Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “mean”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1959

Investigation of a 4.5-Inch-Mean-Diameter Two-Stage Axial-Flow Turbine Suitable for Auxiliary Power Drives

The design and experimental investigation of a 4.5-inch-mean-diameter two-stage turbine are presented herein and used to study the effect of size on the efficiency of turbines in the auxiliary power drive class. The results of the experimental investigation indicated that design specific work was obtained at design speed at a total-to-static efficiency of 0.639. At design pressure ratio, design static-pressure distribution through the turbine was obtained with an equivalent specific work output of 33.2 Btu per pound and an efficiency of 0.656. It was found that, in the design of turbines in the auxiliary power drive class, Reynolds number plays an important part in the selection of the design efficiency. Comparison with theoretical efficiencies based on a loss coefficient and velocity diagrams are presented. Close agreement was obtained between theory and experiment when the loss coefficient was adjusted for changes in Reynolds number to the -1/5 power.

Wong, Robert Y.↗
NASA NTRS1959

An Equation for the Mean Velocity Distribution of Boundary Layers

A general relation, empirical in origin, for the mean velocity distribution of both laminar and turbulent boundary layers is proposed. The equation, in general, accurately describes the profiles in both laminar and turbulent flows. The calculation of profiles is based on a prior knowledge of momentum, displacement, and boundary-layer thickness together with free-stream conditions. The form for turbulent layers agrees with the present concepts of similarity of the outer layer. For the inner region or turbulent boundary layers the present relation agrees very closely with experimental measurements even in cases where the logarithmic law of the wall is inadequate. A unique relation between profile form factors and the ratio of displacement thickness to boundary-layer thickness is obtained for turbulent separation. A similar criterion is also obtained for laminar separation. These relations are demonstrated to serve as an accurate criterion for identifying separation in known profiles.

Sandborn, V. A.↗
NASA NTRS1998

Characterization of Mesoscale Convective Systems by Means of Composite Radar Reflectivity Data

A mesoscale convective system (MCS) is broadly defined as a cloud and precipitation system of mesoscale dimensions (often too large for most aircraft to circumnavigate) with deep-convective activity concentrated in at least part of the MCS, or present during part of its evolution. A large areal fraction of MCSs is stratiform in nature, yet estimates from MCSs over the Great Plains, the Southeast, and tropical waters indicate that at least half of the precipitation is of convective origin. The presence of localized convection is important, because within convective towers cloud particles and hydrometeors are carried upward towards the cloud top. Ice crystals then move over more stratiform regions, either laterally, or through in situ settling over decaying and spreading convection. These ice crystals then grow to precipitation-size particles in mid- to upper tropospheric mesoscale updrafts. The convective portion of a MCS is often a more or less continuous line of thunderstorms, and may be either short-lived or long-lived. Geerts (1997) presents a preliminary climatology of MCSs in the southeastern USA, using just one year of composite digital radar reflectivity data. In this study MCSs are identified and characterized by means of visual inspection of animated images. A total of 398 MCSs were identified. In the warm season MCSs were found to be about twice as frequent as in the cold season. The average lifetime and maximum length of MCSs are 9 hours, and 350 km, respectively, but some MCSs are much larger and more persistent. In the summer months small and short-lived MCSs are relatively more common, whereas in winter larger and longer-lived systems occur more frequently. MCSs occur more commonly in the afternoon, in phase with thunderstorm activity, but the amplitude of the diurnal cycle is small compared to that of observed thunderstorms. It is estimated that in the Southeast more than half of all precipitation and severe weather results from MCSs.

Geerts, Bart↗
NASA NTRS1999

Cloud Liquid Water, Mean Droplet Radius and Number Density Measurements Using a Raman Lidar

A new technique for measuring cloud liquid water, mean droplet radius and droplet number density is outlined. The technique is based on simultaneously measuring Raman and Mie scattering from cloud liquid droplets using a Raman lidar. Laboratory experiments on liquid micro-spheres have shown that the intensity of Raman scattering is proportional to the amount of liquid present in the spheres. This fact is used as a constraint on calculated Mie intensity assuming a gamma function particle size distribution. The resulting retrieval technique is shown to give stable solutions with no false minima. It is tested using Raman lidar data where the liquid water signal was seen as an enhancement to the water vapor signal. The general relationship of retrieved average radius and number density is consistent with traditional cloud physics models. Sensitivity to the assumed maximum cloud liquid water amount and the water vapor mixing ratio calibration are tested. Improvements to the technique are suggested.

