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NASA NTRS1976

Transitions of Zn XXII, Zn XXIII, Zn XXIV, Ge XXIV, and Ge XXV observed in laser-produced plasmas

Wavelengths and wave numbers are presented for zinc lines in the F I, O I, and N I isoelectronic sequences as well as for germanium lines in the F I and O I sequences. The transitions are of the type 2s(2)2p(k)-2s2p(k+1), and the lines were observed near 100 A in the spectra of plasmas produced by focusing the pulse from a high-power glass laser on to solid targets of different elements and compounds. The results are compared with prediction based on extrapolations of semiempirical formulas derived by Edlen (1969). The wavelengths and wave numbers of some of the corresponding transitions in Cu XXI, Cu XXII, Cu XXIII, Ga XXIII, and Ga XXIV are interpolated with a probable wavelength accuracy of at least plus or minus 0.02 A.

Behring, W. E.↗

NASA NTRS2015

Lidar Measurements of Atmospheric CO2 From Regional to Global Scales

Atmospheric CO2 is a critical forcing for the Earth's climate and the knowledge on its distributions and variations influences predictions of the Earth's future climate. Large uncertainties in the predictions persist due to limited observations. This study uses the airborne Intensity-Modulated Continuous-Wave (IMCW) lidar developed at NASA Langley Research Center to measure regional atmospheric CO2 spatio-temporal variations. Further lidar development and demonstration will provide the capability of global atmospheric CO2 estimations from space, which will significantly advances our knowledge on atmospheric CO2 and reduce the uncertainties in the predictions of future climate. In this presentation, atmospheric CO2 column measurements from airborne flight campaigns and lidar system simulations for space missions will be discussed. A measurement precision of approx.0.3 ppmv for a 10-s average over desert and vegetated surfaces has been achieved. Data analysis also shows that airborne lidar CO2 column measurements over these surfaces agree well with in-situ measurements. Even when thin cirrus clouds present, consistent CO2 column measurements between clear and thin cirrus cloudy skies are obtained. Airborne flight campaigns have demonstrated that precise atmospheric column CO2 values can be measured from current IM-CW lidar systems, which will lead to use this airborne technique in monitoring CO2 sinks and sources in regional and continental scales as proposed by the NASA Atmospheric Carbon and Transport â€" America project. Furthermore, analyses of space CO2 measurements shows that applying the current IM-CW lidar technology and approach to space, the CO2 science goals of space missions will be achieved, and uncertainties in CO2 distributions and variations will be reduced.

Lin, Bing↗

NASA NTRS1993

A laboratory study of subjective response to sonic booms measured at White Sands Missile Range

The Sonic Boom Simulator of the Langley Research Center was used to quantify subjective loudness response to boom signatures consisting of: (1) simulator reproductions of booms recently recorded at White Sands Missile Range; (2) idealized N-waves; and (3) idealized booms having intermediate shocks. The booms with intermediate shocks represented signatures derived from CFD predictions. The recorded booms represented those generated by F15 and T38 aircraft flyovers and represented a variety of waveforms reflecting the effects of propagation through a turbulent atmosphere. These waveforms included the following shape categories: N-waves, peaked, rounded, and U-shaped. Results showed that Perceived Level and Zwicker Loudness Level were good estimators of the loudness of turbulence modified sonic booms. No significant differences were observed between loudness responses for the several shape categories when expressed in terms of Perceived Level. Thus, Perceived Level effectively accounted for waveform differences due to turbulence. Idealized booms with intermediate shocks, however, were rated as being approximately 2.7 dB(PL) less loud than the recorded signatures. This difference was not accounted for by PL.

Sullivan, Brenda M.↗

NASA NTRS1990

Development of a computer model to predict platform station keeping requirements in the Gulf of Mexico using remote sensing data

Offshore operations such as oil drilling and radar monitoring require semisubmersible platforms to remain stationary at specific locations in the Gulf of Mexico. Ocean currents, wind, and waves in the Gulf of Mexico tend to move platforms away from their desired locations. A computer model was created to predict the station keeping requirements of a platform. The computer simulation uses remote sensing data from satellites and buoys as input. A background of the project, alternate approaches to the project, and the details of the simulation are presented.

