739 research outputs found
Insect contamination protection for laminar flow surfaces
The ability of modern aircraft surfaces to achieve laminar flow was well-accepted in recent years. Obtaining the maximum benefit of laminar flow for aircraft drag reduction requires maintaining minimum leading-edge contamination. Previously proposed insect contamination prevention methods have proved impractical due to cost, weight, or inconvenience. Past work has shown that insects will not adhere to water-wetted surfaces, but the large volumes of water required for protection rendered such a system impractical. The results of a flight experiment conducted by NASA to evaluate the performance of a porous leading-edge fluid discharge ice protection system operated as an insect contamination protections system are presented. In addition, these flights explored the environmental and atmospheric conditions most suitable for insect accumulation
An experimental and theoretical investigation of deposition patterns from an agricultural airplane
A flight test program has been conducted with a representative agricultural airplane to provide data for validating a computer program model which predicts aerially applied particle deposition. Test procedures and the data from this test are presented and discussed. The computer program features are summarized, and comparisons of predicted and measured particle deposition are presented. Applications of the computer program for spray pattern improvement are illustrated
A New Method for Laminar Boundary Layer Transition Visualization in Flight: Color Changes in Liquid Crystal Coatings
The visualization of laminar to turbulent boundary layer transition plays an important role in flight and wind tunnel aerodynamic testing of aircraft wing and body surfaces. Visualization can help provide a more complete understanding of both transition location as well as transition modes; without visualization, the transition process can be very difficult to understand. In the past, the most valuable transition visualization methods for fight applications included sublimating chemicals and oil flows. Each method has advantages and limitations. In particular, sublimating chemicals are impractical to use in subsonic applications much above 20,000 feet because of the greatly reduced rates of sublimation at lower temperatures (less than -4 degrees Fahrenheit). Both oil flow and sublimating chemicals have the disadvantage of providing only one good data point per flight. Thus, for many important flight conditions, transition visualization has not been readily available. This paper discusses a new method for visualizing transition in fight by the use of liquid crystals. The new method overcomes the limitations of past techniques, and provides transition visualization capability throughout almost the entire altitude and speed ranges of virtually all subsonic aircraft flight envelopes. The method also has wide applicability for supersonic transition visualization in flight and for general use in wind tunnel research over wide subsonic and supersonic speed ranges
Advanced measurement techniques, part 1
In modern laminar flow flight and wind tunnel research, it is important to understand the specific cause(s) of laminar to turbulent boundary layer transition. Such information is crucial to the exploration of the limits of practical application of laminar flow for drag reduction on aircraft. The process of transition involves both the possible modes of disturbance growth, and the environmental conditioning of the instabilities by freestream or surface conditions. The possible modes of disturbance growth include viscous, inviscid, and modes which may bypass these natural ones. Theory provides information on the possible modes of disturbance amplification, but experimentation must be relied upon to determine which of those modes actually dominates the transition process in a given environment. The results to date of research on advanced devices and methods used for the study of transition phenomena in the subsonic and transonic flight and wind tunnel environments are presented
Spatial Correlation Function of X-ray Selected AGN
We present a detailed description of the first direct measurement of the
spatial correlation function of X-ray selected AGN. This result is based on an
X-ray flux-limited sample of 219 AGN discovered in the contiguous 80.7 deg^2
region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at
the 4 sigma level at comoving scales in the interval r = 5-60 h^-1 Mpc. Fitting
the data with a power law of slope gamma=1.8, we find a correlation length of
r_0 = 7.4 (+1.8, -1.9) h^-1 Mpc (Omega_M=0.3, Omega_Lambda=0.7). The median
redshift of the AGN contributing to the signal is z_xi=0.22. This clustering
amplitude implies that X-ray selected AGN are spatially distributed in a manner
similar to that of optically selected AGN. Furthermore, the ROSAT NEP
determination establishes the local behavior of AGN clustering, a regime which
is poorly sampled in general. Combined with high-redshift measures from optical
studies, the ROSAT NEP results argue that the AGN correlation strength
essentially does not evolve with redshift, at least out to z~2.2. In the local
Universe, X-ray selected AGN appear to be unbiased relative to galaxies and the
inferred X-ray bias parameter is near unity, b_X~1. Hence X-ray selected AGN
closely trace the underlying mass distribution. The ROSAT NEP AGN catalog,
presented here, features complete optical identifications and spectroscopic
redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41,
f_X=1.1*10^-13 cgs, and L_X=9.2*10^43 h_70^-2 cgs (0.5-2.0 keV), respectively.
Unobscured, type 1 AGN are the dominant constituents (90%) of this soft X-ray
selected sample of AGN.Comment: 17 pages, 8 figures, accepted for publication in ApJ, a version with
high-resolution figures is available at
http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a
machine-readable version of the ROSAT NEP AGN catalog is available at
http://www.eso.org/~cmullis/research/nep-catalog.htm
Luminous K-band Selected Quasars from UKIDSS
The largest K-band flux-limited sample of luminous quasars to date has been
constructed from the UKIDSS Large Area Survey Early Data Release, covering an
effective area of 12.8 deg^2. Exploiting the K-band excess of all quasars with
respect to foreground stars, including quasars experiencing dust reddening and
objects with non-standard SEDs, a list of targets suitable for spectroscopic
follow-up observations with the AAOmega multi-object spectrograph is
constructed, resulting in more than 200 confirmed AGN. KX-selection
successfully identifies as quasar candidates objects that are excluded from the
SDSS quasar selection algorithm due to their colours being consistent with the
stellar locus in optical colour space (with the space density of the excluded
objects agreeing well with results from existing completeness analyses). Nearly
half of the KX-selected quasars with K<17.0 at z<3 are too faint in the i-band
to have been targeted by the SDSS quasar selection algorithm, revealing a large
population of quasars with red i-K colours. The majority of these objects have
significant amounts of host galaxy light contributing to their K-band
magnitudes, consistent with previous predictions. The remaining objects are
morphologically stellar and have colours consistent with quasars experiencing
SMC-type reddening with 0.10<E(B-V)<0.25. The i-K colour distribution indicates
that <10 per cent of the quasar population is missing from this K-band selected
sample due to dust reddening, and comparisons with simulations strongly favour
an obscured fraction of <20 per cent. (Abridged)Comment: 22 pages, 18 figures, accepted for publication in MNRAS. Resolution
of Figure 2 has been reduced for astro-ph submission. The complete Table 5
can be found at http://www.aip.de/People/nmaddox/KX/catalogue.tx
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
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