Buoyancy Difference Research Articles

Drainage in a Rough Gouge-Filled Fracture

We study experimentally and numerically slow drainage of a non-wetting fluid in a saturated fracture. Facing surfaces of the experimental fracture (25 cm × 25cm) are obtained from the casting of a brittle fracture of a granite block. They are rough but mated leading to a constant aperture where glass beads (1 mm) are spread in a single layer to mimic the influence of gouge particles trapped within the fracture. During injection, snap off might appear owing to the buoyancy difference between fluids, which splits the non-wetting invader into bubbles. Geometry and connectivity of this new fluid structure are characterized and shown to be significantly controlled by the long range spatial correlations of the fracture topography. Two coexisting types of sub structures emerge along percolating clusters: string-like links and compact blobs. Saturation and trapping are shown to be significantly influenced by the buoyancy effect. A numerical model to describe the experiments is introduced. It is based on an invasion percolation algorithm but includes spatially correlated contributions resulting from the roughness of the crack surfaces and gravity. Numerical results are shown to be consistent with experimental observations. The model allows us to extend the analysis to regimes where gravity forces are completely dominating and to obtain statistical results exploring numerous different fractures with the same properties (roughness statistics, pore size distribution, size). A description of the injection pressure evolution is proposed.

Most relevant mechanisms of inclusion growth in an induction–stirred ladle

The purpose of this study has been to theoretically evaluate which inclusion growth mechanisms are important in an inductively stirred ladle. This has been done using data from a computational fluid dynamics model of a real ladle. The data was utilized as the input to the different collision equations. It was concluded that diffusion of oxygen and deoxidant to the inclusion surface, diffusion coalescence and Brownian motion collisions contribute very little to growth of inclusions during the stirring period in the ladle. This was in accordance with earlier findings in the literature. This study also showed that laminar shear collisions could be excluded from a growth model. The major growth mechanism is turbulent collisions. It was also shown that a 700–A stirring current leads to more growth due to turbulent collisions than a 500–A stirring current. The importance of a correct value of the collision efficiency coefficient for turbulent collisions was pointed out. If a large difference in inclusion size exists then collisions due to difference in buoyancy (Stokes collisions) also need to be considered in a growth model. Finally, it is concluded that the variation in fluid flow and turbulent properties in different parts of the ladle should be incorporated into a growth model, since it can affect average turbulent collision volumes by 25–30%.

Experimental Evaluation of Airflow in Naturally Ventilated Active Envelopes

Active envelopes are characterised by two glazed surfaces separated by a ventilated cavity in which a shading device is placed. Thermal performance of active envelopes is governed by the airflow through the cavity. Until now, there is a lack of knowledge about the amplitude and direction of the airflow in naturally ventilated active envelopes. In this paper we report on the airflow in the naturally ventilated active envelope at the Vliet building of the Laboratory of Building Physics in Leuven, Belgium. (1) The airflow was experimentally determined by measuring the pressure difference over a ventilation grid. This method shows several advantages over traditional methods. (2) Thermal buoyancy and wind pressure differences govern the airflow. For both phenomena, measured data were compared with models. It was found that at low windspeeds and upward flow conditions, a linear relation between the pressure and temperature difference could be obtained. For higher windspeeds simple models cannot accurately predict the complexity of the airflow in naturally ventilated active envelopes due to the location of the measurement probes and due to wind washing effects imposed by altering wind directions.

The Daily Behavior of Pressure and Its Influence on the Wind Regime in East Antarctica during the Winters of 1993 and 1994

The analysis of surface pressure data obtained in 1993–94 at the Dumont d'Urville meteorological station and at Automatic Weather Stations Dome C, D-47, and D-80 (East Antarctica) shows the existence of a daily variation. Power spectra are characterized by the presence of a pronounced 1/2-day peak; a 1-day peak is only slightly evident. However, the magnitude of the spectral density at 1 day is about 4–6 times higher than that of 1/2 day. This behavior is different from that observed at lower latitudes, at which two pronounced peaks at 1 and 1/2 day, of similar magnitude, are observed. The yearly averaged daily pressure time series show two maxima, one at 0900 LT and the other at 2100 LT. The magnitude of this variation increases toward the coast, that is, varying from 0.25 hPa at Dome C to 0.4–0.6 hPa at Dumont d'Urville. The difference of the daily pressure behavior at individual sites produces a daily variation in the quasi-meridional pressure gradient between different pairs of stations. This quasi-meridional gradient likely is one of the mechanisms conditioning the daily wind behavior at the near-coastal zones, especially during the winter, when the daily variation of the buoyancy difference between inland and coastal zones is weak. The analysis of the wind velocity daily cycle for different seasons shows that its behavior in winter differs from both the summer and midseason counterparts. In winter, the maximum meridional wind component at Dumont d'Urville is correlated well with the maximum of the pressure gradient. This pressure gradient should be taken into account in modeling the wind regime over Antarctica during winter, when the buoyancy gradient daily variation is negligible.

