6,069 research outputs found
Effects of a nonadiabatic wall on supersonic shock/boundary-layer interactions
Direct numerical simulations are employed to investigate a shock wave impinging on a turbulent boundary layer at free-stream Mach number M=2.28 with different wall thermal conditions, including adiabatic, cooled, and heated, for a wide range of deflection angles. It is found that the thermal boundary condition at the wall has a large effect on the size of the interaction region and on the level of pressure fluctuations. The distribution of the Stanton number shows a good agreement with prior experimental studies and confirms the strong heat transfer and complex pattern within the interaction region. An effort was also made to describe the unsteady features of the flow by means of wall pressure and heat flux spectra. Numerical results indicate that the changes in the interaction length due to the wall thermal condition are mainly linked to the incoming boundary layer, which is in agreement with previous experimental studies
Events leading up to the June 2015 outburst of V404 Cyg
On 2015 June 15 the burst alert telescope (BAT) on board {\em Swift} detected
an X-ray outburst from the black hole transient V404 Cyg. We monitored V404 Cyg
for the last 10 years with the 2-m Faulkes Telescope North in three optical
bands (V, R, and i). We found that, one week prior to this outburst, the
optical flux was 0.1--0.3 mag brighter than the quiescent orbital modulation,
implying an optical precursor to the X-ray outburst. There is also a hint of a
gradual optical decay (years) followed by a rise lasting two months prior to
the outburst. We fortuitously obtained an optical spectrum of V404 Cyg 13 hours
before the BAT trigger. This too was brighter () than
quiescence, and showed spectral lines typical of an accretion disk, with
characteristic absorption features of the donor being much weaker. No He II
emission was detected, which would have been expected had the X-ray flux been
substantially brightening. This, combined with the presence of intense
H emission, about 7 times the quiescent level, suggests that the disk
entered the hot, outburst state before the X-ray outburst began. We propose
that the outburst is produced by a viscous-thermal instability triggered close
to the inner edge of a truncated disk. An X-ray delay of a week is consistent
with the time needed to refill the inner region and hence move the inner edge
of the disk inwards, allowing matter to reach the central BH, finally turning
on the X-ray emission.Comment: Accepted by ApJ Letter, 7 pages, 5 figure
Non-linear macroscopic polarization in III-V nitride alloys
We study the dependence of macroscopic polarization on composition and strain
in wurtzite III-V nitride ternary alloys using ab initio density-functional
techniques. The spontaneous polarization is characterized by a large bowing,
strongly dependent on the alloy microscopic structure. The bowing is due to the
different response of the bulk binaries to hydrostatic pressure, and to
internal strain effects (bond alternation). Disorder effects are instead minor.
Deviations from parabolicity (simple bowing) are of order 10 % in the most
extreme case of AlInN alloy, much less at all other compositions. Piezoelectric
polarization is also strongly non-linear. At variance with the spontaneous
component, this behavior is independent of microscopic alloy structure or
disorder effects, and due entirely to the non-linear strain dependence of the
bulk piezoelectric response. It is thus possible to predict the piezoelectric
polarization for any alloy composition using the piezoelectricity of the parent
binaries.Comment: RevTex 7 pages, 7 postscript figures embedde
Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures
The free-carrier screening of macroscopic polarization fields in wurtzite
GaN/InGaN quantum wells lasers is investigated via a self-consistent
tight-binding approach. We show that the high carrier concentrations found
experimentally in nitride laser structures effectively screen the built-in
spontaneous and piezoelectric polarization fields, thus inducing a
``field-free'' band profile. Our results explain some heretofore puzzling
experimental data on nitride lasers, such as the unusually high lasing
excitation thresholds and emission blue-shifts for increasing excitation
levels.Comment: RevTeX 4 pages, 4 figure
Accurate calculation of polarization-related quantities in semiconductors
We demonstrate that polarization-related quantities in semiconductors can be
predicted accurately from first-principles calculations using the appropriate
approach to the problem, the Berry-phase polarization theory. For III-V
nitrides, our test case, we find polarizations, polarization differences
between nitride pairs, and piezoelectric constants quite close to their
previously established values. Refined data are nevertheless provided for all
the relevant quantities.Comment: RevTeX 4 pages, no figure
Electronic dielectric constants of insulators by the polarization method
We discuss a non-perturbative, technically straightforward, easy-to-use, and
computationally affordable method, based on polarization theory, for the
calculation of the electronic dielectric constant of insulating solids at the
first principles level. We apply the method to GaAs, AlAs, InN, SiC, ZnO, GaN,
AlN, BeO, LiF, PbTiO, and CaTiO. The predicted \einf's agree well
with those given by Density Functional Perturbation Theory (the reference
theoretical treatment), and they are generally within less than 10 % of
experiment.Comment: RevTeX 4 pages, 2 ps figure
First-principles prediction of structure, energetics, formation enthalpy, elastic constants, polarization, and piezoelectric constants of AlN, GaN, and InN: comparison of local and gradient-corrected density-functional theory
A number of diverse bulk properties of the zincblende and wurtzite III-V
nitrides AlN, GaN, and InN, are predicted from first principles within density
functional theory using the plane-wave ultrasoft pseudopotential method, within
both the LDA (local density) and GGA (generalized gradient) approximations to
the exchange-correlation functional. Besides structure and cohesion, we study
formation enthalpies (a key ingredient in predicting defect solubilities and
surface stability), spontaneous polarizations and piezoelectric constants
(central parameters for nanostructure modeling), and elastic constants. Our
study bears out the relative merits of the two density functional approaches in
describing diverse properties of the III-V nitrides (and of the parent species
N, Al, Ga, and In), and leads us to conclude that the GGA approximation,
associated with high-accuracy techniques such as multiprojector ultrasoft
pseudopotentials or modern all-electron methods, is to be preferred in the
study of III-V nitrides.Comment: RevTeX 6 pages, 12 tables, 0 figure
A Virtual Conversational Agent for Teens with Autism: Experimental Results and Design Lessons
We present the design of an online social skills development interface for
teenagers with autism spectrum disorder (ASD). The interface is intended to
enable private conversation practice anywhere, anytime using a web-browser.
Users converse informally with a virtual agent, receiving feedback on nonverbal
cues in real-time, and summary feedback. The prototype was developed in
consultation with an expert UX designer, two psychologists, and a pediatrician.
Using the data from 47 individuals, feedback and dialogue generation were
automated using a hidden Markov model and a schema-driven dialogue manager
capable of handling multi-topic conversations. We conducted a study with nine
high-functioning ASD teenagers. Through a thematic analysis of post-experiment
interviews, identified several key design considerations, notably: 1) Users
should be fully briefed at the outset about the purpose and limitations of the
system, to avoid unrealistic expectations. 2) An interface should incorporate
positive acknowledgment of behavior change. 3) Realistic appearance of a
virtual agent and responsiveness are important in engaging users. 4)
Conversation personalization, for instance in prompting laconic users for more
input and reciprocal questions, would help the teenagers engage for longer
terms and increase the system's utility
- …