Whiteman, David N.↗
NASA NTRS1999

Observations of Sea Surface Mean Square Slope During the Southern Ocean Waves Experiment

For the Southern Ocean Waves Experiment (SOWEX), conducted in June 1992 out of Hobart, Tasmania, the 36 GHz (8.3 mm) NASA Scanning Radar Altimeter (SRA) was shipped to Australia and installed on a CSIRO Fokker F-27 research aircraft instrumented to make comprehensive surface layer measurements of air-sea interaction fluxes. The sea surface mean square slope (mss), which is predominantly caused by the short waves, was determined from the backscattered power falloff with incidence angle measured by the SRA in the plane normal to the aircraft heading. On each flight, data were acquired at 240 m altitude while the aircraft was in a 7 deg roll attitude, interrogating off-nadir incidence angles from -15 deg through nadir to +29 deg. The aircraft turned azimuthally through 810 deg in this attitude, mapping the azimuthal dependence of the backscattered power falloff with incidence angle. Two sets of turning data were acquired on each day, before and after the aircraft measured wind stress at low altitude (12 m to 65 m). Wave topography and backscattered power for mss were also acquired during those level flight segments whenever the aircraft altitude was above the SRA minimum range of 35 m. A unique feature of this experiment was the use of a nadir-directed low-gain horn antenna (35 deg beamwidth) to acquire azimuthally integrated backscattered power data versus incidence angle before and after the turn data.

Walsh, E. J.↗
NASA NTRS2000

Altitude Differentiated Aerosol Extinction Over Tenerife (North Atlantic Coast) During ACE-2 by Means of Ground and Airborne Photometry and Lidar Measurements

Retrievals of spectral aerosol optical depths (tau(sub a)) by means of sun photometers have been undertaken in Tenerife (28 deg 16' N, 16 deg 36' W) during ACE-2 (June-July 1997). Five ground-based sites were located at four different altitudes in the marine boundary layer and in the free troposphere, from 0 to 3570 m asl. The goal of the investigation was to provide estimates of the vertical aerosol extinction over the island, both under clean and turbid conditions. Inversion of spectral tau(sub a) allowed to retrieve size distributions, from which the single scattering albedo omega(sub 0) and the asymmetry factor g could be estimated as a function of altitude. These parameters were combined to calculate aerosol forcing in the column. Emphasis is put on episodes of increased turbidity, which were observed at different locations simultaneously, and attributed to outbreaks of mineral dust from North Africa. Differentiation of tau(sub a) as a function of altitude provided the vertical profile of the extinction coefficient sigma(sub e). For dust outbreaks, aerosol extinction is concentrated in two distinct layers above and below the strong subsidence inversion around 1200 m asl. Vertical profiles of tau(sub a) and sigma(sub e) are shown for July 8. In some occasions, vertical profiles are compared to LIDAR observations, performed both at sea level and in the low free troposphere, and to airborne measurements of aerosol optical depths.

Formenti, P.↗
NASA NTRS2000

BOREAS AFM-06 Mean Temperature Profile Data

The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from the National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) tower from 21 May 1994 to 20 Sep 1994. The data set provides temperature profiles at 15 heights, containing the variables of virtual temperature, vertical velocity, the speed of sound, and w-bar. The data are stored in tabular ASCII files. The mean temperature profile data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).

Wilczak, James↗
NASA NTRS2000

BOREAS AFM-06 Mean Wind Profile Data

The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from the National Oceanic and Atmospheric Administration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) tower from 21 May 1994 to 20 Sep 1994. The data set provides wind profiles at 38 heights, containing the variables of wind speed; wind direction; and the u-, v-, and w-components of the total wind. The data are stored in tabular ASCII files. The mean wind profile data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).

