Showing papers by "Raffaele Ponzini published in 2010"

Outflow Conditions for Image-Based Hemodynamic Models of the Carotid Bifurcation: Implications for Indicators of Abnormal Flow

Abstract: Computational fluid dynamics (CFD) models have become very effective tools for predicting the flow field within the carotid bifurcation, and for understanding the relationship between local hemodynamics, and the initiation and progression of vascular wall pathologies. As prescribing proper boundary conditions can affect the solutions of the equations governing blood flow, in this study, we investigated the influence to assumptions regarding the outflow boundary conditions in an image-based CFD model of human carotid bifurcation. Four simulations were conducted with identical geometry, inlet flow rate, and fluid parameters. In the first case, a physiological time-varying flow rate partition at branches along the cardiac cycle was obtained by coupling the 3D model of the carotid bifurcation at outlets with a lumped-parameter model of the downstream vascular network. Results from the coupled model were compared with those obtained by imposing three fixed flow rate divisions (50/50, 60/40, and 70/30) between the two branches of the isolated 3D model of the carotid bifurcation. Three hemodynamic wall parameters were considered as indicators of vascular wall dysfunction. Our findings underscore that the overall effect of the assumptions done in order to simulate blood flow within the carotid bifurcation is mainly in the hot-spot modulation of the hemodynamic descriptors of atherosusceptible areas, rather than in their distribution. In particular, the more physiological, time-varying flow rate division deriving from the coupled simulation has the effect of damping wall shear stress (WSS) oscillations (differences among the coupled and the three fixed flow partition models are up to 37.3% for the oscillating shear index). In conclusion, we recommend to adopt more realistic constraints, for example, by coupling models at different scales, as in this study, when the objective is the outcome prediction of alternate therapeutic interventions for individual patients, or to test hypotheses related to the role of local fluid dynamics and other biomechanical factors in vascular diseases.

Quantitative Analysis of Bulk Flow in Image-Based Hemodynamic Models of the Carotid Bifurcation: The Influence of Outflow Conditions as Test Case

Abstract: Although flow-driven mechanisms associated with vascular physiopathology also deal with four-dimensional phenomena such as species transport, the majority of the research on the subject focuses primarily on wall shear stress as indicator of disturbed flow. Indeed, the role that bulk flow plays in vascular physiopathology has not been thoroughly investigated, partly because of a lack of descriptors that would be able to reduce the intricacy of arterial hemodynamics. Here, an approach is proposed to investigate, in silico, the bulk flow within the carotid bifurcation. For this purpose, we coupled a three-dimensional model of carotid bifurcation with a lumped model of the downstream vasculature. For the sake of comparison, we also imposed three different fixed flow rate repartitions between the internal and external carotid arteries on the three-dimensional model. The bulk flow was characterized by applying a descriptor of helical motion, the helical flow index (HFI) to the model; the HFI has recently been shown to provide an accurate representation of complex flows. Moreover, a new metric is presented to investigate the vorticity dynamics within the bifurcation. Our results highlight the effectiveness of these metrics in the following contexts: (i) identifying and ranking emerging hemodynamic features and (ii) quantifying the influence of the outflow boundary conditions on the composition of the translational and rotational components of the fluid motion. The metrics applied herein allow for a more comprehensive analysis, which may lead to the development of an instrument to relate the bulk flow to vascular pathophysiological events that involve not only fluid-related forces, but also transport phenomena within blood.

Womersley Number-Based Estimates of Blood Flow Rate in Doppler Analysis: In Vivo Validation by Means of Phase-Contrast MRI

Abstract: A common clinical practice during single-point Doppler analysis is to measure the centerline maximum velocity and to recover the time-averaged flow rate by exploiting an assumption on the shape of velocity profile (a priori formula), either a parabolic or a flat one. In a previous study, we proposed a new formula valid for the peak instant linking the maximum velocity and the flow rate by including a well-established dimensionless fluid-dynamics parameter (the Womersley number), in order to account for the hemodynamics conditions (Womersley number-based formula). Several in silico tests confirmed the reliability of the new formula. Nevertheless, an in vivo confirmation is missing limiting the clinical applicability of the formula. An experimental in vivo protocol using cine phase-contrast MRI (2-D PCMRI) technique has been designed and applied to ten healthy young volunteers in three different arterial districts: the abdominal aorta, the common carotid artery, and the brachial artery. Each PCMRI dataset has been used twice: 1) to compute the value of the blood flow rate used as a gold standard and 2) to estimate the flow rate by measuring directly the maximum velocity and the diameter (i.e., emulating the intravascular Doppler data acquisition) and by applying to these data the a priori and the Womersley number-based formulae. All the in vivo results have confirmed that the Womersley number-based formula provides better estimates of the flow rate at the peak instant with respect to the a priori formula. More precisely, mean performances of the Womersley number-based formula are about three times better than the a priori results in the abdominal aorta, five times better in the common carotid artery, and two times better in the brachial artery.

Mitral Valve Models Reconstructor: a Python based GUI software in a HPC environment for patient-specific FEM structural analysis.

Abstract: A new approach in the biomechanical analysis of the mitral valve (MV) focusing on patient-specific modelling has recently been pursued. The aim is to provide a useful tool to be used in clinic for hypotheses testing in pre-operative surgical planning and post-operative follow-up prediction. In particular, the integration of finite element models (FEMs) with 4D echocardiographic advanced images processing seems to be the key turn in patient-specific modelling. The development of this approach is quite slow and hard, due to three main limitations: i) the time needed for FEM preparation; ii) the high computational costs of FEM calculation; iii) the long learning curve needed to complete the analysis without a unified integrated tool which is not currently available.

Analysis of Aerodynamic Indices for Racing Sailing Yachts: a Computational Study and Benchmark on up to 128 CPUs.

Abstract: This work presents a feasibility study for trustable and affordable CFD analysis of aerodynamic indices of racing sailing yachts. A detailed reconstructed model of a recent America’s Cup class mainsail and asymmetrical spinnaker under light wind conditions has been studied using massive parallel RANS modeling on 128 CPUs. A detailed comparison between computational and experimental data has been performed and discussed, thanks to wind tunnel tests performed with the same geometry under the same wind conditions. The computational grid used was of about 37 millions of tetrahedra and the parallel job has been performed on up to 128 CPUs of a distributed memory Linux cluster using a commercial CFD code. An in deep analysis of the CPU usage has been performed during the computation by means of Ganglia and a complete benchmark of the studied case has been done for 64, 48, 32, 16, 8 and 4 CPUs analyzing the advantages offered by two kind of available interconnection technologies: Ethernet and Infiniband. Besides to this computational benchmark, a sensitivity analysis of the global aerodynamic force components, the lift and the drag, to different grid resolution size has been performed. In particular, mesh size across three orders of magnitude have been investigated: from 0.06 million up to 37 million cells. The computational results obtained here are in great agreement with the experimental data. In particular, the fully tetrahedral meshes allow appreciating the beneficial effect of the increasing of the grid resolution without changing grid topology: a converging trend to the experimental value is observed. In conclusion, the present results confirm the validity of RANS modeling as a design tool and show the advantages and costs of a large tetrahedral mesh for downwind sail design purposes.