1,720,996 research outputs found
Effect of the prosthetic mitral valve on vortex dynamics and turbulence of the left ventricular flow
No full textMechanical heart valves implanted in mitral position have a great effect on the ventricular flow. Changes include alteration of the dynamics of the vortical structures generated during the diastole and the onset of turbulence, possibly affecting the efficiency of the heart pump or causing blood cell damage. Modifications to the hemodynamics in the left ventricle, when the inflow through the mitral orifice is altered, were investigated in vitro using a silicone rubber, flexible ventricle model. Velocity fields were measured in space and time by means of an image analysis technique: feature tracking. Three series of experiments were performed: one with a top hat inflow velocity profile (schematically resembling physiological conditions), and two with mechanical prosthetic valves of different design, mounted in mitral position-one monoleaflet and the other bileaflet. In each series of runs, two different cardiac outputs have been examined by changing the stroke volume. The flow was investigated in terms of phase averaged velocity field and second order moments of turbulent fluctuations. Results show that the modifications in the transmitral flow change deeply the interaction between the coherent structures generated during the first phase of the diastole and the incoming jet during the second diastolic phase. Top hat inflow gives the coherent structures which are optimal, among the compared cases, for the systolic function. The flow generated by the bileaflet valve preserves most of the beneficial features of the top hat inflow, whereas the monoleaflet valve generates a strong jet which discourages the permanence of large coherent structures at the end of the diastole. Moreover, the average shear rate magnitudes induced by the smoother flow pattern of the case of top hat inflow are nearly halved in comparison with the values measured with the mechanical valves. Finally, analysis of the turbulence statistics shows that the monoleaflet valves yield higher turbulence intensity in comparison with the bileaflet and, with top hat inflow, there is not a complete transition to turbulence
A Central Limit Problem for Partially Exchangeable Random Variables
No full textTraduzione inglese: Theory of Probability and its Applications, SIAM (1997
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Turbulence investigation in a laboratory model of the ascending aorta
No full textThis study aims to investigate turbulence inside a model of the
human ascending aorta as a function of the main flow control
parameters. For this purpose, we performed a two-dimensional
in vitro investigation of the pulsatile flow inside a laboratory
model of a healthy aorta by varying both the Reynolds and
Womersley numbers. Our findings indicate that the velocity
fluctuations become significant particularly during the
deceleration phase of the flow, reach the maximum near the
systolic peak and then decay during the rest of the diastole
phase. Higher levels of turbulence were recovered for
increasing Stroke Volumes, in particular maxima of Turbulent
Kinetic Energy occurred in the bulk region while higher
values of Reynolds shear stresses were found in
correspondence of the sinus of Valsalva
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