Showing papers by "Langley Research Center published in 1996"

Clouds and the Earth's Radiant Energy System (CERES): An Earth Observing System Experiment

Abstract: Clouds and the Earth's Radiant Energy System (CERES) is an investigation to examine the role of cloud/radiation feedback in the Earth's climate system. The CERES broadband scanning radiometers are an improved version of the Earth Radiation Budget Experiment (ERBE) radiometers. The CERES instruments will fly on several National Aeronautics and Space Administration Earth Observing System (EOS) satellites starting in 1998 and extending over at least 15 years. The CERES science investigations will provide data to extend the ERBE climate record of top-of-atmosphere shortwave (SW) and longwave (LW) radiative fluxes CERES will also combine simultaneous cloud property data derived using EOS narrowband imagers to provide a consistent set of cloud/radiation data, including SW and LW radiative fluxes at the surface and at several selected levels within the atmosphere. CERES data are expected to provide top-of-atmosphere radiative fluxes with a factor of 2 to 3 less error than the ERBE data Estimates of radiative fluxes at the surface and especially within the atmosphere will be a much greater challenge but should also show significant improvements over current capabilities.

Multidisciplinary Aerospace Design Optimization: Survey of Recent Developments

Abstract: The increasing complexity of engineering systems has sparked increasing interest in multidisciplinary optimization (MDO). This paper presents a survey of recent publications in the field of aerospace where interest in MDO has been particularly intense. The two main challenges of MDO are computational expense and organizational complexity. Accordingly the survey is focused on various ways different researchers use to deal with these challenges. The survey is organized by a breakdown of MDO into its conceptual components. Accordingly, the survey includes sections on Mathematical Modeling, Design- oriented Analysis, Approximation Concepts, Optimization Procedures, System Sensitivity, and Human Interface. With the authors'' main expertise being in the structures area, the bulk of the references focus on the interaction of the structures discipline with other disciplines. In particular, two sections at the end focus on two such interactions that have recently been pursued with a particular vigor: Simultaneous Optimization of Structures and Aerodynamics, and Simultaneous Optimization of Structures Combined With Active Control.

An atmospheric tape recorder: The imprint of tropical tropopause temperatures on stratospheric water vapor

Abstract: We describe observations of tropical stratospheric water vapor q that show clear evidence of large-scale upward advection of the signal from annual fluctuations in the effective "entry mixing ratio" qE of air entering the tropical stratosphere. In other words, air is "marked," on emergence above the highest cloud tops, like a signal recorded on an upward moving magnetic tape. We define qE as the mean water vapor mixing ratio, at the tropical tropopause, of air that will subsequently rise and enter the stratospheric "overworld" at about 400 K. The observations show a systematic phase lag, increasing with altitude, between the annual cycle in qE and the annual cycle in q at higher altitudes. The observed phase lag agrees with the phase lag calculated assuming advection by the transformed Eulerian-mean vertical velocity of a qE crudely estimated from 100-hPa temperatures, which we use as a convenient proxy for tropopause temperatures. The phase agreement confirms the overall robustness of the calculation and strongly supports the tape recorder hypothesis. Establishing a quantitative link between qE and observed tropopause temperatures, however, proves difficult because the process of marking the tape depends subtly on both small- and large-scale processes. The tape speed, or large-scale upward advection speed, has a substantial annual variation and a smaller variation due to the quasi-biennial oscillation, which delays or accelerates the arrival of the signal by a month or two in the middle stratosphere. As the tape moves upward, the signal is attenuated with an e-folding time of about 7 to 9 months between 100 and 50 hPa and about 15 to 18 months between 50 and 20 hPa, constraining possible orders of magnitude both of vertica.1 diffusion Kand of ra.tes of mixing in from the extratropics. For instance, if there were no mixing in, then Kwould be in the range 0.03-0.09 ms-; this is an upper bound on Ifs.

