Showing papers by "Langley Research Center published in 1994"

Applied system identification

Abstract: 1. Introduction. 2. Time-Domain Models. 3. Frequency-Domain Models. 4. Frequency Response Functions. 5. System Realization. 6. Observer Identification. 7. Frequency Domain System ID. 8. Observer/Controller ID. 9. Recursive Techniques. Appendix A: Fundamental Matrix Algebra. Appendix B: Random Variables and Kalman Filter. Appendix C: Data Acquisition.

A Contribution to the Great Riemann Solver Debate

Abstract: The aims of this paper are threefold: to increase the level of awareness within the shock-capturing community of the fact that many Godunov-type methods contain subtle flaws that can cause spurious solutions to be computed; to identify one mechanism that might thwart attempts to produce very-high-resolution simulations; and to proffer a simple strategy for overcoming the specific failings of individual Riemann solvers.

Fourth-order 2N-storage Runge-Kutta schemes

Abstract: A family of five-stage fourth-order Runge-Kutta schemes is derived; these schemes required only two storage locations. A particular scheme is identified that has desirable efficiency characteristics for hyperbolic and parabolic initial (boundary) value problems. This scheme is competitive with the classical fourth-order method (high-storage) and is considerably more efficient and accurate than existing third-order low-storage schemes.

On the dynamics of a shock-bubble interaction

Abstract: We present a detailed numerical study of the interaction of a weak shock wave with an isolated cylindrical gas inhomogeneity. Such interactions have been studied experimentally in an attempt to elucidate the mechanims whereby shock waves propagating through random media enhance mixing. Our study concentrates on the early phases of the interaction process which are dominated by repeated refractions and reflections of acoustic fronts at the bubble interface. Specifically, we have reproduced two of the experiments performed by Haas and Sturtevant.

Optimization of coupled systems: a critical overview of approaches

Abstract: A unified overview is given of problem formulation approaches for the optimization of multidisciplinary coupled systems. The overview includes six fundamental approaches upon which a large number of variations may be made. Consistent approach names and a compact approach notation are given. The approaches are formulated to apply to general nonhierarchic systems. The approaches are compared both from a computational viewpoint and a managerial viewpoint. Opportunities for parallelism of both computation and manpower resources are discussed. Recommendations regarding the need for future research are advanced.

Acetone in the atmosphere: Distribution, sources, and sinks

Abstract: Acetone (CH3COCH3) was found to be the dominant nonmethane organic species present in the atmosphere sampled primarily over eastern Canada (0-6 km, 35 deg-65 deg N) during ABLE3B (July to August 1990). A concentration range of 357 to 2310 ppt (= 10(exp -12) v/v) with a mean value of 1140 +/- 413 ppt was measured. Under extremely clean conditions, generally involving Arctic flows, lowest (background) mixing ratios of 550 +/- 100 ppt were present in much of the troposphere studied. Correlations between atmospheric mixing ratios of acetone and select species such as C2H2, CO, C3H8, C2Cl4 and isoprene provided important clues to its possible sources and to the causes of its atmospheric variability. Biomass burning as a source of acetone has been identified for the first time. By using atmospheric data and three-dimensional photochemical models, a global acetone source of 40-60 Tg (= 10(exp 12) g)/yr is estimated to be present. Secondary formation from the atmospheric oxidation of precursor hydrocarbons (principally propane, isobutane, and isobutene) provides the single largest source (51%). The remainder is attributable to biomass burning (26%), direct biogenic emissions (21%), and primary anthropogenic emissions (3%). Atmospheric removal of acetone is estimated to be due to photolysis (64%), reaction with OH radicals (24%), and deposition (12%). Model calculations also suggest that acetone photolysis contributed significantly to PAN formation (100-200 ppt) in the middle and upper troposphere of the sampled region and may be important globally. While the source-sink equation appears to be roughly balanced, much more atmospheric and source data, especially from the southern hemisphere, are needed to reliably quantify the atmospheric budget of acetone.