Entrainment and mixing in stratified shear flows

The results of a laboratory experiment designed to study turbulent entrainment at sheared density interfaces are described. A stratified shear layer, across which a velocity difference ΔU and buoyancy difference Δb is imposed, separates a lighter upper turbulent layer of depth D from a quiescent, deep lower layer which is either homogeneous (two-layer case) or linearly stratified with a buoyancy frequency N (linearly stratified case). In the parameter ranges investigated the flow is mainly determined by two parameters: the bulk Richardson number RiB = ΔbD/ΔU2 and the frequency ratio fN = ND=ΔU.When RiB > 1.5, there is a growing significance of buoyancy effects upon the entrainment process; it is observed that interfacial instabilities locally mix heavy and light fluid layers, and thus facilitate the less energetic mixed-layer turbulent eddies in scouring the interface and lifting partially mixed fluid. The nature of the instability is dependent on RiB, or a related parameter, the local gradient Richardson number Rig = N2L/ (∂u/∂z)2, where NL is the local buoyancy frequency, u is the local streamwise velocity and z is the vertical coordinate. The transition from the Kelvin–Helmholtz (K-H) instability dominated regime to a second shear instability, namely growing Hölmböe waves, occurs through a transitional regime 3.2 < RiB < 5.8. The K-H activity completely subsided beyond RiB ∼ 5 or Rig ∼ 1. The transition period 3.2 < RiB < 5 was characterized by the presence of both K-H billows and wave-like features, interacting with each other while breaking and causing intense mixing. The flux Richardson number Rif or the mixing efficiency peaked during this transition period, with a maximum of Rif ∼ 0.4 at RiB ∼ 5 or Rig ∼ 1. The interface at 5 < RiB < 5.8 was dominated by ‘asymmetric’ interfacial waves, which gradually transitioned to (symmetric) Hölmböe waves at RiB > 5:8.Laser-induced fluorescence measurements of both the interfacial buoyancy flux and the entrainment rate showed a large disparity (as large as 50%) between the two-layer and the linearly stratified cases in the range 1.5 < RiB < 5. In particular, the buoyancy flux (and the entrainment rate) was higher when internal waves were not permitted to propagate into the deep layer, in which case more energy was available for interfacial mixing. When the lower layer was linearly stratified, the internal waves appeared to be excited by an ‘interfacial swelling’ phenomenon, characterized by the recurrence of groups or packets of K-H billows, their degeneration into turbulence and subsequent mixing, interfacial thickening and scouring of the thickened interface by turbulent eddies.Estimation of the turbulent kinetic energy (TKE) budget in the interfacial zone for the two-layer case based on the parameter α, where α = (−B + ε)/P, indicated an approximate balance (α ∼ 1) between the shear production P, buoyancy flux B and the dissipation rate ε, except in the range RiB < 5 where K-H driven mixing was active.

The life cycle of <i>Halimedusa typus</i>, with discussion of other species closely related to the family Halimedusidae (Hydrozoa, Capitata, Anthomedusae)

