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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A mathematical model of turbulence in flows with uniform stationary velocity gradients

Certain cases of turbulence as a postinstability state of a fluid in motion modeled by the introduction of multivalued velocity fields are examined. The turbulence is regarded as occurring in the form of random pulsations which grow until the external energy input in the average flow is balanced by the dissipated energy of pulsations by means of turbulent friction. Closed form analytic solutions are shown to be possible when the considered velocity fields, the pulsation velocity and the fluid velocity, are decoupled.

Zak, M. A.↗
NASA NTRS1982

Coded error probability evaluation for antijam communication systems

We present a general union-Chernoff bound on the bit error probability for coded communication systems and apply it to examples of antijam systems. The key feature of this bound is the decoupling of the coding aspects of the system from the remaining part of the communication system which includes jamming, suboptimum detectors, and arbitrary decoding metrics which may or may not use jammer state knowledge

Omura, J. K.↗
NASA NTRS1982

A large normal-fault earthquake at the junction of the Tonga trench and the Louisville ridge

Long-period vertical-component Rayleigh waves are inverted in order to determine the source mechanism of the October 10, 1977 earthquake that occurred in the oceanic plate at the junction of the Tonga-Kermadec trench systems with the aseismic Louisville ridge. The cause was predominantly normal faulting on a plane striking roughly parallel to the trench, with a seismic moment of 1.7 x 10 to the 27th dyn cm. A focal depth of 20 km is determined by waveform modeling, but the actual rupture may have extended to 30 or 40 km. Two sources separated by 16 s comprised the event, which experienced an inferred rupture velocity of 3.5 km/sec. The interpretation that the earthquake was caused by gravitational pull due to the sinking slab implies that the Louisville ridge causes some degree of local decoupling between the plates. This event may be associated with the breakup of the Osbourn seamount. Alternatively, the earthquake may have resulted from tensional plate bending stress, as implied by its relatively shallow depth.

Eissler, H.↗
NASA NTRS1982

Parachute suspended solar pointing control system

A high altitude parachute suspended solar pointing control system has been developed and flight tested for use in the altitude range of 30 to 70 kilometers. This development provides an opportunity for extended solar observations at altitudes higher than that attainable by helium balloons. The new system utilizes the NASA high altitude cross parachute to slow the descent of a rocket launched payload allowing observations in the region of interest. Solar pointing is established by using solar sensors in conjunction with a servo controlled platform and cold gas thrusters for payload roll control. The inherent spin of the cross parachute is decoupled by a swivel joint attached to the parachute suspension lines. This paper describes the design, test and flight performance of the new system.

Sakoda, G. T.↗
NASA NTRS1984

Scanning XeCl Laser

Applications of narrow-bandwith laser include remote sensing and high-resolution spectroscopy. Scanning XeCl oscillator/ring-laser amplifier produces narrow spectral bandwidth (less than 0.003nm) over tuning range of 307.5 to 308.5 nm. Ring configuration has following advantages: oscillator decoupled from amplifier, output unidirectional, output beam uniform and inexpensive optics allow variable output coupling and cavity length.

Laudenslager, J. B.↗
NASA NTRS1982

Design of a helicopter autopilot by means of linearizing transformations

An automatic flight control systems design methods for aircraft that have complex characteristics and operational requirements, such as the powered lift STOL and V/STOL configurations are discussed. The method is effective for a large class of dynamic systems that require multiaxis control and that have highly coupled nonlinearities, redundant controls, and complex multidimensional operational envelopes. The method exploits the possibility of linearizing the system over its operational envelope by transforming the state and control. The linear canonical forms used in the design are described, and necessary and sufficient conditions for linearizability are stated. The control logic has the structure of an exact model follower with linear decoupled model dynamics and possibly nonlinear plant dynamics. The design method is illustrated with an application to a helicopter autopilot design.

Meyer, G.↗
NASA NTRS1981

Chromospheric and coronal heating mechanisms

Dissipation mechanisms in the chromosphere were examined. The problem of a heat flux from a cool region of the star to a hot region of the star, which violates our second law of thermodynamics is discussed. It is suggested that this is caused by a nonthermal energy flux. While convection transports the thermal flux, a very small percentage is converted into a nonthermal flux. The major part of the outgoing convective energy is turned back into the radiation field which gets decoupled from the star when the star becomes transparent and the radiant energy escapes to space. The small nonthermal flux is transmitted upwards and becomes the dominant energy flux still coupled to the star. The importance of recycling of energy via advection and conduction is emphasized.

