Gabriel Usera | UdelaR - Academia.edu (original) (raw)

Papers by Gabriel Usera

Optical methods are used to study the following problems in Fluid Mechanics: (1) propagation of s... more Optical methods are used to study the following problems in Fluid Mechanics: (1) propagation of solitary waves in water channels; (2) thermoconvective flow of petroleum-in-water emulsions; (3) flow distribution in combustion chambers; and (4) propagation of blood in human veins.

A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical sur... more A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical surface with a laser beam. The cylindrical surface may be a metallic needle or the core of an optical fiber. In the first case a speckle pattern appears around the light cone. The direction and mean length of light speckles are studied in the text. In the second case the light intensity distribution around the light cone presents bright and dark regions resulting from interference between light rays reflected from and passing through the fiber core. The hollow light cone is transformed into a hollow light cylinder (HLC) by means of a lens whose focus coincides with the cone vertex. The HLC is used to explore the cylindrical layer placed in the neighborhood of the internal wall of a combustion chamber model. Due to the large diameter of the model a Fresnel lens is used to transform the light cone into a light cylinder. This introduces additional optical noise in the system. The HLC is collected again by another Fresnel lens at the model output. A photodetector placed at the focus of the collected beam produces an electrical output showing pulses whenever residual particles pass through the explored region. A quantitative experimental analysis of the performance of the combustion chamber is thus performed.

European Journal of Mechanics B-fluids, 2010

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius d, while it is suppressed for values of d beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius ddd, while it is suppressed for values of ddd beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

Flow Turbulence and Combustion, 2008

An implementation of the finite volume method is presented for the simulation of three dimensiona... more An implementation of the finite volume method is presented for the simulation of three dimensional flows in complex geometries, using block structured body fitted grids and an improved linear interpolation scheme. The interfaces between blocks are treated in a fully implicit manner, through modified linear solvers. The cells across block interfaces can be matching one-to-one or many-to-one. In addition, the use of sliding block interfaces allows the incorporation of moving rigid bodies inside the flow domain. An algebraic multigrid solver has been developed that works with this block structured approach, speeding up the iterations for the pressure. The flow solver is parallelized by domain decomposition using OpenMP, based on the same grid block structure. Application examples are presented that demonstrate these capabilities. This numerical model has been made freely available by the authors.

In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall boun... more In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall bounded flows are addressed. The aim is to exploit the enriched information content of time resolved PIV series of images to overcome some issues that affect the PIV measurements, and to improve the performance and accuracy of the technique, specially near wall boundaries. The use of the time history of local pixel brightness allows the removal of spurious permanent reflections that deteriorate the detection of the correlation peaks. Further, in resolved time series of PIV images the use of multiple image correlation methods is suggested. One of such methods, involving the simultaneous correlation of three consecutive images is proposed, and it is shown that it can enhance the displacement peak in the correlation plane. For the specific case of wall bounded flows, a method to improve the accuracy of PIV image analysing near image boundaries is presented, that can be applied to both iterative and non iterative correlation methods. The correction is based on the accurate computation of the actual placement of the velocity vector estimation when the interrogation area overlaps the image boundary. This computation is based on the relative displacement of the centre of mass of the truncated weighted window, as shown if the accompanying sketch. The resulting correction is then applied to compensate the profile distortion, as in the plot below. It is shown that the proposed method keeps the error level in near boundary zones to the same level found far away form the boundaries, outperforming standard algorithms in this aspect.

European Journal of Mechanics B-fluids, 2006

A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of... more A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of the secondary flow motions observed in the large-eddy simulation of a turbulent square channel flow. The data analysed are the multi-valued time series obtained from sampling the secondary velocity components at a fixed cross-section of the channel, over consecutive time steps. The mean values of the secondary flow motion velocities are one order of magnitude lower than their r.m.s values. The purpose of the conditional sampling scheme used is to replace the picture of the secondary flow motions provided by the unconditional time mean with several ensemble averages. In this way the whole variability of the instantaneous data can be split in two parts: one for the difference between the observed ensemble averages, and the other for the variability within each ensemble. Unlike other conditional sampling schemes which sort only part of the data into one or more families depending on an externally fixed condition, the fuzzy clustering approach used here first determines the optimum number of families or clusters and then classifies all the recorded time steps. The results show that the local turbulence intensities of the ensemble averages obtained from fuzzy clustering can be reduced by one order of magnitude. In addition, the classification of all the time steps into several clusters or families enables the large scale dynamics of the educed structures to be analysed.  2005 Elsevier SAS. All rights reserved.