The little-known Anthomedusa Halimedusa typus has been collected from several locations in California, Oregon, and British Columbia on the Pacific coast of the United States. The adult medusa is redescribed based on new observations of living material and is found to have capitate tentacles. Polyps of H. typus were raised several times after spawning field-collected medusae in the laboratory; the cultures on one occasion lived for more than a year. The capitate polyp is solitary and very tiny, emerging from a basal perisarc measuring 200-300 µm in diameter. One cultured polyp produced a medusa, which is described. The taxonomic positions of several other morphologically-similar Anthomedusae in the Capitata are compared and discussed here. Tiaricodon coeruleus and Urashimea globosa are moved from the Polyorchidae to the Halimedusidae, and the similarity of Boeromedusa auricogonia (Boeromedusidae) to all of these medusae and to the genera Polyorchis, Scrippsia and Spirocodon of the family Polyorchidae is considered. The group of species under consideration is basically restricted to the Pacific Ocean, except for T. coeruleus and U. globosa, which have also been collected in the south Atlantic and south Atlantic/Antarctic. It is noted that medusae of the Halimedusidae are typically found quiescent near the surface, whereas those of the Polyorchidae either rest on the bottom or must continue pulsating to stay up in the water column, indicating a basic underlying difference in buoyancy and resultant behavior between the medusae in these two families.

Diurnal Variations of the Temperature and Their Influenceon Wind Regime in a Confluence Zone of Antarctica

The behaviour of the wind field at the confluence zone of Terra Nova Bay during a transition period from winter to summer is considered. To explain some observed features the influence of diurnal variations are considered to integrate and complete the analysis based on annual and seasonal variations. Diurnal variations in the buoyancy difference between two main air masses coming from the Revees and Priestley glaciers are assumed to contribute to diurnal alteration of the wind direction at the Nansen ice sheet. The data recorded at three automatic weather stations (AWS) available that period are analysed. Using the wavelet analysis procedure, it is shown that the contribution of one-day variations of some basic meteorological parameters and their gradients (in particular of temperature and pressure) is important and comparable with synoptic ones. On the base of the diurnal variations some aspects of the local circulation have been explained. The basis for the further studies related to the evolution of katabatic winds in this area are also presented.

The shallow thermohaline circulation of the Red Sea

The circulation of the Red Sea is driven by strong surface evaporation, but the buoyancy budget alone cannot determine the circulation and thermohaline structure. The circulation could be rapid with weak stratification or slow with strong stratification. These possible states are examined in the dynamical framework of Phillips' similarity model applied to the upper 140 m. A large eddy viscosity in the return flow between 70 and 140 m is required for the solution to match the observed stratification. The circulation for this solution is slow, in accord with the weak geostrophic currents observed in the Red Sea, and partially justifies the neglect of rotation. The internal viscous force required to match the model to observations seems too big to be a proxy for bottom friction on the sloping sides, though the mismatch is not large at the southern end of the Red Sea if the model is modified to allow for decreasing width with depth. The model may also be used to confirm that forcing by the wind stress is of secondary importance. Overall, the Red Sea appears not to be overmixed, though the model displays the counterintuitive result that increased vertical mixing increases the buoyancy difference between surface and bottom.

Encapsidated adenovirus minichromosomes allow delivery and expression of a 14 kb dystrophin cDNA to muscle cells.

Adenovirus-mediated gene transfer to muscle is a promising technology for gene therapy of Duchenne muscular dystrophy (DMD). However, currently available recombinant adenovirus vectors have several limitations, including a limited cloning capacity of approximately 8.5 kb, and the induction of a host immune response that leads to transient gene expression of 3-4 weeks in immunocompetent animals. Gene therapy for DMD could benefit from the development of adenoviral vectors with an increased cloning capacity to accommodate a full-length (approximately 14 kb) dystrophin cDNA. This increased capacity should also accommodate gene regulatory elements to achieve expression of transduced genes in a tissue-specific manner. Additional vector modifications that eliminate adenoviral genes, expression of which is associated with development of a host immune response, might greatly increase long-term expression of virally delivered genes in vivo. We have constructed encapsidated adenovirus minichromosomes theoretically capable of delivering up to 35 kb of non-viral exogenous DNA. These minichromosomes are derived from bacterial plasmids containing two fused inverted adenovirus origins of replication embedded in a circular genome, the adenovirus packaging signals, a beta-galactosidase reporter gene and a full-length dystrophin cDNA regulated by a muscle-specific enhancer/promoter. The encapsidated minichromosomes are propagated in vitro by trans-complementation with a replication-defective (E1 + E3 deleted) helper virus. We show that the minichromosomes can be propagated to high titer (> 10(8)/ml) and purified on CsCl gradients due to their buoyancy difference relative to helper virus. These vectors are able to transduce myogenic cell cultures and express dystrophin in myotubes. These results suggest that encapsidated adenovirus minichromosomes may be useful for gene transfer to muscle and other tissues.