Barber, Bryan↗

NASA NTRS2020

Decadal Changes in the Interannual Variability of Heat Waves in East Asia Caused by Atmospheric Teleconnection Changes

The heat wave in East Asia is examined by using empirical orthogonal function analysis to isolate dominant heat-wave patterns in the ground-based temperature observations over the Korean Peninsula and China and related large-scale atmospheric circulations obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis 1 during 1973–2012. This study focuses particularly on the interannual variability of heat waves and its decadal change. The analysis identifies two major atmospheric teleconnection patterns playing an important role in developing typical heat-wave patterns in East Asia—the Scandinavian (SCAND) and the circumglobal teleconnection (CGT) patterns, which exhibit a significant decadal change in the interannual variability in the mid-1990s. Before the mid-1990s, heat-wave occurrence was closely related to the CGT pattern, whereas the SCAND pattern is more crucial to explain heat-wave variability in the recent period. The stationary wave model experiments suggest an intensification of the SCAND pattern in the recent period driven by an increase in land–atmosphere interaction over Eurasia and decadal change in the dominant heat-wave patterns in East Asia.

heat wave patterns↗

NASA NTRS2008

Initial Results from the Variable Intensity Sonic Boom Propagation Database

An extensive sonic boom propagation database with low- to normal-intensity booms (overpressures of 0.08 lbf/sq ft to 2.20 lbf/sq ft) was collected for propagation code validation, and initial results and flight research techniques are presented. Several arrays of microphones were used, including a 10 m tall tower to measure shock wave directionality and the effect of height above ground on acoustic level. A sailplane was employed to measure sonic booms above and within the atmospheric turbulent boundary layer, and the sailplane was positioned to intercept the shock waves between the supersonic airplane and the ground sensors. Sailplane and ground-level sonic boom recordings were used to generate atmospheric turbulence filter functions showing excellent agreement with ground measurements. The sonic boom prediction software PCBoom4 was employed as a preflight planning tool using preflight weather data. The measured data of shock wave directionality, arrival time, and overpressure gave excellent agreement with the PCBoom4-calculated results using the measured aircraft and atmospheric data as inputs. C-weighted acoustic levels generally decreased with increasing height above the ground. A-weighted and perceived levels usually were at a minimum for a height where the elevated microphone pressure rise time history was the straightest, which is a result of incident and ground-reflected shock waves interacting.

Haering, Edward A., Jr.↗

NASA NTRSDate not supplied

Extreme Precipitation in the Southern US Great Plains in the Spring of 2015: Mechanisms and Prediction

During May of 2015, the southern US Great Plains and adjacent Gulf Coast region experienced more than twice the long-term mean precipitation, making it the wettest May since 1895. We investigate the physical mechanisms associated with this event using a suite of large-ensemble regional replay AGCM simulations from the NASA-GEOS model. In these simulations, certain regions of the globe are constrained to closely follow observations while the remainder of the domain is free running, allowing for the isolation of the remote regions that were important for the event. Preliminary analysis provides evidence that the extreme southern US precipitation was linked in part to positive precipitation anomalies in the central and eastern tropical Pacific via a wave train, which ultimately caused anomalous moisture flux from the Gulf of Mexico. An analysis of Subseasonal Experiment (SubX) model output was conducted to explore the subseasonal prediction skill of the event. Several models are able the predict the presence of positive precipitation anomalies in or near the southern US at lead times exceeding 10 days, albeit with errors in the locations and magnitude of the heaviest precipitation anomalies. A more thorough investigation with version 2 of NASA’s GEOS-S2S model shows that the prediction skill stems from the model’s ability to reasonably predict the positive tropical Pacific precipitation anomalies and the initiation of the Rossby wave train that is believed to be linked to the event. The potential causes for limitations in the prediction skill of this event will be explored.

Great Plains↗

NASA NTRSDate not supplied

Evaluation of Finite Impulse Response Filters for Turbulence Effects on Sonic Booms

Turbulence effects on sonic booms lead to random variability of sonic boom waveforms measured on the ground, complicating the prediction of such waveforms using sonic boom propagation codes that do not account for turbulence effects. The NASA PCBoom software is one such code shown to accurately predict sonic booms above the atmospheric boundary layer but not those on the ground. Analyses of measured sonic booms show that, on average, the turbulence effects result in sonic boom loudness reduction, on average, that increases with propagation distance and turbulence strength. To efficiently account for such effects, a signal processing-based approach has been developed at NASA using finite impulse response filters derived from predicted waveforms by solving a nonlinear parabolic equation. Analyses indicate that the mean loudness reduction obtained using the filters does not increase with propagation distance or with increasing turbulence strength, in disagreement with flight test data. An alternative approach to the filters is currently under development employing the multiple scattering theory (MST) of wave propagation. Available results from the physics-based MST approach are used to evaluate the predictive capability of the filters, suggesting that the filters can severely underestimate the prediction of the MST approach.

