Sokrates Tsangaris

Microscope PIV for velocity-field measurement of particle suspensions flowing inside glass capillaries

We assembled an experimental arrangement for the measurement of the velocity field of glycerol suspensions, in the diametric plane, inside glass capillaries with internal diameter of the order of 200 µm. Glass spheres of mean diameter of 10 µm were used to seed the flow. The velocity fields were determined by the particle image velocimetry (PIV) method, whereby, in contrast to the usual implementations of this method based on light-sheet imaging, a forwards-scattering technique with the entire volume flow illuminated was used and the plane of interest was determined by the objective lens of a microscope. Statistical analysis of acquired experimental results was performed and possible limitations of the proposed technique were investigated. The method, with its present implementation, was used to measure velocities up to 4 mm s-1 and seems to be promising for similar measurements in glass capillaries or in microcirculation.

One-dimensional model of valveless pumping in a closed loop and a numerical solution

The development of a theoretical model of valveless pumping and its numerical solution is presented in this work, applied for the case of a closed hydraulic loop, consisting of a soft and a rigid tube. A periodic compression and decompression of the soft tube causes a unidirectional flow, under certain conditions. The integration of the governing flow equations (continuity and momentum), over the tube cross-sectional area results in a quasi-one-dimensional unsteady model. A system of nonlinear partial differential equations of the hyperbolic type is solved numerically, employing three finite difference schemes: Lax-Wendroff, MacCormack, and Dispersion Relation Preserving, the last being the most accurate one. When the excitation takes place far from the midlength of the soft tube, a phase difference between the pressures at the two edges of each tube is developed, being in advance the one that is closer to the excitation area. Increasing the tube occlusion or the length of the excited part of the loop the...

Assessment of coronary stenoses by Doppler wires: a validation study using in vitro modeling and computer simulations

The present study evaluates the use of intracoronary velocity measurements by Doppler guidewires for assessing coronary obstructions. In vitro experiments were performed in a flow model using acrylic phantoms of coronary stenoses with different configurations (stenosis area: 56%, 75% and 89%; stenosis length: 1 and 5 mm; stenosis border: tapering or abrupt). Nonpulsatile laminar flow conditions of a test fluid were established at flow rates ranging from 0.5 to 2.0 mL/s to simulate baseline flow and flow after vasodilation. Peak Doppler velocity was measured proximal to, within and distal to the model stenoses. Computer simulations were employed to calculate radial flow profiles with and without a Doppler wire aligned with the vessel center. In 84 in vitro flow experiments, peak Doppler velocity correlated well with the average flow velocity as calculated from the actual flow rate and the vessels cross-sectional area proximal to (r = 0.98, SEE = 1.4, p < 0.001) and within (r = 0.97, SEE = 16.4, p < 0.001) the stenosis. However, the ratio of calculated average velocity to Doppler-measured peak velocity was significantly different from 0.5, the expected value for a parabolic flow profile (0.76+/-0.08, 0.81+/-0.14; p < 0.001). Acceptable accuracy was found for the Doppler estimation of stenosis severity using the continuity equation (error: 0.9+/-1.2% and -4.6+/-3.5% for stenosis with a length of 5 mm and 1 mm, respectively). Doppler velocity reserve significantly underestimated the true flow reserve for the 56% and 75% stenoses (p < 0.01). Computer simulations demonstrated significant alterations of flow profiles by the wire, which explained the observed underestimation of the true flow reserve by the Doppler velocity reserve. Thus, Doppler guidewire measurements of intracoronary flow velocities are useful to assess the severity of coronary stenoses. However, the in vitro results and computer simulations indicate that guidewires alter the flow profile, so that Doppler velocity reserve may underestimate the true flow reserve.

Inertial Particle Deposition in a 90° Laminar Flow Bend: An Eulerian Fluid Particle Approach

A numerical model for the simulation of aerosol flows via an Eulerian–Eulerian, one-way coupled, two-phase flow description is presented. An in-house computational fluid dynamics code is used to simulate the gaseous (continuous) phase, whereas a modified convective diffusion equation models particle transport. The convective diffusion equation, which includes inertial, gravitational, and diffusive particle transport, is solved by computational fluid dynamics techniques. The model is validated by comparing the calculated laminar fluid flow and particle deposition fractions to analytical and experimentally studied aerosol flows in a laminar flow 90° bend of circular cross section available in the literature. Model predictions are also compared with numerical predictions of Eulerian-Lagrangian models. Particle concentration profiles at different cross sections are calculated, and deposition sites on the wall boundary are indicated. For the range of studied particle diameters, the Eulerian–Eulerian model predicts deposition fractions satisfactorily, being in good agreement with the experimental data.

Vascular wall flow-induced forces in a progressively enlarged aneurysm model.

The current study is focused on the numerical investigation of the flow field induced by the unsteady flow in the vicinity of an abdominal aortic aneurysm model. The computational fluid dynamics code used is based on the finite volume method, and it has already been used in various bioflow studies. For modelling the rheological behaviour of blood, the Quemada non-Newtonian model is employed, which is suitable for simulating the two-phase character of blood namely a suspension of blood cells in plasma. For examining its non-Newtonian effects a comparison with a corresponding Newtonian flow is carried out. Furthermore, the investigation is focused on the distribution of the flow-induced forces on the interior wall of the aneurysm and in order to study the development of the distribution with the gradual enlargement of the aneurysm, three different degrees of aneurysm-growth have been assumed. Finally and for examining the effect of the distribution on the aneurysm growth, a comparison is made between the pressure and wall shear-stress distributions at the wall for each growth-degree.