Parallel volume rendering using binary-swap compositing

Abstract: We describe a parallel volume-rendering algorithm, which consists of two parts: parallel ray tracing and parallel compositing. In the most recent implementation on Connection Machine's CM-5 and networked workstations, the parallel volume renderer evenly distributes data to the computing resources available. Without the need to communicate with other processing units, each subvolume is ray traced locally and generates a partial image. The parallel compositing process then merges all resulting partial images in depth order to produce the complete image. The compositing algorithm is particularly effective for massively parallel processing, as it always uses all processing units by repeatedly subdividing the partial images and distributing them to the appropriate processing units. Test results on both the CM-5 and the workstations are promising. They do, however, expose different performance issues for each platform. >

Mat - multi-attribute task battery for human operator workload and strategic behavior research

Abstract: The Multi-Attribute Task (MAT) Battery provides a benchmark set of tasks for use in a wide range of lab studies of operator performance and workload. The battery incorporates tasks analogous to activities that aircraft crewmembers perform in flight, while providing a high degree of experimenter control, performance data on each subtask, and freedom to use nonpilot test subjects. Features not found in existing computer based tasks include an auditory communication task (to simulate Air Traffic Control communication), a resource management task permitting many avenues or strategies of maintaining target performance, a scheduling window which gives the operator information about future task demands, and the option of manual or automated control of tasks. Performance data are generated for each subtask. In addition, the task battery may be paused and onscreen workload rating scales presented to the subject. The MAT Battery requires a desktop computer with color graphics. The communication task requires a serial link to a second desktop computer with a voice synthesizer or digitizer card.

Satellite analysis of the severe 1987 forest fires in northern China and southeastern Siberia

Abstract: Meteorological conditions, extremely conducive to fire development and spread in the spring of 1987, resulted in forest fires burning over extremely large areas in the boreal forest zone in northeastern China and the southeastern region of Siberia. The great China fire, one of the largest and most destructive forest fires in recent history, occurred during this period in the Heilongjiang Province of China. Satellite imagery is used to examine the development and areal distribution of 1987 forest fires in this region. Overall trace gas emissions to the atmosphere from these fires are determined using a satellite-derived estimate of area burned in combination with fuel consumption figures and carbon emission ratios for boreal forest fires.

The Physics of Supersonic Turbulent Boundary Layers

Abstract: When a vehicle travels at Mach numbers greater than one, a significant temperature gradient develops across the boundary layer due to the high levels of viscous dissipation near the wall. In fact, the static-temperature variation can be very large even in an adiabatic flow, resulting in a low­ density, high-viscosity region near the wall. In turn, this leads to a skewed mass-flux profile, a thicker boundary layer, and a region in which viscous effects are somewhat more important than at an equivalent Reynolds number in subsonic flow. Intuitively, one would expect to see significant dynamical differences between subsonic and supersonic boundary layers. However, many of these differences can be explained by simply accounting for the fluid-property variations that accompany the temperature

An acoustic/viscous splitting technique for computational aeroacoustics

Abstract: A new technique for the numerical analysis of aerodynamic noise generation is developed. The approach involves first solving for the time-dependent incompressible flow for the given geometry. A “hydrodynamic” density correction to the constant incompressible density is then calculated from knowledge of the incompressible pressure fluctuations. The compressible flow solution is finally obtained by considering perturbations about the “corrected” incompressible flow. This fully nonlinear technique, which is tailored to extract the relevant acoustic fluctuations, appears to be an efficient approach to the numerical analysis of aerodynamic noise generation, particularly in viscous flows. Applications of this technique to some classical acoustic problems of interest, including some with moderately high subsonic Mach numbers, are presented to validate the approach. The technique is then applied to a fully viscous problem where sound is generated by the flow dynamics.

Relativistic Electron Acceleration and Decay Time Scales in the Inner and Outer Radiation Belts: SAMPEX

Abstract: High-energy electrons have been measured systematically in a low-altitude (520 × 675 km), nearly polar (inclination = 82°) orbit by sensitive instruments onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). Count rate channels with electron energy thresholds ranging from 0.4 MeV to 3.5 MeV in three different instruments have been used to examine relativistic electron variations as a function of L-shell parameter and time. A long run of essentially continuous data (July 1992–July 1993) shows substantial acceleration of energetic electrons throughout much of the magnetosphere on rapid time scales. This acceleration appears to be due to solar wind velocity enhancements and is surprisingly large in that the radiation belt “slot” region often is filled temporarily and electron fluxes are strongly enhanced even at very low L-values (L ∼ 2). A superposed epoch analysis shows that electron fluxes rise rapidly for 2.5 ≲ L ≲ 5. These increases occur on a time scale of order 1–2 days and are most abrupt for L-values near 3. The temporal decay rate of the fluxes is dependent on energy and L-value and may be described by J = Ke-t/to with to ≈ 5–10 days. Thus, these results suggest that the Earth's magnetosphere is a cosmic electron accelerator of substantial strength and efficiency.