A Model for a Shear-Free Convective Boundary Layer with Parameterized Capping Inversion Structure

Abstract The paper extends Deardorff's general structure parameterization for a shear-free convective boundary layer. The model suggested employs the mixed layer hypothesis that the buoyancy (which is defined as b = g(ρ0 − &rho)/ρ0, where ρ is the density, ρ0 is the reference density, and g is the acceleration due to gravity) is constant with height within the mixed layer. The buoyancy flux zero-crossing height is taken as the mixed layer depth. The vertical buoyancy profile within the capping inversion, where the buoyancy flux is negative due to entrainment, is made dimensionless, using the buoyancy difference across the inversion and its thickness as appropriate scales. The approach was first suggested by Kitaigorodskii and Miropolsky for the oceanic seasonal thermocline. The authors examine the idea against the data from atmospheric measurements, laboratory experiments with buoyancy-agitated turbulence, and large-eddy simulations. The rate equations for the mixed layer and inversion layer depths are de...

The Role of Indonesian Throughflow in a Global Ocean GCM

The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absence of throughflow. Comparison of the first and second runs shows that the throughflow generally warms the Indian Ocean and cools the Pacific. However, large changes in the surface temperature and heat flux are restricted to certain well-defined regions: the Agulhas Current/outflow, the Leeuwin Current region off western Australia, the Tasman Sea, the equatorial Pacific, and two bands in the midlatitude South Pacific. In contrast, large subsurface temperature changes are widespread across both oceans. Heat budget analysis indicates that the large surface responses are dependent on the subsurface temperature change being brought to the surface, either by strong wind-forced upwelling (as in the equatorial Pacific) or by vigorous mixing in convective mixed layers (as in the other regions). Over most of both oceans, such mechanisms are absent and surface heat-flux changes are small (a few W m−2). There, subsurface temperature perturbations are largely insulated from the surface and may extend via direct advection or baroclinic wave propagation. The additional beat is released upon encounter with upwelling or a convective mixed layer, which may be far removed from the source of the perturbation. Atlantic and far Southern Ocean effects are mostly very small, possibly because of our use of restoring upper boundary conditions. The third and fourth runs break the throughflow into its baroclinic and barotropic components. The baroclinic (buoyancy-driven) component affects surface beat flux strongly in the Leeuwin Current region but relatively weakly in the Agulhas Current and Tasman Sea. The barotropic component has the converse effect. Interocean heat exchange is discussed; the full throughflow transports a net 0.63 petawatts out of the Pacific Ocean, which represents about one-third of the total heat input into the model's equatorial Pacific.

On the turbulent fluxes of buoyancy, heat and moisture at the air‐sea interface at low wind speeds

Under low‐wind, unstable conditions over water, the Liu, Katsaros and Businger (LKB) estimate of the buoyancy flux into the atmosphere has the form ν1/3 ΔB4/3 g(ξ), where 22.7ξ is the ratio of the Monin‐Obukhov length to the calm‐water roughness length; ΔB is the buoyancy difference between air just above the ocean surface and the ambient air above, and ν is viscosity. An analytic expression for g(ξ) is obtained, for low winds; g(ξ) has a minimum at about ξ=40 (corresponding over tropical oceans to wind speeds of about 0.3–0.6 m s−1). For lower values of x the assumptions underlying the LKB algorithm are no longer valid. For a reasonable choice of dimensionless constants, g(ξ) varies only by 8% over 40<ξ<200, being just above the minimum. Thus the LKB flux estimates are nearly independent of wind speed at their lower limit of applicability; it seems reasonable to identify this LKB minimum flux with laboratory observations of fluxes at zero wind. These show that the buoyancy flux is given by C* Pr−⅔ ν1/3 ΔB4/3, where measured values of C* are about 0.14±0.01 (Golitsin and Grachev, 1986). With Pr=0.71 for air, this implies that g(0) = 0.19 — well within the allowable range of values for the minimum of g(ξ). However, the minimum in g(ξ) varies as (γAT)1/3, where γ is the dimensionless constant in the Dyer‐Hicks‐Businger stability profile and AT the constant in the scalar roughness length for low winds; this can be used to fix AT. In field applications, buoyant motions in the planetary boundary layer ensure that r.m.s. wind speeds wG just above the surface are generally greater than the wind speed at the minimum of g(ξ), even for dry convection — though this would not be true if the planetary boundary layer were typically 100 m thick, instead of 1000 m. This “gustiness” is allowed for by replacing mean wind speed U by (U2+wG2)½ in the bulk formulae.