Flow Turbulence and Combustion, 2006

In this paper an approach to the validation of transient numerical simulations of turbulent flows... more In this paper an approach to the validation of transient numerical simulations of turbulent flows through the use of time resolved PIV data is presented and applied to the case of turbulent flow within a simplified model of printed circuit board (PCB) enclosure. The comparison between numerical and experimental data is not limited here to the time averaged fields, but transient flow structures are also addressed. With this purpose a set of conditionally sampled averages, or cluster averages, are obtained through a Fuzzy Clustering technique. Further, the time history of the flow is analyzed on the base of this set of cluster averages, yielding time sequence relations among them that simplify their interpretation in terms of flow dynamics. In this way, events of alternate vortex shedding have been identified in a selected portion of the flow, and showed to exist in both numerical and experimental sets of data.

International Journal of Thermal Sciences

Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations... more Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations inside Printed Circuit Board (PCB) enclosures to understand turbulent flows, heat transport and design techniques to improve cooling effectiveness of electronic devices. The numerical method caffa3d [1] uses, a spatial discretization based on block-structured, non-orthogonal body fitted grids. Numerical simulations for two different PCB geometry configurations are analyzed using two Reynolds numbers. In the top sections for both configurations, the dynamic field shows flow structures such as a pair of quasisymmetric counter-rotating vortices while at the bottom sections the flow behavior depends on the geometry. In the closed geometry the pair of quasi-symmetric counter-rotating vortices remains instead in the open geometry those structures disappear giving way to smaller ones. The temperature field is practically the same above the plate encasing the PCB, differences appear below the plate. While analyzing the plane just below for the open geometry the main consideration is the presence of a hot zone.

European Journal of Mechanics B-fluids, 2010

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius d, while it is suppressed for values of d beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius ddd, while it is suppressed for values of ddd beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

Flow Turbulence and Combustion, 2008

An implementation of the finite volume method is presented for the simulation of three dimensiona... more An implementation of the finite volume method is presented for the simulation of three dimensional flows in complex geometries, using block structured body fitted grids and an improved linear interpolation scheme. The interfaces between blocks are treated in a fully implicit manner, through modified linear solvers. The cells across block interfaces can be matching one-to-one or many-to-one. In addition, the use of sliding block interfaces allows the incorporation of moving rigid bodies inside the flow domain. An algebraic multigrid solver has been developed that works with this block structured approach, speeding up the iterations for the pressure. The flow solver is parallelized by domain decomposition using OpenMP, based on the same grid block structure. Application examples are presented that demonstrate these capabilities. This numerical model has been made freely available by the authors.

In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall boun... more In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall bounded flows are addressed. The aim is to exploit the enriched information content of time resolved PIV series of images to overcome some issues that affect the PIV measurements, and to improve the performance and accuracy of the technique, specially near wall boundaries. The use of the time history of local pixel brightness allows the removal of spurious permanent reflections that deteriorate the detection of the correlation peaks. Further, in resolved time series of PIV images the use of multiple image correlation methods is suggested. One of such methods, involving the simultaneous correlation of three consecutive images is proposed, and it is shown that it can enhance the displacement peak in the correlation plane. For the specific case of wall bounded flows, a method to improve the accuracy of PIV image analysing near image boundaries is presented, that can be applied to both iterative and non iterative correlation methods. The correction is based on the accurate computation of the actual placement of the velocity vector estimation when the interrogation area overlaps the image boundary. This computation is based on the relative displacement of the centre of mass of the truncated weighted window, as shown if the accompanying sketch. The resulting correction is then applied to compensate the profile distortion, as in the plot below. It is shown that the proposed method keeps the error level in near boundary zones to the same level found far away form the boundaries, outperforming standard algorithms in this aspect.

