Showing papers by "Langley Research Center published in 1988"

Biomass‐burning emissions and associated haze layers over Amazonia

Abstract: The characteristics of haze layers, which were visually observed over the central Amazon Basin during many of the Amazon Boundary Layer Experiment 2A flights in July/August 1985, were investigated by remote and in situ measurements, using the broad range of instrumentation and sampling equipment on board the Electra aircraft. It was found that these layers strongly influenced the chemical and optical characteristics of the atmosphere over the eastern Amazon Basin. Relative to the regional background, the concentrations of CO, CO2, O3, and NO were significantly elevated in the plumes and haze layers, with the NO/CO ratio in fresh plumes much higher than in the aged haze layers. The haze aerosol was composed predominantly of organic material, NH4, K(+), NO3(-), SO4(2-), and organic anions (formate, acetate, and oxalate).

An analytical study of the hydrogen-air reaction mechanism with application to scramjet combustion

Abstract: A chemical kinetic mechanism for the combustion of hydrogen has been assembled and optimized by comparing the observed behavior as determined in shock tube and flame studies with that predicted by the mechanism. The reactions contained in the mechanism reflect the current state of knowledge of the chemistry of the hydrogen/air system, and the assigned rate coefficients are consistent with accepted values. It was determined that the mechanism is capable of satisfactorily reproducing the experimental results for a range of conditions relevant to scramjet combustion. Calculations made with the reaction mechanism for representative scramjet combustor conditions at Mach 8, 16, and 25 showed that chemical kinetic effects can be important and that combustor models which use nonequilibrium chemistry should be used in preference to models that assume equilibrium chemistry. For the conditions examined the results also showed the importance of including the HO2 chemistry in the mechanism. For Mach numbers less than 16, the studies suggest that an ignition source will most likely be required to overcome slow ignition chemistry. At Mach 25, the initial temperature and pressure was high enough that ignition was rapid and the presence of an ignition source did not significantly affect reaction rates.

Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperate site

Abstract: Tropospheric concentrations of formic and acetic acids in the gas, the aerosol, and the rainwater phases were determined in samples collected 1-2 m above ground level at an open field site in eastern Virginia. These acids were found to occur principally (98 percent or above) in the gas phase, with a marked annual seasonality, averaging 1890 ppt for formate and 1310 ppt for acetate during the growing season, as compared to 695 ppt and 700 ppt, respectively, over the nongrowing season. The data support the hypothesis that biogenic emissions from vegatation are important sources of atmospheric formic and acetic acid during the local growing season. The same time trends were observed for precipitation, although with less defined seasonality. The relative increase of the acetic acid/formic acid ratio during the nongrowing season points to the dominance of anthropogenic inputs of acetic acid from motor vehicles and biomass combustion in the wintertime.

Formic and acetic acid over the central Amazon region, Brazil: 1. Dry season

Abstract: The concentrations of formic and acetic acids in the gas phase, atmospheric aerosol, and rainwater samples collected in Amazonia at ground level and in the atmosphere during the Amazon Boundary Layer Experiment in July/August 1985 were analyzed by ion exchange chromatography. The diurnal behavior of both acids at ground level and their vertical distribution in the forest canopy point to the existence of vegetative sources as well as to production by chemical reactions in the atmosphere. The concentrations of formic and acetic acids in the gas phase were about 2 orders of magnitude higher than the corresponding concentrations in the atmospheric aerosol. In rainwater, the total formate and acetate represented about one half of the anion equivalents, in contrast to less than 10 percent of the soluble anionic equivalents contributed by these acids in the atmospheric aerosol. The observed levels of these ions in rainwater are considered to be the result of a combination of chemical reactions in hydrometeors and the scavenging of the gaseous acids by cloud droplets.

A virtual crack-closure technique for calculating stress intensity factors for cracked three dimensional bodies

Abstract: A three-dimensional virtual crack-closure technique is presented which calculates the strain energy release rates and the stress intensity factors using only nodal forces and displacements from a standard finite element analysis. The technique is an extension of the Rybicki-Kanninen (1977) method, and it assumes that any continuous function can be approximated by a finite number of straight line segments. Results obtained by the method for surface cracked plates with and without notches agree favorably with previous results.

