2,442 research outputs found
Cutting the Cord: Removing the CMRS Spectrum Cap to Promote Wireless-Landline Convergence and Wireless Alernatives in the Local Loop
Alien Registration- Beckwith, Robert C. (Portland, Cumberland County)
https://digitalmaine.com/alien_docs/24141/thumbnail.jp
Alien Registration- Beckwith, Robert C. (Portland, Cumberland County)
https://digitalmaine.com/alien_docs/24141/thumbnail.jp
Low-disturbance wind tunnels
During the past years, there was an extensive program under way at the Langley Research Center to upgrade the flow quality in several of the large wind tunnels. This effort has resulted in significant improvements in flow quality in these tunnels and has also increased the understanding of how and where changes in existing and new wind tunnels are most likely to yield the desired improvements. As part of this ongoing program, flow disturbance levels and spectra were measured in several Langley tunnels before and after modifications were made to reduce acoustic and vorticity fluctuations. A brief description of these disturbance control features is given for the Low-Turbulence Pressure Tunnel, the 4 x 7 Meter Tunnel, and the 8 Foot Transonic Pressure Tunnel. To illustrate typical reductions in disturbance levels obtained in these tunnels, data from hot-wire or acoustic sensors are presented. A concept for a subsonic quiet tunnel designed to study boundary layer stability and transition is also presented. Techniques developed at Langley in recent years to eliminate the high intensity and high-frequency acoustic disturbances present in all previous supersonic wind tunnels are described. In conclusion, the low-disturbance levels present in atmospheric flight can now be simulated in wind tunnels over the speed range from low subsonic through high supersonic
Upper limit for the D2H+ ortho-to-para ratio in the prestellar core 16293E (CHESS)
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+, which are very useful probes of the early
stages of star and planet formation. The ground-state rotational ortho-D2H+
111-000 transition at 1476.6 GHz in the prestellar core 16293E has been
searched for with the Herschel/HIFI instrument, within the CHESS (Chemical
HErschel Surveys of Star forming regions) Key Program. The line has not been
detected at the 21 mK km/s level (3 sigma integrated line intensity). We used
the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in
this source to determine an upper limit on the ortho-to-para D2H+ ratio as well
as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison
between our chemical modeling and the observations suggests that the CO
depletion must be high (larger than 100), with a density between 5e5 and 1e6
cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas
temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times
higher than the value estimated from the chemical modeling, making it
impossible to detect this high frequency transition with the present state of
the art receivers.Comment: Accepted in A&
Design and preliminary test results at Mach 5 of an axisymmetric slotted sound shield
The basic theory and sound attenuation mechanisms, the design procedures, and preliminary experimental results are presented for a small axisymmetric sound shield for supersonic wind tunnels. The shield consists of an array of small diameter rods aligned nearly parallel to the entrance flow with small gaps between the rods for boundary layer suction. Results show that at the lowest test Reynolds number (based on rod diameter) of 52,000 the noise shield reduced the test section noise by about 60 percent ( or 8 db attenuation) but no attenuation was measured for the higher range of test reynolds numbers from 73,000 to 190,000. These results are below expectations based on data reported elsewhere on a flat sound shield model. The smaller attenuation from the present tests is attributed to insufficient suction at the gaps to prevent feedback of vacuum manifold noise into the shielded test flow and to insufficient suction to prevent transition of the rod boundary layers to turbulent flow at the higher Reynolds numbers. Schlieren photographs of the flow are shown
Symptomatology under storm conditions in the north atlantic in control subjects and in persons with bilateral labyrinthine defects
Motion sickness under conditions of stress and anxiety - role of vestibular apparatu
Recollimation Boundary Layers in Relativistic Jets
We study the collimation of relativistic hydrodynamic jets by the pressure of
an ambient medium in the limit where the jet interior has lost causal contact
with its surroundings. For a jet with an ultrarelativistic equation of state
and external pressure that decreases as a power of spherical radius, p \propto
r^(-eta), the jet interior will lose causal contact when eta > 2. However, the
outer layers of the jet gradually collimate toward the jet axis as long as eta
< 4, leading to the formation of a shocked boundary layer. Assuming that
pressure-matching across the shock front determines the shape of the shock, we
study the resulting structure of the jet in two ways: first by assuming that
the pressure remains constant across the shocked boundary layer and looking for
solutions to the shock jump equations, and then by constructing self-similar
boundary-layer solutions that allow for a pressure gradient across the shocked
layer. We demonstrate that the constant-pressure solutions can be characterized
by four initial parameters that determine the jet shape and whether the shock
closes to the axis. We show that self-similar solutions for the boundary layer
can be constructed that exhibit a monotonic decrease in pressure across the
boundary layer from the contact discontinuity to the shock front, and that the
addition of this pressure gradient in our initial model generally causes the
shock front to move outwards, creating a thinner boundary layer and decreasing
the tendency of the shock to close. We discuss trends based on the value of the
pressure power-law index eta.Comment: 10 pages, 8 figures. Accepted to MNRAS; minor revisions from original
submitted versio
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