Wilczak, James↗
NASA NTRS2000

Predictability of Seasonal Means in the NSIPP-1 AGCM

The predictability of seasonal means during 1980-99 is examined from 720 seasonal integrations with version 1 of NASA's Seasonal to Interanual Prediction Project (NSIPP-1) atmospheric general circulation model (AGCM). The AGCM is the atmospheric (and land) component of the NSIPP fully-coupled atmosphere-land-ocean model. For these runs, the initial atmospheric conditions are taken from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses, and sea surface temperatures and sea ice are specified from observations. Results from the northern winter integrations show that the model has a very realistic El Nino/Southern Oscillation (ENSO) response in both the tropics and extratropics. Early results from the northern summer runs show a robust extratropical ENSO response in the Southern Hemisphere Pacific, as well as, a pronounced zonally-symmetric component to the ENSO response in both hemispheres. Results will be presented at the meeting highlighting the seasonal evolution in predictability and skill of the NSIPP model on both global and regional scales. A further analysis of the zonally-symmetric response will be discussed in terms of its potential impact on the predictability of middle-latitude climate variability on seasonal to interannual time scales.

Schubert, Siegfried↗
NASA NTRS2000

An Assessment of the Predictability of Northern Winter Seasonal Means with the NSIPP 1 AGCM

This atlas assesses the predictability of January-February-March (JFM) means using version 1 of the NASA Seasonal-to-Interannual Prediction Project Atmospheric General Circulation Model (the NSIPP 1 AGCM). The AGCM is part of the NSIPP coupled atmosphere-land-ocean model. For these results, the atmosphere was run uncoupled from the ocean, but coupled with an interactive land model. The results are based on 20 ensembles of nine JFM hindcasts for the period 1980-1999, with sea surface temperature (SST) and sea ice specified from observations. The model integrations were started from initial atmospheric conditions (taken from NCEP/NCAR reanalyses) centered on December 15. The analysis focuses on 200 mb height, precipitation, surface temperature, and sea-level pressure. The results address issues of both predictability and forecast skill. Various signal-to-noise measures are computed to demonstrate the potential for skillful prediction on seasonal time scales under the assumption of a perfect model and perfectly known oceanic boundary forcings. The results show that the model produces a realistic ENSO response in both the tropics and extratropics.

Suarez, Max J.↗
NASA NTRS2001

A Chronology of Annual-Mean Effective Radii of Stratospheric Aerosols from Volcanic Eruptions During the Twentieth Century as Derived From Ground-based Spectral Extinction Measurements

Stratospheric extinction can be derived from ground-based spectral photometric observations of the Sun and other stars (as well as from satellite and aircraft measurements, available since 1979), and is found to increase after large volcanic eruptions. This increased extinction shows a characteristic wavelength dependence that gives information about the chemical composition and the effective (or area weighted mean) radius of the particles responsible for it. Known to be tiny aerosols constituted of sulfuric acid in a water solution, the stratospheric particles at midlatitudes exhibit a remarkable uniformity of their column-averaged effective radii r(sub eff) in the first few months after the eruption. Considering the seven largest eruptions of the twentieth century, r(sub eff) at this phase of peak aerosol abundance is approx. 0.3 micrometers in all cases. A year later, r(sub eff) either has remained about the same size (almost certainly in the case of the Katmai eruption of 1912) or has increased to approx. 0.5 micrometers (definitely so for the Pinatubo eruption of 1991). The reasons for this divergence in aerosol growth are unknown.

Strothers, Richard B.↗
NASA NTRS2001

Evidence for Large Decadal Variability in the Tropical Mean Radiative Energy Budget

It is widely assumed that variations in the radiative energy budget at large time and space scales are very small. We present new evidence from a compilation of over two decades of accurate satellite data that the top-of-atmosphere (TOA) tropical radiative energy budget is much more dynamic and variable than previously thought. We demonstrate that the radiation budget changes are caused by changes In tropical mean cloudiness. The results of several current climate model simulations fall to predict this large observed variation In tropical energy budget. The missing variability in the models highlights the critical need to Improve cloud modeling in the tropics to support Improved prediction of tropical climate on Inter-annual and decadal time scales. We believe that these data are the first rigorous demonstration of decadal time scale changes In the Earth's tropical cloudiness, and that they represent a new and necessary test of climate models.

