Initial Density Gradient Research Articles

A Model Study of the Buoyancy-Driven Circulation in the Gulf of Maine

Abstract A quasi-linear prognostic numerical model has been used to study the three-dimensional baroclinic circulation in the Gulf of Maine region, with a focus on the buoyancy-driven circulation in the inner basins and near the coast where the influence of relatively fresh water from rivers and the Scotian shelf is important. The model uses a horizontal grid with 6-km resolution and a terrain-following vertical coordinate with ten levels to represent the bathymetry and stratification. Several model experiments explore the influences of fluxes from the principal rivers, tidal mixing, and shelf water inflow, beginning with a case with no initial horizontal density gradients and ending with cases initialized by temperature and salinity data from cruises in June of 1982 and 1983, augmented by climatological data. With composite initialization, the model develops a circulation containing realistic elements, including an anticlockwise gyre in the interior of the gulf and a prominent southwestward coastal curre...

A visual study of liquid motion in vertical capillaries under conditions of inverse density gradients

A visual study applying close-up photography was used to study the motions of aqueous KMnO 4 solutions inside vertical capillaries under conditions of inverse density gradients. The method permitted the recording of the disturbance modes during the evolution of the convective instability. In the present work three types of such modes were identified. Irrespective of their sequence the final quasi-steady state was observed to be always the same. A criterion of neutral stability of these motions was proposed by Taylor for the measurement of molecular diffusivity of liquids. In this investigation, initial density gradients, capillary radius and duration of experimental runs proved to be important parameters for such measurements. It was found that Taylor's assumption of negligible axial molecular diffusion compared to convection is not valid under conditions of low initial density gradients combined with narrow capillaries. In addition, a criterion for the determination of the appropriate experimental run time was established.

Nuclear magnetic resonance imaging of miscible fingering in porous media

Nuclear Magnetic Resonance Imaging (MRI) can noninvasively map the spatial distribution of Nuclear Magnetic Resonance (NMR)-sensitive nuclei. This can be utilized to investigate the transport of fluids (and solute molecules) in three-dimensional model systems. In this study, MRI was applied to the buoyancy-driven transport of aqueous solutions, across an unstable interface in a three-dimensional box model in the limit of a small Peclet number (Pe<0.4). It is demonstrated that MRI is capable of distinguishing between convective transport (‘fingering’) and molecular diffusion and is able to quantify these processes. The results indicate that for homogeneous porous media, the total fluid volume displaced through the interface and the amplitude of the fastest growing finger are linearly correlated with time. These linear relations yielded mean and maximal displacement velocities which are related by a constant dimensionless value (2.4±0.1). The mean displacement velocity (U) allows us to calculate the media permeability which was consistent between experiments (1.4±0.1×10−7cm2).U is linearly correlated with the initial density gradient, as predicted by theory. An extrapolation of the density gradient to zero velocity enables an approximate determination of the critical density gradient for the onset of instability in our system (0.9±0.3×10−3 g/cm3), a value consistent with the value predicted by a calculation based upon the modified Rayleigh number. These results suggest that MRI can be used to study complex fluid patterns in three-dimensional box models, offering a greater flexibility for the simulation of natural conditions than conventional experimental modelling methods.

Slippery Bottom Boundary Layers on a Slope

Abstract The turbulent bottom boundary layer for rotating, stratified flow along a slope is explored through theory and numerical simulation. The model flow begins with a uniform current along constant-depth contours and with flat isopycnals intersecting the slope. The boundary layer is then allowed to evolve in time and in distance from the boundary. Ekman transport up or down the slope advects the initial density gradient, eventually giving rise to substantial buoyancy forces. The rearranged density structure opposes the cross-slope flow, causing the transport to decay exponentially from its initial value (given by Ekman theory) to near zero, over a time scale proportional to f/(Nα)2, where f is the Coriolis frequency, N is the buoyancy frequency, and α is the slope angle. The boundary stress slowing the along-slope flow decreases simultaneously, leading to a very “slippery” bottom boundary compared with that predicted by Ekman theory.

