7,197 research outputs found
Numerical Investigation of Second Mode Attenuation over Carbon/Carbon Surfaces on a Sharp Slender Cone
We have carried out axisymmetric numerical simulations of a spatially
developing hypersonic boundary layer over a sharp 7-half-angle cone
at inspired by the experimental investigations by Wagner (2015).
Simulations are first performed with impermeable (or solid) walls with a
one-time broadband pulse excitation applied upstream to determine the most
convectively-amplified frequencies resulting in the range 260kHz -- 400kHz,
consistent with experimental observations of second-mode instability waves.
Subsequently, we introduce harmonic disturbances via continuous periodic
suction and blowing at 270kHz and 350kHz. For each of these forcing frequencies
complex impedance boundary conditions (IBC), modeling the acoustic response of
two different carbon/carbon (C/C) ultrasonically absorptive porous surfaces,
are applied at the wall. The IBCs are derived as an output of a pore-scale
aeroacoustic analysis -- the inverse Helmholtz Solver (iHS) -- which is able to
return the broadband real and imaginary components of the surface-averaged
impedance. The introduction of the IBCs in all cases leads to a significant
attenuation of the harmonically-forced second-mode wave. In particular, we
observe a higher attenuation rate of the introduced waves with frequency of
350kHz in comparison with 270kHz, and, along with the iHS impedance results, we
establish that the C/C surfaces absorb acoustic energy more effectively at
higher frequencies.Comment: AIAA-SciTech 201
Drift and Diffusion of Spins Generated by the Spin Hall Effect
Electrically generated spin accumulation due to the spin Hall effect is
imaged in n-GaAs channels using Kerr rotation microscopy, focusing on its
spatial distribution and time-averaged behavior in a magnetic field.
Spatially-resolved imaging reveals that spin accumulation observed in
transverse arms develops due to longitudinal drift of spin polarization
produced at the sample boundaries. One- and two-dimensional drift-diffusion
modeling is used to explain these features, providing a more complete
understanding of observations of spin accumulation and the spin Hall effect.Comment: 9 pages, 3 figure
Quality assurance in stereotactic radiosurgery/radiotherapy according to DIN 6875-1
The new DIN (' Deutsche Industrie- Norm') 6875- 1, which is currently being finalised, deals with quality assurance ( QA) criteria and tests methods for linear accelerator and Gamma Knife stereotactic radiosurgery/ radiotherapy including treatment planning, stereotactic frame and stereotactic imaging and a system test to check the whole chain of uncertainties. Our existing QA program, based on dedicated phantoms and test procedures, has been refined to fulfill the demands of this new DIN. The radiological and mechanical isocentre corresponded within 0.2 mm and the measured 50% isodose lines were in agreement with the calculated ones within less than 0.5 mm. The measured absorbed dose was within 3%. The resultant output factors measured for the 14-, 8- and 4- mm collimator helmet were 0.9870 +/- 0.0086, 0.9578 +/- 0.0057 and 0.8741 +/- 0.0202, respectively. For 170 consecutive tests, the mean geometrical accuracy was 0.48 +/- 0.23 mm. Besides QA phantoms and analysis software developed in- house, the use of commercially available tools facilitated the QA according to the DIN 6875- 1 with which our results complied. Copyright (C) 2004 S. Karger AG, Basel
Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet/Semiconductor Heterostructures
We demonstrate methods to locally control the spin rotation of moving
electrons in a GaAs channel. The Larmor frequency of optically-injected spins
is modulated when the spins are dragged through a region of spin-polarized
nuclei created at a MnAs/GaAs interface. The effective field created by the
nuclei is controlled either optically or electrically using the ferromagnetic
proximity polarization effect. Spin rotation is also tuned by controlling the
carrier traverse time through the polarized region. We demonstrate coherent
spin rotations exceeding 4 pi radians during transport.Comment: 15 pages, 4 figure
SDSSJ143244.91+301435.3 at VLBI: a compact radio galaxy in a narrow-line Seyfert 1
We present VLBI observations, carried out with the European Very Long
Baseline Interferometry Network (EVN), of SDSSJ143244.91+301435.3, a radio-loud
narrow-line Seyfert 1 (RLNLS1) characterized by a steep radio spectrum. The
source, compact at Very Large Array (VLA) resolution, is resolved on the
milliarcsec scale, showing a central region plus two extended structures. The
relatively high brightness temperature of all components (5x10^6-1.3x10^8 K)
supports the hypothesis that the radio emission is non-thermal and likely
produced by a relativistic jet and/or small radio lobes. The observed radio
morphology, the lack of a significant core and the presence of a low frequency
(230 MHz) spectral turnover are reminiscent of the Compact Steep Spectrum
sources (CSS). However, the linear size of the source (~0.5kpc) measured from
the EVN map is lower than the value predicted using the turnover/size relation
valid for CSS sources (~6kpc). This discrepancy can be explained by an
additional component not detected in our observations, accounting for about a
quarter of the total source flux density, combined to projection effects. The
low core-dominance of the source (CD<0.29) confirms that
SDSSJ143244.91+301435.3 is not a blazar, i.e. the relativistic jet is not
pointing towards the observer. This supports the idea that
SDSSJ143244.91+301435.3 may belong to the "parent population" of flat-spectrum
RLNLS1 and favours the hypothesis of a direct link between RLNLS1 and compact,
possibly young, radio galaxies.Comment: 8 pages, 3 figures, accepted for publication in MNRA
Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies
Sensitive images of low-level, Mpc-sized radio cocoons offer new
opportunities to probe large scale intergalactic gas flows outside clusters of
galaxies. New radio images of high surface brightness sensitivity at
strategically chosen wavelengths of the giant radio galaxy NGC 315 (Mack et al.
1997,1998) reveal significant asymmetries and particularities in the
morphology, radio spectrum and polarization of the ejected radio plasma. We
argue that the combination of these signatures provides a sensitive probe of an
environmental shock wave. Analysis of optical redshifts in NGC 315 vicinity
confirms its location to be near, or at a site of large-scale flow collisions
in the 100 Mpc sized Pisces-Perseus Supercluster region. NGC 315 resides at the
intersection of several galaxy filaments, and its radio plasma serves there as
a `weather station' (Burns 1998) probing the flow of the elusive and previously
invisible IGM gas. If our interpretation is correct, this is the first
indication for a shock wave in flows caused by the cosmological large scale
structure formation, which is located in a filament of galaxies. The
possibility that the putative shock wave is a source of gamma-rays and ultra
high energy cosmic rays is briefly discussed.Comment: accepted by Astrophysical Journal Letters, 4 pages, 3 figures (incl.
2 color), uses emulateapj5.sty (included), aastex.sty (included) and
psfig.st
Oscillatory Spin Polarization and Magneto-Optic Kerr Effect in Fe3O4 Thin Films on GaAs(001)
The spin dependent properties of epitaxial Fe3O4 thin films on GaAs(001) are
studied by the ferromagnetic proximity polarization (FPP) effect and
magneto-optic Kerr effect (MOKE). Both FPP and MOKE show oscillations with
respect to Fe3O4 film thickness, and the oscillations are large enough to
induce repeated sign reversals. We attribute the oscillatory behavior to
spin-polarized quantum well states forming in the Fe3O4 film. Quantum
confinement of the t2g states near the Fermi level provides an explanation for
the similar thickness dependences of the FPP and MOKE oscillations.Comment: to appear in Phys. Rev. Let
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