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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 325 records · Page 18
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NASA NTRS1981

Perspectives on jet noise

Jet noise is a byproduct of turbulence. Until recently turbulence was assumed to be known statistically, and jet noise was computed therefrom. As a result of new findings though on the behavior of vortices and instability waves, a more integrated view of the problem has been accepted lately. After presenting a simple view of jet noise, the paper attempts to resolve the apparent differences between Lighthill's and Lilley's interpretations of mean-flow shear, and examines a number of ad hoc approaches to jet noise suppression.

Ribner, H. S.↗
NASA NTRS1981

On the interaction of a sound pulse with the shear layer of an axisymmetric jet

The behavior of a sound pulse from a simulated source in a jet is investigated both experimentally and numerically. Both approaches show that in the low and medium frequencies the far field acoustic power exhibits and marked amplification as the flow velocity increases. Experimentally this changes to an attenuation at the higher frequencies which cannot be computed by the numerical model. This amplification is traced to shear noise terms which trigger the instability waves that are inherent within the flow.

Maestrello, L.↗
NASA NTRS1981

Acoustic and turbulence measurements of a tone-excited jet with and without flight simulation

Acoustic as well as turbulence measurements were made of tone-excited jets to obtain an understanding of the broadband noise augmentation mechanism. Results for both heated and unheated jets with and without flight simulation are presented for a range of excitation frequencies and levels, as well as for the zero order and first order spinning modes. It is argued that although the phase-locked large-scale turbulence structure is at the root of the noise amplification process, the actual noise generation mechanism lies in the small-scale turbulence. Results show that (1) the phase velocity of the excited large scale instability waves is subsonic relative to the ambient fluid and (2) broadband jet noise is almost uniform at all frequencies. Moreover, when the large-scale and small-scale structures are plotted against forward velocity (for a given upstream excitation level of 141 dB), the change in large-scale turbulence is negligible with forward velocity, whereas the small-scale turbulence decreases for both excited and unexcited jets. Also, the corresponding difference in far-field noise does not appear to change significantly, indicating that the changes in small-scale turbulence are responsible for jet noise amplification.

Ahuja, K. K.↗
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 NTRS1983

On the interaction of a sound pulse with the shear layer of an axisymmetric jet. II - Heated jets

The fluctuating field of a heated jet excited by a sound pulse is simulated numerically. The fluctuations in both the flow field and the far field are studied. The flow field results depend crucially on the stability properties of the heated jet. The altered stability properties due to the heating cause a reduction in the overall fluctuations and a shift of the instability waves into lower frequencies. These changes cause a similar downward shift in the far field spectrum and a reduction in the total far field noise. The results provide a partial explanation in terms of stability theory of experimentally observed properties of the noise of heated jets.

Bayliss, A.↗
NASA NTRS1983

A model of the excitation of large scale fluctuations in a shear layer

The acoustical excitation of instability waves in free shear layers is investigated theoretically and experimentally. A configuration with a thin shear layer leaving from a semi-infinite plate is considered. The excitation by sound sources in various locations, e.g., far away from the trailing edge or very near the edge lip, is analyzed. The influence of additional walls (like those in a channel) is also predicted. The theory is extended to include two streams on both sides of the shear layer having different velocities and densities. A reference quantity for the artificial excitation is suggested. The experimental data compare well with the theory. The theory does not contain any empirical constants.

Bechert, D. W.↗
NASA NTRS1983

Plasmapause diffusion

The Bohm diffusion coefficient and observed electrostatic wave scattering are used as the bases of estimates of the smoothing effect that diffusion may have on steep plasmapause density gradients. The estimate for diffusion resulting from scattering by observed electrostatic waves is found to be much lower than that of the perpendicular Bohm diffusion coefficient for characteristic plasma temperatures and magnetic fields. This diffusion rate estimate may be too small, however, if the wave amplitudes are significantly higher for steep plasmapauses. The effects are therefore negligible for most considerations of macroscopic plasmapause dynamics, but may be significant in limiting drift wave instabilities and similar phenomena driven by the steepness of the plasmapause density gradient.

Horwitz, J. L.↗
NASA NTRS1984

Boundary-layer linear stability theory

Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer reached a stage of development which permit the direct solution of the primary differential equations, numerical results were obtained from the linear theory during the next 10 years for many different boundary layer flows: three dimensional boundary layers; free convention boundary layers; compressible boundary layers; boundary layers on compliant walls; a recomputation of Falkner-Skan flows; unsteady boundary layers; and heated wall boundary layers.

Leslie M Mack↗
NASA NTRS1985

Sudden stratospheric warmings as catastrophes

The sudden stratospheric warming (SSW) process is qualitatively studied using a conceptual and numerical approach guided by catastrophe theory. A simple example of a catastrophe taken from nonlinear dynamics is given, and results from previous modelling studies of SSW are interpreted in light of catastrophe theory. Properties of this theory such as hysteresis, cusp, and triggering essential to SSW are numerically demonstrated using the truncated quasi-geostrophic beta-plane model of Holton and Mass (1976). A qualitative explanation of the transition from the steady regime to the vacillation regime is given for the Holton and Mass model in terms of the topographically induced barotropic Rossby wave instability. Some implications for the simulation and prediction of SSW are discussed.