Computational Models for Sandwich Panels and Shells

Abstract: The focus of this review is on the hierarchy of computational models for sandwich plates and shells, predictor-corrector procedures, and the sensitivity of the sandwich response to variations in the different geometric and material parameters. The literature reviewed is devoted to the following application areas: heat transfer problems; thermal and mechanical stresses (including boundary layer and edge stresses); free vibrations and damping; transient dynamic response; bifurcation buckling, local buckling, face-sheet wrinkling and core crimping; large deflection and postbuckling problems; effects of discontinuities (eg, cutouts and stiffeners), and geometric changes (eg, tapered thickness); damage and failure of sandwich structures; experimental studies; optimization and design studies. Over 800 relevant references are cited in this review, and another 559 references are included in a supplemental bibliography for completeness. Extensive numerical results are presented for thermally stressed sandwich panels with composite face sheets showing the effects of variation in their geometric and material parameters on the accuracy of the free vibration response, and the sensitivity coefficients predicted by eight different modeling approaches (based on two-dimensional theories). The standard of comparison is taken to be the analytic three-dimensional thermoelasticity solutions. Some future directions for research on the modeling of sandwich plates and shells are outlined.

Multidisciplinary aerospace design optimization: Survey of recent developments

Abstract: The increasing complexity of engineering systems has sparked increasing interest in multidisciplinary optimization (MDO). This paper presents a survey of recent publications in the field of aerospace where interest in MDO has been particularly intense. The two main challenges of MDO are computational expense and organizational complexity. Accordingly the survey is focussed on various ways different researchers use to deal with these challenges. The survey is organized by a breakdown of MDO into its conceptual components. Accordingly, the survey includes sections on Mathematical Modeling, Design-oriented Analysis, Approximation Concepts, Optimization Procedures, System Sensitivity, and Human Interface. With the authors' main expertise being in the structures area, the bulk of the references focus on the interaction of the structures discipline with other disciplines. In particular, two sections at the end focus on two such interactions that have recently been pursued with a particular vigor: Simultaneous Optimization of Structures and Aerodynamics, and Simultaneous Optimization of Structures Combined With Active Control.

On the Two Components of Turbulent Mixing Noise from Supersonic Jets

Abstract: It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.

Origin of ozone and NOx in the tropical troposphere: A photochemical analysis of aircraft observations over the South Atlantic basin

Abstract: The photochemistry of the troposphere over the South Atlantic basin is examined by modeling of aircraft observations up to 12-km altitude taken during the TRACE A expedition in September–October 1992. A close balance is found in the 0 to 12-km column between photochemical production and loss of O3, with net production at high altitudes compensating for weak net loss at low altitudes. This balance implies that O3 concentrations in the 0–12 km column can be explained solely by in situ photochemistry; influx from the stratosphere is negligible. Simulation of H2O2, CH3OOH, and CH2O concentrations measured aboard the aircraft lends confidence in the computations of O3 production and loss rates, although there appears to be a major gap in current understanding of CH2O chemistry in the marine boundary layer. The primary sources of NOx over the South Atlantic Basin appear to be continental (biomass burning, lightning, soils). There is evidence that NOx throughout the 0 to 12-km column is recycled from its oxidation products rather than directly transported from its primary sources. There is also evidence for rapid conversion of HNO3 to NOx in the upper troposphere by a mechanism not included in current models. A general representation of the O3 budget in the tropical troposphere is proposed that couples the large-scale Walker circulation and in situ photochemistry. Deep convection in the rising branches of the Walker circulation injects NOx from combustion, soils, and lightning to the upper troposphere, leading to O3 production; eventually, the air subsides and net O3 loss takes place in the lower troposphere, closing the O3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NOx emissions in the tropics.

Quadrature-free implementation of discontinuous galerkin method for hyperbolic equations

Abstract: A discontinuous Galerkin formulation that avoids the use of discrete quadrature formulas is described and applied to linear and nonlinear test problems in one and two space dimensions. This approach requires less computational time and storage than conventional implementations but preserves the compactness and robustness inherent in the discontinuous Galerkin method. Test problems include the linear and nonlinear one-dimensional scalar advection of smooth initial value problems that are discretized by using unstructured grids with varying degrees of smoothness and regularity, and two-dimensional linear Euler solutions on unstructured grids.

An overview of LITE: NASA's Lidar In-space Technology Experiment

Abstract: The Lidar In-space Technology Experiment (LITE) is a three-wavelength backscatter lidar developed by NASA Langley Research Center to fly on the Space Shuttle. LITE flew on Discovery in September 1994 as part of the STS-64 mission. The goals of the LITE mission were to validate key lidar technologies for spaceborne applications, to explore the applications of space lidar, and to gain operational experience which will benefit the development of future systems on free-flying satellite platforms. The performance of the LITE instrument was excellent, resulting in the collection of over 40 GBytes of data. These data present us with our first highly detailed global view of the vertical structure of cloud and aerosol from the Earth's surface through the middle stratosphere. This paper will discuss the LITE instrument, the LITE mission, and briefly present some results from the Experiment. These preliminary results highlight the benefits to be obtained from long duration satellite lidars.

