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At least 1,693 records · Page 94

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NASA NTRS1981

Helicopter rotor trailing edge noise

A two dimensional section of a helicopter main rotor blade was tested in an acoustic wind tunnel at close to full-scale Reynolds numbers to obtain boundary layer data and acoustic data for use in developing an acoustic scaling law and testing a first principles trailing edge noise theory. Results were extended to the rotating frame coordinate system to develop a helicopter rotor trailing edge noise prediction. Comparisons of the calculated noise levels with helicopter flyover spectra demonstrate that trailing edge noise contributes significantly to the total helicopter noise spectrum at high frequencies. This noise mechanism is expected to control the minimum rotor noise. In the case of noise radiation from a local blade segment, the acoustic directivity pattern is predicted by the first principles trailing edge noise theory. Acoustic spectra are predicted by a scaling law which includes Mach number, boundary layer thickness and observer position. Spectrum shape and sound pressure level are also predicted by the first principles theory but the analysis does not predict the Strouhal value identifying the spectrum peak.

Schlinker, R. H.↗

NASA NTRS1982

Field interface module software description

The software for the field interface module (FIM) is designed to fully optimize the hardware capabilities of the FIM. Software routines exist which enable the FIM to monitor data, control components, communicate with an uplink computer, and perform real-time and local self-diagnostics. The FIM program is not dependent upon any particular uplink computer and is adaptable to various applications with a miniumum of modifications.

Valtier, H.↗

NASA NTRS1985

Numerical simulation of boundary layer excitation by surface heating/cooling

The concept of active control of growing disturbances in an unstable compressible flow by using time periodic, localized surface heating is studied numerically. The simulations are calculated by a fourth-order accurate solution of the compressible, laminar Navier-Stokes equations. Fourth-order accuracy is particularly important for this problem because the solution must be computed over many wavelengths. The numerical results demonstrate the growth of an initially small fluctuation into the nonlinear regime where a local breakdown into smaller scale disturbances can be observed. It is shown that periodic surface heating over a small strip can reduce the level of the fluctuation provided that the phase of the heating current is properly chosen.

Bayliss, A.↗

NASA NTRS1986

Numerical simulation of boundary-layer excitation by surface heating/cooling

Bayliss, A.↗

NASA NTRS1987

The evolution of the Mission Control Center

A discussion of the function and history of the Space Shuttle Mission Control Center describes the communications and data processing facilities, with an emphasis on display system changes. The current display system will be replaced by new technology as part of a project called the Mission Control Center Upgrade. That upgrade will implement a set of high-performance Local Area Networks (LANs) which transmit real-time and historical data to engineering workstations for processing and display. The benefits from such an architecture include a more flexible software development environment and a more modular system which will promote future upgrades. The proposed upgrade architecture is explored, and the engineering and project management challenges of that effort are described, emphasizing the local area networks task.

Kearney, Michael W., III↗

NASA NTRS1988

Analysis and design of gain scheduled control systems

Gain scheduling, as an idea, is to construct a global feedback control system for a time varying and/or nonlinear plant from a collection of local time invariant designs. However in the absence of a sound analysis, these designs come with no guarantees on the robustness, performance, or even nominal stability of the overall gain schedule design. Such an analysis is presented for three types of gain scheduling situations: (1) a linear parameter varying plant scheduling on its exogenous parameters, (2) a nonlinear plant scheduling on a prescribed reference trajectory, and (3) a nonlinear plant scheduling on the current plant output. Conditions are given which guarantee that the stability, robustness, and performance properties of the fixed operating point designs carry over to the global gain scheduled designs, such as the scheduling variable should vary slowly and capture the plants nonlinearities. Finally, an alternate design framework is proposed which removes the slowing varying restriction or gain scheduled systems. This framework addresses some fundamental feedback issues previously ignored in standard gain.

