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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Asymmetry of a circular jet observed in near and far fields

Turbulence intensity measurements were made in three different-size jets at various Mach numbers and the aerodynamic jet axis, defined as the locus of points in the flow at which the turbulence intensity is minimum, was determined. The deviations from the geometrical jet axis were on the order of 2-4% of the nozzle diameter, and the pure jet axis had the form of a helical wave. A similar asymmetry in the radiated noise was also noted. The measured circumferential distribution of rms fluctuating velocity signals confirmed the asymmetry of even a circular jet.

Yamamoto, K.↗
NASA NTRS1980

Far-field radiation of aft turbofan noise

Approximate expressions were developed for the noise radiation from the aft duct. The results of approximate aft radiation equation compare favorably to more exact Wiener-Hopf radiation results. Refraction as well as convective effects in the multiple flow streams is considered. The peak in the radiation pattern, which occurs nearly at engine sideline, is composed of modes with relatively large cut-off ratios. This implies that aft fan radiation will be inherently more difficult to suppress that the fan inlet noise. The theoretical multimodal radiation pattern is compared to experimental data for the first two harmonics of blade passage frequency for three full scale fans at two speeds. The agreement between theory and experiment is quite good.

Rice, E. J.↗
NASA NTRS1981

Far field conditions for compressible flows

The numerical treatment of exterior regions requires a method to convert the problem to one in a bounded region. One method is to truncate the unbounded region at some finite, artificial surface. This creates a finite computational region at the expense of imposing boundary conditions at the artificial boundary. If the problem has wave-like solutions near infinity, then these boundary conditions must simulate the radiation of energy out of the computational domain and towards infinity. Incorrect specification of these radiation boundary conditions can cause spurious reflected waves to be generated at the artificial boundary. Radiation boundary conditions which have the following properties were constructed: to accurately simulate the radiation of energy out of the computational domain; to accomplish the simulation with an accuracy which improves as fast as feasible as the artificial surface is moved outward; and to accelerate convergence to the steady state (by minimizing spurious reflections).

Bayliss, A.↗
NASA NTRS1981

Flow and far field acoustic amplification properties of heated and unheated jets

The interaction of an acoustic pulse with the experimentally determined mean flow field of a spreading jet is simulated numerically. The simulation is obtained through solving the Euler equations linearized about the spreading jet. The model reveals a small, sustained oscillation long after the original pulse has passed. This remnant is considered a continual shedding of vortices from the nozzle lip, together with the generation of acoustic ripples. IT is shown that the jet also acts as an amplifier of sound. This amplification is traced to the jet's stability characteristics. It is demonstrated that some of the observed differences in the spectra of heated and unheated jets can be attributed to differences in the stability characteristics of the jets.

Maestrello, L.↗
NASA NTRS1981

Enhancement of Far Field Sound Levels by Refractive Focusing

The enhancement of sound pressure levels resulting from refractive focusing was calculated for meteorological conditions representative of those observed at the MOD-1 site near Boone, N.C. The results show that 10 to 20dB enhancements can occur over ranges of several hundred meters. Localized enhancements in excess of 20dB can occur but will probably be of limited duration as a consequence of normal temporally varying meteorological conditions.

Thomson, D. W.↗
NASA NTRS1982

Analysis of the effect of heated jet flow on the far field radiation from a noise source

The influence on jet noise radiation of the interaction with the heated moving flow of a parallel twin jet is investigated. An analytical model of jet shielding is developed which consists of the sound field emitted from a stationary, discrete frequency point source impinging on a cylinder of locally parallel flow. The zones in which the various shielding mechanisms dominate are analyzed using the model. The effects of parameters such as jet temperature and flow speed are examined. It is found that the results obtained using the model are comparable to the experimental results for a point noise source impinging on a subsonic isothermal air jet.

Gerhold, C.↗
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 NTRS1984

Far-field brainstem responses evoked by vestibular and auditory stimuli exhibit increases in interpeak latency as brain temperature is decreased

The effect of decreasing of brain temperature on the brainstem auditory evoked response (BAER) in rats was investigated. Voltage pulses, applied to a piezoelectric crystal attached to the skull, were used to evoke stimuli in the auditory system by means of bone-conducted vibrations. The responses were recorded at 37 C and 34 C brain temperatures. The peaks of the BAER recorded at 34 C were delayed in comparison with the peaks from the 37 C wave, and the later peaks were more delayed than the earlier peaks. These results indicate that an increase in the interpeak latency occurs as the brain temperature is decreased. Preliminary experiments, in which responses to brief angular acceleration were used to measure the brainstem vestibular evoked response (BVER), have also indicated increases in the interpeak latency in response to the lowering of brain temperature.

Hoffman, L. F.↗
NASA NTRS1986

On uniformly valid high-frequency far-field asymptotic solutions of the Helmholtz equation

An asymptotic, large wave number approximation for the Helmholtz equation is derived. The theory is an extension of the geometric acoustic theory, and provides corrections to that theory in the form of multiplicative functions which satisfy parabolic equations. A simple example is used both to illustrate failure of the geometric theory for large propagation distances, and to show the improvement obtained by use of the new theory.

