Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “density estimation”

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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Hydrometeor Size Distribution Measurements by Imaging the Attenuation of a Laser Spot

The optical extinction of a laser due to scattering of particles is a well-known phenomenon. In a laboratory environment, this physical principle is known as the Beer-Lambert law, and is often used to measure the concentration of scattering particles in a fluid or gas. This method has been experimentally shown to be a usable means to measure the dust density from a rocket plume interaction with the lunar surface. Using the same principles and experimental arrangement, this technique can be applied to hydrometeor size distributions, and for launch-pad operations, specifically as a passive hail detection and measurement system. Calibration of a hail monitoring system is a difficult process. In the past, it has required comparison to another means of measuring hydrometeor size and density. Using a technique recently developed for estimating the density of surface dust dispersed during a rocket landing, measuring the extinction of a laser passing through hail (or dust in the rocket case) yields an estimate of the second moment of the particle cloud, and hydrometeor size distribution in the terrestrial meteorological case. With the exception of disdrometers, instruments that measure rain and hail fall make indirect measurements of the drop-size distribution. Instruments that scatter microwaves off of hydrometeors, such as the WSR-88D (Weather Surveillance Radar 88 Doppler), vertical wind profilers, and microwave disdrometers, measure the sixth moment of the drop size distribution (DSD). By projecting a laser onto a target, changes in brightness of the laser spot against the target background during rain and hail yield a measurement of the DSD's second moment by way of the Beer-Lambert law. In order to detect the laser attenuation within the 8-bit resolution of most camera image arrays, a minimum path length is required. Depending on the intensity of the hail fall rate for moderate to heavy rainfall, a laser path length of 100 m is sufficient to measure variations in optical extinction using a digital camera. For hail fall only, the laser path may be shorter because of greater scattering due to the properties of hailstones versus raindrops. A photodetector may replace the camera in automated installations. Laser-based rain and hail measurement systems are available, but they are based on measuring the interruption of a thin laser beam, thus counting individual hydrometeors. These systems are true disdrometers since they also measure size and velocity. The method reported here is a simple method, requiring far less processing, but it is not a disdrometer.

Lane, John↗
NASA NTRS1985

Upper limits to (O) in the lower thermosphere from airglow

A method for estimating the upper limit to oxygen atom density in the lower thermosphere is presented. The method is based on an analytical equation for the energy density of recombination process, and the radiation of O2 atoms at excited levels. As an example of the method, upper limits of oxygen atom density are estimated for the CIRA (O2) reference atmosphere of 1972. The estimates are given in a table.

Sharp, W. E.↗
NASA NTRS1990

Galactic H I and the interstellar medium in Ursa Major

Sensitive neutral hydrogen measurements have been made over a 100 square degree field in Ursa Major. The observations cover the area which has the least neutral hydrogen and probably the least interstellar matter of any direction in the sky. It is likely that this data set contains every line of sight that intersects a total H I column density of less than 6.5 x 10 to the 19th/sq cm. The smallest colum density is estimated to be 4.4 + or - 0.5 x 10 to the 19th/sq cm. Observations at 10 arcmin angular resolution reveal little structure that is not seen at 21 arcmin resolution, and it is believed that the estimates of the total H I column density are accurate on yet smaller angular scales. The lowest N(H I) area is also extremely deficient in H-alpha emission, and shows no evidence for significant amounts of H2.

Jahoda, Keith↗
NASA NTRS2018

Investigation of Transient Gas Phase Column Density Due to Droplet Evaporation

Analytical expressions are developed for estimating column density near a rapidly evaporating droplet along general paths. The influence for instantaneous evaporation is created first as a limiting case, where the peak value occurs at the time it takes a wave of vapor to reach the closest point along the optical path traveling at its most probable thermal speed. Next the case for finite-period evaporation is evaluated for constant conditions. Compared to the instantaneous case, peak column density occurs shortly after droplet extinction at a lower intensity. A new mathematical function is discovered that solves the integrals associated with this case. Finally, ways to account for droplet motion and changes in evaporation rate with size and temperature are discussed.

transient↗
NASA NTRS2018

Investigation of Transient Gas Phase Column Density Due to Droplet Evaporation

Analytical expressions are developed for estimating column density near a rapidly evaporating droplet along general paths. The influence for instantaneous evaporation is created first as a limiting case, where the peak value occurs at the time it takes a wave of vapor to reach the closest point along the optical path traveling at its most probable thermal speed. Next the case for finite-period evaporation is evaluated for constant conditions. Compared to the instantaneous case, peak column density occurs shortly after droplet extinction at a lower intensity. A new mathematical function is discovered that solves the integrals associated with this case. Finally, ways to account for droplet motion and changes in evaporation rate with size and temperature are discussed.

