17,552 research outputs found
An experimental investigation of the effect of boundary layer refraction on the noise from a high-speed propeller
Models of supersonic propellers were previously tested for acoustics in the Lewis 8- by 6-Foot Wind Tunnel using pressure transducers mounted in the tunnel ceiling. The boundary layer on the tunnel ceiling is believed to refract some of the propeller noise away from the measurement transducers. Measurements were made on a plate installed in the wind tunnel which had a thinner boundary layer than the ceiling boundary layer. The plate was installed in two locations for comparison with tunnel ceiling noise data and with fuselage data taken on the NASA Dryden Jetstar airplane. Analysis of the data indicates that the refraction increases with: increasing boundary layer thickness; increasing free stream Mach number; increasing frequency; and decreasing sound radiation angle (toward the inlet axis). At aft radiation angles greater than about 100 deg there was little or no refraction. Comparisons with the airplane data indicated that not only is the boundary layer thickness important but also the shape of the velocity profile. Comparisons with an existing two-dimensional theory, using an idealized shear layer to approximate the boundary layer, showed that the theory and data had the same trends. Analysis of the data taken in the tunnel at two different distances from the propeller indicates a decay with distance in the wind tunnel at high Mach numbers but the decay at low Mach numbers is not as clear
Enzyme activity in terrestrial soil in relation to exploration of the Martian surface
Urease activity in soil is persistent for long periods under low water, low temperature, and sterile regimes, and it was suggested that some form of enzyme-protective mechanism exists in soil. Dublin soil was extracted by sonication in water followed by adding a mixture of salts. Urease activity is associated with the organo-mineral complex thus obtained and is resistant to the activities of proteolytic enzymes. Clay free soil organic matter prepared subsequently by filtration also exhibits urease activity which is resistant to proteolysis. Models consisting of enzymes with bentonite and lignin were found to mimic this resistance to proteolysis. A model system is presented which suggests both the origin and location of soil ureases and a reason for their persistence in nature
Great exaptations
Long interspersed nuclear elements (LINEs) are among the most successful parasitic genetic sequences in higher organisms. Recent work has discovered many instances of LINE incorporation into exons, reminding us of the hazards they pose to genes in their vicinity as well as their potential to be co-opted for the host's purposes
Sandstone Pore Aspect Ratio Spectra from Direct Observations and Velocity Inversion
Measurements of pore shapes from Scanning Electron Microscope (SEM) images
for three sandstone samples (the Navajo Sandstone, the Weber Sandstone, and the
Kayenta Sandstone) are compared to the aspect ratio spectra obtained from
inverting laboratory velocity versus pressure data using the method of Cheng and
Toksoz (1979). The results indicate that the inversion method is in very good
agreement with the observations at high aspect ratios ( ex > 0.0 1). At low aspect ratios the agreement is very good for the clean Navajo Sandstone sample, but poor
for the Weber and Kayenta samples which contain clay. The Navajo sample is
composed chiefly of quartz with significant pressure dissolution apparent along grain
contacts resulting in smooth, flat cracks between grains. The Weber and Kayenta
samples have rougher crack surfaces as well as tapered pore edges, indicating that
asperities ,and non-elliptical pore shapes may result in an over estimation of low
aspect ratio cracks by velocity inversion. The presence of degraded feldspars may
also play a role.Phillips Petroleum Fellowshi
Lithologic Cycles And Paleo Fluid Flow Channels In Old Oceanic Crust From Geophysical Logs At ODP Site 418A
Using some new processing of the multichannel sonic (MCS) log data from Site 418A,
the resulting P, S, and Stoneley wave velocity estimates and apparent attenuation were
integrated with the natural gamma, spectral gamma, resistivity, neutron, density, and
caliper logs and core lithology information for interpretation of lithologic cycles and
possible paleo fluid flow intervals. These data indicate the presence of multiple breccia
or rubble zones in the lower portions of the borehole. These zones are interpreted
as the last stages of eruptive cycles as described by Hyndman and Salisbury (1984).
The results of permeability (packer) tests and temperature gradient measurements at
Sites 395A and 504B indicate that fluid flow in the crust at those sites is localized
to brecciated zones which occur below massive flow basalts. By analogy, the breccia
zones interpreted at Site 418A may have acted as fluid flow channels at an earlier time.
