Effect Of Density Gradient Research Articles

An Infrared Study of the Velocity Structure of the Cometary Compact H II Region G29.96-0.02

We have mapped the velocity structure of the cometary compact H II region G29.96-0.02 using long-slit echelle spectra of the Brγ line. This technique detects line emission over a much wider area at the necessary spatial resolution compared to radio recombination line observations. Significant structure in both the velocity centroids and the line widths is seen over the entire nebula. Large line widths are seen ahead of the bow and in the tail, which may be due to turbulent motions in shocked and interface regions, respectively. We construct analytic models of the density and velocity structure in order to attempt to distinguish between the bow shock and champagne flow models that have been put forward to explain the cometary morphology of many compact H II regions. The bow shock model is unable to explain the large velocity gradient that we see right across the tail of the cometary region, which can be explained only by the streaming motions toward low-density regions in the champagne model. However, our approximation to the champagne model is also not able to fit all of the features of the data. More realistic versions of this model that include the effects of stellar winds and density gradients may be able to provide a better match to these data.

A model of the effects of horizontal density gradients on mixing processes in the oceanic surface layer

A diurnal Eulerian model of the upper mixed layer incorporating the effects of horizontal density gradients is developed based on that of Janowitz and Kamykowski [1991] and is utilized to study the effects of wind‐driven cross‐isopycnal transport on mixing processes in the layer. Four runs of the model are conducted for a constant horizontal density gradient of 2.5 × 10−5 kg m−4 with wind directions at 0°, 90°, 180°, and 270° to this gradient. Two runs are conducted with half this density gradient and wind directions 90° to the left and right of the gradient and one run with no density gradient. In all cases the model is first run with a constant heat loss rate of 150 W m−2 through the sea surface for 11 hours to simulate nighttime cooling and to provide initial conditions for daytime runs. The model is then run for 24 hours with a constant wind speed of 7 m s−1 and a maximum solar radiation of 900 W m−2 during the first 13 hours of the 24‐hour simulation. The results show that the shallowest mixed layer occurs when the wind direction is 90° to the left of the density gradient and the deepest layer occurs with the wind direction 90° to the right of the gradient. After 24 hours, the ratio of the deepest to the shallowest mixed layer depth is 2.35 for the stronger gradient and 1.83 for the weaker gradient. These results are explained by the effect of vertical shear of the wind‐driven advection of horizontal density variations, which changes the vertical density gradient and can destabilize or stabilize the mixed layer. The results indicate that wind‐driven, cross‐isopycnal transport can have significant effects on mixing processes in the layer. Dimensionless parameters are developed to estimate the relative importance of cross‐isopycnal, wind‐driven transport on mixed layer properties.

Effect of density gradient and porosity on HMX/TNT pressure cast shaped charge performance

AbstractPrecision shaped charges are prepared adopting pressure casting technique. Porosity and voids in such charges which may not be detected by X‐ray radiography would substantially effect the shaped charge performance. The effect of porosity on the parameters like density, VOD, CJ pressure and jet tip velocity have been predicted. Density variation in the charges prepared under different experimental conditions has been determined and compared with that of the predicted values in this paper.

Effect of density gradients in confined supersonic shear layers. II. Three-dimensional modes

The effect of density gradients on three-dimensional (3-D) supersonic acoustic modes and 3-D subsonic modes in a compressible confined shear layer were investigated using linear analysis. The compressible Rayleigh equation with the solution of the steady compressible boundary layer equations as its basic profiles was solved for two different density ratios, 1.398 and 3.0, and several spanwise wave numbers. For a density ratio of 1.398, the 2-D C01 mode had the highest growth rate for a fixed aspect ratio B/H=2. At the higher-density ratio, the 3-D mode had a slightly higher maximum growth rate than the two-dimensional (2-D) mode, and the maximum growth rate was at a location where the phase speed was supersonic with respect to the fast stream.