Methane flux from the central Amazonian floodplain

Abstract: A total of 186 methane measurements from the three primary Amazon floodplain environments of open water lakes, flood forests, and floating grass mats were made over the period 18 July through 2 September 1985. These data indicate that emissions were lowest over open water lakes. Flux from flooded forests and grass mats was significantly higher. At least three transport processes contribute to tropospheric emissions: ebullition from sediments, diffusion along the concentration gradient from sediment to overlaying water to air, and transport through the roots and stems of aquatic plants. Measurements indicate that the first two of these processes are most significant. It was estimated that on the average bubbling makes up 49% of the flux from open water, 54% of that from flooded forests, and 64% of that from floating mats. If the measurements were applied to the entire Amazonian floodplain, it is calculated that the region could supply up to 12% of the estimated global natural sources of methane.

Effects of independent variation of Mach and Reynolds numbers on the low-speed aerodynamic characteristics of the NACA 0012 airfoil section

Abstract: A comprehensive data base is given for the low speed aerodynamic characteristics of the NACA 0012 airfoil section. The Langley low-turbulence pressure tunnel is the facility used to obtain the data. Included in the report are the effects of Mach number and Reynolds number and transition fixing on the aerodynamic characteristics. Presented are also comparisons of some of the results with previously published data and with theoretical estimates. The Mach number varied from 0.05 to 0.36. The Reynolds number, based on model chord, varied from 3 x 10 to the 6th to 12 x 10 to the 6th power.

Singular Perturbation Methodology in Control Systems

Abstract: This book presents the twin topics of singular perturbation methods and time scale analysis to problems in systems and control. The heart of the book is the singularly perturbed optimal control systems, which are notorious for demanding excessive computational costs. The book addresses both continuous control systems (described by differential equations) and discrete control systems (characterised by difference equations).

The subgrid‐scale modeling of compressible turbulence

Abstract: A subgrid‐scale model recently derived by Yoshizawa [Phys. Fluids 29, 2152 (1986)] for use in the large‐eddy simulation of compressible turbulent flows is examined from a fundamental theoretical and computational standpoint. It is demonstrated that this model, which is only applicable to compressible turbulent flows in the limit of small density fluctuations, correlates somewhat poorly with the results of direct numerical simulations of compressible isotropic turbulence at low Mach numbers. An alternative model, based on Favre‐filtered fields, is suggested which appears to reduce these limitations.

Carbon-carbon - An overview

Abstract: In nonoxidizing high-temperature environments, carbon-carbon composites retain room temperature properties to more than 2225 C; in oxidizing environments, the variety of coatings thus far developed limits maximum operating temperatures to about 1600 C. The high thermal conductivity and low thermal expansion of these composites renders them ideal for applications encountering thermal shocks. In addition, the variety of fibers, weave patterns, and layup procedures that can be used for the composites allows mechanical properties to be carefully tailored over a wide range to fit the application in question.

Carbon dioxide in the atmosphere over the Amazon Basin

Abstract: As a part of the NASA's Amazon Boundary Layer Experiment 2A mission, the cycle of atmospheric CO2 over the Amazon Basin was examined using measured vertical profiles of CO2 concentrations in the canopy and aloft, and direct measurements of CO2 emissions from soils. The results provide a detailed picture of daily exchanges of air between the tropical forest (0-30) and the atmospheric boundary layer (30-2000 m). A comparison of atmospheric CO2 distributions over forests, wetlands, and rivers shows that the lower atmosphere over forests functions separately from that over rivers or wetlands during the night and to some extent during the day; the basic diurnal cycle of CO2 over wetlands is much weaker than over forests, and the cycle is almost absent over rivers. This result is consistent with expectations based on the biogeochemistry of organic carbon in these systems.

Tropospheric methane from an Amazonian floodplain lake

Abstract: The sources of methane and its flux to the troposphere from the Amazonian floodplain were investigated during the dry season of July and August 1985, using measurements of methane concentration gradients obtained aboard a houseboat laboratory anchored in Lago Calado, a stratified dendritic lake of about 6-sq km area located near the center of the Amazon Basin. Methane concentrations in the mixed layer of the lake were found to vary from 0.0001 to 0.0055 mM, with no consistent temporal trend. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH4/sq m per day, consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70 percent to the average total flux. The source of methane to the lake and, ultimately, to the troposphere is the benthic sediments.

Measurements of odd nitrogen compounds in the stratosphere by the ATMOS experiment on Spacelab 3

Abstract: Spacelab 3's Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment has obtained 30 deg N and 48 deg S vertical profiles of reservoir gases, source gases, and other trace molecules that are important in the middle atmosphere's odd nitrogen, odd chlorine, and odd hydrogen chemical families. The abundances of individual gases and total odd nitrogen levels measured by ATMOS have been compared with prior results obtained from balloon and satellite platforms. The lower-limit profile agrees with ATMOS data to within 16 percent up to 42 km altitude.

