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NASA NTRS1992

Two-dimensional viscous accretion disk models. I - On meridional circulations in radiative regions

It has been suggested that thin Keplerian disks, like rotating stars, cannot attain hydrostatic and thermal equilibrium simultaneously if the angular velocity is a function of radius only. To address the problem of vertical structure and stability of thin accretion disks, we performed 2D axisymmetric calculations by solving the set of fully nonlinear hydrodynamic equations, including radiation transport. The results indicate that the variation of the rotational velocity with height is too small to sustain hydrostatic equilibrium. Instead, the variation of the radial (inflow) velocity with height suffices to establish a stationary state. In particular, we find that for low values of the viscosity parameter alpha, there is mass outflow in the central parts of the disk close to the equatorial plane, and inflow only near the surface. Only for modestly high values of alpha, the flow throughout the disk is directed inward. Thus, the flow within the disk is the result of viscous radial inflow plus a circulatory flow directed outward in the midplane and inward near the surface.

Kley, W.↗

NASA NTRS2016

Origin of the Two Scales of Wind Ripples on Mars

Earth's sandy deserts host two main types of bedforms - decimeter-scale ripples and larger dunes. Years of orbital observations on Mars also confirmed the existence of two modes of active eolian bedforms - meter-scale ripples, and dunes. By analogy to terrestrial ripples, which are thought to form from a grain mechanism, it was hypothesized that large martian ripples also formed from grain impacts, but spaced further apart due to elongated saltation trajectories from the lower martian gravity and different atmospheric properties. However, the Curiosity rover recently documented the coexistence of three scales of bedforms in Gale crater. Because a grain impact mechanism cannot readily explain two distinct and coeval ripple modes in similar sand sizes, a new mechanism seems to be required to explain one of the scales of ripples. Small ripples are most similar to Earth's impact ripples, with straight crests and subdued profiles. In contrast, large martian ripples are sinuous and asymmetric, with lee slopes dominated by grain flows and grainfall deposits. Thus, large martian ripples resemble current ripples formed underwater on Earth, suggesting that they may form from a fluid-drag mechanism. To test this hypothesis, we develop a scaling relation to predict the spacing of fluid-drag ripples from an extensive flume data compilation. The size of large martian ripples is predicted by our scaling relation when adjusted for martian atmospheric properties. Specifically, we propose that the wavelength of martian wind-drag ripples arises from the high kinematic viscosity of the low-density atmosphere. Because fluid density controls drag-ripple size, our scaling relation can help constrain paleoatmospheric density from wind-drag ripple stratification.

Lapotre, Mathieu G. A.↗

NASA NTRS2001

High-Flow PMR-Polymide Composites Developed With Mechanical Properties Comparable to Other High-Temperature Systems

PMR polyimides, in particular PMR-15, are well known for their excellent high-temperature stability and performance, and solvent resistance. However, the processing of these materials is limited, for the most part, to prepreg-based methods, such as compression or autoclave processing. These methods involve substantial amounts of hand labor, and as a result, manufacturing costs for components made from PMR polyimides can be high. In cost-sensitive applications, these high manufacturing costs can make the use of PMR polyimide-based components cost prohibitive. Lower cost manufacturing methods, such as resin transfer molding (RTM) and resin film infusion, have been demonstrated to reduce manufacturing costs by as much as 50 percent over prepreg-based methods. However, these processes are only amenable to materials with melt viscosities below 30 poise. Most PMR polyimides have melt viscosities on the order of 100 poise or higher. Recent efforts at the NASA Glenn Research Center have focused on chemical modifications to PMR polyimides to reduce their melt viscosity to the point where they could be processed by these low-cost manufacturing methods without adversely affecting their high-temperature properties and performance. These efforts have led to a new family of PMR polyimides that have melt viscosities significantly lower than that of PMR-15. Reductions in melt viscosity are brought about through the introduction of molecular twists in the polymer backbone. Carbon fiber (T650- 35) composites were prepared from one of these polyimides, designated PMR-Flex, by compression molding. The properties of these composites are presented below and compared with comparable composites made from PMR-15 and PETI-RTM, a new low-melt-viscosity polyimide.

