1,720,976 research outputs found
Fluid flow in a helical vessel in presence of a stenosis
Full text linkLarge arteries are not straight and rather present curvature and torsion. The present study analyzed fluid flow in a helical vessel without and with a stenosis in comparison with an analogous rectilinear vessel. The analysis is performed by threedimensional numerical simulation of the Navier–Stokes equations under steady conditions considering stenosis as an axially symmetric reduction of vessel lumen. Results show that the double curvature gives rise to persistent secondary motion which combines with the vorticity separated behind the constriction to develop a complex three-dimensional vorticity structure. The curved streamlines and the three-dimensional vortex wake result in a increase of energetic losses in helical vessels. However, the same symmetry break due to the double curvature improves the capacity of self-cleaning and allows a more rapid wash-out of the flowing blood
Optimal helical entry flow in a helical vessel
No full textMany large arteries present a double curvature that is known to reduce the
development of flow stagnating regions, which are related to the formation of
arteriosclerosis. It is not known, however, how a helical geometry affects the
energetic properties of the flowing blood and whether these depend on the
presence of a rotation in the entry flow. This work analyzes the flow in a finite
length helical vessel of constant cross-sectional area in presence of a swirl at
the entrance. The analysis is performed by numerical simulations for one
geometric condition that is typical for large arteries. As expected, the optimal
entry flow rotation is found in correspondence of the natural helical twist of
the the vessel geometry. The energetic losses were minimized in a small range
between optimal rotation and no rotation. In any case, a helical vessel presents
a larger dissipation of kinetic energy than those found in a corresponding
straight vessel. Although limited to steady flow and one geometric configuration, this result provides a preliminary basis for considering whether the
rotation observed in the flow ejected from the heart ventricles could be part or
not of a physiological optimization
Flow about a circular cylinder between parallel walls
No full textThe flow about a body placed inside a channel differs from its unbounded counterpart
because of the effects of wall confinement, shear in the incoming velocity profile, and
separation of vorticity from the channel walls. The case of a circular cylinder placed
between two parallel walls is here studied numerically with a finite element method
based on the vorticity–streamfunction formulation for values of the Reynolds number
consistent with a two-dimensional assumption.The transition from steady flow to a periodic vortex shedding regime has been
analysed: transition is delayed as the body approaches one wall because the interaction
between the cylinder wake and the wall boundary layer vorticity constrains the
separating shear layer, reducing its oscillations. The results confirm previous observations
of the inhibition of vortex shedding for a body placed near one wall. The
unsteady vortex shedding regime changes, from a pattern similar to the von Kármán
street (with some differences) when the body is in about the centre of the channel, to
a single row of same-sign vortices as the body approaches one wall. The separated
vortex dynamics leading to this topological modification is presented.The mean drag coefficients, once they have been normalized properly, are comparable
when the cylinder is placed at different distances from one wall, down to
gaps less than one cylinder diameter. At smaller gaps the body behaves similarly to
a surface-mounted obstacle. The lift force is given by a repulsive component plus an
attractive one. The former, well known from literature, is given by the deviation of
the wake behind the body. Evidence of the latter, which is a consequence of the shear
in front of the body, is given.</jats:p
Development and application of high-order discontinuous CVFEM algorithms
No full textThe discontinuous control-volume finite-element method is applied to the one-dimensional advection-diffusion equation and
validated on relevant test cases. The technique merges the features of the classical finite-volume method, as robustness and local
conservation properties [1], with those of the discontinuous Galerkin finite-element method, known for the capability of handling
large gradients or discontinuities with high accuracy [2]. On the other hand, most finite-volume methods attain relatively low
orders of spatial accuracy and resolution characteristics, particularly on unstructured meshes. To achieve high-order accuracy,
the proposed technique adopts polynomial shape functions of any degree as in spectral finite-element methods [3]. In many
applications high resolution is not needed in the whole domain, which results also in a loss of computational resources. We thus
apply an automatic p-refinement technique which adapts the polynomial order at element level, according to the local behavior
of the computed solution. Element-wise p-adaption can be easily achieved with discontinuous Galerkin methods, where the
inter-element continuity is imposed in weak form
Improving the convergence order of the meshless approach for the Cell Method for numerical integration of discrete conservation laws
No full textIn thiswork, the problem of increasing the convergence order of
the integral meshless method already proposed by the same authors
is addressed. Solutions are determined through equations directly
written in discrete form over a tributary region represented by the
circle with center in the generic node and radius given by the average
of the distances between the node itself and its neighbors,
thus allowing a considerable ease in writing the discrete form of
the governing equations. The proposed approach, besides avoiding
global mesh generation, adopts interpolating polynomials, which
exactly reproduce nodal values of field variables, and eliminates
some problems typically encountered when posing Dirichlet and
Neumann boundary conditions with the Finite Element Method.
