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We propose to develop and enhance a workstation based grid analysis and display software system for Earth science dataset browsing, sampling and manipulation. The system will be coupled to a supercomputer in a distributed computing environment for near real-time interaction between scientists and computational results.
Electron cyclotron resonance (ECR) based ion thrusters with carbon based ion optics can potentially satisfy lifetime requirements for long duration missions (approximately 10 years) because grid erosion and cathode insert depletion issues are virtually eliminated. Though the ECR plasma discharge has been found to typically operate at slightly higher discharge losses than conventional DC ion thrusters (for high total thruster power applications), the discharge power fraction is small (less than 1 percent at 25 kW). In this regard, the benefits of increased life, low discharge plasma potentials, and reduced complexity are welcome tradeoffs for the associated discharge efficiency decrease. Presented here are results from discharge characterization of a large area ECR plasma source for gridded ion thruster applications. These measurements included load matching efficacy, bulk plasma properties via Langmuir probe, and plasma uniformity as measured using current probes distributed at the exit plane. A high degree of plasma uniformity was observed (flatness greater than 0.9). Additionally, charge state composition was qualitatively evaluated using emission spectroscopy. Plasma induced emission was dominated by xenon ion lines. No doubly charged xenon ions were detected.
The goal of NASA's Land Long Term Iiata Record (LTDR) project is to produce a consistent long term data set from the AVHRR and MODIS instruments for land climate studies. The project will create daily surface reflectance and normalized difference vegetation index (NDVI) products at a resolution of 0.05 deg., which is identical to the Climate Modeling Grid (CMG) used for MODIS products from EOS Terra and Aqua. Higher order products such as burned area, land surface temperature, albedo, bidirectional reflectance distribution function (BRDF) correction, leaf area index (LAI), and fraction of photosyntheticalIy active radiation absorbed by vegetation (fPAR), will be created. The LTDR project will reprocess Global Area Coverage (GAC) data from AVHRR sensors onboard NOAA satellites by applying the preprocessing improvements identified in the AVHRR Pathfinder Il project and atmospheric and BRDF corrections used in MODIS processing. The preprocessing improvements include radiometric in-flight vicarious calibration for the visible and near infrared channels and inverse navigation to relate an Earth location to each sensor instantaneous field of view (IFOV). Atmospheric corrections for Rayleigh scattering, ozone, and water vapor are undertaken, with aerosol correction being implemented. The LTDR also produces a surface reflectance product for channel 3 (3.75 micrometers). Quality assessment (QA) is an integral part of the LTDR production system, which is monitoring temporal trands in the AVHRR products using time-series approaches developed for MODIS land product quality assessment. The land surface reflectance products have been evaluated at AERONET sites. The AVHRR data record from LTDR is also being compared to products from the PAL (Pathfinder AVHRR Land) and GIMMS (Global Inventory Modeling and Mapping Studies) systems to assess the relative merits of this reprocessing vis-a-vis these existing data products. The LTDR products and associated information can be found at http://ltdr.nascom.nasa.gov/ltdr/ltdr.html.
We have used a global MHD simulation with high spatial resolution to investigate the origin and properties of turbulence in the plasma sheet. In this simulation we imposed a steady southward IMF with a magnitude of 5 nT at the upstream simulation boundary for more than three hours followed by ninety minutes of northward IMF of the same magnitude. The solar wind number density was 20 cm-3, the thermal pressure was 20 pPa, and the velocity was 500 km/s in the x direction. The moderately high dynamic pressure confined the magnetotail to the high-grid resolution region. Even for these nominal solar wind parameters and steady driving the plasma sheet became turbulent. The power spectral densities and probability distribution functions computed from the simulations were comparable to those obtained from spacecraft observations. The largest scale vortices were associated with reconnection outflows and, in the southward IMF case, with the diversion of high speed flows in the near-Earth region. Both time and space domain analyses revealed that there were three scales present, the large scale of the driving processes, the intermediate inertial scale and the dissipative scale.
