Transverse Density Gradients Research Articles

Stability properties of cold mantle systems for current-driven modes

The stability problem of the boundary regions of cold blanket systems with induced currents parallel to the lines of force is formulated. Particular interest is focused on two types of mode: first electrostatic modes driven by the combined effects of a transverse resistivity gradient due to a spatially non-uniform electron temperature and a longitudinal current, second electromagnetic kink-like modes driven by the torque arising from a transverse current density gradient and magnetic field perturbations. It is found that the combination of various dissipative and neutral gas effects introduces strong stabilizing effects within specific parameter ranges. For particular steady-state models investigated it is shown that these effects become of importance in laboratory plasmas at relatively high densities, low temperatures and moderate magnetic field strengths. Stability diagrams based on specific steady-state cold plasma blanket models will be presented.

Plasma wave and second harmonic generation

Presents an investigation of the generation of a plasma wave at pump wave frequency and second harmonic generation caused by a self induced transverse inhomogeneity introduced by a Gaussian electromagnetic beam in a hot collisionless plasma. In the presence of a Gaussian beam the carriers get redistributed from the high field region to the low field region by ponderomotive force and a transverse density gradient is established in the plasma. When the electric vector of the main beam is parallel to this density gradient, a plasma wave at the pump wave frequency is generated. In addition to this the transverse intensity gradient of the electromagnetic wave also contributes significantly to the plasma wave generation. The power of the plasma wave exhibits a maximum and minimum with the power of the pump wave (at z=0). The generated plasma wave interacts with the electromagnetic wave and leads to the generation of a second harmonic.

Transverse Mixing in Partially Stratified Flow

The paper reports the results of a laboratory study on the rate of transverse mixing in oscillatory estuary-type flow with density gradients typical of those found in real estuaries. Two sets of experiments were performed: one in a uniform channel with a trapezoidal cross section, and one in a channel with an irregular cross section. In the uniform channel density stratification reduced the rate of transverse mixing. A plot of the dimensionless transverse mixing coefficient versus Richardson Number is given. In the irregular channel, the density stratification had an opposite effect: the irregular side wall generated vertical mixing which induced transverse density gradients and transverse flows. It is concluded that real estuaries are probably similar to the irregular channel and that density stratification in real estuaries may hasten rather than impede transverse mixing.

Surface reaction with combined forced and free convection

AbstractThe influence of combined forced and free convection on the surface reaction of a laminarly flowing species in a rectangular channel is studied. The free convective motion, which is superimposed upon the main axial flow, arises from a transverse density gradient produced by the release of a reaction product, or heat, at the channel wall. A numerical stream function‐vorticity method is employed to solve the three‐dimensional conservation equations in the case of large Schmidt number. Effects of aspect ratio, diffusivity ratio, surface reaction rate, and Rayleigh number upon the overall reactant conversion rate and local Sherwood number are examined. Reasonable agreement is obtained with experimental data for a hydrochloric acid‐calcium carbonate surface reaction.

The effect of a transverse density gradient on the interference fringe shape in the hook method

Application of the hook method to the study of inhomogenous objects with large density gradients perpendicular to the axis of observation is considered. The reasons causing hook distortion are explained by a graphical representation. An expression is given to determine the density gradient from the characteristic points in the interference fringes.

The effect of a density difference on shear-layer instability

Measurements of mass flow rate and mean density have been made in separated laminar boundary layers with large transverse density gradients. Two-dimensional shear layers were formed by exhausting a half-jet of one gas into a reservoir of another gas with a different molecular weight. Two freons with a density ratio of 1-98 and unusual properties which permitted the measurement of the mass flow rate with a single hot wire were used. A n analysis of the mass flow rate fluctuations showed that a negative density gradient (i.e. light gas flowing into heavy) increases the amplification rate of the instability oscillations and reduces the frequency and wave number. Opposite trends were observed when the density gradient was positive. These findings are in agreement with recent theoretical predictions.

