Showing papers by "Langley Research Center published in 1985"
TL;DR: A new approach is introduced in conjunction with the singular value decomposition technique to derive the basic formulation of minimum order realization which is an extended version of the Ho-Kalman algorithm.
Abstract: A method, called the Eigensystem Realization Algorithm (ERA), is developed for modal parameter identification and model reduction of dynamic systems from test data. A new approach is introduced in conjunction with the singular value decomposition technique to derive the basic formulation of minimum order realization which is an extended version of the Ho-Kalman algorithm. The basic formulation is then transformed into modal space for modal parameter identification. Two accuracy indicators are developed to quantitatively identify the system modes and noise modes. For illustration of the algorithm, examples are shown using simulation data and experimental data for a rectangular grid structure.
2,366 citations
TL;DR: In this article, a flux-splitting method in generalized coordinates was developed and applied to quasi-one-dim ensional transonic flow in a nozzle and two-dimensional subsonic, transonic, and supersonic flow over airfoils.
Abstract: A flux-splitting method in generalized coordinates has been developed and applied to quasi-one-dim ensional transonic flow in a nozzle and two-dimensional subsonic, transonic, and supersonic flow over airfoils. Computational results using the Steger-Warming and Van Leer flux splittings are compared. Discussed are several advantages of a MUSCL-type approach (differencing followed by flux splitting) over a standard flux differencing approach (flux splitting followed by differencing) . With an approximately factored implicit scheme, spectral radii of 0.978-0.930 for a series of airfoil computations are obtained, generally decreasing as a larger portion of the flow becomes supersonic. The Van Leer splitting leads to higher convergence rates and a sharper representation of shocks, with at most two (but more often, one) zones in the shock transition. The second-order accurate one-sided-difference model is extended to a third-order upwind-biased model with a small additional computational effort. The results for both the second- and third-order schemes agree closely in overall features to a widely used central difference scheme, although the shocks are resolved more accurately with the flux splitting approach.
856 citations
TL;DR: A condition under which a multiversion system is a better strategy than relying on a single version is given and some differences between the coincident errors model developed here and the model that assumes independent failures of component verions are studied.
Abstract: Fundamental to the development of redundant software techniques (known as fault-tolerant software) is an understanding of the impact of multiple joint occurrences of errors, referred to here as coincident errors. A theoretical basis for the study of redundant software is developed which 1) provides a probabilistic framework for empirically evaluating the effectiveness of a general multiversion strategy when component versions are subject to coincident errors, and 2) permits an analytical study of the effects of these errors. An intensity function, called the intensity of coincident errors, has a central role in this analysis. This function describes the propensity of programmers to introduce design faults in such a way that software components fail together when executing in the application environment. We give a condition under which a multiversion system is a better strategy than relying on a single version and we study some differences between the coincident errors model developed here and the model that assumes independent failures of component verions.
339 citations
TL;DR: In this article, two simple and improved models were developed to calculate impact force and duration during low velocity impact of circular composite plates, including the contact deformation of the plate and the impactor as well as bending, transverse shear, and membrane deformations.
Abstract: Two simple and improved models--energy-balance and spring-mass--were developed to calculate impact force and duration during low velocity impact of circular composite plates. Both models include the contact deformation of the plate and the impactor as well as bending, transverse shear, and membrane deformations of the plate. The plate was transversely isotropic graphite/epoxy composite laminate and the impactor was a steel sphere. Calculated impact forces from the two analyses agreed with each other. The analyses were verified by comparing the results with reported test data.
300 citations
TL;DR: The concept of synergetic plane change was introduced by London in 1961 as mentioned in this paper, and has been used extensively in a wide variety of Earthorbital and planetary missions since then.
