Physical Sciences

DNS of a Transitional Supercritical C7H16/N2 Mixing Layer

This report discusses direct numerical simulations (DNS) of a mixing layer, between a nitrogen stream and a heptane stream initially flowing at different velocities, under supercritical conditions and undergoing a transition to turbulence. Thermodynamically, supercritical conditions prevail when either the temperature or the pressure exceeds its critical value; the critical regime is in particular characterized by the existence of a single phase. The governing conservation equations were formulated according to fluctuation-dissipation (FD) theory, in which the low-pressure typical transport properties (viscosity, diffusivity, and thermal conductivity) are complemented, at high pressure, by a thermal-diffusion factor.

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Built-in "Health Check" for Pressure Transducers

Calibrations could be verified approximately, without removing transducers to calibration laboratories. "Health check" would be built into pressure transducers, according to a proposal, to enable occasional, rapid, in situ testing of the transducers between normal pressure-measurement operations. The health check would include relatively simple devices that, upon command, would provide known stimuli to the transducers. The responses of the pressure transducers to these stimuli would be analyzed to quantify (at least approximately) deviations from the responses expected from previous rigorous calibrations. On the basis of such an analysis, a given pressure transducer could be removed from service, rigorously recalibrated, or continued in use with corrections applied for calibration drift. The use of the health check could provide timely warnings of pressure-transducer malfunctions and make it possible to retain confidence in the calibrations of pressure transducers while reducing the frequency with which they are replaced or subjected to full laboratory recalibration.

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All-Pressure Fluid-Drop Model Applied to a Binary Mixture

A report presents a computational study of the subcritical and supercritical behaviors of a drop of heptane surrounded by nitrogen, using the fluid-drop model described in "Model of a Drop of O2 Surrounded by H2 at High Pressure" (NPO-20220) and "The Lewis Number Under Supercritical Conditions" (NPO-20256), NASA Tech Briefs, Vol. 23, No. 3 (March 1999), pages 66-70. In this model, the differences between subcritical and supercritical behaviors are identified with length scales. The report compares results of the computations with data from microgravity experiments on large drops at temperatures and pressures in the sub- and supercritical regimes.

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Validation of All-Pressure Fluid-Drop Model

A report presents a computational study of the subcritical and supercritical behaviors of a drop of heptane surrounded by nitrogen. The subject matter is basically same as that of the report described in the preceding article, except that the Lewis-number issue is not addressed in detail; however, this article presents the full set of equations which lack in the former. As in the preceding case, the results of the computations are compared with data from microgravity experiments on drops of heptane evaporating in nitrogen at temperatures and pressures in the sub- and supercritical regimes, and conclusions are drawn regarding the accuracy of (1) the mathematical model used in the present study and (2) the limitation on accuracy of a traditional model (known as the d2 law) at supercritical pressures. The conclusions stated in the report are essentially a subset of the conclusions stated in the report described in the preceding article.

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Subgrid Analysis of Mixing Layer With Evaporating Droplets

This report presents an analysis of a database from computational simulations of a droplet-laden mixing layer (i.e., evaporating droplets of a liquid fuel in air) undergoing a transition to turbulence. The basic governing equations were those of transport of discrete droplets through a flowing gas; the droplets were followed in a Lagrangian frame whereas the gas was followed in an Eulerian frame. The analysis involved the extraction of subgrid scale (SGS) models from flow fields generated using the direct numerical simulation (DNS) approach, in which the governing equations are solved directly at the relevant length scales.

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Validated Model of a Fluid Drop for All Pressures

The report "A Validated All-Pressure Fluid Drop Model and Lewis Number Effects for a Binary Mixture" presents one in a series of theoretical and computational studies of the subcritical and subpercritical behaviors of a drop of fluid and, in particular, a drop of heptane surrounded by nitrogen The study is based on a fluid-drop model in which, among other things, the differences between subcritical and supercritical behaviors are identified with length scales. It is shown that in the subcritical regime and for a large rate of evaporation from the drop, there exists a mass0fraction "Film layer" immediately below the drop surface and the solution of the model equations has a convective-diffusive character. In the supercritical regime, there is no material surface to follow and this introduces an indeterminancy in the boundary conditions. To resolve the indeterminancy, one must follow an arbitrary boundary, which, in this case, is that of the initial fluid drop. The solution has then a purely diffusive character, and from this solution, one calculates the location of the highest density gradient, which location is identified with the optically observable boundary. It is also shown that the classical calculation of the Lewis number gives qualitatively erroneous results at supercritical conditions, but that an effective Lewis number previously defined gives qualitatively correct estimates of the length scales for heat and mass transfer at all pressures.

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Numerical Index for Quantifying Aircraft Icing Hazards

This index would offer several advantages over the present four-level index. A new method for assessing and communicating aviation in-flight icing hazards has been proposed. This methodology creates a simple numerical index for quantifying hazard severity. The index is traceable to flight-level meteorology and aircraft-specific, icing-induced reductions in aircraft performance. It also provides a connection to a statistical data base of icing meteorology. This system will clarify the terminology used to describe the degree of danger posed by specific meteorological conditions. The relationship between hazard severity and meteorology is related by measured ice accumulation rates observed on a standard airfoil under prescribed conditions. This system has greater fidelity than the existing system and is applicable to all types of air vehicles.

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