A. Theodorakakos - Academia.edu (original) (raw)

Papers by A. Theodorakakos

… Symposium on Turbulence and Shear Flow …, 2005

Oblique flow past a cube or low rise structure is of importance in designing and constructing str... more Oblique flow past a cube or low rise structure is of importance in designing and constructing structures and buildings, among other applications. In the present study the large eddy simulation (LES) methodology is applied on an unstructured grid with locally refined resolution in order to study the surface pressure field and the conical vortices that develop on the roof of a low rise cubicle. Three simulations are performed, evaluating the effect of temporal and spatial resolution and results are compared to experimental measurements and to a point vortex theory for the conical vortex structure. Spatial resolution is found to be of importance in correct representation of the conical vortex structure as well as the high frequency spectrum of the roof-top pressure fluctuations.

SAE Technical Paper Series, 2007

... damage has been investigated using enlarged transparent nozzle replicas and computational flu... more ... damage has been investigated using enlarged transparent nozzle replicas and computational fluid dynamics ... This has been further supported by CFD predictions obtained using the measured ... New Modelling of Cavitating Flows - a Numerical Study of Unsteady Cavitation on a ...

International Journal of Heat and Fluid Flow, 2005

The paper presents results of a numerical investigation of the impingement process of a droplet n... more The paper presents results of a numerical investigation of the impingement process of a droplet normally onto a wall film. The numerical method is based on the finite volume solution of the Navier-Stokes equations, in their axisymmetric formulation, expressing the flow field of the two phases, liquid and gas, coupled by the volume of fluid method (VOF). The latter allows the tracking of the fluid-gas interfaces. The results are compared with available experimental data for integral quantities such as the lamella development in time, whilst the method gives details of the flow fields not hitherto available and which shed light on the droplet interaction with the gas and the mixing mechanisms of the droplet and the wall film during the initial stages of impingement. The latter information is obtained via a 2-VOF model for the prediction of the distribution of mass in the region of lamella during the splash.

Computers and Geotechnics, 2012

Composites Part B: Engineering, 2011

In this study, a novel reinforcing method for glass-fiber reinforced composites, which consists o... more In this study, a novel reinforcing method for glass-fiber reinforced composites, which consists of inter-adherend glass fibers (IAF) that get through the composite adherend like a pin, was investigated experimentally and numerically. Two types of joints were investigated. One is ...

Atomization and Sprays, 2010

This paper presents an experimental study of the impact of a single water droplet onto a stationa... more This paper presents an experimental study of the impact of a single water droplet onto a stationary liquid bulk built by the previous impact of one or two droplets. The experiments were performed with two different film thicknesses, three different Weber (We) numbers, and two surface contact angles. In both cases we have hydrophilic conditions, which do not allow for a good investigation of this parameter. The morphology of the drop impact was studied using a chargecoupled device (CCD) camera, and the corresponding qualitative and quantitative characteristics regarding the time evolution of the phenomena, such as the diameter and height of the evolving crown, were obtained by image analysis. Analysis of the experimental data evidences that the phenomenon has a strong similarity to the impact of a single drop on shallow films, although the effect of the surface wetting characteristics plays, in this case, a negligible role. The regimes of deposition and splashing are identified as a function of the We number and the maximum thickness of the steady film, which is affected by the surface wettability properties.

Circulation. Cardiovascular interventions, 2012

The ideal bifurcation stenting technique is not established, and data on the hemodynamic characte... more The ideal bifurcation stenting technique is not established, and data on the hemodynamic characteristics at stented bifurcations are limited. 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 (t(r)). 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) m(2), t(r)=5.57·10(-4) m(2)/Pa) with bifurcational area subjected to OSI values >0.25, >0.35, and >0.45 calculated as 0.36 mm(2), 0.04 mm(2), and 0 mm(2), respectively. Extended bifurcation areas subjected to these OSI values were seen after T-stenting: 0.61 mm(2), 0.18 m...

Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2008

Journal of Colloid and Interface Science, 2006

The detachment of liquid droplets from porous material surfaces used with proton exchange membran... more The detachment of liquid droplets from porous material surfaces used with proton exchange membrane (PEM) fuel cells under the influence of a cross-flowing air is investigated computationally and experimentally. CCD images taken on a purpose-built transparent fuel cell have revealed that the water produced within the PEM is forming droplets on the surface of the gas-diffusion layer. These droplets are swept away if the velocity of the flowing air is above a critical value for a given droplet size. Static and dynamic contact angle measurements for three different carbon gasdiffusion layer materials obtained inside a transparent air-channel test model have been used as input to the numerical model; the latter is based on a Navier-Stokes equations flow solver incorporating the volume of fluid (VOF) two-phase flow methodology. Variable contact angle values around the gas-liquid-solid contact-line as well as their dynamic change during the droplet shape deformation process, have allowed estimation of the adhesion force between the liquid droplet and the solid surface and successful prediction of the separation line at which droplets loose their contact from the solid surface under the influence of the air stream flowing around them. Parametric studies highlight the relevant importance of various factors affecting the detachment of the liquid droplets from the solid surface.