Convective transport of biomass burning emissions over Brazil during TRACE A

Abstract: A series of large mesoscale convective systems that occurred during the Brazilian phase of GTE/TRACE A (Transport and Atmospheric Chemistry near the Equator-Atlantic) provided an opportunity to observe deep convective transport of trace gases from biomass burning. This paper reports a detailed analysis of flight 6, on September 27, 1992, which sampled cloud- and biomass-burning-perturbed regions north of Brasilia. High-frequency sampling of cloud outflow at 9-12 km from the NASA DC-8 showed enhancement of CO mixing ratios typically a factor of 3 above background (200- 300 parts per billion by volume (ppbv) versus 90 ppbv) and significant increases in NOx and hydrocarbons. Clear signals of lightning-generated NO were detected; we estimate that at least 40% of NO x at the 9.5-km level and 32% at 11.3 km originated from lightning. Four types of model studies have been performed to analyze the dynamical and photochemical characteristics of the series of convective events. (1) Regional simulations for the period have been performed with the NCAR/Penn State mesoscale model (MM5), including tracer transport of carbon monoxide, initialized with observations. Middle-upper tropospheric enhancements of a factor of 3 above background are reproduced. (2) A cloud-resolving model (the Goddard cumulus ensemble (GCE) model) has been run for one representative convective cell during the September 26-27 episode. (3) Photochemical calculations (the Goddard tropospheric chemical model), initialized with trace gas observations (e.g., CO, NO x, hydrocarbons, 03) observed in cloud outflow, show appreciable 0 3 formation postconvection, initially up to 7-8 ppbv O3/d. (4) Forward trajectories from cloud outflow levels (postconvective conditions) put the ozone-producing air masses in eastern Brazil and the tropical Atlantic within 2-4 days and over the Atlantic, Africa, and the Indian Ocean in 6-8 days. Indeed, 3-4 days after the convective episode (September 30, 1992), upper tropospheric levels in the Natal ozone sounding show an average increase of -30 ppbv (3 Dobson units (DU) integrated) compared to the September 28 sounding. Our simulated net 0 3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms. Integrated over the 8- to 16-km cloud outflow layer, the postconvection net 0 3 production (-5-6 DU over 8 days) accounts for -25% of the excess 03 (15-25 DU) over the South Atlantic. Comparison of TRACE A Brazilian ozonesondes and the frequency of deep convection with climatology (Kirchhoff et al., this issue) suggests that the late September 1992 conditions represented an unusually active period for both convection and upper tropospheric ozone formation.

Where did tropospheric ozone over southern Africa and the tropical Atlantic come from in October 1992? Insights from TOMS, GTE TRACE A, and SAFARI 1992

Abstract: The seasonal tropospheric ozone maximum in the tropical South Atlantic, first recognized from satellite observations (Fishman et al., 1986, 1991), gave rise to the IGAC/ STARE/SAFARI 1992/TRACE A campaigns (International Global Atmospheric Chemistry/South Tropical Atlantic Regional Experiment/Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry Near the Equator- Atlantic) in September and October 1992. Along with a new TOMS-based method for deriving tropospheric column ozone, we used the TRACE A/SAFARI 1992 data set to put together a regional picture of the 0 3 distribution during this period. Sondes and aircraft profiling showed a troposphere with layers of high O3 (->90 ppbv) all the way to the tropopause. These features extend in a band from 0 o to 25oS, over the SE Indian Ocean, Africa, the Atlantic, and eastern South America. A combination of trajectory and photochemical modeling (the Goddard (GSFC) isentropic trajectory and tropospheric point model, respectively) shows a strong connection between regions of high ozone and concentrated biomass burning, the latter identified using satellite-derived fire counts (Justice et al., this issue). Back trajectories from a high-O3 tropical Atlantic region (column ozone at Ascension averaged 50 Dobson units (DU)) and forward trajectories from fire- rich and convectively active areas show that the Atlantic and southern Africa are supplied with O3 and O3-forming trace gases by midlevel easterlies and/or recirculating air from Africa, with lesser contributions from South American burning and urban pollution. Limited sampling in the mixed layer over Namibia shows possible biogenic sources of NO. High-level westerlies from Brazil (following deep convective transport of ozone precursors to the upper troposphere) dominate the upper tropospheric 03 budget over Natal, Ascension, and Okaukuejo (Namibia), although most enhanced O3 (75% or more) equatorward of 10oS was from Africa. Deep convection may be responsible for the timing of the seasonal tropospheric 0 3 maximum: Natal and Ascension show a 1- to 2-month lag relative to the period of maximum burning (cf. Baldy et al., this issue; Olson et al., this issue). Photochemical model calculations constrained with TRACE A and SAFARI airborne observations of O3 and 03 precursors (NOx, CO, hydrocarbons) show robust ozone formation (up to 15 ppbv O3/d or several DU/d) in a widespread, persistent, and well-mixed layer to 4 km. Slower but still positive net 03 formation took place throughout the tropical upper troposphere (cf. Pickering et al., this issue (a); Jacob et al., this issue). Thus whether it is faster rates of 0 3 formation in source regions with higher turnover rates or slower 03 production in long-lived stable layers ubiquitous in the TRACE A region, 10-30 DU tropospheric 03 above a -25-DU background can be accounted for. In summary, the 03 maximum studied in October 1992 was caused by a coincidence of abundant 03 precursors from biomass fires, a long residence time of stable air parcels over the eastern Atlantic and southern Africa, and deep convective transport of biomass burning products, with additional NO from lightning and occasionally biogenic sources.