Abstract: Introduction I N 1961, Howard London presented a paper at the 29th Annual Meeting of the Institute of the Aeronautical Sciences in which he demonstated the possibility of using aerodynamic forces to produce an orbital plane change with an expenditure of energy significantly smaller than that associated with an extra-atmospheric propulsive maneuver. Since that time, numerous studies of aeroassisted orbit transfer have been carried out for a wide variety of Earthorbital and planetary missions. Almost any mission that involves changes in orbital altitude and/or inclination in the vicinity of an atmosphere-bearing planet is a candidate for aeroassist. For ease of discussion these many possible mission applications may be categorized as 1) synergetic plane change, 2) planetary mission applications, and 3) orbital transfer vehicle applications. The term, synergetic plane change, is commonly used to denote a maneuver in which a change in orbit inclination is accomplished through a combination of aerodynamic and propulsive forces rather than through propulsion alone. While the maneuver is applicable to any planet with a suitable atmosphere, practically all studies to date have dealt with low Earth orbit missions. As illustrated in Fig. 1, a retrorocket burn causes the vehicle to leave its initial low Earth orbit (LEO) and dip into the atmosphere. During the atmospheric pass, the vehicle is banked so that the lift vector is in a lateral direction, thus producing an aerodynamic turn. As will be discussed subsequently, this turn is often accomplished with continuous thrusting sufficient to balance the aerodynamic drag. Following the aerodynamic turn, the vehicle is reboosted to the desired orbital altitude and a rocket burn is effected to circularize the orbit (shown by the dashed low Earth orbit in Fig. 1). The typical orbital transfer vehicle (OTV) maneuver is in many ways similar to the synergetic plane change. In the OTV case, however, the vehicle is initially in a high Earth orbit (HEO)—geosynchronous in most studies—and must undergo a sizeable velocity decrement in its transfer to LEO as illustrated in Fig. 1. This velocity decrement is more efficiently achieved through a combination of aerodynamic drag and propulsion than through propulsion alone. The OTV mission usually involves some change in orbital plane—for instance, the return from geosynchronous orbit to a typical Shuttle orbit involves a plane change of 28.5 deg. If the OTV is a lifting configuration, some of the required plane change can be achieved by banking the vehicle as in the synergetic plane change maneuver. In planetary mission applications, aeroassist is again used to produce a velocity decrement through aerodynamic drag. For these missions two distinctly different applications of aeroassist have been studied: 1) aerocapture in which the velocity decrement achieved in a single deep atmospheric pass is sufficient to transfer the vehicle from its hyperbolic approach trajectory to a target orbit about the planet (i.e., the vehicle is captured by the planet's gravitational field); and 2) multipass aerobraking in which the vehicle is transferred from its approach trajectory to a highly elliptic orbit about the planet by a rocket burn and then the orbit is circularized by many high-altitude atmospheric passes, each followed by small corrective rocket burns at apoapsis to maintain the periapsis altitude low enough to produce the desired drag decrement but high enough to avoid excessive aerodynamic heating. The first type of planetary application, as illustrated in Fig. 1, is similar to the OTV maneuver except that it usually has a larger velocity excess and no plane change. The second type of planetary application is illustrated in Fig. 2. As is also illustrated in Fig. 2, this type of multipass orbit lowering has also been studied for the GEO-to-LEO OTV mission. In the intervening years since London presented his paper in 1961, many studies have been carried out and a rich literature has developed. In the process of researching the present paper, an extensive bibliography was compiled. It lists the relevant publications in chronological order for each of the three vehicle classes considered. While certainly incomplete (new and additional references keep turning up), this collection of papers is believed to present an accurate overall picture of the evolution of aeroassist technology. Examination of this bibliography shows that the various aeroassist mission applications received their most intensive study in different time periods. As was mentioned previously, the serious study of synergetic plane change maneuvers began with London's paper in 1961. Actually, there were studies published prior to 1961 (e.g., Ref. 1) but London's paper appears to be the first to convincingly demonstrate a significant performance gain. During the 1960s, synergetic plane change was studied extensively with the peak activity occurring around 1967. The initial
268 citations
TL;DR: The Strato-spheric Aerosol and Gas Experiment II (SAGE II) instrument as mentioned in this paper was designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide by measuring the ex-tinction of solar radiation through the earth's atmosphere during the ERBS observatory solar occultations.