Shamma, Jeff S.↗

NASA NTRS1988

Shock-induced melting and shear banding in single-crystal NaCl

Radiative color temperatures were measured in single-crystal sodium chloride under shock compression parallel to 100-line orientation over a pressure range from 20 to 35 GPa. Color temperatures from 2500 to 4500 K and emittances from 0.003 to 0.3 were determined by fitting observed spectra (450-850 nm) to the Planck greybody radiation law. These data support a heterogeneous shock deformation model of shocked halite in this pressure range. A 2500-K temperature rise, of unknown origin, is observed over the B1-B2 mixed phase region from 25 to 30 GPa. Assuming that shock deformation occurs via yielding in localized planar zones which become melt and the melting temperature at high pressure controls the temperature, it is inferred that the temperature of the B2 fusion curve from 30 to 35 GPa rises from 3200 to 3300 K. The B1-B2-liquid triple point is predicted to be at a temperature of 2250 K and 23.5 GPa.

Schmitt, Douglas R.↗

NASA NTRS1988

Source localization of Jupiter's Io dependent radio emissions

The peak frequencies of the Io-dependent part of the Jovian emissions are compared with the surface gyrofrequency determined from Jovian magnetic models in order to localize the source of Jovian radio emissions. The bulk of the Io-controlled emissions was found to be delayed by up to 70 deg of equatorial longitude from the predicted instantaneous position of the Io flux tube, with the L and S emissions both displaying this same unexpected behavior. It is suggested that the source of these emissions is delayed substantially with respect to Io either as an Alfven-wave delay or because of errors in the magnetic field models.

Aubier, Monique G.↗

NASA NTRS1989

A decentralized approach to vibration suppression in segmented reflector telescopes

The problem of vibration suppression in segmented reflector telescopes is considered. The decomposition of the structure into smaller components is discussed, and control laws for vibration suppression as well as conditions for stability at the local level are derived. These conditions and the properties of the interconnecting patterns are then utilized to obtain sufficient conditions for global stability.

Ryaciotaki-Boussalis, Helen A.↗

NASA NTRS1989

Interset: A natural language interface for teleoperated robotic assembly of the EASE space structure

A teleoperated robot was used to assemble the Experimental Assembly of Structures in Extra-vehicular activity (EASE) space structure under neutral buoyancy conditions, simulating a telerobot performing structural assembly in the zero gravity of space. This previous work used a manually controlled teleoperator as a test bed for system performance evaluations. From these results several Artificial Intelligence options were proposed. One of these was further developed into a real time assembly planner. The interface for this system is effective in assembling EASE structures using windowed graphics and a set of networked menus. As the problem space becomes more complex and hence the set of control options increases, a natural language interface may prove to be beneficial to supplement the menu based control strategy. This strategy can be beneficial in situations such as: describing the local environment, maintaining a data base of task event histories, modifying a plan or a heuristic dynamically, summarizing a task in English, or operating in a novel situation.

Boorsma, Daniel K.↗

NASA NTRS1990

Modal identification of a deployable space truss

Work performed under a collaborative research effort between NASA and the German Aerospace Research Establishment (DLR) is summarized. The objective is to develop and demonstrate advanced technology for system identification of future large space structures. Recent experiences using the eigensystem realization algorithm (ERA) for modal identification of Mini-Mast are reported. Mini-Mast is a 20-meter-long deployable space truss used for structural dynamics and active-vibration control research at the NASA Langley Research Center. Due to nonlinearities and numerous local modes, modal identification of Mini-Mast proved to be surprisingly difficult. Methods available with ERA for obtaining detailed, high-confidence results are illustrated.