Effect of density gradients in confined supersonic shear layers. I. Two-dimensional disturbances

The effect of density gradients on two-dimensional (2-D) supersonic wall modes (acoustic modes) of a 2-D confined compressible shear layer are investigated using linear analysis. Due to the inadequacies of the hyperbolic tangent velocity profile, the boundary layer basic flow profiles are used. First, a test case is taken with the same parameters as in a previous analysis by Tam and Hu [J. Fluid Mech. 203, 51 (1989)], who used a hyperbolic tangent profile. For the boundary layer profiles, three generalized inflection points are found giving rise to three modes. The first two show similar properties to those found by Tam and Hu, whereas the third is a new mode that can have a higher growth rate than the others. As the density ratio is increased above that of the test case, the smallest of the three neutral phase speeds tends toward the speed of the lower-velocity stream, and the other two eventually coalesce and then disappear. These two effects lead to a linear resonance between some of the modes that increases the cutoff frequency and growth rate of the lowest mode. In fact, growth rates of two to four times the test case were found as the density ratio was increased to 7. A similar trend (linear resonance) is observed as the density ratio is decreased from the test case, but the growth rate is only slightly changed from the test case.

Long Packed Column Supercritical Fluid Chromatography: Influence of Pressure Drop on Apparent Efficiency

Supercritical fluid chromatography, when performed on a packed column, is a powerful and fast separation technique. To enhance the number of theoretical plates (TP) available, long packed columns (>1 m) have been used successfully, despite controversy over the effect on column efficiency of the density gradient induced by the pressure drop. Peak broadening and deformation were reported, and packings with larger particle diameter than those used in liquid chromatography (10 μm instead of 3−5 μm) were advised in order to reduce the column pressure drop. Velocity gradient induced by the density gradient was reported to reduce efficiency. This paper presents the results of investigations on the influence of density gradient on the apparent efficiency obtained on a series of four 25-cm × 4.6-mm-i.d. Nucleosil C18 columns connected in series (particle size, 5 μm). Apparent column efficiency is found to vary from less than 10 000 TP to more than 100 000 TP versus the density and the density gradient. The higher the density gradient, the higher the efficiency loss. A model is presented which accounts for the effect of linear velocity and density gradients on peak broadening. It confirms that it is the linear velocity variation rather than the variation of the density which causes band broadening and allows prediction of conditions for which apparent efficiency loss occurs. To reduce the density gradient induced by column pressure drop, one can compensate for pressure gradient by a superimposed temperature gradient (multitemperature control of the mobile phase via three column ovens). It allows one to obtain the highest efficiency and to use CO2 at lower density without any loss of efficiency. When methanol is added to the CO2, no pressure drop compensation is required in order to obtain the highest apparent efficiency. As density gradient compensation via multitemperature control of the mobile phase provides higher apparent efficiency and, consequently, higher resolution than in isothermal operation, it is especially useful for separation of complex oil samples.

Experimental and mathematical studies of natural convection for solid‐salt dissolution in water

AbstractThe phenomena of natural convection caused by a concentration gradient usually creates currents as a result of buoyancy forces. These forces are formed from the combined effect of a fluid density gradient and the body force (gravitational force). The above phenomenon was studied experimentally in a circular glass column of 0.08 m diameter and 1 m length. This column was erected vertically and filled with distilled water with a salt bag placed on top of it and partially immersed in water. The dissolution mechanism was then modelled and a finite difference method was used to solve the formulated equations by using the implicit scheme of MacCormack (Anderson et al., 1984). The experimental results and the numerical predictions are represented graphically and give compatible agreement.

