Search NASA⌕ Search

Engineering topics

Aggarwal, H. R.

Publications and source records attributed to Aggarwal, H. R..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Atmospheric correction of remotely sensed image data by a simplified model

The interaction of radiation with the atmosphere is complex and has proved difficult to calculate without reference to measurements made at or close to the time and location of interest. In this paper we describe the use of data from an airborne-tracking sunphotometer mounted on a NASA C-130 aircraft to derive optical properties of the atmospheric aerosols above the aircraft when it is on the ground as well as when the aircraft is at altitude collecting remote sensing data. Furthermore, we describe the use of these optical properties for quantitative atmospheric correction of Landsat Thematic Mapper (TM) and NS001 TM simulator radiances in a simplified radiative transfer model appropriate for each pixel of entire images. Both qualitative and quantitative comparisons of results from use of the model show it provides good to excellent correction for atmospheric effects in TM and NS001 TM simulator data. Quantitative comparisons with measurements of surface radiances measured near the time of the TM overpass showed agreement within a few percent for bands 2, 3, and 4. Band 1 yielded radiances within 15 percent of the surface radiances. We think the simplified atmospheric correction model shows great promise for applying atmospheric corrections to entire images instead of individual points within images to yield surface radiances within a few percent in a computationally efficient manner.

Wrigley, R. C.↗
NASA NTRS1984

Helicopter noise as predicted by three-dimensional monopole and quasi-steady full-potential dipole sources

A three-dimensional quasi-steady full-potential rotor-flow-analysis program, called ROT22 developed at NASA Ames Research Center is run in conjunction with Farassat's (1981) helicopter-noise-prediction code to assess the thickness and loading noises made by a rotor blade. As a model example, the case of a 1/7th UH-1H NACA-0012-profile straight nonlifting rotor blade in hover is studied. Results for tip Mach numbers ranging from .4 to .962 are presented. Also presented is the effect of blade-tip loading on the overall noise and of profile curvature on the thickness noise. The study confirms that the blade volume displacement is a dominant source of helicopter noise and further concludes that the blade tip makes only a small noise contribution that steadily decreases with increasing tip Mach numbers, and that the inclusion of blade profile curvature tends to improve the negative peak amplitudes overpredicted by Schmitz and Yu (1983).

Aggarwal, H. R.↗
NASA NTRS1984

The calculation of transonic rotor noise

It is pointed out that an accurate prediction of the high-speed impulsive rotor noise is very difficult since the noise field depends on many complex factors. Schmitz and Yu (1981) have conducted a study with the aim to include local aerodynamic nonlinearities in a calculation regarding the acoustic shock. In the present investigation, the hover case of the model rotor studied by Schmitz and Yu was recomputed without certain implicit assumptions made in the earlier calculation. The obtained results show that the Schmitz-Yu computations are very much base-support area dependent. Attention is given to the quadrupole integral, and the pressure-time curves for the model rotor in hover.

Aggarwal, H. R.↗
NASA NTRS1983

Derivation of the fundamental equation of sound generated by moving aerodynamic surfaces

Ffowcs Williams and Hawkings (1969) based their derivation of the fundamental equation of the sound generated by arbitrarily moving aerodynamic surfaces on the study of mass and momentum balance of a control volume imbedding a mathematical surface(s) exactly corresponding to real surface(s). These investigators also sketched an alternative method, employing generalized functions, for its derivation. This latter method, which was later developed by Farassat (1975), is purely mathematical and formal. Goldstein (1976) used the free-space Green function to produce an implicit derivation of the Ffowcs Williams and Hawkings equation. In the study presented here, Lowson's (1965) concept of moving point singularities is generalized to moving surface singularities, and a new derivation is given of the fundamental equation. The derivation is based on topological considerations of the underlying space, the fluid medium, and the integral properties of the Dirac delta function.

Aggarwal, H. R.↗
NASA NTRS1979

Monte Carlo simulation of lunar megaregolith and implications

A realistic Monte Carlo model closely simulating the evolution of the lunar megaregolith over a large area of 67 million sq. km of the front surface of the moon is presented. Craters larger than 100 km in diameter observed over the entire surface of the moon and those less than 100 km lying in the referenced area are included in the simulation. A total of 21,664 craters are processed. The model predicts the average thickness of the megaregolith to be about 1.9-2.0 km. Curves for the variation of the regolith thickness across the simulated area are given and show that about 50% of the area is covered with regolith less than 1 km thick. The model produces crater structures similar to the ones observed in the lunar highlands, it partially supports the layering theory for crater structures that the variations in strength and density of target materials may be responsible for the observed differences in the morphologies of lunar craters, and rules out the possibility that all craters when formed are bowl-shaped with a fixed depth/diameter ratio characteristic of small craters.

Aggarwal, H. R.↗
NASA NTRS1978

Size-frequency distributions of primary and secondary lunar impact craters

The graphs of diameter vs frequency plotted in the present study for primary impact craters produced in four time intervals exhibit an appreciable change in form with age. A deficiency of small craters relative to an extrapolation from the large-diameter parts of the curves is greatest and extends to the largest sizes in the oldest population (pre-Nectarian), whose curve approximates a log-normal form. Successively younger populations (Nectarian, Imbrian, Copernican plus Eratosthenian) have successively more small craters relative to larger ones and more nearly log-log distributions. Many of the craters thought to be small old primaries are here identified as secondary craters of basins, on the basis of previously developed criteria.