sonic booms↗

NASA NTRSDate not supplied

Evaluation of Finite Impulse Response Filters for Turbulence Effects on Sonic Booms

sonic booms↗

NASA NTRS1991

Wind tunnel wall effects in a linear oscillating cascade

Experiments in a linear oscillating cascade reveal that the wind tunnel walls enclosing the airfoils have, in some cases, a detrimental effect on the oscillating cascade aerodynamics. In a subsonic flow field, biconvex airfoils are driven simultaneously in harmonic, torsion-mode oscillations for a range of interblade phase angle values. It is found that the cascade dynamic periodicity - the airfoil to airfoil variation in unsteady surface pressure - is good for some values of interblade phase angle but poor for others. Correlation of the unsteady pressure data with oscillating flat plate cascade predictions is generally good for conditions where the periodicity is good and poor where the periodicity is poor. Calculations based upon linearized unsteady aerodynamic theory indicate that pressure waves reflected from the wind tunnel walls are responsible for the cases where there is poor periodicity and poor correlation with the predictions.

Buffum, Daniel H.↗

NASA NTRS1990

Experimental flaw detection by scattering of plate waves

The surface displacements of elastic waves scattered from a surface-breaking crack are measured with a capacitive transducer on a 2D plate, and comparisons with theoretical predictions are reported. The noncontacting detector has a wide and flat frequency response which measures the response of a glass plate to the propagation of Rayleigh-Lamb waves. A theoretical simulation of the surface displacements is generated by multiplying experimental traces with a cosine window, and Green's function is used to compute the source function. Power spectra are also recorded from the experiment to compare the results with a finite-element model of the crack, and good agreement is reported for results in both the time and frequency domains. The crack can be detected in the far field when the spacing of the receiver locations is small, and the transducer/probe appears to be a practical flaw-detection instrument.

Spetzler, Hartmut↗

NASA NTRS1991

Wind tunnel wall effects in a linear oscillating cascade

Experiments in a linear oscillating cascade reveal that the wind tunnel walls enclosing the airfoils have, in some cases, a detrimental effect on the oscillating cascade aerodynamics. In a subsonic flowfield, biconvex airfoils are driven simultaneously in harmonic, torsion-mode oscillations for a range of interblade phase angle values. It is found that the cascade dynamic periodicity - the airfoil to airfoil variation in unsteady surface pressure - is good for some values of interblade phase angle but poor for others. Correlation of the unsteady pressure data with oscillating flat plate cascade predictions is generally good for conditions where the periodicity is good and poor where the periodicity is poor. Calculations based upon linearized unsteady aerodynamic theory indicate that pressure waves reflected from the wind tunnel walls are responsible for the cases where there is poor periodicity and poor correlation with the predictions.

Buffum, D. H.↗

NASA NTRS1998

Confined States in Large-Aspect-Ratio Thermosolutal Convection

Two-dimensional thermosolutal convection with no-slip boundary conditions is studied using numerical simulations in a periodic domain. The domain is large enough to follow the evolution of phase instabilities of fully nonlinear traveling waves. In the parameter regime studied these instabilities evolve, without loss of phase or hysteresis, into a series of confined states or pulses characterized by locally enhanced heat and solute transport. The wavelength and phase velocity of the traveling rolls within a pulse differ substantially from those in the background. The pulses drift in the same direction as the convection rolls on which they ride but more slowly, and are characterized by an exponential leading front and an oscillatory trailing end. Multiple, apparently stable, states are found for identical parameter values. The qualitative properties of the pulses are in good agreement with the predictions of a third-order phase equation which accounts for the relation between wave number and phase velocity, the oscillatory tails and the multiplicity of states. These properties of the pulses are shown to be a consequence of Shil'nikov dynamics in the spatial domain.

Spina, Alejandro↗

NASA NTRS1978

Nonlinear and linear bottom interaction effects in shallow water

The paper examines wave-energy dissipation rates in shallow water calculated from measured wave spectra at different distances from the shore. Different linear and nonlinear transfer and dissipation mechanisms are discussed. The various data sets are interpreted in terms of prevailing mechanisms at the respective sites. The incorporation of different processes in a predictive shallow-water model is outlined. The analysis suggests that bottom motion is primarily responsible for wave-energy dissipation in the Delta Region of the Gulf of Mexico, that friction is mainly responsible for wave-energy dissipation in Marineland, Panama City and Melkbosstrand, and that percolation is probably the dominant mechanism in the JONSWAP area of the North Sea.