International Journal of Thermal Sciences, 2011

Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a b... more Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a bottleneck to the system's efficiency and even durability under certain operating conditions. Due to the small scale and complex geometry of the materials involved, detailed insight into the pore scale phenomena that take place are difficult to measure or simulate. In the present effort, a direct 3D microscale model of a portion of the PEM cathode channel and carbon cloth GDL is used to parametrically investigate local heat and fluid flow at the GDL's pore scale and their effects on condensation of water vapour that leads to flooding. The 3D simulation through the microscale geometry is among the first appearing in the international literature. The NaviereStokes, energy and water vapour transport equations are solved at steady state and in three-dimensional space for a range of inlet velocities and cloth fibre material properties, using a conjugate heat transfer approach to calculate the temperature field within the solid fibres. Psychrometric calculations are applied to provide indications of the conditions and areas most prone to condensation based on the calculated local temperatures and water vapour concentration.

International Journal of Heat and Mass Transfer, 2009

The paper presents results from a numerical investigation of the non-central binary collision of ... more The paper presents results from a numerical investigation of the non-central binary collision of two equal size droplets in a gaseous phase. The flow field is two phase and three dimensional; the investigation is based on the finite volume numerical solution of the Navier–Stokes equations, coupled with the Volume of Fluid Method (VOF), expressing the unified flow field of the

International Journal of Heat and Fluid Flow, 1996

A model for diesel spray wall impaction is presented, which is assessed against experiments for a... more 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

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 ...

Atomization and Sprays, 2010

… Symposium on Turbulence and Shear Flow …, 2005

Oblique flow past a cube or low rise structure is of importance in designing and constructing str... more Oblique flow past a cube or low rise structure is of importance in designing and constructing structures and buildings, among other applications. In the present study the large eddy simulation (LES) methodology is applied on an unstructured grid with locally refined resolution in order to study the surface pressure field and the conical vortices that develop on the roof of a low rise cubicle. Three simulations are performed, evaluating the effect of temporal and spatial resolution and results are compared to experimental measurements and to a point vortex theory for the conical vortex structure. Spatial resolution is found to be of importance in correct representation of the conical vortex structure as well as the high frequency spectrum of the roof-top pressure fluctuations.

… Symposium on Turbulence and Shear Flow …, 2005

Oblique flow past a cube or low rise structure is of importance in designing and constructing str... more Oblique flow past a cube or low rise structure is of importance in designing and constructing structures and buildings, among other applications. In the present study the large eddy simulation (LES) methodology is applied on an unstructured grid with locally refined resolution in order to study the surface pressure field and the conical vortices that develop on the roof of a low rise cubicle. Three simulations are performed, evaluating the effect of temporal and spatial resolution and results are compared to experimental measurements and to a point vortex theory for the conical vortex structure. Spatial resolution is found to be of importance in correct representation of the conical vortex structure as well as the high frequency spectrum of the roof-top pressure fluctuations.

SAE Technical Paper Series, 2007

... damage has been investigated using enlarged transparent nozzle replicas and computational flu... more ... damage has been investigated using enlarged transparent nozzle replicas and computational fluid dynamics ... This has been further supported by CFD predictions obtained using the measured ... New Modelling of Cavitating Flows - a Numerical Study of Unsteady Cavitation on a ...

International Journal of Heat and Fluid Flow, 2005

The paper presents results of a numerical investigation of the impingement process of a droplet n... more The paper presents results of a numerical investigation of the impingement process of a droplet normally onto a wall film. The numerical method is based on the finite volume solution of the Navier-Stokes equations, in their axisymmetric formulation, expressing the flow field of the two phases, liquid and gas, coupled by the volume of fluid method (VOF). The latter allows the tracking of the fluid-gas interfaces. The results are compared with available experimental data for integral quantities such as the lamella development in time, whilst the method gives details of the flow fields not hitherto available and which shed light on the droplet interaction with the gas and the mixing mechanisms of the droplet and the wall film during the initial stages of impingement. The latter information is obtained via a 2-VOF model for the prediction of the distribution of mass in the region of lamella during the splash.

Computers and Geotechnics, 2012

Composites Part B: Engineering, 2011

In this study, a novel reinforcing method for glass-fiber reinforced composites, which consists o... more In this study, a novel reinforcing method for glass-fiber reinforced composites, which consists of inter-adherend glass fibers (IAF) that get through the composite adherend like a pin, was investigated experimentally and numerically. Two types of joints were investigated. One is ...

