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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 235 records · Page 13

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NASA NTRS1978

The effects of eccentricities on the fracture of off-axis fiber composites

Finite element analyses were performed to investigate theoretically the effects of in-plane and out-of plane eccentricities, bending or twisting, and thickness nonuniformity on the axial stress and strain variations across the width of off-axis specimens. The results are compared with measured data and are also used to access the effects of these eccentricities on the fracture stress of off-axis fiber composites. Guidelines for detecting and minimizing the presence of eccentricities are described.

Chamis, C. C.↗

NASA NTRS1979

Fan-shaped coma, orientation of rotation axis, and surface structure of a cometary nucleus. I - Test of a model on four comets

Pronounced anisotropy in the outgassing from comets, especially from short-period ones, appears to be a factor responsible for the frequent occurrence of a fan-shaped coma, extending in the general direction of the sun. It is suggested that the pattern of deviations from the sunward direction contains information on the orientation of the spin axis and on the time lag in the sublimation process, which in turn provides insight into the nature of the nuclear surface. A simple model of a spherical rotating nucleus is formulated and a trial-and-error technique is devised to determine the axis-orientation constants and a lag angle, a measure of the time lag in units of the rotation period. The method is applied to the comets Encke, Tempel 2, Borrelly, and Schwassmann-Wachmann 3.

Sekanina, Z.↗

NASA NTRS1980

Nonlinear Aeroelastic Equations of Motion of Twisted, Nonuniform, Flexible Horizontal-Axis Wind Turbine Blades

The second-degree nonlinear equations of motion for a flexible, twisted, nonuniform, horizontal axis wind turbine blade were developed using Hamilton's principle. A mathematical ordering scheme which was consistent with the assumption of a slender beam was used to discard some higher-order elastic and inertial terms in the second-degree nonlinear equations. The blade aerodynamic loading which was employed accounted for both wind shear and tower shadow and was obtained from strip theory based on a quasi-steady approximation of two-dimensional, incompressible, unsteady, airfoil theory. The resulting equations had periodic coefficients and were suitable for determining the aeroelastic stability and response of large horizontal-axis wind turbine blades.

Kaza, K. R. V.↗

NASA NTRS1980

On the secular decrease in the semimajor axis of Lageos orbit

The semimajor axis of the Lageos orbit is decreasing secularly at the rate of -1.1 mm/day due to an unknown force. Nine possible mechanisms are investigated. Five of the mechanisms, resonance with the Earth's gravitational field, gravitational radiation, the Poynting-Robertson effect, transfer of spin angular momentum to the orbital angular momentum, and drag from near Earth dust are ruled out because they are too small to require unacceptable assumptions to account for the observed rate. Three other mechanisms, the Yarkovsky effect, the Schach effect, and terrestrial radiation pressure could possibly give the proper order of magnitude for the decay rate, but the characteristic signatures of these perturbations do not agree with the observed secular decrease. Atmospheric drag from a combination of charged and neutral particles is the most likely cause for the orbital decay. This mechanism explains at least 71 percent of the observed rate of decrease of the semimajor axis.

Rubincam, D. P.↗

NASA NTRS1979

Comet Encke - Precession of the spin axis, nongravitational motion, and sublimation

From the observed light curve of P/Encke the jet force from sublimation is calculated both as a (precessing) torque and as a (perturbing) force transverse to the radius vector. An integral iteration is carried out over 59 perihelion passages, 1786-1977, to fit the previously determined nongravitational transverse force and to derive the precession of the spin axis. It is shown that the spin axis turned more than 100 degrees in longitude and almost 30 degrees in latitude from 1786 to 1977, but appears to have been almost fixed in direction for hundreds of revolutions before 1700. It is suggested that ejected meteoroidal debris accumulated on the currently less active hemisphere, insulating it to maintain a low activity level. A tentative rotation period of 6 h 33 min is derived, using Whipple's halo method. The suggested spinup rate is 21 min/century, while the current rate of relative mass loss by sublimation is 0.09% of the comet's mass per revolution. Moreover, the mass of the nucleus is estimated at less than 10 to the 16th grams, and its oblateness at less than 4%.

