1,215 research outputs found
Some experiences using wind-tunnel models in active control studies
A status report and review of wind tunnel model experimental techniques that have been developed to study and validate the use of active control technology for the minimization of aeroelastic response are presented. Modeling techniques, test procedures, and data analysis methods used in three model studies are described. The studies include flutter mode suppression on a delta-wing model, flutter mode suppression and ride quality control on a 1/30-size model of the B-52 CCV airplane, and an active lift distribution control system on a 1/22 size C-5A model
Active controls: A look at analytical methods and associated tools
A review of analytical methods and associated tools for active controls analysis and design problems is presented. Approaches employed to develop mathematical models suitable for control system analysis and/or design are discussed. Significant efforts have been expended to develop tools to generate the models from the standpoint of control system designers' needs and develop the tools necessary to analyze and design active control systems. Representative examples of these tools are discussed. Examples where results from the methods and tools have been compared with experimental data are also presented. Finally, a perspective on future trends in analysis and design methods is presented
Holographic metastability
We show how supersymmetric QCD in a slice of AdS can naturally acquire
metastable vacua. The formulation closely follows that of Intriligator, Seiberg
and Shih (ISS), with an "electric" sector on the UV brane and a "magnetic"
sector on the IR brane. However the 't Hooft anomaly matching that constrains
the Seiberg duality central to ISS is replaced by anomaly inflow and
cancellation, and the source of strong coupling is the CFT to which the theory
couples rather than the gauge groups. The theory contains an anomaly free
R-symmetry that, when broken by UV effects, leads to an O'Raifeartaigh model on
the IR brane. In contrast to ISS, the R-symmetry breaking in the UV can be
maximal, and yet the R-symmetry breaking in the IR theory remains under strict
control: there is no need for retrofitting of small parameters.Comment: 20 pages, 2 figure
The signature of the first stars in atomic hydrogen at redshift 20
Dark and baryonic matter moved at different velocities in the early Universe,
which strongly suppressed star formation in some regions. This was estimated to
imprint a large-scale fluctuation signal of about 2 mK in the 21-cm spectral
line of atomic hydrogen associated with stars at a redshift of 20, although
this estimate ignored the critical contribution of gas heating due to X-rays
and major enhancements of the suppression. A large velocity difference reduces
the abundance of halos and requires the first stars to form in halos of about a
million solar masses, substantially greater than previously expected. Here we
report a simulation of the distribution of the first stars at z=20 (cosmic age
of ~180 Myr), incorporating all these ingredients within a 400 Mpc box. We find
that the 21-cm signature of these stars is an enhanced (10 mK) fluctuation
signal on the 100-Mpc scale, characterized by a flat power spectrum with
prominent baryon acoustic oscillations. The required sensitivity to see this
signal is achievable with an integration time of a thousand hours with an
instrument like the Murchison Wide-field Array or the Low Frequency Array but
designed to operate in the range of 50-100 MHz.Comment: 27 pages, 5 figures, close (but not exact) match to accepted version.
