10,830 research outputs found
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Densely distributed and real-time scour hole monitoring using piezoelectric rod sensors
This study aims to validate a piezoelectric driven-rod scour monitoring system that can sense changes in scour depth along the entire rod at its instrumented location. The proposed sensor is a polymeric slender rod with a thin strip of polyvinylidene fluoride that runs through its midline. Extraction of the fundamental frequency allows the direct calculation of the exposed length (or scour depth) of the slender rod undergoing fluid flow excitation. First, laboratory validation in dry conditions is presented. Second, hydrodynamic testing of the sensor system in a soil-bed flume is discussed. Each rod was installed using a three-dimensional-printed footing designed for ease of installation and stabilization during testing. The sensors were installed in a layout designed to capture symmetric scour conditions around a scaled pier. In order to analyze the system out of steady-state conditions, water velocity was increased in stages during testing to induce different degrees of scour. As ambient water flow excited the portion of the exposed rods, the embedded piezoelectric element outputted a time-varying voltage signal. Different methods were then employed to extract the fundamental frequency of each rod, and the results were compared. Further testing was also performed to characterize the relationship between frequency outputs and flow velocity, which were previously thought to be independent. In general, the proposed driven-rod scour monitoring system successfully captured changing frequencies under varied flow conditions
Structure estimation for discrete graphical models: Generalized covariance matrices and their inverses
We investigate the relationship between the structure of a discrete graphical
model and the support of the inverse of a generalized covariance matrix. We
show that for certain graph structures, the support of the inverse covariance
matrix of indicator variables on the vertices of a graph reflects the
conditional independence structure of the graph. Our work extends results that
have previously been established only in the context of multivariate Gaussian
graphical models, thereby addressing an open question about the significance of
the inverse covariance matrix of a non-Gaussian distribution. The proof
exploits a combination of ideas from the geometry of exponential families,
junction tree theory and convex analysis. These population-level results have
various consequences for graph selection methods, both known and novel,
including a novel method for structure estimation for missing or corrupted
observations. We provide nonasymptotic guarantees for such methods and
illustrate the sharpness of these predictions via simulations.Comment: Published in at http://dx.doi.org/10.1214/13-AOS1162 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
High-dimensional regression with noisy and missing data: Provable guarantees with nonconvexity
Although the standard formulations of prediction problems involve
fully-observed and noiseless data drawn in an i.i.d. manner, many applications
involve noisy and/or missing data, possibly involving dependence, as well. We
study these issues in the context of high-dimensional sparse linear regression,
and propose novel estimators for the cases of noisy, missing and/or dependent
data. Many standard approaches to noisy or missing data, such as those using
the EM algorithm, lead to optimization problems that are inherently nonconvex,
and it is difficult to establish theoretical guarantees on practical
algorithms. While our approach also involves optimizing nonconvex programs, we
are able to both analyze the statistical error associated with any global
optimum, and more surprisingly, to prove that a simple algorithm based on
projected gradient descent will converge in polynomial time to a small
neighborhood of the set of all global minimizers. On the statistical side, we
provide nonasymptotic bounds that hold with high probability for the cases of
noisy, missing and/or dependent data. On the computational side, we prove that
under the same types of conditions required for statistical consistency, the
projected gradient descent algorithm is guaranteed to converge at a geometric
rate to a near-global minimizer. We illustrate these theoretical predictions
with simulations, showing close agreement with the predicted scalings.Comment: Published in at http://dx.doi.org/10.1214/12-AOS1018 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
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Bio-Inspired Active Skins for Surface Morphing.
Mechanical metamaterials that leverage precise geometrical designs and imperfections to induce unique material behavior have garnered significant attention. This study proposes a Bio-Inspired Active Skin (BIAS) as a new class of instability-induced morphable structures, where selective out-of-plane material deformations can be pre-programmed during design and activated by in-plane strains. The deformation mechanism of a unit cell geometrical design is analyzed to identify how the introduction of hinge-like notches or instabilities, versus their pristine counterparts, can pave way for controlling bulk BIAS behavior. Two-dimensional arrays of repeating unit cells were fabricated, with notches implemented at key locations throughout the structure, to harvest the instability-induced surface features for applications such as camouflage, surface morphing, and soft robotic grippers
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Characterizing the Conductivity and Enhancing the Piezoresistivity of Carbon Nanotube-Polymeric Thin Films.
The concept of lightweight design is widely employed for designing and constructing aerospace structures that can sustain extreme loads while also being fuel-efficient. Popular lightweight materials such as aluminum alloy and fiber-reinforced polymers (FRPs) possess outstanding mechanical properties, but their structural integrity requires constant assessment to ensure structural safety. Next-generation structural health monitoring systems for aerospace structures should be lightweight and integrated with the structure itself. In this study, a multi-walled carbon nanotube (MWCNT)-based polymer paint was developed to detect distributed damage in lightweight structures. The thin film's electromechanical properties were characterized via cyclic loading tests. Moreover, the thin film's bulk conductivity was characterized by finite element modeling
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Distributed Strain Sensing Using Electrical Time Domain Reflectometry With Nanocomposites
Estimating Third-Order Moments for an Absorber Catalog
Thanks to the recent availability of large surveys, there has been renewed
interest in third-order correlation statistics. Measures of third-order
clustering are sensitive to the structure of filaments and voids in the
universe and are useful for studying large-scale structure. Thus, statistics of
these third-order measures can be used to test and constrain parameters in
cosmological models. Third-order measures such as the three-point correlation
function are now commonly estimated for galaxy surveys. Studies of third-order
clustering of absorption systems will complement these analyses. We define a
statistic, which we denote K, that measures third-order clustering of a data
set of point observations and focus on estimating this statistic for an
absorber catalog. The statistic K can be considered a third-order version of
the second-order Ripley K-function and allows one to study the abundance of
various configurations of point triplets. In particular, configurations
consisting of point triplets that lie close to a straight line can be examined.
Studying third-order clustering of absorbers requires consideration of the
absorbers as a three-dimensional process, observed on QSO lines of sight that
extend radially in three-dimensional space from Earth. Since most of this
three-dimensional space is not probed by the lines of sight, edge corrections
become important. We use an analytical form of edge correction weights and
construct an estimator of the statistic K for use with an absorber catalog. We
show that with these weights, ratio-unbiased estimates of K can be obtained.
Results from a simulation study also verify unbiasedness and provide
information on the decrease of standard errors with increasing number of lines
of sight.Comment: 19 pages, 4 figure
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