143 research outputs found
Fluid dynamics of aortic root dilation in Marfan syndrome
Aortic root dilation and propensity to dissection are typical manifestations
of the Marfan Syndrome (MS), a genetic defect leading to the degeneration of
the elastic fibres. Dilation affects the structure of the flow and, in turn,
altered flow may play a role in vessel dilation, generation of aneurysms, and
dissection. The aim of the present work is the investigation in-vitro of the
fluid dynamic modifications occurring as a consequence of the morphological
changes typically induced in the aortic root by MS. A mock-loop reproducing the
left ventricle outflow tract and the aortic root was used to measure time
resolved velocity maps on a longitudinal symmetry plane of the aortic root. Two
dilated model aortas, designed to resemble morphological characteristics
typically observed in MS patients, have been compared to a reference, healthy
geometry. The aortic model was designed to quantitatively reproduce the change
of aortic distensibility caused by MS. Results demonstrate that vorticity
released from the valve leaflets, and possibly accumulating in the root, plays
a fundamental role in redirecting the systolic jet issued from the aortic
valve. The altered systolic flow also determines a different residual flow
during the diastole.Comment: Accepted versio
On the flow field generated by a gradually varying flow through an orifice
The motion of a vortex ring generated by gradually varied flows through a thin-edged orifice has been investigated experimentally using particle image velocimetry. This flow reproduces the primary characteristics of many biological flows, such as cardiac flows through valves or jellyfish and squid propulsion. Even though vortex ring formation has been extensively studied, there is still interest in gradually varying inflows, i.e. the ones that are mostly found in previous conditions. The main purpose of this paper is to extend the time scaling already proposed in the literature to the entire cycle of vortex ring formation, pinch-off and free motion. To this end, eight inflow time laws have been tested, with different acceleration and deceleration phases. They have been selected in relation to practical applications by their resemblance to the main characteristics of cardiovascular and pulsed locomotion flows. Analysis of measured velocity and vorticity fields suggested a general criterion to establish the instant of vortex pinch-off directly from the imposed velocity program. This allows the proper scaling of the entire time evolution of the vortex ring for all tested inflows. Since it is quite easy to identify this instant experimentally, these results give a simple, practical rule for the computation of scales in vortex ring formation and development in the case of gradual inflows. The ``slug model\u27\u27 has been used to test the proposed scaling and to obtain predictions for the vortex position, circulation and vorticity which are in agreement with experimental data
A non-intrusive and continuous-in-space technique to investigate the wave transformation and breaking over a breakwater
To design longshore breakwaters, the evaluation of the wave motion transformations over the structures and of the energy they are able to absorb, dissipate and reflect is necessary. To characterize features and transformations of monochromatic wave trains above a breakwater, both submerged and emerged, we have designed and developed a non-intrusive and continuous-in-space technique, based on Image Analysis, and carried out an experimental campaign, in a laboratory flume equipped with a wave-maker, in order to test it. The investigation area was lighted with a light sheet and images were recorded by a video-camera. The working fluid was seeded with non buoyant particles to make it bright and clearly distinct from dark background and breakwater. The technique, that is based on a robust algorithm to identify the free surface, has showed to properly work also in prohibitive situations for traditional resistive probes (e.g., very shallow waters and/or breaking waves) and to be able to measure the free surface all over the investigation field in a non-intrusive way. Two kind of analysis were mainly performed, a statistical and a spectral one. The peculiarities of the measurement technique allowed to describe the whole wave transformation and to supply useful information for design purposes
Turbulence investigation in a laboratory model of the ascending aorta
This 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
INFUSION PRESSURE CONTROL SYSTEM
An infusion pressure control system for ophthalmic surgery includes: - intraocular pressure detection means; - arterial pressure detection means; and a control unit operatively connected to said intraocular pressure detection means and said arterial pressure detection means and configured for calculating the mean ocular perfusion pressure value based on the intraocular pressure and arterial pressure values provided by said intraocular pressure detection means and by said arterial pressure detection means, and for comparing said calculated mean ocular perfusion pressure value and at least one predetermined threshold value
Water-channel study of flow and turbulence past a two-dimensional array of obstacles
A neutral boundary layer was generated in the laboratory to analyze the mean
velocity field and the turbulence field within and above an array of
two-dimensional obstacles simulating an urban canopy. Different geometrical
configurations were considered in order to investigate the main characteristics
of the flow as a function of the aspect ratio (AR) of the canopy. To this end,
a summary of the two-dimensional fields of the fundamental turbulence
parameters is given for AR ranging from 1 to 2. The results show that the flow
field depends strongly on AR only within the canyon, while the outer flow seems
