8,423 research outputs found
In Vitro Flow Modelling for Mitral Valve Leakage Quantification
In this study particle image velocimetry (PIV) is used to
measure and visualise the blood flow through a leaking mitral
heart valve. The results are compared with the results from
Doppler echocardiography and computational fluid dynamics
(CFD). Using CAD, five-axis milling and Rapid Prototyping
Machining (RPM) technology, a hydraulic in vitro flow model
was developed and constructed which is compatible with flow
investigation with 2D normal speed PIV and 2D Doppler
echocardiography. The same CAD model was used to conduct the CFD analysis. PIV results compared successfully with Doppler echo and CFD results, both in the upstream
converging region and downstream the turbulent regurgitated
jet zone. These results are expected to improve the assessment of mitral valve regurgitation severity with Doppler echocardiography in clinical practice
Fundamental Relativistic Rotator. Hessian singularity and the issue of the minimal interaction with electromagnetic field
There are two relativistic rotators with Casimir invariants of the
Poincar\'{e} group being fixed parameters. The particular models of spinning
particles were studied in the past both at the classical and quantum level.
Recently, a minimal interaction with electromagnetic field has been considered.
We show that the dynamical systems can be uniquely singled out from among other
relativistic rotators by the unphysical requirement that the Hessian referring
to the physical degrees of freedom should be singular. Closely related is the
fact that the equations of free motion are not independent, making the
evolution indeterminate. We show that the Hessian singularity cannot be removed
by the minimal interaction with the electromagnetic field. By making use of a
nontrivial Hessian null space, we show that a single constraint appears in the
external field for consistency of the equations of motion with the Hessian
singularity. The constraint imposes unphysical limitation on the initial
conditions and admissible motions. We discuss the mechanism of appearance of
unique solutions in external fields on an example of motion in the uniform
magnetic field. We give a simple model to illustrate that similarly constrained
evolution cannot be determinate in arbitrary fields.Comment: 16 pages, in v2: shortened, improved presentation, proofs moved to
Appendices, in v3: further text permutations and a comment added concerning
hamiltonization, in v4: language corrections, final for
Frameworks, Symmetry and Rigidity
Symmetry equations are obtained for the rigidity matrix of a bar-joint
framework in R^d. These form the basis for a short proof of the Fowler-Guest
symmetry group generalisation of the Calladine-Maxwell counting rules. Similar
symmetry equations are obtained for the Jacobian of diverse framework systems,
including constrained point-line systems that appear in CAD, body-pin
frameworks, hybrid systems of distance constrained objects and infinite
bar-joint frameworks. This leads to generalised forms of the Fowler-Guest
character formula together with counting rules in terms of counts of
symmetry-fixed elements. Necessary conditions for isostaticity are obtained for
asymmetric frameworks, both when symmetries are present in subframeworks and
when symmetries occur in partition-derived frameworks.Comment: 5 Figures. Replaces Dec. 2008 version. To appear in IJCG
The separate neural control of hand movements and contact forces
To manipulate an object, we must simultaneously control the contact forces exerted on the object and the movements of our hand. Two alternative views for manipulation have been proposed: one in which motions and contact forces are represented and controlled by separate neural processes, and one in which motions and forces are controlled jointly, by a single process. To evaluate these alternatives, we designed three tasks in which subjects maintained a specified contact force while their hand was moved by a robotic manipulandum. The prescribed contact force and hand motions were selected in each task to induce the subject to attain one of three goals: (1) exerting a regulated contact force, (2) tracking the motion of the manipulandum, and (3) attaining both force and motion goals concurrently. By comparing subjects' performances in these three tasks, we found that behavior was captured by the summed actions of two independent control systems: one applying the desired force, and the other guiding the hand along the predicted path of the manipulandum. Furthermore, the application of transcranial magnetic stimulation impulses to the posterior parietal cortex selectively disrupted the control of motion but did not affect the regulation of static contact force. Together, these findings are consistent with the view that manipulation of objects is performed by independent brain control of hand motions and interaction forces
Cortical spatio-temporal dimensionality reduction for visual grouping
The visual systems of many mammals, including humans, is able to integrate
the geometric information of visual stimuli and to perform cognitive tasks
already at the first stages of the cortical processing. This is thought to be
the result of a combination of mechanisms, which include feature extraction at
single cell level and geometric processing by means of cells connectivity. We
present a geometric model of such connectivities in the space of detected
features associated to spatio-temporal visual stimuli, and show how they can be
used to obtain low-level object segmentation. The main idea is that of defining
a spectral clustering procedure with anisotropic affinities over datasets
consisting of embeddings of the visual stimuli into higher dimensional spaces.
Neural plausibility of the proposed arguments will be discussed
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