere geometry. This connection is exploited to extend Zuber's result to ellipsoidal bubbles. The hyperbolicity of effective equations derived from a variational principle is analysed for various bubble configurations. Without bubble clustering the equations are ill-posed (unstable). However, when the bubbles group into ellipsoidal clusters the resulting effective equations are well-posed for a wide range of parameter values.
Two-layer hydraulics with comparable internal wave speeds
Authors: Williams Richard , Armi Laurence
Published: April 2006

Two-layer hydraulics is developed for problems in which the moving layers can have stagnant layers above and below, the two internal wave modes can have comparable speeds and the total depth of the moving layers may vary. The general development allows both Boussinesq and non-Boussinesq problems to be studied. Solutions are presented in the Froude-number plane and the effect of different layer densities on the form of the solution space is shown. The theory is applied to two-layer plunging flows and a variety of controlled solutions are found. Solutions for the 2½-layer theory and the plunging flow theory are demonstrated experimentally. Shear instability is often observed in the divergent section of the channel.
Vortex ring bubbles
Authors: Lundgren T. S. , Mansour N. N.
Published: April 2006

Toroidal bubbles with circulation are studied numerically and by means of a physically motivated model equation. Two series of computations are performed by a boundary-integral method. One set shows the starting motion of an initially spherical bubble as a gravitationally driven liquid jet penetrates through the bubble from below causing a toroidal geometry to develop. The jet becomes broader as surface tension increases and fails to penetrate if surface tension is too large. The dimensionless circulation that develops is not very dependent on the surface tension. The second series of computations starts from a toroidal geometry, with circulation determined from the earlier series, and follows the motion of the rising and spreading vortex ring. Some modifications to the boundary-integral formulation were devised to handle the multiply connected geometry. The computations uncovered some unexpected rapid oscillations of the ring radius. These oscillations and the spreading of the ring are explained by the model equation which provides a more general description of vortex ring bubbles than previously available.