Column Density↗
NASA NTRS2019

Evaluating a Regional Climate Model Simulation of Greenland Ice Sheet Snow and Firn Density for Improved Surface Mass Balance Estimates

Modeling vertical profiles of snow and firn density near the surface of the Greenland ice sheet (GrIS) is key to estimating GrIS mass balance, and by extension, global sea level change. To understand sources of error in simulated GrIS density, we compare GrIS density profiles from a leading regional climate model with coincident in situ measurements. We identify key contributors to model density and mass balance biases, including underestimated simulated fresh snow density (which leads to underestimation of density in the top 1 m of snow by ~10%). In areas undergoing frequent melting, positive density biases (of 7% in the top 1 m, and 10% between 1 and 10 m) are likely associated with errors in representing meltwater production, retention and refreezing. The results highlight the importance of accurately capturing fresh snow density and meltwater processes in models used to estimate GrIS mass balance change.

P M Alexander↗
NASA NTRS2006

Estimating tropical-forest density profiles from multibaseline interferometric SAR

Vertical profiles of forest density are potentially robust indicators of forest biomass, fire susceptibility and ecosystem function. Tropical forests, which are among the most dense and complicated targets for remote sensing, contain about 45% of the world's biomass. Remote sensing of tropical forest structure is therefore an important component to global biomass and carbon monitoring. This paper shows preliminary results of a multibasline interfereomtric SAR (InSAR) experiment over primary, secondary, and selectively logged forests at La Selva Biological Station in Costa Rica. The profile shown results from inverse Fourier transforming 8 of the 18 baselines acquired. A profile is shown compared to lidar and field measurements. Results are highly preliminary and for qualitative assessment only. Parameter estimation will eventually replace Fourier inversion as the means to producing profiles.

profiles↗
NASA NTRS2000

The density of apical cells of dark-grown protonemata of the moss Ceratodon purpureus

Determinations of plant or algal cell density (cell mass divided by volume) have rarely accounted for the extracellular matrix or shrinkage during isolation. Three techniques were used to indirectly estimate the density of intact apical cells from protonemata of the moss Ceratodon purpureus. First, the volume fraction of each cell component was determined by stereology, and published values for component density were used to extrapolate to the entire cell. Second, protonemal tips were immersed in bovine serum albumin solutions of different densities, and then the equilibrium density was corrected for the mass of the cell wall. Third, apical cell protoplasts were centrifuged in low-osmolarity gradients, and values were corrected for shrinkage during protoplast isolation. Values from centrifugation (1.004 to 1.015 g/cm3) were considerably lower than from other methods (1.046 to 1.085 g/cm3). This work appears to provide the first corrected estimates of the density of any plant cell. It also documents a method for the isolation of protoplasts specifically from apical cells of protonemal filaments.

NASA Discipline Plant Biology↗
NASA NTRS1985

Birds and insects as radar targets - A review

A review of radar cross-section measurements of birds and insects is presented. A brief discussion of some possible theoretical models is also given and comparisons made with the measurements. The comparisons suggest that most targets are, at present, better modeled by a prolate spheroid having a length-to-width ratio between 3 and 10 than by the often used equivalent weight water sphere. In addition, many targets observed with linear horizontal polarization have maximum cross sections much better estimated by a resonant half-wave dipole than by a water sphere. Also considered are birds and insects in the aggregate as a local radar 'clutter' source. Order-of-magnitude estimates are given for many reasonable target number densities. These estimates are then used to predict X-band volume reflectivities. Other topics that are of interest to the radar engineer are discussed, including the doppler bandwidth due to the internal motions of a single bird, the radar cross-section probability densities of single birds and insects, the variability of the functional form of the probability density functions, and the Fourier spectra of single birds and insects.

