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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Numerical Comparison of Active Acoustic and Structural Noise Control in a Stiffened Double Wall Cylinder

The active acoustic and structural noise control characteristics of a double wall cylinder with and without ring stiffeners were numerically evaluated. An exterior monopole was assumed to acoustically excite the outside of the double wall cylinder at an acoustic cavity resonance frequency. Structural modal vibration properties of the inner and outer shells were analyzed by post-processing the results from a finite element analysis. A boundary element approach was used to calculate the acoustic cavity response and the coupled structural-acoustic interaction. In the frequency region of interest, below 500 Hz, all structural resonant modes were found to be acoustically slow and the nonresonant modal response to be dominant. Active sound transmission control was achieved by control forces applied to the inner or outer shell, or acoustic control monopoles placed just outside the inner or outer shell. A least mean square technique was used to minimize the interior sound pressures at the nodes of a data recovery mesh. Results showed that single acoustic control monopoles placed just outside the inner or outer shells resulted in better sound transmission control than six distributed point forces applied to either one of the shells. Adding stiffeners to the double wall structure constrained the modal vibrations of the shells, making the double wall stiffer with associated higher modal frequencies. Active noise control obtained for the stiffened double wall configurations was less than for the unstiffened cylinder. In all cases, the acoustic control monopoles controlled the sound transmission into the interior better than the structural control forces.

Grosveld, Ferdinand W.↗
NASA NTRS1980

Observations of electron gyroharmonic waves and the structure of the Io torus

Narrow-banded emissions were observed by the Planetary Radio Astronomy experiment on the Voyager 1 spacecraft as it traversed the Io plasma torus. These waves occur between harmonics of the electron gyrofrequency and are the Jovian analogue of electrostatic emissions observed and theoretically studied for the terrestrial magnetosphere. The observed frequencies always include the component near the upper hybrid resonant frequency, (fuhr) but the distribution of the other observed emissions varies in a systematic way with position in the torus. A refined model of the electron density variation, based on identification of the fuhr line, is included. Spectra of the observed waves are analyzed in terms of the linear instability of an electron distribution function consisting of isotropic cold electrons and hot losscone electrons. The positioning of the observed auxiliary harmonics with respect to fuhr is shown to be an indicator of the cold to hot temperature ratio. It is concluded that this ratio increases systematically by an overall factor of perhaps 4 or 5 between the inner and outer portions of the torus.

Birmingham, T. J.↗
NASA NTRS1982

Extending the frequency of response of lightly damped second order systems: Application to the drag force anemometer

It is shown that a conventional electronic frequency compensator does not provide adequate compensation near the resonant frequency of a lightly damped second order system, such as the drag force anemometer. The reason for this is discussed, and a simple circuit modification is presented which overcomes the difficulty. The improvement is shown in theoretical frequency response curves as well as in the experimental results from some typical drag force anemometers.

Fralick, G. C.↗
NASA NTRS1992

Comparison of analysis and experiment for gearbox noise

Low contact ratio spur gears were tested in the NASA gear-noise rig to study the noise radiated from the top of the gearbox. Experimental results were compared with a NASA acoustics code to validate the code for predicting transmission noise. The analytical code is based on the boundary element method (BEM) which models the gearbox top as a plate in an infinite baffle. Narrow band vibration spectra measured at 63 nodes on the gearbox top were used to produce input data for the BEM model. The BEM code predicted the total sound power based on the measured vibration. The measured sound power was obtained from an acoustic intensity scan taken near the surface of the gearbox at the same 63 nodes used for vibration measurement. Analytical and experimental results were compared at four different speeds for sound power at each of the narrow band frequencies over the range of 400 to 3200 Hz. Results are also compared for the sound power level at meshing frequency plus three sideband pairs and at selected gearbox resonant frequencies. The difference between predicted and measure sound power is typically less than 3 dB with the predicted value generally less than the measured value.

Oswald, Fred B.↗
NASA NTRS1940

Resonance vibrations in intake and exhaust pipes of in-line engines III : the inlet process of a four-stroke-cycle engine

Using a previously developed method, the boundary process of four-stroke-cycle engines are set up. The results deviate considerably from those obtained under the assumption that the velocity fluctuation is proportional to the cylinder piston motion. The deviation is less at the position of resonance frequencies. By the method developed, the effect of the resonance vibrations on the volumetric efficiency can be demonstrated.