European Journal of Mechanics B-fluids, 2006

A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of... more A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of the secondary flow motions observed in the large-eddy simulation of a turbulent square channel flow. The data analysed are the multi-valued time series obtained from sampling the secondary velocity components at a fixed cross-section of the channel, over consecutive time steps. The mean values of the secondary flow motion velocities are one order of magnitude lower than their r.m.s values. The purpose of the conditional sampling scheme used is to replace the picture of the secondary flow motions provided by the unconditional time mean with several ensemble averages. In this way the whole variability of the instantaneous data can be split in two parts: one for the difference between the observed ensemble averages, and the other for the variability within each ensemble. Unlike other conditional sampling schemes which sort only part of the data into one or more families depending on an externally fixed condition, the fuzzy clustering approach used here first determines the optimum number of families or clusters and then classifies all the recorded time steps. The results show that the local turbulence intensities of the ensemble averages obtained from fuzzy clustering can be reduced by one order of magnitude. In addition, the classification of all the time steps into several clusters or families enables the large scale dynamics of the educed structures to be analysed.  2005 Elsevier SAS. All rights reserved.

Flow Turbulence and Combustion, 2006

In this paper an approach to the validation of transient numerical simulations of turbulent flows... more In this paper an approach to the validation of transient numerical simulations of turbulent flows through the use of time resolved PIV data is presented and applied to the case of turbulent flow within a simplified model of printed circuit board (PCB) enclosure. The comparison between numerical and experimental data is not limited here to the time averaged fields, but transient flow structures are also addressed. With this purpose a set of conditionally sampled averages, or cluster averages, are obtained through a Fuzzy Clustering technique. Further, the time history of the flow is analyzed on the base of this set of cluster averages, yielding time sequence relations among them that simplify their interpretation in terms of flow dynamics. In this way, events of alternate vortex shedding have been identified in a selected portion of the flow, and showed to exist in both numerical and experimental sets of data.

International Journal of Thermal Sciences

Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations... more Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations inside Printed Circuit Board (PCB) enclosures to understand turbulent flows, heat transport and design techniques to improve cooling effectiveness of electronic devices. The numerical method caffa3d [1] uses, a spatial discretization based on block-structured, non-orthogonal body fitted grids. Numerical simulations for two different PCB geometry configurations are analyzed using two Reynolds numbers. In the top sections for both configurations, the dynamic field shows flow structures such as a pair of quasisymmetric counter-rotating vortices while at the bottom sections the flow behavior depends on the geometry. In the closed geometry the pair of quasi-symmetric counter-rotating vortices remains instead in the open geometry those structures disappear giving way to smaller ones. The temperature field is practically the same above the plate encasing the PCB, differences appear below the plate. While analyzing the plane just below for the open geometry the main consideration is the presence of a hot zone.

A general purpose incompressible flow solver, called caffa3d.MBRi, is presented which features a ... more A general purpose incompressible flow solver, called caffa3d.MBRi, is presented which features a block structured framework to accommodate both a flexible approach to geometry representation and a straightforward implementation of parallel capabilities through the MPI library. Representation of complex geometries can be handled semi automatically through a combination of body fitted blocks of grids and the immersed boundary condition strategy over both Cartesian and body fitted grid blocks. The parallelization strategy is based on the same block structured framework, by means of encapsulated MPI calls performing a set of conceptually defined high level communication tasks. A set of real world applications ranging from bioengineering to microclimate scenarios is presented to demonstrate the capabilities of the solver, which is open source and freely available through the web page.

Optical methods are used to study the following problems in Fluid Mechanics: (1) propagation of s... more Optical methods are used to study the following problems in Fluid Mechanics: (1) propagation of solitary waves in water channels; (2) thermoconvective flow of petroleum-in-water emulsions; (3) flow distribution in combustion chambers; and (4) propagation of blood in human veins.

