Andrea Boghi

Preliminary Numerical Solutions of the Evolution of Free Jets

The present paper presents preliminary numerical solutions of the flow evolution of a two dimensional rectangular free jet. The numerical simulations in a two-dimensional domain are carried out with Open-FOAM, the open-source code, and compare the numerical results with the experimental visualizations performed in the same laboratory with the shadowgraph technique. The evolution of a two-dimensional submerged free jet is reported in the literature by the presence of two regions of flow: the potential core, where the centerline velocity maintains equal to that on the slot exit, and the turbulent or mixing region, where the centerline velocity decreases with the distance from the exit. Previous anemometric measurements, carried out in this laboratory with an air jet emerging from a rectangular channel, showed the presence of a region of flow, just outside the exit and before the potential core, where velocity and turbulence remain almost equal to those measured on the exit, and it has been called “undisturbed region of flow” because is present also in turbulent conditions. Previous and present shadowgraph visualizations show a jet which has the same height along the undisturbed region of flow and increases its height afterwards. The length of the undisturbed region depends on the Reynolds number of the flow and on the presence of turbulence promoters, e.g. metallic grids, at the exit of the slot. The undisturbed region is becoming nil with the increase of the Reynolds numbers, in agreement to the literature. The present two dimensional numerical solutions, carried out at Re numbers equal to 25,000 and 60,000 confirm the results obtained with the shadow visualizations.

Three-Dimensional Numerical Simulation of Blood Flow in Two Coronary Stents

The aim of the present study is to perform computational fluid dynamics (CFD) simulations of blood flow in two coronary stents in a realistic three-dimensional geometry under physiological conditions. The two stents, similar to real coronary ones, are both made of 12 rings but are different as far as the positions of the struts are concerned. One type of stent has parallel-connectors and the other transverse-ones. Time variation of the parameters correlated to neo-intimal hyperplasia, such as wall shear stress, magnitude of wall shear stress gradient, and oscillatory shear index are investigated with the conclusion that the stent with parallel-connectors has a better fluid dynamic behavior.

Numerical solution of three-dimensional rectangular submerged jets with the evidence of the undisturbed region of flow

ABSTRACT The evolution of turbulent rectangular submerged free jets has been investigated numerically with a two-dimensional (2D) approach by the present authors and, by using the large eddy simulations (LES) at several Reynolds numbers. The average numerical results confirmed the presence of the undisturbed region of flow (URF) located between the slot exit and the beginning of the potential core region (PCR) previously observed experimentally at the University of Rome “Tor Vergata” by Gori and coworkers. The 2D study of the present authors carried out under the conditions previously investigated in the literature, showed that the URF has a self-similar behavior, and proposed a new law for the evolution of the momentum. The present paper extends the LES to three-dimensional (3D) rectangular submerged free jets, in the range from Re = 5,000 to Re = 40,000, showing that the self-similar behavior of URF is also present in the 3D numerical simulations, as well as in the PCR and in the fully developed region (FDR).

Three-dimensional numerical simulation of the fluid dynamics in a coronary stent

Stents are commonly used to restore blood flow in patients with severe coronary artery disease. Local hemodynamic variables, as wall shear stress, have an important role in the restenosis and their distribution depends on the stent geometry. The objective of the present study is to carry out CFD simulations in a realistic 3D geometry of a coronary stent in physiological conditions. A comparison is performed between two reconstructed stents, made of 12 rings and similar to the real coronary ones, which differ by the position of the struts, where the first type is with closed cells and the second one with open cells. The artery is modeled as a cylinder with rigid walls and the blood is assumed as incompressible Newtonian fluid in laminar flow with constant physical properties. The commercial computational fluid dynamic code FLUENT is used with a mesh composed of non uniform tetrahedrons. The simulations are performed in steady and unsteady state. Wall shear stresses, WSS, as well as its time variations, are investigated in unsteady state with the conclusion that the stent with closed cells have a better fluid dynamic behavior.Copyright

Numerical evidence of an undisturbed region of flow in a turbulent rectangular submerged free jet

ABSTRACT The evolution of turbulent rectangular submerged free jets is described in the literature by the presence of two regions of flow: the potential core region (PCR) and the fully developed region (FDR). However, experiments carried out in the last decade showed that a third region of flow is present, the undisturbed region of flow (URF), so-called in the average visualization, or the negligible disturbances flow (NDF) plus the small disturbances flow (SDF), so-called in the instant visualization. The URF is located between the slot exit and the beginning of the PCR. The main characteristics of URF, and NDF, are that velocity and turbulence profiles remain almost equal to those measured on the slot exit, and the height of the jet remains equal to the slot one. In the SDF the jet height undergoes small variations, i.e., contractions or expansions, but without formation of the vortex. To date, no numerical evidence of the presence of URF has been given by the literature. The present study, which concerns a two-dimensional jet, presents Large Eddy Simulations (LES), carried out at four Reynolds numbers, which are able to predict and characterize URF. The present numerical results are compared to previous theoretical approaches and confirm the presence of URF, between the slot exit and the PCR. Moreover, URF has a self-similar behavior and a new law for the evolution of the momentum is proposed.