Stratification and wind mixing in the Southern Kattegat

Abstract Changes in salt stratification were investigated by using temperature and fluorescence measurements as tracers. By doing this, information on the short term history of the water column was revealed. Further the dominant processes in mixing and stratification of the upper metres of the water column in the southern Kattegat were identified. Due to the salt content, density increased with depth over most of the water column during the period of measurement., A 50 km intrusion of warm, phytoplankton-rich water from the Great Belt outlet was identified. The density increase was maintained due to the buoyancy input given by the combined effect of a horizontal density gradient and vertical shear. Hence the vertical temperature inhomogeneities lasted for 3–7 days and were not mixed even under an event of strong wing mixing. In this period the nutrient and salt transport are uncoupled, since most of the mixing energy is used to redistribute salt within the nutrient-poor surface layer. A reduction of the horizontal density gradient reduced the buoyancy input and allowed for the homogenization of the upper 15 metres within a few hours. One sharp interface was created. After the formation of the sharp interface salt and nutrient transport can be calculated by using the same entrainment function for both salt and nutrient transport. Only during approximately 5 % of the time there was a direct entrainment of bottom water and only 30% of the available mixing energy could be used to give this entrainment.

Validity of Laplace formula and dependence of surface tension on curvature in second gradient fluids

Experiments on very small bubbles showed that classical Gibbs-Laplace theory on surface tension cannot be applied when the dimension of the interface becomes comparable with bubble radi[1]. On the other hand the pressure jump across the interface is from a experimental point of view the most suitable quantity to be measured in order to know surface tension and determine its dependence on curvature. Therefore Laplace formula needs to be generalized to the case of very small bubbles. The theory of second gradient fluids (Germain cf.[2]) accounts the effects of high density gradients (occurring in fluid interface) on fluid pressure. In [3] it is shown that second gradient theory allows the determination of the simplest consistent generalization of Laplace formula and an equivalent bubble theory in which surface tension and radius of a bubble are determined in terms of the density spatial field. This paper complements the quoted results analyzing them numerically. We find that our model: i) predicts the existence of a (minimal) nucleation radius, i.e. a radius which is a minimum possible for the equilibrium of a small bubble; ii) permits, through the new Laplace formula, to evaluate the dependence of surface tension on curvature with results very similar to actual experiments; iii) allows the determination of the range of validity of classical Laplace formula and a theoretical prediction on the departure of experimental data from it.

Absorption of laser light in overdense plasmas by sheath inverse bremsstrahlung

The original sheath inverse bremsstrahlung model [P. J. Catto and R. M. More, Phys. Fluids 20, 704 (1977)] is modified by including the v×B term in the equation of motion, as the evanescent magnetic field in an overdense plasma is greater than the corresponding electric field. It is shown that the present results are significantly different from those derived without the v×B term. The v×B term is also important in interpreting the absorption mechanism. If the v×B term were neglected, the absorption of the light would be incorrectly interpreted as an increase in the transverse components of the canonical momentum, in the case of a normally incident laser light. It is also shown that both the sheath inverse bremsstrahlung and the anomalous skin effect are limiting cases of the same collisionless absorption mechanism. Results from particle-in-cell (PIC) plasma simulations are compared with the absorption coefficient calculated from the linear theory. Finally, the effects of finite density gradients are investigated by PIC simulations.

Effect of density-gradients on the binding of photosensitizing agents to plasma proteins

The binding of photosensitizing agents to low-density lipoprotein is considered an important factor in tumor localization. We examined the affinity of a group of photosensitizers, varying in charge and hydrophobicity, for LDL, under conditions designed to determine whether the high salt concentration involved in conventional KBr gradients affected the results. Density-gradients containing KBr vs D 2O were evaluated; the latter can delineate VLDL and LDL from other plasma components, while the KBr gradient readily resolved VLDL, LDL, HDL and albumin. Distribution of the photosensitizers to plasma fractions was assessed, along with the effect of Cremophor EL, an emulsifier used for formulation of water-insoluble drugs. Both the D 2O and KBr gradients provided similar results with regard to the affinity of anionic, neutral or cationic photosensitizers for LDL. The use of Cremophor EL for drug formulation was associated with an altered electrophoretic lipoprotein profile. In some cases, affinity of CRM-solubilized sensitizers for plasma components varied with the density-gradient employed. The high salt concentration used in density-gradient fractionation had little effect on the affinity of photosensitizing agents to low-density lipoprotein but may introduce artifacts when emulsifiers are used in drug formulation.

