Air Volume Fraction Research Articles

Water content dependence of trapped air in two soils

An improved air pycnometer method was used to examine the water content dependence of trapped‐air volumes in two repacked, nonswelling soils. Trapped‐air volumes were determined at a series of hydrostatic equilibrium stages which were attained during water pressure‐controlled wetting and drying cycles over a range of 0 to −10 kPa for a sand and 0 to −20 kPa for a loam. Small pressure perturbations, between 0.2 and 0.6 kPa, were used in the air pycnometer method. Volumes of trapped air obtained at each hydrostatic equilibrium stage were independent of perturbation level and remained relatively constant over the time required to make repeated determinations. In contrast with most of the results obtained in previous studies, which often showed irregular relations, in this study the volume fraction of trapped air was found to be a regular, monotonically increasing (though possibly hysteretic) function of water content. For the soils studied, the function definitely exceeded zero only at water contents greater than 70% of saturation. However, during the initial drying from complete water saturation, the volume fraction of trapped air was virtually zero. Air trapping influenced the water retention curves significantly only at water contents higher than about 60% of saturation. Except at zero water pressure, however, not all of the differences between the initial and the other drying retention curves were accounted for by observed differences in trapped‐air volumes. Air trapping was not required for the onset of hysteresis in the water retention relation for the cases studied, i.e., when drying‐to‐wetting reversals were imposed at about 27% and 40% of saturation for the sand and loam soils, respectively.

Sea-ice characterization measurements needed for testing of microwave remote sensing models

The nature and accuracy of ice-characterization measurements needed to test two microwave backscattering models are clarified by examining the sensitivities of these models to variations in the geophysical parameters they require as input. First, the Bragg, or small perturbation, model for rough surface scattering, which appears appropriate for backscattering from new ice types at L-band, is considered. The sensitivities of this model to variations in the dielectric constant of the ice and to the power spectrum of surface roughness are examined. The dense-medium radiation-transfer model at X-band is considered for backscattering from air bubbles embedded in multilayer ice. The sensitivities of this model to air-bubble size, air-volume fraction, and dielectric loss in the ice are examined. Based on these sensitivities, quantitative characterization guidelines for model testing are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Enhanced nonlinearity in a bubbly liquid considered as a mixture

During the past decade there has been an increasing interest in the nonlinear behavior of gas bubbles in liquids. Recent papers by Soviet investigators [e.g., Kustov et al., Sov. Phys. Acoust. 32(6), 500–503 (1986)] have concentrated on phenomena observed when sound interacts with a bubbly layer. These phenomena can include acoustic solitons, parametric radiation, self‐focusing, phase conjugation, and other effects that occur because the system is highly nonlinear. An investigation of the acoustic nonlinear parameter of mixtures was conducted to help infer the volume fractions of the mixture components. When a bubbly medium of known air volume fraction is considered as a two‐component mixture, its effective nonlinear parameter may be calculated using mixture methodologies [R. Apfel, J. Acoust. Soc. Am. 74, 1866–1869 (1983)]. The mixture analysis in this work, which is independent of any assumptions regarding bubble oscillation, yields nonlinear parameter values comparable to those of the Soviet investigators. For certain volume fractions, values of the nonlinear parameter of a bubbly liquid may be hundreds of times that of either the gas or the liquid host. Both experimental and theoretical work will be presented and the implications of this enhanced nonlinearity will be discussed. [Work supported by NIH through Grant 5‐R01‐GM30419.]

Grain growth in ice

Grain growth is studied in polycrystalline ice, consisting of elongated grains, of (200÷300) μm mean width\(\bar w\) and (2÷3) mm mean length\(\bar l\). The samples are annealed at different temperatures, between 0°C and −10°C. It is found that\(\bar l\) is not affected by annealing, while\(\bar w\) increases with the annealing time. Below the melting point,\(\bar w\)(t) tends to a limit value\(\bar w_s \). This behaviour is related to the pinning action of air bubbles, which would be similar to that found for solid inclusions in metals. By assuming\(\bar w_s = {{\bar d} \mathord{\left/ {\vphantom {{\bar d} f}} \right. \kern-\nulldelimiterspace} f}\) where\(\bar d\) is the mean bubble diameter andf is the volume fraction of air dissolved in water, reasonable values are found for\(\bar d\). The activation energy of the phenomenon is evaluated on the basis of the present and of Jellinek and Gouda's results. It is foundQ=0.6 eV, which value approximately coincides with that for bulk self-diffusion as it occurs for metals, several degrees below the melting point. This coincidence suggests that, for ice, grain growth would be controlled by bulk impurity diffusion up to the very melting point.

