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Global existence result for thermoviscoelastic problems with hysteresis : dedicated to the memory of M. Schatzman
We consider viscoelastic solids undergoing thermal expansion and exhibiting hysteresis effects due to plasticity or phase transformations. Within the framework of generalized standard solids, the problem is described in a 3D setting by the momentum equilibrium equation, the flow rule describing the dependence of the stress on the strain history, and the heat transfer equation. Under appropriate regularity assumptions on the data, a local existence result for this thermodynamically consistent system is established, by combining existence results for ordinary differential equations in Banach spaces with a fixed-point argument. Then global estimates are obtained by using both the classical energy estimate and more specific techniques for the heat equation introduced by Boccardo and Gallouet. Finally a global existence result is derived
Aeroacoustic Measurements of the Bell 699 Rotor on the Tiltrotor Test Rig in the National Full-Scale Aerodynamics Complex 40- by 80-Foot Wind Tunnel
The Tiltrotor Test Rig (TTR) with the Bell 699 Rotor was tested in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel from 2017 to 2018. The primary goal of the test was to understand the operational capabilities of the TTR while also acquiring research data, including acoustic data. A data quality study revealed that the NFAC 40- by 80-Foot Wind Tunnel is an adequate acoustic environment to test the Bell 699 Rotor for helicopter, conversion, and airplane configurations. Representative acoustic data are presented, and selected acoustic data and corresponding test conditions are included
Breaking a chaos-noise-based secure communication scheme
This paper studies the security of a secure communication scheme based on two
discrete-time intermittently-chaotic systems synchronized via a common random
driving signal. Some security defects of the scheme are revealed: 1) the key
space can be remarkably reduced; 2) the decryption is insensitive to the
mismatch of the secret key; 3) the key-generation process is insecure against
known/chosen-plaintext attacks. The first two defects mean that the scheme is
not secure enough against brute-force attacks, and the third one means that an
attacker can easily break the cryptosystem by approximately estimating the
secret key once he has a chance to access a fragment of the generated
keystream. Yet it remains to be clarified if intermittent chaos could be used
for designing secure chaotic cryptosystems.Comment: RevTeX4, 11 pages, 15 figure
Accelerated freshening of Antarctic Bottom Water over the last decade in the Southern Indian Ocean
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science Advances 3 (2017): e1601426, doi:10.1126/sciadv.1601426.Southern Ocean abyssal waters, in contact with the atmosphere at their formation sites around Antarctica, not only bring signals of a changing climate with them as they move around the globe but also contribute to that change through heat uptake and sea level rise. A repeat hydrographic line in the Indian sector of the Southern Ocean, occupied three times in the last two decades (1994, 2007, and, most recently, 2016), reveals that Antarctic Bottom Water (AABW) continues to become fresher (0.004 ± 0.001 kg/g decadeâ1), warmer (0.06° ± 0.01°C decadeâ1), and less dense (0.011 ± 0.002 kg/m3 decadeâ1). The most recent observations in the Australian-Antarctic Basin show a particularly striking acceleration in AABW freshening between 2007 and 2016 (0.008 ± 0.001 kg/g decadeâ1) compared to the 0.002 ± 0.001 kg/g decadeâ1 seen between 1994 and 2007. Freshening is, in part, responsible for an overall shift of the mean temperature-salinity curve toward lower densities. The marked freshening may be linked to an abrupt iceberg-glacier collision and calving event that occurred in 2010 on the George V/AdĂ©lie Land Coast, the main source region of bottom waters for the Australian-Antarctic Basin. Because AABW is a key component of the global overturning circulation, the persistent decrease in bottom water density and the associated increase in steric height that result from continued warming and freshening have important consequences beyond the Southern Indian Ocean.The 2016 I08S cruise and the analysis and science performed at sea, as well as the individual principal investigators were funded through multiple National Oceanic and Atmospheric Administration (NOAA) and NSF grants including NSF grant OCE-1437015. The research for this article was mainly completed at sea. For land-based work, V.V.M. relied on her postdoctoral funding through NSF grant OCE-1435665, and A.M.M. was supported in part by NSF grant OCE-1356630 and NOAA grant NA11OAR4310063
Hot-Film and Hot-Wire Anemometry for a Boundary Layer Active Flow Control Test
Unsteady active flow control (AFC) has been used experimentally for many years to minimize bluff-body drag. This technology could significantly improve performance of rotorcraft by cleaning up flow separation. It is important, then, that new actuator technologies be studied for application to future vehicles. A boundary layer wind tunnel was constructed with a 1ft-x-3ft test section and unsteady measurement instrumentation to study how AFC manipulates the boundary layer to overcome adverse pressure gradients and flow separation. This unsteady flow control research requires unsteady measurement methods. In order to measure the boundary layer characteristics, both hot-wire and hot-film Constant Temperature Anemometry is used. A hot-wire probe is mounted in the flow to measure velocity while a hot-film array lays on the test surface to measure skin friction. Hot-film sensors are connected to an anemometer, a Wheatstone bridge circuit with an output that corresponds to the dynamic flow response. From this output, the time varying flow field, turbulence, and flow reversal can be characterized. Tuning the anemometers requires a fan test on the hot-film sensors to adjust each output. This is a delicate process as several variables drastically affect the data, including control resistance, signal input, trim, and gain settings
Reflection and Ducting of Gravity Waves Inside the Sun
Internal gravity waves excited by overshoot at the bottom of the convection
zone can be influenced by rotation and by the strong toroidal magnetic field
that is likely to be present in the solar tachocline. Using a simple Cartesian
model, we show how waves with a vertical component of propagation can be
reflected when traveling through a layer containing a horizontal magnetic field
with a strength that varies with depth. This interaction can prevent a portion
