14,714 research outputs found
Growth mechanisms of perturbations in boundary layers over a compliant wall
The temporal modal and nonmodal growth of three-dimensional perturbations in
the boundary-layer flow over an infinite compliant flat wall is considered.
Using a wall-normal velocity/wall-normal vorticity formalism, the dynamic
boundary condition at the compliant wall admits a linear dependence on the
eigenvalue parameter, as compared to a quadratic one in the canonical
formulation of the problem. This greatly simplifies the accurate calculation of
the continuous spectrum by means of a spectral method, thereby yielding a very
effective filtering of the pseudospectra as well as a clear identification of
instability regions. The regime of global instability is found to be matching
the regime of the favorable phase of the forcing by the flow on the compliant
wall so as to enhance the amplitude of the wall. An energy-budget analysis for
the least-decaying hydroelastic (static-divergence, traveling-wave-flutter and
near-stationary transitional) and Tollmien--Schlichting modes in the parameter
space reveals the primary routes of energy flow. Moreover, the flow exhibits a
slower transient growth for the maximum growth rate of a superposition of
streamwise-independent modes due to a complex dependence of the wall-boundary
condition with the Reynolds number. The initial and optimal perturbations are
compared with the boundary-layer flow over a solid wall; differences and
similarities are discussed. Unlike the solid-wall case, viscosity plays a
pivotal role in the transient growth. A slowdown of the maximum growth rate
with the Reynolds number is uncovered and found to originate in the transition
of the fluid-solid interaction from a two-way to a one-way coupling. Finally, a
term-by-term energy budget analysis is performed to identify the key
contributors to the transient growth mechanism
Effects of wall compliance on the laminar–turbulent transition of torsional Couette flow
Torsional Couette flow between a rotating disk and a stationary wall is studied experimentally. The surface of the disk is either rigid or covered with a compliant coating. The influence of wall compliance on characteristic flow instabilities and on the laminar–turbulent flow transition is investigated. Data obtained from analysing flow visualizations are discussed. It is found that wall compliance favours two of the three characteristic wave patterns associated with the transition process and broadens the parameter regime in which these patterns are observed. The results for the effects of wall compliance on the third pattern are inconclusive. However, the experiments indicate that the third pattern is not a primary constituent of the laminar–turbulent transition process of torsional Couette flow
Ysovar: The First Sensitive, Wide-area, Mid-infrared Photometric Monitoring of the Orion Nebula Cluster
We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs
Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster
We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)
and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most
massive star in the Orion Nebula Cluster (ONC). At the age and distance of this
cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo)
occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our
photometry is complete for source detection at the 7 sigma level to K~17.5 mag
and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual
extinction values as high as 10 magnitudes. We use the observed magnitudes,
colors, and star counts to constrain the shape of the inner ONC stellar mass
function across the hydrogen burning limit. After determining the stellar age
and near-infrared excess properties of the optically visible stars in this same
inner ONC region, we present a new technique that incorporates these
distributions when extracting the mass function from the observed density of
stars in the K-(H-K) diagram. We find that our data are inconsistent with a
mass function that rises across the stellar/sub-stellar boundary. Instead, we
find that the most likely form of the inner ONC mass function is one that rises
to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit
with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply
to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC
as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal.
Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc
Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements
The complex elastic compliance and dielectric susceptibility of
(Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the
composition range between pure NBT and the morphotropic phase boundary
included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the
rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the
determination of the tetragonal region of the phase diagram, up to now
impossible due to the strong lattice disorder and small distortions and
polarizations involved. In spite of ample evidence of disorder and structural
heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C
transition merges into the diffuse and relaxor-like transition associated with
broad maxima of the dielectric and elastic susceptibilities. An attempt is made
at relating the different features in the anelastic and dielectric curves to
different modes of octahedral rotations and polar cation shifts. The
possibility is also considered that the cation displacements locally have
monoclinic symmetry, as for PZT near the morphotropic phase boundary.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Dirac Gauginos, Negative Supertraces and Gauge Mediation
In an attempt to maximize General Gauge Mediated parameter space, I propose
simple models in which gauginos and scalars are generated from disconnected
mechanisms. In my models Dirac gauginos are generated through the supersoft
mechanism, while independent R-symmetric scalar masses are generated through
operators involving non-zero messenger supertrace. I propose several new
methods for generating negative messenger supertraces which result in viable
positive mass squareds for MSSM scalars. The resultant spectra are novel,
compressed and may contain light fermionic SM adjoint fields.Comment: 16 pages 3 figure
BVRI Light Curves for 22 Type Ia Supernovae
We present 1210 Johnson/Cousins B,V,R, and I photometric observations of 22
recent type Ia supernovae (SNe Ia): SN 1993ac, SN 1993ae, SN 1994M, SN 1994S,
SN 1994T, SN 1994Q, SN 1994ae, SN 1995D, SN 1995E, SN 1995al, SN 1995ac, SN
1995ak, SN 1995bd, SN 1996C, SN 1996X, SN 1996Z, SN 1996ab, SN 1996ai, SN
1996bk, SN 1996bl, SN 1996bo, and SN 1996bv. Most of the photometry was
obtained at the Fred Lawrence Whipple Observatory (FLWO) of the
Harvard-Smithsonian Center for Astrophysics in a cooperative observing plan
aimed at improving the data base for SN Ia. The redshifts of the sample range
from =1200 to 37000 km s with a mean of =7000 km s.Comment: Accepted to the Astronomical Journal, 41 pages, 8 figure
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