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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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Sources, control, and effects of noise from aircraft propellers and rotors

Source noise predictions are compared with measurements for conventional low-speed propellers, for new high speed propellers (propfans), and for a helicopter. Results from a light aircraft demonstration program are described, indicating that about 5-dB reduction of flyover noise can be obtained without significant performance penalty. Sidewall design studies are described for interior noise control in light general aviation aircraft and in large transports using propfan propulsion. The weight of the added acoustic treatment is estimated and tradeoffs between weight and noise reduction are discussed. A laboratory study of passenger response to combined broadband and tonal propeller like noise is described. Subject discomfort ratings of combined tone broadband noises are compared with ratings of broadband (boundary layer) noise alone, and the relative importance of the propeller tones is examined.

Mixson, J. S.↗
NASA NTRS1981

Sources, control, and effects of noise from aircraft propellers and rotors

Recent NASA and NASA sponsored research on the prediction and control of propeller and rotor source noise, on the analysis and design of fuselage sidewall noise control treatments, and on the measurement and quantification of the response of passengers to aircraft noise is described. Source noise predictions are compared with measurements for conventional low speed propellers, for new high speed propellers (propfans), and for a helicopter. Results from a light aircraft demonstration program are considered which indicates that about 5 dB reduction of flyover noise can be obtained without significant performance penalty. Sidewall design studies are examined for interior noise control in light general aviation aircraft and in large transports using propfan propulsion. The weight of the added acoustic treatment is estimated and tradeoffs between weight and noise reduction are discussed. A laboratory study of passenger response to combined broadband and tonal propeller-like noise is described. Subject discomfort ratings of combined tone broadband noises are compared with ratings of broadband (boundary layer) noise alone and the relative importance of the propeller tones is examined.

Mixson, J. S.↗
NASA NTRS1981

Turbomachinery noise studies of the AiResearch QCGAT engine with inflow control

The AiResearch Quiet Clean General Aviation Turbofan engine was tested on a vertical lift fan facility to measure the acoustic performance of two inflow control devices (ICD) of similar design, and three inlet lips of different external shape. Far-field directivity patterns calculated by existing analyses were compared with the measured fan fundamental blade passing frequency (BPF/F/) and broadband data. Installing an ICD on an engine with hardwall ducts reduced the BPF(F) tone everywhere in the far-field. When the ICD was installed on an engine with active acoustic panels, tone reduction in the forward quadrant was comparable to that in the hardwall tests; in the aft quadrant, however, tone noise was attenuated by the large acoustic panels in the bypass duct to such a degree that the ICD had little effect. Tests to compare performance of ICDs with hardwall inlet ducts showed only minor differences in the BPF(F) directivity patterns, while broadband noise was the same for both. Forward-quadrant BPF(F) and broadband directivity patterns were found to be similar for the inlet lips tested with a hardwall inlet duct. At high fan speeds, however, the shape of the analytical multimodal tone pattern from the exhaust nozzle was flatter than the measured patterns. The sources of lobes from several propagating single modes found in the forward-quadrant BPF(F) data were attributed to rotor/strut interaction and the rotor-alone pressure field.

Mcardle, J. G.↗
NASA NTRS1982

Tone-excited jet: Theory and experiments

A detailed study to understand the phenomenon of broadband jet-noise amplification produced by upstream discrete-tone sound excitation has been carried out. This has been achieved by simultaneous acquisition of the acoustic, mean velocity, turbulence intensities, and instability-wave pressure data. A 5.08 cm diameter jet has been tested for this purpose under static and also flight-simulation conditions. An open-jet wind tunnel has been used to simulate the flight effects. Limited data on heated jets have also been obtained. To improve the physical understanding of the flow modifications brought about by the upstream discrete-tone excitation, ensemble-averaged schlieren photographs of the jets have also been taken. Parallel to the experimental study, a mathematical model of the processes that lead to broadband-noise amplification by upstream tones has been developed. Excitation of large-scale turbulence by upstream tones is first calculated. A model to predict the changes in small-scale turbulence is then developed. By numerically integrating the resultant set of equations, the enhanced small-scale turbulence distribution in a jet under various excitation conditions is obtained. The resulting changes in small-scale turbulence have been attributed to broadband amplification of jet noise. Excellent agreement has been found between the theory and the experiments. It has also shown that the relative velocity effects are the same for the excited and the unexcited jets.