Scalability of parallel algorithm-machine combinations

Abstract: Scalability has become an important consideration in parallel algorithm and machine designs. The word scalable, or scalability, has been widely and often used in the parallel processing community. However, there is no adequate, commonly accepted definition of scalability available. Scalabilities of computer systems and programs are difficult to quantify, evaluate, and compare. In this paper, scalability is formally defined for algorithm-machine combinations. A practical method is proposed to provide a quantitative measurement of the scalability. The relation between the newly proposed scalability and other existing parallel performance metrics is studied. A harmony between speedup and scalability has been observed. Theoretical results show that a large class of algorithm-machine combinations is scalable and the scalability can be predicted through premeasured machine parameters. Two algorithms have been studied on an nCUBE 2 multicomputer and on a MasPar MP-1 computer. These case studies have shown how scalabilities can be measured, computed, and predicted. Performance instrumentation and visualization tools also have been used and developed to understand the scalability related behavior. >

Influence of plumes from biomass burning on atmospheric chemistry over the equatorial and tropical South Atlantic during CITE 3

Abstract: During all eight flights conducted over the equatorial and tropical South Atlantic in the course of the Chemical Instrumentation Test and Evaluation (CITE 3) experiment, we observed haze layers with elevated concentrations of aerosols, O3, CO, and other trace gases related to biomass burning emissions. They occurred at altitudes between 1000 and 5200 m and were usually only some 100-300 m thick. These layers extended horizontally over several 100 km and were marked by the presence of visible brownish haze. Air mass trajectories indicate that these layers originate in the biomass burning regions of Africa and South America and typically have aged at least 10 days since the time of emission. In the haze layers, O3 and CO concentrations up to 90 and 210 ppb were observed, respectively. The two species were highly correlated. The ratio concentrations in plume minus background concentrations of O3/CO is typically in the range 0.2-0.7, much higher than the ratios in the less aged plumes investigated previously in Amazonia. In most cases, aerosol (0.12-3 micrometer diameter) number concentrations were also elevated by up to 400/cu cm in the layers; aerosol enrichments were also strongly correlated with elevated CO levels. Clear correlations between CO and NO(x) enrichments were not apparent due to the age of the plumes, in which most NO(x) would have already reacted away within 1-2 days. Only in some of the plumes could clear correlations between NO(y) and CO be identified; the absence of a general correlation between NO(y) and CO may be due to instrumental limitations and to variable sinks for NO(y). The average enrichment of the ratio concentrations in plume minus background concentrations of NO(y)/CO was quite high, consistent with the efficient production of ozone observed in the plumes. The chemical characteristics of the haze layers, together with remote sensing information and trajectory calculations, suggest that fire emissions (in Africa and/or South America) are the primary source of the haze layer components.

Separation control on high-lift airfoils via micro-vortex generators

Abstract: An experimental investigation has been conducted to evaluate boundary-layer separation control on a twodimensional single-flap, three-element, high-lift system at near-flight Reynolds numbers with small surfacemounted vortex generators. The wind-tunnel testing was carried out in the NASA Langley Low-Turbulence Pressure Tunnel as part of a cooperative program between McDonnell Douglas Aerospace and NASA Langley Research Center to develop code validation data bases and to improve physical understanding of multielement airfoil flows. This article describes results obtained for small (subboundary-layer) vane-type vortex generators mounted on a multielement airfoil in a landing configuration. Measurements include lift, drag, surface pressure, wake profile, and fluctuating surface heat fluxes. The results reveal that vortex generators as small as 0.18% of reference (slat and flap stowed) wing chord ("micro-vortex generators") can effectively reduce boundarylayer separation on the flap for landing configurations. Reduction of flap separation can significantly improve performance of the high-lift system by reducing drag and increasing lift for a given approach angle of attack. At their optimum chordwise placement on the flap, the micro-vortex generators are hidden inside the wing when the flap is retracted, thus extracting no cruise drag penalty.