Functional and metabolic heterogeneity among normal neutrophil subpopulations

In this study, normal human peripheral blood neutrophils were separated into subpopulations by centrifugation on discontinuous Percoll density gradients and further characterized according to various functional and metabolic capabilities. Chemotaxis was measured in response to optimal concentrations (20 nM) of the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). According to a population ratio analysis, the majority of cells that contain the highest proportion of fast migrating cells are isolated between the densities of 1.093 and 1.096 gm/ml. Arachidonic acid (AA) metabolism was also determined in neutrophil subpopulations. On initial density gradients, cells isolated from 1.084-1.087 gm/ml and incubated in the presence of exogenous [3H]AA produce 50% of the total [3H]LTB4 produced in response to ionophore A23187 stimulation; whereas after prelabeling with [3H]AA for 2 hr at 37 degrees C, the cells between 1.087 and 1.093 gm/ml are the most active at both releasing [3H]AA from the cellular phospholipids and synthesizing [3H]LTB4 (dpm/cell). These studies demonstrate the significant functional and metabolic heterogeneity that exists among normal neutrophil subpopulations.

Early‐time plasma expansion characteristics of ionized clouds in the ionosphere

The collisionless expansion of high‐density Ba+, Li+, and Ba+‐Li+ plasma clouds into low‐density O+ background plasmas was studied using a one‐dimensional Vlasov‐Poisson model in order to elucidate the very early‐time plasma expansion characteristics. Simulations were conducted for a range of Ba+‐Li+ mixtures, cloud/background density ratios, and electron/ion temperature ratios. Simulations were also conducted for cloud expansions against both stationary and flowing O+ background plasmas to simulate gas releases from moving spacecraft. The following results were obtained from the numerical simulations: (1) The front of an expanding high‐density Ba+ cloud acts as an “electrostatic snowplow,” and both O+ density and temperature peaks are pushed ahead of the expanding Ba+ cloud. (2) The strength of the electrostatic snowplow is increased for elevated cloud electron temperatures. (3) The effect of a flowing O+ background plasma is too slow the Ba+ expansion and change the O+ response. For small O+ drift velocities the Ba+ snowplow still occurs, for moderate O+ drift velocities ion density peaks propagate into and away from the cloud, and for large O+ drift velocities the O+ plasma quickly penetrates the Ba+ cloud and there are small density perturbations. (4) The Li+ cloud expansion is faster than the Ba+ expansion by approximately the square root of the heavy‐to‐light ion mass ratio, and the Li+ electrostatic snowplow is weaker. (5) As with a Ba+ cloud, an expanding Li+ cloud pushes an O+ density enhancement ahead of it, but some of the light Li+ ions can penetrate this O+ enhancement with the result that an Li+ plateau forms and propagates ahead of the propagating O+ enhancement. (6) For Li+ plasma expansions against a rapidly drifting O+ plasma, the two plasmas quickly penetrate each other with minor density perturbations. (7) For Ba+‐Li+ cloud expansions, with Li+ minor, into an O+ plasma, the Ba+‐O+ interactions are not affected by the presence of the light minor ion. This expansion scenario is led by suprathermal forerunner Li+ ions, then a propagating Li+ density plateau, then a propagating O+ density peak, and finally the main front of the expanding Ba+ cloud. A linear instability analysis including a constant magnetic field indicates that some of the expansion scenarios with elevated electron temperatures are unstable. When the plasma is unstable, the ion‐ion acoustic wave is the most unstable mode. Depending on the conditions, waves can be excited in the expanding plasma cloud by penetrating O+ ions and in the background O+ plasma by penetrating cloud ions. The numerical simulations are limited in that the simulation domain is relatively small, the initial plasma density gradients are fairly steep, the expansions are one‐dimensional, collisions are ignored, and only very early times are simulated.

