Papers by Andreas Theodorakakos
Physics in medicine and biology, Jan 21, 2008
This study aimed at investigating the effect of myocardial motion on pulsating blood flow distrib... more 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 ...
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Catheterization and Cardiovascular Interventions, 2008
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Metallurgical and Materials Transactions B, 1998
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SAE Technical …, Jan 1, 2007
Cavitation formation and development inside various types of nozzles for close-spacing spray-guid... more Cavitation formation and development inside various types of nozzles for close-spacing spray-guided fuel injection systems is predicted using a computational fluid dynamics cavitation model. The fuel injection systems investigated include generic geometries of multi-hole nozzles and outwards opening pintle injectors. Model validation is performed against experimental data reported elsewhere in large-scale transparent nozzle replicas. The results confirm that cavitation strongly depends on the geometry of the nozzle and the operating conditions. For multi-hole nozzles, cavitation structures similar to those realised in Diesel injectors are formed. These include the needle seat cavitation realised at low needle lifts, the geometrically-induced hole entry cavitation and string cavitation developing inside the sac volume. A more chaotic and less understood cavitation pattern develops at the sealing area of inward seal band outwards opening nozzles. Vapour pockets have been found to develop around the circumferential area of the needle sealing area in a transient mode. Parametric studies obtained under realistic injection and back pressure conditions reveal the effect of nozzle design on the different nozzle flow patterns that may form during the injection timing.
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The onset and development of cavitation in a new Diesel injector nozzle design is investigated bo... more The onset and development of cavitation in a new Diesel injector nozzle design is investigated both computationally through use of CFD and experimentally using transparent nozzle replicas. The injector design eliminates the sac volume and isolates the flow path that links adjacent holes. It is proved that this results to elimination of vortex cavitation and profound spray stability. Unlike most existing nozzle designs, geometric cavitation becomes a controlled flow characteristic that can be used to determine fuel atomisation and near-nozzle spray angle. Modelling of cavitation is performed using various sub-models for nucleation and bubble formation, further bubble growth and collapse, as well as bubble break-up and transport are incorporated into the model. Simulations are performed both under fixed and transient needle lift conditions. Model validation is performed against experimental data performed in transparent nozzle replicas operating under steady-state flow conditions. Measurements include, in addition to nozzle discharge coefficient, images of the geometric hole cavitation at various combinations of needle lifts, Reynolds and cavitation number.
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Pressurization of Diesel fuel in modern common-rail injectors in excess of 2000bar can result to ... more Pressurization of Diesel fuel in modern common-rail injectors in excess of 2000bar can result to increased temperatures and significant variation of the fuel physical properties (density, viscosity, heat capacity and thermal conductivity) relative to those under atmospheric pressure and room temperature conditions. Moreover, due to the sharp de-pressurization experienced by the fuel at the inlet of the injection holes, significant gradients of the above properties are established. The subsequent fuel acceleration at velocities reaching 700m/s is also inducing further wall friction and thus heating. Consequently, the characteristics of cavitation taking place at the entrance to the injection holes are altered while the volumetric efficiency of the nozzle is significantly affected. The present study quantifies the role of these effects in mini sac-type Diesel injectors operating at pressures up to 2400bar through use of a RANS cavitation CFD model. The flow solver is accordingly modified to account for such effects during the solution of the flow conservation equations. Two different injector designs have been considered, both based on the same mini sac-type nozzle body; one with sharp-inlet cylindrical holes and one with tapered holes with inlet rounding. The results indicate significant changes in terms of the details of the flow development but also to bulk flow characteristics such as the volumetric efficiency of the injectors and the mean fuel injection temperature relative to the isothermal/constant properties case.
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Physics of Fluids, Jan 1, 2009
The cavitation structures formed inside enlarged transparent replicas of tapered Diesel valve cov... more 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 allowed estimation of their frequency of appearance, lifetime, and size along the injection hole length, as function of cavitation and Reynolds numbers and needle lift.
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… processes in diesel …, Jan 1, 2008
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Introduction: The functional behavior and hemodynamic characteristics of percutaneously implanted... more Introduction: The functional behavior and hemodynamic characteristics of percutaneously implanted bioprosthetic
valves are not known.
Methods: We created aortic models after the simulated implantation of two of the most widely used bioprosthetic
valves: the Edwards SAPIEN, and the Medtronic CoreValve. By using computational fluid dynamics
analysis we sought to investigate variations in the aortic flow patterns induced by the two valve designs
and their association with detrimental phenomena such as vascular remodeling, vascular wall damage and
thrombosis.