to be less sensitive to this parameter. This is not true for the vertical
momentum flux, which is one of the parameters most affected by AR, both within
and outside the canyon. The experiments also indicate that, when (i.e. the
skimming flow regime), the roughness sub-layer extends up to a height equal to
1.25 times the height of the obstacles (H), surmounted by an inertial sub-layer
that extends up to 2.7 H. In contrast, for (i.e. the wake-interference regime)
the inertial sub-layer is not present. This has significant implications when
using similarity laws for deriving wind and turbulence profiles in canopy
flows. Furthermore, two estimations of the viscous dissipation rate of
turbulent kinetic energy of the flow are given. The first one is based on the
fluctuating strain rate tensor, while the second is related to the mean strain
rate tensor. It is shown that the two expressions give similar results, but the
former is more complicated, suggesting that the latter might be used in
numerical models with a certain degree of reliability. Finally, the data
presented can also be used as a dataset for the validation of numerical models
A parallel interaction potential approach coupled with the immersed boundary method for fully resolved simulations of deformable interfaces and membranes
In this paper we show and discuss the use of a versatile interaction
potential approach coupled with an immersed boundary method to simulate a
variety of flows involving deformable bodies. In particular, we focus on two
kinds of problems, namely (i) deformation of liquid-liquid interfaces and (ii)
flow in the left ventricle of the heart with either a mechanical or a natural
valve. Both examples have in common the two-way interaction of the flow with a
deformable interface or a membrane. The interaction potential approach (de
Tullio & Pascazio, Jou. Comp. Phys., 2016; Tanaka, Wada and Nakamura,
Computational Biomechanics, 2016) with minor modifications can be used to
capture the deformation dynamics in both classes of problems. We show that the
approach can be used to replicate the deformation dynamics of liquid-liquid
interfaces through the use of ad-hoc elastic constants. The results from our
simulations agree very well with previous studies on the deformation of drops
in standard flow configurations such as deforming drop in a shear flow or a
cross flow. We show that the same potential approach can also be used to study
the flow in the left ventricle of the heart. The flow imposed into the
ventricle interacts dynamically with the mitral valve (mechanical or natural)
and the ventricle which are simulated using the same model. Results from these
simulations are compared with ad- hoc in-house experimental measurements.
Finally, a parallelisation scheme is presented, as parallelisation is
unavoidable when studying large scale problems involving several thousands of
simultaneously deforming bodies on hundreds of distributed memory computing
processors
Experimental Studies for the characterization of the mixing processes in negative buoyant jets
A negatively buoyant jet (NBJ) corresponds to the physical phenomenon that develops when a fluid is discharged upwards into a lighter environment or downwards into a heavier receptor fluid. In a NBJ the flow is initially driven mostly by the momentum, so it basically behaves as a simple jet released withthe same angle, while far from the outlet the buoyancy prevails, bending the jet axis down and making it similar to a plume. The coexistence in the same phenomenon of both the characteristics of simple jets and plumes makes the NBJs a phenomenon still not entirely explained but, considering also the numerous practical applications, very interesting to study. Here some of the experimental results are presented. The laboratory experiment were obtained on a model simulating a typical sea discharge of brine from desalination plants: a pipe laid down on the sea bottom, with orifices on its lateral wall, releasing brine (heavier than the sea water) with a certain angle to the horizontal, in order to increase the jet path before sinking to the seafloor. A non-intrusive image analysis technique, namely Feature Tracking Velocimetry, is applied to measure velocity fields, with the aim at understanding the influence of some non-dimensional parameters driving the phenomenon (e.g. Reynolds number, release angle) on the structure of the NBJ and of the turbulence. © Owned by the authors, published by EDP Sciences, 2013
A street graph-based morphometric characterization of two large urban areas
Urban microclimate modelling, both numerical and in the laboratory, has strong
implications in many relevant health and life-style management issues e.g., in studies for assessment
and forecast of air quality (for both outdoor and, as boundary conditions, indoor investigations), for
thermometric trend analysis in urban zones, in cultural heritage preservation, etc. Moreover, the
study of urban microclimate modelling is largely promoted and encouraged by international
institutions for its implication in human health protection. In the present work, we propose and
discuss an adaptive street graph-based method aimed at automatically computing the geometrical
parameters adopted in atmospheric turbulent flow modelling. This method has been applied to two
real cases, the Italian cities of Rome and Cagliari, and its results has been compared with the ones
from traditional methods based on regular grids. Results show that the proposed method leads to a
more accurate determination of the urban canyon parameters (Canyon Aspect Ratio and Building
Aspect Ratio) and morphometric parameters (Planar Area Index and Frontal Area Index) compared
to traditional regular grid-based methods, at least for the tested cases. Further investigations on a
larger number of different urban contexts are planned to thoroughly test and validate the proposed
algorithm
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