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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 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Numerical Experiment with Time and Spatial Accuracy of Navier-Stokes Computation For Helicopter Problems

Helicopter flowfields are highly unsteady, nonlinear and three-dimensional. In forward flight and in hover, the rotor blades interact with the tip vortex and wake sheet developed by either itself or the other blades. This interaction, known as blade-vortex interactions (BVI), results in unsteady loading of the blades and can cause a distinctive acoustic signature. Accurate and cost-effective computational fluid dynamic solutions that capture blade-vortex interactions can help rotor designers and engineers to predict rotor performance and to develop designs for low acoustic signature. Such a predictive method must preserve a blade's shed vortex for several blade revolutions before being dissipated. A number of researchers have explored the requirements for this task. This paper will outline some new capabilities that have been added to the NASA Ames' OVERFLOW code to improve its overall accuracy for both vortex capturing and unsteady flows. To highlight these improvements, a number of case studies will be presented. These case studies consist of free convection of a 2-dimensional vortex, dynamically pitching 2-D airfoil including light-stall, and a full 3-D unsteady viscous solution of a helicopter rotor in forward flight In this study both central and upwind difference schemes are modified to be more accurate. Central difference scheme is chosen for this simulation because the flowfield is not dominated by strong shocks. The feature of shock-vortex interaction in such a flow is less important than the dominant blade-vortex interaction. The scheme is second-order accurate in time and solves the thin-layer Navier-Stokes equations in fully-implicit manner at each time-step. The spatial accuracy is either second and fourth-order central difference or third-order upwind difference using Roe-flux and MUSCLE scheme. This paper will highlight and demonstrate the methods for several sample cases and for a helicopter rotor. Preliminary computations on a rotor were performed by using this method and are in the process of documentation.

Ahmad, Jasim↗
NASA NTRS1973

Transmission of sound from a nozzle carrying a compressible subsonic flow

The effect of an aperture on an impinging plane wave in a duct is extended to the case where the duct carries a compressible mean flow. The effect of the mean flow is to induce the closing mechanical impedance to have a significant resistive component in addition to the reactive component due to the 'attached mass' effect. The effect of the reactive load on the aperture is shown to be especially important for large contraction ratios. Of special interest is the acoustic particle velocity at the exit plane of the aperture. This is the 'piston velocity' that drives the spherical acoustic waves in the far field. The effect of aperture geometry on the piston velocity is established, and a criterion for minimum acoustic radiation is presented. An experimental arrangement for measuring the piston velocity with a hot-wire anemometer is described.

Davis, S. S.↗
NASA NTRS1996

Computation of Acoustic Waves Through Sliding-Zone Interfaces Using an Euler/Navier-Stokes Code

The effect of a patched sliding-zone interface on the transmission of acoustic waves is examined for two- and three-dimensional model problems. A simple but general interpolation scheme at the patched boundary passes acoustic waves without distortion, provided that a sufficiently small time step is taken. A guideline is provided for the maximum permissible time step or zone speed that gives an acceptable error introduced by the sliding-zone interface.

Rumsey, Christopher L.↗
NASA NTRS1980

Mechanical suppression: Modern technology applied to an old art

Various suppressor configurations were analyzed and evaluated in an effort to reduce combustion instability and improved ballistic reproducibility of the MK 12 motor. A static firing test series of eight field return boosters featured two baseline motor firings with the existing suppressor in each motor, followed by two tests each of two different prototype suppressor designs. Results were analyzed and used to generate the design for the final two firings. Acoustic pressure pulsers were utilized to measure the damping effectiveness of the final design. The reduction in motor vibration and acoustic pressure levels between the baseline tests and the final two improved lightweight tests was significant. The average acceleration level of the motors equipped with flightweight rods was reduced to only 5% of the level in the unmodified motors; the average acoustic pressure level was reduced to 20%; maximum values were reduced to 6% for acceleration and 23% for pressure.

Crockford, W. H.↗
NASA NTRS1994

Use of acoustic wave travel-time measurements to probe the near-surface layers of the Sun

The variation of solar p-mode travel times with cyclic frequency nu is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (approximately equals 5.5 mHz) are inverted to yield the local acoustic cutoff frequency nu(sub c) as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with nu greater than 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation.