This work summarizes the results of FUN3D analyses conducted for the 1st AIAA Stability and Control Workshop on behalf of participants from the NASA Langley Research Center. The workshop was created to establish best practices for the prediction of stability and control derivatives using computational fluid dynamics and assess the limitations of these methods when those best practices are applied. The inaugural workshop considered the ONERA version of the NASA/Boeing Common Research Model, which includes the wing, body, horizontal tail, and a vertical tail designed by ONERA. Wind tunnel data at small sideslip angles remain unpublished and served as ‘blind’ data for computational comparisons. The present research generated workshop test case data using the NASA FUN3D code, which is a parallelized, unstructured, node-based, finite-volume discretization, Reynolds-averaged Navier-Stokes flow solver. Steady- state numerical simulations were conducted for workshop test cases investigating the following: grid convergence, Mach number effect on static stability, wind tunnel sting increments, static stability-derivative calculations, and a sideslip angle sweep. Results were generated for two series of unstructured, mixed-element grids, one set provided by the workshop and another set created using the HeldenMesh grid generation software. The results provided include total- and component-level breakdowns of the force and moment coefficients, in addition to sectional pressure distributions for the wing and tail components for comparisons to wind tunnel data.
Uncertainty in top-of-atmosphere (TOA) radiation fluxes observations are larger in the Arctic than in other regions. These uncertainties are due to the low sun angles and the highly reflective, anisotropic, and heterogeneous surface conditions. Quantifying, attributing, and reducing Arctic TOA radiative flux uncertainty enables a better understanding of the rapidly changing Arctic. To advance this goal, we compare the Cloud and Earth’s Radiant Energy System (CERES) TOA radiative fluxes with Arctic Radiation-IceBridge Sea and Ice Experiment (ARISE) campaign measurements collected in September 2014. We compare CERES TOA and aircraft radiative flux measurements using two complementary approaches: grid box average fluxes and instantaneously matched footprints. The grid box mean flux comparison indicates an agreement between CERES and aircraft measurements within 2 uncertainty (calibration and inversion) in the longwave for all five grid boxes and for four-of-five grid boxes in the shortwave; shortwave and longwave mean differences are -7.9 and +2.3 Wm‑2, respectively. The comparison of 36 instantaneously matched footprints with aircraft measurements reveals mean differences of -10.5 and 0.4 Wm‑2 in the shortwave and longwave, respectively. To further explore the persistent negative difference in the shortwave, we further quantify the effects of temporal and spatial sampling differences, angular distribution models, and scene identification to CERES-aircraft differences. Our analysis indicates that sampling differences (including scene evolution) account for an additional 1.8 and 1.7% uncertainty in the shortwave and longwave, respectively and indicates no bias. After accounting for this sampling uncertainty, all CERES-aircraft grid box mean fluxes agree within 2 uncertainty. Scene identification errors due to sea ice concentration data set differences exhibit no bias in the shortwave flux difference and indicate the possibility of substantial differences in the CERES fluxes in specific cases with large spatial heterogeneity. Considering the instantaneously matched footprints, we find that the angular distribution models account may account for up to ‑7.3 Wm-2 of the persistent CERES-aircraft shortwave flux difference due to systematic differences in the anisotropy for sea ice partly cloudy scenes. Additional analysis using a special scan model with the CERES FM2 instrument suggests a significant view zenith angle dependence of the CERES fluxes for sea ice partly cloudy scenes where shortwave fluxes systematically decrease with increasing view zenith angle; no dependence is found for other scene types. We conclude that (1) spatial heterogeneity and scene temporal evolution substantially limit our ability to use aircraft measurements to place strong constraints on CERES TOA fluxes and (2) that the representation of anisotropy in sea ice partly cloudy scenes is a significant factor contributing to the persistent negative CERES-aircraft shortwave flux difference in this comparison and require additional data to analysis fully quantify the potential bias.
Working across U.S. federal agencies, international agencies, and multiple worldwide data centers, and spanning seven international network organizations, the Earth System Grid Federation (ESGF) allows users to access, analyze, and visualize data using a globally federated collection of networks, computers, and software. Its architecture employs a system of geographically distributed peer nodes that are independently administered yet united by common federation protocols and application programming interfaces (APIs). The full ESGF infrastructure has now been adopted by multiple Earth science projects and allows access to petabytes of geophysical data, including the Coupled Model Intercomparison Project (CMIP) output used by the Intergovernmental Panel on Climate Change assessment reports. Data served by ESGF not only include model output (i.e., CMIP simulation runs) but also include observational data from satellites and instruments, reanalyses, and generated images. Metadata summarize basic information about the data for fast and easy data discovery.
