Andreas Theodorakakos

Three-dimensional numerical investigation of a droplet impinging normally onto a wall film

The paper presents a three-dimensional numerical investigation of a droplet impinging normally onto a wall film. The numerical method is based on the finite volume solution of the Navier-Stokes equations coupled with the volume of fluid method (VOF) and utilizing an adaptive local grid refinement technique for tracking more accurately the liquid-gas interface. The results are compared with available experimental data for integral quantities such as the lamella temporal development. Two mechanisms are identified leading to secondary droplet formation; in the initial and intermediate stages of splashing secondary droplet formation is according to Rayleigh instability while at later times surface tension effects contribute further to secondary atomization. Moreover, the influence of Weber number on the impingement process is investigated and correlations for the diameter and number of secondary droplets are proposed.

Characterization of string cavitation in large-scale Diesel nozzles with tapered holes

The cavitation structures formed inside enlarged transparent replicas of tapered Diesel valve covered orifice nozzles have been characterized using high speed imaging visualization. Cavitation images obtained at fixed needle lift and flow rate conditions have revealed that although the conical shape of the converging tapered holes suppresses the formation of geometric cavitation, forming at the entry to the cylindrical injection hole, string cavitation has been found to prevail, particularly at low needle lifts. Computational fluid dynamics simulations have shown that cavitation strings appear in areas where large-scale vortices develop. The vortical structures are mainly formed upstream of the injection holes due to the nonuniform flow distribution and persist also inside them. Cavitation strings have been frequently observed to link adjacent holes while inspection of identical real-size injectors has revealed cavitation erosion sites in the area of string cavitation development. Image postprocessing has...

Evaluation of the Effect of Droplet Collisions on Spray Mixing

A spray model, implemented in a three-dimensional computational fluid dynamics (CFD) code has been used to evaluate the effect of droplet collisions on spray mixing resulting from the overlapping of liquid spray cones produced by two parallel hollow-cone nozzles under the influence of a cross-flow. The computations are compared with experimental results from phase Doppler anemometer (PDA) measurements in mixing steady sprays. The results show that the droplet collisions, which mainly occur in the mixing area of the two different sprays, have great influence on the droplet size and, as a consequence, on the predicted droplet velocities, especially at distances far from the spray nozzles. Information about the collision mechanisms as well as about droplet velocities and droplet dispersion due to collisions is also presented.

A new method of three‐dimensional coronary artery reconstruction from X‐ray angiography: Validation against a virtual phantom and multislice computed tomography

Objective: To develop and implement a method for three‐dimensional (3D) reconstruction of coronary arteries from conventional monoplane angiograms. Background: 3D reconstruction of conventional coronary angiograms is a promising imaging modality for both diagnostic and interventional purposes. Methods: Our method combines image enhancement, automatic edge detection, an iterative method to reconstruct the centerline of the artery and reconstruction of the diameter of the vessel by taking into consideration foreshortening effects. The X‐Ray‐based 3D coronary trees were compared against phantom data from a virtual arterial tree projected into two planes as well as computed tomography (CT)‐based coronary artery reconstructions in patients subjected to coronary angiography. Results: Comparison against the phantom arterial tree demonstrated perfect agreement with the developed algorithm. Visual comparison against the CT‐based reconstruction was performed in the 3D space, in terms of the direction angle along the centerline length of the left anterior descending and circumflex arteries relative to the main stem, and location and take‐off angle of sample bifurcation branches from the main coronary arteries. Only minimal differences were detected between the two methods. Inter‐ and intraobserver variability of our method was low (intra‐class correlation coefficients > 0.8). Conclusion: The developed method for coronary artery reconstruction from conventional angiography images provides the geometry of coronary arteries in the 3D space.

Simulation of cardiac motion on non-Newtonian, pulsating flow development in the human left anterior descending coronary artery.

This study aimed at investigating the effect of myocardial motion on pulsating blood flow distribution of the left anterior descending coronary artery in the presence of atheromatous stenosis. The moving 3D arterial tree geometry has been obtained from conventional x-ray angiograms obtained during the heart cycle and includes a number of major branches. The geometry reconstruction model has been validated against projection data from a virtual phantom arterial tree as well as with CT-based reconstruction data for the same patient investigated. Reconstructions have been obtained for a number of temporal points while linear interpolation has been used for all intermediate instances. Blood has been considered as a non-Newtonian fluid. Results have been obtained using the same pulse for the inlet blood flow rate but with fixed arterial tree geometry as well as under steady-state conditions corresponding to the mean flow rate. Predictions indicate that myocardial motion has only a minor effect on flow distribution within the arterial tree relative to the effect of the blood pressure pulse.

Flow Patterns at Stented Coronary Bifurcations Computational Fluid Dynamics Analysis

Background— The ideal bifurcation stenting technique is not established, and data on the hemodynamic characteristics at stented bifurcations are limited. Methods and Results— We used computational fluid dynamics analysis to assess hemodynamic parameters known affect the risk of restenosis and thrombosis at coronary bifurcations after the use of various single- and double-stenting techniques. We assessed the distributions and surface integrals of the time averaged wall shear stress (TAWSS), oscillatory shear index (OSI), and relative residence time (tr). Single main branch stenting without side branch balloon angioplasty or stenting provided the most favorable hemodynamic results (integrated values of TAWSS=4.13·10−4 N, OSI=7.52·10−6 m2, tr=5.57·10−4 m2/Pa) with bifurcational area subjected to OSI values >0.25, >0.35, and >0.45 calculated as 0.36 mm2,0.04 mm2, and 0 mm2, respectively. Extended bifurcation areas subjected to these OSI values were seen after T-stenting: 0.61 mm2, 0.18 mm2, and 0.02 mm2, respectively. Among the considered double-stenting techniques, crush stenting (integrated values of TAWSS=1.18·10−4 N, OSI=7.75·10−6 m2, tr=6.16·10−4 m2/Pa) gave the most favorable results compared with T-stenting (TAWSS=0.78·10−4 N, OSI=10.40·10−6 m2, tr=6.87·10−4 m2/Pa) or the culotte technique (TAWSS=1.30· 10−4 N, OSI=9.87·10−6 m2, tr=8.78·10−4 m2/Pa). Conclusions— In the studied models of computer simulations, stenting of the main branch with our without balloon angioplasty of the side branch offers hemodynamic advantages over double stenting. When double stenting is considered, the crush technique with the use of a thin-strut stent may result in improved immediate hemodynamics compared with culotte or T-stenting.