Ahuja, K. K.↗
NASA NTRS1982

Analysis of combustion spectra containing organ pipe tone by cepstral techniques

Signal reinforcements and cancellations due to standing waves may distort constant bandwidth combustion spectra. Cepstral techniques previously applied to the ground reflection echo problem are used to obtain smooth broadband data and information on combustion noise propagation. Internal fluctuating pressure measurements made using a J47 combustor attached to a 6.44 m long duct are analyzed. Measurements made with Jet A and hydrogen fuels are compared. The acoustic power levels inferred from the measurements are presented for a range of low heat release rate operating conditions near atmospheric pressure. For these cases, the variation with operating condition of the overall acoustic broadband power level for both hydrogen and Jet A fuels is consistent with previous results showing it was proportional to the square of the heat release rate. However, the overall acoustic broadband power level generally is greater for hydrogen than for Jet A.

Miles, J. H.↗
NASA NTRS1981

High-sensitivity circular polarimetry of the Sun, 0.5 - 1.7 microns

Using an isometric (symmetrical) photoelastic-modulator polarimeter and a Cassegrain telescope, broadband polarization measures of the Sun were obtained with unprecedented sensitivity. With an instrumental background of x 10 to the minus 6th power differential effects delta q = V/I of 1 x 10 to the minus 7th power were detectable. Some results: (1) the lambda dependence of broadband circular polarization in spots was observed out to 1.7 mu, extending the visible-light measurements of Illing, Landman, and Mickey. The q drops to of the order of 1 x 10 to the minus 5 power at 1.2 mu (in a strong spot), then rises at 1.6 mu. While molecules or other special mechanisms must play a role in the visible-light polarization, at lambda 1.5 mu the q values seem consistent with gray magneto-opacity. (2) By surveying inactive regions upper limits are set to broadband polarization due to global or deep-seated magnetic fields. For a double-toroidal pattern a differential measure is found, q sub b = (q sub ne + q sub sw - q sub nw - q sub - se) 14, at 1.7 mu, of (+1.2 + or = 0.7) x 10 to the minus 7th power a mean from three series in 1981. Based on a simple estimate by L. Biermann this would correspond to deep-down toroidal fields of or approx 2000 gauss. Extension of this work are discussed.

Kemp, J. C.↗
NASA NTRS1983

An experimental study of supersonic jet shock-associated noise

A new method of screech tone reduction in underexpanded supersonic jets is utilized to determine if the amplitude of screech is related to the level of shock-associated broadband noise. Acoustic measurements show that as the screech tone is reduced, the broadband shock-associated noise may be increased by up to 4dB at some locations. Several experiments were conducted with disturbances put into the flow upstream of the final contraction. Results from these tests support the concept of linkage between the broadband shock noise and screech tones. Some interesting preliminary results were obtained with a disturbance of the flow at several locations downstream of the nozzle. These results are also discussed.

Nagel, R. T.↗
NASA NTRS1984

Spectrophotometry of extreme helium stars - Ultraviolet fluxes and effective temperatures

Ultraviolet flux distributions are presented for the extremely helium rich stars BD +10 deg 2179, HD 124448, LSS 3378, BD -9 deg 4395, LSE 78, HD 160641, LSIV -1 deg 2, BD 1 deg 3438, HD 168476, MV Sgr, LS IV-14 deg 109 (CD -35 deg 11760), LSII +33 deg 5 and BD +1 deg 4381 (LSIV +2 deg 13) obtained with the International Ultraviolet Explorer (IUE). Broadband photometry and a newly computed grid of line blanketed model atmospheres were used to determine accurate angular diameters and total stellar fluxes. The resultant effective temperatures are in most cases in satisfactory agreement with those based on broadband photometry and/or high resolution spectroscopy in the visible. For two objects, LSII +33 deg 5 and LSE 78, disagreement was found between the IUE observations and broadband photometry: the colors predict temperatures around 20,000 K, whereas the UV spectra indicate much lower photospheric temperatures of 14,000 to 15,000 K. The new temperature scale for extreme helium stars extends to lower effective temperatures than that of Heber and Schoenberner (1981) and covers the range from 8,500 K to 32,000 K. Previously announced in STAR as N83-24433

Heber, U.↗
NASA NTRS1985

Far-field noise of a subsonic jet under controlled excitation

The role of large-scale coherent structures in broadband jet noise suppression and amplification under controlled excitation are studied experimentally. It is found that suppression occurs only at low Reynolds and Mach numbers, when the boundary layer at the jet exit is laminar; the optimum St(Theta) (St, based on the initial shear-layer momentum thickness) value is 0.017, at which the excitation results in a quick roll-up and transition of the laminar shear-layer vortices, yielding coherent structures similar to those existing at high speeds. At the asymptotic level the broadband jet noise can only be amplified by the excitation. The amplification is inferred to be maximum for excitation in the St(D) (St based on the jet diameter) range of 0.65-0.85. Finally, it is noted that the pairing process induced by the excitation is at the origin of the broadband noise amplification.