Use of the Collaborative Optimization Architecture for Launch Vehicle Design

Abstract: Collaborative optimization is a new design architecture specifically created for large-scale distributed-analysis applications. In this approach, a problem is decomposed into a user-defined number of subspace optimization problems that are driven towards interdisciplinary compatibility and the appropriate solution by a system-level coordination process. This decentralized design strategy allows domain-specific issues to be accommodated by disciplinary analysts, while requiring interdisciplinary decisions to be reached by consensus. The present investigation focuses on application of the collaborative optimization architecutre to the multidisciplinary design of a single-stage-to-orbit launch vehicle. Vehicle design, trajectory, and cost issues are directly modeled. Posed to suit the collaborative architecture, the design problem is characterized by 95 design variables and 16 constraints. Numerous collaborative solutions are obtained. Comparison of these solutions demonstrates the influence which an a priori ascent-abort criterion has on development cost. Similarly, objective-function selection is discussed, demonstrating the difference between minimum weight and minimum cost concepts. The operational advantages of the collaborative optimization architecutre in a multidisciplinary design environment are also discussed.

User''s Manual for the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA)

Abstract: This user''s manual provides detailed instructions for the installation and the application of version 4.1 of the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) (refs. 2 and 3), which is a program for obtaining the simulations discussed above. Earlier versions of LAURA were predominantly research codes, and they had minimal (or no) documentation. This manual describes UNIX-based utilities for customizing the code for special applications that also minimize system resource requirements. The algorithm is reviewed, and the various program options are related to specific equations and variables in the theoretical development.

The Atmospheric Trace Molecule Spectroscopy (ATMOS) Experiment: Deployment on the ATLAS Space Shuttle Missions

Abstract: The ATMOS Fourier transform spectrometer was flown for a fourth time on the Space Shuttle as part of the ATLAS-3 instrument payload in November 1994. More than 190 sunrise and sunset occultation events provided measurements of more than 30 atmospheric trace gases at latitudes 3–49°N and 65–72°S, including observations both inside and outside the Antarctic polar vortex. The instrument configuration, data retrieval methodology, and mission background are described to place in context analyses of ATMOS data presented in this issue.

Global to microscale evolution of the Pinatubo volcanic aerosol derived from diverse measurements and analyses