Dynamic pressure loads associated with twin supersonic plume resonance

Abstract: The phenomenon of twin supersonic plume resonance is defined and studied as it pertains to high level dynamic loads in the inter-nozzle region of aircraft like the F-15 and B1-A. Using a 1/40th scale model twin jet nacelle with powered choked nozzles, it is found that intense internozzle dynamic pressures are associated with the synchrophased coupling of each plume's jet flapping mode. This condition is found most prevalent when each plume's jet flapping mode has constituent elements composed of the B-type helical instability. Suppression of these fatigue bearing loads was accomplished by simple geometric modifications to only one plume's nozzle. These modifications disrupt the natural selection of the B-type mode and thereby decouple the plumes.

Polar stratospheric clouds and the Antarctic ozone hole

Abstract: A theoretical model for the formation and growth of polar stratospheric clouds (PSCs) has been developed. Results for the calculated temperature dependence of optical backscattering are found to agree well with values obtained during two Arctic airborne-lidar experiments. Results for PSC formation in Antarctica show that at the 70-mbar level, about 80 percent of the HNO3 and about 30 percent of the H2O vapor available may be sequestered in relatively large PSC particles at a temperature near 189 K.

Experimental results for the Eppler 387 airfoil at low Reynolds numbers in the Langley low-turbulence pressure tunnel

Abstract: Experimental results were obtained for an Eppler 387 airfoil in the Langley Low Turbulence Pressure Tunnel. The tests were conducted over a Mach number range from 0.03 to 0.13 and a chord Reynolds number range for 60,000 to 460,000. Lift and pitching moment data were obtained from airfoil surface pressure measurements and drag data for wake surveys. Oil flow visualization was used to determine laminar separation and turbulent reattachment locations. Comparisons of these results with data on the Eppler 387 airfoil from two other facilities as well as the Eppler airfoil code are included.

Regional aerosol chemistry of the Amazon Basin during the dry season

Abstract: The distribution and chemical composition of the atmospheric aerosol over the Amazon Basin forest were determined during the 1985 July-August dry season, using data on the aerosol chemical constituent concentration collected during the NASA Global Tropospheric Experiment Amazon Boundary Layer Experiment 2A mission. The results of the analyses suggest that there is a remarkable compositional and spatial homogeneity of the atmospheric aerosol on an extensive regional scale. Particulate organic carbon is the dominant component of the atmospheric aerosol, exhibiting an average concentration of about 740 nmol/cu m in the mixed layer and about 220 nmol/cu m in free tropospheric air. Oxalate and SO4(2-) exhibited the greatest enrichment in the mixed layer, while Cl(-) showed essentially no enrichment. The aerosol in the Amazonian atmosphere is essentially acid-base neutral, primarily as a result of incorporation of NH(+), which is presumably derived from NH3 released by the forest ecosystem.

A water tunnel study of Gurney flaps

Abstract: Several Gurney flap configurations were tested in the NASA Langley 16 x 24 inch Water Tunnel. These devices provided an increased region of attached flow on a wing upper surface relative to the wing without the flaps. The recirculation region behind the flap was visualized and shown to be consistent with hypotheses stated in previous research. Although the test Reynolds number for this study was several orders of magnitude below those in previous investigations, the effect of the Gurney flaps is in qualitative agreement with them. This is as would be expected from first order effects for high lift devices.

Enhanced biogenic emissions of nitric oxide and nitrous oxide following surface biomass burning

Abstract: Recent measurements indicate significantly enhanced biogenic soil emissions of both nitric oxide (NO) and nitrous oxide (N2O) following surface burning. These enhanced fluxes persisted for at least 6 months following the burn. Simultaneous measurements indicate enhanced levels of exchangeable ammonium in the soil following the burn. Biomass burning is known to be an instantaneous source of NO and N2O resulting from high-temperature combustion. Now we find that biomass burning also results in significantly enhanced biogenic emissions of these gases, which persist for months following the burn.

