Low Fr Research Articles

Gas Dispersion in Rheologically-Evolving Model Fluids by Hybrid Dual Mixing Systems

Gas dispersion experiments (0.18≤Fr≤0.71, 0.02≤Fl≤0.09) were carried out using a hybrid dual mixing system, which included a helical ribbon impeller and either a Smith or a Rushton turbine. Newtonian and non-Newtonian model fluids were used as rheologically-evolving fluids to evaluate changes in gas dispersion performance. A motionless helical ribbon agitator was used as a baffle in low-viscosity Newtonian fluids. Both Smith and Rushton turbines produced a vortex, which was eliminated by the motionless helical ribbon impeller. Gas dispersion in low-viscosity fluids was enhanced when the helical ribbon agitator and turbine of the dual hybrid mixing system was kept at a rotational speed ratio of 10 (N T /N HR =10), which allowed dispersion at a lower Fr than the turbine alone. For moderate-viscosity Newtonian fluids, gas dispersion was achieved at Fr≤0.71 and Fl≤0.05. Flow properties of non-Newtonian fluids played an important role in gas dispersion; transition from dispersing to flooding stages was observed for the fluids that were more shear-thinning (n≤0.38).

Effects of drift angle on model ship flow

The effects of drift angle on model ship flow are investigated through towing tank tests for the Series 60 CB=0.6 cargo/container model ship. Resistance, side force, drift moment, sinkage, trim, and heel data are procured for a range of drift angles β and Froude numbers (Fr) and the model free condition. Detailed free-surface and mean velocity and pressure flow maps are procured for high and low Fr=0.316 and 0.16 and β=5° and 10° (free surface) and β=10° (mean velocity and pressure) for the model fixed condition (i.e. fixed with zero sinkage, trim, and heel). Comparison of results at high and low Fr and previous data for β=0° enables identification of important free-surface and drift effects. Geometry, conditions, data, and uncertainty analysis are documented in sufficient detail so as to be useful as a benchmark for computational fluid dynamics (CFD) validation. The resistance increases linearly with β with same slope for all Fr, whereas the increases in the side force, drift moment, sinkage, trim, and heel with β are quadratic. The wave profile is only affected near the bow, i.e. the bow wave amplitude increases/decreases on the windward/leeward sides, whereas the wave elevations are affected throughout the entire wave field. However, the wave envelope angle on both sides is nearly the same as β=0°, i.e. the near-field wave pattern rotates with the hull and remains within a similar wave envelope as β=0°. The wave amplitudes are significantly increased/decreased on the windward/leeward sides. The wake region is also asymmetric with larger wedge angle on the leeward side. The boundary layer and wake are dominated by the hull vortex system consisting of fore body keel, bilge, and wave-breaking vortices and after body bilge and counter-rotating vortices. The occurrence of a wave-breaking vortex for breaking bow waves has not been previously documented in the literature. The trends for the maximum vorticity, circulation, minimum axial velocity, and trajectories are discussed for each vortex.

Scaling relations for granular flow in quasi-two-dimensional rotating cylinders.

An experimental study of the flow of different materials (steel balls, glass beads, and sand) in quasi-two-dimensional rotating cylinders is carried out using flow visualization. The flow in the rotating cylinder comprises of a thin-flowing surface layer with the remaining particles rotating as a fixed bed. Experimental results indicate that the scaled layer thickness increases with increasing Froude number (Fr=omega(2)R/g, where omega is the angular speed, R is the cylinder radius, and g the acceleration due to gravity) and with increase in size ratio (s=d/R, where d is the particle diameter). The free surface profile, is nearly flat at low Fr and becomes increasingly S shaped with increasing Fr. The layer thickness profiles, which are symmetric at low Fr become skewed at high values of Fr and small s. The dynamic angles of repose for all the materials studied show a near-linear increase with rotational speed (omega). Scaling analysis of the experimental data shows that the shape of the scaled surface profiles and the scaled layer thickness profiles are nearly identical when Froude number and size ratio are held constant, for each material. The surface profiles and layer thickness profiles are also found to be nearly independent of the material used. The dynamic angle of repose (beta), however, does not scale with Fr and s and depends on the particle properties. The experimental results are compared to continuum models for flow in the layer. The models of Elperin and Vikhansky [Europhys. Lett. 42, 619 (1998)] and Makse [Phys. Rev. Lett. 83, 3186 (1999)] show good agreement at low Fr while that of Khakhar et al. [Phys. Fluids, 9, 31 (1997)] gives good predictions over the entire range of parameters considered. An analysis of the data indicate that the velocity gradient (gamma;) is nearly constant along the layer at low Fr, and the value calculated at the layer midpoint varies as gamma;(0)~[g sin(beta(0)-beta(s))/d cos beta(s)](1/2) for all the experimental data, where beta(s) is the static angle of repose and beta(0) is the interface angle at the layer midpoint. An extension of "heap" models (BCRE, BRdG) is used to predict the interface angle profiles, which are in reasonable agreement with experimental measurements.