Lutz, O↗
NASA NTRS1997

Characteristics of the NASA-Ames Laminar Flow Supersonic Wind Tunnel for Unique Mach 1.6 Transition Studies

Flow quality measurements have been performed in the unique Laminar Flow Supersonic Wind Tunnel (LFSWT) to examine both mean and dynamic characteristics. The intent was to provide the necessary flow information about this ground test facility, to support meaningful transition research at Mach 1.6 and flight unit Reynolds numbers. This paper is intended to assist other experimentalists with similar goals of characterizing low-supersonic test environments. An array of instrumentation has been used to highlight the importance of proper selection of pressure instruments and data acquisition procedures. We conclude that the test section is low-disturbance (based on classical standards of pressure disturbances less than 0.1% with no specified data bandwidth), and has uniform flow. This is confirmation that the quiet design features of the LFSWT are effective. However, characterization of the test section flow over a 0.25k-5Ok bandwidth shows that the disturbance levels can be greater than classical standards particularly for stagnation pressures less than 9.5 psia (0.65 bar) with low stagnation temperatures. Variability of the flow disturbances in the settling chamber and test section is contained in a narrow frequency bandwidth below 5k Hz, which is associated with resonant frequencies from the pressure reduction system. So far, these disturbances have not impacted transition along the tunnel walls or a 10 degrees cone. However, continual vigilance is required to maintain a known low-disturbance environment for transition research in the LFSWT. Furthermore, the formation of standards for flow quality measurements is strongly recommended, so that transition research can be better isolated from tunnel disturbances.

Wolf, Stephen W. D.↗
NASA NTRS2010

Tunable Reduced Size Planar Folded Slot Antenna Utilizing Varactor Diodes

A tunable folded slot antenna that utilizes varactor diodes is presented. The antenna is fabricated on Rogers 6006 Duriod with a dielectric constant and thickness of 6.15 and 635 m, respectively. A copper cladding layer of 17 m defines the antenna on the top side (no ground on backside). The antenna is fed with a CPW 50 (Omega) feed line, has a center frequency of 3 GHz, and incorporates Micrometrics microwave hyper-abrupt 500MHV varactors to tune the resonant frequency. The varactors have a capacitance range of 2.52 pF at 0 V to 0.4 pF at 20 V; they are placed across the radiating slot of the antenna. The tunable 10 dB bandwidth of the 3 GHz antenna is 150 MHz. The varactors also reduce the size of the antenna by 30% by capacitively loading the resonating slot line. At the center frequency, 3 GHz, the antenna has a measured return loss of 44 dB and a gain of 1.6 dBi. Full-wave electromagnetic simulations using HFSS are presented that validate the measured data. Index Terms capacitive loading, Duriod, folded slot antenna, varactor.

Scardelletti, Maximilian C.↗
NASA NTRS1986

Power conversion distribution system using a resonant high-frequency AC link

Static power conversion systems based on a resonant high frequency (HF) link offers a significant reduction in the size and weight of the equipment over that achieved with conventional approaches, especially when multiple sources and loads are to be integrated. A faster system response and absence of audible noise are the other principal characteristics of such systems. A conversion configuration based on a HF link which is suitable for applications requiring distributed power is proposed.

Sood, P. K.↗
NASA NTRS1977

A CW ultrasonic bolt-strain monitor

There exists a need for a relatively inexpensive system for measuring strain in bolts. The torque wrench is one technique for straining bolts which has been widely applied. Unfortunately, friction in the bolt threads and between the nut and the work tend to make such a simple system inaccurate. In practice, a torque wrench is unacceptable for many situations where strain is critical. In this article, an ultrasonic technique is described which can indicate changes in bolt strain to better than one part in 10,000. The technique is based on the one-dimensional propagating-ultrasonic-wave model and uses a new ultrasonic instrument called a Reflection Oscillator Ultrasonic Spectrometer which is a closed-loop feedback marginal-oscillator system that frequency locks the device to the peak of a mechanical resonance in the bolt. The instrument indicates a shift in the bolt resonance frequency due to elongation and changes in velocity of sound due to strain. Data are presented comparing a standard torque wrench to the ultrasonic monitor for different measured stresses on the bolt as well as for different bolt conditions. The strain instrument can be used to monitor changing stresses, to measure material properties and may be applied as a strain gage or load cell.

Heyman, J. S.↗
NASA NTRS1982

Study of the damping characteristics of general aviation aircraft panels and development of computer programs to calculate the effectiveness of interior noise control treatment, part 1

Tests were carried out on 20 inch x 20 inch panels at different test conditions using free-free panels, clamped panels, and panels as installed in the KU-FRL acoustic test facility. Tests with free-free panels verified the basic equipment set-up and test procedure. They also provided a basis for comparison. The results indicate that the effect of installed panels is to increase the damping ratio at the same frequency. However, a direct comparison is not possible, as the fundamental frequency of a free-free panel differs from the resonance frequency of the panel when installed. The damping values of panels installed in the test facility are closer to the damping values obtained with fixed-fixed panels. Effects of damping tape, stiffeners, and bonded and riveted edged conditions were also investigated. Progress in the development of a simple interior noise level control program is reported.

Navaneethan, R.↗
NASA NTRS1993

Microwave characterization of slotline on high resistivity silicon for antenna feed network

Conventional silicon wafers have low resistivity and consequently unacceptably high value of dielectric attenuation constant. Microwave circuits for phased array antenna systems fabricated on these wafers therefore have low efficiency. By choosing a silicon substrate with sufficiently high resistivity it is possible to make the dielectric attenuation constant of the interconnecting microwave transmission lines approach those of GaAs or InP. In order for this to be possible, the transmission lines must be characterized. In this presentation, the effective dielectric constant (epsilon sub eff) and attenuation constant (alpha) of a slotline on high resistivity (5000 to 10 000 ohm-cm) silicon wafer will be discussed. The epsilon sub eff and alpha are determined from the measured resonant frequencies and the corresponding insertion loss of a slotline ring resonator. The results for slotline will be compared with microstrip line and coplanar waveguide.