Acoustic harmonic generation from fatigue-induced dislocation dipoles

Abstract: A model is presented of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue. The model predicts the generation of a substantial acoustic second harmonic that depends on the distance between the glide planes of the dipole pair, on the dipole density, and on the particular arrangement and volume fraction of dipoles in a given substructure of the fatigued solid. Experimental evidence which strongly supports the essential features of the model is presented for fatigued aluminium alloy 2024-T4.

Intrusions into the lower stratospheric Arctic vortex during the winter of 1991–1992

Abstract: Investigations of the kinematics of the lower stratospheric Arctic vortex during the winter of 1991-1992 using the contour advection with surgery technique reveal three distinct events in which there was substantial intrusion of midlatitude air into the vortex, in apparent contradiction of the view that the polar vortex constitutes an isolated air mass. Two of these events, in late January and mid-February, were well documented. They were predicted in high-resolution forecasts by the European Centre for Medium-Range Weather Forecasts, most clearly in experimental forecasts with reduced diffusion. Direct confirmation of the presence of the intrusions and of their calculated locations was provided by aerosol observations from the airborne differential absorption laser lidar aboard the NASA DC-8, taken as part of the second Airborne Arctic Stratospheric Expedition campaign; aerosol-rich air of midlatitude origin was seen in the expected position of the intrusions. The reality of the February event was also confirmed by in situ measurements from the NASA ER-2. Such events may be significant for the chemical processes taking place within the winter vortex. The intrusions were evidently related to the meteorology of the northern stratosphere during this winter and in particular to persistent tropospheric blocking over the northeastern Atlantic Ocean and western Europe and concomitant ridging into the lower stratospheric vortex in this region. Nevertheless, preliminary investigations have indicated that such events are not uncommon in other northern hemisphere winters, although no such events were found in the southern hemisphere during the Antarctic winter of 1987.

Recent Progress Toward a Three-Dimensional Unstructured Navier-Stokes Flow Solver

Abstract: Progress has been made toward solving three-dimensional viscous flow problems with unstructured grids. Enhancements have been added to an established unstructured grid flow solver that upgrade its formal accuracy to second order, improve the code efficiency and speed, and accurately solve the Navier-Stokes equations on tetrahedral cells. Accuracy is increased by a pseudo-Laplacian weight averaging algorithm which produces more robust convergence, and permits higher-order boundary conditions to be used, which is important on highly- stretched cells. An existing time-implicit Gauss-Seidel algorithm is installed which results in an six-fold reduction in total computer time over that of an explicit algorithm for inviscid-flow calculations. Finally, an unstructured laminar solution on the ONERA M6 wing has been validated against a structured- grid solution with encouraging results.

Structural damage detection of space truss structures using best achievable eigenvectors

Abstract: A method is presented by which measured modes and frequencies from a modal test can be used to determine the location and magnitude of damage in a space struss structure. The damage is located by computing the Euclidean distances between the measured mode shapes and the best achievable eigenvectors. The best achievable eigenvectors are the projection of the measured mode shapes onto the subspace defined by the refined analytical model of the structure and the measured frequencies. Loss of both stiffness and mass properties can be located and quantified. To examine the performance of the method when experimentally measured modes are employed, various damage detection studies using a laboratory eight-bay truss structure were conducted. The method performs well even though the measurement errors inevitably make the damage location more difficult.