Repeat breeding and subsequent reproductive performance in Swedish Landrace and Swedish Yorkshire sows

The objective of the present study was to analyse the association between repeat breeding (RB) in gilts/sows and their subsequent reproductive performance as well as the impact of interactions between repeat breeding and factors like parity number, boar breed, season and mating type (MT) on subsequent reproductive performance in Swedish Landrace (L) and Swedish Yorkshire (Y) sows. Data analysed included 7040 sows (3654 L and 3386 Y), farrowing during January 1994 until December 1999 in 11 L and 8 Y nucleus herds. The study was assigned as a cohort design and the aim was to study gilts/sows from their first mating as gilts until mating after third parity. Analysis of variance was applied to continuous data and logistic regression was applied to categorical data. Percentages of litters as a result of repeat breeding in sow parities 1–3 were 6.1, 12.0 and 6.3% for L sows and 6.7, 13.1 and 7.4% for Y sows. For parity 3, the incidence of litters resulting from repeat breeding was significantly higher ( P<0.001) in Y than in L sows. The proportion of irregular return to oestrus (>24 days after first mating) was higher ( P<0.01) in primiparous sows than in multiparous sows (69% versus 61%). On average, litters resulting from repeat breeding were larger ( P<0.001) than litters resulting from non-repeat breeding (NR) (about 0.5 piglets per litter) in both L and Y sows. For Y sows, if the previous litter was a result of repeat breeding, the subsequent reproductive cycle had 2.7% higher RR ( P<0.05) and 2.4% lower FR (N.S.) compared with sows that were not repeat bred. The same trend was found in L sows (1.4% higher RR and 1.3% lower FR) but the differences were not significant. Among the sows removed from the herds, about 24% of L and 28% of Y were culled due to reproductive problems (gilts not included). In addition, a number of sows from these nucleus herds were also culled due to low breeding value and poor conformation.

Large Eddy Simulation of a Flow Past a Free Surface Piercing Circular Cylinder

Interactions between surface waves and underlying viscous wake are investigated for a turbulent flow past a free surface piercing circular cylinder at Reynolds number Re=2.7×104 using large eddy simulation (LES). The computations have been performed for three Froude numbers Fr=0.2, 0.5 and 0.8 in order to examine the influence of the Froude number. A second-order finite volume method coupled with a fractional step method is used for solving the grid-filtered incompressible Navier-Stokes equations. The computational results are found to be in good agreement with the available experimental data. At low Froude numbers Fr=0.2 and 0.5, the amplitude of generated surface wave is small and the influence on the wake is not evident. On the other hand, strong wave-wake interactions are present at Fr=0.8, when the generated free surface wave is very steep. It is shown that structures of the underlying vortical flow correlate closely with the configuration of the free surface. Computational results show presence of a recirculation zone starting at the point where the surface slope changes discontinuously. Above this zone the surface elevation fluctuates intensively. The computed intensity of the surface fluctuation is in good agreement with the measurements. It is also shown that the periodic vortex shedding is attenuated near the free surface at a high Froude number. The region in which the periodic vortex shedding is hampered extends to about one diameter from the mean water level. It is qualitatively shown that the separated shear layers are inclined outward near the free surface due to the generation of the surface waves. This change in the relation between two shear layers is suggested to be responsible for the attenuation of the periodic vortex shedding.