Whipple, F. L.↗

NASA NTRS1980

Stability of large horizontal-axis axisymmetric wind turbines

The stability of large horizontal axis, axi-symmetric, power producing wind turbines was examined. The analytical model used included the dynamic coupling of the rotor, tower and power generating system. The aerodynamic loading was derived from blade element theory. Each rotor blade was permitted tow principal elastic bending degrees of freedom, one degree of freedom in torsion and controlled pitch as a rigid body. The rotor hub was mounted in a rigid nacelle which may yaw freely or in a controlled manner. The tower can bend in two principal directions and may twist. Also, the rotor speed can vary and may induce perturbation reactions within the power generating equipment. Stability was determined by the eigenvalues of a set of linearized constant coefficient differential equations. All results presented are based on a 3 bladed, 300 ft. diameter, 2.5 megawatt wind turbine. Some of the parameters varied were; wind speed, rotor speed structural stiffness and damping, the effective stiffness and damping of the power generating system and the principal bending directions of the rotor blades. Unstable or weakly stable behavior can be caused by aerodynamic forces due to motion of the rotor blades and tower in the plane of rotation or by mechanical coupling between the rotor system and the tower.

Hirschbein, M. S.↗

NASA NTRS1981

Slosh dynamics of a spin-stabilized spacecraft comprising off-axis tanks filled partially with liquid propellant

The fundamental nonlinear equations of motion were derived and the specialized to a steady-state rotation of the vehicle about a given axis of rotation. A thrust about the spin axis was introduced. A perturbation solution was derived which linearizes the problem. The effect of the centrifugal and coriolis accelerations together with vorticity are implicitly taken into consideration in the formulation. A variational formulation of the associated boundary conditions is presented. For practical cases it is shown that the simple classical pendulum representation for slosh is not very appealing for a spinning spacecraft unless severe restrictions are allowed.

Fontenot, L. L.↗

NASA NTRS1980

A very rapid turning of the plasma-tail axis of comet Bradfield 19791 on 1980 February 6

Schmidt camera photographs of comet Bradfield 19791 obtained at the Joint Observatory for Cometary Research (JOCR) indicate that a rapid change took place in the comet's plasma tail on 1980 February 6. On that date, a sequence of photographs spanning 27.5 minutes shows a 10 deg shift occurring in the plasma-tail axis between the first and last exposures. The speed of this tail-turning event greatly exceeds that of any other known event and even exceeds turning rates for individual tail streamers. An interpretation based on the windsock theory of plasma tails is that the comet entered a region of rapidly changing solar-wind flow direction. While the search for an associated solar-wind event from near-earth spacecraft observations is a future activity, the present analysis shows that a 50 km/s change in the polar component of the solar-wind velocity, from about 30 km/s northward to about 20 km/s southward, would have produced the 10 deg shift in the tail axis.

Brandt, J. C.↗

NASA NTRS1981

Repetitive Q-switched operation of x-axis Nd:La2Be2O5

The repetitive Q-switched performances of an x-axis neodymium-doped lanthanum beryllate (Nd:BeL) laser and the current state-of-the-art Nd:YAG laser are compared. Output beam pulse energy, peak power, pulse width, beam divergence and mode pattern were measured for a gold-plated single elliptical cavity using a xenon flashlamp for excitation. The output of the Nd:BeL laser is found to exceed that of the Nd:YAG laser by a factor of two at Q-switched repetition rates on the order of 1 Hz, due to the smaller stimulated emission cross section of the x-axis Nd:BeL. When the repetition rate is increased to 10 Hz, the Nd:BeL output falls to a level three fifths lower than its low-rate value, while Nd:YAG output remained essentially constant. Analysis reveals that the decrease in output may be attributed to a depolarization loss due to thermally induced stress birefringence.