A novel method for the generation of multi-block computational structured grids from medical imaging of arterial bifurcations

In this study a description of a new approach, for the generation of multi-block structured computational grids on patient-specific bifurcation geometries is presented. The structured grid generation technique is applied to data obtained by medical imaging examination, resulting in a surface conforming, high quality, multi-block structured grid of the branching geometry. As a case study application a patient specific abdominal aorta bifurcation is selected. For the evaluation of the grid produced by the novel method, a grid convergence study and a comparison between the grid produced by the method and unstructured grids produced by commercial meshing software are carried out.

Modeling and computational analysis of the hemodynamic effects of crossing the limbs in an aortic endograft ("ballerina" position).

Purpose To evaluate the displacement forces acting on an aortic endograft when the iliac limbs are crossed (“ballerina” position). Methods An endograft model was computationally reconstructed based on data from a patient whose infrarenal aortic aneurysm had an endovascular stent-graft implanted with the iliac limbs crossed. Computational fluid dynamics analysis determined the maximum displacement force on the endograft and separately on the bifurcation and iliac limbs. Its analogue model was reconstructed for comparison, assuming the neck, main body, and total length constant but considering the iliac limbs to be deployed in the usual bifurcated mode. Calculations were repeated after developing “idealized” models of both the bifurcated and crossed-limbs endografts with straight main bodies and no neck angulation or curved iliac segments. Results The vector of the total force was directed anterocaudal for both the typical bifurcated and the crossed-limbs configurations, with the forces in the latter slightly reduced and the vertical component accounting for most of the force in both configurations. Idealized crossed-limbs and bifurcated configurations differed only in the force on the iliac limbs, but this difference disappeared in the realistic models. Conclusion Crossing of the iliac limbs can slightly affect the direction of the displacement forces. Although this configuration can exert larger forces on the limbs than in the bifurcated mode, this effect can be blunted by concomitant modifications in the geometry of the main body and other parts of the endograft, making its hemodynamic behavior resemble that of a typically positioned endograft.

Motion of deformable drops in pipes and channels using Navier–Stokes equations

The motion of deformable drops in pipes and channels is studied using a level set approach in order to capture the interface of two fluids. The interface is described as the zero level set of a smooth function, which is defined to be the signed normal distance from the interface. In order to solve the Navier-Stokes equations, a second-order projection method is used. The dimensionless parameters of the problem are the relative size of the drop to the size of the pipe or channel cross-section, the ratio of the drop viscosity to the viscosity of the suspending fluid and the relative magnitude of viscous forces to the surface tension forces. The shape of the drop, the velocity field and the additional pressure loss due to the presence of the drop, varying systematically with the above-mentioned dimensionless parameters, are computed

Development of an artificial compressibility methodology using flux vector splitting

An implicit, upwind arithmetic scheme that is efficient for the solution of laminar, steady, incompressible, two-dimensional flow fields in a generalised co-ordinate system is presented in this paper. The developed algorithm is based on the extended flux-vector-splitting (FVS) method for solving incompressible flow fields. As in the case of compressible flows, the FVS method consists of the decomposition of the convective fluxes into positive and negative parts that transmit information from the upstream and downstream flow field respectively. The extension of this method to the solution of incompressible flows is achieved by the method of artificial compressibility, whereby an artificial time derivative of the pressure is added to the continuity equation. In this way the incompressible equations take on a hyperbolic character with pseudopressure waves propagating with finite speed. In such problems the ‘information’ inside the field is transmitted along its characteristic curves. In this sense, we can use upwind schemes to represent the finite volume scheme of the problems governing equations. For the representation of the problem variables at the cell faces, upwind schemes up to third order of accuracy are used, while for the development of a time-iterative procedure a first-order-accurate Euler backward-time difference scheme is used and a second-order central differencing for the shear stresses is presented. The discretized Navier–Stokes equations are solved by an implicit unfactored method using Newton iterations and Gauss–Siedel relaxation. To validate the derived arithmetical results against experimental data and other numerical solutions, various laminar flows with known behaviour from the literature are examined.

Geometric Factors Affecting the Displacement Forces in an Aortic Endograft With Crossed Limbs: A Computational Study

Purpose To compare the hemodynamic behavior between an aortic endograft model in the “crossed-limbs” configuration and the customary bifurcated deployment position under the influence of several geometric factors. Methods A crossed-limbs graft and its analogue model with uncrossed limbs were computationally reconstructed. The displacement forces acting over the entire endograft and at the bifurcation and iliac sites separately were calculated using a fluid structure interaction simulation under a range of specific geometric characteristics, namely, the lateral and anteroposterior (AP) neck angulation, the iliac bifurcation angulation, and the endograft curvature. Results The variations of lateral neck angulation caused a constantly higher total displacement force for the crossed-limbs graft, whereas the force at the bifurcation of the two configurations differed only within a narrow range of 30° to 50°. On the contrary, the displacement force at the iliac site was higher in the crossed-limbs configuration only with lateral neck angulation >50°, reaching its highest value at 70°. The variations of AP neck angulation also caused higher total displacement forces in the crossed-limbs graft. Increasing AP neck angulation values caused generally lower forces at the crossed iliac limbs and higher at its bifurcation, respectively, compared to the uncrossed limbs model. Similarly, the influence of high iliac bifurcation angulation and endograft curvature was associated with slightly elevated forces over the entire crossed-limbs graft and its bifurcation, whereas the opposite held true at the iliac site. Conclusion Apart from minor differentiations due to geometric alterations, the customary bifurcated and crossed-limbs endografts present similar hemodynamic performance. Further clinical studies should be conducted to confirm the clinical applicability of these findings.