Abstract: We assemble data on the Pinatubo aerosol from space, air, and ground measurements, develop a composite picture, and assess the consistency and uncertainties of measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements agree with theoretical calculations in showing a dominant composition of H2SO4-H20 mixture, with H2SO4 weight fraction of 65-80% for most stratospheric temperatures and humidities. Important exceptions are (1) volcanic ash, present at all heights initially and just above the tropopause until at least March 1992, and (2) much smaller H2SO4 fractions at the low temperatures of high-latitude winters and the tropical tropopause. Laboratory spectroscopy and calculations yield wavelength- and temperature-dependent refractive indices for the H2SO4-H20 droplets. These permit derivation of particle size information from measured optical depth spectra, for comparison to impactor and optical-counter measurements. All three techniques paint a generally consistent picture of the evolution of R(sub eff), the effective radius. In the first month after the eruption, although particle numbers increased greatly, R(sub eff) outside the tropical core was similar to preeruption values of approx. 0.1 to 0.2 microns, because numbers of both small (r 0.6 microns) particles increased. In the next 3-6 months, extracore R(sub eff) increased to approx. 0.5 microns, reflecting particle growth through condensation and coagulation. Most data show that R(sub eff) continued to increase for about 1 year after the eruption. R(sub eff) values up to 0.6 - 0.8 microns or more are consistent with 0.38 - 1 micron optical depth spectra in middle to late 1992 and even later. However, in this period, values from in situ measurements are somewhat less. The difference might reflect in situ undersampling of the very few largest particles, insensitivity of optical depth spectra to the smallest particles, or the inability of flat spectra to place an upper limit on particle size. Optical depth spectra extending to wavelengths lambda > 1 micron are required to better constrain R(sub eff), especially for R(sub eff) > 0.4 microns. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with lambda(sub max) 0.3 microns) and relatively flat extinction spectra (0.4 - 1 microns) are among the longest-lived indicators of Pinatubo volcanic influence. They persist for years after the peaks in number, mass, surface area, and optical depth at all wavelengths <= 1 microns. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between global maps of 0.5- and 1.0-micron optical depth derived from the Advanced Very High Resolution Radiometer (AVHRR) and Stratospheric Aerosol and Gas Experiment (SAGE) satellite sensors. However, there are important differences between the AVHRR and SAGE midvisible optical thickness products. We discuss possible reasons for these differences and how they might be resolved.

A Parameterization for Computing Grid-Averaged Solar Fluxes for Inhomogeneous Marine Boundary Layer Clouds. Part II: Validation Using Satellite Data.

Abstract: The Independent Pixel approximation (IPA) is one of the simplest methods of computing solar radiative fluxes for inhomogencous clouds. It claims that if p(τ) is a normalized probability density function for cloud optical depth τ and Rpp(τ) is plane-parallel, homogeneous (PPH) albedo, mean cloud albedo can be approximated by integrating p(τ)Rpp(τ) over all τ. The purpose of this study is to assess the ability of the gamma distribution function to represent p(τ) for marine boundary layer clouds and to examine the accuracy of the ensuing gamma IPA albedos. In a separate study, pixel values of τ were inferred from high spatial resolution Landsat imagery of marine boundary layer clouds. The present study utilizes 45 images, each measuring (58 km)2. For each image, a density function pobs(τ) is estimated, and, using the mean τ¯ and variance of τ, a corresponding truncated gamma distribution function pγ(τ) is defined. For a diverse range of clouds, pγ(τ) usually approximate pobs(τ) well. The best result...

Validation of UARS Microwave Limb Sounder Ozone Measurements

Abstract: This paper describes the validation of ozone data from the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS). The MLS ozone retrievals are obtained from the calibrated microwave radiances (emission spectra) in two separate bands, at frequencies near 205 and 183 GHz. Analyses described here focus on the MLS Version 3 data (the first set of files made publicly available). We describe results of simulations performed to assess the quality of the retrieval algorithms, in terms of both mixing ratio and radiance closure. From actual MLS observations, the 205-GHz ozone retrievals give better closure (smaller radiance residuals) than that from the 183-GHz measurements and should be considered more accurate from the calibration aspects. However, the 183-GHz data are less noise limited in the mesosphere and can provide the most useful scientific results in that region. We compare the retrieved 205-GHz ozone profiles in the middle-to lower stratosphere to ozonesonde measurements at a wide range of latitudes and seasons. Ground-based lidar data from Table Mountain, California, provide a good reference for comparisons at higher altitudes. Based on these analyses, comparisons with balloon-borne measurements and others, as well as a detailed budget of estimated uncertainties, MLS results appear to be generally of high quality, with some biases worth mentioning. Results for the lowermost stratosphere (approx. 50 to 100 bPa) are still in need of improvement. A set of estimated precision and accuracy values is derived for the MLS ozone data sets. We also comment on recent updates in the retrieval algorithms and their impact on ozone values.

