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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

A further study of the Comptonization of X-rays by low-temperature electrons

The hard X-ray transfer problem is considered for a monochromatic point source at the center of a spherical cloud of Compton scatterers. The problem is solved by a Monte Carlo calculation which provides solutions accurate to within a few percent with a reasonable amount of computation. It is shown that the method proposed by Illarionov et al. (1979) makes some significant errors when applied to atmospheres with a Thompson depth of 3. It is demonstrated that second-order corrections to the cross section are important at 50 keV, while first-order corrections are important even at 7 keV.

Langer, S. H.↗
NASA NTRS1979

The effect of accuracy, conservation and filtering on numerical weather forecasting

Considerations leading to the numerical design of the GLAS fourth-order global atmospheric model are discussed, including changes recently introduced into the model. The computation time and memory requirements for the fourth-order model are similar to those of the present second-order GLAS model with the same 4 deg latitude, 5 deg longitude, and 9 vertical-level resolution. However, the fourth-order model forecast skill is significantly better than that of the current GLAS model, and after three days it is comparable to the 2.5 by 3 deg version of the GLAS model in the sea level pressure maps, and has less phase errors in the 500 mb maps.

Kalnay-Rivas, E.↗
NASA NTRS1980

Numerical simulation of three-dimensional boattail afterbody flow fields

The thin shear layer approximations of the three-dimensional, compressible Navier-Stokes equations are solved for subsonic, transonic, and supersonic flow over axisymmetric boattail bodies at moderate angles of attack. The plume is modeled by a solid body configuration identical to those used in experimental tests. An implicit algorithm of second-order accuracy is used to solve the equations on the ILLIAC IV computer. The turbulence is expressed by an algebraic model applicable to three-dimensional flow fields with moderate separation. The computed results compare favorably with three different sets of experimental data reported by Reubush, Shrewsbury, and Benek, respectively

Deiwert, G. S.↗
NASA NTRS1980

Effect of pressure gradients on Gortler instability

Gortler instability for boundary-layer flows over generally curved walls is considered. The full linearized disturbance equations are obtained in an orthogonal curvilinear coordinate system. A perturbation procedure to account for second-order effects is used to determine the effects of the displacement thickness and the variation of the streamline curvature on the neutral stability of the Blasius flow. The pressure gradient in the mean flow is accounted for by solving the nonsimilar boundary-layer equations. Growth rates are obtained for the actual mean flow and the Falkner-Skan flows. The results demonstrate the strong influence of the pressure gradient and the nonsimilarity of the basic flow on the stability characteristics.

Ragab, S. A.↗
NASA NTRS1980

An ab initio calculation of the zero-field splitting parameters of the 3A-double prime state of formaldehyde

The spin dipole-dipole and spin-orbit contributions to the zero-field splitting of the 3A-double prime state of formaldehyde have been evaluated at the excited state experimental geometry. Ab initio CI wave functions were generated from a Dunning double zeta plus polarization bases set using 3A-double prime rhf orbitals. Twelve states of each symmetry were used to evaluate the second-order spin-orbit effect. The resulting values of D and E were 0.19 and 0.03 kayser with the principal magnetic axes rotated 36 deg from the CO bond. The values of alpha and beta relative to the inertial axes were calculated to be 0.03 and 0.01 kayser compared to the experimental values of 0.05 plus or minus 0.01 and 0.02 plus or minus 0.02 kayser.

Davidson, E. R.↗
NASA NTRS1980

A new approach to active control of rotorcraft vibration

A state-variable feedback approach is utilized for active control of rotorcraft vibration. Fuselage accelerations are passed through undamped second-order filters with resonant frequencies at N/rev. The resulting outputs contain predominantly the N/rev vibration components, phase shifted by 180 deg, and are used to drive the blade pitch to cancel this component of fuselage vibration. The linear-quadratic-gaussian (LQG) method is used to design a feedback control system utilizing these filtered accelerations. The design is based on a nine-degree-of-freedom linear model of the Rotor System Research Aircraft (RSRA) in hover and is evaluated on a nonlinear blade-element simulation of the RSRA for this flight condition. The system is shown to essentially eliminate vibrations at N/rev in all axes. The required blade-pitch amplitude is within the capability of conventional actuators at the N/rev frequency.