Temporal pitch perception and the binaural system.

Two experiments examined the relationship between temporal pitch (and, more generally, rate) perception and auditory lateralization. Both used dichotic pulse trains that were filtered into the same high (3,900-5,400-Hz) frequency region in order to eliminate place-of-excitation cues. In experiment 1, a 1-s periodic pulse train of rate Fr was presented to one ear, and a pulse train of rate 2Fr was presented to the other. In the "synchronous" condition, every other pulse in the 2Fr train was simultaneous with a pulse in the opposite ear. In each trial, subjects concentrated on one of the two binaural images produced by this mixture: they matched its perceived location by adjusting the interaural level difference (ILD) of a bandpass noise, and its rate/pitch was then matched by adjusting the rate of a regular pulse train. The results showed that at low Fr (e.g., 2 Hz), subjects heard two pulse trains of rate Fr, one in the "higher rate" ear, and one in the middle of the head. At higher Fr (>25 Hz) subjects heard two pulse trains on opposite sides of the midline, with the image on the higher rate side having a higher pitch than that on the "lower rate" side. The results were compared to those in a control condition, in which the pulses in the two ears were asynchronous. This comparison revealed a duplex region at Fr > 25 Hz, where across-ear synchrony still affected the perceived locations of the pulse trains, but did not affect their pitches. Experiment 2 used a 1.4-s 200-Hz dichotic pulse train, whose first 0.7 s contained a constant interaural time difference (ITD), after which the sign of the ITD alternated between subsequent pulses. Subjects matched the location and then the pitch of the "new" sound that started halfway through the pulse train. The matched location became more lateralized with increasing ITD, but subjects always matched a pitch near 200 Hz, even though the rate of pulses sharing the new ITD was only 100 Hz. It is concluded from both experiments that temporal pitch perception is not driven by the output of binaural mechanisms.

VLBA Observations of a Sample of Nearby FR I Radio Galaxies

We observed 17 nearby low luminosity FR I radio galaxies using the NRAO Very Long Baseline Array (VLBA) at 1.67 GHz, as part of a multi-wavelength study of a complete sample of 21 sources selected by radio flux density from the Uppsala General Catalogue of Galaxies. We detected radio emission from all 17 galaxies. At a FWHM resolution of ~ 10x4 mas, five galaxies show only an unresolved radio core, ten galaxies show core-jet structures, and two galaxies show twin-jet structures. Comparing these VLBA images with images previously obtained with the NRAO VLA, we find that all detected VLBA jets are well-aligned on parsec scales with the VLA jets on kilo parsec scales, and that the jet to counter-jet surface brightness ratios, or the sidedness, decreases systematically with increasing distance along the jet. We attribute the sidedness to the Doppler boosting effect and its decline to the deceleration of the jets. We show that a distribution of Lorentz factor centered near Gamma = 5 can reproduce our VLBA detection statistics for core, core-jet and twin-jet sources. We also note that the luminosity per unit length, L_j, of the VLBA jets drops quickly with distance, r, along the jet, approximately as L_j being proportional to r^(-2.0). We discuss three different mechanisms to explain this jet fading: (1) the decrease of Doppler boosting due to jet deceleration, (2) synchrotron losses, and (3) expansion losses in constant velocity but adiabatically spreading jets. Mechanisms (1) and (2) are inconsistent with the observations, while mechanism (3) is consistent with the observations provided the magnetic field lines in the jets are aligned perpendicular to the jet axis. This implies that the deceleration of the jets required by the Unified Scheme does not occur on the tens of parsec scales, but must occur on larger scales.

An amino acid substitution in the putative second extracellular loop of RBC band 3 accounts for the Froese blood group polymorphism.