Leibacher, J.↗
NASA NTRS1983

Efficient Algorithms for Computing Trim and Small-Disturbance Equations of Motion of Aircraft Coordinated and Uncoordinated, Steady, Steep Turns

The development of computational algorithms that permit efficient calculation of aircraft trim states and of the associated small disturbance equations of motion for a systematic investigation of the statics and dynamics of aircraft in coordinated and uncoordinated, steady, steep turning flight is reported. The efficiency in the trim computation is realized by decoupling the governing equations. The small disturbance equations of motion, which are given in a general body axis system, include aerodynamic acceleration derivatives; they are cast in a familiar first order, vector matrix format of modern system theory. These algorithms were applied to a variety of rotorcraft simulation models. Results pertaining to a simulated hingeless rotor helicopter are also presented

Chen, Robert T. N.↗
NASA NTRS1983

Research on the control of large space structures

The research effort on the control of large space structures at the University of Houston has concentrated on the mathematical theory of finite-element models; identification of the mass, damping, and stiffness matrix; assignment of damping to structures; and decoupling of structure dynamics. The objective of the work has been and will continue to be the development of efficient numerical algorithms for analysis, control, and identification of large space structures. The major consideration in the development of the algorithms has been the large number of equations that must be handled by the algorithm as well as sensitivity of the algorithms to numerical errors.

Denman, E. D.↗
NASA NTRS1982

Experimental gas-fired pulse-combustion studies

Experimental studies conducted at Argonne National Laboratory on a gas-fired, water-cooled, Helmholtz-type pulse combustion burner are discussed. In addition to the experimental work, information is presented on the evolution of pulse combustion, the types of pulse combustion burners and their applications, and the types of fuels used. Also included is a survey of other pertinent studies of gas-fired pulse combustion. The burner used in the Argonne research effort was equipped with adjustable air and gas flapper valves and was operated stably over a heat-input range of 30,000 to 200,000 Btu/h. The burner's overall heat transfer in the pulsating mode was 22 to 31% higher than when the unit was operated in the steady mode. Important phenomena discussed include (1) effects on performance produced by inserting a corebustor to change tailpipe diameter, (2) effects observed following addition of an air-inlet decoupling chamber to the unit, and (3) occurrence of carbon monoxide in the exhaust gas.

Blomquist, C. A.↗
NASA NTRS1983

Modeling of flexible structures for active control

If a flexible structure has a plane of symmetry, the equations of motion can be split into two uncoupled sets, one for symmetrical motions and one for anti-symmetric motions. If there are m controls, it is often convenient to assign the linear combinations of controls that enter into the m lowest frequency modes and new controls. As an example the feed-support structure of a spacecraft antenna is considered. It is modeled as a tetrahedron made up of flexible bars and connected to the spacecraft by six short flexible legs containing force actuators and displacement sensors. Due to the three-sided symmetry of this structure, both the symmetric and the anti-symmetric equations of motion can be decoupled into two subsystems. The resulting four subsystems are: (1) pitch/fore-aft motions with four degrees of freedom (DOF), two controls, and one output (the fore-aft motion of the feed); (2) vertical motions with three DOF, one control, and one output (the vertical motion of the feed); (3) roll/lateral motions with four DOF, two controls, and one output (the lateral motions of the feed); and (4) yaw motion with one DOF, one control, and no output (the feed does not move during yaw motion).

Bryson, A. E., Jr.↗
NASA NTRS1982

Implicit numerical integration for periodic solutions of autonomous nonlinear systems

A change of variables that stabilizes numerical computations for periodic solutions of autonomous systems is derived. Computation of the period is decoupled from the rest of the problem for conservative systems of any order and for any second-order system. Numerical results are included for a second-order conservative system under a suddenly applied constant load. Near the critical load for the system, a small increment in load amplitude results in a large increase in amplitude of the response.