A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical sur... more A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical surface with a laser beam. The cylindrical surface may be a metallic needle or the core of an optical fiber. In the first case a speckle pattern appears around the light cone. The direction and mean length of light speckles are studied in the text. In the second case the light intensity distribution around the light cone presents bright and dark regions resulting from interference between light rays reflected from and passing through the fiber core. The hollow light cone is transformed into a hollow light cylinder (HLC) by means of a lens whose focus coincides with the cone vertex. The HLC is used to explore the cylindrical layer placed in the neighborhood of the internal wall of a combustion chamber model. Due to the large diameter of the model a Fresnel lens is used to transform the light cone into a light cylinder. This introduces additional optical noise in the system. The HLC is collected again by another Fresnel lens at the model output. A photodetector placed at the focus of the collected beam produces an electrical output showing pulses whenever residual particles pass through the explored region. A quantitative experimental analysis of the performance of the combustion chamber is thus performed.

European Journal of Mechanics B-fluids, 2010

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius d, while it is suppressed for values of d beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius ddd, while it is suppressed for values of ddd beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

Flow Turbulence and Combustion, 2008

An implementation of the finite volume method is presented for the simulation of three dimensiona... more An implementation of the finite volume method is presented for the simulation of three dimensional flows in complex geometries, using block structured body fitted grids and an improved linear interpolation scheme. The interfaces between blocks are treated in a fully implicit manner, through modified linear solvers. The cells across block interfaces can be matching one-to-one or many-to-one. In addition, the use of sliding block interfaces allows the incorporation of moving rigid bodies inside the flow domain. An algebraic multigrid solver has been developed that works with this block structured approach, speeding up the iterations for the pressure. The flow solver is parallelized by domain decomposition using OpenMP, based on the same grid block structure. Application examples are presented that demonstrate these capabilities. This numerical model has been made freely available by the authors.

In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall boun... more In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall bounded flows are addressed. The aim is to exploit the enriched information content of time resolved PIV series of images to overcome some issues that affect the PIV measurements, and to improve the performance and accuracy of the technique, specially near wall boundaries. The use of the time history of local pixel brightness allows the removal of spurious permanent reflections that deteriorate the detection of the correlation peaks. Further, in resolved time series of PIV images the use of multiple image correlation methods is suggested. One of such methods, involving the simultaneous correlation of three consecutive images is proposed, and it is shown that it can enhance the displacement peak in the correlation plane. For the specific case of wall bounded flows, a method to improve the accuracy of PIV image analysing near image boundaries is presented, that can be applied to both iterative and non iterative correlation methods. The correction is based on the accurate computation of the actual placement of the velocity vector estimation when the interrogation area overlaps the image boundary. This computation is based on the relative displacement of the centre of mass of the truncated weighted window, as shown if the accompanying sketch. The resulting correction is then applied to compensate the profile distortion, as in the plot below. It is shown that the proposed method keeps the error level in near boundary zones to the same level found far away form the boundaries, outperforming standard algorithms in this aspect.

European Journal of Mechanics B-fluids, 2006

A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of... more A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of the secondary flow motions observed in the large-eddy simulation of a turbulent square channel flow. The data analysed are the multi-valued time series obtained from sampling the secondary velocity components at a fixed cross-section of the channel, over consecutive time steps. The mean values of the secondary flow motion velocities are one order of magnitude lower than their r.m.s values. The purpose of the conditional sampling scheme used is to replace the picture of the secondary flow motions provided by the unconditional time mean with several ensemble averages. In this way the whole variability of the instantaneous data can be split in two parts: one for the difference between the observed ensemble averages, and the other for the variability within each ensemble. Unlike other conditional sampling schemes which sort only part of the data into one or more families depending on an externally fixed condition, the fuzzy clustering approach used here first determines the optimum number of families or clusters and then classifies all the recorded time steps. The results show that the local turbulence intensities of the ensemble averages obtained from fuzzy clustering can be reduced by one order of magnitude. In addition, the classification of all the time steps into several clusters or families enables the large scale dynamics of the educed structures to be analysed.  2005 Elsevier SAS. All rights reserved.

