Uniform Density Profile Research Articles

Boundary mixing in a stratified fluid

A vertically oscillating grid is used to simulate boundary mixing in the laboratory. The oscillation of the grid creates a turbulent mixing region in its vicinity, and mixing within this region creates a step-like structure in an initial density distribution which varies linearly with depth. If the initial density varies only at the boundary between two homogeneous layers, the same grid turbulence generates additional steps above and below the initial one. The steps, in turn, drive multiple intrusions of mixed fluid away from the boundary and into the non-turbulent interior of the fluid. A compensat- ing return flow carries fluid from the interior into the turbulent mixing region. From the data, the inference is made that the intrusions make a negligible direct contribution to the vertical mass transport. An analytical model of the intrusions, which employs only molecular values of the transport coefficients and also demonstrates negligible vertical mass transport, is consissent with the laboratory observations.Nevertheless, the data indicate that the fluid eventually reaches a homogeneous density by means of the gradual change of the gradient at a rate which is essentially the same both near the grid and far from it. For an initially uniform density profile this change occurs at all heights simultaneously, and for an initial density step it occurs preferentially near the step. Thus in both cases the interior flow must include slow vertical advection away from the horizontal centre plane. These advective currents can be made part of a consistent dynamic model, the buoyant equivalent of the spin- down in a rotating flow, provided that the net effect of the grid mixing includes a decrease in the local slope of the density gradient. This mode of adjustment explains satisfactorily the experimentally observed negligible horizontal density gradients. For the cme of an initially uniform density stratification, the shape of the evolving density gradient is not accounted for. In particular, it is not clear why the gradient changes at mid-depth almost simultaneously with variations at top and bottom boundaries.The vertical mass flux is found to be independent of container length, and it increases with grid frequency of oscillation, amplitude of oscillation and with the mean density gradient.

Effect of a magnetic filter on hydrogen ion species in a multicusp ion source

Hydrogen ion species and discharge characteristics have been compared for two different magnet geometries in a multicusp ion source. One magnet configuration indicated that the H2+ ion percentage in the extracted beam could be reduced by eliminating the high energy ionizing electrons near the ion extraction region. To accomplish this and maintain the desired features of both magnet geometries, a magnetic filter was installed near the source exit. With this combined arrangement, we found that the H+ ion percentage and discharge condition were much improved and a more uniform density profile across the extraction plane was achieved. The atomic species fraction can then be further enhanced by the addition of low-energy (∼16 eV) primary electrons into the source plasma chamber.

Optimization of Earth crust profiles by collocation

All gravimetric problems dealing with the inner gravity field of the Earth are ill-posed, ambiguous. Thus, we must complement the gravity data with other information, particularly with the results of deep seismic soundings and with the notions of the g-'.ologists of the structure of the Earth's crust if we want to solve our problems. Our task, however, is not solved for the actual Earth, but for its model. The most important information for us is the distribution of densities within the Earth's body. Thz construction of an uniform normal vertical density profile is of no use, since at least in the upper layers of the Earth's body the density is not a function of the d¢-pth only, but depends on the geological structure of the crust and varies from point to point. According to our opinion it would be better to use a different procedure. We have at our disposal the characteristics of the outer Earth's gravity field, and we are able to estimate its accuracy. We know the depths of the interfaces discovered by deep seismic soundings and, finally, there are some geological hypotheses about the structure of the Earth's crust in the place under consideration. Our task is now to process these three quite different sets of information using a suitable mathematical apparatus. We consider the new methods of mathematical statistics to be the most expectant for this purpose. In preparing for the extensive task mentioned above, we started with a quite simple case. We tried to derive a mathematical aid for optimizing the Earth's crust profiles, designed by seismologists and geologists. The collocation [1] was used to solve the problem. This method includes adjustment, filtering and data prediction.

Effect of Total Load on Subsurface Soil Compaction

ABSTRACT TWO different size tires carrying unequal total loads that resulted in approximately equal soil surface pressures were run over soil pressure transducers buried at 18-, 30-, and 50-cm depths. The larger tire consistent-ly produced higher soil pressures at these depths. Froehlich's equation approximately described the soil pressure distribution in both a sandy soil and a clay soil with uniform density profiles. However, when a com-pacted layer was inserted in the density profile, this equation was no longer suitable. The data indicated no simple relationship between soil pressures and bulk densities.

Effects of a multipole magnetic field on characteristics of a multifilament plasma source

A multipole magnetic field is applied in a multifilament plasma source. By changing the strength of the magnetic field, the instability of the discharge, the change of the density profile, and the increment of the ion density are investigated. It is shown that the stable plasma with a rather high density on the order of 1012 cm−3 and with a uniform density profile within ±5% over the 9-cm diameter is obtained in the lower discharge current compared with the discharge without the magnetic field, and the total heater current of the filament can be reduced by about 10%. The ratio (0.93) of the discharge current to the heater current without a magnetic field is improved to 1.8 with the field.

A mathematical model of wood pyrolysis

This is a theoretical study of the pyrolysis of a wood slab, one side of which is heated and the other side insulated and impervious. The physical processes contained in the model include: (1) transient conduction, (2) internal heat convection of volatiles, (3) Arrhenius decomposition of active material into volatiles and residual char, (4) endothermicity of the decomposition process, and finally (5) variable density, specific heat, and thermal conductivity. The problem formulation leads to a system of coupled-nonlinear-parabolic partial differential equations which have been solved iteratively by the Crank-Nicolson method. Results are reported for three slab thicknesses-1 cm, 0.2 cm, and 0.02 cm. The larger and intermediate thicknesses produce a pyrolysis wave which progressively penetrates into the solid. This wave is strongly influenced by both the outward convection of volatiles and the char conductivity. On the other hand, the thinnest slab produces rather uniform density and temperature profiles which are insensitive to the convection of volatiles and char conductivity. Finally the decomposition endothermicity has a pronounced effect on the total rate of mass loss. At present, the results have not been compared with experiment; however the precision and generality of the solution method should prove useful for testing various hypotheses in comparison with experiment.