Results: The simulated implantation of models that resemble the two valves resulted in different aortic flow
conditions. Vortex formation in the upper ascending aorta was more persistent in the case of the simulated
Medtronic valve. The ranges of average wall shear stress (WSS) values were 2.4-3.5 Pa for Edwards and
3.0-5.3 Pa for Medtronic; the calculated WSS values induced endothelial quiescence and an atheroprotective
setting in both valves. The average shear stress on the simulated valve leaflets was low; however, hotspots
were present in both valves (155.0 Pa for Edwards and 250.0 Pa for Medtronic) which would in theory be
able to cause platelet activation and thus promote thrombosis. The pressure drops along the aorta were
slightly lower for the Edwards compared to the Medtronic valve (198.0 Pa versus 218.0 Pa).
Conclusions: The presented method allows the assessment of aortic flow conditions following the implantation
of bioprosthetic valves. It may be useful in predicting detrimental flow phenomena, thus facilitating the
selection of appropriate valve designs.
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SAE SP, 2007
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Thermo- and Fluid Dynamic Processes in Diesel Engines 2, 2004
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Proceedings of the 8th International Symposium on Cavitation, 2012
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Atomization and Sprays, 2009
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The functional behavior and hemodynamic characteristics of percutaneously implanted bioprosthetic... more The functional behavior and hemodynamic characteristics of percutaneously implanted bioprosthetic valves are not known. The objective of this study is to evaluate aortic flow conditions after the simulated implantation of two of the most widely used bioprosthetic valves, the Edwards SAPIEN, and the Medtronic CoreValve, at the same patient-specific aorta geometry, by using computational fluid dynamics (CFD) analysis. The simulated implantation on models that resemble the two valves results in different aortic flow conditions. Vortex formation at the upper ascending aorta is more persistent in the case of the simulated Medtronic valve. Ranges of average wall shear stress values are 2.4 – 3.5 Pa for Edwards and 3.0 – 5.3 Pa for Medtronic; the calculated WSS values induce endothelial quiescence and an atheroprotective setting at both valves. The average shear stress on the simulated valve leaflets are low, however hot-spots are present at both valves (155.0 Pa for Edwards and 250.0 Pa f...
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Papers by Andreas Theodorakakos
valves are not known.
Methods: We created aortic models after the simulated implantation of two of the most widely used bioprosthetic
valves: the Edwards SAPIEN, and the Medtronic CoreValve. By using computational fluid dynamics
analysis we sought to investigate variations in the aortic flow patterns induced by the two valve designs
and their association with detrimental phenomena such as vascular remodeling, vascular wall damage and
thrombosis.
Results: The simulated implantation of models that resemble the two valves resulted in different aortic flow
conditions. Vortex formation in the upper ascending aorta was more persistent in the case of the simulated
Medtronic valve. The ranges of average wall shear stress (WSS) values were 2.4-3.5 Pa for Edwards and
3.0-5.3 Pa for Medtronic; the calculated WSS values induced endothelial quiescence and an atheroprotective
setting in both valves. The average shear stress on the simulated valve leaflets was low; however, hotspots
were present in both valves (155.0 Pa for Edwards and 250.0 Pa for Medtronic) which would in theory be
able to cause platelet activation and thus promote thrombosis. The pressure drops along the aorta were
slightly lower for the Edwards compared to the Medtronic valve (198.0 Pa versus 218.0 Pa).
Conclusions: The presented method allows the assessment of aortic flow conditions following the implantation
of bioprosthetic valves. It may be useful in predicting detrimental flow phenomena, thus facilitating the
selection of appropriate valve designs.
valves are not known.
Methods: We created aortic models after the simulated implantation of two of the most widely used bioprosthetic
valves: the Edwards SAPIEN, and the Medtronic CoreValve. By using computational fluid dynamics
analysis we sought to investigate variations in the aortic flow patterns induced by the two valve designs
and their association with detrimental phenomena such as vascular remodeling, vascular wall damage and
thrombosis.
Results: The simulated implantation of models that resemble the two valves resulted in different aortic flow
conditions. Vortex formation in the upper ascending aorta was more persistent in the case of the simulated
Medtronic valve. The ranges of average wall shear stress (WSS) values were 2.4-3.5 Pa for Edwards and
3.0-5.3 Pa for Medtronic; the calculated WSS values induced endothelial quiescence and an atheroprotective
setting in both valves. The average shear stress on the simulated valve leaflets was low; however, hotspots
were present in both valves (155.0 Pa for Edwards and 250.0 Pa for Medtronic) which would in theory be
able to cause platelet activation and thus promote thrombosis. The pressure drops along the aorta were
slightly lower for the Edwards compared to the Medtronic valve (198.0 Pa versus 218.0 Pa).
Conclusions: The presented method allows the assessment of aortic flow conditions following the implantation
of bioprosthetic valves. It may be useful in predicting detrimental flow phenomena, thus facilitating the
selection of appropriate valve designs.