2,943 research outputs found
An analysis of the normal accelerations and airspeeds of several Lockheed Constellation L-649 airplanes in postwar commercial transport operations over the eastern part of the United States
Atmospheric Turbulence Measurements Obtained from Airplane Operations at Altitudes Between 20,000 and 75,000 Feet for Several Areas in the Northern Hemisphere
Measurements of clear-air turbulence by use of airplane-borne instrumentation have been obtained from NASA VGH recorders during research flights of Lockheed U-2 airplanes to altitudes of 75,000 feet over several areas of the Northern Hemisphere. An analysis of these data has indicated that for the higher altitudes (50,000 to 75,000 feet), turbulence is both less frequent and less severe than for the lower altitudes (20,000 to 50,000 feet). Turbulence appears to be present at the high altitudes (60,000 to 75,000 feet) less than 1 percent of the time. Moderately heavy turbulence appears to exist on occasion at altitudes of about 50,000 feet over Japan. As a consequence, the gust experience appears to be more severe for operations over Japan than for the other areas. Less than 50 percent of the turbulent areas exceeded 10 miles in length
Preliminary measurements of atmospheric turbulence at high altitude as determined from acceleration measurements on Lockheed U-2 airplane
Dispersal of protoplanetary discs: How stellar properties and the local environment determine the pathway of evolution
We study the evolution and final dispersal of protoplanetary discs that
evolve under the action of internal and external photoevaporation, and
different degrees of viscous transport. We identify five distinct dispersal
pathways, which are i) very long lived discs (Myr), ii) inside-out
dispersal where internal photoevaporation dominates and opens inner holes, iii)
outside-in dispersal where external photoevaporation dominates through disc
truncation and two intermediate regimes characterised by lingering material in
the inner disc with the outer disc dispersed predominantly by either internal
or external photoevaporation. We determine how the lifetime, relative impact of
internal and external winds and clearing pathway varies over a wide, plausible,
parameter space of stellar/disc/radiation properties. There are a number of
implications, for example in high UV environments because the outer disc
lifetime is shorter than the time-scale for clearing the inner disc we do not
expect transition discs to be common, which appears to be reflected in the
location of transition disc populations towards the Orion Nebular Cluster.
Irrespective of environment, we find that ongoing star formation is required to
reproduce observed disc fractions as a function of stellar cluster age. This
work demonstrates the importance of including both internal and external winds
for understanding protoplanetary disc evolution.Comment: Submitted to MNRAS. 19 pages, 15 figure
Airplane Measurements of Atmospheric Turbulence for Altitudes Between 20,000 and 55,000 Feet over the Western Part of the United States
An analysis of acceleration, airspeed, and gust-velocity data from a four-engine transport airplane in operations on an eastern United States route
Maneuver accelerations experienced by five types of commercial transport airplanes during routine operations
An analysis of normal-acceleration and airspeed data from a four-engine type of transport airplane in commercial operation on an eastern united states route from november 1947 to february 1950
Planet formation via pebble accretion in externally photoevaporating discs
We demonstrate that planet formation via pebble accretion is sensitive to
external photoevaporation of the outer disc. In pebble accretion, planets grow
by accreting from a flux of solids (pebbles) that radially drift inwards from
the pebble production front. If external photoevaporation truncates the outer
disc fast enough, it can shorten the time before the pebble production front
reaches the disc outer edge, cutting off the supply of pebble flux for
accretion, hence limiting the pebble mass reservoir for planet growth.
Conversely, cloud shielding can protect the disc from strong external
photoevaporation and preserve the pebble reservoir. Because grain growth and
drift can occur quickly, shielding even on a short time-scale (<1 Myr) can have
a non-linear impact on the properties of planets growing by pebble accretion.
For example a planetary seed at 25 au stays at 25 au with a
lunar mass if the disc is immediately irradiated by a G field, but
grows and migrates to be approximately Earth-like in both mass and orbital
radius if the disc is shielded for just 1 Myr. In NGC 2024, external
photoevaporation is thought to happen to discs that are <0.5 Myr old, which
coupled with the results here suggests that the exact planetary parameters can
be very sensitive to the star forming environment. Universal shielding for
time-scales of at least Myr would be required to completely nullify
the environmental impact on planetary architectures.Comment: Accepted for publication in mnras, 12 pages, 8 figure
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