Jefferies, S. M.↗
NASA NTRS1975

Optimization of duct acoustic liners of finite length

Lining design procedures generally ignore the effects of discontinuous impedance changes on reflection and modal redistribution of incident energy. The present study involves identification of the mechanisms of optimal suppression by segmented liners, including impedance discontinuity effects. The mathematical model involves rectangular ducts in the absence of mean flow. Segmented acoustic fields are represented by duct eigenfunction expansions, and mode matching is used to ensure continuity of the total field. Extensive parameter studies were used to identify attenuation mechanisms and to define preliminary liner configurations. These are used as starting values in conjugate-gradient optimization studies. Optimal segmented liners are presented and it is shown that, if the sound source is sufficiently well-defined, conventional infinite duct optimum attenuation rates can be improved upon by taking advantage of wall impedance discontinuities. Finally, follow-on work which will involve experimental verification and extension to the case with mean flow in the duct is described.

Beckemeyer, R. J.↗
NASA NTRS1981

An experimental investigation of a large delta P settling chamber for a supersonic pilot quiet tunnel

The mean streamwise flow distributions and turbulence levels across the chamber were measured with a hot wire anemometer downstream of a series of porous Rigimesh plates which were shown to be an effective means of reducing the chamber acoustic disturbance levels due to upstream pipe and valve systems. Tests made with various types of flow conditioners downstream of the porous plates showed that a series of screens was the most effective means of achieving the objective of a uniform mean flow distribution with reduced vorticity levels downstream of the porous components. Frequency spectra obtained across the series of screens show that they reduce vorticity over a wide frequency range for several different initial upstream vorticity conditions. Improvements in the mechanical installation of the porous plates and damping screens and the use of porous plates with more uniform porosity should reduce the freestream velocity fluctuations to the minimum acoustic levels of about 0.5 percent.

Piatt, M.↗
NASA NTRS1994

Properties of acoustic sources in the Sun

The power spectrum of solar acoustic oscillations shows peaks extending out to frequencies much greater than the acoustic cutoff frequency of approximately 5.3 mHz, where waves are no longer trapped. Kumar & Lu (1991) proposed that these peaks arise from the interference of traveling waves which are generated by turbulent convection. According to this model, the frequencies of the peaks in the power spectrum depend on the static structure of the Sun as well as the radial location of the sources. Kumar & Lu used this idea to determine the depth of the acoustic sources. However, they ignored dissipative effects and found that the theoretically computed power spectrum was falling off much more rapidly than the observed spectrum. In this paper, we include the interaction of radiation with acoustic waves in the computation of the power spectrum. We find that the theoretically calculated power spectra, when radiative damping is included are in excellent agreement with the observed power spectra over the entire observed frequency range of 5.3 to 7.5 mHz above the acoustic cutoff frequency. Moreover, by matching the peak frequencies in the observed and theoretical spectra we find the mean depth of acoustic sources to be 140 +/- 60 km below the photosphere. We show that the spectrum of solar turbulence near the top of the solar convection zone is consistent with the Kolmogorov spectrum, and that the observed high frequency power spectrum provides strong evidence that the acoustic sources in the Sun are quadrupolar. The data, in fact, rules out dipole sources as significant contributors to acoustic wave generation in the Sun. The radial extent of the sources is poorly determined and is estimated to be less than about 550 km.

Kumar, Pawan↗
NASA NTRS1974

Flight velocity effects on the jet noise of several variations of a 104-tube suppressor nozzle

At the relatively high takeoff speeds of supersonic transport aircraft, an important question concerns whether the flight speed affects the noise of suppressor nozzles. To answer this question, flyover and static tests using a modified F-106B aircraft were conducted on a 104-tube suppressor nozzle. Comparison of adjusted flyover and static spectra indicated that flight velocity had a small adverse effect on the suppression of the 104-tube suppressor. The adverse effect was larger with the acoustic shroud installed than without it.

Burley, R. R.↗
NASA NTRS1975

Thermal acoustic oscillations, volume 2

A number of thermal acoustic oscillation phenomena and their effects on cryogenic systems were studied. The conditions which cause or suppress oscillations, the frequency, amplitude and intensity of oscillations when they exist, and the heat loss they induce are discussed. Methods of numerical analysis utilizing the digital computer were developed for use in cryogenic systems design. In addition, an experimental verification program was conducted to study oscillation wave characteristics and boiloff rate. The data were then reduced and compared with the analytical predictions.