Meador, Michael A.↗

NASA NTRS1987

Analysis of subgrid-scale eddy viscosity with use of results from direct numerical simulations

Without resort to any modeling, subgrid-scale eddy viscosity is computed from the results of high-resolution (64 cubed and 128 cubed grid points) direct numerical simulations of three-dimensional homogeneous isotropic decaying turbulence. In these simulations the eddy viscosity peaks sharply at the cutoff wavenumber, in rough agreement with the results of Kraichnan (1976). In addition, in the low-wavenumber range the eddy viscosity may be negative, contrary to the generally accepted concept of subgrid-scale eddy viscosity. Some possible explanations of this behavior are discussed.

Domaradzki, J. Andrzej↗

NASA NTRS1984

Nitrogen threshold experiment

In order to obtain data on Earth geologic processes associated with wind erosion, Mar's and Venus' atmospheres were simulated. Experiments were designed to observe the effects of pressure, gravity, and density on sand movement. An experiment designed to test the operator variance was a comparative test between nitrogen and carbon dioxide. Pressure and therefore density was increased until threshold was noted. Threshold was determined at precisely the same densities of the two gases. If viscosity were a contributing factor in grain saltation the effect would be noted at low pressures. The experiment showed no such variance between carbon dioxide and nitrogen runs for low pressure (density). The largest difference in viscosity between carbon dioxide and nitrogen occurs at low pressures (densities).

Treat, C. L.↗

NASA NTRS2002

Numerical Simulation of Flow Field Within Parallel Plate Plastometer

Parallel Plate Plastometer (PPP) is a device commonly used for measuring the viscosity of high polymers at low rates of shear in the range 10 4 to 10 9 poises. This device is being validated for use in measuring the viscosity of liquid glasses at high temperatures having similar ranges for the viscosity values. PPP instrument consists of two similar parallel plates, both in the range of 1 inch in diameter with the upper plate being movable while the lower one is kept stationary. Load is applied to the upper plate by means of a beam connected to shaft attached to the upper plate. The viscosity of the fluid is deduced from measuring the variation of the plate separation, h, as a function of time when a specified fixed load is applied on the beam. Operating plate speeds measured with the PPP is usually in the range of 10 -3 cm/s or lower. The flow field within the PPP can be simulated using the equations of motion of fluid flow for this configuration. With flow speeds in the range quoted above the flow field between the two plates is certainly incompressible and laminar. Such flows can be easily simulated using numerical modeling with computational fluid dynamics (CFD) codes. We present below the mathematical model used to simulate this flow field and also the solutions obtained for the flow using a commercially available finite element CFD code.

Basil N. Antar↗

NASA NTRS2003

Effect of Viscosity on the Crystallization of Undercooled Liquids

There have been numerous studies of glasses indicating that low-gravity processing enhances glass formation. NASA PI s are investigating the effect of low-g processing on the nucleation and crystal growth rates. Dr. Ethridge is investigating a potential mechanism for glass crystallization involving shear thinning of liquids in 1-g. For shear thinning liquids, low-g (low convection) processing will enhance glass formation. The study of the viscosity of glass forming substances at low shear rates is important to understand these new crystallization mechanisms. The temperature dependence of the viscosity of undercooled liquids is also very important for NASA s containerless processing studies. In general, the viscosity of undercooled liquids is not known, yet knowledge of viscosity is required for crystallization calculations. Many researchers have used the Turnbull equation in error. Subsequent nucleation and crystallization calculations can be in error by many orders of magnitude. This demonstrates the requirement for better methods for interpolating and extrapolating the viscosity of undercooled liquids. This is also true for undercooled water. Since amorphous water ice is the predominant form of water in the universe, astrophysicists have modeled the crystallization of amorphous water ice with viscosity relations that may be in error by five orders-of-magnitude.

Source record↗

NASA NTRS1961

Alignment Charts for Transport Properties Viscosity, Thermal Conductivity, and Diffusion Coefficients for Nonpolar Gases and Gas Mixtures at Low Density

Alignment charts (nomographs) designed to permit rapid calculations of the low-pressure transport properties of nonpolar gases and gas mixtures are presented. Calculations for pure gases are based on the rigorous kinetic theory of gases as applied to a realistic intermolecular force law. Mixture viscosities and conductivities are calculated from good approximations derived from rigorous theory. The properties are calculable with a precision of 2 percent or better. Accuracy depends on how well the constants characterizing the Intermolecular force law are known; if constants are derived from experimental data, results should be accurate to 5 percent or better. Force constants are tabulated for 65 gases.