Plasma instabilities in the lower E-region observed during the DYANA campaign

During the DYANA campaign a number of instrumented sounding rockets were launched from Andøya Rocket Range (ARR) in Norway (69°N) to study the dynamics of the mesosphere and lower thermosphere during winter conditions. The EISCAT UHF radar in Tromsø, located 130 km to the east of ARR, was used to map the electric field and the wind field in the same height region. During some of the launches, plasma irregularities with scales in the range from a few metres to tens of metres were observed. We focus our attention on an event which took place on 8/9 March 1990. Two instrumented payloads code-named DAT-73 and DAT-76 were flown 92 min apart during different levels of disturbance in the E-region. We have studied the plasma fluctuations observed during these flights in terms of the modified two-stream and gradient-drift instabilities using linear fluid theory. The observed height extent of the disturbed region is in excellent agreement with theoretical predictions based upon measurements of background plasma parameters. We also find that the instability is much stronger during the first part of the event as a consequence of larger drift velocities. This is evident by the large plasma fluctuations measured in situ as well as the electron temperature measured by EISCAT. An adequate description of the observations must include effects of electron density gradients and recombinational damping. During the flight of DAT-73, the plasma waves must propagate in a direction different from 90° with respect to the magnetic field, while the DAT-76 observations can be explained by plasma waves propagating normal to the magnetic field.

Rapid flow of granular materials with density and fluctuation energy gradients

A constitutive relationship for the stress tensor of granular materials during the rapid flow regime that includes the effects of density gradient and fluctuating velocity is proposed. Explicit expressions for various stress components are obtained and simple examples are presented. It is shown that this continuum-based model is capable of predicting normal stress differences. The model also contains many earlier models as its special cases. The results also show that the model may be suitable for analyzing rapid, as well as slow, motions of granular materials.

Propagation characteristics of the ELF emissions observed by the satellite Akebono in the magnetic equatorial region

Emissions with their frequencies below 100 Hz are often observed by the Akebono satellite in the vicinity of the geomagnetic equatorial plane. These ELF emissions are classified into two types: One is an ion cyclotron wave below the local proton cyclotron frequency and the other is assumed to be magnetosonic wave observed not only below but also above the local proton cyclotron frequency. The wave normal directions of the latter type of emissions were estimated by using the wave distribution function method. It is found that the emissions are propagating with their wave normal direction nearly perpendicular to the meridian plane. The propagation characteristics of these emissions are also examined by ray tracing including the effects of ions. The ray tracing study clarified that the wave can propagate around the plasmapause because of the trapping effect of density gradient. In this paper we propose that these emissions are generated just outside the plasmapause and are propagating around the plasmapause with their wave normal nearly perpendicular to the geomagnetic field line and to the meridian plane.

Particle dispersion in a binary solid‐liquid fluidized bed

AbstractThe mixing and segregation behavior of liquid fluidized beds containing two different‐sized particle species are described mathematically. This model is based on a mass balance, rather than a volume balance, which has been used by most previous researchers. The model also includes the effects of density gradients that are present in the bed. Particle dispersion coefficients in a binary‐solid fluidized bed are evaluated using this model. Further, a correlation that allows the calculation of these particle dispersion coefficients is presented, which describes not only dispersion coefficients calculated using the above model, but also those presented by other researchers. It is applicable over a broad range of variables of practical interest and is based on relevant dimensionless parameters to account for the influence of important particle and fluid properties on the particle dispersion characteristics.