Abstract: Design and performance data are presented for the Strato-spheric Aerosol and Gas Experiment II (SAGE II) instrument, which has been developed for the Earth Radiation Budget Satellite (ERBS). SAGE II is designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, and nitrogen dioxide by measuring the ex-tinction of solar radiation through the earth's atmosphere during the ERBS observatory solar occultations. Solar radiation is reflected from a flat scanning mirror into a Cassegrain telescope, which forms a solar image on the entrance slit of a grating spectrometer. The SAGE II instan-taneous field of view is scanned along the vertical solar diameter by the elevation scan mirror. The entire optical system is contained within an azimuth gimbal that tracks the solar radiometric centroid during the data event. This spectrometer, with help from three interference filters, iso-lates seven spectral wavelengths ranging from 0.385 um to 1.02 um. All seven channels use silicon photodiode detectors operated in the photo-voltaic mode. Detector outputs are multiplexed into a serial data stream for readout by the ERBS telemetry system. Each output is sampled 64 times per second and digitized to 12 bit resolution. SAGE II is a third generation instrument, following the highly successful Stratospheric Aerosol Measurement II (SAM II) and SAGE programs.
265 citations
TL;DR: The movement of methane (CH4) from anaerobic sediments through the leaves, stems, and flowers of aquatic plants and into the atmosphere was found to provide a significant pathway for the emission of CH4 from the aquatic substrates of flooded wetlands as discussed by the authors.
Abstract: The movement of methane (CH4) from anaerobic sediments through the leaves, stems, and flowers of aquatic plants and into the atmosphere was found to provide a significant pathway for the emission of CH4 from the aquatic substrates of flooded wetlands. Methane concentrations well above the surrounding ambient air levels were found in the mesophyll of 16 varies of aquatic plants and are attributed to transpiration, diffusion, and pressure-induced flow of gaseous CH4 from the roots when they are embedded in CH4-saturated anaerobic sediments. Methane emissions from the emergent parts of aquatic plants were measured using floating chamber techniques and by enclosing the plants in polyethylene bags of known volume. Concentration changes were monitored in the trapped air using syringes and gas chromatographic techniques. Vertical profiles of dissolved CH4 in sediment pore water surrounding the aquatic plants' rhizomes were obtained using an interstitial sampling technique. Methane emissions from the aquatic plants studied varied from 14.8 mg CH4/d to levels too low to be detectable. Rooted and unrooted freshwater aquatic plants were studied as well as saltwater and brackish water plants. Included in the experiment is detailed set of measurements on CH4 emissions from the common cattail (Typha latifolia). This paper illustrates that aquatic plants play an important gas exchange role in the C cycle between wetlands and the atmosphere.
259 citations
TL;DR: In this paper, a decomposition method for decomposing an optimization problem into a set of subproblems and a coordination problem that preserves coupling between the sub-problems is described.
Abstract: A method for decomposing an optimization problem into a set of subproblems and a coordination problem that preserves coupling between the subproblems is described The decomposition is achieved by separating the structural element optimization subproblems from the assembled structural optimization problem Each element optimization and optimum sensitivity analysis yields the cross-sectional dimensions that minimize a cumulative measure of the element constraint violation as a function of the elemental forces and stiffness The assembled structural optimization produces the overall mass and stiffness distributions optimized for minimum total mass subject to constraints that include the cumulative measures of the element constraint violations extrapolated linearly with respect to the element forces and stiffnesses The method is introduced as a special case of a multilevel, multidisciplinary system optimization and its algorithm is fully described for two-level optimization for structures assembled of finite elements of arbitrary type Numerical results are given as an example of a framework to show that the decomposition method converges and yields results comparable to those obtained without decomposition It is pointed out that optimization by decomposition should reduce the design time by allowing groups of engineers using different computers to work concurrently on the same large problem
228 citations
01 Jun 1985
TL;DR: In this paper, a collection of spectral reflectances of 156 natural targets is presented in a uniform format, where a graphical plot and a digital tabulation of reflectance is given.