Phase-field modelling of a miscible system in spinning droplet tensiometer

The spinning drop tensiometry is used for measurements of surface tension coefficients, especially, when interfaces are characterised by low and ultra-low interfacial stresses. A droplet of lighter liquid is introduced into a rotating capillary that was initially saturated with another heavier liquid. The tube is subject to axial rotation that results in droplets elongation along the tubes axis. The equilibrium shape of the droplet is used to determine the surface tension coefficient. In this work, the evolution of a slowly miscible droplet introduced into a spinning capillary is investigated. This technique is frequently employed for studies of the dynamics of miscible systems, even despite the fact that a strict equilibrium is never achieved in a mixture of fully miscible liquids. The numerical modelling of a miscible droplet is fulfilled on the basis of the phase-field (Cahn-Hilliard) approach. The numerical results are compared against the experimental data pursuing two objectives: (i) to verify the use of the phase-field approach as a consistent physics-based approach capable of accurate tracking of the short- and long-term evolution of miscible systems, and (ii) to estimate the values of the phenomenological parameters introduced within the phase-field approach, so making this approach a practical tool for modelling of thermohydrodynamic changes in miscible systems within various configurations.

Numerical Simulation of Blood Flow through Different Stents in Stenosed and Non-Stenosed Vessels

Fluid dynamics in two different stent configurations, peak-to-valley (S1) and peak-to-peak (S2), within a fully expanded situation and a 30% restenosis is investigated. Numerical simulations are carried out in order to evaluate the conditions promoting atherosclerotic events when a self-expanding bare metal stent is applied. The conclusions are that the two configurations, S1 and S2, have a similar fluid dynamics behavior as far as wall shear stress is concerned, but oscillatory shear and relative residence time maps suggest that the peak-to-peak configuration, S2, has a better behavior than the peak-to-valley, S1.

Three-dimensional numerical simulation of a failed coronary stent implant at different degrees of residual stenosis. Part II: Apparent viscosity and wall permeability

ABSTRACT The influence of the degree of residual stenosis (DOR) on the hemodynamics inside coronary arteries is investigated through three-dimensional (3D) numerical simulations. The vascular wall permeability is investigated and the effect of the non-Newtonian viscosity discussed. The results agree in predicting an abrupt increase in wall permeability above 45% DOR, indicating that the implant could lead to a massive restenosis. This behavior is considered to be due to the shift of the regions involved by low and oscillatory wall shear stress (WSS), from the zone adjacent to the struts toward the center of the stent meshes.

On a New Passive Scalar Equation With Variable Mass Diffusivity: Flow Between Parallel Plates

The present work investigates mass conservation equations in turbulent flow between parallel plates with variable mass diffusivity. Species conservation equations are relative to the average concentration, as well as to the concentration variance. The product of fluctuating mass diffusivity and space gradient of concentration fluctuation is appearing in the equation of mean and concentration variance. A physical interpretation is given to the different terms. The assumption of a relation between mass diffusivity and concentration allows writing expressions for average and fluctuating mass diffusivity, which can be simplified on the basis of theoretical considerations. The new mass flux is expressed as a function of mass diffusivity and a gradient of concentration variance. Further considerations make it possible to model the new terms appearing in the concentration variance equation. The mass conservation equation can be solved when coupled to the equation of concentration variance. The equations are solved numerically for flow between parallel plates in order to evaluate the influence of the new terms. [DOI: 10.1115/1.4002743]

Three-dimensional numerical simulation of a failed coronary stent implant at different degrees of residual stenosis. Part I: Fluid dynamics and shear stress on the vascular wall

ABSTRACT The influence of the degree of residual stenosis on the hemodynamics inside coronary arteries is investigated through three-dimensional (3D) numerical simulations. The present paper, which is the first of a series of two, focuses on the influence that the degree of residual stenosis (DOR) has on the fluid dynamics and the shear stresses acting on the stent and the artery wall. The pulsatile nature of the blood flow and its non-Newtonian features are taken into account. Four models of artery are investigated. The results show that the wall shear stress (WSS) increases monotonically, but not linearly, with the DOR.