11,894 research outputs found

    Wind tunnel supplementary Mach number minimum section insert

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    A device is described which changes the Mach number capability of a wind tunnel without permanently altering the existing nozzle of the tunnel. An insert is removably attached to the wall of the existing nozzle expansion area thereby creating a second minimum section upstream of the model test section. The added insert may be removed without complicated and expensive changes to the basic wind tunnel. In one embodiment, a removable insert is disposed within wind tunnel nozzle walls with a portion of the flow boundary layer being bled off from the tunnel via passageway and tunnel exit to reduce the extent of separated flow normally occuring upstream of the insert contraction section

    The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions

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    The neutrino-heated "gain layer" immediately behind the stalled shock in a core-collapse supernova is unstable to high-Reynolds-number turbulent convection. We carry out and analyze a new set of 19 high-resolution three-dimensional (3D) simulations with a three-species neutrino leakage/heating scheme and compare with spherically-symmetric (1D) and axisymmetric (2D) simulations carried out with the same methods. We study the postbounce supernova evolution in a 1515-MM_\odot progenitor star and vary the local neutrino heating rate, the magnitude and spatial dependence of asphericity from convective burning in the Si/O shell, and spatial resolution. Our simulations suggest that there is a direct correlation between the strength of turbulence in the gain layer and the susceptability to explosion. 2D and 3D simulations explode at much lower neutrino heating rates than 1D simulations. This is commonly explained by the fact that nonradial dynamics allows accreting material to stay longer in the gain layer. We show that this explanation is incomplete. Our results indicate that the effective turbulent ram pressure exerted on the shock plays a crucial role by allowing multi-D models to explode at a lower postshock thermal pressure and thus with less neutrino heating than 1D models. We connect the turbulent ram pressure with turbulent energy at large scales and in this way explain why 2D simulations are erroneously exploding more easily than 3D simulations.Comment: 13 pages, 8 figures, accepted by Ap

    Apparatus for aiding a pilot in avoiding a midair collision between aircraft

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    An apparatus for aiding a pilot in avoiding a midair collision between aircraft is described. A protected aircraft carries a transmitter, a transponder, a receiver, and a data processor; and an intruding cooperating aircraft carries a transponder. The transmitter of the protected aircraft continuously transmits a signal to the transponders of all intruding aircraft. The transponder of each of the intruding aircraft adds the altitude of the intruding aircraft to the signal and transmits it back to the receiver of the protected aircraft. The receiver selects only the signal from the most hazardous intruding aircraft and applies it to the data processor. From this selected signal the data processor determines the closing velocity between the protected and intruding aircraft, the range between the two aircraft, their altitude difference and the time to a possible collision

    Black-white wage inequality in the 1990s: a decade of progress

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    Using Current Population Survey data, we find that the gap between wages by black and white males declined during the 1990s at a rate of 0.59 percentage point per year. The reduction in occupational crowding appears to be most important in explaining this trend. Recent wage convergence was most rapid among younger workers with less than 10 years experience; for this group the black-white wage gap declined by 1.40 percentage points per year. Among younger workers greater occupational diversity and a reduction in unexplained or residual differences are important in explaining this trend. For both younger and older workers, general wage inequality tempered the rate of wage convergence between blacks and whites during the 1990s.Income distribution ; Wages