Drift wave vortices in nonuniform plasmas with sheared magnetic fields

Nonlinear coherent structures governed by the coupled drift wave–ion-acoustic mode equations in nonuniform plasmas with sheared magnetic fields are studied analytically and numerically. A solitary vortex equation that includes the effects of density and temperature gradients and magnetic shear is derived and analyzed. The analytic and numerical studies show that for a plasma in a sheared magnetic field, even without the temperature and drift velocity gradients, solitary vortex solutions are possible; however, these solutions are not exponentially localized due to the presence of a nonstructurally stable perturbative tail that connects to the core of the vortex. The new coherent vortex structures are dipolelike in their symmetry, but are not the modons of Larichev and Reznik. In the presence of a small temperature or drift velocity gradient, the new shear-induced dipole cannot survive and will separate into monopoles, like the case of the modon in a sheared drift velocity as studied in Su et al. [Phys. Fluids B 3, 921 (1991)]. The solitary solutions are found from the nonlinear eigenvalue problem for the effective potential in a quasi-one-dimensional approximation. The numerical simulations are performed in two dimensions with the coupled vorticity and parallel mass flow equations.

The horizontal density gradient in hydrodynamic models of salinity around river mouths

Research described in this paper looks at the effect of horizontal density gradient in the vicinity of a river mouth. The model results based on the measured prototype data in Apalachicola Bay, Florida, indicate that horizontal density in the vicinity of a river mouth. The horizontal density gradient terms have been added to a two-dimensional depth averaged model and a two-layer hydrodynamic and salinity model. The salinity results from model with and without the horizontal density gradient terms are presented and compared with prototype data. The statistical analysis of salinity variance at each gage station around the river mouth also demonstrates the effect of the horizontal salinity gradients.

Airflow through large openings

The subject of airflow through large openings includes a large number of different problems, ranging from steady gravitational flows to fluctuating flows due to wind turbulence, and including recirculating flows caused by boundary layer effects in a thermally driven cavity. After a review describing the various physical phenomena, and the existing solutions available in the literature to solve this problem, we present in this paper a general solution to predict airflow through large vertical openings. This solution is able to take into account effects of density gradients and turbulence on gravitational flows, and is compatible with a pressure network description for multizone modelling of airflow in buildings.

Effect of impurity density gradients on low-frequency electrostatic fluctuations

For a plasma with impurities, the density scale lengths for electrons and bulk ions become different. Here it is shown that for drift-wave-type instabilities, the electron density scale length arises in the equations of motion (to lowest order in ion gyroradius), not the density scale length of the bulk ions, as is commonly believed. Thus, when comparing experiments with theoretical predictions, it is appropriate to take the drift frequency as ω*eω*e= −(Ti/Te)ω*i = −(kθcTe/eB)d ln n0e/dr, and the parameter ηi≡d ln Ti/d ln n0e (that is, with ne replacing the more commonly used ni in these terms).

Wind Mixing In a Turbulent Surface Layer in the Presence of a Horizontal Density Gradient

The effect of a horizontal density gradient of buoyancy on the turbulent kinetic energy budget of the surface mixed layer in the ocean is discussed. The combination of a horizontal buoyancy gradient and a vertical shear of the horizontal velocity within the mixed layer will give rise to a “shear flow dispersion” effect within the layer. A vertical flux of buoyancy will be induced. This will act as a source or a sink of energy, depending on the relative direction between the horizontal buoyancy gradient and the surface velocity. An expression for the vertical flux of buoyancy due to this process is derived and discussed in relation to the shape of the velocity profile in the mixed layer. The contribution to the energy budget is calculated and specialized to the case when the transport in the layer is given by the Ekman solution. A length scale LA of the same significance as the Monin–Obukhov length is derived from the friction velocity u*. and the “shear flow buoyancy flux.” With the horizontal transport given by the Ekman solution it is shown that LA = u*f|∂b/∂x|−1, where f is the Coriolis parameter and b = gg(ρ0 − ρ)/ρ0 is the buoyancy. The shear flow buoyancy flux should be included in the turbulent kinetic energy budget for the mixed layer when the mixed layer depth is of magnitude LA. If the effect of the shear flow buoyancy flux dominates, the mixed layer depth is determined by LA.