Flow Turbulence and Combustion, 2006

In this paper an approach to the validation of transient numerical simulations of turbulent flows... more In this paper an approach to the validation of transient numerical simulations of turbulent flows through the use of time resolved PIV data is presented and applied to the case of turbulent flow within a simplified model of printed circuit board (PCB) enclosure. The comparison between numerical and experimental data is not limited here to the time averaged fields, but transient flow structures are also addressed. With this purpose a set of conditionally sampled averages, or cluster averages, are obtained through a Fuzzy Clustering technique. Further, the time history of the flow is analyzed on the base of this set of cluster averages, yielding time sequence relations among them that simplify their interpretation in terms of flow dynamics. In this way, events of alternate vortex shedding have been identified in a selected portion of the flow, and showed to exist in both numerical and experimental sets of data.

International Journal of Thermal Sciences

Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations... more Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations inside Printed Circuit Board (PCB) enclosures to understand turbulent flows, heat transport and design techniques to improve cooling effectiveness of electronic devices. The numerical method caffa3d [1] uses, a spatial discretization based on block-structured, non-orthogonal body fitted grids. Numerical simulations for two different PCB geometry configurations are analyzed using two Reynolds numbers. In the top sections for both configurations, the dynamic field shows flow structures such as a pair of quasisymmetric counter-rotating vortices while at the bottom sections the flow behavior depends on the geometry. In the closed geometry the pair of quasi-symmetric counter-rotating vortices remains instead in the open geometry those structures disappear giving way to smaller ones. The temperature field is practically the same above the plate encasing the PCB, differences appear below the plate. While analyzing the plane just below for the open geometry the main consideration is the presence of a hot zone.

European Journal of Mechanics B-fluids, 2010

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius d, while it is suppressed for values of d beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is s... more The effect of fixed cylindrical rods located at the centerline axis on vortex breakdown (VB) is studied experimentally and numerically. We find that the VB is enhanced for very small values of the rod radius ddd, while it is suppressed for values of ddd beyond a critical value. In order to characterize this effect, the critical Reynolds number for the appearance of vortex breakdown as a function of the radius of the fixed rods and the different aspect ratios was accurately determined, using digital particle image velocimetry. The numerical and experimental results are compared showing an excellent agreement. In addition, a simple model in order to show that this effect also appears in open pipe flows is presented in the appendix.

Flow Turbulence and Combustion, 2008

An implementation of the finite volume method is presented for the simulation of three dimensiona... more An implementation of the finite volume method is presented for the simulation of three dimensional flows in complex geometries, using block structured body fitted grids and an improved linear interpolation scheme. The interfaces between blocks are treated in a fully implicit manner, through modified linear solvers. The cells across block interfaces can be matching one-to-one or many-to-one. In addition, the use of sliding block interfaces allows the incorporation of moving rigid bodies inside the flow domain. An algebraic multigrid solver has been developed that works with this block structured approach, speeding up the iterations for the pressure. The flow solver is parallelized by domain decomposition using OpenMP, based on the same grid block structure. Application examples are presented that demonstrate these capabilities. This numerical model has been made freely available by the authors.

In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall boun... more In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall bounded flows are addressed. The aim is to exploit the enriched information content of time resolved PIV series of images to overcome some issues that affect the PIV measurements, and to improve the performance and accuracy of the technique, specially near wall boundaries. The use of the time history of local pixel brightness allows the removal of spurious permanent reflections that deteriorate the detection of the correlation peaks. Further, in resolved time series of PIV images the use of multiple image correlation methods is suggested. One of such methods, involving the simultaneous correlation of three consecutive images is proposed, and it is shown that it can enhance the displacement peak in the correlation plane. For the specific case of wall bounded flows, a method to improve the accuracy of PIV image analysing near image boundaries is presented, that can be applied to both iterative and non iterative correlation methods. The correction is based on the accurate computation of the actual placement of the velocity vector estimation when the interrogation area overlaps the image boundary. This computation is based on the relative displacement of the centre of mass of the truncated weighted window, as shown if the accompanying sketch. The resulting correction is then applied to compensate the profile distortion, as in the plot below. It is shown that the proposed method keeps the error level in near boundary zones to the same level found far away form the boundaries, outperforming standard algorithms in this aspect.

