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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Stability analysis of the accretion line

The stability of the flow along the accretion line in the Bondi-Hoyle-Lyttleton-type accretion flow is studied. The WKB approximation is used to analyze short wavelength linear perturbations of the axisymmetric steady state flow. The viscosity terms in the gasdynamical equations are included in the analysis. The radial modes are studied both in two- and three-dimensional accretion flow. Since a displaced accretion line cannot be defined in three-dimensional flow, the tangential modes are only analyzed in the two-dimensional flow. The analysis of the perturbed gasdynamical equations are made simpler as a result of the radial and tangential modes being decoupled in the linear regime. As was shown before, the three-dimensional accretion flow is unstable against radial modes. The two-dimensional accretion flow is found to be unstable against tangential modes, as well as against radial modes. Using typical numerical parameters which are in common use, the wavelength which gives the maximum growth rate of a small perturbation is found to be about 0.5 r(a), where r(a) is the accretion radius.

Soker, Noam↗
NASA NTRS1972

Liapunov functions for non-linear difference equation stability analysis.

Liapunov functions to determine the stability of non-linear autonomous difference equations can be developed through the use of auxiliary exact difference equations. For this purpose definitions are introduced for the gradient of an implicit function of a discrete variable, a principal sum, a definite sum and an exact difference equation, and a theorem for exactness of a difference form is proved. Examples illustrate the procedure.

Park, K. E.↗
NASA NTRS2018

Thermal Stability Analysis of Gevo Jet Fuel Using Ellipsometry

Thermal stability is an important characteristic of alternative fuels that must be evaluated before they can be used in aviation engines. This characteristic is of great importance to the effectiveness of the fuel as a coolant and to the engine's combustion performance. The thermal stability of Gevo fuel, an alcohol to jet fuel made from plant feedstock was studied. This analysis was used to make comments on the effectiveness of the current thermal stability test standard. This work was performed using a spectroscopic ellipsometer to measure the thickness of deposits left on aluminum substrates. It was observed that Gevo deposit thickness increased slowly up to 375 C and much more rapidly after that point. Similar behavior was observed in JP-8 fuel. Comparisons were also made between color standard ratings and ellipsometric thickness measurements, and it was found that in some cases, darker colors did not indicate thicker deposits. Reference tubes were used to validate the optical models used in this work, and different optical constants were found to best model the results than what are published in the ASTM D3241 test method for thermal stability.

Ellipsometry↗
NASA NTRS1973

Stability analysis of cylinders with circular cutouts

The stability of axially compressed cylinders with circular cutouts is analyzed numerically. An extension of the finite-difference method is used which removes the requirement that displacement components be defined in the directions of the grid lines. The results of this nonlinear analysis are found to be in good agreement with earlier experimental results.

Almroth, B. O.↗
NASA NTRS1974

Linear viscous stability analysis of the stratified Bickley jet

The stability of the stably stratified Bickley jet to small disturbances in a viscous, heat conducting fluid is explored by numerical integration of the governing differential equation. Curves of neutral stability are presented in the alpha-R plane and the parametric dependence of these curves upon Richardson number and Prandtl number is examined. The results show a curious bimodal behavior of the curve of neutral stability for small Richardson numbers. In addition, closed neutral curves are found for Richardson numbers in excess of the critical value for the inviscid flow. The unstable regions in the interior of these curves increase with decreasing Prandtl number. Energy balances are computed which show the relative magnitude of the Reynolds stress production, buoyancy dissipation, and viscous dissipation terms across the flow. As the thermal diffusivity becomes large compared with the kinematic viscosity, the buoyancy term is suppressed and the thermal stratification becomes irrelevant.

Gage, K. S.↗
NASA NTRS1993

Efficient computation of spatial eigenvalues for hydrodynamic stability analysis

The simple procedure presented for spatial stability computations can substantially reduce the computational requirements of such analyses, as illustrated for the cases of both internal and external cases of compressible and incompressible flows, and both viscous and inviscid instability modes. Excellent estimates of spatial eigenvalues are obtained.