Vortex formation and recirculation zones in left anterior descending artery stenoses: computational fluid dynamics analysis

Flow patterns may affect the potential of thrombus formation following plaque rupture. Computational fluid dynamics (CFD) were employed to assess hemodynamic conditions, and particularly flow recirculation and vortex formation in reconstructed arterial models associated with ST-elevation myocardial infraction (STEMI) or stable coronary stenosis (SCS) in the left anterior descending coronary artery (LAD). Results indicate that in the arterial models associated with STEMI, a 50% diameter stenosis immediately before or after a bifurcation creates a recirculation zone and vortex formation at the orifice of the bifurcation branch, for most of the cardiac cycle, thus allowing the creation of stagnating flow. These flow patterns are not seen in the SCS model with an identical stenosis. Post-stenotic recirculation in the presence of a 90% stenosis was evident at both the STEMI and SCS models. The presence of 90% diameter stenosis resulted in flow reduction in the LAD of 51.5% and 35.9% in the STEMI models and 37.6% in the SCS model, for a 10 mmHg pressure drop. CFD simulations in a reconstructed model of stenotic LAD segments indicate that specific anatomic characteristics create zones of vortices and flow recirculation that promote thrombus formation and potentially myocardial infarction.

Modelling of internal and near-nozzle flow of a pintle-type outwards-opening gasoline piezo-injector

Abstract Computational fluid dynamics (CFD) methods have been used to investigate the internal nozzle flow of an outwards-opening piezo-driven pintle injector designed for spray-guided direct injection gasoline engines. The internal nozzle flow has been investigated for various nozzle designs, with emphasis placed on the effect of manufacturing tolerances on the internal and near-nozzle flow characteristics. The methods employed include features such as moving wall boundaries and time-dependent pressure or flowrate inlet conditions, cavitation as well as Eulerian and Lagrangian near-nozzle and spray models. The results reveal that not only the nozzle internal geometric details and manufacturing tolerances influence significantly the flow conditions at the exit of the pintle injector, but the actual spray characteristics are significantly influenced by the external geometry of the nozzle housing and the pintle shape. The atomization process of the liquid emerging from the pintle nozzle seems to be different from that realized in other nozzle designs used in direct injection gasoline engines; a so-called string-type of spray is formed at the nozzle exit, as confirmed by near-nozzle CCD spray images. The mechanism of string formation is attributed to the limited liquid volume passing through the needle seat area and partly occupying the available volume, while the details of the geometry in this location may enhance local air entrainment. The velocity differences along the circumference of the nozzle exit magnify those flow instabilities and have a predictable effect on the dispersion of the liquid droplets near the nozzle exit.

Modeling wall impaction of diesel sprays

Abstract A model for diesel spray wall impaction is presented, which is assessed against experiments for a number of test cases, including normal or angled injection to a wall into a quiescent space or a cross-flowing gas at various gas pressures. New relationships are given for the velocities of the droplets rebounding from the wall. These relationships take into account the wall roughness and the possible break-up of the droplets during their impingement. The impingement model was incorporated in a spray model based on the stochastic particle technique (Dukowicz 1980) and accounts for the phenomena of droplet injection, break-up, collision and coalescence, turbulent dispersion, and evaporation. The spray model was incorporated in a recently developed three-dimensional (3-D) computational fluid dynamics (CFD) code that simulates the unsteady compressible flow of the gas in internal combustion engines by solving the full Navier-Stokes equations. It was found that the motion of the surrounding gas caused by the spray injection plays a minor role on the predicted results. The latter concern the wall spray radius and the wall spray height. The validity of the spray model is demonstrated through extensive comparisons with experiments over a wide range of gas conditions.

Prediction of Liquid and Vapor Penetration of High Pressure Diesel Sprays

A dense-particle Eulerian-Lagrangian stochastic methodology, able to resolve the dense spray formed at the nozzle exit has been applied to the simulation of evaporating diesel sprays. Local grid refinement at the area where the spray evolves allows use of cells having sizes from 0.6 down to 0.075mm. Mass, momentum and energy source terms between the two phases are spatially distributed to cells found within a distance from the droplet centre; this has allowed for grid-independent interaction between the Eulerian and the Lagrangian phases to be reached. Additionally, various models simulating the physical processes taking place during the development of sprays are considered. The cavitating nozzle flow is used to estimate the injection velocity of the liquid while its effect on the spray formation is considered through an atomisation model predicting the initial droplet size. Various vaporisation models have been tested, including high-pressure and non-equilibrium effects and chemical composition change. Different droplet break-up and droplet aerodynamic drag models are used to assess the predicted results. In particular, the increased surface area of the droplets associated with their fragmentation process is found to play a major role on the exchange of heat and mass between the evaporating liquid and the surrounding air. This has allowed for calculation of liquid penetration length independent of the injection pressure. The model has been successfully validated against experimental data for the liquid and vapour penetration under a variety of injection pressure, back pressure and temperature, injection hole diameter and fuel initial temperature and composition.