Results for “Critical” — Page 68

NASA NTRS1988

Structure of a fluid interface near the critical point

The structure of the liquid-liquid interface of three very different mixtures (carbon disulfide + methanol, methanol + cyclohexane + deuterated cyclohexane, and nitrobenzene + n-decane) has been studied using ellipsometry in the reduced temperature range between 0.0009 and 0.042. Although the elliptical thickness varies by an order of magnitude between these mixtures, the data from all three mixtures can be scaled to the same universal constant by a combined mean-field plus capillary-wave model of the interface. The universal constant determined experimentally is significantly less than the theoretical value.

Schmidt, James W.↗

NASA NTRS1988

The human factors of color in environmental design: A critical review

The literature on environmental color to enhance habitability in the design of Space Station interiors is reviewed. Some 200 studies were examined to determine the relative contributions of the three dimensions of color (hue, saturation, and brightness or lightness) to responses to environmental colorations. Implications of the study for color usage in novel settings and locales include: (1) There are no hard-wired linkages between environmental colors and particular judgmental or emotional states; (2) Perceptual impressions of color applications can, however, affect experiences and performances in settings; (3) Color behavior studies cannot yet specify an optimal color scheme, but instead must consider differing objectives, the relative importance of each, and design features such as the coordination of geometry, color, texture, etc.; (4) Some color-behavior effects are governed by low-level retinal and limbal mechanisms as well as by cognitive processes; and (5) Colors should first be specified in terms of what they are to do instead of what they are. Some exercise of choice is therefore needed to establish a sense of personal competence in the setting, since color must be ultimately be accepted by the people who are to live with it.

Wise, Barbara K.↗

NASA NTRS1989

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of their predictions.

Speziale, Charles G.↗

NASA NTRS1988

What does C II lambda 2325 A emission tell us about chromospheres of red supergiants? - A critical test using Zeta Aurigae-type K supergiants

The limitations of the Carpenter et al. (1985) C II intercombination multiplet method of determining the density and geometric extent of red giant chromospheres are presently tested through observation of the C II 2325 A emission of two K-type supergiants whose empirical model chromospheres have been derived by high-resolution IUE observations at eclipse phases. While the observed C II emission fluxes are well reproduced, much of this emission originates in the high-density lower chromosphere.

Schroeder, K.-P.↗

NASA NTRS1989

Synchronous critical speed tracking in hydrostatic bearing supported rotors

Hydrostatic bearings used in advanced turbopump designs use the pumped propellant as the working fluid and supply the propellant to the bearing from pump discharge. The resulting rotordynamic coefficients are highly speed-dependent and in some instances can cause system natural frequencies to coincide with spin speed over a wide speed range. This paper discusses this 'synchronous tracking' phenomenon. The factors affecting it are defined, and specific examples are presented. Methods which identify synchronous tracking issues early in the design process are reported, and techniques for eliminating this undesirable characteristic are addressed.

Henderson, Thomas W.↗

NASA NTRS1989

Design, operation, and critical issues of the U.S. Space Station Freedom propulsion system

The U.S. Space Station Freedom Manned Base (SSFMB) propulsion system is a gaseous hydrogen/oxygen-based system for primary reboost, attitude control, and station contingencies using electrolyzed water as a propellant. A secondary propulsion reboost system employs multipropellant resistojets which utilize the various waste gases generated during normal station operations. The hydrogen/oxygen propulsion system is comprised of several modules which contain thrusters, propellant storage tanks, regulation subsystems, water electrolysis units, electronic controls, and fluid plumbing. The resistojet system is comprised of one module containing the resistojets, regulators, electronic controls, and fluid plumbing. The waste gas propellant storage takes place in the Fluid Management System.