Gupta, N. K.↗
NASA NTRS1980

Pressure shift in the 170-micron emission of the CW optically pumped CH3OH laser

Pressure shifts of +15 MHz torr were observed in 16(8)-16(7) 170-micron CW CH3OH optically pumped laser emission. The experiments were performed using a harmonic mixing technique in a Schottky diode. The results are explained in terms of a second-order dipole-dipole interaction in a statistical formulation.

Lawandy, N. M.↗
NASA NTRS1980

A computer simulation of the midlatitude plasmasphere and ionosphere

A computer model has been developed to simulate species density, temperature, and plasma flow in the ionosphere and plasmasphere. A new approach, the flux preserving method involving a Newton iteration, is used to solve a system of governing equations comprising four second-order partial differentials. Tests are performed to demonstrate that the simulation converges to a stable steady state solution, then steady state ion fluxes are analyzed. Finally, simulations of the collapse of the sunset ionosphere are presented. Comparisons with satellite and radar data show good agreement in a number of cases.

Young, E. R.↗
NASA NTRS1980

Alternating direction implicit methods for parabolic equations with a mixed derivative

Alternating direction implicit (ADI) schemes for two-dimensional parabolic equations with a mixed derivative are constructed by using the class of all A(0)-stable linear two-step methods in conjunction with the method of approximate factorization. The mixed derivative is treated with an explicit two-step method which is compatible with an implicit A(0)-stable method. The parameter space for which the resulting ADI schemes are second-order accurate and unconditionally stable is determined. Some numerical examples are given.

Beam, R. M.↗
NASA NTRS1980

Hypersonic and planetary entry flight mechanics

The book treats hypersonic flight trajectories and atmospheric entry flight mechanics in light of their importance for space shuttle entry. Following a review of the structures of planetary atmospheres and aerodynamic forces, equations are derived for flight over a spherical planet, and the performance of long-range hypervelocity vehicles in extra-atmospheric flight is analyzed. Consideration is then given to vehicle trajectories in the powered and atmospheric reentry phases of flight, and several first-order solutions are derived for various planetary entry situations. The second-order theory of Loh for entry trajectories is presented along with the classical theories of Yaroshevskii and Chapman for entry into planetary atmospheres, and the thermal problems encountered in hypersonic flight are analyzed. A unified theory for entry into planetary atmospheres is then introduced which allows the performance of a general type of lifting vehicle to be studied, and applied to the analysis of orbit contraction due to atmospheric drag, flight with lift modulation and lateral maneuvers.

Vinh, N. X.↗
NASA NTRS1979

Analysis and design of an adaptive multi-loop controlled two winding buck/boost regulator

Small signal low frequency linear average model is derived for a multi-loop controlled two-winding buck/boost converter employing average techniques and the describing function method. The model reveals that a well-designed multi-loop control can provide a second-order zero adaptive to output filter parameter changes due to component tolerances, temperature changes, aging, and the effect of duty cycle modulation. It also can provide stabilization effect by shifting the positive zero to the left-half S-plane. Design guidelines are formulated to optimize regulator-loop dependent characteristics.

Mahmoud, M. F.↗
NASA NTRS1980

A numerical study of a marine subtropical stratus cloud layer and its stability

A model for numerical simulation of stratus cloud layers is constructed by combining a second-order closure, turbulent transfer model with a thermal radiative transfer model. The turbulent transfer model allows water vapor saturation. The combined turbulence-radiation model is applied to both a horizontally uniform one-dimensional case and a horizontally nonuniform two-dimensional case. In the latter, the dynamics of mesoscale circulations are also incorporated. Results of the two-dimensional simulation show that the layer cloud instability occurs where the sea surface temperature is high and the large-scale subsidence is weak. The simulated instability is analyzed in view of an instability criterion, the eddy kinetic energy budget, and evaporative cooling near the cloud top.