Thurston, G. A.↗
NASA NTRS1982

Single event upset vulnerability of selected 4K and 16K CMOS static RAM's

Upset thresholds for bulk CMOS and CMOS/SOS RAMS were deduced after bombardment of the devices with 140 MeV Kr, 160 MeV Ar, and 33 MeV O beams in a cyclotron. The trials were performed to test prototype devices intended for space applications, to relate feature size to the critical upset charge, and to check the validity of computer simulation models. The tests were run on 4 and 1 K memory cells with 6 transistors, in either hardened or unhardened configurations. The upset cross sections were calculated to determine the critical charge for upset from the soft errors observed in the irradiated cells. Computer simulations of the critical charge were found to deviate from the experimentally observed variation of the critical charge as the square of the feature size. Modeled values of series resistors decoupling the inverter pairs of memory cells showed that above some minimum resistance value a small increase in resistance produces a large increase in the critical charge, which the experimental data showed to be of questionable validity unless the value is made dependent on the maximum allowed read-write time.

Kolasinski, W. A.↗
NASA NTRS1983

Dynamics of magnetosphere-ionosphere coupling including turbulent transport

The dynamics of magnetosphere-ionosphere coupling has been investigated by means of a two-dimensional two-fluid MHD model including anomalous resistivity. When field-aligned current is generated on auroral field lines, the disturbance propagates toward the ionosphere in the form of a kinetic Alfven wave. When the current exceeds a critical value, microscopic turbulence is produced, which modifies the propagation of the Alfven wave. This process is modeled by a nonlinear collision frequency, which increases with the excess of the drift velocity over the critical value. The system evolves toward an electrostatic structure, with the perpendicular electric field having a shorter scale than the field-aligned current. The approach to a steady state is strongly dependent on the presence or absence of the turbulence and on the boundary conditions imposed in the generator. As current is increased or scale size is decreased, the turbulent region reflects and absorbs most of the Alfven wave energy, decoupling the generator from the ionosphere.

Lysak, R. L.↗
NASA NTRS1983

Control of the constrained planar simple inverted pendulum

Control of a constrained planar inverted pendulum by eigenstructure assignment is considered. Linear feedback is used to stabilize and decouple the system in such a way that specified subspaces of the state space are invariant for the closed-loop system. The effectiveness of the feedback law is tested by digital computer simulation. Pre-compensation by an inverse plant is used to improve performance.

Bavarian, B.↗
NASA NTRS1982

Controller design for flexible, distributed parameter mechanical arms via combined state space and frequency domain techniques

The potential benefits of the ability to control more flexible mechanical arms are discussed. A justification is made in terms of speed of movement. A new controller design procedure is then developed to provide this capability. It uses both a frequency domain representation and a state variable representation of the arm model. The frequency domain model is used to update the modal state variable model to insure decoupled states. The technique is applied to a simple example with encouraging results.

Book, W. J.↗
NASA NTRS1983

Calculation of unsteady small disturbance transonic flow at arbitrary reduced frequency using an extended integral equation method.

A novel analytical-numerical method for calculating unsteady small disturbance transonic flow over airfoils has been developed. The method uses an extended integral equation technique, based on both the velocity potential and the acceleration potential, to predict unsteady aerodynamic loading on airfoils oscillating in subcritical transonic free stream conditions. The formulation is an extension of the work of Sivaneri and Harris (1980) for steady, non-lifting flows and utilizes the linear theory of Landahl (1961) for decoupling of steady and unsteady components. The analytical-numerical procedure involves several intnegrating schemes and applies to general frequencies of oscillations. The technique is illustrated by computing the transonic flow about parabolic arc airfoils. Specific unsteady results for reduced frequencies based on semi-chord of 0.01, 0.1, 0.3, 0.4 and 0.6 are given. Comparison of results with those obtained by an ADI finite difference scheme is made.

Lamah, C. A.↗
NASA NTRS1983

Decay of long-lived particles in the early universe

It is pointed out that radiative decay of massive fermions can distort the cosmic background radiation. The present investigation is concerned with a study of decay lifetimes in the range from 10 to 100,000 years. Attention is given to the physics involved in determining the effect of radiative decay of massive fermions on observed photon backgrounds. The case of particles which decoupled when the effective number of species in equilibrium was in the range from 50 to 100 is considered, and constraints on particle masses and lifetimes are placed on the basis of observed photon fluxes. This approach provides results with special applications to particles predicted by supersymmetry theories and to right-handed neutrinos. Implications for galaxy formation are also discussed.

Silk, J.↗