Zaman, K. B. M. Q.↗
NASA NTRS1986

Noise predictions of a high bypass turbofan engine using the Lockheed Near-Field Noise Prediction Program

The prediction of engine noise during cruise using the Near-Field Noise Prediction Program developed by Lockheed is examined. Test conditions were established which simulate the operation of a high bypass turbofan engine under a wide range of operating conditions. These test conditions include variations in altitude, flight Mach number and thrust setting. Based on the results of noise prediction made using the Lockheed program, an evaluation of the impact of these test conditions on the overall sound pressure level(OASPL)and the one-third octave band spectra is made. An evaluation of the sensitivity of flight condition parameters is also made. The primary noise source from a high bypass turbofan was determined to be fan broadband shock noise. This noise source can be expected to be present during normal cruising conditions. When present, fan broadband shock noise usually dominates at all frequencies and all directivity angles. Other noise sources of importance are broadband shock noise from the primary jet, fan noise, fan mixing noise and turbine noise.

Rawls, J. W., Jr.↗
NASA NTRS1985

The generation of electrostatic noise in the plasma sheet boundary layer

The one and two ion beam instability is considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions are present, no broadband waves are excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam acoustic mode, but the growth rates of the waves are significantly enhanced when hot boundary layer ions are present. (Both the slow and fast beam acoustic modes can be excited, depending on the relative ion drift.) This model predicts that the wave intensity of the broad band noise should peak in the plasma sheet boundary layer. Observations of less intense electrostatic waves in the lobes and plasma sheet are likely a result of the absence of hot ion beams or large ion temperatures, respectively, which result in smaller growth rates. The ion beam instability may play an important role in the formation of the central plasma sheet.

Dusenbery, P. B.↗
NASA NTRS1985

Tone excited jets. IV - Acoustic measurements

Attention is given to acoustic results from an experiment designed to deepen insight into the noise generated by a tone-excited jet, with emphasis on the mechanism of broadband jet noise amplification. Results are presented for both heated and unheated jets, with and without the effect of forward flight simulation over a range of excitation frequencies and levels, and for zero-order and first-order spinning modes. Broadband jet noise amplifications of up to 5 dB have been observed in this study. Relative velocity effects are the same for both excited and unexcited jets. It is concluded that small scale turbulence generates the additional broadband noise upon excitation of the jet by an upstream-injected discrete tone acoustic wave.

Ahuja, K. K.↗
NASA NTRS1987

AMPTE IRM observations of waves associated with flux transfer events in the magnetosphere

The AMPTE IRM wave instrument has been applied to the study of flux transfer events (FTE's). This initial investigation concentrates on FTE's observed in the magnetosphere during the fall of 1984. The wave morphology consists of four significant features: at frequencies below the ion gyrofrequency, magnetic fluctuations occur with amplitudes of the order of 1 nT; at frequencies from a few hertz to a few hundred hertz, electric field fluctuations are observed which have a broadband amplitude of a few millivolts per meter, which are perpendicularly polarized at the lowest frequencies, and which are partly electrostatic and partly electromagnetic; in the frequency range from 100 Hz to 10 kHz, broadband spikelike waves occur with a time-averaged amplitude of about 0.1 mV/m; and near the electron plasma frequency, bursts of waves occur at the boundaries of FTE's. In none of the 25 events surveyed does the total broadband amplitude of all the waves exceed a few millivolts per meter, an amplitude far too small to provide the dissipation required by 'traditional' reconnection models in which the dissipation region has a thickness the order of an ion gyroradius. Thus, either all of the FTE's are observed at some distance from the diffusion region, or the observable waves play no significant role in the diffusion process, or 'traditional' reconnection models do not apply to reconnection in FTE's.

Labelle, J.↗
NASA NTRS1988

Apparent brightness of stars and lasers

Foremost among issues affecting the potential use of astrometric techniques to locate and track laser-carrying spacecraft is the apparent brightness of a laser relative to reference stars. Broadband detectors offer improved sensitivity to stars, but not to lasers. The important and fundamental step of calibrating laser and star brightnesses according to detector spectral sensitivity is performed for four representative kinds of broadband detectors, located above and below the atmosphere. Stars are identified according to parameters traditionally used by astronomers: temperature and apparent brightness at visible wavelengths. These are translated into an energy and photon flux for each kind of broadband detector and are then compared with the corresponding flux from a laser. The comparisons are also given as magnitude correction factors. Astrometrists typically characterize the sensitivity of their instruments in terms of the precision with which they can make a relative measurement of position and the minimum brightness needed to achieve that precision. Given this information and the instruments's spectral sensitivity, the calculations described can be used to infer the detectability of a laser and the precision with which it could be located and tracked. Since the limiting brightness quoted for a particular instrument is typically firm to within 4.0 dB, judging a laser's detectability by its visible brightness alone could lead to serious underestimation of the requirements on its effective radiated power. The laser brightness corrections given solve this problem.