Simons, Rainee N.↗
NASA NTRS2010

Estimation of Geodetic and Geodynamical Parameters with VieVS

Since 2008 the VLBI group at the Institute of Geodesy and Geophysics at TU Vienna has focused on the development of a new VLBI data analysis software called VieVS (Vienna VLBI Software). One part of the program, currently under development, is a unit for parameter estimation in so-called global solutions, where the connection of the single sessions is done by stacking at the normal equation level. We can determine time independent geodynamical parameters such as Love and Shida numbers of the solid Earth tides. Apart from the estimation of the constant nominal values of Love and Shida numbers for the second degree of the tidal potential, it is possible to determine frequency dependent values in the diurnal band together with the resonance frequency of Free Core Nutation. In this paper we show first results obtained from the 24-hour IVS R1 and R4 sessions.

Spicakova, Hana↗
NASA NTRS2006

Automatic Alignment of Displacement-Measuring Interferometer

A control system strives to maintain the correct alignment of a laser beam in an interferometer dedicated to measuring the displacement or distance between two fiducial corner-cube reflectors. The correct alignment of the laser beam is parallel to the line between the corner points of the corner-cube reflectors: Any deviation from parallelism changes the length of the optical path between the reflectors, thereby introducing a displacement or distance measurement error. On the basis of the geometrical optics of corner-cube reflectors, the length of the optical path can be shown to be L = L(sub 0)cos theta, where L(sub 0) is the distance between the corner points and theta is the misalignment angle. Therefore, the measurement error is given by DeltaL = L(sub 0)(cos theta - 1). In the usual case in which the misalignment is small, this error can be approximated as DeltaL approximately equal to -L(sub 0)theta sup 2/2. The control system (see figure) is implemented partly in hardware and partly in software. The control system includes three piezoelectric actuators for rapid, fine adjustment of the direction of the laser beam. The voltages applied to the piezoelectric actuators include components designed to scan the beam in a circular pattern so that the beam traces out a narrow cone (60 microradians wide in the initial application) about the direction in which it is nominally aimed. This scan is performed at a frequency (2.5 Hz in the initial application) well below the resonance frequency of any vibration of the interferometer. The laser beam makes a round trip to both corner-cube reflectors and then interferes with the launched beam. The interference is detected on a photodiode. The length of the optical path is measured by a heterodyne technique: A 100- kHz frequency shift between the launched beam and a reference beam imposes, on the detected signal, an interferometric phase shift proportional to the length of the optical path. A phase meter comprising analog filters and specialized digital circuitry converts the phase shift to an indication of displacement, generating a digital signal proportional to the path length.

Halverson, Peter↗
NASA NTRS1978

Interaction of a turbulent-jet noise source with transverse modes in a rectangular duct

A turbulent jet was used to excite transverse acoustic modes in a rectangular duct. The pressure spectrum showed asymmetric singularities (pressure spikes) at the resonant frequencies of the duct modes. This validates previously published theoretical results. These pressure spikes occurred over a range of jet velocities, orientations, and inlet turbulence levels. At the frequency of the spike, the measured transverse pressure shape matched the resonant mode shape.

Succi, G. P.↗
NASA NTRS1978

Noise transmission through flat rectangular panels into a closed cavity

Five panels backed by a closed cavity were studied experimentally and analytically to determine the noise transmission characteristics of the coupled panel-cavity system. The closed cavity was studied both with and without fiber-glass lining to provide either an absorbent or a reverberant acoustic space. The effects on noise reduction of cavity absorption, measurement location within the cavity, panel mass, and panel stiffness were examined. Results indicated that both measurement location and absorption in the cavity have significant effects on the noise reduction. Increasing panel mass improves the noise reduction at almost all frequencies, and increasing panel stiffness improves nose reduction below the fundamental resonance frequency. A simple, one dimensional analytical model was developed which provided good agreement with the experimental results.

Barton, C. K.↗
NASA NTRS1993

CAFS: A Cesium Atomic Frequency Standard for GPS block IIR

Kernco, Inc. was selected to design the Cesium Atomic Frequency Standards (CAFS) for the GPS Block IIR NAVSTAR satellites. These spacecraft are scheduled to be launched in the mid-1990's to replenish and upgrade the existing constellation of Global Positioning System satellites. The Block IIR CAFS output frequency is 13.4003378 MHz, the 686th submultiple of the cesium atomic resonance frequency. Using an integer submultiple simplifies the design of the atomic frequency standard's rf multiplier circuits, eliminating the secondary frequency synthesizer needed in previous designs. The GPS Block IIR CAFS design, particularly the improvements made on our earlier Block II design is described. Test results are included.

Wisnia, Jeffry A.↗