Spradley, L. W.↗
NASA NTRS1975

Space shuttle orbital maneuvering engine platelet injector program

A platelet face injector for the Orbit Maneuvering Engine (OME) on the space shuttle was evaluated as a means of obtaining additional design margin and lower cost. The program was conducted in three phases. The first phase evaluated single injection elements, or unielements; it involved visual flow studies, mixing experiments using propellant simulants, and hot firings to assess combustion efficiency, chamber wall compatibility, and injector face temperatures. In the second phase, subscale units producing 600 lbf thrust were used to further evaluate the orifice patterns chosen on the basis of unielement testing. In addition to combustion efficiency, chamber and injector heat transfer, the subscale testing provided a preliminary indication of injector stability. Full scale testing of the selected patterns at 6,000 lbf thrust was performed in the third phase. Performance, heat transfer, and combustion stability were evaluated over the anticipated range of OMS operating conditions. The effects on combustion stability of acoustic cavity configuration, including cavity depth, open area, inlet contour, and other parameters, were investigated.

Source record↗
NASA NTRS1977

A research to reduce interior noise in general aviation airplanes. General aviation interior noise study

The construction, calibration, and properties of a facility for measuring sound transmission through aircraft type panels are described along with the theoretical and empirical methods used. Topics discussed include typical noise source, sound transmission path, and acoustic cabin properties and their effect on interior noise. Experimental results show an average sound transmission loss in the mass controlled frequency region comparable to theoretical predictions. The results also verify that transmission losses in the stiffness controlled region directly depend on the fundamental frequency of the panel. Experimental and theoretical results indicate that increases in this frequency, and consequently in transmission loss, can be achieved by applying pressure differentials across the specimen.

Roskam, J.↗
NASA NTRS1981

GE MOD-1 noise study

Noise studies of the MOD-1 Wind Turbine Generator are summarized, and a simple mathematical noise is presented which is adequate to correlate the sound levels found near the machine. A simple acoustic measure is suggested for use in evaluating far field sound levels. Use of this measure as input to a currently available sound complaint prediction program is discussed. Results of a recent statistical survey relative to the far field variation of this acoustic measure because of atmospheric effects are described.

Wells, R. J.↗
NASA NTRS1986

Improved acoustic emission locations

A technique for locating acoustic emission in brittle rock specimens is described. An array of eight piezoelectric transducers was used to detect acoustic emission and to receive velocity measurement pulses. Axial and transverse velocities were calculated from the transmission data; the use of the velocity data to locate the acoustic emissions is discussed. Waveforms were recorded for about 140 acoustic emissions during the deformation of the Westerly granite specimen and about 100 acoustic emissions were located. The effect of transducer size and incident angle on apparent arrival times is evaluated using a lead zirconium titinate transducer bonded to a small brass rider with a curved surface matched to the brass specimen. The relation between location accuracy in various regions of the specimen and the geometry of the receiver array is studied. It is observed that for incidence angles away from the normal, the velocity appears to be higher than for normal incidences.

Getting, I. C.↗
NASA NTRS1988

Wind turbine acoustics research bibliography with selected annotation

Citations of documents are included, which represent the state-of-the-art of technology in each of the following acoustics subject areas: Prediction of Wind Turbine Noise; Acoustic Measurements for Wind Tunnels; Effect of Wind Turbine Noise on Building Structures, People and Communities; Atmospheric Propagation; and Measurement Technology Including Wind Screens. Documents are listed in chronological order in each section of the paper, with key documents and associated annotation listed first. The sources are given along with acquisition numbers, when available, to expedite the acquisition of copies of the documents.

Hubbard, Harvey H.↗
NASA NTRS1988

Dynamics of localized ion-acoustic waves in a magnetized plasma

The evolution of negative potential pulses in a magnetized plasma is studied. A three-dimensional nonlinear ion-acoustic wave equation, including nonstationary effects of reflected electrons, has been derived from the Poisson-Vlasov equations with uniform magnetic field. The studies show that a negative potential pulse can be enhanced by drifting electrons. The growing pulse develops asymmetrically with an oscillatory precursor and a local potential jump resembling the early phase of weak double layer formation.

Qian, S.↗