Atomization and Sprays, 2010

This paper presents an experimental study of the impact of a single water droplet onto a stationa... more This paper presents an experimental study of the impact of a single water droplet onto a stationary liquid bulk built by the previous impact of one or two droplets. The experiments were performed with two different film thicknesses, three different Weber (We) numbers, and two surface contact angles. In both cases we have hydrophilic conditions, which do not allow for a good investigation of this parameter. The morphology of the drop impact was studied using a chargecoupled device (CCD) camera, and the corresponding qualitative and quantitative characteristics regarding the time evolution of the phenomena, such as the diameter and height of the evolving crown, were obtained by image analysis. Analysis of the experimental data evidences that the phenomenon has a strong similarity to the impact of a single drop on shallow films, although the effect of the surface wetting characteristics plays, in this case, a negligible role. The regimes of deposition and splashing are identified as a function of the We number and the maximum thickness of the steady film, which is affected by the surface wettability properties.

Circulation. Cardiovascular interventions, 2012

The ideal bifurcation stenting technique is not established, and data on the hemodynamic characte... more The ideal bifurcation stenting technique is not established, and data on the hemodynamic characteristics at stented bifurcations are limited. 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 (t(r)). 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) m(2), t(r)=5.57·10(-4) m(2)/Pa) with bifurcational area subjected to OSI values >0.25, >0.35, and >0.45 calculated as 0.36 mm(2), 0.04 mm(2), and 0 mm(2), respectively. Extended bifurcation areas subjected to these OSI values were seen after T-stenting: 0.61 mm(2), 0.18 m...

Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2008

Journal of Colloid and Interface Science, 2006

The detachment of liquid droplets from porous material surfaces used with proton exchange membran... more The detachment of liquid droplets from porous material surfaces used with proton exchange membrane (PEM) fuel cells under the influence of a cross-flowing air is investigated computationally and experimentally. CCD images taken on a purpose-built transparent fuel cell have revealed that the water produced within the PEM is forming droplets on the surface of the gas-diffusion layer. These droplets are swept away if the velocity of the flowing air is above a critical value for a given droplet size. Static and dynamic contact angle measurements for three different carbon gasdiffusion layer materials obtained inside a transparent air-channel test model have been used as input to the numerical model; the latter is based on a Navier-Stokes equations flow solver incorporating the volume of fluid (VOF) two-phase flow methodology. Variable contact angle values around the gas-liquid-solid contact-line as well as their dynamic change during the droplet shape deformation process, have allowed estimation of the adhesion force between the liquid droplet and the solid surface and successful prediction of the separation line at which droplets loose their contact from the solid surface under the influence of the air stream flowing around them. Parametric studies highlight the relevant importance of various factors affecting the detachment of the liquid droplets from the solid surface.

International Journal of Thermal Sciences, 2011

Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a b... more Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a bottleneck to the system's efficiency and even durability under certain operating conditions. Due to the small scale and complex geometry of the materials involved, detailed insight into the pore scale phenomena that take place are difficult to measure or simulate. In the present effort, a direct 3D microscale model of a portion of the PEM cathode channel and carbon cloth GDL is used to parametrically investigate local heat and fluid flow at the GDL's pore scale and their effects on condensation of water vapour that leads to flooding. The 3D simulation through the microscale geometry is among the first appearing in the international literature. The NaviereStokes, energy and water vapour transport equations are solved at steady state and in three-dimensional space for a range of inlet velocities and cloth fibre material properties, using a conjugate heat transfer approach to calculate the temperature field within the solid fibres. Psychrometric calculations are applied to provide indications of the conditions and areas most prone to condensation based on the calculated local temperatures and water vapour concentration.

International Journal of Heat and Mass Transfer, 2009

The paper presents results from a numerical investigation of the non-central binary collision of ... more The paper presents results from a numerical investigation of the non-central binary collision of two equal size droplets in a gaseous phase. The flow field is two phase and three dimensional; the investigation is based on the finite volume numerical solution of the Navier–Stokes equations, coupled with the Volume of Fluid Method (VOF), expressing the unified flow field of the

International Journal of Heat and Fluid Flow, 1996

A model for diesel spray wall impaction is presented, which is assessed against experiments for a... more 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

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 ...

Atomization and Sprays, 2010

… Symposium on Turbulence and Shear Flow …, 2005

Oblique flow past a cube or low rise structure is of importance in designing and constructing str... more Oblique flow past a cube or low rise structure is of importance in designing and constructing structures and buildings, among other applications. In the present study the large eddy simulation (LES) methodology is applied on an unstructured grid with locally refined resolution in order to study the surface pressure field and the conical vortices that develop on the roof of a low rise cubicle. Three simulations are performed, evaluating the effect of temporal and spatial resolution and results are compared to experimental measurements and to a point vortex theory for the conical vortex structure. Spatial resolution is found to be of importance in correct representation of the conical vortex structure as well as the high frequency spectrum of the roof-top pressure fluctuations.