of the downward-traveling wave energy flux from reaching the deep solar
interior. If a highly reflecting magnetized layer is located some distance
below the convection zone base, a duct or wave guide can be set up, wherein
vertical propagation is restricted by successive reflections at the upper and
lower boundaries. The presence of both upward- and downward-traveling
disturbances inside the duct leads to the existence of a set of horizontally
propagating modes that have significantly enhanced amplitudes. We point out
that the helical structure of these waves makes them capable of generating an
alpha-effect, and briefly consider the possibility that propagation in a shear
of sufficient strength could lead to instability, the result of wave growth due
to over-reflection.Comment: 23 pages, 5 figures. Accepted for publication in Solar Physic
Modelling element distributions in the atmospheres of magnetic Ap stars
In recent papers convincing evidence has been presented for chemical
stratification in Ap star atmospheres, and surface abundance maps have been
shown to correlate with the magnetic field direction. Radiatively driven
diffusion in magnetic fields is among the processes responsible for these
inhomogeneities. Here we explore the hypothesis that equilibrium
stratifications can, in a number of cases, explain the observed abundance maps
and vertical distributions of the various elements. The investigation of
equilibrium stratifications in stellar atmospheres with temperatures from 8500K
to 12000K and fields up to 10 kG reveals considerable variations in the
vertical distribution of the 5 elements studied (Mg, Si, Ca, Ti, Fe), often
with zones of large over- or under-abundances and with indications of other
competing processes (such as mass loss). Horizontal magnetic fields can be very
efficient in helping the accumulation of elements in higher layers. A
comparison between our calculations and the vertical abundance profiles and
surface maps derived by magnetic Doppler imaging reveals that equilibrium
stratifications are in a number of cases consistent with the main trends
inferred from observed spectra. However, it is not clear whether such
equilibrium solutions will ever be reached during the evolution of an Ap star.Comment: 7 pages, 6 figures, the paper will be published in Astronomy &
Astrophysics, on November 200
Gravity Waves in the Sun
We present numerical simulations of penetrative convection and gravity wave
excitation in the Sun. Gravity waves are self-consistently generated by a
convective zone overlying a radiative interior. We produce power spectra for
gravity waves in the radiative region as well as estimates for the energy flux
of gravity waves below the convection zone. We calculate a peak energy flux in
waves below the convection zone to be three orders of magnitude smaller than
previous estimates for m=1. The simulations show that the linear dispersion
relation is a good approximation only deep below the convective-radiative
boundary. Both low frequency propagating gravity waves as well as higher
frequency standing modes are generated; although we find that convection does
not continually drive the standing g-mode frequencies.Comment: 22 pages, 14 figures, submitted to MNRA
On the origin of solar wind. Alfven waves induced jump of coronal temperature
Absorbtion of Alfven waves is considered to be the main mechanism of heating
in the solar corona. It is concluded that the sharp increase of the plasma
temperature by two orders of magnitude is related to a self-induced opacity
with respect to Alfven waves. The maximal frequency for propagation of Alfven
waves is determined by the strongly temperature dependent kinematic viscosity.
In such a way the temperature jump is due to absorption of high frequency
Alfven waves in a narrow layer above the solar surface. It is calculated that
the power per unit area dissipated in this layer due to damping of Alfven waves
blows up the plasma and gives birth to the solar wind. A model short
wave-length (WKB) evaluation takes into account the 1/f^2 frequency dependance
of the transversal magnetic field and velocity spectral densities. Such
spectral densities agree with old magnetometric data taken by Voyager 1 and
recent theoretical calculations in the framework of Langevin-Burgers MHD. The
present theory predicts existence of intensive high frequency MHD Alfven waves
in the cold layer beneath the corona. It is briefly discussed how this
statement can be checked experimentally. It is demonstrated that the magnitude
of the Alfven waves generating random noise and the solar wind velocity can be
expressed only in terms of satellite experimental data. It is advocated that
investigation of properties of the solar surface as a random driver by optical
methods is an important task for future solar physics. Jets of accretion disks
are speculated as a special case of the wind from magnetized turbulent plasma.Comment: 4 pages, no figures, minor corrections, final version for EPJ
The evolution of X-ray emission in young stars
We study the relation between age and magnetic activity in late-type pre-main
sequence (PMS) stars, for the first time using mass-stratified subsamples. The
effort is based on the Chandra Orion Ultradeep Project (COUP) which provides
very sensitive and homogenous X-ray data on a uniquely large sample of 481
optically well-characterized low-extinction low-mass members of the Orion
Nebula Cluster, for which individual stellar masses and ages could be
determined. More than 98 percent of the stars in this sample are detected as
X-ray sources. Within the PMS phase for stellar ages in the range
Myr, we establish a mild decay in activity with stellar age roughly as
. On longer timescales, when the Orion stars are
compared to main sequence stars, the X-ray luminosity decay law for stars in
the M mass range is more rapid with over the wide range of ages yr. The
magnetic activity history for M stars with masses is
distinctly different. Only a mild decrease in X-ray luminosity, and even a mild
increase in and , is seen over the 1-100 Myr
range, though the X-ray emission does decay over long timescales on the main
sequence. Together with COUP results on the absence of a rotation-activity
relation in Orion stars, we find that the activity-age decay is strong across
the entire history of solar-type stars but is not attributable to rotational
deceleration during the early epochs. A combination of tachocline and
distributed convective dynamos may be operative in young solar-type stars. The
results for the lowest mass stars are most easily understood by the dominance
of convective dynamos during both the PMS and main sequence phases.Comment: accepted for ApJS, COUP special issu
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