Six major paleo fluid flow zones are interpreted at Site 418A. These breccia zones have
low velocities (P, S, and Stoneley), increased apparent attenuation, and an increase
in gamma activity. These intervals are interpreted as permeable pathways which may
have been altered by the second stage of oxidizing alteration as described by Holmes
(1988). Breccia units occur just below massive basalt flow units. The massive basalt
flow units are also easily identified in the MCS data. The resistivity log data suggest
that each major eruptive cycle trend is made up of several smaller sub-cycles. The
MCS data provides much insight into the variations in lithology in ODP boreholes.
The trace energy provides a stable measure of apparent attenuation which may be
related to alteration, fracturing, or permeability (if there are open fractures). Velocity estimates for P, S, and Stoneley waves provide useful information about lithologic variability if interpreted in detail.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumNational Science Foundation (U.S.) (Grant OCE-89000316
Photoinduced time-resolved electrodynamics of superconducting metals and alloys
The photoexcited state in superconducting metals and alloys was studied via
pump-probe spectroscopy. A pulsed Ti:sapphire laser was used to create the
non-equilibrium state and the far-infrared pulses of a synchrotron storage
ring, to which the laser is synchronized, measured the changes in the material
optical properties. Both the time- and frequency- dependent photoinduced
spectra of Pb, Nb, NbN, Nb{0.5}Ti{0.5}N, and Pb{0.75}Bi{0.25} superconducting
thin films were measured in the low-fluence regime. The time dependent data
establish the regions where the relaxation rate is dominated either by the
phonon escape time (phonon bottleneck effect) or by the intrinsic quasiparticle
recombination time. The photoinduced spectra measure directly the reduction of
the superconducting gap due to an excess number of quasiparticles created by
the short laser pulses. This gap shift allows us to establish the temperature
range over which the low fluence approximation is valid.Comment: 12 pages with 10 figure
Energy Partitioning and Attenuation of Guided Waves in a Radially Layered Borehole
Recently published results (Tubman et al., 1984; Baker,. 1984) indicate that
synthetic full waveform acoustic logs generated in cased or damaged boreholes
differ significantly from those generated in an open hole with the same formation
parameters. In particular, the guided waves appear to be the most affected by such
radial layering. In order to gain some understanding of these effects, the amplitude
response and energy distribution of the pseudo-Rayleigh and Stoneley waves are
studied for the cased and invaded borehole models. The expressions derived by
Cheng et al. (1 982) are used to calculate partition coefficients (partial derivatives
of phase velocity with respect to body wave velocities) for the guided wave modes.
The attenuation of the guided wave can then be represented by the sum of the layer
attenuation values weighted by their respective partition coefficients. The results
indicate that the attenuation of the Stoneley wave is dominated by the fluid
attenuation at all frequencies in fast formations, both in the open hole geometry and
in the presence of casing or invaded zones. In a slow formation, the Stoneley wave
attenuation becomes more sensitive to the shear wave attenuation of the formation
at higher frequencies in both the open and cased hole situations. For the pseudo-Rayleigh
wave, the introduction of casing reduces the effect of the fluid attenuation,
while the presence of an invaded zone reduces the effect of the formation shear
attenuation. Plots of the partition coefficients indicate that the casing and invasion
layers are most important over a limited frequency range which is related to the
thickness of the layer. Radial displacement curves illustrate the depth of penetration
of the various frequency components of the pseudo-Rayleigh wave.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Homomorphic Processing Of Acoustic Logging Data
A new processing method, which we developed for the guided waves generated during
acoustic logging, accurately estimates the wavenumber when only a few seismograms
are available or when the seismograms are irregularly spaced. The estimates of the
attenuation coefficient are seemingly accurate when many seismograms are available
but are inaccurate when only a few seismograms are available. The new method does
not generate any spurious estimates as the Prony-based method does.Halliburton CompanyMassachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Investigation of optimization of attitude control systems, volume ii
Attitude control system optimization - computer programs, listings and subroutine
Investigation of optimization of attitude control systems, volume i
Optimization of attitude control systems by development of mathematical model and computer program for space vehicle simulatio
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