97,997 research outputs found
A study of the local pressure field in turbulent shear flow and its relation to aerodynamic noise generation Semiannual status report, 1 Aug. 1970 - 31 Jan. 1971
Relation of Eulerian and Lagrangian structure of pseudosound pressure and velocity fields in turbulent shear flow to aerodynamic noise generatio
The design and development of a solar tracking unit
The solar tracking unit was developed to support the Laser Heterodyne Spectrometer (LHS) airborne instrument, but has application to a general class of airborne solar occultation research instruments. The unit consists of a mirror mounted on two gimbals, one of which is hollow. The mirror reflects a 7.6 cm (3.0 in.) diameter beam of sunlight through the hollow gimbal into the research instrument optical axis. A portion of the reflected sunlight is directed into a tracking telescope which uses a four quadrant silicon detector to produce the servo error signals. The colinearity of the tracker output beam and the research instrument optical axis is maintained to better than + or - 1 arc-minute. The unit is microcomputer controlled and is capable of stand alone operation, including automatic Sun acquisition or operation under the control of the research instrument
Jupiter - Friend or Foe? IV: The influence of orbital eccentricity and inclination
For many years, it was assumed that Jupiter prevented the Earth from being
subject to a punishing impact regime that would greatly hinder the development
of life. Here, we present the 4th in a series of studies investigating this
hypothesis. Previously, we examined the effect of Jupiter's mass on the impact
rate experienced by Earth. Here, we extend that approach to consider the
influence of Jupiter's orbital eccentricity and inclination on the impact rate.
We first consider scenarios in which Jupiter's orbital eccentricity was
somewhat higher and somewhat lower than that in our Solar System. We find that
Jupiter's orbital eccentricity plays a moderate role in determining the impact
flux at Earth, with more eccentric orbits resulting in a higher impact rate of
asteroids than for more circular orbits. This is particularly pronounced at
high "Jupiter" masses. For short-period comets, the same effect is clearly
apparent, albeit to a lesser degree. The flux of short-period comets impacting
the Earth is slightly higher for more eccentric Jovian orbits.
We also consider scenarios in which Jupiter's orbital inclination was greater
than that in our Solar System. Increasing Jupiter's orbital inclination greatly
increased the flux of asteroidal impactors. However, at the highest tested
inclination, the disruption to the Asteroid belt was so great that the belt
would be entirely depleted after an astronomically short period of time. In
such a system, the impact flux from asteroid bodies would therefore be very
low, after an initial period of intense bombardment. By contrast, the influence
of Jovian inclination on impacts from short-period comets was very small. A
slight reduction in the impact flux was noted for the moderate and high
inclination scenarios considered in this work - the results for inclinations of
five and twenty-five degrees were essentially identical.Comment: 5 figures, plus 12 as an appendi
Application to rotary wings of a simplified aerodynamic lifting surface theory for unsteady compressible flow
A general method of predicting airloads is applied to helicopter rotor blades on a full three-dimensional basis using the general theory developed for a rotor blade at the psi = pi/2 position where flutter is most likely to occur. Calculations of aerodynamic coefficients for use in flutter analysis are made for forward and hovering flight with low inflow. The results are compared with values given by two-dimensional strip theory for a rigid rotor hinged at its root. The comparisons indicate the inadequacies of strip theory for airload prediction. One important conclusion drawn from this study is that the curved wake has a substantial effect on the chordwise load distribution
First-principles Calculations of Engineered Surface Spin Structures
The engineered spin structures recently built and measured in scanning
tunneling microscope experiments are calculated using density functional
theory. By determining the precise local structure around the surface
impurities, we find the Mn atoms can form molecular structures with the binding
surface, behaving like surface molecular magnets. The spin structures are
confirmed to be antiferromagnetic, and the exchange couplings are calculated
within 8% of the experimental values simply by collinear-spin GGA+U
calculations. We can also explain why the exchange couplings significantly
change with different impurity binding sites from the determined local
structure. The bond polarity is studied by calculating the atomic charges with
and without the Mn adatoms
Prospects for Extrasolar "Earths" in Habitable Zones
We have shown that Earth-mass planets could survive in variously restricted
regions of the habitable zones (HZs) of most of a sample of nine of the 102
main-sequence exoplanetary systems confirmed by 19 November 2003. In a
preliminary extrapolation of our results to the other systems, we estimate that
roughly a half of these systems could have had an Earth-mass planet confined to
the HZ for at least the most recent 1000 Ma. The HZ migrates outwards during
the main-sequence lifetime, and so this proportion varies with stellar age.