Wilhelms, D. E.↗
NASA NTRS1977

Comparative studies of Lunar, Martian, and Mercurian craters and plains

Lunar features provide a valuable interpretation data base for comparison with features on other planets which are believed to have been formed by the cratering process. The paper adopts the comparative approach by examining the smooth plains of the moon and Mercury, with special emphasis on crater degradation on the moon, Mercury, and Mars. A possible cause for the observed deficiency of craters with diameters no more than 50 km on certain areas of lunar uplands and on the entire uplands of Mercury and Mars is discussed. It is suggested that pertinent differences can be predicted on the basis of new concepts for production population and of effects of changes in planetary conditions on the secondary cratering process. It is concluded that the deficiency of craters with diameters no more than 50 km on parts of the lunar uplands and on the Mercurian and Martian uplands may be evidence for a deficiency in production of primary craters in this size range. Origin of the late heavy bombardment in the inner solar system is examined.

Oberbeck, V. R.↗
NASA NTRS1977

Topographic analysis of lunar secondary craters of Copernicus and implications

An analysis is conducted of the topography of lunar secondary craters and the associated herringbone pattern observed on lunar topophotomaps. The topography and the patterns are compared with those of crater pairs produced in the laboratory. The results are used to identify secondaries on the lunar uplands. The chain of craters that was selected for mapping and which is described is known to be a secondary impact crater chain produced by material ejected from Copernicus Crater because it lies on a well-developed ray system of Copernicus. Oberbeck et al. (1977) had hypothesized that most lunar areas exhibit more craters smaller than 50 km than are observed on Mars and Mercury because lower lunar gravity permitted more widespread distribution of secondaries for the moon. After removal of basin secondaries it is found that the surfaces of the lunar uplands are only sparsely populated by craters between 5 and 50 km. The lunar uplands appear then similar to the Mercurian terrain.

Oberbeck, V. R.↗
NASA NTRS1975

Comparative studies of lunar, Martian, and Mercurian craters and plains

The spatial distribution of lunar smooth plains is not consistent with experimental simulations of melt rock emplacement during cratering in layered materials. Nor is it consistent with the location of melt rocks (suevite) near the Ries basin. Lunar smooth plains surrounding Imbrium are most extensive in areas where pre-existing craters are most degraded. This observation suggests that plains form by impact of basin and local primary crater ejecta, together with deposition of debris excavated by the resultant secondary cratering events. Craters within the belt of smooth plains surrounding the Caloris basin on Mercury are most degraded nearest the basin; this suggests that Mercurian smooth plains must, at least in part, be emplaced in a manner similar to plains surrounding the Imbrium basin. Mercurian uplands have a primary crater population deficient in small crater diameters (less than approximately 30 km). Lunar uplands far from major basins also have a crater population deficient in small crater sizes. Martian cratered terrain exhibits a similar crater deficiency, which was previously interpreted as due to obliteration of small craters (less than approximately 30 km) by some surface process. A crater size distribution deficient in small sizes (less than approximately 30 km) on the Mercurian, lunar, and Martian uplands has implications for the origin of debris bombarding the inner solar system during the period recorded by these surfaces. It is proposed that during late heavy bombardment, the inner solar system was inundated with bodies that broke up under tidal fission as they approached the planets. Such a mechanism would lend to production of a crater population deficient in small crater sizes, and it would also explain the large degree of spatial clustering of primary craters on Mercury, the moon, and Mars.

Oberbeck, V. R.↗
NASA NTRS1974

Roche limit of a solid body

Tidal fission of both impacting and orbiting linear elastic solid bodies based on Kelvin's theory of earth tides is considered. It is shown that there can be more than one mutually exclusive modes of fracture - the particular mode in which a body fractures depending on its size and strength. The analysis gives a vivid picture of the propagation of the fracture with a decreasing distance from the planet. Expressions for the initiation and completion of fracture are obtained which are displayed graphically for a rigid body. The effect of elasticity on the breakup altitude is discussed. For orbiting solid bodies, the study gives the upper limit of the breakup altitude as 0.38R (where R is the radius of planet), which is much less than the value 1.44R used for such bodies in the past. The results presented include a previously given theory by Sekiguchi as a part. For the special case of a liquid body, comparison is made with Roche's calculation and the difference explained.

Aggarwal, H. R.↗
NASA NTRS1972

Rayleigh wave effects in an elastic half-space.

Consideration of Rayleigh wave effects in a homogeneous isotropic linearly elastic half-space subject to an impulsive uniform disk pressure loading. An approximate formula is obtained for the Rayleigh wave effects. It is shown that the Rayleigh waves near the center of loading arise from the portion of the dilatational and shear waves moving toward the axis, after they originate at the edge of the load disk. A study is made of the vertical displacement due to Rayleigh waves at points on the axis near the surface of the elastic half-space.

Aggarwal, H. R.↗