Direct tracking of coral larvae: Implications for dispersal studies of planktonic larvae in topographically complex environments

Abstract The larvae of broadcast spawning corals form visible surface aggregations following the annual spring mass spawning on the Great Barrier Reef. Aerial surveys revealed that these aggregations had patchy distributions which were related to local hydrodynamic features. Aggregations were observed along fronts associated with buoyancy differences between water masses, with boundary mixing at reef surfaces, and with island wakes. Secondary currents associated with these fronts accumulated the buoyant larvae into linear slicks. Fronts may be found in proximity to islands, reefs and coastal headlands. Hence, local topography may determine where larval aggregations occur as well as the fine-scale structure of these patches. Once aggregated into coral spawn slicks, larvae were advected away from reefs in complex, inter-reefal currents. Wind patterns determined the general direction of larval transport and overall degree of larval patchiness. These findings have important implications for modelling studies of larval dispersal and highlight the importance of topographically controlled fronts in structuring larval distributions.

Emptying filling boxes: the fluid mechanics of natural ventilation

This paper describes the fluid mechanics of the natural ventilation of a space connected to a large body of stationary ambient fluid. The flows are driven by buoyancy differences between the interior and exterior fluids. Connections with the ambient fluid are high level and low level openings. Two main forms of ventilation are identified: mixing ventilation and displacement ventilation. Mixing ventilation occurs when the incoming ambient fluid mixes with the fluid within the space, as is, the case if dense fluid enters through a high level inlet. In this case vertical stratification is weak. Displacement ventilation occurs when dense fluid enters at low levels and displaces the lighter fluid within the space out through high level openings. A strong stable stratification develops in this case, and there is little mixing between the incoming fluid and that in the interior. Both of these modes of ventilation are studied theoretically and the results are compared with laboratory experiments. Transient draining flows which occur when a space initially contains fluid of a density different from the ambient are examined.The presence of internal sources of buoyancy allows steady states to be established, and the effects of point, line and vertically distributed sources are studied. These steady states are extensions of filling box models, with the addition of continuous exchange of fluid with the environment outside the space. A major result of this work is that the form of the stratification within the space depends on the entrainment caused by the convective elements (plumes) produced by the buoyancy sources, but is independent of the strength of the sources. The strength of the stratification and the magnitudes of the velocities do, however, depend on the source strength. The effects of opening size(s) and configurations are determined, and criteria for producing a particular stratification within the space are established. Applications of this work to the ventilation of buildings are presented.

Gravity Waves, Compensating Subsidence and Detrainment around Cumulus Clouds

Abstract Gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field. We explore this role in a two-dimensional numerical experiment on a simple moist convecting system consisting of an isolated long-lasting nonprecipitating cloud in a calm atmosphere with no surface forcing. The cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic. The buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence. Unlike the circulations inside clouds, this adjustment takes place without turbulent mixing. Hence, the “buoyancy adjustment time” T1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the “mixing time” T2 which it takes a tracer, initially concentrated at s...

A fracture-loop thermal balance model of black smoker circulation

This time-dependent model examines the thermal balance between hot rocks and hydrothermal circulation during the formation of volcanogenic ore deposits, in order to establish whether the major heat supply to black smoker systems comes from the rock or from an underlying magma chamber. Flow is modelled as an open thermosyphon in a single fault, which is parallel and very close to the mid-ocean ridge axis. This configuration is based on seafloor and ophiolite evidence of black smoker systems and their structural location. The fault is represented in the model by a series of pipes carrying turbulent flow, which is driven by the buoyancy difference between upflow and downflow. Hydraulic resistance to flow is predominantly in the discharge zone. The fault is allowed to propagate downwards. The horizontal conductive heat flux from the rock is computed analytically, with the water flow treated digitally, so that the heat exchange at the rock-water interface at every position in the loop can be calculated for each time step. The model thus provides the exit water temperature and mass flow rate throughout the lifespan of a hot spring, enabling the resulting ore mass to be estimated. Results show that this system cannot obtain sufficient heat from a thin layer of rock (such as the 1–2 km thick layer of lavas and dykes overlying an axial magma chamber) to sustain long-lived, ore-forming black smokers. If the circulation penetrates through a greater volume of hot rock to the base of a recently-solidified magma chamber, ore masses of a few million tonnes can be formed if conditions are particularly favourable. We conclude that systems producing larger ore masses must tap a magmatic heat supply.

