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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Force on a heated sphere in a horizontal plane acoustic standing wave field

The force on a heated sphere in a horizontal plane acoustic standing wave field is the subject of this investigation. The heated sphere produces a thermal gradient in the resonance chamber. The force on the sphere in a direction perpendicular to that of gravity is measured. This force is enhanced in the region near the pressure node, and is weakened in the region near the pressure antinode. Measurements of the force on a heated sphere with sound pressure levels between 148 and 156 dB are presented.

Leung, E. W.↗
NASA NTRS1975

Some results of the hydrodynamic theory of finite-amplitude waves

Standing waves can be interpreted as free waves between two vertical walls; for a standing wave in an unlimited container there is no transport of liquid mass in the horizontal direction. Standing waves of finite amplitude also possess the following properties not possessed by the waves of the linear theory: (1) there are no motionless points, (2) the points of maximum amplitude (crests and troughs) are fixed, and coincide with the antinodes of the original linear wave, (3) the ordinates of the wave crests are greater in absolute magnitude than the ordinates of the troughs; the wave is similar to a trochoid curve; (4) the wave profile never becomes straight; and (5) the frequency of the oscillations depends not only on the wave length but also on the amplitude.

Sekerzh-Zenkovich, Y. I.↗
NASA NTRS1972

Laser-induced line-narrowing effects in coupled Doppler-broadened transitions. II - Standing-wave features.

Previous theoretical results on the influence of a laser on the line shape of a coupled transition (laser-induced line narrowing) have been restricted to the case where the laser is detuned from the center of its atomic gain profile or is in the form of a traveling wave. The present paper extends these results to the case where the laser is an intense standing-wave field tunable to the center of its atomic gain profile (conditions for Lamb dip). A theoretical solution of the problem is developed, and a detailed discussion of line shapes and physical processes involved is included.

Feldman, B. J.↗
NASA NTRS2002

IMAGE-EUV Observation of Large Scale Standing Wave Pattern in the Nightside Plasmasphere

We present analyses of a nightside plasmaspheric pattern of bifurcated, filamentary He(+) 30.4-nm emission enhancements observed by IMAGE EUV between approximately 19:40-22:13 UT on 28 June 2000 that indicate the presence of a large-scale, global ULF standing wave pattern. Analysis of coincident IMAGE magnetometer chain data reveals that these ULF waves extend across the magnetic latitude-longitude range of the chain and possess multiple spectral features between 0.6-5-mHz (3-30 minute period). Additionally, analysis of ACE SWE data reveals similarly structured spectral components in the solar wind. Collectively, these analyses lead to the conclusion that the observed large-scale ULF wave pattern is the result of solar wind pressure pulses 'ringing' the inner-magnetosphere.

Six, N. Frank↗
NASA NTRS2014

Impact of Acoustic Standing Waves on Structural Responses

For several decades large reverberant chambers and most recently direct field acoustic testing have been used in the aerospace industry to test larger structures with low surface densities such as solar arrays and reflectors to qualify them and to detect faults in the design and fabrication. It has been reported that in reverberant chamber and direct acoustic testing, standing acoustic modes may strongly couple with the fundamental structural modes of the test hardware (Reference 1). In this paper results from a recent reverberant chamber acoustic test of a composite reflector are discussed. These results provide further convincing evidence of the acoustic standing wave and structural modes coupling phenomenon. The purpose of this paper is to alert test organizations to this phenomenon so that they can account for the potential increase in structural responses and ensure that flight hardware undergoes safe testing. An understanding of the coupling phenomenon may also help minimize the over and/or under testing that could pose un-anticipated structural and flight qualification issues.

Reverberant acoustic field↗
NASA NTRS2012

Impact of Acoustic Standing Waves on Structural Responses: Reverberant Acoustic Testing (RAT) vs. Direct Field Acoustic Testing (DFAT)

Loudspeakers have been used for acoustic qualification of spacecraft, reflectors, solar panels, and other acoustically responsive structures for more than a decade. Limited measurements from some of the recent speaker tests used to qualify flight hardware have indicated significant spatial variation of the acoustic field within the test volume. Also structural responses have been reported to differ when similar tests were performed using reverberant chambers. To address the impact of non-uniform acoustic field on structural responses, a series of acoustic tests were performed using a flat panel and a 3-ft cylinder exposed to the field controlled by speakers and repeated in a reverberant chamber. The speaker testing was performed using multi-input-single-output (MISO) and multi-input-multi-output (MIMO) control schemes with and without the test articles. In this paper the spatial variation of the acoustic field due to acoustic standing waves and their impacts on the structural responses in RAT and DFAT (both using MISO and MIMO controls for DFAT) are discussed in some detail.

speakers↗
NASA NTRS1982

Standing waves along a microwave generated surface wave plasma

Two surface wave plasma columns, generated by microwave power in argon at gas pressures of 0.05 torr to 330 torr, interact in the same discharge tube to form standing surface waves. Radial electric field and azimuthal magnetic field outside the discharge tube are measured to be 90 deg out of phase with respect to axial position and to decay exponentially with radial distance from the tube axis. Maximum light emission occurs at the position of maximum azimuthal magnetic field and minimum radial electric field. Electron temperature and density are measured at low pressures with double probes inserted into the plasma at a null of radial electric field. Measured electron densities compare well with those predicted by Gould-Trivelpiece surface wave theory.