Schumaker, B. L.↗
NASA NTRS1988

VLF wave emissions by pulsed and dc electron beams in space. I - Spacelab 2 observations

The properties of radio waves generated by electron beams in space were investigated using data from the wideband wave receiver on the Spacelab 2. The VLF observations were found to confirm the results of the STS 3/OSS-1 mission. It was found that a 1-keV electron beam injected from the orbiter produced copious broadband electromagnetic emissions. When the electron beam was square-wave modulated, narrow-band emissions at the pulsing frequency and harmonics of that frequency were produced along with the broadband emissions. The observations indicated that dc 50-mA electron beams and pulsed 50-percent duty-cycle 100-mA beams produce broadband radiation which is comparable in intensity and spectral shape at all points for which the wave field was sampled.

Reeves, G. D.↗
NASA NTRS1989

Average electric wave spectra across the plasma sheet and their relation to ion bulk speed

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer of the wave experiment and the three-dimensional plasma instrument on board the AMPTE/IRM satellite, a statistical survey on the electric wave spectral density in the earth's plasma sheet has been conducted. More than 50,000 10-s-averaged electric wave spectra were analyzed with respect to differences between their values in the inner and outer central plasma sheet and the plasma sheet boundary layer as well as their dependence on radial distance and ion bulk speed. High-speed flows are dominated by broadband electrostatic noise with highest spectral densities in the plasma sheet boundary, where broadband electrostatic noise also exists during periods of low-speed flows. The broadband electrostatic noise has a typical spectral index of about -2. During low-speed flows the spectra in the central plasma sheet show distinct emissions at the electron cyclotron odd half-harmonic and upper hybrid frequency. Wave intensities during episodes of fast perpendicular flows are higher than those associated with fast parallel flows.

Baumjohann, W.↗
NASA NTRS1990

ERBE and AVHRR Cirrus cloud fire study

Understanding the impact of cirrus clouds on the global radiation budget is essential to determining the role of clouds in the process of climate change. The ongoing Earth Radiation Budget Experiment (ERBE) is charged with measuring the global radiation balance at the top of the atmosphere. The International Satellite Cloud Climatology Project (ISCCP) is measuring global cloud amounts and properties over a time frame similar to ERBE. Specific cloud properties are absent from the ERBE program, while ISCCP lacks the broadband radiances necessary to determine the total radiation fields. Together, results from these two global programs have the potential for improving the knowledge of the relationship between cirrus clouds and the Earth radiation balance. The First ISCCP Regional Experiment (FIRE), especially its cirrus Intensive Field Observations (IFO), provides opportunities for studying radiation measurements from the ERBE taken over areas with known cirrus cloud properties. Satellite measurements taken during the IFO are used to determine the broadband radiation fields over cirrus clouds and to examine the relationship between narrowband and broadband radiances over various known scenes. The latter constitutes the link between the ERBE and the ISCCP.

Harrison, Edwin F.↗
NASA NTRS1990

Radiative properties of Cirrus clouds: FIRE IFO case October 28, 1986

A description of the radiative properties of two cirrus clouds sampled on 10/28/88 in the FIRE cirrus IFO is presented. The clouds are characterized in terms of the broadband infrared effective emittance, cloud fractional absorptance, shortwave heating rate, cloud albedo and vertical velocity. The broadband fluxes used in these calculations were obtained from measurements made by pyranometers and pyrgeometers. The shortwave irradiances were corrected to a horizontal plane and normalized to the same time by taking into account Sabreliner flight information (i.e., pitch, roll, heading and angle of attack), as well as sun-earth geometry considerations. Since only one aircraft was used, broadband fluxes at different levels in the cloud were not measured simultaneously. As a result, sampling errors may occur due to the nonsteady state of the cloud field or due to the possibility that the flight legs were not flown directly above or below each other. To minimize these errors and to simplify the analysis, the necessary variables were averaged and the averages used in the calculations. The downwelling shortwave and longwave irradiances were used as selection criteria to remove cloud free data encountered along the data sampling leg.

Hein, Paul F.↗