This study evaluates the performance of NASA's Modern-Era Retrospective Analysis for Research and Applications (MERRA) precipitation product in reproducing the trend and distribution of extreme precipitation events. Utilizing the extreme value theory, time-invariant and time-variant extreme value distributions are developed to model the trends and changes in the patterns of extreme precipitation events over the contiguous United States during 1979-2010. The Climate Prediction Center (CPC) U.S.Unified gridded observation data are used as the observational dataset. The CPC analysis shows that the eastern and western parts of the United States are experiencing positive and negative trends in annual maxima, respectively. The continental-scale patterns of change found in MERRA seem to reasonably mirror the observed patterns of change found in CPC. This is not previously expected, given the difficulty in constraining precipitation in reanalysis products. MERRA tends to overestimate the frequency at which the 99th percentile of precipitation is exceeded because this threshold tends to be lower in MERRA, making it easier to be exceeded. This feature is dominant during the summer months. MERRA tends to reproduce spatial patterns of the scale and location parameters of the generalized extreme value and generalized Pareto distributions. However, MERRA underestimates these parameters, particularly over the Gulf Coast states, leading to lower magnitudes in extreme precipitation events. Two issues in MERRA are identified: 1) MERRA shows a spurious negative trend in Nebraska and Kansas, which is most likely related to the changes in the satellite observing system over time that has apparently affected the water cycle in the central United States, and 2) the patterns of positive trend over the Gulf Coast states and along the East Coast seem to be correlated with the tropical cyclones in these regions. The analysis of the trends in the seasonal precipitation extremes indicates that the hurricane and winter seasons are contributing the most to these trend patterns in the southeastern United States. In addition, the increasing annual trend simulated by MERRA in the Gulf Coast region is due to an incorrect trend in winter precipitation extremes.
Uncertainty in top-of-atmosphere (TOA) radiation fluxes observations are larger in the Arctic than in other regions. These uncertainties are due to the low sun angles and the highly reflective, anisotropic, and heterogeneous surface conditions. Quantifying, attributing, and reducing Arctic TOA radiative flux uncertainty enables a better understanding of the rapidly changing Arctic. To advance this goal, we compare the Cloud and Earth’s Radiant Energy System (CERES) TOA radiative fluxes with Arctic Radiation-IceBridge Sea and Ice Experiment (ARISE) campaign measurements collected in September 2014. We compare CERES TOA and aircraft radiative flux measurements using two complementary approaches: grid box average fluxes and instantaneously matched footprints. The grid box mean flux comparison indicates an agreement between CERES and aircraft measurements within 2σ uncertainty (calibration and inversion) in the longwave for all five grid boxes and for four-of-five grid boxes in the shortwave; shortwave and longwave mean differences are -7.9 and +2.3 Wm 2, respectively. The comparison of 36 40 instantaneously matched footprints with aircraft measurements reveals mean differences of -120.25 and -1.00.4 Wm 2 in the shortwave and longwave, respectively. To further explore the persistent negative difference in the shortwave, Wwe further quantify the effects of temporal and spatial sampling differences, angular distribution models, and scene identification to CERES-aircraft differences. Our analysis indicates that sampling differences (including scene evolution) account for an additional 1.8 and 1.7% uncertainty in the shortwave and longwave, respectively and , but indicates no bias. After accounting for this sampling uncertainty, all CERES-aircraft grid box mean fluxes agree within 2σ uncertainty. Scene identification errors due to sea ice concentration data set differences exhibit no bias in the shortwave flux difference and indicate the possibility of substantial differences in the CERES fluxes in specific cases with large spatial heterogeneity. Considering the instantaneously matched footprints, we find that the angular distribution models account may account for up to 7.3 Wm-2 of the persistent CERES-aircraft shortwave flux difference due to systematic differences in the anisotropy for sea ice partly cloudy scenes. Additional analysis using a special programmable scan model with the CERES FM2 instrument suggests a significant view zenith angle dependence of the CERES fluxes for sea ice partly cloudy scenes where shortwave fluxes systematically decrease with increasing view zenith angle; no dependence is found for other scene types. We conclude that (1) spatial heterogeneity and scene temporal evolution substantially limit our ability to use aircraft measurements to place strong constraints on CERES TOA fluxes and (2) that the representation of anisotropy in sea ice partly cloudy scenes is likely a significant factor contributing to the persistent negative CERES-aircraft shortwave flux difference in this comparison and require additional data to analysis fully quantify the potential bias.