Frontogenesis in Mixed Layers

Abstract A unified theory of frontogenesis in mixed layers is proposed. Analytical solutions of the nonlinear mass and density balances in a mixed layer are found in the Lagrangian frame for a wide class of entrainment parameterizations. These solutions show mixed-layer depth and density to be functions of particle position and separation, where mixing is represented by an integral over time following a particle. Thus, given a knowledge of only the crossfront velocity field, the likelihood of frontal formation can be predicted. Both surface convergence, and divergence in conjunction with vertical mixing can be frontogenetic. Three different types of fronts, distinguished by their associated crossfront velocity fields, are discussed. Large-scale fronts, such as the subtropical front, are described by a convergent surface flow acting upon an initial horizontal density gradient. The powerful divergence caused by an offshore Ekman flow forced to zero at a coast causes upwelling fronts. The divergence associat...

Purification of very high density lipoproteins by differential density gradient ultracentrifugation

Differential density gradient ultracentrifugation procedures, utilizing a vertical rotor, were developed for the preparative purification of very high density lipoproteins (VHDL, density > 1.21 g/ml). The VHDLs of several insect species were purified as follows. An initial density gradient ultracentrifugation step removed lipoproteins of lower density from the VHDL-fraction, which partially separated from the nonlipoproteins present in the infranatant. A complete separation was achieved by a second centrifugation step employing a modified gradient system. The use of a vertical rotor and specially designed discontinuous gradients allows a relatively fast, efficient, and economical isolation of the class of very high density lipoproteins. Similar gradient systems should be useful for the detection and purification of VHDLs from other sources.

STUDIES ON THE STRUCTURE AND DYNAMICS OF SEA-BREEZE FRONTS

Characteristics of sea-breeze fronts are investigated by field research and laboratory experiments. Field research was conducted at the Kasumigaura Lake where a sea-breeze front shows its plane figure through water surface ripples. Experiments were carried out using water tank with heating device below the inland part. Both front structures observed at the field and the laboratory water tank are closely similar to each other. A consideration is given to the celerity of sea-breeze front as a function of the initial density gradient and the density flux from the bottom.

海風フロントの構造の伝ぱ特性について

Characteristics of sea-breeze fronts are investigated by field research and laboratory experiments. Field research was conducted at the Kasumigaura Lake where a sea-breeze front shows its plane figure through water surface ripples. Experiments were carried out using water tank with heating device below the inland part. Both front structures observed at the field and the laboratory water tank are closely similar to each other. A consideration is given to the celerity of sea-breeze front as a function of the initial density gradient and the density flux from the bottom.

Nonlinear theory of the E × B instability with an inhomogeneous electric field

Using analytical and numerical simulation techniques, the nonlinear evolution of the E × B instability with an inhomogeneous electric field has been studied. For the case where the electric field component parallel to the density gradient is inhomogeneous, we find that the inhomogeneous E × B instability in the nonlinear regime (1) evolves into large‐scale anisotropic fingerlike structures, (2) can be described by power laws P(kx) ∝ kx−nx, nx ≃ 2, P(ky) ∝ ky−ny, ny ≃ 2–3, where P(kx) and P(ky) are the one‐dimensional power spectra parallel and perpendicular to the initial density gradient, and (3) can be stabilized by quasi‐linear mechanisms in which the initial density gradient is modified by a finite‐amplitude wave spectrum. Application is made to naturally occurring plasma density fluctuations in the auroral ionosphere, but the results are applicable to plasma jet and striation formation in artificially produced ionospheric plasma clouds.