The low incidence RBC antigen Fr(a) has been excluded from 17 of the 25 established blood group systems. Previous genetic analysis assigned the gene controlling Fr(a) expression to the same chromosomal region as the solute carrier family 4, anion exchanger member 1 gene (SLC4A1). Because SLC4A1 encodes RBC band 3 and controls the expression of Diego blood group system antigens, the possible relationship of Fr(a) to the Diego blood group system was investigated by molecular analysis of SLC4A1. Blood samples were obtained from the members of two unrelated Mennonite kindreds segregating for Fr(a). DNA was extracted, amplified by PCR using intronic primer sets flanking exons 11-20 of SLC4A1, and screened by single-strand conformation polymorphism (SSCP) analysis. Those exons displaying SSCPs were subjected to DNA sequence analysis. An exon 13 SSCP mobility shift was observed in the DNA from all Fr(a+) persons that was not seen in the DNA from Fr(a-) family members or control subjects. Linkage between the exon 13 SSCP and FR:(a) was established, with peak lods = 3.62 at theta = 0.00 for combined paternal and maternal meioses. DNA sequencing revealed a GAG --> AAG mutation that underlies a Glu480Lys substitution in RBC band 3. A point mutation in exon 13 of SLC4A1 accounting for a Glu480Lys substitution in band 3 controls Fr(a) expression. On the basis of these our results, the International Society of Blood Transfusion Working Party on Terminology for Red Cell Surface Antigens has assigned Fr(a) to the Diego blood group system, with the designation DI20.

Genetic structure through surnames in Campobasso Province, Italy.

The population of Campobasso Province shows a level of inbreeding that is distinct from most Italian rural populations, regardless of their geographic location (Fr=0.0040; Fn=0.0102; Ft=0.0142). The genetic structure of the Italian-Greek communities of Lecce and Reggio Calabria Provinces does not appear to be affected by ethnicity. The level of inbreeding in Italian-Greeks of Reggio Calabria Province is similar to other Italians of Campobasso Province (Fr=0.0041; Fn=0.0127; Ft=0.0168). The Italian-Greeks of Lecce Province show random mating, and their inbreeding is in fact very low (Fr=0.0038; Fn=0.0024; Ft=0.0062).

An Alignment Effect in FR I Radio Galaxies:U‐Band Polarimetry of the Abell 2597 Cluster Central Galaxy

We have obtained U-band polarimetry of the spatially extended optical continuum associated with the FR I radio source PKS 2322-122, located in the Abell 2597 cluster central galaxy. We find a 3 σ upper limit to the degree of optical continuum polarization of less than 6%. The accuracy of the measurement is limited primarily by our ability to measure the amount of diluting galactic starlight. This limit is inconsistent with the continuum being primarily scattered light or synchrotron radiation. We can therefore exclude models that attribute the continuum to scattered light from an active nucleus that is hidden from direct view. Our result does not support the unification paradigm for BL Lac objects and FR I radio sources. Essentially all of the data pertaining to the continuum along the radio source—the blue lobes—indicate that they are regions of recent star formation. The spatial coincidence between the lobes and disturbances in the radio source suggests that star formation may have been induced by an interaction between the radio source and the cool (less than 104 K) surrounding gas. This result, in addition to the results of a similar study of the A1795 cluster central galaxy, shows that under the appropriate conditions FR I radio sources may be capable of inducing significant episodes of star formation in elliptical galaxies. We compare the rest-frame U-band polarized luminosities and 1.4 GHz radio powers of A2597 and A1795 to those for several high-redshift radio galaxies exhibiting the alignment effect. We find that if the polarized luminosities of radio galaxies scale in proportion to their radio luminosities, we would not have detected a polarized signal in either A2597 or A1795. We suggest that the strength of the active galactic nucleus is the fundamental property distinguishing some powerful high-redshift radio galaxies exhibiting the alignment effect from the lower power FR I radio galaxies in cooling flows that exhibit the lobe phenomenon. While both FR I and FR II radio sources seem to be capable of triggering star formation, FR I radio sources seem to be incapable of producing a large enough polarized luminosity to contribute significantly to the aligned continuum radiation.