Abstract: A collection of spectral reflectances of 156 natural targets is presented in a uniform format. For each target both a graphical plot and a digital tabulation of reflectance is given. The data were taken from the literature and include laboratory, field, and aircraft measurements. A discussion of the different measurements of reflectance is given, along with the changes in apparent reflectance when targets are viewed through the atmosphere. The salient features of the reflectance curves of common target types are presented and discussed.
225 citations
TL;DR: In this article, upwind relaxation algorithms for obtaining efficient steady-state solutions to the compressible Navier-Stokes equations are described, using third order flux splitting of the pressure and convective terms and second-order central differencing for shear and heat flux terms.
Abstract: The development of upwind relaxation algorithms for obtaining efficient steady-state solutions to the compressible Navier-Stokes equations is described. The method is second-order accurate spatially and naturally disipative, using third-order flux splitting of the pressure and convective terms and second-order central differencing for shear and heat flux terms. A line Gauss-Seidel relaxation approach, shown to be unconditionally stable for model convection and diffusion equations, is used. The algorithm is demonstrated for several flows using the thin-layer form of the equations, including the problem of shock-induced separation over a flat plate.
198 citations
TL;DR: This article used data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) for the period October 25, 1978-May 28, 1979 for a descriptive study of ozone variations in the middle stratosphere.
Abstract: Data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) for the period October 25, 1978-May 28, 1979 are used in a descriptive study of ozone variations in the middle stratosphere. It is shown that the ozone distribution is strongly influenced by irreversible deformation associated with large amplitude planetary-scale waves. This process, which has been described by McIntyre and Palmer as planetary wave breaking, takes place throughout the 3-30 mb layer, and poleward transport of ozone within this layer occurs in narrow tongues drawn on the tropics and subtropics in association with major and minor warming events. These events complement the zonal mean diabatic circulation in producing significant changes in the total column amount of ozone.
TL;DR: In this paper, the buckling of an elliptic delamination embedded near the surface of a thick quasi-isotropic laminate was predicted using finite element and Rayleigh-Ritz methods.
Abstract: The buckling of an elliptic delamination embedded near the surface of a thick quasi-isotropic laminate was predicted. The thickness of the delaminated ply group (the sublaminate) was assumed to be small compared to the total laminate thickness. Finite-element and Rayleigh-Ritz methods were used for the analyses. The Rayleigh-Ritz method was found to be simple, inexpensive, and accurate, except for highly anisotropic delaminated regions. Effects of delamination shape and orientation, material anisotropy, and layup on buckling strains were examined. Results show that: (1) the stress state around the delaminated region is biaxial, which may lead to buckling when the laminate is loaded in tension; (2) buckling strains for multi-directional fiber sublaminates generally are bounded by those for the 0 deg and 90 deg unidirectional sublaminates; and (3) the direction of elongation of the sublaminate that has the lowest buckling strain correlates with the delamination growth direction.
TL;DR: The International Satellite Cloud Climatology Project (ISCCP) project as discussed by the authors conducted a pilot study initiated in late 1981 to evaluate currently available cloud analysis algorithms, which focused on the most fundamental step of any cloud algorithm, namely cloud detection.