European Journal of Mechanics B-fluids, 2006

A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of... more A conditional sampling technique, based on fuzzy clustering, is used to educe the organization of the secondary flow motions observed in the large-eddy simulation of a turbulent square channel flow. The data analysed are the multi-valued time series obtained from sampling the secondary velocity components at a fixed cross-section of the channel, over consecutive time steps. The mean values of the secondary flow motion velocities are one order of magnitude lower than their r.m.s values. The purpose of the conditional sampling scheme used is to replace the picture of the secondary flow motions provided by the unconditional time mean with several ensemble averages. In this way the whole variability of the instantaneous data can be split in two parts: one for the difference between the observed ensemble averages, and the other for the variability within each ensemble. Unlike other conditional sampling schemes which sort only part of the data into one or more families depending on an externally fixed condition, the fuzzy clustering approach used here first determines the optimum number of families or clusters and then classifies all the recorded time steps. The results show that the local turbulence intensities of the ensemble averages obtained from fuzzy clustering can be reduced by one order of magnitude. In addition, the classification of all the time steps into several clusters or families enables the large scale dynamics of the educed structures to be analysed.  2005 Elsevier SAS. All rights reserved.

Flow Turbulence and Combustion, 2006

In this paper an approach to the validation of transient numerical simulations of turbulent flows... more In this paper an approach to the validation of transient numerical simulations of turbulent flows through the use of time resolved PIV data is presented and applied to the case of turbulent flow within a simplified model of printed circuit board (PCB) enclosure. The comparison between numerical and experimental data is not limited here to the time averaged fields, but transient flow structures are also addressed. With this purpose a set of conditionally sampled averages, or cluster averages, are obtained through a Fuzzy Clustering technique. Further, the time history of the flow is analyzed on the base of this set of cluster averages, yielding time sequence relations among them that simplify their interpretation in terms of flow dynamics. In this way, events of alternate vortex shedding have been identified in a selected portion of the flow, and showed to exist in both numerical and experimental sets of data.

International Journal of Thermal Sciences

Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations... more Fully implicit finite volume method is used for solving 3D incompressible NaviereStokes equations inside Printed Circuit Board (PCB) enclosures to understand turbulent flows, heat transport and design techniques to improve cooling effectiveness of electronic devices. The numerical method caffa3d [1] uses, a spatial discretization based on block-structured, non-orthogonal body fitted grids. Numerical simulations for two different PCB geometry configurations are analyzed using two Reynolds numbers. In the top sections for both configurations, the dynamic field shows flow structures such as a pair of quasisymmetric counter-rotating vortices while at the bottom sections the flow behavior depends on the geometry. In the closed geometry the pair of quasi-symmetric counter-rotating vortices remains instead in the open geometry those structures disappear giving way to smaller ones. The temperature field is practically the same above the plate encasing the PCB, differences appear below the plate. While analyzing the plane just below for the open geometry the main consideration is the presence of a hot zone.

A general purpose incompressible flow solver, called caffa3d.MBRi, is presented which features a ... more A general purpose incompressible flow solver, called caffa3d.MBRi, is presented which features a block structured framework to accommodate both a flexible approach to geometry representation and a straightforward implementation of parallel capabilities through the MPI library. Representation of complex geometries can be handled semi automatically through a combination of body fitted blocks of grids and the immersed boundary condition strategy over both Cartesian and body fitted grid blocks. The parallelization strategy is based on the same block structured framework, by means of encapsulated MPI calls performing a set of conceptually defined high level communication tasks. A set of real world applications ranging from bioengineering to microclimate scenarios is presented to demonstrate the capabilities of the solver, which is open source and freely available through the web page.