Vaughn, C. R.↗
NASA NTRS1981

A computer program for estimating the power-density spectrum of advanced continuous simulation language generated time histories

A computer program for performing frequency analysis of time history data is presented. The program uses circular convolution and the fast Fourier transform to calculate power density spectrum (PDS) of time history data. The program interfaces with the advanced continuous simulation language (ACSL) so that a frequency analysis may be performed on ACSL generated simulation variables. An example of the calculation of the PDS of a Van de Pol oscillator is presented.

Dunn, H. J.↗
NASA NTRS2003

The O+ Density Trough at 5000 km Altitude in the Polar Cap

At altitudes near 5000 km over the Southern polar cap region of the terrestrial magnetospherehonosphere, the Thermal Ion Dynamics Experiment (TIDE) onboard the Polar satellite has observed O+ ion density trough regions, in which the O+ densities were at least one order of magnitude lower than the surrounding O+ densities. In the O+ demify trough regions, the estimated O+ densities were generally lower than 0.01 per cc. The boundaries between normal density level regions and the trough density regions were usually abrupt transitions. From December 1, 1997 to November 30, 1998, polar cap O+ troughs in Polar/TIDE observations occurred at a frequency of about 48%. Statistical examination of the Polar perigee observations from December 1 , 1997 to November 30, 1998 shows that the Polar perigee passes evenly covered the southern polar cap region, while the O+ density trough was always located on the nightside portion of the polar cap magnetospherehonosphere, and that invariant latitude spans of such troughs could be as large as 230 in extent. The trough occurrence displayed strong seasonal dependence; in the winter season (e.g. for July in the southern hemisphere) the O+ ion density trough occurrence frequency ranged up to 92%, while in the summer season (e.g. for January in the southern hemisphere) it decreased to as infrequent as 15%. The O+ ion density trough occurrence appeared relatively independent of the geomagnetic Kp index, and IMF Bz, By conditions. However, as suggested by the seasonal dependence, the O+ ion density trough occurrence was strongly related to the solar zenith angle (SZA). In the SZA range 500 to 1250, the trough occurrence increased monotonically with SZA. Also, case-by-case examinations of near-simultaneous O+ densities and vertical velocities observed by the DMSP satellite group orbiting at 840 km altitude indicate that the O+ density troughs observed at 5000 km altitude exhibit moderate correlation or anti-correlation with topside ionosphere density or velocity features at 840 km altitude for approximately half of the near-conjunction cases examined.

Zeng, W.↗
NASA NTRS1996

An Estimate of Solar Wind Density and Velocity Profiles in a Coronal Hole and a Coronal Streamer

Using the total electron content data obtained by the Ulysses Solar Corona Experiment (SCE) during the first solar conjunction in summer 1991, two data sets were selected, one associated with a coronal hole and the other associated with coronal streamer crossings. In order to determine coronal streamer density profiles, the electron content of the tracking passes embedded in a coronal streamer were corrected for the contributions from coronal hole densities.

coronal density coronal velocity velocity profiles↗
NASA NTRS2008

Apollo Video Photogrammetry Estimation Of Plume Impingement Effects

The Constellation Project's planned return to the moon requires numerous landings at the same site. Since the top few centimeters are loosely packed regolith, plume impingement from the Lander ejects the granular material at high velocities. Much work is needed to understand the physics of plume impingement during landing in order to protect hardware surrounding the landing sites. While mostly qualitative in nature, the Apollo Lunar Module landing videos can provide a wealth of quantitative information using modem photogrammetry techniques. The authors have used the digitized videos to quantify plume impingement effects of the landing exhaust on the lunar surface. The dust ejection angle from the plume is estimated at 1-3 degrees. The lofted particle density is estimated at 10(exp 8)- 10(exp 13) particles per cubic meter. Additionally, evidence for ejection of large 10-15 cm sized objects and a dependence of ejection angle on thrust are presented. Further work is ongoing to continue quantitative analysis of the landing videos.

Immer, Christopher↗