Brokaw, Richard S.↗

NASA NTRS1972

The evaporation of various lubricant fluids in vacuum.

The weight-loss rate in vacuum of 18 lubricating liquids and greases is reported for temperatures up to 120 C. Eleven types of 'medium-viscosity' (20 to 52 centistokes at 40 C) fluids were studied. In addition, a variety of perfluoro ether liquids having viscosities of 350 to 500 centistokes at 40 C were investigated, as were some commercial greases based on these liquids. The results showed that a methylchlorophenyl silicone, a high-viscosity-index perfluoro ether research fluid, and a polyoctene have suitably low outgassing properties for long-term space applicaitons requiring a medium-viscosity lubricant. Perfluoro ether greases also displayed low outgassing, but batch-to-batch variation was great. Mass spectrometer measurements indicated that the outgassed species were characterized by much lower molecular weights than that expected for the liquids. This suggests that lubricant transfer techniques which rely on evaporation from a reservoir may not be desirable for long-term space applications.

Hamilton, D. B.↗

NASA NTRS2010

Viscosity Measurement via Drop Coalescence: A Space Station Experiment

The concept of using low gravity experimental data together with CFD simulations for measuring the viscosity of highly viscous liquids was recently validated on onboard the International Space Station (ISS). A series of microgravity tests were conducted for this purpose on the ISS in July, 2004 and in May of 2005. In these experiments two liquid drops were brought manually together until they touched and were allowed to coalesce under the action of the capillary force alone. The coalescence process was recorded photographically from which the contact radius speed of the merging drops was measured. The liquid viscosity was determined by fitting the measured data with accurate numerical simulation of the coalescence process. Several liquids were tested and for each liquid several drop diameters were employed. Experimental and numerical results will be presented in which the viscosity of several highly viscous liquids were determined using this technique.

Antar, Basil↗

NASA NTRS1984

Nightward ion flow in the Venus ionosphere - Implications of momentum balance

Using global empirical models of Venus ionospheric conditions, the plasma flow field consistent with the horizontal momentum equation in both viscid and inviscid forms is solved for. It is found that plasma viscosity is negligible except at low altitudes and that the observed plasma flows are consistent with the inviscid solution above 300 km but are larger than the calculated flows at lower altitudes. This is probably due to downward momentum advection. The strong vertical shear in the calculated and observed flows just beyond the terminator may produce turbulence there, manifested as the observed transterminator waves in plasma density and magnetic field. The implications of ionospheric superrotation are discussed.

Elphic, R. C.↗

NASA NTRS1988

Containment of a silicone fluid free surface in reduced gravity using barrier coatings

In support of the Surface Tension Driven Convection Experiment planned for flight aboard the Space Shuttle, tests were conducted under reduced gravity in the 2.2-sec Drop Tower and the 5.0-sec Zero-G facility at the NASA Lewis Research Center. The dynamics of controlling the test fluid, a 10-cSt viscosity silicone fluid in a low gravity environment were investigated using different container designs and barrier coatings. Three container edge designs were tested without a barrier coating; a square edge, a sharp edge with a 45-deg slope, and a sawtooth edge. All three edge designs were successful in containing the fluid below the edge. G-jitter experiments were made in scaled down containers subjected to horizontal accelerations. The data showed that a barrier coating is effective in containing silicone fluid under g-levels up to 10 sup -1 sub g sub 0. In addition, a second barrier coating was found which has similar anti-wetting characteristics and is also more durable.

Pline, Alexander D.↗

NASA NTRS1989

Effects of lubrication on the performance of high speed spur gears

An experimental analysis was conducted to determine power loss and gear noise of high speed spur gears with long addendum under various conditions of load, speed, and oil jet pressure for into mesh lubrication. Power losses were calculated from temperature measurements of lubricating oil, gears, gear box, and oil flow rate. Furthermore, power loss was divided into windage loss, friction loss and churning loss. The results show that windage loss and churning loss were the main components of gear power loss of high gear speed. In addition, lubricating conditions had some influences on gear noise especially under low oil temperature or high viscosity.