Abstract: Research related to the International Satellite Cloud Climatology Project (ISCCP) began with a pilot study initiated in late 1981 to evaluate currently available cloud analysis algorithms. Other objectives of this study are related to a test of the effects of data volume compression schemes and the design of the operational algorithms for ISCCP. The present paper summarizes the pilot study by focusing on the most fundamental step of any cloud algorithm, that of cloud detection. An outline is provided of the objectives and limitations of the pilot study, and a description is given of the criteria used to design the operational analysis algorithm. Attention is given to the pilot study data set, a cloud detection intercomparison, a cloud analysis, and ISCCP cloud algorithm design.
TL;DR: A general theoretical analysis of the errors that result from differences between the assumed and actual extinction/backscattering ratio profiles is presented.
Abstract: The Bernoulli solution of the lidar equation with the assumption of a constant extinction/backscattering ratio can lead to errors in the derived aerosol extinction and backscattering profiles. This paper presents a general theoretical analysis of the errors that result from differences between the assumed and actual extinction/backscattering ratio profiles. Examples of the influence of the constant extinction/backscattering ratio assumption on the lidar derived aerosol extinction profile are presented for various laser wavelengths.
TL;DR: In this paper, the first survey of CH4 flux from northern peatlands of the United States was presented, with emissions ranging from 0.003 to 1.94 g CH4 m−2 day−1.
Abstract: The concentration of methane (CH4) in the global troposphere is increasing. Ambient air measurements document an approximate rate of increase of 1–2% yr−1 over the past decade1–4. Measurements of CH4 in air bubbles trapped in polar ice indicate that tropospheric concentrations of CH4 several hundred years ago may have been ∼45% of present levels5–7. To understand and assess possible causes of the atmospheric CH4 increase requires improved quantitative knowledge of global sources and sinks of CH4. Previous attempts to estimate sources of atmospheric CH4, based on very few measurements, have suggested that natural and agricultural wetlands are major sources8,9. The major wetland regions of the world are in boreal, low Arctic and tropical ecosystems10. It is these regions, particularly in peatland habitats where major accumulations of organic materials occur under anaerobic conditions, that should be significant sources of global tropospheric CH4. The most extensive peatlands in the world occur in the boreal taiga zone between ∼45° and 65° N latitude. More than 95% of world peat resources occur in the Soviet Union, Canada, the United States, Sweden, Norway, Finland and the United Kingdom10. We report here the first survey of CH4 flux from northern peatlands of the United States. Emission rates ranged from 0.003 to 1.94 g CH4 m−2 day−1, with half of these values between 0.1 and 0.4 g CH4 m−2 day−1. The frequency distribution is log normal (Fig. 1) and the mean emission rate is 0.337 g CH4 m−2 day−1. Such fluxes are higher than most values reported for other ecosystems, suggesting that northern peatlands may be an important source of global tropospheric CH4.
TL;DR: In this article, a general circulation model (GCM) study is presented in which cloud radiative properties are computed from cloud liquid water content inferred from the GCM hydrological cycle.
Abstract: A general circulation model (GCM) study is presented in which cloud radiative properties are computed from cloud liquid water content inferred from the GCM hydrological cycle. Model-generated and satellite albedos are in rough agreement. Analysis of the cloud radiative forcing indicates that cloud albedo effects overcome cloud infrared opacity effects in most regions. Both computed and observed albedo of clouds decrease from low to high altitudes. The model with variable cloud optics produces significantly different regional albedos from the same one with fixed cloud optics, especially over the tropics. The cloud droplet size distribution also has a significant impact on the model albedos. The temperature of the tropical upper troposphere is somewhat sensitive to the microphysical characteristics of the model cirrus clouds.
TL;DR: In this paper, the authors provided models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, for the height interval from 100 to 3,500 km.
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01 Jan 1985TL;DR: In this paper, a dynamic hybrid finite-element analysis for interfacial cracks in composites, energy release rate during delamination crack growth in composite laminates, matrix deformation and fracture in graphite-reinforced epoxies, and the role of delamination and damage development on the strength of thick notched Laminates is presented.
