436 research outputs found
MinoanER: Schema-Agnostic, Non-Iterative, Massively Parallel Resolution of Web Entities
Entity Resolution (ER) aims to identify different descriptions in various
Knowledge Bases (KBs) that refer to the same entity. ER is challenged by the
Variety, Volume and Veracity of entity descriptions published in the Web of
Data. To address them, we propose the MinoanER framework that simultaneously
fulfills full automation, support of highly heterogeneous entities, and massive
parallelization of the ER process. MinoanER leverages a token-based similarity
of entities to define a new metric that derives the similarity of neighboring
entities from the most important relations, as they are indicated only by
statistics. A composite blocking method is employed to capture different
sources of matching evidence from the content, neighbors, or names of entities.
The search space of candidate pairs for comparison is compactly abstracted by a
novel disjunctive blocking graph and processed by a non-iterative, massively
parallel matching algorithm that consists of four generic, schema-agnostic
matching rules that are quite robust with respect to their internal
configuration. We demonstrate that the effectiveness of MinoanER is comparable
to existing ER tools over real KBs exhibiting low Variety, but it outperforms
them significantly when matching KBs with high Variety.Comment: Presented at EDBT 2001
Investigation of Submergence Depth and Wave-Induced Effects on the Performance of a Fully Passive Energy Harvesting Flapping Foil Operating Beneath the Free Surface
This paper investigates the performance of a fully passive flapping foil
device for energy harvesting in a free surface flow. The study uses numerical
simulations to examine the effects of varying submergence depths and the impact
of monochromatic waves on the foil's performance. The results show that the
fully passive flapping foil device can achieve high efficiency for submergence
depths between 4 and 9 chords, with an "optimum" submergence depth where the
flapping foil performance is maximised. The performance was found to be
correlated with the resonant frequency of the heaving motion and its proximity
to the damped natural frequency. The effects of regular waves on the foil's
performance were also investigated, showing that waves with a frequency close
to that of the natural frequency of the flapping foil aided energy harvesting.
Overall, this study provides insights that could be useful for future design
improvements for fully passive flapping foil devices for energy harvesting
operating near the free surface
On the role of laminar/turbulent interface on energy transfer between scales in bypass transition
We investigate the role of laminar/turbulent interface in the interscale
energy transfer in a boundary layer undergoing bypass transition, with the aid
of the Karman-Howarth-Monin-Hill (KHMH) equation. A local binary indicator
function is used to detect the interface and employed subsequently to define
two-point intermittencies. These are used to decompose the standard-averaged
interscale and interspace energy fluxes into conditionally-averaged components.
We find that the inverse cascade in the streamwise direction reported in an
earlier work arises due to events across the downstream or upstream interfaces
(head or tail respectively) of a turbulent spot. However, the three-dimensional
energy flux maps reveal significant differences between these two regions: in
the downstream interface, inverse cascade is stronger and dominant over a
larger range of streamwise and spanwise separations. We explain this finding by
considering a propagating spot of simplified shape as it crosses a fixed
streamwise location. We derive also the conditionally-averaged KHMH equation,
thus generalising similar equations for single-point statistics to two-point
statistics. We compare the three-dimensional maps of the conditionally-averaged
production and total energy flux within turbulent spots against the maps of
standard-averaged quantities within the fully turbulent region. The results
indicate remarkable dynamical similarities between turbulent spots and the
fully turbulent region for two-point statistics. This has been known only for
single-point quantities, and we show here that the similarity extends to
two-point quantities as well
Reconstruction of irregular flow dynamics around two square cylinders from sparse measurements using a data-driven algorithm
We propose a data-driven algorithm for reconstructing the irregular, chaotic
flow dynamics around two side-by-side square cylinders from sparse,
time-resolved, velocity measurements in the wake. We use Proper Orthogonal
Decomposition (POD) to reduce the dimensionality of the problem and then
explore two different reconstruction approaches: in the first approach, we use
the subspace system identification algorithm n4sid to extract a linear
dynamical model directly from the data (including the modelling and measurement
error covariance matrices) and then employ Kalman filter theory to synthesize a
linearly optimal estimator. In the second approach, the estimator matrices are
directly identified using n4sid. A systematic study reveals that the first
strategy outperforms the second in terms of reconstruction accuracy, robustness
and computational efficiency. We also consider the problem of sensor placement.
A greedy approach based on the QR pivoting algorithm is compared against
sensors placed at the POD mode peaks; we show that the former approach is more
accurate in recovering the flow characteristics away from the cylinders. We
demonstrate that a linear dynamic model with a sufficiently large number of
states and relatively few measurements, can recover accurately complex flow
features, such as the interaction of the irregular flapping motion of the jet
emanating from the gap with the vortices shed from the cylinders as well as the
convoluted patterns downstream arising from the amalgamation of the individual
wakes. The proposed methodology is entirely data-driven, does not have tunable
parameters, and the resulting matrices are unique (to within a linear
coordinate transformation of the state vector). The method can be applied
directly to either experimental or computational data.Comment: 33 pages, 23 figures, 1 tabl
Sensitivity analysis of chaotic systems using a frequency-domain shadowing approach
We present a frequency-domain method for computing the sensitivities of
time-averaged quantities of chaotic systems with respect to input parameters.
Such sensitivities cannot be computed by conventional adjoint analysis tools,
because the presence of positive Lyapunov exponents leads to exponential growth
of the adjoint variables. The proposed method is based on the least-square
shadowing (LSS) approach [1], that formulates the evaluation of sensitivities
as an optimisation problem, thereby avoiding the exponential growth of the
solution. However, all existing formulations of LSS (and its variants) are in
the time domain and the computational cost scales with the number of positive
Lyapunov exponents. In the present paper, we reformulate the LSS method in the
Fourier space using harmonic balancing. The new method is tested on the
Kuramoto-Sivashinski system and the results match with those obtained using the
standard time-domain formulation. Although the cost of the direct solution is
independent of the number of positive Lyapunov exponents, storage and computing
requirements grow rapidly with the size of the system. To mitigate these
requirements, we propose a resolvent-based iterative approach that needs much
less storage. Application to the Kuramoto-Sivashinski system gave accurate
results with very low computational cost. The method is applicable to large
systems and paves the way for application of the resolvent-based shadowing
approach to turbulent flows. Further work is needed to assess its performance
and scalability
Sensitivity-enhanced generalized polynomial chaos for efficient uncertainty quantification
We present an enriched formulation of the Least Squares (LSQ) regression
method for Uncertainty Quantification (UQ) using generalised polynomial chaos
(gPC). More specifically, we enrich the linear system with additional equations
for the gradient (or sensitivity) of the Quantity of Interest with respect to
the stochastic variables. This sensitivity is computed very efficiently for all
variables by solving an adjoint system of equations at each sampling point of
the stochastic space. The associated computational cost is similar to one
solution of the direct problem. For the selection of the sampling points, we
apply a greedy algorithm which is based on the pivoted QR decomposition of the
measurement matrix. We call the new approach sensitivity-enhanced generalised
polynomial chaos, or se-gPC. We apply the method to several test cases to test
accuracy and convergence with increasing chaos order, including an aerodynamic
case with stochastic parameters. The method is found to produce accurate
estimations of the statistical moments using the minimum number of sampling
points. The computational cost scales as , instead of
of the standard LSQ formulation, where is the number of stochastic
variables and the chaos order. The solution of the adjoint system of
equations is implemented in many computational mechanics packages, thus the
infrastructure exists for the application of the method to a wide variety of
engineering problems
- …