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All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Solutions of one-dimensional steady nozzle flow revisited

A formulation is presented by which any iteration process for obtaining the entropy increase in the flow of a one-dimensional steady nozzle is eliminated, and the simple solution of a quadratic equation is obtained. The proper parameters are then explicitly seen in the equation, and their effects on the solution are easily determined. Since only one root of the equation is physically admissible, entropy production, and therefore the shock wave, are uniquely determined by this set of parameters.

Liou, Meng-Sing↗
NASA NTRS1989

H2O masers in star-forming regions

A comprehensive model is presented for the powerful H2O masers observed in star-forming regions. The shocks are the result of interaction of dense, high-velocity clumps with dense shells or clumps in the ambient medium. The shocked material is a thin slab in physical space, but breaks up into a network of filaments in velocity space. These filaments are the individual H2O maser features. The results of maser model calculations are presented which provide the maximum brightness temperatures that can be produced in single-temperature environments of any kind.

Elitzur, Moshe↗
NASA NTRS1992

Relaxation and turbulence effects on sonic boom signatures

The rudimentary theory of sonic booms predicts that the pressure signatures received at the ground begin with an abrupt shock, such that the overpressure is nearly abrupt. This discontinuity actually has some structure, and a finite time is required for the waveform to reach its peak value. This portion of the waveform is here termed the rise phase, and it is with this portion that this presentation is primarily concerned. Any time characterizing the duration of the rise phase is loosely called the 'rise time.' Various definitions are used in the literature for this rise time. In the present discussion the rise time can be taken as the time for the waveform to rise from 10 percent of its peak value to 90 percent of its peak value. The available data on sonic booms that appears in the open literature suggests that typical values of shock over-pressure lie in the range of 30 Pa to 200 Pa, typical values of shock duration lie in the range of 150 ms to 250 ms, and typical values of the rise time lie in the range of 1 ms to 5 ms. The understanding of the rise phase of sonic booms is important because the perceived loudness of a shock depends primarily on the structure of the rise phase. A longer rise time typically implies a less loud shock. A primary question is just what physical mechanisms are most important for the determination of the detailed structure of the rise phase.

Pierce, Allan D.↗
NASA NTRS1994

Al III, Si IV, and C IV absorption toward zeta Ophiuchi: Evidence for photionized and collisionally ionized gas

We present Goddard High-Resolution Spectrograph observations at 3.5 km/s resolution and signal-to-noise ratios of 30 to 60 for the Al III, Si IV, and N V absorption lines in the far-ultraviolet spectrum of the O9.5 V star zeat Ophiuchi. The measurement reveal three types of highly ionized gas along the 140 pc line of sight. (1) Narrow components of Al III (b = 4.3 km/s, the mean value of (v(helio)) = -7.8 km/s; b = 3.2 km/s, the mean value of (v(sub helio)) = -14.4 km/s) and Si IV (b = 5.3 km/s, the mean value of (v(sub helio)) = -15.0 km/s) trace photionized gas in the expanding H II region surrounding zeta Oph. The observed magnitude and direction of the velocity offset between the Al III and Si IV profiles can be explained by models of H II regions that incorporate expansion. Narrow C IV absorption associated with the H II region is not detected. Predictions of the expected amounts of Si IV and C IV overestimate the column densities of these ions by factors of 30 and more than 10, respectively. The discrepancy may be due to the effects of elemental depletions in the gas and/or to the interaction of the stellar wind with surrounding matter. (2) Broad (b = 15 to 18 km/s) and weak Si IV and C IV absorption components are detected near the mean value of (v(sub helio)) = -26 km/s. The high-ionization species associated with these absorption components are probably produced by electron collisional ionization in a heated gas. This absorption may be physically related to the zeta Oph bow shock ot to a cloud complex situated within the local interstellar medium at d less than 60 pc. The C IV to Si IV column density ratio in this gas is 8, a factor of 6 less than conductive interface models predict, but this discrepancy may be removed by considering the effects of self-photoionization within the cooling gas in the model calculations. (3) A broad (b = 13 km/s) and weak C IV absorption feature detected at the mean value of (v(sub helio)) = -61 km/s is not seen in other species. We tentatively ascribe this absorption to gas in a postshock ragion of an optically thin shock in the zeta Oph stellar wind.