Tucker, A. W.↗

NASA NTRS1981

Pressure distributions and shock shapes for 12.84 deg/7 deg on-axis and bent-nose biconics in air at Mach 6

Pressure distributions and shock shapes on a spherically blunted, 12.84 deg /7 deg on axis biconic and a spherically blunted, 12.84 deg/7 deg bent nose biconic at Mach 6 in air were measured. The angle of attack, referenced to the axis of aft cone, was varied from 0 deg to 25 deg in nominal 5 deg increments. Two values of free stream Reynolds number based on model length were tested. Predictions from simple, theories and from a supersonic, three dimensional, external invsicid code (STEIN) are compared with measured values. Predicted STEIN shock shapes and windward pressures are in agreement with measured values for both biconics over the present range of angle of attack.

Miller, C. G., III↗

NASA NTRS1982

Effect of rotor configuration on guyed tower and foundation designs and estimated costs for intermediate site horizontal axis wind turbines

Three designs of a guyed cylindrical tower and its foundation for an intermediate size horizontal axis wind turbine generator are discussed. The primary difference in the three designs is the configuration of the rotor. Two configurations are two-blade rotors with teetering hubs - one with full span pitchable blades, the other with fixed pitch blades. The third configuration is a three-bladed rotor with a rigid hub and fixed pitch blades. In all configurations the diameter of the rotor is 38 meters and the axis of rotation is 30.4 meters above grade, and the power output is 200 kW and 400 kW. For each configuration the design is based upon for the most severe loading condition either operating wind or hurricane conditions. The diameter of the tower is selected to be 1.5 meters (since it was determined that this would provide sufficient space for access ladders within the tower) with guy rods attached at 10.7 meters above grade. Completing a design requires selecting the required thicknesses of the various cylindrical segments, the number and diameter of the guy rods, the number and size of soil anchors, and the size of the central foundation. The lower natural frequencies of vibration are determined for each design to ensure that operation near resonance does not occur. Finally, a cost estimate is prepared for each design. A preliminary design and cost estimate of a cantilever tower (cylindrical and not guyed) and its foundation is also presented for each of the three configurations.

Frederick, G. R.↗

NASA NTRS1981

Downwind rotor horizontal axis wind turbine noise prediction

NASA and industry are currently cooperating in the conduct of extensive experimental and analytical studies to understand and predict the noise of large, horizontal axis wind turbines. This effort consists of (1) obtaining high quality noise data under well controlled and documented test conditions, (2) establishing the annoyance criteria for impulse noise of the type generated by horizontal axis wind turbines with rotors downwind of the support tower, (3) defining the wake characteristics downwind of the axial location of the plane of rotation, (4) comparing predictions with measurements made by use of wake data, and (5) comparing predictions with annoyance criteria. The status of work by Hamilton Standard in the above areas which was done in support of the cooperative NASA and industry studies is briefly summarized.

Metzger, F. B.↗

NASA NTRS1982

Simplified aeroelastic modeling of horizontal axis wind turbines

Certain aspects of the aeroelastic modeling and behavior of the horizontal axis wind turbine (HAWT) are examined. Two simple three degree of freedom models are described in this report, and tools are developed which allow other simple models to be derived. The first simple model developed is an equivalent hinge model to study the flap-lag-torsion aeroelastic stability of an isolated rotor blade. The model includes nonlinear effects, preconing, and noncoincident elastic axis, center of gravity, and aerodynamic center. A stability study is presented which examines the influence of key parameters on aeroelastic stability. Next, two general tools are developed to study the aeroelastic stability and response of a teetering rotor coupled to a flexible tower. The first of these tools is an aeroelastic model of a two-bladed rotor on a general flexible support. The second general tool is a harmonic balance solution method for the resulting second order system with periodic coefficients. The second simple model developed is a rotor-tower model which serves to demonstrate the general tools. This model includes nacelle yawing, nacelle pitching, and rotor teetering. Transient response time histories are calculated and compared to a similar model in the literature. Agreement between the two is very good, especially considering how few harmonics are used. Finally, a stability study is presented which examines the effects of support stiffness and damping, inflow angle, and preconing.