Shemdin, O.↗

NASA NTRS1983

Propagation of sound in highly porous open-cell elastic foams

This work presents both theoretical predictions and experimental measurements of attenuation and progressive phase constants of sound in open-cell, highly porous, elastic polyurethane foams. The foams are available commercially in graded pore sizes for which information about the static flow resistance, thermal time constant, volume porosity, dynamic structure factor, and speed of sound is known. The analysis is specialized to highly porous foams which can be efficient sound absorbers at audio frequencies. Negligible effect of internal wave coupling on attenuation and phase shift for the frequency range 16-6000 Hz was predicted and no experimentally significant effects were observed in the bulk samples studied. The agreement between predictions and measurements in bulk materials is excellent. The analysis is applicable to both the regular and compressed elastic open-cell foams.

Lambert, R. F.↗

NASA NTRS1972

Prediction and measurement of heat transfer rates for the shock-induced unsteady laminar boundary layer on a flat plate

The unsteady laminar boundary layer induced by the flow-initiating shock wave passing over a flat plate mounted in a shock tube was theoretically and experimentally studied in terms of heat transfer rates to the plate for shock speeds ranging from 1.695 to 7.34 km/sec. The theory presented by Cook and Chapman for the shock-induced unsteady boundary layer on a plate is reviewed with emphasis on unsteady heat transfer. A method of measuring time-dependent heat-transfer rates using thin-film heat-flux gages and an associated data reduction technique are outlined in detail. Particular consideration is given to heat-flux measurement in short-duration ionized shocktube flows. Experimental unsteady plate heat transfer rates obtained in both air and nitrogen using thin-film heat-flux gages generally agree well with theoretical predictions. The experimental results indicate that the theory continues to predict the unsteady boundary layer behavior after the shock wave leaves the trailing edge of the plate even though the theory is strictly applicable only for the time interval in which the shock remains on the plate.

Cook, W. J.↗

NASA NTRS1997

The Stability of Radiatively Cooling Jets. 2: Nonlinear Evolution

We use two-dimensional time-dependent hydrodynamical simulations to follow the growth of the Kelvin-Helmholtz (K-H) instability in cooling jets into the nonlinear regime. We focus primarily on asymmetric modes that give rise to transverse displacements of the jet beam. A variety of Mach numbers and two different cooling curves are studied. The growth rates of waves in the linear regime measured from the numerical simulations are in excellent agreement with the predictions of the linear stability analysis presented in the first paper in this series. In the nonlinear regime, the simulations show that asymmetric modes of the K-H instability can affect the structure and evolution of cooling jets in a number of ways. We find that jets in which the growth rate of the sinusoidal surface wave has a maximum at a so-called resonant frequency can be dominated by large-amplitude sinusoidal oscillations near this frequency. Eventually, growth of this wave can disrupt the jet. On the other hand, nonlinear body waves tend to produce low-amplitude wiggles in the shape of the jet but can result in strong shocks in the jet beam. In cooling jets, these shocks can produce dense knots and filaments of cooling gas within the jet. Ripples in the surface of the jet beam caused by both surface and body waves generate oblique shock "spurs" driven into the ambient gas. Our simulations show these shock "spurs" can accelerate ambient gas at large distances from the jet beam to low velocities, which represents a new mechanism by which low-velocity bipolar outflows may be driven by high-velocity jets. Rapid entrainment and acceleration of ambient gas may also occur if the jet is disrupted. For parameters typical of protostellar jets, the frequency at which K-H growth is a maximum (or highest frequency to which the entire jet can respond dynamically) will be associated with perturbations with a period of - 200 yr. Higher frequency (shorter period) perturbations excite waves associated with body modes that produce internal shocks and only small-amplitude wiggles within the jet. The fact that most observed systems show no evidence for large-amplitude sinusoidal oscillation leading to disruption is indicative that the perturbation frequencies are generally large, consistent with the suggestion that pro- tostellar jets arise from the inner regions (r less than 1 AU) of accretion disks.

Stone, James M.↗

NASA NTRS1986

The evolution of nonlinear Alfven waves subject to growth and damping

The effects of wave amplification (by streaming particle distributions) and damping (by ion-cyclotron resonance absorption) on the nonlinear evolution of Alfven waves are investigated theoretically. The results of numerical simulations based on the derivative-Schroedinger-equation model of Spangler and Sheerin (1983 and 1985) are presented graphically and characterized in detail, with an emphasis on astrophysical applications. Three phases of wave-packet evolution (linear, nonlinear-saturation, and postsaturation quasi-steady) are identified, and nonlinearity is found to transfer wave energy from growing or amplified wavenumbers to wavenumbers affected by damping. It is pointed out that although there are similarities between the solitonlike pulses predicted by the simulations and short-wavelength shocklet structures observed in the earth bow shock, the model does not explain why low-frequency waves stop growing in the vicinity of the bow shock.

Spangler, S. R.↗