Sembach, Kenneth R.↗
NASA NTRS1989

Fast Fermi and gradient drift acceleration of electrons at nearly perpendicular collisionless shocks

Electrons can be reflected and effectively energized at quasi-perpendicular shocks. At a curved shock this process is most prominent where the upstream magnetic field and the shock surface are tangent. A theoretical explanation of the underlying physical mechanism in terms of fast Fermi acceleration has been proposed in the literature. The theory uses properties of the de Hoffmann-Teller frame and assumes conservation of magnetic moment in a static, simplified shock profile. Here, the discussion is extended in order to clarify certain aspects of the reflection process and to pinpoint the physical mechanisms that are operative and dominant from the viewpoint of the normal incidence frame. By reducing the analysis to the essential physical content and solving the pertinent energy equation, the equivalence of fast Fermi and gradient drift acceleration is shown.

Krauss-Varban, D.↗
NASA NTRSDate not supplied

Cross-Measurement Comparisons for a CFD Validation Dataset on Mach 2.5 Axisymmetric Turbulent Shock-Wave/Boundary-Layer Interactions

Experimental data for a shock-wave/boundary-layer interaction has been collected using multiple measurement techniques. Unfortunately, diversity of methods for acquisition begets an aggregation of data which does not directly quantify the same properties of the flowfield. The objective in this paper was to (a) present the collection of flowfield measurements in one venue in a format more usable for those validating CFD models against the data, (b) evaluate the degree to which the experimental data support each other, and (c) highlight the differences and relative advantages/shortcomings of each measurement techniques. To present the various measured quantities into common format, CFD results from a companion paper also submitted for presentation at this meeting are utilized. For the case where the boundary layer remains attached, there is agreement between the various measurements as well as with Reynolds-averaged Navier-Stokes simulation solutions. As the impinging shock strength is increased beyond the point of separating the boundary layer, the congruity of the data wanes. Generally, agreement among the measurements exceeds the degree to which the CFD solutions agree with experiments. This suggests that unmodeled physical phenomena, such as transient motion of the reflected shock and separation bubble, give rise to the discrepancies observed in the computational results.

Shock-wave Boundary-layer interaction↗
NASA NTRS2004

Fact Sheet: Range Complex

NASA Ames has a long tradition in leadership with the use of ballistic ranges and shock tubes for the purpose of studying the physics and phenomena associated with hypervelocity flight. Cutting-edge areas of research run the gamut from aerodynamics, to impact physics, to flow-field structure and chemistry. This legacy of testing began in the NACA era of the 1940's with the Supersonic Free Flight Tunnel, and evolved dramatically up through the late 1950s with the pioneering work in the Ames Hypersonic Ballistic Range. The tradition continued in the mid-60s with the commissioning of the three newest facilities: the Ames Vertical Gun Range (AVGR) in 1964, the Hypervelocity Free Flight Facility (HFFF) in 1965 and the Electric Arc Shock Tube (EAST) in 1966. Today the Range Complex continues to provide unique and critical testing in support of the Nation's programs for planetary geology and geophysics; exobiology; solar system origins; earth atmospheric entry, planetary entry, and aerobraking vehicles; and various configurations for supersonic and hypersonic aircraft.

Cornelson, C.↗
NASA NTRS1992

Diabatic dynamic initialization

A generalized dynamical adjustment procedure was applied to a diabatic model to produce balanced initial conditions. Namely, backward adiabatic model integration is followed by forward diabatic model integration, with a high frequency (low pass) filter in the form of the Euler-backward time differencing scheme being applied throughout the whole integration. As a result of the application of such a diabatic dynamic initialization procedure within the Goddard Laboratory for Atmospheres (GLA) 4-D data assimilation system, the following properties of forecasts from initialized fields are achieved right from the beginning of the usual forecast integration: (1) the forecast tendencies (and fields) are free of any noise due to imbalance in initial conditions; (2) the shocks related to an initial imbalance between model physics and dynamics, and especially the substantial initial imbalance of precipitation and evaporation fields, or the initial spin-up effect, are practically removed. Diabatic dynamic initialization has been compared with implicit nonlinear normal mode initialization, and found to be superior in removing the initial spin-up effect, and also in improving the tropical structure. The diabatic dynamic initialization procedure was successfully tested for the GLA system with the use of all conventional data and the GLA satellite data retrievals. It allows a smooth data insertion without any shocks or imbalances, which is highly desirable for efficient functioning of 4-D data assimilation systems. The developed initialization procedure is computationally efficient and in principle easily applicable to different forecast models.