Airborne lidar observations of Arctic polar stratospheric clouds: Indications of two distinct growth stages

Abstract: Airborne lidar observations of Arctic polar stratospheric clouds (PSCs) during January 1984 and January 1986 show contrast suggestive of two distinct PSC growth stages delineated by the frost-point temperature. Results obtained at temperatures 2-6 K above the frost point indicate a stage of significant, but limited, particle growth such as proposed in recent models of PSC formation by co-deposition of HNO3 and H2O vapors. Results obtained at a temperature near the frost point indicate the formation of somewhat larger crystalline particles.

Studies of aerothermal loads generated in regions of shock/shock interaction in hypersonic flow

Abstract: Experimental studies were conducted to examine the aerothermal characteristics of shock/shock/boundary layer interaction regions generated by single and multiple incident shocks. The presented experimental studies were conducted over a Mach number range from 6 to 19 for a range of Reynolds numbers to obtain both laminar and turbulent interaction regions. Detailed heat transfer and pressure measurements were made for a range of interaction types and incident shock strengths over a transverse cylinder, with emphasis on the 3 and 4 type interaction regions. The measurements were compared with the simple Edney, Keyes, and Hains models for a range of interaction configurations and freestream conditions. The complex flowfields and aerothermal loads generated by multiple-shock impingement, while not generating as large peak loads, provide important test cases for code prediction. The detailed heat transfer and pressure measurements proved a good basis for evaluating the accuracy of simple prediction methods and detailed numerical solutions for laminar and transitional regions or shock/shock interactions.

On the sensitivity of complex, internally coupled systems

Abstract: A method is presented for computing sensitivity derivatives with respect to independent (input) variables for complex, internally coupled systems, while avoiding the cost and inaccuracy of finite differencing performed on the entire system analysis. The method entails two alternative algorithms: the first is based on the classical implicit function theorem formulated on residuals of governing equations, and the second develops the system sensitivity equations in a new form using the partial (local) sensitivity derivatives of the output with respect to the input of each part of the system. A few application examples are presented to illustrate the discussion.

Structure of rough-wall turbulent boundary layers

Abstract: Recent experiments have shown that, in rough-wall turbulent boundary layers, drag varies systematically with the spanwise aspect ratio lambda(z) (span/height) of roughness elements. In this paper, the effect of lambda(z) on turbulence structure has been examined. Based on lambda(z), the roughness in a transversely grooved surface with lambda(z) much greater than 1 is the opposite extreme of model plant canopies with lambda(z) much less than 1, studied in wind tunnels, whereas sandgrain is an intermediate type. Second-, third-, and fourth-order turbulence moments have been measured in turbulent boundary layers over transversely grooved and smooth surfaces and compared with available turbulence structure measurements over other types of surfaces. The near-wall turbulence structure is found to vary with lambda(z). The instantaneous motions involved in the flux of shear stress near the wall in smooth and transversely grooved surfaces are opposite in sign to those in three-dimensional roughness. The former is explained in terms of hairpin vortices alone, while the latter group is modeled to have an additional vortex (the so-called necklace vortex which straddles a three-dimensional roughness element near its base).

Nonlinear programming extensions to rational function approximation methods for unsteady aerodynamic forces

Abstract: The approximation of unsteady generalized aerodynamic forces in the equations of motion of a flexible aircraft are discussed. Two methods of formulating these approximations are extended to include the same flexibility in constraining the approximations and the same methodology in optimizing nonlinear parameters as another currently used extended least-squares method. Optimal selection of nonlinear parameters is made in each of the three methods by use of the same nonlinear, nongradient optimizer. The objective of the nonlinear optimization is to obtain rational approximations to the unsteady aerodynamics whose state-space realization is lower order than that required when no optimization of the nonlinear terms is performed. The free linear parameters are determined using the least-squares matrix techniques of a Lagrange multiplier formulation of an objective function which incorporates selected linear equality constraints. State-space mathematical models resulting from different approaches are described and results are presented that show comparative evaluations from application of each of the extended methods to a numerical example.

Modern wing flutter analysis by computational fluid dynamics methods

Abstract: The application and assessment of the recently developed CAP-TSD transonic small-disturbance code for flutter prediction is described. The CAP-TSD code has been developed for aeroelastic analysis of complete aircraft configurations and was previously applied to the calculation of steady and unsteady pressures with favorable results. Generalized aerodynamic forces and flutter characteristics are calculated and compared with linear theory results and with experimental data for a 45 deg sweptback wing. These results are in good agreement with the experimental flutter data which is the first step toward validating CAP-TSD for general transonic aeroelastic applications. The paper presents these results and comparisons along with general remarks regarding modern wing flutter analysis by computational fluid dynamics methods.