Abstract: The general topics consist of stress analysis, mechanical behavior, and fractography/NDI of composite laminates. Papers are presented on a dynamic hybrid finite-element analysis for interfacial cracks in composites, energy release rate during delamination crack growth in composite laminates, matrix deformation and fracture in graphite-reinforced epoxies, and the role of delamination and damage development on the strength of thick notched laminates. In addition, consideration is given to a new ply model for interlaminar stress analysis, a fracture mechanics approach for designing adhesively bonded joints, the analysis of local delaminations and their influence on composite laminate behavior, and moisture and temperature effects on the mixed-mode delamination fracture of unidirectional graphite/epoxy.
TL;DR: In this paper, an analytical procedure for the static shape control of flexible space structures subjected to thermal distortions is developed which is based on prescribing temperatures in control elements having much higher coefficients of thermal expansion than the main structure.
Abstract: An analytical procedure for the static shape control of flexible space structures subjected to thermal distortions is developed which is based on prescribing temperatures in control elements having much higher coefficients of thermal expansion than the main structure. The temperatures at the control elements are defined so as to minimize the overall thermal distortion of the structure from its ideal shape, and a matrix equation is obtained which can be solved for the set of optimum control temperatures. A formulation of the procedure for continuous structures governed by differential equations and a formulation for discrete (finite element modeled) structures governed by matrix equations are presented. The equations from the continuous formulation are employed for the shape control of a simple beam distorted by nonuniform heating, and the discrete formulation is applied in a general purpose finite-element structural analysis computer program for the shape control of a 750 m radiometer antenna reflector dish subjected to orbital heating. A reduction in thermal distortion by a factor of nearly 50 was obtained with the use of only seven control elements. Results for four different sets of control locations for the antenna are presented in which reductions in distortion of up to a factor of four were obtained.
TL;DR: In this paper, the effect of turbulence in the mixing layer of the jet is simulated by the addition of turbulent eddy-viscosity terms to the momentum equation, and a comparison of the numerical results with experimental data is given to the mean flow profile and the numerical solution.
Abstract: The present investigation is concerned with the development of an analytical model of the quasi-periodic shock-cell structure of an imperfectly expanded supersonic jet. The investigation represents a part of a program to develop a mathematical theory of broadband shock-associated noise of supersonic jets. Tam and Tanna (1982) have suggested that this type of noise is generated by the weak interaction between the quasi-periodic shock cells and the downstream-propagating large turbulence structures in the mixing layer of the jet. In the model developed in this paper, the effect of turbulence in the mixing layer of the jet is simulated by the addition of turbulent eddy-viscosity terms to the momentum equation. Attention is given to the mean-flow profile and the numerical solution, and a comparison of the numerical results with experimental data.
TL;DR: In this paper, a Bernoulli solution for determining the relative distribution of aerosol backscattering in the UV region is presented, and scattering ratio boundary values for these solutions are discussed.
Abstract: The differential absorption lidar (DIAL) technique generally assumes that atmospheric optical scattering is the same at the two laser wavelengths used in the DIAL measurement of a gas concentration profile. Errors can arise in this approach when the wavelengths are significantly separated, and there is a range dependence in the aerosol scattering distribution. This paper discusses the errors introduced by large DIAL wavelength separations and spatial inhomogeneity of aerosols in the atmosphere. A Bernoulli solution for determining the relative distribution of aerosol backscattering in the UV region is presented, and scattering ratio boundary values for these solutions are discussed. The results of this approach are used to derive a backscatter correction to the standard DIAL analysis method. It is shown that for the worst cases of severe range dependence in aerosol backscattering, the residual errors in the corrected DIAL O3 measurements were <10 ppbv for DIAL wavelengths at 286 and 300 nm.
TL;DR: In this article, an experimental study of the transitional flow over a flat disk rotating in quiescent ambient air has been conducted using digitized hot-wire data, the axes of the stationary spiral vortices, which are the primary instability mechanisms for the disk flow, have been mapped out in terms of both spatial coordinates and velocity fluctuations.