Fox-Rabinovitz, Michael S.↗
NASA NTRS1992

Particle acceleration, transport and turbulence in cosmic and heliospheric physics

In this progress report, the long term goals, recent scientific progress, and organizational activities are described. The scientific focus of this annual report is in three areas: first, the physics of particle acceleration and transport, including heliospheric modulation and transport, shock acceleration and galactic propagation and reacceleration of cosmic rays; second, the development of theories of the interaction of turbulence and large scale plasma and magnetic field structures, as in winds and shocks; third, the elucidation of the nature of magnetohydrodynamic turbulence processes and the role such turbulence processes might play in heliospheric, galactic, cosmic ray physics, and other space physics applications.

Matthaeus, W.↗
NASA NTRS2010

Analysis of Nozzle Jet Plume Effects on Sonic Boom Signature

An axisymmetric full Navier-Stokes computational fluid dynamics (CFD) study was conducted to examine nozzle exhaust jet plume effects on the sonic boom signature of a supersonic aircraft. A simplified axisymmetric nozzle geometry, representative of the nozzle on the NASA Dryden NF-15B Lift and Nozzle Change Effects on Tail Shock (LaNCETS) research airplane, was considered. The highly underexpanded nozzle flow is found to provide significantly more reduction in the tail shock strength in the sonic boom N-wave pressure signature than perfectly expanded and overexpanded nozzle flows. A tail shock train in the sonic boom signature, similar to what was observed in the LaNCETS flight data, is observed for the highly underexpanded nozzle flow. The CFD results provide a detailed description of the nozzle flow physics involved in the LaNCETS nozzle at different nozzle expansion conditions and help in interpreting LaNCETS flight data as well as in the eventual CFD analysis of a full LaNCETS aircraft. The current study also provided important information on proper modeling of the LaNCETS aircraft nozzle. The primary objective of the current CFD research effort was to support the LaNCETS flight research data analysis effort by studying the detailed nozzle exhaust jet plume s imperfect expansion effects on the sonic boom signature of a supersonic aircraft. Figure 1 illustrates the primary flow physics present in the interaction between the exhaust jet plume shock and the sonic boom coming off of an axisymmetric body in supersonic flight. The steeper tail shock from highly expanded jet plume reduces the dip of the sonic boom N-wave signature. A structured finite-volume compressible full Navier-Stokes CFD code was used in the current study. This approach is not limited by the simplifying assumptions inherent in previous sonic boom analysis efforts. Also, this study was the first known jet plume sonic boom CFD study in which the full viscous nozzle flow field was modeled, without coupling to a sonic boom propagation analysis code, from the stagnation chamber of the nozzle to the far field external flow, taking into account all nonisentropic effects in the shocks, boundary layers, and free shear layers, and their interactions at distances up to 30 times the nozzle exit diameter from the jet centerline. A CFD solution is shown in Figure 2. The flow field is very complicated and multi-dimensional, with shock-shock and shockplume interactions. At the time of this reporting, a full three-dimensional CFD study was being conducted to evaluate the effects of nozzle vectoring on the aircraft tail shock strength.

Bui, Trong↗
NASA NTRS2007

A Shock-Refracted Acoustic Wave Model for the Prediction of Screech Amplitude in Supersonic Jets

A physical model is proposed for the estimation of the screech amplitude in underexpanded supersonic jets. The model is based on the hypothesis that the interaction of a plane acoustic wave with stationary shock waves provides amplification of the transmitted acoustic wave upon traversing the shock. Powell's discrete source model for screech incorporating a stationary array of acoustic monopoles is extended to accommodate variable source strength. The proposed model reveals that the acoustic sources are of increasing strength with downstream distance. It is shown that the screech amplitude increases with the fuiiy expanded jet Mach number. Comparisons of predicted screech amplitude with available test data show satisfactory agreement. The effect of variable source strength on directivity of the fundamental (first harmonic, lowest frequency mode) and the second harmonic (overtone) is found to be unimportant with regard to the principal lobe (main or major lobe) of considerable relative strength, and is appreciable only in the secondary or minor lobes (of relatively weaker strength

Kandula, Max↗
NASA NTRS2010

The Effect of Nondeterministic Parameters on Shock-Associated Noise Prediction Modeling

Engineering applications for aircraft noise prediction contain models for physical phenomenon that enable solutions to be computed quickly. These models contain parameters that have an uncertainty not accounted for in the solution. To include uncertainty in the solution, nondeterministic computational methods are applied. Using prediction models for supersonic jet broadband shock-associated noise, fixed model parameters are replaced by probability distributions to illustrate one of these methods. The results show the impact of using nondeterministic parameters both on estimating the model output uncertainty and on the model spectral level prediction. In addition, a global sensitivity analysis is used to determine the influence of the model parameters on the output, and to identify the parameters with the least influence on model output.