Mcaninch, G. L.↗
NASA NTRS1987

Computational aeroacoustics of propeller noise in the near and far field

Techniques for applying the NASPROP-E computer code (Bober et al., 1983) to characterize the acoustic field of a transonic propfan are described and demonstrated for the case of the SR-3 propfan. It is pointed out that NASPROP E accounts for the nonlinear quadrupole, monopole, and dipole noise sources. The approach used, based on that of White (1984) and Korkan et al. (1985 and 1986), is described in detail, and the results of simulations employing different (reflective and nonreflective) inflow-outflow boundary conditions and azimuthal mesh spacings are presented in graphs and briefly discussed.

Forsyth, D. W.↗
NASA NTRS1989

Measured far-field flight noise of a counterrotation turboprop at cruise conditions

Modern high speed propeller (advanced turboprop) aircraft are expected to operate on 50 to 60 percent less fuel than the 1980 vintage turbofan fleet while at the same time matching the flight speed and performance of those aircraft. Counterrotation turboprop engines offer additional fuel savings by means of upstream propeller swirl recovery. This paper presents acoustic sideline results for a full-scale counterrotation turboprop engine at cruise conditions. The engine was installed on a Boeing 727 aircraft in place of the right-side turbofan engine. Acoustic data were taken from an instrumented Learjet chase plane. Sideline acoustic results are presented for 0.50 and 0.72 Mach cruise conditions. A scale model of the engine propeller was tested in a wind tunnel at 0.72 Mach cruise conditions. The model data were adjusted to flight acquisition conditions and were in general agreement with the flight results.

Woodward, Richard P.↗
NASA NTRS1989

Bounds on current collection from the far field by plasma clouds in the ionosphere

Plasma clouds can enhance the collection of current to a charged body in the ionosphere in two ways. The first is by providing a large collection area for ionospheric electrons to be collected. The second is by ionization of neutral gas in the vicinity of the plasma cloud. The collection of electrons across a magnetic field is examined, and it is shown that the effective collection area is limited to the region of the cloud where the magnetic field effects are overwhelmed by the electric field effects from the charged body. Upper and lower bounds are obtained for the radius of the core region where the electric field influence dominates. From these, upper and lower bounds are obtained for the current collection.

Hastings, D. E.↗
NASA NTRS1989

Determination of near and far field acoustics for advanced propeller configurations

A method has been studied for predicting the acoustic field of the SR-3 transonic propfan using flow data generated by two versions of the NASPROP-E computer code. Since the flow fields calculated by the solvers include the shock-wave system of the propeller, the nonlinear quadrupole noise source term is included along with the monopole and dipole noise sources in the calculation of the acoustic near field. Acoustic time histories in the near field are determined by transforming the azimuthal coordinate in the rotating, blade-fixed coordinate system to the time coordinate in a nonrotating coordinate system. Fourier analysis of the pressure time histories is used to obtain the frequency spectra of the near-field noise.

Korkan, K. D.↗
NASA NTRS1990

The ROTONET prediction system and initial comparisons with far-field acoustics measurements for the XV-15 tilt-rotor aircraft

The NASA Langley Research Center is developing the ROTONET prediction system, a comprehensive computer program for the prediction of full system rotorcraft noise, to provide an analytical tool for estimating the total noise signature of rotorcraft. It contains models of many rotorcraft noise generating mechanisms. NASA is also performing a series of flight tests to provide comprehensive validation data bases for the ROTONET System. A joint NASA/Bell Helicopter Textron Incorporated test of an XV-15 tilt-rotor aircraft is one of several flight test which has been performed. A data base consisting of spectra, noise level time histories, and effective perceived noise levels, incorporating actual meteorological conditions, and tilt-order aircraft flight dynamics, is being produced from this test. The ROTONET prediction system is described, the flight test methodology is explained, and initial comparisons are shown of ROTONET predictions with data from the flight test. EPNL, noise level time history, and narrowband noise spectra comparisons demonstrate the overall capabilities of the prediction system.

Golub, Robert A.↗
NASA NTRS1992

Faster Computation Of Far Fields Of Dish Antennas

Modified Zernike-polynomial approach reduces number of terms in numerical integration. Yields greater or equal accuracy with fewer terms in series expansion and fewer sampling points across aperture and, less computation. Method based on Jacobi-Bessel-expansion concept.

Jamnejad, Vahraz↗
NASA NTRS1992

Far-field noise and internal modes from a ducted propeller at simulated aircraft takeoff conditions

The ducted propeller offers structural and acoustic benefits typical of conventional turbofan engines while retaining much of the aeroacoustic benefits of the unducted propeller. A model Advanced Ducted Propeller (ADP) was tested in the NASA Lewis Low-Speed Anechoic Wind Tunnel at a simulated takeoff velocity of Mach 0.2. The ADP model was designed and manufactured by the Pratt and Whitney Division of United Technologies. The 16-blade rotor ADP was tested with 22- and 40-vane stators to achieve cut-on and cut-off criterion with respect to propagation of the fundamental rotor-stator interaction tone. Additional test parameters included three inlet lengths, three nozzle sizes, two spinner configurations, and two rotor rub strip configurations. The model was tested over a range of rotor blade setting angles and propeller axis angles-of-attack. Acoustic data were taken with a sideline translating microphone probe and with a unique inlet microphone probe which identified inlet rotating acoustic modes. The beneficial acoustic effects of cut-off were clearly demonstrated. A 5 dB fundamental tone reduction was associated with the long inlet and 40-vane sector, which may relate to inlet duct geometry. The fundamental tone level was essentially unaffected by propeller axis angle-of-attack at rotor speeds of at least 96 percent design.

Woodward, Richard P.↗