Morano, Joseph S.↗

NASA NTRS1989

A parametric study of space radiation exposures to critical body organs for low earth orbit missions

It is shown that it is possible to calculate the extent of the exposure to space radiation of a specific body organ. Using a computerized anatomical man model and models of radiation environments and spacecraft shielding, a parametric study was carried out for several Space-Shuttle scenarios. The dose equivalent data for several blood-forming organ locations are presented (for a solar-minimum epoch) as a function of altitude and the shielding material, and the results are compared with the NASA crew exposure limits for these body organs.

Atwell, W.↗

NASA NTRS1990

Implantable Electrode For Critical Locations

Implantable electrode holds itself in place until body tissue grows around it and retains it. Sheath covers electrode during implantation. Partially retracted after implantation and completely retracted after growth of tissue onto electrode. Intended to provide electrical stimuli to muscles having become inactive through accident or disease. Helps paraplegics and others to exercise voluntary control over arms and legs.

Collins, Earl R., Jr.↗

NASA NTRS1990

Critical evaluation of Jet-A spray combustion using propane chemical kinetics in gas turbine combustion simulated by KIVA-2

Jet-A spray combustion has been evaluated in gas turbine combustion with the use of propane chemical kinetics as the first approximation for the chemical reactions. Here, the numerical solutions are obtained by using the KIVA-2 computer code. The KIVA-2 code is the most developed of the available multidimensional combustion computer programs for application of the in-cylinder combustion dynamics of internal combustion engines. The released version of KIVA-2 assumes that 12 chemical species are present; the code uses an Arrhenius kinetic-controlled combustion model governed by a four-step global chemical reaction and six equilibrium reactions. Researchers efforts involve the addition of Jet-A thermophysical properties and the implementation of detailed reaction mechanisms for propane oxidation. Three different detailed reaction mechanism models are considered. The first model consists of 131 reactions and 45 species. This is considered as the full mechanism which is developed through the study of chemical kinetics of propane combustion in an enclosed chamber. The full mechanism is evaluated by comparing calculated ignition delay times with available shock tube data. However, these detailed reactions occupy too much computer memory and CPU time for the computation. Therefore, it only serves as a benchmark case by which to evaluate other simplified models. Two possible simplified models were tested in the existing computer code KIVA-2 for the same conditions as used with the full mechanism. One model is obtained through a sensitivity analysis using LSENS, the general kinetics and sensitivity analysis program code of D. A. Bittker and K. Radhakrishnan. This model consists of 45 chemical reactions and 27 species. The other model is based on the work published by C. K. Westbrook and F. L. Dryer.

Nguyen, H. L.↗

NASA NTRS1990

The Mach number of the cosmic flow - A critical test for current theories

A new cosmological, self-contained test using the ratio of mean velocity and the velocity dispersion in the mean flow frame of a group of test objects is presented. To allow comparison with linear theory, the velocity field must first be smoothed on a suitable scale. In the context of linear perturbation theory, the Mach number M(R) which measures the ratio of power on scales larger than to scales smaller than the patch size R, is independent of the perturbation amplitude and also of bias. An apparent inconsistency is found for standard values of power-law index n = 1 and cosmological density parameter Omega = 1, when comparing values of M(R) predicted by popular models with tentative available observations. Nonstandard models based on adiabatic perturbations with either negative n or small Omega value also fail, due to creation of unacceptably large microwave background fluctuations.

Ostriker, Jeremiah P.↗

NASA NTRS1990

A critical analysis of the modified equation technique of Warming and Hyett

The modified equation technique of Warming and Hyett (1974), which evaluates the behavior of a difference scheme by means of the coefficients of a certain modified equation, established a link between such coefficients and the multiplication factor obtained from the von Neumann analysis. It follows from this pioneering work that modified equations should be derived from the difference scheme, rather than from the original differential equation; a rigorous investigation is presently made of this link without resort to their original interpretation. Only partial validity is establishable for the case of multilevel schemes.