Schenk, Axel↗

NASA NTRS1993

Hypervelocity impact facility for simulating materials exposure to impact by space debris

As a result of man's venturing into space, the local debris contributed by his presence exceeds, at some orbital altitudes, that of the natural component. Man's contribution ranges from fuel residue to large derelect satellites that weigh many kilograms. Current debris models are able to predict the growth of the problem and suggest that spacecraft must employ armor or bumper shields for some orbital altitudes now, and that, the problem will become worse as a function of time. The practical upper limit to the velocity distribution is on the order of 40 km/s and is associated with the natural environment. The maximum velocity of the man-made component is in the 14-16 km/s range. The Long Duration Exposure Facility (LDEF) has verified that the 'high probability of impact' particles are in the microgram to milligram range. These particles can have significant effects on coatings, insulators, and thin metallic layers. The surface of thick materials becomes pitted and the local debris component is enhanced by ejecta from the debris spectrum in a controlled environment. The facility capability is discussed in terms of drive geometry, energetics, velocity distribution, diagnostics, and projectile/debris loading. The facility is currently being used to study impact phenomena on Space Station Freedom's solar array structure, other solar array materials, potential structural materials for use in the station, electrical breakdown in the space environment, and as a means of clarifying or duplicating the impact phenomena on the LDEF surfaces. The results of these experiments are described in terms of the mass/velocity distribution incident on selected samples, crater dynamics, and sample geometry.

Rose, M. F.↗

NASA NTRS1993

Development of generalized 3-D braiding machines for composite preforms

The operating principles of two prototype braiding machines for the production of generalized braid patterns are described. Both processes afford previously unachievable control of the interlacing of fibers within a textile structure that make them especially amenable to the fabrication of textile preforms for composite materials. They enable independent control of the motion of the individual fibers being woven, thereby enabling the greatest possible freedom in controlling fiber orientation within a structure. This freedom enables the designer to prescribe local fiber orientation to better optimize material performance. The processes have been implemented on a very small scale but at a level that demonstrates their practicality and the soundness of the principles governing their operation.

Huey, Cecil O., Jr.↗

NASA NTRS1994

Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs

Strong x-ray emission detected in Large Magellanic Cloud (LMC) superbubbles has been explained as the result of interior supernova remnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot be found using conventional criteria. We thus investigate the possibility of using the interstellar absorption properties in the ultraviolet (UV) as a diagnostic of hidden SNR shocks. The International Ultraviolet Explorer (IUE) archives provide the database for this pilot study. They contain high-dispersion spectra of several stars in x-ray bright superbubbles. To distinguish the effects of SNR shocks from those of local stellar winds and a global hot halo around the LMC, we included control objects in different environments. We find that almost all interstellar absorption properties can be explained by the interstellar environment associated with the objects. Summarizing the two most important results of this study: (1) a large velocity shift between the high-ionization species (C IV and Si IV) and the low-ionization species (S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however, the absence of a velocity shift does not preclude the existence of SNR shocks; (2) there is no evidence that the LMC is uniformly surrounded by hot gas; hot gas is preferentially found associated with large interstellar structures like superbubbles and supergiant shells, which may extend to large distances from the plane.

Chu, You-Hua↗

NASA NTRS1996

A Numerical Study of Self-Similarity in a Turbulent Plane Wake Using Large-Eddy Simulation

Turbulent wakes are known to develop self-similarly sufficiently far downstream from obstacles that generate them. It has long been assumed that the spreading rate of the wake in the self-similar regime is independent of the details of the body generating the wake, being dependent only on the total drag (or momentum deficit). This assumption seems to be in contradiction with some recent experiments. In this study we attempt to complement these experimental investigations through a numerical study of a time-developing wake. A numerical study has the advantage of eliminating many of the uncontrolled factors present in experiments and allowing precise control of initial conditions. Large-eddy simulations employing the recently developed dynamic localization model are used to extend previous results from direct numerical simulations. The large-eddy simulation results are compared to the direct numerical simulation database, wherever such comparisons are feasible, as a check of the method. Like the experiments, the large-eddy simulations suggest that non-unique self-similar states, characterized by different spreading rates and turbulent statistics, are possible and that they can be maintained for significant time periods. The study also demonstrates the predictive capability of the dynamic localization subgrid model.