Moeng, C.-H.↗
NASA NTRS1952

Preliminary Investigation of Cyclic De-Icing of an Airfoil Using an External Electric Heater

An investigation was conducted in the NACA Lewis icing research tunnel to determine the characteristics and requirements of cyclic deicing of a 65,2-216 airfoil by use of an external electric heater. The present investigation was limited to an airspeed of 175 miles per hour. Data are presented to show the effects of variations in heat-on and heat-off periods, ambient air temperature, liquid-water content, angle of attack, and. heating distribution on the requirements for cyclic deicing. The external heat flow at various icing and heating conditions is also presented. A continuously heated parting strip at the airfoil leading edge was found necessary for quick, complete, and consistent ice removal. The cyclic power requirements were found to be primarily a function of the datum temperature and heat-on time, with the other operating and meteorological variables having a second-order effect. Short heat-on periods and high power densities resulted in the most efficient ice removal, the minimum energy input, and the minimum runback ice formations. The optimum chordwise heating distribution pattern was found to consist of a uniform distribution of cycled power density in the impingement region. Downstream of the impingement region the power density decreased to the limits of heating which, for the conditions investigated, extended from 5.7 percent chord on the upper surface of the airfoil to 8.9 percent chord on the lower surface. Ice removal did not take place at a heater surface temperature of 32 F; surface temperatures of approximately 50 to 100 F were required to effect removal. Better de-icing performance and greater energy savings would be possible with a heater having a higher thermal efficiency.

Lewis, James P.↗
NASA NTRS1953

Theoretical pressure distribution and wave drags for conical boattails

Afterbody pressure distributions and wave drag were calculated using a second-order theory for a variety of conical boattails at zero angle of attack. Results are presented for Mach numbers from 1.5 to 4.5, area ratios from 0.200 to 0.800, and boattail angle from 3 degrees to 11 degrees. The results indicate that for a given boattail angle, the wave drag decreases with increasing Mach number and area ratio. The wave drag, for a constant area ratio, increases with increasing boattail angle. For a specific Mach number, area ratio, and fineness ratio, a comparison of the wave-drag coefficients for conical, tangent-parabolic, and secant-parabolic boattails showed the conical boattail to have the smallest wave drag.

Jack, John R↗
NASA NTRS1954

Aerodynamics of Slender Bodies at Mach Number of 3.12 and Reynolds Numbers from 2 x 10(exp 6) to 15 x 10(exp 6) IV : Aerodynamic Characteristics of Series of Four Bodies Having Near-parabolic Noses and Cylindrical Afterbodies

Pressure distributions and forces for a series of four bodies of revolution having nose-fineness ratios varying from 4 to 10 have been obtained and compared with theory for a Mach number of 3.12, a Reynolds number range of 2x10(sup)6 to 14x10(sup)6, and angles of attack from zero to 9 degrees. In general, a comparison of the experimental data with a second-order theory showed good agreement for the range of variables investigated.

BODIES - FITNESS RATIO↗
NASA NTRS1954

Aerodynamics of Slender Bodies at Mach Number of 3.12 and Reynolds Numbers from 2 x 10(exp 6) to 15 x 10(exp 6) V : Aerodynamic Load Distributions for a Series of Four Boattailed Bodies

Pressure distributions for a series of four boattailed bodies of revolution were obtained and compared with theory for a Mach number of 3.12, a Reynolds number range of 2 x 10 to 6th power to 14 x 10 to the 6th power, and angles of attack from zero to 9 degrees. Second-order theory adequately predicted the pressure distribution for regions free of the effects of cross-flow separation.

Moskowitz, Barry↗
NASA NTRS1956

An analog study of a shock-position diffuser control on a supersonic turbojet engine

Shock position was assumed to be controlled by air bleed through a diffuser-exit bypass door operated by a second-order servomotor. Pressure disturbance resulting from changes in bypass-door position had whereas a disturbance in engine speed had a large effect on diffuser pressure and could impose severe requirements on diffuser control response. The controlled diffuser response improved with faster servomotor response and smaller diffuser dead time and lag. The effect of diffuser dead time on diffuser response was greater than the effect of diffuser lag.

Novik, David↗
NASA NTRS1958

Investigation of a Nonlinear Control System

A discontinuous variation of coefficients of the differential equation describing the linear control system before nonlinear elements are added is studied in detail. The nonlinear feedback is applied to a second-order system. Simulation techniques are used to study performance of the nonlinear control system and to compare it with the linear system for a wide variety of inputs. A detailed quantitative study of the influence of relay delays and of a transport delay is presented.

Flugge-Lotz, I↗