Low Ozone in the Marine Boundary Layer of the Tropical Pacific Ocean

Abstract: Aircraft measurements of ozone, its key precursors, and a variety of chemical tracers were made in the troposphere of the western and central Pacific in October 1991. These data are presented and analyzed to examine the occurrence of low ozone concentrations in the remote marine boundary layer of the tropical and equatorial Pacific Ocean. The data from these flights out of Guam, covering an area extending from the equator to 20 N and from south of the Philippines to Hawaii, show average O3 concentrations as low as 8-9 ppb (ppb=10(exp-9)v/v) at altitudes of 0.3-0.5 km in the boundary layer. Individual measurements as low as 2-5 ppb were recorded. Low O3 concentrations do not always persist in space and time. High O3, generally associated with the transport of upper tropospheric air, was also encountered in the boundary layer. In practically all cases, O3 increased to values as large as 25-30 ppb within 2 km above the boundary layer top. Steady state model computations are used to suggest that these low O3 concentrations are a result of net photochemical O3 destruction in a low NO environment, sea-surface deposition, and extremely low net entrainment rates (1-2 mm per second) from the free troposphere. Day/night measurements of ethane, propane, gaseous and aerosol Cl suggest that daytime (morning) Cl atom concentrations in the vicinity of 10(exp 5) molecules per cubic centimeter may be present in the marine boundary layer. This Cl atom abundance can be rationalized only if sea salt aerosols can release free chlorine (Cl2) to the gas phase in the presence of sun light (and possibly O3). These Cl atom concentrations, however, are still insufficient and Cl (or Br) chemistry is not likely to be an important cause of the observed low O3.

AE Source Orientation by Plate Wave Analysis

Abstract: Lead breaks (Hsu-Neilsen source) were used to generate simulated acoustic emission signals in an aluminum plate at angles of 0, 30, 60, and 90 degrees with respect to the plane of the plate. This was accomplished by breaking the lead on slots cut into the plate at the respective angles. The out-of-plane and in-plane displacement components of the resulting signals were detected by broad band transducers and digitized. Analysis of the waveforms showed them to consist of the extensional and flexural plate modes. The amplitude of both components of the two modes was dependent on the source orientation angle. This suggests that plate wave analysis may be used to determine the source orientation of acoustic emission sources.

Polar stratospheric cloud climatology based on Stratospheric Aerosol Measurement II observations from 1978 to 1989

Abstract: The probability of polar stratospheric cloud (PSC) occurrence in the Antarctic and Arctic has been estimated using Stratospheric Aerosol Measurement (SAM) II aerosol extinction data from 1978 to 1989. Antarctic PSCs are typically observed by SAM II from mid-May to early November, with a maximum zonal average probability of about 0.6 at 18-20 km in August. The typical Arctic PSC season extends only from late November to early March, with a peak zonal average probability of about 0.1 in early February at 20-22 km. There is considerable year-to-year variability in Arctic PSC sightings because of changes in the dynamics of the northern polar vortex. Year-to-year variability in Antarctic sightings is most prominent in the number of late season clouds. Maximum PSC sighting probabilities in both polar regions occur in the region from 90 deg W through the Greenwich meridian to 90 deg E, where temperatures are coldest on average. Arctic sighting probabilities approach zero outside this region, but clouds have been sighted in the Antarctic at all longitudes during most months. Inferred PSC formation temperatures remain constant throughout the Arctic winter and are similar to those in early Antarctic winter. PSC formation temperatures in the Antarctic drop markedly in the 15 to 20-km region by September, a pattern consistent with the irreversible loss of HNO3 and H2O vapor in sedimenting PSC particles.

Introduction to Generalized Functions With Applications in Aerodynamics and Aeroacoustics

Abstract: Generalized functions have many applications in science and engineering. One useful aspect is that dis-break continuous functions can be handled as easily as continuous or differentiable functions and provide a powerful tool in formulating and solving many problems of aerodynamics and acoustics. Furthermore, generalized function theory elucidates and unifies many ad hoc mathematical approaches used by engineers and scientists. We define generalized functions as continuous linear functionals on the space of infinitely differentiable functions with compact support, then introduce the concept of generalized differentiation. Generalized differentiation is the most important concept in generalized function theory and the applications we present utilize mainly this concept. First, some results of classical analysis, are derived with the generalized function theory. Other applications of the generalized function theory in aerodynamics discussed here are the derivations of general transport theorems for deriving governing equations of fluid mechanics, the interpretation of the finite part of divergent integrals, the derivation of the Oswatitsch integral equation of transonic flow, and the analysis of velocity field discontinuities as sources of vorticity. Applications in aeroacoustics include the derivation of the Kirchhoff formula for moving surfaces, the noise from moving surfaces, and shock noise source strength based on the Ffowcs Williams--Hawkings equation.