Several numerical schemes adopting extended or compact computational
cells are proposed and tested for the Laplace equation, in
line with the previous papers. Results show that, when using interpolating
polynomials that satisfy also the differential operator
in some nodes, compact computational cells characterized by the
fifth-order of convergence may be constructed
Optimal location and control of pressure reducing valves in water networks
No full textThis paper addresses the problem of optimal pressure management in water distribution systems through the introduction and
regulation of pressure reducing valves. Reduction in pressure is aimed at controlling water leakages which, being in some cases a high
proportion of the total volume supplied, are nowadays one of the major concerns for water utilities. The determination of the number,
location, and setting of such valves is formulated as a two criteria optimization problem and is solved with multiobjective genetic
algorithms. In particular, the first criterion is represented by the minimization of the number of valves, and the second is the minimization
of the total leakage in the system, when maintaining the required pressure at each node. The great advantage of the multiobjective
approach resides in the fact that, in one run, several trade-off alternatives are found, thus providing the set of the optimal solutions with
a different level of compromise between the conflicting objectives. At the same time, data necessary for practical choice and operation of
pressure reducing valves may be determined
Simplified mitral valve modeling for prospective clinical application of left ventricular fluid dynamics
Full text linkThe fluid dynamics inside the left ventricle of the human heart is considered a potential indicator of long term cardiovascular outcome. In this respect, numerical simulations can play an important role for integrating existing technology to reproduce flow details and even conditions associated to virtual therapeutic solutions. Nevertheless, numerical models encounter serious practical difficulties in describing the interaction between flow and surrounding tissues due to the limited information inherently available in real clinical applications.
This study presents a computational method for the fluid dynamics inside the left ventricle designed to be efficiently integrated in clinical scenarios. It includes an original model of the mitral valve dynamics, which describes an asymptotic behavior for tissues with no elastic stiffness other than the constrain of the geometry obtained from medical imaging; in particular, the model provides an asymptotic description without requiring details of tissue properties that may not be measurable in vivo.
The advantages of this model with respect to a valveless orifice and its limitations with respect to a complete tissue modeling are verified. Its performances are then analyzed in details to ensure a correct interpretation of results. It represents a potential option when information about tissue mechanical properties is insufficient for the implementations of a full fluid-structure interaction approach
Pulsatile flow inside moderately elastic arteries, its modelling and effects of elasticity
No full textInfluence of mitral valve elasticity on flow development in the left ventricle
Full text linkThe Mitral valve of the human heart has a great relevance for numerous cardiac pathologies; however, the knowledge of relationships between valvular properties and cardiac function is still limited. On one side, this is partly due to the limited resolution of clinical imaging technologies that do not allow routinely visualization of the valve during its motion. On the other, its modeling presents serious challenges either due to the strong flow–tissueinteraction or because the mechanical properties of its constitutive elements are complex and not measurable in vivo. This work introduces a parametric model of the Mitral valve where the interaction with the blood flow obeys global balances and the overall elastic properties are summarized into a single functional parameter. This is integrated into a numerical model of left ventricular fluid dynamics with the aim to study the effect of varying the valvular stiffness. Results show that the elasticity of the valve influences the amplitude of the mitral opening, while the timings of opening/closure are driven by the transmitral blood flow due to the ventricular dynamics. In addition, the increase of stiffness increases the transvalvular pressure gradients required to ensure the same flow. These results are discussed in relation to parameters for monitoring valvular stiffness that are accessible through clinical imaging
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