Dynamics of counter-rotating vortex pairs in stratified and sheared environments

The evolution of a vertically propagating vortex pair in stratified and sheared environments is studied with a two-dimensional numerical model. We consider a range of Froude (Fr) and Richardson (Ri) numbers, and a limited number of Reynolds numbers (Re). We find that stratification causes the formation of counter-sign vorticity around each of the original vortices through baroclinic production. At higher Fr, this wake vorticity advects the primary vortices closer together, decreasing their separation distance and increasing their vertical propagation speed, as predicted by Crow (1974) and Scorer &amp; Davenport (1970). For these higher values of Fr, the wake vorticity also participates in an instability of the primary vortex pair, with the direction of propagation of the pair oscillating about the vertical. We term this instability the vortex head instability to distinguish it from the jet instabilities to which the wake itself is also susceptible. At lower Fr, internal gravity wave radiation dominates, and the intensity and spatial coherence of each vortex is rapidly reduced.When a mean horizontal flow having constant shear is present in an unstratified fluid, we find that the vortices eventually rotate about one another with the same rotational sense as the background shear flow, as predicted in Lissaman et al. (1973). When stratification is also present, we find that the distribution of baroclinically generated wake vorticity is asymmetric, which sometimes leads to the emergence of a solitary vortex with the same sign as the background shear vorticity (depending on the values of Fr, Ri, and Re). Our limited survey of parameter space indicates that a solitary vortex emerges more rapidly for smaller values of Ri, smaller values of Fr, and/or larger values of Re.

Very Low Fruit : Flower Ratios in Grevillea (Proteaceae) are Independent of Breeding System

Members of the family Proteaceae have extremely low mature fruit : flower (FR : FL) ratios (range 0.001–0.163) compared with other temperate, hermaphroditic, woody perennials. Sutherland’s (1986) survey of FR : FL ratios indicated that compatibility was an important factor explaining levels of fruit set. The role of compatibility in regulating FR : FL ratios was tested in five closely related species of Grevillea (Proteaceae). Species-specific compatibility was compared using the self-compatibility index (SI = ratio of selfed fruit set to crossed fruit set) calculated at fruit initiation to minimise the confounding effect of other post-fertilisation fruit losses, such as inbreeding depression and pre-dispersal predation. Fruit : flower ratios at initiation ranged from 0.041–0.249, and at maturity 0.015–0.096. Grevillea species showed highly variable breeding systems: G. linearifolia was self-incompatible (SI = 0.003), G. sphacelata, G. mucronulata, and G. oleoides were partially self-compatible (SI = 0.07–0.28) and G. longifolia was self-compatible (SI = 0.61). Intrapopulation variability in the level of self-incompatibility was high in all species but G. linearifolia. The correlation between SI and FR: FL ratios was non-significant, indicating that compatibility has a minimal effect on fruit set in the Grevillea species studied, and that these data, together with other data on proteaceous species do not support trends observed in Sutherland’s survey. Low FR : FL ratios resulted from of a combination of pollen limitation, and high levels of flower and fruit predation.

New vocalization definitively linked to the minke whale

The description of minke whale (Balaenoptera acutorostrata) vocalizations is sparse with little recent study. In addition, the minke and some of its assumed vocalizations are not definitively linked. To address this problem, recordings were made at a unique site off the Great Barrier Reef where minkes maintain long, close contacts (to 10 h) with vessels. During these encounters a 200-m, four-element hydrophone array (Hi-Tech HTI-SSQ41B, 10 Hz–30 kHz) was deployed and digitally recorded onto a Tascam DA-38 (0–22 kHz). Time of arrival differences between hydrophones were used to obtain hyperbolic fixes on nearby sound sources. With this method, a new vocalization consisting of three short pulses (pulse duration 100 ms, peak energy 200–250 Hz, with significant energy between 1.5 and 2 kHz) followed by a slight upsweep (trail of 1 s) has been definitively linked to the minke. Source levels of at least 155 dB re: 1 μPa were estimated. Similar vocalizations with varying pulse numbers (to at least 7), and low frequency grunts and moans (80–4000 Hz) were recorded. Approximately 19 h were recorded in the close presence of 45 minke whales along with 18 h of ambient recordings. These recordings greatly contribute to the sparse database of known minke vocalizations. [Work supported by ONR and the R/V Undersea Explorer.]

