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At least 325 records · Page 18

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NASA NTRS1975

Poisson equations of rotational motion for a rigid triaxial body with application to a tumbling artificial satellite

A mathematical model is developed for studying the effects of gravity gradient torque on the attitude stability of a tumbling triaxial rigid satellite. Poisson equations are used to investigate the rotation of the satellite (which is in elliptical orbit about an attracting point mass) about its center of mass. An averaging method is employed to obtain an intermediate set of differential equations for the nonresonant, secular behavior of the osculating elements which describe the rotational motions of the satellite, and the averaged equations are then integrated to obtain long-term secular solutions for the osculating elements.

Liu, J. J. F.↗

NASA NTRS1976

Timing of solar cycles by rigid internal rotations

The so-called 11-year cycle of solar activity is really more complex and contains many periods of greatly different lengths. Periods as long as 178 years and as short as 3.1 years are predicted by a theory based on beats between rigidly rotating, inertially oscillating g-modes inside the sun. Most of the beat periods are then confirmed to about 1 percent accuracy in sunspot observations. Since the agreement is of high statistical significance, one can conclude that approximate alignment of major solar oscillation modes contributes to high solar activity. The theory receives further support when tested against an independent class of observations - the large-scale magnetic sector structure. Predicted rotation rates of at least four solar oscillation modes are detected in the sector data with discrepancies all less than 0.3 percent. As a by-product of these successful fits to observation, the mean rotation of the entire solar mass becomes known. Its rotation frequency is 4.49 by 10 to the -7th power Hz, which is a sidereal period of 25.8 days. Magnetic fields have played no role in calculating the length of any of these solar cycles.

Wolff, C. L.↗

NASA NTRS1976

An experimental study of the aeroacoustics of a subsonic jet impinging normal to a large rigid surface

This paper presents results from an experimental study of unsteady surface pressures and far-field noise produced by a subsonic jet impinging normal to a large, rigid, flat surface. The tests were performed in an anechoic room for jet Mach numbers from 0.54 to 0.85, and for jet-to-surface heights of from 5 to 10 jet diameters. Results showed that the root-mean-square surface pressure levels were proportional to jet dynamic pressure and were independent of jet height for radial distances from the stagnation point greater than 3 jet diameters. Far-field measurements indicated a significant increase in noise over that of a free jet for all cases of impingement. Cross spectral calculations between the surface and the far field suggested that at a Mach number of 0.70 and a height of 5 jet diameters the additional noise originated mainly in the outer edge of the impingement region between 1 and 3 jet diameters from the stagnation point.

Preisser, J. S.↗

NASA NTRS1976

Elasto-plastic impact of hemispherical shell impacting on hard rigid sphere

An analysis of plastic stress waves for cylindrical metallic projectile in impact is extended to an analysis of a hemispherical shell suffereing plastic deformation during the process of impact. It is assumed that the hemispherical shell with a prescribed launch velocity impinges a fixed rigid sphere of diameter equal to the internal diameter of the shell. The dynamic biaxial state of stress present in the shell during deformation is investigated. The analysis is valuable for studying the state of stress during large plastic deformation of a hemispherical shell.

Raftopoulos, D. D.↗

NASA NTRS1976

Diffraction of sound by nearly rigid barriers

The diffraction of sound by barriers with surfaces of large, but finite, acoustic impedance was analyzed. Idealized source-barrier-receiver configurations in which the barriers may be considered as semi-infinite wedges are discussed. Particular attention is given to situations in which the source and receiver are at large distances from the tip of the wedge. The expression for the acoustic pressure in this limiting case is compared with the results of Pierce's analysis of diffraction by a rigid wedge. An expression for the insertion loss of a finite impedance barrier is compared with insertion loss formulas which are used extensively in selecting or designing barriers for noise control.