Results from the Sixth AIAA CFD Drag Prediction Workshop Common Research Model Cases 2 to 5 are presented. As with past workshops, numerical calculations are performed using industry-relevant geometry, methodology, and test cases. Cases 2 to 5 focused on force/moment and pressure predictions for the NASA Common Research Model wing-body and wing-body-nacelle-pylon configurations, including Case 2 - a grid refinement study and nacelle-pylon drag increment prediction study; Case 3 - an angle-of-attack buffet study; Case 4 - an optional wing-body grid adaption study; and Case 5 - an optional wing-body coupled aero-structural simulation. The Common Research Model geometry differed from previous workshops in that it was deformed to the appropriate static aeroelastic twist and deflection at each specified angle-of-attack. The grid refinement study used a common set of overset and unstructured grids, as well as user created Multiblock structured, unstructured, and Cartesian based grids. For the supplied common grids, six levels of refinement were created resulting in grids ranging from 7x10(exp 6) to 208x10(exp 6) cells. This study (Case 2) showed further reduced scatter from previous workshops, and very good prediction of the nacelle-pylon drag increment. Case 3 studied buffet onset at M=0.85 using the Medium grid (20 to 40x10(exp 6) nodes) from the above described sequence. The prescribed alpha sweep used finely spaced intervals through the zone where wing separation was expected to begin. Although the use of the prescribed aeroelastic twist and deflection at each angle-of-attack greatly improved the wing pressure distribution agreement with test data, many solutions still exhibited premature flow separation. The remaining solutions exhibited a significant spread of lift and pitching moment at each angle-of-attack, much of which can be attributed to excessive aft pressure loading and shock location variation. Four Case 4 grid adaption solutions were submitted. Starting with grids less than 2x10(exp 6) grid points, two solutions showed a rapid convergence to an acceptable solution. Four Case 5 coupled aerostructural solutions were submitted. Both showed good agreement with experimental data. Results from this workshop highlight the continuing need for CFD improvement, particularly for conditions with significant flow separation. These comparisons also suggest the need for improved experimental diagnostics to guide future CFD development.
This paper reports the findings from a study that applies wall-resolved large-eddy simulation to investigate flow separation over the NASA wall-mounted hump geometry. Despite its conceptually simple flow configuration, this benchmark problem has proven to be a challenging test case for various turbulence simulation methods that have attempted to predict flow separation arising from the adverse pressure gradient on the aft region of the hump. The momentum-thickness Reynolds number of the incoming boundary layer has a value that is near the upper limit achieved by recent direct numerical simulation and large-eddy simulation of incompressible turbulent boundary layers. The high Reynolds number of the problem necessitates a significant number of grid points for wall-resolved calculations. The present simulations show a significant improvement in the separation-bubble length prediction compared to Reynolds-Averaged Navier-Stokes calculations. The current simulations also provide good overall prediction of the skin-friction distribution, including the relaminarization observed over the front portion of the hump due to the strong favorable pressure gradient. We discuss a number of problems that were encountered during the course of this work and present possible solutions. A systematic study regarding the effect of domain span, subgrid-scale model, tunnel back pressure, upstream boundary layer conditions and grid refinement is performed. The predicted separation-bubble length is found to be sensitive to the span of the domain. Despite the large number of grid points used in the simulations, some differences between the predictions and experimental observations still exist (particularly for Reynolds stresses) in the case of the wide-span simulation, suggesting that additional grid resolution may be required.
As part of NASA’s Urban Air Mobility (UAM) mission, the Revolutionary Vertical Lift Technology (RVLT) project is creating rotorcraft reference designs that can be used by the rotorcraft community to develop quiet, efficient, and safer air vehicles. At NASA, this effort is made possible by the use of the RVLT toolchain software. Airfoil tables are often a critical part of the conceptual design process as they inform both comprehensive analysis and Computational Fluid Dynamics (CFD) codes of 2D aerodynamic coefficients, such as lift, drag, and moment coefficients. These airfoil tables, typically generated during experimental testing, are often proprietary and thus not widely distributable; further, there is not always clear or available reference documentation that provides information on the test conditions. The airfoil tables from these experiments are referred to in this work as Legacy airfoil tables. The scope of this study is to generate airfoil tables with the NASA OVERset grid CFD FLOW solver (OVERFLOW) coupled with the AirFoil Table Generator (AFTGen) software and validate these airfoil tables based on performance predictions calculated in Comprehensive Hierarchical Aeromechanics Rotorcraft Model (CHARM). The overall goal is to provide CFD generated airfoil tables, validate them through comprehensive analysis, and publish the airfoil tables for public distribution. The RVLT quadrotor reference model was selected as the basis for all analyses. The quadrotor uses the VR-12 and SSC-A09 airfoils for its rotor blades. Simulations were run in OVERFLOW for the VR-12 and SSC-A09 airfoils for ±20° angles of attack, Mach numbers from 0.3 to 1.0, and a Reynolds-Mach proportionality constant of 1.023x107. The C81 format airfoil tables from AFTGen were then blended with Legacy NACA 0012 airfoil table data for ±20° to ±180° angle of attack range. CHARM was used to compute thrust coefficient, power coefficient, and figure of merit for an isolated rotor in hover using proprietary legacy airfoil tables for the VR-12 and SSC-A09, as well as with the OVERFLOW generated airfoil tables for the VR-12 and SSC-A09. Overall, the OVERFLOW and Legacy performance predictions agreed well, with the best correlation between simulated and Legacy data observed with the VR-12 airfoil tables.