A Multiscale Retinex for Color Rendition and Dynamic Range Compression

Abstract: The human vision system performs the tasks of dynamic range compression and color constancy almost effortlessly. The same tasks pose a very challenging problem for imaging systems whose dynamic range is restricted by either the dynamic response of film, in case of analog cameras, or by the analog-to-digital converters, in the case of digital cameras. The images thus formed are unable to encompass the wide dynamic range present in most natural scenes (often < 500:1). Whereas the human visual system is quite tolerant to spectral changes in lighting conditions, these strongly affect both the film response for analog cameras and the filter responses for digital cameras, leading to incorrect color formulation in the acquired image. Our multiscale retinex, based in part on Edwin Land''s work on color constancy, provides a fast, simple, and automatic technique for simultaneous dynamic range compression and accurate color rendition. The retinex algorithm is non-linear, and global---output at a point is also a function of its surround---in extent. A comparison with conventional dynamic range compression techniques such as the application of point non-linearities, e.g. log(x,y), and global histogram equalization and/or modification shows that the multiscale retinex simultaneously provides the best dynamic range compression and color rendition. The applications of such an algorithm are many; from medical imaging to remote sensing; and from commercial photography to color transmission.

Coherent lidar airborne wind sensor II: flight-test results at 2 and 10 νm.

Abstract: The use of airborne laser radar (lidar) to measure wind velocities and to detect turbulence in front of an aircraft in real time can significantly increase fuel efficiency, flight safety, and terminal area capacity. We describe the flight-test results for two coherent lidar airborne shear sensor (CLASS) systems and discuss their agreement with our theoretical simulations. The 10.6-μm CO2 system (CLASS-10) is a flying brassboard; the 2.02-μm Tm:YAG solid-state system (CLASS-2) is configured in a rugged, lightweight, high-performance package. Both lidars have shown a wind measurement accuracy of better than 1 m/s. © 1996 Optical Society of America

Accurate universal parameterization of absorption cross sections.

Abstract: We present a simple universal parameterization of total reaction cross sections for any system of colliding nuclei valid for the entire energy range from a few A MeV to a few A GeV. The universal picture presented here treats the proton-nucleus collision as a special case of the nucleus-nucleus collision, where the projectile has charge and mass number one. The parameters are associated with the physics of the collision system. In general terms Coulomb interaction modifies cross sections at lower energies and the effects of Pauli blocking are important at higher energies. The agreement between the calculated and experimental data is better than all earlier published results.

Neural generalized predictive control

Abstract: An efficient implementation of generalized predictive control using a multilayer feedforward neural network as the plant's nonlinear model is presented. By using Newton-Raphson as the optimization algorithm, the number of iterations needed for convergence is significantly reduced from other techniques. The main cost of the Newton-Raphson algorithm is in the calculation of the Hessian, but even with this overhead the low iteration numbers make Newton-Raphson faster than other techniques and a viable algorithm for real-time control. This paper presents a detailed derivation of the neural generalized predictive control algorithm with Newton-Raphson as the minimization algorithm. Simulation results show convergence to a good solution within two iterations and timing data show that real-time control is possible. Comments about the algorithm's implementation are also included.

Boundary-layer stability measurements in a hypersonic quiet tunnel

Abstract: Hypersonic boundary-layer measurements were conducted over a flared cone in a quiet wind tunnel. The flared cone was tested at a freestream unit Reynolds number of 2.82 x 10 6 /ft in a Mach 6 flow. This Reynolds number provided laminar-to-transitional flow over the model in a low-disturbance environment. Point measurements with a single hot wire using a novel constant voltage anemometry system were used to measure the boundary-layer disturbances. Surface temperature and schlieren measurements were also conducted to characterize the laminar-to-transitional state of the boundary layer and to identify instability modes. Results suggest that the second mode is the dominant mode of instability. The integrated growth rates of the second mode compared well with linear stability theory in the linear stability regime. Furthermore, the existence of higher harmonics of the fundamental suggests that nonlinear disturbances are not associated with high freestream disturbance levels.

Preconditioning Methods for Low-Speed Flows.

Abstract: We consider the steady-state equations for a compressible fluid. For low-speed flow, the system is stiff because the ratio of the convective speed to the speed of sound is quite small. To overcome the difficulty, we alter the time evolution of the equations but retain the same steady-state analytic equations. To achieve high numerical resolution, we also alter the artifical viscosity of the numerical scheme, which is implemented conveniently by using other sets of variables in addition to the conservative variables. We investigate the effect of the artificial dissipation within this preconditioned system. We consider both the nonconservative and conservative formulations for artificial viscosity and examine their effect on the accuracy and convergence of the numerical solutions. The numerical results for viscous three-dimensional wing flows and two-dimensional multi-element airfoil flows indicate that efficient multigrid.