34,635 research outputs found
Subsonic balance and pressure investigation of a 60 deg delta wing with leading edge devices
Low supersonic wave drag makes the thin highly swept delta wing the logical choice for use on aircraft designed for supersonic cruise. However, the high-lift maneuver capability of the aircraft is limited by severe induced-drag penalties attributed to loss of potential flow leading-edge suction. This drag increase may be alleviated through leading-edge flow control to recover lost aerodynamic thrust through either retention of attached leading-edge flow to higher angles of attack or exploitation of the increased suction potential of separation-induced vortex flow. A low-speed wind-tunnel investigation was undertaken to examine the high-lift devices such as fences, chordwise slots, pylon vortex generators, leading-edge vortex flaps, and sharp leading-edge extensions. The devices were tested individually and in combinations in an attempt to improve high-alpha drag performance with a minimum of low-alpha drag penalty. This report presents an analysis of the force, moment, and static pressure data obtained in angles of attack up to 23 deg, at Mach and Reynolds numbers of 0.16 and 3.85 x 10 to the 6th power per meter, respectively. The results indicate that all the devices produced drag and longitudinal/lateral stability improvements at high lift with, in most cases, minor drag penalties at low angles of attack
The star formation history of damped Lyman alpha absorbers
The local power law relationship between the surface densities of neutral
hydrogen gas and star formation rate (SFR) can be used to explore the SFR
properties of damped Lyman alpha (DLA) systems at higher redshift. We find that
while the SFR densities for DLA systems are consistent with luminous star
forming galaxies at redshifts below z~0.6, at higher redshifts their SFR
density is too low for them to provide a significant contribution to the cosmic
star formation history (SFH). This suggests that the majority of DLAs may be a
distinct population from the Lyman break galaxies (LBGs) or submillimeter
star-forming galaxies that together dominate the SFR density at high redshift.
It is also possible that the DLAs do not trace the bulk of the neutral gas at
high redshift. The metallicity properties of DLAs are consistent with this
interpretation. The DLAs show a metal mass density lower by two orders of
magnitude at all redshifts than that inferred from the SFH of the universe.
These results are consistent with DLAs being dominated by low mass systems
having low SFRs or a late onset of star formation, similar to the star
formation histories of dwarf galaxies in the local universe.Comment: 9 pages, 5 figures, accepted for publication in Ap
Has the Rajiv Aarogyasri Community Health Insurance Scheme of Andhra Pradesh Addressed the Educational Divide in Accessing Health Care?
Lattice Green's function for crystals containing a planar interface
Flexible boundary condition methods couple an isolated defect to a
harmonically responding medium through the bulk lattice Green's function; in
the case of an interface, interfacial lattice Green's functions. We present a
method to compute the lattice Green's function for a planar interface with
arbitrary atomic interactions suited for the study of line defect/interface
interactions. The interface is coupled to two different semi-infinite bulk
regions, and the Green's function for interface-interface, bulk-interface and
bulk-bulk interactions are computed individually. The elastic bicrystal Green's
function and the bulk lattice Green's function give the interaction between
bulk regions. We make use of partial Fourier transforms to treat in-plane
periodicity. Direct inversion of the force constant matrix in the partial
Fourier space provides the interface terms. The general method makes no
assumptions about the atomic interactions or crystal orientations. We simulate
a screw dislocation interacting with a twin boundary in Ti using
flexible boundary conditions and compare with traditional fixed boundary
conditions results. Flexible boundary conditions give the correct core
structure with significantly less atoms required to relax by energy
minimization. This highlights the applicability of flexible boundary conditions
methods to modeling defect/interface interactions by \textit{ab initio}
methods
Shear flow induced isotropic to nematic transition in a suspension of active filaments
We study the effects of externally applied shear flow on a model of
suspensions of motors and filaments, via the equations of active hydrodynamics
[PRL {\bf 89} (2002) 058101; {\bf 92} (2004) 118101]. In the absence of shear,
the orientationally ordered phase of {\it both} polar and apolar active
particles is always unstable at zero-wavenumber. An imposed steady shear large
enough to overcome the active stresses stabilises both apolar and moving polar
phases. Our work is relevant to {\it in vitro} studies of active filaments, the
reorientation of endothelial cells subject to shear flow and shear-induced
motility of attached cells.Comment: 8 pages, 4 figures submitted to Europhysics Letter
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