Simultaneous activity of two different mechanisms of folate transport in ovarian carcinoma cell lines

We investigated whether the folate receptor alpha-isoform (FR alpha), which is overexpressed on ovarian carcinoma cells, is functionally active in internalizing the physiological form et folate, 5-methyl tetrahydrofolate (THF). Six ovarian tumor cell lines, expressing different levels of FR alpha (COR > > OVCAR3 > IGROV1 > OVCAR4 > SKOV3 > OVCAR5), were maintained in folate-depleted medium and internalization of 10 nM evaluated as acid-resistant radioactivity at 0 degree and 37 degrees C. The amount of 5-methyl[1H]THF present in this fraction was not strictly related to the number of membrane receptors, since even cell lines with low FR alpha expression, e.g., OVCAR4, showed efficient internalization. Time-course studies indicated that, whereas no uptake was detected at 0 degree C, at 37 degrees C the internalized fraction showed a slow and constant increase, until 4 h. At this time the internalized radioactivity represented < 50% of the total bound in COR, OVCAR3 and IGROV1 cells, whereas the other cell lines tested internalized fourfold more folate than their surface binding capacity. The incubation in the presence of a concentration (50 nM) of 5-methyl[3H]THF, which best ensures receptors saturation on cells with highest FR levels (COR and OVCAR3), had slight effect on surface binding of all the tested cell lines, including IGROV1 and SKOV3. In contrast, the increase of the uptake was more pronounced, particularly in SKOV3 cells. These results, together with the accumulation curves of folic acid (FA) and 5-methylTHF at 37 degrees C, suggested the presence of a molecule on ovarian carcinoma cells with high affinity for reduced folates, possibly a reduced folate carrier (RFC). Measurement of radioactivity present in the supernatant of IGROV1 and SKOV3 cells, subjected to hypotonic lysis and cell fractionation, further indicated that 5-methyl[3H]THF was translocated to the cytosol and, despite differences in membrane levels of FR alpha expression this internalized fraction was similar in both cell lines. Inhibition experiments to selectively block FR alpha or RFC activity showed a differential sensitivity of the two pathways depending on the cell line examined. Internalization was more consistently inhibited on IGROV1 than on SKOV3 cells by treatments that disrupt FR alpha activity, e.g., incubation with excess FA and phosphatidylinositol specific phospholipase C, whereas Probenecid, which preferentially inhibits the carrier-mediated pathway, showed a strong inhibitory effect on both cell lines. These findings suggest that the internalization of 5-methylTHF in these tumor cells depends not only on the level of overexpressed FR alpha, but another transport route, with features characteristic for RFC, is functional and participates in folate uptake.

Arcsecond Scale Polarization of BL Lac Objects

BL Lac objects are an enigmatic class of active galactic nuclei. They are characterized by high luminosity, a flat radio spectrum that steepens at higher energies, relatively high optical and radio polarization, rapid variability and an optical continuum with weak or absent emission lines (see Urry and Padovani, 1995 for a recent review).These properties have been interpreted in terms of a relativistic jet closely aligned to the line of sight (Blandford and Rees, 1978, Ghisellini et al., 1993). This model, known as the beaming model, implies that there must be a so called “parent population” of radio sources intrinsically identical to BL Lac objects, but with the jets oriented at large angles to the line of sight. Browne (1983) was the first to propose the low luminosity FR I radio galaxies as the most likely candidates for the “parent population” of the core dominated BL Lac objects. An outcome of the beaming model is that all the properties not depending on orientation should be shared by the BL Lac objects and the FR I radio galaxies.