Wendell, J. H.↗

NASA NTRS1982

Development of methodology for horizontal axis wind turbine dynamic analysis

Horizontal axis wind turbine dynamics were studied. The following findings are summarized: (1) review of the MOSTAS computer programs for dynamic analysis of horizontal axis wind turbines; (2) review of various analysis methods for rotating systems with periodic coefficients; (3) review of structural dynamics analysis tools for large wind turbine; (4) experiments for yaw characteristics of a rotating rotor; (5) development of a finite element model for rotors; (6) development of simple models for aeroelastics; and (7) development of simple models for stability and response of wind turbines on flexible towers.

Dugundji, J.↗

NASA NTRS1983

Aircraft body-axis rotation measurement system

A two gyro four gimbal attitude sensing system having gimbal lock avoidance is provided with continuous azimuth information, rather than roll information, relative to the magnetic cardinal headings while in near vertical attitudes to allow recovery from vertical on a desired heading. The system is comprised of a means for stabilizing an outer roll gimbal that is common to a vertical gyro and a directional gyro with respect to the aircraft platform which is being angularly displaced about an axis substantially parallel to the outer roll gyro axis. A means is also provided for producing a signal indicative of the magnitude of such displacement as an indication of aircraft heading. Additional means are provided to cause stabilization of the outer roll gimbal whenever the pitch angle of the aircraft passes through a threshold prior to entering vertical flight and destabilization of the outer roll gimbal upon passing through the threshold when departing vertical flight.

Cowdin, K. T.↗

NASA NTRS1984

Crack growth direction in unidirectional off-axis graphite epoxy

An anisotropic elasticity crack tip stress analysis is implemented using three crack extension direction criteria (the normal stress ratio, the tensor polynominal and the strain energy density) to predict the direction of crack extension in unidirectional off axis graphite-epoxy. The theoretical predictions of crack extension direction are then compared with experimental results for 15 deg off axis tensile coupons with center cracks. Specimens of various aspect ratios and crack orientations are analyzed. It is shown that only the normal stress ratio criterion predicts the correct direction of crack growth.

Herakovich, C. T.↗

NASA NTRS1984

Relative potentials of concentrating and two-axis tracking flat-plate photovoltaic arrays for central-station applications

The purpose of this study is to assess the relative economic potentials of concenrating and two-axis tracking flat-plate photovoltaic arrays for central-station applications in the mid-1990's. Specific objectives of this study are to provide information on concentrator photovoltaic collector probabilistic price and efficiency levels to illustrate critical areas of R&D for concentrator cells and collectors, and to compare concentrator and flat-plate PV price and efficiency alternatives for several locations, based on their implied costs of energy. To deal with the uncertainties surrounding research and development activities in general, a probabilistic assessment of commercially achievable concentrator photovoltaic collector efficiencies and prices (at the factory loading dock) is performed. The results of this projection of concentrator photovoltaic technology are then compared with a previous flat-plate module price analysis (performed early in 1983). To focus this analysis on specific collector alternatives and their implied energy costs for different locations, similar two-axis tracking designs are assumed for both concentrator and flat-plate options.

Borden, C. S.↗

NASA NTRS1986

Summary of tower designs for large horizontal axis wind turbines

Towers for large horizontal axis wind turbines, machines with a rotor axis height above 30 meters and rated at more than 500 kW, have varied in configuration, materials of construction, type of construction, height, and stiffness. For example, the U.S. large HAWTs have utilized steel truss type towers and free-standing steel cylindrical towers. In Europe, the trend has been to use only free-standing and guyed cylindrical towers, but both steel and reinforced concrete have been used as materials of construction. These variations in materials of construction and type of construction reflect different engineering approaches to the design of cost effective towers for large HAWTs. Tower designs are the NASA/DOE Mod-5B presently being fabricated. Design goals and requirements that influence tower configuration, height and materials are discussed. In particular, experiences with United States large wind turbine towers are elucidated. Finally, current trends in tower designs for large HAWTs are highlighted.

Frederick, G. R.↗