Hadden, W. J., Jr.↗

NASA NTRS1976

An analytical study of noise generation by subsonic flows in the presence of rigid surfaces

A general analytical method is developed for determination of the radiated noise field from highly sheared, subsonic flows in the presence of rigid surfaces. The method is specifically formulated to use measurable flow field properties as the basis input data for calculations. The governing equations of a viscous, incompressible fluid are employed to determine approximately the so-called shear noise source strength distribution in the flow. Mean rates of strain and fluctuating velocity covariances are the fundamental input data for calculations. The method is applied to the wall jet region of a circular jet impinging at 45 deg to an infinite plane surface. Velocity field measurements of Foss are used to describe the required flow properties. Comparisons of calculated results with measured surface pressures and sound pressure levels are given and discussed.

Wooley, J. P.↗

NASA NTRS1976

A theoretical evaluation of rigid baffles in suppression of combustion instability

An analytical technique for the prediction of the effects of rigid baffles on the stability of liquid propellant combustors is presented. This analysis employs a three dimensional combustor model characterized by a concentrated combustion source at the chamber injector and a constant Mach number nozzle. An eigenfunction-matching method is used to solve the linearized partial differential equations describing the unsteady flow field. Boundary layer corrections to this unsteady flow are used in a mechanical energy dissipation model to evaluate viscous and turbulence effects within the flow. An integral stability relationship is then employed to predict the decay rate of the oscillations. Results of this analysis agree qualitatively with experimental observations and show that sufficient dissipation exists to indicate that the proper mechanism of baffle damping is a fluid dynamic loss. The response of the dissipation model to varying baffle blade length, mean flow Mach number and oscillation amplitude is examined.

Combustion instability↗

NASA NTRS1977

Vibration characteristics of a large wind turbine tower on non-rigid foundations

Vibration characteristics of the Mod-OA wind turbine supported by nonrigid foundations were investigated for a range of soil rigidities. The study shows that the influence of foundation rotation on the fundamental frequency of the wind turbine is quite significant for cohesive soils or loose sand. The reduction in natural frequency can be greater than 20 percent. However, for a foundation resting on well graded, dense granular materials or bedrock, such effect is small and the foundation can be treated as a fixed base.

Yee, S. T.↗

NASA NTRS1977

Strength and rigidity of the elastic lunar lithosphere and implications for present-day mantle convection in the moon

Lunar gravity data are used to estimate the thickness of the elastically defined lunar lithosphere, and the load-bearing capability of lithosphere with respect to lunar mascons is analyzed by representing the elastic lithosphere as a plane elastic plate subject to spatially concentrated loads from above and buoyant support from below. Gravity perturbations are computed and compared with observations, the spectrum of lunar gravity perturbations is derived, and effects of curvature are considered. The magnitude of the elastic stresses associated with a lithospheric flexure model maintaining the irregularities of the lunar gravity field is shown to be quite similar to that of the lithospheric stresses found on earth. It is concluded that associated differences in gravity, heat flow, and lithospheric thickness all combine to make the lunar lithosphere much more rigid and stronger than that of earth. Implications for present-day lunar convection are discussed.

Kuckes, A. F.↗

NASA NTRS1977

Prediction of elastic-airplane lateral dynamics from rigid-body aerodynamics

Control-configured vehicle technology has increased the demand for detailed analysis of dynamic stability and control, handling and ride qualities, and control system dynamics at the early stages of preliminary design and development. For these early analyses an approximate, but reasonably accurate, set of equations of motion for elastic airplanes is needed. Such a formulation is developed for the lateral dynamics of elastic airplanes. It makes use of rigid-body aerodynamic stability derivatives and the antisymmetric elastic mode shapes and frequencies in formulating the forces and moments due to elastic motion. Verification of accuracy was made by comparison with B-1 airplane dynamics obtained by other methods. Frequencies and damping ratios of the coupled modes agree acceptably well with four antisymmetric elastic modes included.