Mizutani, Hachiro↗

NASA NTRS1989

Effects of lubrication on the performance of high speed spur gears

Mizutani, H.↗

NASA NTRS1979

Studies for improved high temperature coatings for Space Shuttle application

Improvement of the current Class 2 Space Shuttle Orbiter RCG coating was experimentally investigated.Coatings, which are applied to LI-900 or LI-2200 tiles, were prepared to provide increased performance in thermal expansion, impact, residual strain and increased viscosity. Turbulent duct arc-plasma tests at NASA/Ames Research Center are continuing on two candidates that show improved low residual strain and increased high temperature viscosity. A coating system with lower fusion-temperature (1950 F) was identified which has the potential of improving tile yield through reduced LI-900 shrinkage and distortion since it can be fused at 250 F lower than the present Class 2 coating.

Creedon, J.↗

NASA NTRS2007

Fluid Merging Viscosity Measurement (FMVM) Experiment on the International Space Station

The concept of using low gravity experimental data together with fluid dynamical numerical simulations for measuring the viscosity of highly viscous liquids was recently validated on the International Space Station (ISS). After testing the proof of concept for this method with parabolic flight experiments, an ISS experiment was proposed and later conducted onboard the ISS in July, 2004 and subsequently in May of 2005. In that experiment a series of two liquid drops were brought manually together until they touched and then were allowed to merge under the action of capillary forces alone. The merging process was recorded visually in order to measure the contact radius speed as the merging proceeded. Several liquids were tested and for each liquid several drop diameters were used. It has been shown that when the coefficient of surface tension for the liquid is known, the contact radius speed can then determine the coefficient of viscosity for that liquid. The viscosity is determined by fitting the experimental speed to theoretically calculated contact radius speed for the same experimental parameters. Experimental and numerical results will be presented in which the viscosity of different highly viscous liquids were determined, to a high degree of accuracy, using this technique.

Antar, Basil N.↗

NASA NTRS1995

Development of a two-equation turbulence model for hypersonic flows: Evaluation of a low Reynolds number correction to the Kappa - epsilon two equation compressible turbulence model - Volume 1

The objective of the current research is the development of an improved k-epsilon two-equation compressible turbulence model for turbulent boundary layer flows experiencing strong viscous-inviscid interactions. The development of an improved model is important in the design of hypersonic vehicles such as the National Aerospace Plane (NASP) and the High Speed Civil Transport (HSCT). Improvements have been made to the low Reynolds number functions in the eddy viscosity and dissipation of solenoidal dissipation of the k-epsilon turbulence mode. These corrections offer easily applicable modifications that may be utilized for more complex geometries. The low Reynolds number corrections are functions of the turbulent Reynolds number and are therefore independent of the coordinate system. The proposed model offers advantages over some current models which are based upon the physical distance from the wall, that modify the constants of the standard model, or that make more corrections than are necessary to the governing equations. The code has been developed to solve the Favre averaged, boundary layer equations for mass, momentum, energy, turbulence kinetic energy, and dissipation of solenoidal dissipation using Keller's box scheme and the Newton spatial marching method. The code has been validated by removing the turbulent terms and comparing the solution with the Blasius solution, and by comparing the turbulent solution with an existing k-epsilon model code using wall function boundary conditions. Excellent agreement is seen between the computed solution and the Blasius solution, and between the two codes. The model has been tested for both subsonic and supersonic flat-plate turbulent boundary layer flow by comparing the computed skin friction with the Van Driest II theory and the experimental data of Weighardt; by comparing the transformed velocity profile with the data of Weighardt, and the Law of the Wall and the Law of the Wake; and by comparing the computed results of an adverse pressure gradient with the experimental data of Fernando and Smits. Good agreement is obtained with the experimental correlations for all flow conditions.

Knight, Doyle D.↗

NASA NTRS2003

High-Pressure Transport Properties Of Fluids: Theory And Data From Levitated Drops At Combustion-Relevant Temperatures

Although the high pressure multicomponent fluid conservation equations have already been derived and approximately validated for binary mixtures by this PI, the validation of the multicomponent theory is hampered by the lack of existing mixing rules for property calculations. Classical gas dynamics theory can provide property mixing-rules at low pressures exclusively. While thermal conductivity and viscosity high-pressure mixing rules have been documented in the literature, there is no such equivalent for the diffusion coefficients and the thermal diffusion factors. The primary goal of this investigation is to extend the low pressure mixing rule theory to high pressures and validate the new theory with experimental data from levitated single drops. The two properties that will be addressed are the diffusion coefficients and the thermal diffusion factors. To validate/determine the property calculations, ground-based experiments from levitated drops are being conducted.

Bellan, Josette↗