Chao, W. C.↗
NASA NTRS1986

The near and far acoustic fields of broadband shock associated noise

A stochastic model theory is presented which suggests that the broadband shock associated noise of supersonic jets is generated by the weak interaction between downstream propagating large turbulence structures and the quasiperiodic shock cells in the jet plume. The large turbulence structures are modelled by a superposition of the intrinsic instability waves of the mean flow of the jet, and the quasiperiodic shock cells are broken down into time-independent waveguide modes of the jet flow using multiple scales expansion. The observed broadband shock associated noise consists of a superposition of the spectra generated by the different waveguide modes. Starting from the equations of motion of a compressible fluid, formulas for calculating the far field and near field noise are developed. Good general agreement is found with previous results.

Tam, C. K. W.↗
NASA NTRS1985

Near field pressure fluctuations in a circular jet

The vortical structures in shear layers govern many dynamic aspects of the jet, including entrainment of mass, transfer of momentum and noise generation. These structures develop from instability waves and undergo a merging process. Vortex merging reduces the time scale and increases the length scale of the shear layer. At the same time pressure fluctuations are induced by the unsteady motion. These pressure perturbations can exert high level dynamic loading on nearby structures, and radiate to the far field as noise. In this study, the generation mechanism of the pressure fluctuations and the radiation process are examined.

Ho, C. M.↗
NASA NTRS1987

A laboratory investigation of potential double layers

In a triple plasma device, the injection of electron current from the source chamber to the target chamber causes the formation of a potential double layer. At a low current density, the space charge of the injected current produces a virtual cathode-type potential double layer. This double layer is stable, and various wave instabilities are observed to associate with this double layer. As the current density is increased, the double layer becomes unstable, and a moving double layer results. As the current density is increased further, the enhanced ionization causes the neutralization of the space charge of the electron beam, and the beam plasma discharge is ignited.

Leung, Philip↗
NASA NTRS1985

Atmospheric circulation of Venus

Observational data of Venus are utilized to study rotational effects on atmospheric circulations. The high surface temperature and planetary-scale turbulent motion at cloud tops, and the relation between energy and momentum budget are examined. The limited amount of data available on the vertical and horizontal distribution of net radiative heating, the zonal wind structure, and waves affects the study of the temperature and motion on Venus. The limitations of the scaling analysis used to estimate the properties of the circulation as regards the cyclostrophic balance, the extent of the Hadley circulation, large-scale wave transport, vertical propagation of waves, convection, and turbulence are considered. Hypotheses concerned with the deep, cloud-level, and upper atmospheres of Venus are proposed. Future research in the areas of propagating planetary- scale waves, zonal flow and planetary-scale wave instability processes, and convection is suggested.

Rossow, W. B.↗
NASA NTRS1986

Waves generated during electron beam emissions from the Space Shuttle

Observations from the SEPAC (Space Experiments with Particle Accelerators) experiment on Spacelab 1 Shuttle mission of waves generated during electron beam emissions are presented. The dependence of the wave intensity on the beam parameters and the Shuttle attitude is outlined, and the shape of the wave spectra are discussed. It is found that the noise in the 0.75- to 10 kHz band has a spectral shape that can be characterized by an f exp -n law, and that the VLF signal level depends on the beam angle to the magnetic field, the strongest emissions being observed for parallel beams. These features are found to be consistent with a drift wave instability.

Neubert, T.↗
NASA NTRS1987

Dynamics of the middle atmosphere

Research performed in the U.S. from October 1982 to May 1986 on the middle-atmosphere dynamics is reviewed. The topics discussed include extratropical large- and medium-scale waves and instabilities, gravity waves, tropical motions, and the general circulation. In addition, consideration is given to the studies of atmosphere constituent transport. A number of studies in the latter area have shed light on the specific seasonal behavior of dynamically controlled important constituents in the stratosphere, such as ozone, water vapor, and aerosols.

Schoeberl, Mark R.↗
NASA NTRS1987

A laboratory investigation of potential double layers

In a triple plasma device, the injection of electron current from the source chamber to the target chamber causes the formation of a potential double layer. At a low current density, the space charge of the injected current produces a virtual cathode-type potential double layer. This double layer is stable and various wave instabilities are observed to associate with this double layer. As the current density is increased, the double layer becomes unstable and a moving double layer results. As the current density is increased further, the enhanced ionization causes the neutralization of the space charge of the electron beam and the 'beam plasma discharge' is ignited.

Leung, Philip↗
NASA NTRS1987

Stochastic model theory of broadband shock associated noise from supersonic jets

A method based on the work of Tam and Tanna (1982) for calculating the near field noise spectrum and the spatial distribution of broadband shock associated noise from supersonic jets is proposed. Multiple-scales expansion is used to decompose the quasi-periodic shock cells into time-independent waveguide modes of the jet flow, and the interaction of the instability waves with each of the waveguide modes is shown to generate unsteady disturbances which become part of the broadband shock associated noise when radiated to the far field. The observed broadband shock associated noise is composed of a superposition of the various distinct spectra of the different waveguide modes, and the multispectra can be easily identified in many of the existing far and near field noise measurements.

Tam, C. K. W.↗
NASA NTRS1987

Anisotropic interface kinetics and tilted cells in unidirectional solidification

A nonlinear evolution equation is derived which governs the cellular structure of a binary alloy with a small segregation coefficient, including the effects of anisotropic interface kinetics. This equation, applicable to long-wave instabilities of a planar interface, describes the spatial pattern of the growing disturbances. The presence of anisotropy causes the cells to grow at an angle to the normal of the planar front. This transition to a cellular morphology is shown to be a subcritical instability.

Young, G. W.↗