Schedule-Controlled Responding of Two Persons Under a Single Schedule of Reinforcement

In four experiments, dyad members made noncompetitive responses for a mutual reinforcer. Satisfying reinforcement requirements delivered a single reinforcer (points) to both subjects as a team. The purpose of this research was two-fold: First, to discover the degree to which individual behavior is sensitive to team-like contingencies, and second, to ask how the social context itself can modulate the effects of team-like contingencies on individual behavior. Variables manipulated included fixed-ratio size, conjunctive reinforcement, reinforcer magnitude, and social context. Results showed that (1) individual subjects responded consistently in an alternating all-or-none pattern at lower FR values, (2) experimental contingencies embedded within the FR schedule exerted powerful control over individual responding, (3) changes in the reinforcer magnitude also radically altered how each subject responded, and (4) changing the social context to not allow subjects to converse did not significantly alter subjects’ behavior. These results suggest that team-like contingencies can control individual patterns of behavior and that particular social aspects of the environment do not necessarily mediate individual behavior in such situations.

Periphyton responses to a hydraulic gradient in a regulated river in New Zealand

SUMMARY1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analysed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1–0.5m depth, 0.1–1.5ms−1 velocity, and 0.01–1.5 Froude number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis herculeana having the highest relative biovolumes.2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co‐dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid‐velocity range (velocity = 0.8–1.2ms−1).3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1–2mm) at low velocities and Fr (in pool habitats), to thick (approximately 10mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash‐free dry mass (APDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in % chlorophyll a over the gradient.4. The results of experiments conducted in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms−1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions.5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient‐poor streams.

Theoretical analysis of the onset of liquid entrainment for orifices of finite diameter

The onset of liquid entrainment during discharge from large reservoirs containing a stratified mixture of two immiscible fluids through a side orifice of finite diameter is considered theoretically. A previously reported analysis by Craya, in which the orifice was simulated by a point sink, has been extended to account for the finite size of the orifice. The model resulting from the present analysis is expressed in terms of two simple equations suitable for hand calculations. The ratio of the critical height to the orifice diameter is found to be dependent only on Froude number (Fr). The present model approaches the correct physical limits at low Fr and it converges to Craya's predictions at high Fr. Some experimental verification of the present theoretical trends is also provided.

Mixing, Dispersion, and Resuspension in Vicinity of Ocean Wastewater Plume

Buoyant plumes discharged from ocean outfalls are important for dispersing municipal wastewater into the marine environment in many coastal areas, although field studies of operating outfall systems are rare. Here, we report on an extensive field study of the effluent plume from one of the largest wastewater outfalls on the west coast of the United States. This study shows that the dispersion and mixing levels of the plume depend upon the local current speed and ambient density stratification. Under these highly stratified, late‐summer conditions, we find that a combination of temperature, salinity, turbidity, and chlorophyll fluorescence measurements are required to unambiguously identify plume waters. Over the course of these observations, the lcoal current speed at plume depth varied from 0.01 m/s to 0.07 m/s, corresponding to a Froude number Fr range of 10-3 to 0.2. A limited number of dilution estimates are made for the lower Fr case, and these fall in the range 110 to 160, which is within the design range for this type of diffuser. The observed maximum height of rise and wastewater field thickness are in reasonable agreement with laboratory results for zero current speed (Fr = 0). At the higher current speed, turbidity layers originating at the sea floor are observed which result from resuspension of bottom sediments.

Developmental responses of portulaca seedlings to conflicting spectral signals.

Portulaca oleracea seedlings avoid growing in the direction of neighbouring plants even when they are very small or remote. The present study was designed to determine the relative effect on the development of Portulaca seedlings of light availability (i.e. the resource level) as compared with spectral composition (i.e. the signal of future competition for the resource). The plants were subjected to various intensities of photosynthetic light and red/far-red (R/FR) ratios from opposite directions. The seedlings became recumbent preferentially towards the direction with the lower FR light, even when this meant growing towards plastic that absorbed 20 times more photosynthetic light. A preference for the direction with higher photosynthetic light over lower FR was also found, though only under extreme light differences. The response of the seedlings was not absolute: the orientation chosen depended on the light received from other alternative directions.