About two thirds of the systems could have such a planet confined to the HZ for
at least 1000 Ma at sometime during the main-sequence lifetime. Clearly, these
systems should be high on the target list for exploration for terrestrial
planets. We have reached this conclusion by launching putative Earth-mass
planets in various orbits and following their fate with mixed-variable
symplectic and hybrid integrators. Whether the Earth-mass planets could form in
the HZs of the exoplanetary systems is an urgent question that needs further
study.Comment: 7 pages, 2 figure
Planck pre-launch status: High Frequency Instrument polarization calibration
The High Frequency Instrument of Planck will map the entire sky in the millimeter and sub-millimeter domain from 100 to 857 GHz with unprecedented sensitivity to polarization (ΔP/T_(cmb) ~ 4 × 10^(-6) for P either Q or U and T_(cmb) ≃ 2.7 K) at 100, 143, 217 and 353 GHz. It will lead to major improvements in our understanding of the cosmic microwave background anisotropies and polarized foreground signals. Planck will make high resolution measurements of the E-mode spectrum (up to l ~ 1500) and will also play a prominent role in the search for the faint imprint of primordial gravitational waves on the CMB polarization. This paper addresses the effects of calibration of both temperature (gain) and polarization (polarization efficiency and detector orientation) on polarization measurements. The specific requirements on the polarization parameters of the instrument are set and we report on their pre-flight measurement on HFI bolometers. We present a semi-analytical method that exactly accounts for the scanning strategy of the instrument as well as the combination of different detectors. We use this method to propagate errors through to the CMB angular power spectra in the particular case of Planck-HFI, and to derive constraints on polarization parameters. We show that in order to limit the systematic error to 10% of the cosmic variance of the E-mode power spectrum, uncertainties in gain, polarization efficiency and detector orientation must be below 0.15%, 0.3% and 1° respectively. Pre-launch ground measurements reported in this paper already fulfill these requirements
Laser Interferometer Gravitational-Wave Observatory beam tube component and module leak testing
Laser Interferometer Gravitational-Wave Observatory (LIGO) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology funded by the National Science Foundation. The project is designed to detect gravitational waves from astrophysical sources such as supernova and black holes. The LIGO project constructed observatories at two sites in the U.S. Each site includes two beam tubes (each 4 km long) joined to form an "L" shape. The beam tube is a 1.25 m diam 304 L stainless steel, ultrahigh vacuum tube that will operate at 1×10^–9 Torr or better. The beam tube was manufactured using a custom spiral weld tube mill from material processed to reduce the outgassing rate in order to minimize pumping costs. The integrity of the beam tube was assured by helium mass spectrometer leak testing each component of the beam tube system prior to installation. Each 2 km long, isolatable beam tube module was then leak tested after completion
Indianapolis New Department of Transportation: Part II-A:: Management of Street Improvement Projects
Infrared astronomy research and high altitude observations
Highlights are presented of studies of the emission mechanisms in the 4 to 8 micron region of the spectrum using a circular variable filter wheel spectrometer with a PbSnTe photovoltaic detector. Investigations covered include the spectroscopy of planets, stellar atmospheres, highly obscured objects in molecular clouds, planetary nebulae, H2 regions, and extragalactic objects
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