Dahl, Milo D.↗
NASA NTRS1986

Shock-wave boundary layer interactions

Presented is a comprehensive, up-to-date review of the shock-wave boundary-layer interaction problem. A detailed physical description of the phenomena for transonic and supersonic speed regimes is given based on experimental observations, correlations, and theoretical concepts. Approaches for solving the problem are then reviewed in depth. Specifically, these include: global methods developed to predict sudden changes in boundary-layer properties; integral or finite-difference methods developed to predict the continuous evolution of a boundary-layer encountering a pressure field induced by a shock wave; coupling methods to predict entire flow fields; analytical methods such as multi-deck techniques; and finite-difference methods for solving the time-dependent Reynolds-averaged Navier-Stokes equations used to predict the development of entire flow fields. Examples are presented to illustrate the status of the various methods and some discussion is devoted to delineating their advantages and shortcomings. Reference citations for the wide variety of subject material are provided for readers interested in further study.

Delery, J.↗
NASA NTRS1976

Density wave theory

The prospect that density waves and galactic shock waves are present on the large-scale in disk-shaped galaxies has received support in recent years from both theoretical and observational studies. Large-scale galactic shock waves in the interstellar gas are suggested to play an important, governing role in star formation, molecule formation, and the degree of development of spiral structure. Through the dynamics of the interstellar gas and the galactic shock wave phenomenon, a new insight into the physical basis underlying the morphological classification system of galaxies is suggested.

Roberts, W. W., Jr.↗
NASA NTRS1977

Density wave theory

The prospect that density waves and galactic shock waves are present on the large scale in disk shaped galaxies has received support in recent years from both theoretical and observational studies. Large-scale galactic shock waves in the interstellar gas are suggested to play an important governing role in star formation, molecule formation, and the degree of development of spiral structure. Through the dynamics of the interstellar gas and the galactic shock-wave phenomenon, a new insight into the physical basis underlying the morphological classification system of galaxies is suggested.

Roberts, W. W., Jr.↗
NASA NTRS1983

Numerical computations of turbulence amplification in shock wave interactions

Numerical computations are presented which illustrate and test various effects pertinent to the amplification and generation of turbulence in shock wave turbulent boundary layer interactions. Several fundamental physical mechanisms are identified. Idealizations of these processes are examined by nonlinear numerical calculations. The results enable some limits to be placed on the range of validity of existing linear theories.

Zang, T. A.↗
NASA NTRS1991

Darkening in gas-rich ordinary chondrites: Spectral modelling and implications for the regoliths of ordinary chondrite parent bodies

The dark fine grained matrix of gas-rich ordinary chondrites replicates many of the physical, morphological, and spectral characteristics of the highly shocked and optically altered black chondrites. Spectral mixture modeling shows that the darkening and spectral attenuation seen in the dark matrix can be simulated with realistic mass fractions of light host material and black chondritic material. All these factors point to the conclusion that the dark matrix of gas-rich ordinary chondrites is dark due to the same processes that darkens black chondrites, shock distributed small particle size FeNi metal and troilite. Because the darkening is not seen in any of the non-gas-rich light portions and is only seen in the gas rich grains of the meteorite, the shock darkening would have to occur as part of the matrix's exposure to regolith processes. Since all gas-rich grains are darkened, it follows that darkening is not only common, but pervasive in asteroidal regoliths. These results imply that the upper, optically active layer of an ordinary chondrite parent body should have the spectral characteristics of a black chondrite, which are a dark, relatively featureless spectrum with modest red slope in the infrared. These are the characteristics of special type C asteroids.

Britt, Daniel T.↗
NASA NTRS1993

Diabatic dynamic initialization

A version of the dynamical adjustment procedure is generalized in order to make the procedure applicable to a diabatic model. In this new initialization procedure, backward adiabatic model integration is followed by forward diabatic model integration, with a high-frequency (low-pass) time filter in the form of the Euler backward time-differencing scheme applied throughout the integration. The new procedure was applied to the Goddard Laboratory for Atmospheres 4D data-assimilation system. It was found that, right from the beginning of the forecast integration, the forecast tendencies (and fields) were free of any noise due to imbalance in initial conditions, and the shocks related to an initial imbalance between model physics and dynamics, or the initial spinup effect, were practically removed.

Fox-Rabinovitz, Michael S.↗