Rogers, J.↗
NASA NTRS1982

Circumferentially segmented duct lines optimized for axisymmetric and standing wave sources

Optimum and off-optimum properties of circumferentially segmented duct liners are compared with those of uniform liners to identify any potential benefits of circumferentially segmented liners. High- and low-order spinning-mode sources are considered in the study. The solution for the segmented liner is obtained by a multimodal expansion of the segmented-liner eigenmodes in terms of a series of hardwall duct models. The coefficients in the hard-wall series are obtained by using Galerkin's method. Results show that for some frequencies and duct lengths, circumferentially segmented liners scatter energy equally between a higher and lower order circumferential wave number. Studies for higher order spinning-mode sources show that an optimized segmented liner with a hard-wall/soft-wal admittance variation representing an optimum configuration gives better performance than an optimized uniform liner. Overall, the greatest benefit of the segmented liner over the uniform liner occurs under off-optimum conditions. The optimized segmented liner gives more effective broadband performance than the optimized uniform liner.

Watson, W. R.↗
NASA NTRS1989

Distortion effects in spacecraft observations of MHD toroidal standing waves - Theory and observations

This paper considers the distortion effect of the simultaneous occurrence of locally resonant toroidal pulsations over a range of L shells on the interpretation of spacecraft observations of these waves. A theoretical model is developed which yields quantitative predictions of the phase shear effect that causes a shift from the true pulsation frequency measured from a spacecraft moving radially across L shells. Calculations with this model indicate that, due to this effect, the AMPTE/CCE satellite should observe 8-30 percent lower frequencies on outbound passes than on inbound passes. A comparison of fundamental mode frequency measurements made on 10 inbound and 9 outbound passes of the AMPTE/CCE satellite in the morning sector confirms the predictions of the model.

Anderson, B. J.↗
NASA NTRS1990

Toroidal standing waves excited by a storm sudden commencement - DE 1 observations

A 74-nT sudden commencement on July 13, 1982, was observed in the magnetosphere, with instruments on the Dynamics Explorer 1 satellite. Inbound, near L = 4.5, the satellite was located at 1524 magnetic local time and 20 deg magnetic latitude. The sudden commmencement established a strong, east-west oscillation, with 100-s period, which was observed in the magnetic field, the electric field, and the plasma flow velocity records. There was also a compressional component of this 100-s oscillation and a rapidly damped 300-s compressional pulsation. The compressional oscillations may be an evidence of cavity resonances, excited by the sudden commencement. The cavity waves may, in turn, couple to toroidal waves in field line resonance at the satellite location. In addition, the sudden commencement caused the onset of waves with frequencies from 0.1 up to at least 0.5 Hz. The observations are compared with similar reports from earlier pulsations related to sudden commencements.

Cahill, L. J., Jr.↗
NASA NTRS2002

An Experimental Study of Nonlinear Standing Waves in Resonators with Numerical Comparison

Lawrenson et. al. [Journal of the Acoustic Society of America, Nov. 1998] described the generation of shock-free high-amplitude pressure waves in closed cavities using large equipment and resonators to produce the reported effects. An attempt is made to generate shock-free high-amplitude pressure waves using relatively small resonators. Ambient air is used as the working fluid. A small cylindrical resonator is tested resulting in the lack of a shocked waveform while a larger model of the same shape produces shock waves. A small conical resonator produces shock-free pressure waves at resonance, but the amplitude of these waves is small. A larger cone resonator model produces shock-free pressure waves of higher amplitude. A large horn-cone resonator also produces shock-free high amplitude pressure waves, A numerical model is used to compare the experimental results to theoretical results. The effects of structural resonances on the production of shock-free high-amplitude pressure waves are discussed, especially concerning difficulties encountered when these resonances were in the frequency ranges of interest. Identifying features of a structural resonance are presented.

Finkbeiner, Joshua R.↗
NASA NTRS2004

Modeling Nonlinear Acoustic Standing Waves in Resonators: Theory and Experiments

The overall goal of the cooperative research with NASA Glenn is to fundamentally understand, computationally model, and experimentally validate non-linear acoustic waves in enclosures with the ultimate goal of developing a non-contact acoustic seal. The longer term goal is to transition the Glenn acoustic seal innovation to a prototype sealing device. Lucas and coworkers are credited with pioneering work in Resonant Macrosonic Synthesis (RMS). Several Patents and publications have successfully illustrated the concept of Resonant Macrosonic Synthesis. To utilize this concept in practical application one needs to have an understanding of the details of the phenomenon and a predictive tool that can examine the waveforms produced within resonators of complex shapes. With appropriately shaped resonators one can produce un-shocked waveforms of high amplitude that would result in very high pressures in certain regions. Our goal is to control the waveforms and exploit the high pressures to produce an acoustic seal. Note that shock formation critically limits peak-to-peak pressure amplitudes and also causes excessive energy dissipation. Proper shaping of the resonator is thus critical to the use of this innovation.

Raman, Ganesh↗