Geostrophic Adjustment: A Mechanism for Frontogenesis

Abstract A simple two-dimensional model is used to determine the geostrophically adjusted state of an initially motionless fluid with lateral density gradient. It is an extension of the much studied two-layer models to the case when the initial stratification is continuous, and allows a more detailed examination of the density and current fields. In addition, the model has demonstrated that geostrophic adjustment with its accompanying transverse circulation can produce regions of convergence and thus steepen the initial density gradient. In cases when the initial density transition is sharp enough, fronts, or density discontinuities, are formed at these convergence regions, along which the surface fluid particles drawn in from both sides of the fronts move into the interior. The density jump across the fronts is the greatest at the top and bottom surface and diminishes toward the interior where differential vertical motion during the adjustment can significantly level the isopycnals. One observed frontal ...

Ion transport theory for a slightly ionized rarefied gas in a strong electric field. II.

Solutions to the initial-value problem of a weakly ionized rarefied gas in a uniform electrostatic field are found explicitly for the lowest three hydrodynamic levels indicated by a moment method previously described. The resulting sets of equations are solved by a completely implicit Crank-Nicholson scheme. For very light ions, or moderately inelastic electrons, a Monte Carlo solution is also found and compared to the moment solutions delineating their range of validity. The deviation of the higher-order theory from the diffusion theory is mapped on a parametric plane bounding the region of validity of the diffusion equation as derivable from the Boltzmann equation. The Hornbeck experiment is analyzed neglecting a cathode boundary, thus illustrating the effects of large initial density gradients on the transient current. (AIP)

Myelination in rat brain: method of myelin isolation.

Abstract— A procedure is described for the preparation from rat brain of myelin having the same degree of purity at all ages. Such a procedure is essential for the study of myelin composition during development. Microsomal contamination was successfully eliminated by adjusting the method to maintain a constant amount of brain per unit volume in the initial density gradient step, and by including two osmotic shocks and two low‐speed centrifugation steps. Myelin prepared in this way from animals ranging from 15 days to 14months of age had a total ATPase activity of 0.3‐2.0 μmol of Pi.h−1.mg−1 dry wt of myelin, representing 0.1‐1.2 per cent recovery of the total homogenate activity; a Na+, K+‐ ATPase activity of 0.1‐1.6 μfnol of Pi.h−1.mg−1 dry wt, representing 0.04‐1.5 per cent recovery; a nucleic acid content of 0.2‐0.7 per cent of myelin dry wt, representing 0.2‐2.0 per cent recovery; and a ganglioside NANA content of 0.04‐0.07 per cent of myelin dry wt. representing 0.2‐4.6 per cent recovery. The myelin prepared from 20‐day animals had the highest content of the first three constituents; otherwise the values of the four constituents were relatively constant per unit weight of myelin. The amounts of nucleic acid and ganglioside recovered in the myelin fractions increased with increasing age and myelin yield.

The free convection of a fluid with an initial vertical density gradient and lateral heating

Some results are given of an experimental investigation into the free convection in solutions with density which is nonuniform with height. It is shown that convective motion in solutions with a vertical density gradient and lateral heating is cellular in nature. The height of the cells is determined as a function of the temperature head and the density gradient.

Disappearance of polyribosomes during in vivo erythroid maturation and nuclease activity.

DEBELLIS and I have compared the disappearance of ribosomes and polyribosomes during the maturation in vivo of erythroid cells in the rabbit1. This involved injecting twice labelled cells, obtained from a donor animal, into a recipient animal in which the bone marrow was repressed by continuing injections of colchi-cine. The events which occurred, including disappearance of total erythroid cell RNA, of reticulocytes, and of ribosomes and polyribosomes, were the result of maturation of the donor cells in the recipient circulation, for evidence was presented to show that the donor cells were not removed from the circulation during the time of the study. In contrast to previous work2–4 there was no disproportionate loss of polyribosomes during erythroid cell maturation. Rowley5 criticized these studies because the initial and subsequent sucrose density gradient centrifugation patterns of ribosomes (Fig. 1, upper) resembled those obtained by other workers6,7 with polyribosomes treated with small amounts of ribonuclease. He concluded that “the similarity in (sucrose density gradient) patterns (as a) function of age simply represents degradation to a uniform state”. No experimental evidence is offered to support this conclusion.