Abstract: An experimental study of the transitional flow over a flat disk rotating in quiescent ambient air has been conducted. Using digitized hot-wire data, the axes of the stationary spiral vortices, which are the primary instability mechanisms for the disk flow, have been mapped out in terms of both spatial coordinates and velocity fluctuations. Data are presented for a clean disk and a disk with a single, isolated roughness element. The data show that the spiral vortices are generated at discrete roughness disturbance sites on the disk and that they propagate and grow as wave packets. The familiar vortex pattern of 30 or so vortices results only when these wave packets have merged and filled the entire circumference. The appearance of stationary, secondary instabilities prior to turbulent breakdown has also been observed.
TL;DR: In this paper, the von Karman strain-displacement equations were used to calculate plate load and ply stresses and the failure region and modes were calculated using the Tsai-Wu and the maximum stress criteria, respectively.
Abstract: Clamped circular composite plates were analyzed for static equivalent impact loads. Three plate sizes—25.4, 38.1, and 50.8 mm radii—made of quasi-isotropic graphite/epoxy laminate were analyzed. The analysis was based on the minimum total potential energy method and used the von Karman strain-displacement equations. A step-by-step incremental transverse displacement procedure was used to calculate plate load and ply stresses. The ply failure region and modes (splitting and fiber break) were calculated using the Tsai-Wu and the maximum stress criteria, respectively. Reduced moduli were then used in the failed region in subsequent increments of analyses. The analysis predicted that the failure would initiate as splitting in the bottom-most ply and then progress to other plies. Larger radii plates had a lower splitting threshold (load or energy) and a higher first-fiber failure threshold. The size and shape of the ply damage regions were different for different plies. The bottom ply damage was the largest and elongated in its ply-fiber direction. Calculated splitting damage for a 25.4 mm radius plate agreed with reported test data.
TL;DR: In this article, the role of large-scale coherent structures in broadband jet noise suppression and amplification under controlled excitation is studied experimentally, and it is found that suppression occurs only at low Reynolds and Mach numbers, when the boundary layer at the jet exit is laminar.
Abstract: The role of large-scale coherent structures in broadband jet noise suppression and amplification under controlled excitation are studied experimentally. It is found that suppression occurs only at low Reynolds and Mach numbers, when the boundary layer at the jet exit is laminar; the optimum St(Theta) (St, based on the initial shear-layer momentum thickness) value is 0.017, at which the excitation results in a quick roll-up and transition of the laminar shear-layer vortices, yielding coherent structures similar to those existing at high speeds. At the asymptotic level the broadband jet noise can only be amplified by the excitation. The amplification is inferred to be maximum for excitation in the St(D) (St based on the jet diameter) range of 0.65-0.85. Finally, it is noted that the pairing process induced by the excitation is at the origin of the broadband noise amplification.
TL;DR: In this paper, the postbuckling response and failure characteristics of undamaged panels and panels damaged by low-speed impact are described, and the analytical modeling detail necessary to predict accurately the response of a panel is described.
Abstract: Results of an experimental study of the postbuckling behavior of selected flat stiffened graphite-epoxy panels loaded in compression are presented. The postbuckling response and failure characteristics of undamaged panels and panels damaged by low-speed impact are described. Each panel had four equally-spaced I-spaced stiffeners and 16- or 24-ply quasi-isotropic skins. Panels with three different stiffener spacings were tested. Some undamaged specimens supported as much as three times their initial buckling load before failing. Failure of all panels initiated in a skin-stiffener interface region. Analytical results obtained from a nonlinear general shell finite element analysis computer code correlate well with typical postbuckling test results up to failure. The analytical modeling detail necessary to predict accurately the response of a panel is described. Test results show that low-speed impact damage can reduce the postbuckling strength of a stiffened panel and that the skin-stiffener interface region is more sensitive to impact damage than the skin midway between stiffeners.