Heat transfer from a vertical tube bundle under natural circulation conditions

A rectangular loop (thermosyphon) was used to measure the average heat transfer coefficients for water at atmospheric pressure under natural circulation conditions. A twenty-one tube bundle with tubes 1.65 m long and 9.55 mm in diameter, and a pitch-to-diameter ratio of 1.33, was used as a test heat exchanger in one of the vertical legs of the loop. A natural circulation flow in the loop developed due to buoyancy differences of the fluid in its two vertical legs. Flow visualization experiments were performed to determine the flow regimes associated with natural circulation flow longitudinal to a tube bundle. Empirical correlations for the average Nusselt number have been developed and are reported. Grid spacers arranged on tube bundles were shown to enhance heat transfer, especially for laminar flow, without any noticeable increase in pressure drop.

Micro-lipid-droplet encapsulation of Bacillus thuringiensis subsp. israelensis delta-endotoxin for control of mosquito larvae

The crystal delta-endotoxin of Bacillus thuringiensis subsp. israelensis is less toxic to larvae of Anopheles freeborni than to larvae of Aedes aegypti. However, when solubilized crystal was used, larvae from both species showed similar sensitivities. This effect presumably was due to the differences in feeding behavior between the two mosquito larvae when crystal preparations are used. A procedure is described whereby both crystal and solubilized B. thuringiensis subsp. israelensis toxin were emulsified with Freund incomplete adjuvant, with retention of toxicity. The use of Freund incomplete adjuvant also allowed one to assay the solubilized toxin at a low nanogram level. Furthermore, coating the toxin with lipophilic material altered the buoyancy of the toxin and reversed the sensitivities of the two mosquito larvae toward the B. thuringiensis subsp. israelensis toxin. This difference in buoyancy was determined by using an enzyme-linked immunosorbent assay that was specific for the toxic peptides. These data indicate that economically feasible buoyant formulations for the B. thuringiensis subsp. israelensis crystal can be developed.

Temps d'ajustement de la flottabilité des saumons atlantiques (Salmo salar) d'élevage et flottabilité comparée de saumons atlantiques d'élevage et sauvages

We determined the time of adjustment of buoyancy of juvenile hatchery-reared Atlantic salmon (Salmo salar) and investigated whether if their higher position off the substratum, compared with wild fish, might better result from a higher buoyancy. In flowing water, hatchery-reared Atlantic salmon adjust their buoyancy within 24 h. No difference in buoyancy exists between juvenile hatchery-reared and wild Atlantic salmon in still water. However, juvenile hatchery-reared Atlantic salmon are more buoyant in flowing water than wild Atlantic salmon. Even after 3 mo in a stream, the buoyancy of juvenile hatchery-reared Atlantic salmon did not change. Acquired hatchery behaviour appears to be responsible for their difference in buoyancy.

The growth of a grid-generated turbulent mixed layer in a two-fluid system

This paper describes a laboratory experiment designed to compare measurements with published theoretical ideas of the mixed-layer growth of a two-layer system in which the turbulence is induced by an oscillating grid. Experimental results show excellent agreement with an earlier theory by one of us (Long), in which the mixed-layer depth D* measured from a virtual origin is given by $D_{*}\sim V_0^{-\frac{7}{11}}K^{\frac{9}{11}}t^{\frac{2}{11}}$, where K is action, t is time and V0 is a characteristic velocity of the problem. The experiments also verify Long's theoretical entrainment relation E = α2Ri−7/4, where E is the entrainment coefficient and $Ri = D^3_{*}\Delta b/K^2$, and Δb is the buoyancy difference between the two layers. The interfacial-layer thickness was observed to be proportional to the depth of the mixed layer, as also predicted by Long. After a certain depth, the entrainment law tends to deviate from Long's theory. The deviation may be due to wall effects.