Space-Charge Effects by Nonthermal Ions in a Magnetized Plasma

In combination with transverse density gradients the finite Larmor radius produces space-charge effects both in the interior and at the boundaries of a magnetized plasma. In a thermal plasma having moderately large density gradients, electric quasi-neutrality can be preserved by a smoothly varying electric potential of limited amplitude. It is not obvious that a plasma containing groups of nonthermal ions will behave in a similar way when there are steep density gradients. An example is given here where a strongly oscillating potential of sheath-like structure arises from nonthermal ions which are peaked at a singular velocity. Whenever nonthermal effects of this or a similar kind occur, they should introduce fundamental changes in the plasma behavior which cannot be described by elementary plasma theory. Possible applications are given by the motion of a plasma through a neutral gas, where groups of nonthermal ions arise from charge exchange and ionizing collisions. The electric fields generated by the nonthermal particles may affect the ionization process and become connected with the mechanisms of ionizing waves and with the observed voltage limitation in rotating plasmas and in certain types of plasma guns.

Generalized Formulation of Electrostatic Oscillations and Streaming Instabilities in Bounded and Unbounded Inhomogeneous Vlasov Plasmas

A general formulation for the study of electrostatic oscillations and streaming instabilities in bounded and unbounded Vlasov plasmas with transverse density gradients in a strong axial magnetic field is presented. It is found that the behavior of the plasma depends on a parameter which is a function of geometry, scale of inhomogeneity, and wavelength. Using this formulation, several experimentally realistic configurations have been studied. Two results obtained are of special interest. They are (1) for waves with the same wavelength, a density inhomogeneity and/or boundary increase Landau damping. (2) Under appropriate conditions, depending on the geometry and the degree of inhomogeneity, it is not possible to excite the two-stream instability for any drift velocity. Previous theories of uniform plasmas appear as special cases in this general treatment.

An effect of a transverse temperature gradient on turbulent pipe flow

IT IS USUAL, when investigating the distribution of velocity and its relation to shear stress in non-isothermal turbulent flow through pipes or channels, either to neglect the influence of a temperature gradient or to consider only its effect on the regions near the heated boundaries. This is reasonable since the highest temperature gradients normally occur here; moreover fluid viscosity, which is usually strongly temperature dependent, plays a more important role in these regions than in the turbulent core. In recent years nuclear engineers have had to deal with turbulent flow and heat transfer in ducts of various shapes, including annulii and channels containing bundles of rods. In such multiply-connected shapes, containing perhaps elements having roughened or extended surfaces, it is important to be able to isolate the contribution of an individual element to the overall heat transfer and pressure drop. A way of doing so involves locating the position of the “surface of zero shear” which surrounds the element in question [I]. In dealing with simple shapes it is assumed that zero shear occurs at the section where the mean velocity is a maximum, on the basis that the shear stress is proportional to cm dU/dy where E~ is the usual “eddy viscosity”, #is the mean velocity and y is an appropriate co-ordinate normal to 0. With real fluids, however, and especially with gases, the existence of a transverse temperature gradient implies a transverse density gradient; the validity of the technique therefore needs investigation. If a Reynolds-type substitution is made in the NavierStokes equations for steady compressible flow of a fluid having a constant viscosity, and time-averages are taken, one obtains:

Noise in Single-Crystal Tellurium from Thermal or Optical Excitation

The electrical noise of carefully prepared single-crystal tellurium samples was measured in the extrinsic temperature range. The effect of temperature variations on the noise power spectrum is interpreted quantitatively as the result of transitions between the valence band and four species of traps lying ∼0.045 eV above it. The additional noise due to steady optical excitation at 77°K is attributed to transverse carrier density gradients which result from nonuniform excitation.

Galactic and extragalactic studies at the Harvard Observatory

1. The facilities of the Harvard Observatory at its northern and southern stations have permitted a varied attack on problems of the distribution, dimensions, and structure of galaxies. With the publication of the twenty-second paper in a series devoted to the researches in this field, [1], carried out with the aid of several assistants during many years, the major parts of the programmes have been completed. The present report outlines remaining projects that are currently on the programme, as listed below, and gives some preliminary results. 2. 1. The Anti-centre Octant. 3. 2. The Stellar Corona. 4. 3. The Centaurus Window 5. 4. The Inner Metagalaxy. 6. 5. Transverse Density Gradients. 7. 6. The Clouds of Magellan. 8. 7. The Fourth Quadrant Variables. 9. The extensive investigations of the Milky Way structure by Dr. B. J. Bok and his associates are not reported in this article.