Swaim, R. L.↗

NASA NTRS1977

Theory of the rotation of the rigid earth

Equations of motion for a triaxial rigid earth are derived in Andoyer variables. The reference plane is the ecliptic of date which is moving as a result of planetary perturbations. By using this noninertial system, the development of the disturbing function for the sun and moon is simplified, with an additional term appearing in the Hamiltonian which, however, contributes only to precessional motion. The nutation terms derived are compared with those of Woolard.

Kinoshita, H.↗

NASA NTRS1978

Rigid coupling is also flexible

Spring-loaded coupling is rigid under light loads and swivels under higher loads. Break-out point can be set at any desired value by selecting appropriate preload springs. Coupling requires no cushions or elastomeric joints that limit temperature range.

Appleberry, W. T.↗

NASA NTRS1977

Method of adhering bone to a rigid substrate using a graphite fiber reinforced bone cement

A method is described for adhering bone to the surface of a rigid substrate such as a metal or resin prosthesis using an improved surgical bone cement. The bone cement has mechanical properties more nearly matched to those of animal bone and thermal curing characteristics which result in less traumatization of body tissues and comprises a dispersion of short high modulus graphite fibers within a bonder composition including polymer dissolved in reactive monomer such as polymethylmethacrylate dissolved in methylmethacrylate monomer.

Knoell, A. C.↗

NASA NTRS1978

Computer program for determining mass properties of a rigid structure

A computer program was developed for the rapid computation of the mass properties of complex structural systems. The program uses rigid body analyses and permits differences in structural material throughout the total system. It is based on the premise that complex systems can be adequately described by a combination of basic elemental shapes. Simple geometric data describing size and location of each element and the respective material density or weight of each element were the only required input data. From this minimum input, the program yields system weight, center of gravity, moments of inertia and products of inertia with respect to mutually perpendicular axes through the system center of gravity. The program also yields mass properties of the individual shapes relative to component axes.

Hull, R. A.↗

NASA NTRS1978

Stress intensity factors in a cracked infinite elastic wedge loaded by a rigid punch

A plane elastic wedge-shaped solid was split through the application of a rigid punch. It was assumed that the coefficient of friction on the the contact area was constant, and the problem had a plane of symmetry with respect to loading and geometry, with the crack in the plane of symmetry. The problem was formulated in terms of a system of integral equations with the contact stress and the derivative of the crack surface displacement as the unknown functions. The solution was obtained for an internal crack and for an edge crack. The results include primarily the stress intensity factors at the crack tips, and the measure of the stress singularity at the wedge apex, and at the end points of the contact area.

Erdogan, F.↗

NASA NTRS1978

Estimation of payload loads using rigid body interface accelerations

In the design/analysis process of a payload structural system, the accelerations at the payload/launch vehicle interface obtained from a system analysis using a rigid payload are often used as the input forcing function to the elastic payload to obtain structural design loads. Such an analysis is at best an approximation since the elastic coupling effects are neglected. This paper develops a method wherein the launch vehicle/rigid payload interface accelerations are modified to account for the payload elasticity. The advantage of the proposed method, which is exact to the extent that the physical system can be described by a truncated set of generalized coordinates, is that the complete design/analysis process can be performed within the organization responsible for the payload design. The method requires the updating of the system normal modes to account for payload changes, but does not require a complete transient solution using the composite system model. An application to a real complex structure, the Viking Spacecraft System, is given.

Chen, J. C.↗

NASA NTRS1978

Flash-fire propensity of rigid foam insulation

Twelve samples of rigid foam insulation were evaluated for flash-fire propensity, using the USF flash-fire screening test method. These materials exhibited little or no flash-fire propensity under these particular test conditions.

Hilado, C. J.↗

NASA NTRS1979

Hinge-connected rigid bodies

Package of subroutines solve minimum dimension sets of discrete coordinate equations of motion for arbitrary number of hinge-connected rigid bodies assembled in tree topology.

Fleischer, C. E.↗