Ghosal, Sandip↗

NASA NTRS1997

Reduction of Helicopter Blade-Vortex Interaction Noise by Active Rotor Control Technology

Helicopter blade-vortex interaction noise is one of the most severe noise sources and is very important both in community annoyance and military detection. Research over the decades has substantially improved basic physical understanding of the mechanisms generating rotor blade-vortex interaction noise and also of controlling techniques, particularly using active rotor control technology. This paper reviews active rotor control techniques currently available for rotor blade vortex interaction noise reduction, including higher harmonic pitch control, individual blade control, and on-blade control technologies. Basic physical mechanisms of each active control technique are reviewed in terms of noise reduction mechanism and controlling aerodynamic or structural parameters of a blade. Active rotor control techniques using smart structures/materials are discussed, including distributed smart actuators to induce local torsional or flapping deformations, Published by Elsevier Science Ltd.

Yu, Yung H.↗

NASA NTRS2001

Atmospheric Constraints on Landing Site Selection

The Martian atmosphere is a significant part of the environment that the Mars Exploration Rovers (MER) will encounter. As such, it imposes important constraints on where the rovers can and cannot land. Unfortunately, as there are no meteorological instruments on the rovers, there is little atmospheric science that can be accomplished, and no scientific preference for landing sites. The atmosphere constrains landing site selection in two main areas, the entry descent and landing (EDL) process and the survivability of the rovers on the surface. EDL is influenced by the density profile and boundary layer winds (up to altitudes of 5 to 10 km). Surface survivability involves atmospheric dust, temperatures and winds. During EDL, the atmosphere is used to slow the lander down, both ballistically and on the parachute. This limits the maximum elevation of the landing site to -1.3 km below the MOLA reference aeroid. The landers need to encounter a sufficiently dense atmosphere to be able to stop, and the deeper the landing site, the more column integrated atmosphere the lander can pass through before reaching the surface. The current limit was determined both by a desire to be able to reach the hematite region and by a set of atmosphere models we developed for EDL simulations. These are based on Thermal Emission Spectrometer (TES) atmospheric profile measurements, Ames Mars General Circulation Model (MGCM) results, and the 1-D Ames GCM radiative/convective model by J. Murphy. The latter is used for the near surface diurnal cycle. The current version of our model encompasses representative latitude bands, but we intend to make specific models for the final candidate landing sites to insure that they fall within the general envelope. The second constraint imposed on potential landing sites through the EDL process is the near surface wind. The wind in the lower approximately 5 km determines the horizontal velocity that the landers have when they land. Due to the mechanics of the landing process, the total velocity (including both the horizontal and vertical components) determines whether or not the landers are successful. Unfortunately, the landing system has no easy way to nullify any horizontal velocity imparted by the wind, so the landing sites selected need to have as little wind as possible. In addition to the mean wind velocity, the landing system is sensitive to vertical wind shear in the lowest kilometer or so. Wind shear can deflect the retro rockets (RADs) from their nominal vertical orientation producing unwanted horizontal spacecraft velocities. Both mean velocity and wind shear are dominated by the the local topography and other surface properties (in particular albedo and thermal inertia which control the surface temperature). This is seen even in simplified 2-D mesoscale models. The effects in a fully 3-D model are expected to he even more topographically dependent. In particular there is potential for wind channeling in canyons and other terrain features. Boundary layer winds and wind shear are currently being modeled based on terrestrial data and boundary layer scaling laws modified for Martian conditions. We hope to supplement this with mesoscale model results (from several sources) once the number of landing sites is reduced to a manageable number.

Kass, David M.↗

NASA NTRS1996

Fuzzy Q-Learning for Generalization of Reinforcement Learning

Fuzzy Q-Learning, introduced earlier by the author, is an extension of Q-Learning into fuzzy environments. GARIC is a methodology for fuzzy reinforcement learning. In this paper, we introduce GARIC-Q, a new method for doing incremental Dynamic Programming using a society of intelligent agents which are controlled at the top level by Fuzzy Q-Learning and at the local level, each agent learns and operates based on GARIC. GARIC-Q improves the speed and applicability of Fuzzy Q-Learning through generalization of input space by using fuzzy rules and bridges the gap between Q-Learning and rule based intelligent systems.

Berenji, Hamid R.↗