Wielicki, Bruce A.↗
NASA NTRS2001

Observations of Sea Surface Mean Square Slope During the Southern Ocean Waves Experiment

For the Southern Ocean Waves Experiment (SOWEX), conducted in June 1992 out of Hobart, Tasmania, the NASA Scanning Radar Altimeter (SRA) was shipped to Australia and installed on a CSIRO Fokker F-27 research aircraft instrumented to make comprehensive surface layer measurements of air-sea interaction fluxes. The SRA sweeps a radar beam of P (two-way) half-power width across the aircraft ground track over a swath equal to 0.8 of the aircraft height, simultaneously measuring the backscattered power at its 36 GHz (8.3 mm) operating frequency and the range to the sea surface at 64 cross-track positions. In realtime, the slant ranges are multiplied by the cosine of the off-nadir incidence angles (including the effect of aircraft roll attitude) to determine the vertical distances from the aircraft to the sea surface. These distances are subtracted from the aircraft height to produce a sea-surface elevation map, which is displayed on a monitor in the aircraft to enable real-time assessments of data quality and wave properties. The sea surface mean square slope (mss), which is predominantly caused by the short waves, was determined from the backscattered power falloff with incidence angle measured by the SRA in the plane normal to the aircraft heading. On each flight, data were acquired at 240 m altitude while the aircraft was in a 7 degree roll attitude, interrogating off-nadir incidence angles from -15 degrees through nadir to +29 degrees. The aircraft turned azimuthally through 810 degrees in this attitude, mapping the azimuthal dependence of the backscattered power falloff with incidence angle. Two sets of turning data were acquired on each day, before and after the aircraft measured wind stress at low altitude (12 meters to 65 meters). Wave topography and backscattered power for mss were also acquired during those level flight segments whenever the aircraft altitude was above the SRA minimum range of 35 m. Data were collected over a wide range of wind and sea conditions, from quiescent to gale force winds with 9 meter wave height.

Walsh, E. J.↗
NASA NTRS2002

Comparison of Mean Climate Trends in the Northern Hemisphere Between N.C.E.P. and Two Atmosphere-Ocean Model Forced Runs

Results are presented for two greenhouse gas experiments of the Goddard Institute for Space Studies Atmosphere-Ocean Model (AOM). The computed trends of surface pressure, surface temperature, 850, 500 and 200 mb geopotential heights and related temperatures of the model for the time frame 1960-2000 are compared to those obtained from the National Centers for Environmental Prediction observations. A spatial correlation analysis and mean value comparison are performed, showing good agreement. A brief general discussion about the statistics of trend detection is presented. The domain of interest is the Northern Hemisphere (NH) because of the higher reliability of both the model results and the observations. The accuracy that this AOM has in describing the observed regional and NH climate trends makes it reliable in forecasting future climate changes.

Lucarini, Valerio↗
NASA NTRS2003

Mean Flow Boundary Conditions for Computational Aeroacoustics

In this work, a new type of boundary condition for time-accurate Computational Aeroacoustics solvers is described. This boundary condition is designed to complement the existing nonreflective boundary conditions while ensuring that the correct mean flow conditions are maintained throughout the flow calculation. Results are shown for a loaded 2D cascade, started with various initial conditions.

Hixon, R.↗
NASA NTRS1947

Wind Tunnel Development of Means to Alleviate Buffeting of the North American XP-82 Airplane at High Speeds

This report presents the results of wind-tunnel tests of a 0.22-scale model of the North American XP-82 airplane with several modifications designed to reduce the buffeting of the airplane. The effects of various modifications on the air flow over the model are shown by means of photographs of tufts. The drag, lift, and pitching-moment coefficients of the model with several of the modifications are shown. The result indicate that, by reflexing the trailing edge of the center section of the wing and modifying the radiator air-scoop gutter and the inboard lower-surface wing fillets, the start of buffeting can be delayed from a Mach number of 0.70 to 0.775, and that the diving tendency of the airplane would be eliminated up to a Mach number of 0.80.

Anderson, Joseph L.↗
NASA NTRS2004

Mean Flow and Noise Prediction for a Separate Flow Jet With Chevron Mixers

Experimental and numerical results are presented here for a separate flow nozzle employing chevrons arranged in an alternating pattern on the core nozzle. Comparisons of these results demonstrate that the combination of the WIND/MGBK suite of codes can predict the noise reduction trends measured between separate flow jets with and without chevrons on the core nozzle. Mean flow predictions were validated against Particle Image Velocimetry (PIV), pressure, and temperature data, and noise predictions were validated against acoustic measurements recorded in the NASA Glenn Aeroacoustic Propulsion Lab. Comparisons are also made to results from the CRAFT code. The work presented here is part of an on-going assessment of the WIND/MGBK suite for use in designing the next generation of quiet nozzles for turbofan engines.

Koch, L. Danielle↗