Computations are presented for a Lambda = 75 deg delta wing in a supersonic freestream under two conditions which lead to leading-edge vortices. For one condition, analysis of the computed vortical flow reveals a closed streamline in the core. From varying computational parameters, it appears that this is due to truncation error of the convective derivatives. For the other condition, comparisons are made with wind-tunnel data, and good agreement is noted for pitot pressure distributions, flow angles on the symmetry plane, and the position of an embedded shock. Many of the aerodynamic parameters are shown to be insensitive to grid spacing.
Three-dimensional transonic viscous flow computations are presented for a generic high-speed accelerator model which includes wing, body, fillets, and a no-flow through engine nacelle. Solutions are obtained from an algorithm for the compressible Navier-Stokes equations which incorporates an upwind-biased, flux-vector-splitting approach along with longitudinally-patched grids. Results are presented for fully turbulent flow assumptions and include correlations with wind tunnel data. A good quantitative agreement for the forebody surface pressure distribution is achieved between computations and the available wind-tunnel measurements at freestream Mach number equal to 0.9. Furthermore, it is demonstrated that the flow is stagnating around the boattail region due to separation from the aft-engine cowl lip.
Experimental x-wire measurements of the flowfield above a 70 degree and 75 degree flat plate delta wing were performed at a Reynolds number of 250,000. Grids were taken normal to the wing at various chordwise locations for angles of attack of 20 degrees and 30 degrees. Axial and azimuthal vorticity distributions were derived from the velocity fields. The dependence of circulation on distance from the vortex core and on chordwise location was also examined. The effects of nondimensionalization in comparison with other experimental data is made. The results indicate that the circulation distribution scales with the local semispan and grows in a nearly linear fashion in the chordwise direction. The spanwise distribution of axial vorticity is severely altered through the breakdown region and the amount of vorticity present appears to reach a maximum immediately preceding breakdown. The axial velocity components with a negative sense, such as that found in the secondary vortex, seem to remain unaffected by changes in wing sweep or angle of attack, in direct contrast to the visible components. In addition, the inclusion of the local wing geometry into a previously derived correlation parameter allows the circulation of growing leading edge vortex flows to be reduced to a single curve.
Many large-scale computational problems are based on unstructured computational domains. Primary examples are unstructured grid calculations based on finite volume methods in computational fluid dynamics, or structural analysis problems based on finite element approximations. The question of how to distribute such unstructured computational domains over a large number of processors in a MIMD machine with distributed memory is addressed. A graph theoretical framework for these problems is established. Based on this framework three decomposition algorithms are introduced. In particular a new decomposition algorithm is discussed, which is based on the computation of an eigenvector of the Laplacian matrix associated with the graph. Numerical comparisons on large-scale two- and three-dimensional problems demonstrate the superiority of the new spectral bisection algorithm.
In the present study, errors in using Taylor's hypothesis to transform measurements obtained in a temporal (or phase) frame onto a spatial one were evaluated. For the first time, phase-averaged ('real') spanwise and streamwise vorticity data measured on a three-dimensional grid were compared directly to those obtained using Taylor's hypothesis. The results show that even the qualitative features of the spanwise and streamwise vorticity distributions given by the two techniques can be very different. This is particularly true in the region of the spanwise roller pairing. The phase-averaged spanwise and streamwise peak vorticity levels given by Taylor's hypothesis are typically lower (by up to 40%) compared to the real measurements.
In the present study, errors in using Taylor's hypothesis to transform measurements obtained in a temporal (or phase) frame onto a spatial one were evaluated. For the first time, phase-averaged ('real') spanwise and streamwise vorticity data measured on a three-dimensional grid were compared directly to those obtained using Taylor's hypothesis. The results show that even the qualitative features of the spanwise and streamwise vorticity distributions given by the two techniques can be very different. This is particularly true in the region of the spanwise roller pairing. The phase-averaged spanwise and streamwise peak vorticity levels given by Taylor's hypothesis are typically lower (by up to 40%) compared to the real measurements.