Sound speed in liquid-gas mixtures: Water-air and water-steam

The sound speed of a two-phase fluid, such as a magma-gas, water-air, or water-steam mixture, is dramatically different from the sound speed of either pure component. In numerous geologic situations the sound speed of such two-phase systems may be of interest: in the search for magma reservoirs, in seismic exploration of geothermal areas, in prediction of P wave velocity decreases prior to earthquakes, and in inversion of crustal and upper mantle seismic records. Probably most dramatically, fluid flow characteristics during eruptions of volcanoes and geysers are strongly dependent on the sound speed of erupting two-phase (or multiphase) fluids. In this paper the sound speeds of water, air, steam, water-air mixtures, and water-steam mixtures are calculated. It is demonstrated that sound speeds calculated from classical acoustic and fluid dynamics analyses agree with results obtained from finite amplitude ‘vaporization wave’ theory. To the extent that air and steam are represented as perfect gases with an adiabatic exponent γ, independent of temperature, their sound speeds vary in a simple manner directly with the square root of the absolute temperature. The sound speed of pure liquid water is a complex function of pressure and temperature and is given here to 8 kbar, 900°C. In pure water at all pressures the sound speed attains a maximum value near 100°C and decreases at higher temperatures; at high pressures the decrease is continuous, but at pressures below 1 kbar the sound speed reaches a minimum value in the vicinity of 500°–600°C, above which it again increases. The sound speed of a water-air mixture depends on the pressure, the void or mass fraction of air, the frequency of the sound wave, and, if surface tension effects are included, on bubble radius. The admixture of small volume fractions of air causes a dramatic lowering of the sound speed by nearly 3 orders of magnitude. The sound speeds of the pure liquid and gas end-members are nearly independent of pressure, but the sound speed of a mixture is highly dependent on pressure. Calculated values for water-air mixtures are in good agreement with measured values. The sound speed in a single-component two-phase system, such as a water-steam mixture, depends on whether or not equilibrium between the phases on the saturation curve is maintained. Heat and mass transfer which occur when equilibrium is maintained cause the sound speed to be much lower than under non-equilibrium conditions in which heat and mass transfer are absent. The sound speed in a water-steam mixture may be as low as 1 m s−1.

Gas-Vapor Separation in a Ranque-Hilsch Vortex Tube

Industrial process air is used as the energy source for Ranque-Hilsch vortex tubes applied to personal protective equipment and in turn supplies breathing air for the individual wearing the protective clothing. The possibility that contaminants in the supply air might be concentrated by gas separation hi the hot or cold air discharges was investigated. Mercury vapor at low concentrations was introduced into the supply air, and the mercury concentrations were monitored at the inlet and at each of the outlets of the vortex tube. A statistical analysis of variance of the data showed that a very small but insignificant (p<0.01) gas separation occurred. The influence of pressure, air volume, and cold air fraction on the mercury concentrations at the outlets was insignificant. This study indicates that the use of industrial process air, meeting accepted air quality standards for breathing purposes, would not increase the hazard to personnel using the vortex tube in conjunction with personal protective equipment.

Thermal Properties of a Pumice Soil

AbstractDacite pumice materials deposited by the eruption of Mount Mazama cover a land area for as much as 161 km (100 miles) north and east of Crater Lake, Oregon. Frequent night frosts occur in this region. Thermal properties of the pumice material were investigated to determine their affect on the occurrence of low temperatures in the area and thus gain a better understanding of the factors governing the distribution of lodgepole (Pinus contorta Dougl.) and ponderosa pine (Pinus ponderosa Laws). Thermal conductivities were determined experimentally with a line heat source and calculated theoretically. The theoretical calculations were based on methods which take into account the volume fractions of solid, air, and water of the pumice and the thermal conductivity of each one of these components. The thermal conductivities of dacite pumice were found to be very low, which may account for the frequent occurrence of night frost in this region.

Porosity of Surface Soil Aggregates at Various Moisture Contents

AbstractPorosity of individual surface soil aggregates at moisture contents ranging from saturation to air‐dry was determined by glass bead displacement. Measurements were made on aggregates having a diameter range from 0.5 to 10 mm at air‐dry moisture content and 2.0 to 10 mm at suctions ≤ 15 bars. The coefficient of variation for a determination of aggregate density (g cm‐3) ranged from 1 to 11% depending on diameter and moisture content. The major contributor to precision loss was heterogeneity of soil aggregates and not precision errors of the method.More than a 50% decrease in specific total pore volume (cm3 g‐1 of oven‐dry aggregate) upon desorption from saturation to air‐dry was found for three aggregate sizes each from a Chernozem and a Chestnut soil. In both soils, the percentage of pores having diameters &gt; 29 µ increased as aggregate diameter increased. The volume fraction of air at a given moisture content increased as the diameter of aggregate increased. For all aggregate sizes, the volume fraction of air was ≥ 0.10 at 0.33 bar suction. The two soils exhibited different types of shrinkage curves.