TL;DR: The nebulization-reflex concentration technique is more efficient (maximum flow of gas through the minimum volume of extractant) than conventional bubbler/impinger gas extraction techniques and is offered as an alternative method.
Abstract: A new concentration technique for the extraction and enrichment of water-soluble atmospheric trace gases has been developed. The gas scrubbing technique efficiently extracts soluble gases from a large volume flow rate of air sample into a small volume of refluxed trapping solution. The gas scrubber utilizes a small nebulizing nozzle that mixes the incoming air with an aqueous extracting solution to form an air/droplet mist. The mist provides excellent interfacial surface areas for mass transfer. The resulting mist sprays upward through the reaction chamber until it impinges upon a hydrophobic membrane that virtually blocks the passage of droplets but offers little resistance to the existing gas flow. Droplets containing the scrubbed gases coalesce on the membrane and drip back into the reservoir for further refluxing. After a suitable concentration period, the extracting solution containing the analyte can be withdrawn for analysis. The nebulization-reflex concentration technique is more efficient (maximum flow of gas through the minimum volume of extractant) than conventional bubbler/impinger gas extraction techniques and is offered as an alternative method.
14 Jan 1985
TL;DR: A class of explicit multistage time-stepping schemes is used to construct an algorithm for solving the compressible Navier-Stokes equations and Flexibility in treating arbitrary geometries is obtained with a finite-volume formulation.
Abstract: A class of explicit multistage time-stepping schemes is used to construct an algorithm for solving the compressible Navier-Stokes equations. Flexibility in treating arbitrary geometries is obtained with a finite-volume formulation. Numerical efficiency is achieved by employing techniques for accelerating convergence to steady state. Computer processing is enhanced through vectorization of the algorithm. The scheme is evaluated by solving laminar and turbulent flows over a flat plate and an NACA 0012 airfoil. Numerical results are compared with theoretical solutions or other numerical solutions and/or experimental data.
TL;DR: In this paper, the results of a study of direct measurements of methane fluxes to the atmosphere from salt marsh soils and of indirect determinations of fluxes from tidal creek waters are presented.
Abstract: It is thought that biological methanogenesis in natural and agricultural wetlands and enteric fermentation in animals are the dominant sources of global tropospheric methane. It is pointed out that the anaerobic soils and sediments, where methanogenesis occurs, predominate in coastal marine wetlands. Coastal marine wetlands are generally believed to be approximately equal in area to freshwater wetlands. For this reason, coastal marine wetlands may be a globally significant source of atmospheric methane. The present investigation is concerned with the results of a study of direct measurements of methane fluxes to the atmosphere from salt marsh soils and of indirect determinations of fluxes from tidal creek waters. In addition, measurements of methane distributions in coastal marine wetland sediments and water are presented. The results of the investigation suggest that marine wetlands provide only a minor contribution to atmospheric methane on a global scale.
TL;DR: In this article, a Fourier-Chebyshev spectral method for the incompressible Navier-Stokes equations is described, which is applicable to a variety of problems including some with fluid properties which vary strongly both in the normal direction and in time.
01 Jan 1985
TL;DR: In this paper, an equation was derived for the strain energy release rate associated with local delamination growth from a matrix ply crack and a simple technique for predicting strain concentrations in the primary load bearing plies near local delaminations was developed.
Abstract: An equation was derived for the strain energy release rate, G, associated with local delamination growth from a matrix ply crack. The critical GC for edge delamination onset in 25/902s graphite epoxy laminates was measured and used in this equation to predict local delamination onset strains in 25/90ns, n = 4, 6, 8 laminates. A simple technique for predicting strain concentrations in the primary load bearing plies near local delaminations was developed. These strain concentrations were responsible for reduced laminate nominal failure strains in laminates containing local delaminations. The influence of edge delamination and matrix crack tip delamination on laminate stiffness and strength was compared.