Chang, Sin-Chung↗

NASA NTRS1990

Water vapor absorption coefficients in the 8-13-micron spectral region - A critical review

Measurements of water vapor absorption coefficients in the thermal IR atmospheric window (8-13 microns) during the past 20 years obtained by a variety of techniques are reviewed for consistency and compared with computed values based on the AFGL spectral data tapes. The methods of data collection considered were atmospheric long path absorption with a CO2 laser or a broadband source and filters, a White cell and a CO2 laser or a broadband source and a spectrometer, and a spectrophone with a CO2 laser. Advantages and disadvantages of each measurement approach are given as a guide to further research. Continuum absorption has apparently been measured accurately to about the 5-10 percent level in five of the measurements reported.

Grant, William B.↗

NASA NTRS1989

Crew workload-management strategies - A critical factor in system performance

This paper reviews the philosophy and goals of the NASA/USAF Strategic Behavior/Workload Management Program. The philosophical foundation of the program is based on the assumption that an improved understanding of pilot strategies will clarify the complex and inconsistent relationships observed among objective task demands and measures of system performance and pilot workload. The goals are to: (1) develop operationally relevant figures of merit for performance, (2) quantify the effects of strategic behaviors on system performance and pilot workload, (3) identify evaluation criteria for workload measures, and (4) develop methods of improving pilots' abilities to manage workload extremes.

Hart, Sandra G.↗

NASA NTRS1989

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of thier predictions.

Speziale, Charles G.↗

NASA NTRS1990

Spherically symmetric near-critical accretion onto neutron stars

Numerical and approximate analytic solutions for time-independent, spherically symmetric, radiation pressure-dominated accretion flows are presented. For flows with luminosities at infinity, L-infinity, sufficiently close to the Eddington limit L-crit, the flow velocity profile is qualitatively different from the modified free-fall profile v(r) = (1 - L-infinity/L-crit)exp 1/2 (2GM/r)exp 1/2. Advective contributions to the comoving radiation flux decelerate the flow within a criical radius, and, in this settling region, the velocity of the flow decreases linearly with decreasing radius.

Miller, Guy S.↗

NASA NTRS1990

A critical evaluation of two-equation models for near wall turbulence

A basic theoretical and computational study of two-equation models for near-wall turbulent flows was conducted. Two major problems established for the K-epsilon model are discussed, the lack of natural boundary conditions for the dissipation rate and the appearance of higher-order correlations in the balance of terms for the dissipation rate at the wall. The K-omega equation is shown to have two problems also: an exact viscous term is missing, and the destruction of the dissipation term is not properly damped near the wall. A new K-tau model (where tau = 1/omega is the turbulent time scale) was developed by inclusion of the exact viscous term, and by introduction of new wall damping functions with improved asymptotic behavior. A preliminary test of the new model yields improved predictions for the flat-plate turbulent boundary layer.

Speziale, Charles G.↗

NASA NTRS1990

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of their predictions.

Speziale, Charles G.↗

NASA NTRS1990

Critical evaluation of Jet-A spray combustion using propane chemical kinetics in gas turbine combustion simulated by KIVA-II

Numerical solutions of the Jet-A spray combustion were obtained by means of the KIVA-II computer code after Jet-A properties were added to the 12 chemical species the program had initially contained. Three different reaction mechanism models are considered. The first model consists of 131 reactions and 45 species; it is evaluated by comparing calculated ignition delay times with available shock tube data, and it is used in the evaluation of the other two simplified models. The simplified mechanisms consider 45 reactions and 27 species and 5 reactions and 12 species, respectively. In the prediction of pollutants NOx and CO, the full mechanism of 131 reactions is considered to be more reliable. The numerical results indicate that the variation of the maximum flame temperature is within 20 percent as compared with that of the full mechanism of 131 reactions. The chemical compositions of major components such as C3H8, H2O, O2, CO2, and N2 are of the same order of magnitude. However, the concentrations of pollutants are quite different.

Nguyen, H. L.↗