Khorrami, Mehdi R.↗
NASA NTRS1978

Performance and stability analysis of a photovoltaic power system

The performance and stability characteristics of a 10 kVA photovoltaic power system are studied using linear Bode analysis and a nonlinear analog simulation. Power conversion efficiencies, system stability, and system transient performance results are given for system operation at various levels of solar insolation. Additionally, system operation and the modeling of system components for the purpose of computer simulation are described.

Merrill, W. C.↗
NASA NTRS1978

Ion acoustic stability analysis of the earth's bow shock

A marginal stability criterion for ion acoustic waves is applied to the analysis of five thin bow shock crossings observed by OGO-5; the observations began with the satellite in the magnetosheath. The assumption of marginally stable ion acoustic wave generation in the shock ramp provides the basis for determining the thickness of the magnetic gradients for subcritical, quasi-perpendicular shock features.

Greenstadt, E. W.↗
NASA NTRS1977

FLUT - A program for aeroelastic stability analysis

A computer program (FLUT) that can be used to evaluate the aeroelastic stability of aircraft structures in subsonic flow is described. The algorithm synthesizes data from a structural vibration analysis with an unsteady aerodynamics analysis and then performs a complex eigenvalue analysis to assess the system stability. The theoretical basis of the program is discussed with special emphasis placed on some innovative techniques which improve the efficiency of the analysis. User information needed to efficiently and successfully utilize the program is provided. In addition to identifying the required input, the flow of the program execution and some possible sources of difficulty are included. The use of the program is demonstrated with a listing of the input and output for a simple example.

Johnson, E. H.↗
NASA NTRS1982

A stability analysis of AVE-4 severe weather soundings

The stability and vertical structure of an average severe storm sounding, consisting of both thermodynamic and wind vertical profiles, were investigated to determine if they could be distinguished from an average lag sounding taken 3 to 6 hours prior to severe weather occurrence. The term average is defined here to indicate the arithmetic mean of a parameter, as a function of altitude, determined from a large number of available observations taken either close to severe weather occurrence, or else more than 3 hours before it occurs. The investigative computations were also done to help determine if a severe storm forecast or index could possibly be used or developed. These mean vertical profiles of thermodynamic and wind parameters as a function of severity of the weather, determined from manually digitized radar (MDR) categories are presented. Profile differences and stability index differences are presented along with the development of the Johnson Lag Index (JLI) which is determined entirely upon environmental vertical parameter differences between conditions 3 hours prior to severe weather, and severe weather itself.

Johnson, D. L.↗
NASA NTRS1988

Stability analysis and control of tethered satellites

Issues of stability and stabilization of the Shuttle/Tethered Satellite System (TSS) are considered. The basic, nonlinear equations of motion of the TSS are derived using the system Lagrangian. A tension control strategy in feedback form is introduced to stabilize the TSS during station-keeping. In addition, the stability properties of a constant angle control method for deployment and retrieval are investigated using Liapunov stability theory.

Liaw, D.-C.↗
NASA NTRS2018

Practical Methodology for the Inclusion of Nonlinear Slosh Damping in the Stability Analysis of Liquid-Propelled Space Vehicles

A method is presented that allows for the prediction of the magnitude of limit cycles due to adverse control-slosh interaction in liquid propelled space vehicles using non-linear slosh damping. Such a method is an alternative to the industry practice of assuming linear damping and relying on: mechanical slosh baffles to achieve desired stability margins; accepting minimal slosh stability margins; or time domain non-linear analysis to accept time periods of poor stability. Sinusoidal input describing functional analysis is used to develop a relationship between the non-linear slosh damping and an equivalent linear damping at a given slosh amplitude. In addition, a more accurate analytical prediction of the danger zone for slosh mass locations in a vehicle under proportional and derivative attitude control is presented. This method is used in the control-slosh stability analysis of the NASA Space Launch System.

Ottander, John A.↗