6,336 research outputs found
Airfoil interaction with impinging vortex
The tip of a finite-span airfoil was used to generate a streamwise vortical flow, the strength of which could be varied by changing the incidence of the airfoil. The vortex that was generated traveled downstream and interacted with a second airfoil on which measurements of lift, drag, and pitching moment were made. The flow field, including the vortex core, was visualized in order to study the structural alterations to the vortex resulting from various levels of encounter with the downstream airfoil. These observations were also used to evaluate the accuracy of a theoretical model
Suppression of dynamic stall with a leading-edge slat on a VR-7 airfoil
The VR-7 airfoil was experimentally studied with and without a leading-edge slat at fixed angles of attack from 0 deg to 30 deg at Re = 200,000 and for unsteady pitching motions described by alpha equals alpha(sub m) + 10 deg(sin(wt)). The models were two dimensional, and the test was performed in a water tunnel at Ames Research Center. The unsteady conditions ranged over Re equals 100,000 to 250,000, k equals 0.001 to 0.2, and alpha(sub m) = 10 deg to 20 deg. Unsteady lift, drag, and pitching-moment measurements were obtained along with fluorescent-dye flow visualizations. The addition of the slat was found to delay the static-drag and static-moment stall by about 5 degrees and to eliminate completely the development of a dynamic-stall vortex during unsteady motions that reached angles as high as 25 degrees. In all of the unsteady cases studied, the slat caused a significant reduction in the force and moment hysteresis amplitudes. The reduced frequency was found to have the greatest effect on the results, whereas the Reynolds number had little effect on the behavior of either the basic or the slatted airfoil. The slat caused a slight drag penalty at low angles of attack, but generally increased the lift/drag ratio when averaged over the full cycle of oscillation
Optimal Topological Test for Degeneracies of Real Hamiltonians
We consider adiabatic transport of eigenstates of real Hamiltonians around
loops in parameter space. It is demonstrated that loops that map to nontrivial
loops in the space of eigenbases must encircle degeneracies. Examples from
Jahn-Teller theory are presented to illustrate the test. We show furthermore
that the proposed test is optimal.Comment: Minor corrections, accepted in Phys. Rev. Let
Resonant Subband Landau Level Coupling in Symmetric Quantum Well
Subband structure and depolarization shifts in an ultra-high mobility
GaAs/Al_{0.24}Ga_{0.76}As quantum well are studied using magneto-infrared
spectroscopy via resonant subband Landau level coupling. Resonant couplings
between the 1st and up to the 4th subbands are identified by well-separated
anti-level-crossing split resonance, while the hy-lying subbands were
identified by the cyclotron resonance linewidth broadening in the literature.
In addition, a forbidden intersubband transition (1st to 3rd) has been
observed. With the precise determination of the subband structure, we find that
the depolarization shift can be well described by the semiclassical slab plasma
model, and the possible origins for the forbidden transition are discussed.Comment: 4 pages, 2 figure
Magneto-Infrared Spectroscopic Study of Ultrathin BiTe Single Crystals
Ultrathin BiTe single crystals laid on Scotch tape are
investigated by Fourier transform infrared spectroscopy at K and in a
magnetic field up to T. The magneto-transmittance spectra of the Bi%
Te/tape composite are analyzed as a two-layer system and the optical
conductivity of BiTe at different magnetic fields are extracted. We
find that magnetic field modifies the optical conductivity in the following
ways: (1) Field-induced transfer of the optical weight from the lower frequency
regime (cm) to the higher frequency regime (cm) due
to the redistribution of charge carriers across the Fermi surface. (2) Evolving
of a Fano-resonance-like spectral feature from an anti-resonance to a resonance
with increasing magnetic field. Such behavior can be attributed to the
electron-phonon interactions between the optical phonon mode and
the continuum of electronic transitions. (3) Cyclotron resonance resulting from
the inter-valence band Landau level transitions, which can be described by the
electrodynamics of massive Dirac holes
Heavy Quark Mass Effects in Deep Inelastic Scattering and Global QCD Analysis
A new implementation of the general PQCD formalism of Collins, including
heavy quark mass effects, is described. Important features that contribute to
the accuracy and efficiency of the calculation of both neutral current (NC) and
charged current (CC) processess are explicitly discussed. This new
implementation is applied to the global analysis of the full HERA I data sets
on NC and CC cross sections, with correlated systematic errors, in conjunction
with the usual fixed-target and hadron collider data sets. By using a variety
of parametrizations to explore the parton parameter space, robust new parton
distribution function (PDF) sets (CTEQ6.5) are obtained. The new quark
distributions are consistently higher in the region x ~ 10^{-3} than previous
ones, with important implications on hadron collider phenomenology, especially
at the LHC. The uncertainties of the parton distributions are reassessed and
are compared to the previous ones. A new set of CTEQ6.5 eigenvector PDFs that
encapsulates these uncertainties is also presented.Comment: 32 pages, 12 figures; updated, Publication Versio
Effect of an extendable slat on the stall behavior of a VR-12 airfoil
Experimental and computational tests were performed on a VR-12 airfoil to determine if the dynamic-stall behavior that normally accompanies high-angle pitch oscillations could be modified by segmenting the forward portion of the airfoil and extending it ahead of the main element. In the extended position the configuration would appear as an airfoil with a leading-edge slat, and in the retracted position it would appear as a conventional VR-12 airfoil. The calculations were obtained from a numerical code that models the vorticity transport equation for an incompressible fluid. These results were compared with test data from the water tunnel facility of the Aeroflightdynamics Directorate at Ames Research Center. Steady and unsteady flows around both airfoils were examined at angles of attack between 0 and 30 deg. The Reynolds number was fixed at 200,000 and the unsteady pitch oscillations followed a sinusoidal motion described by alpha = alpha(sub m) + 10 deg sin(omega t). The mean angle (alpha(sub m)) was varied from 10 to 20 deg and the reduced frequency from 0.05 to 0.20. The results from the experiment and the calculations show that the extended-slat VR-12 airfoil experiences a delay in both static and dynamic stall not experienced by the basic VR-12 airfoil
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