Basic results unchanged from first submitted version, but justification
strengthened, title and abstract modified, and substantial Supplementary
Material added. Originally first submitted for publication on Oct. 12, 201
CP Violation in Supersymmetry with Effective Minimal Flavour Violation
We analyze CP violation in supersymmetry with Effective Minimal Flavour
Violation, as recently proposed in arXiv:1011.0730. Unlike the case of standard
Minimal Flavour Violation, we show that all the phases allowed by the flavour
symmetry can be sizable without violating existing Electric Dipole Moment
constraints, thus solving the SUSY CP problem. The EDMs at one and two loops
are precisely analyzed as well as their correlations with the expected CP
asymmetries in B physics.Comment: 22 pages, 7 figures. v2: Discussion in section 2 extended,
conclusions unchanged. Matches published versio
Size and frequency of natural forest disturbances and Amazon carbon balance
Forest inventory studies in the Amazon indicate a large terrestrial carbon sink. However, field plots may fail to represent forest mortality processes at landscape-scales of tropical forests. Here we characterize the frequency distribution of disturbance events in natural forests from 0.01 ha to 2,651 ha size throughout Amazonia using a novel combination of forest inventory, airborne lidar and satellite remote sensing data. We find that small-scale mortality events are responsible for aboveground biomass losses of B1.28 Pg C y 1 over the entire Amazon region. We also find that intermediate-scale disturbances account for losses of B0.01 Pg C y 1 , and that the largest-scale disturbances as a result of blow-downs only account for losses of B0.003 Pg C y 1 . Simulation of growth and mortality indicates that even when all carbon losses from intermediate and large-scale disturbances are considered, these are outweighed by the net biomass accumulation by tree growth, supporting the inference of an Amazon carbon sink
General Gauge Mediation at the Weak Scale
We completely characterize General Gauge Mediation (GGM) at the weak scale by
solving all IR constraints over the full parameter space. This is made possible
through a combination of numerical and analytical methods, based on a set of
algebraic relations among the IR soft masses derived from the GGM boundary
conditions in the UV. We show how tensions between just a few constraints
determine the boundaries of the parameter space: electroweak symmetry breaking
(EWSB), the Higgs mass, slepton tachyons, and left-handed stop/sbottom
tachyons. While these constraints allow the left-handed squarks to be
arbitrarily light, they place strong lower bounds on all of the right-handed
squarks. Meanwhile, light EW superpartners are generic throughout much of the
parameter space. This is especially the case at lower messenger scales, where a
positive threshold correction to coming from light Higgsinos and winos is
essential in order to satisfy the Higgs mass constraint.Comment: 43 pages, 20 figures, mathematica package included in the sourc
Electroweak Symmetry Breaking Beyond the Standard Model
In this talk, I shall address two key issues related to electroweak symmetry
breaking. First, how fine-tuned different models are that trigger this
phenomenon? Second, even if a light Higgs boson exists, does it have to be
necessarily elementary? After a brief introduction, I shall first review the
fine-tuning aspects of the MSSM, NMSSM, generalized NMSSM and GMSB scenarios. I
shall then compare and contrast the little Higgs, composite Higgs and the
Higgsless models. Finally, I shall summarize by giving a broad overview on
where we stand at the end of 2011.Comment: Plenary talk at the International Lepton-Photon Conference, Mumbai,
August 2011. To appear in the proceedings (special issue of PRAMANA
Cosmological vacuum selection and metastable susy breaking
We study gauge mediation in a wide class of O'Raifeartaigh type models where
supersymmetry breaking metastable vacuum is created by gravity and/or quantum
corrections. We examine their thermal evolution in the early universe and the
conditions under which the susy breaking vacuum can be selected. It is
demonstrated that thermalization typically makes the metastable supersymmetry
breaking cosmologically disfavoured but this is not always the case. Initial
conditions with the spurion displaced from the symmetric thermal minimum and a
small coupling to the messenger sector can result in the realization of the
susy breaking vacuum even if the reheating temperature is high. We show that
this can be achieved without jeopardizing the low energy phenomenology. In
addition, we have found that deforming the models by a supersymmetric mass term
for messengers in such a way that the susy breaking minimum and the susy
preserving minima are all far away from the origin does not change the
conclusions. The basic observations are expected to hold also in the case of
models with an anomalous U(1) group.Comment: 28 pages, 4 figures, plain Latex, journal versio
Closed-loop separation control over a sharp edge ramp using Genetic Programming
We experimentally perform open and closed-loop control of a separating
turbulent boundary layer downstream from a sharp edge ramp. The turbulent
boundary layer just above the separation point has a Reynolds number
based on momentum thickness. The goal of the
control is to mitigate separation and early re-attachment. The forcing employs
a spanwise array of active vortex generators. The flow state is monitored with
skin-friction sensors downstream of the actuators. The feedback control law is
obtained using model-free genetic programming control (GPC) (Gautier et al.
2015). The resulting flow is assessed using the momentum coefficient, pressure
distribution and skin friction over the ramp and stereo PIV. The PIV yields
vector field statistics, e.g. shear layer growth, the backflow area and vortex
region. GPC is benchmarked against the best periodic forcing. While open-loop
control achieves separation reduction by locking-on the shedding mode, GPC
gives rise to similar benefits by accelerating the shear layer growth.
Moreover, GPC uses less actuation energy.Comment: 24 pages, 24 figures, submitted to Experiments in Fluid
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