1stState of the art and Challenges of Research Efforts (S.C.O.R.E.) at(@) POLIBA 3rd- 5th December 2014 Scheda dei gruppi di ricerca GRUPPO DI RICERCA • COMPLEX FLOW SIMULATION ATTIVO DAL • COMPONENTI 2013_7 • Professori (3): Pietro De Palma, PO, IND-IND/08; Michele Napolitano, PO, IND-IND/08; Giuseppe Pascazio, PO, IND-IND/06 (responsabile scientifico). • Ricercatori (1): Marco D. de Tullio, ING-IND/06. • Dottorandi (3):Dott. Ing. Giovanni Caramia, 27°,ING-IND/08;Dott. Alessandro Coclite, 27°, ING-IND/06; Dott. Ing. DarioDe Marinis, 28°, ING-IND/08. SSD SETTORI ERC (European Research Council) ING-IND/06, Fluidodinamica ING-IND/08, Macchine a Fluido PE8_1 - Aerospace engineering PE8_4 - Computational engineering PE8_5 - Fluid mechanics, hydraulic-, turbo-, and piston engines PE8_13 - Industrial bioengineering TEMATICA: • Simulazione fluidodinamica di flussi complessi LINEE DI RICERCA • Modellistica di gas reali reagenti e non reagenti. • Interazione fluido-struttura e applicazioni biomediche. • Stabilità dei flussi e transizione al regime turbolento. • Turbomacchine. RISULTATI DELLA RICERCA 2013 (co–autori in corsivo) PRODUZIONE SCIENTIFICA • Contributi in rivista: De Palma, 4; de Tullio, 4; Pascazio, 4. • Contributi in volume: • Monografie: • Proceedings:Caramia, 1; Coclite, 2; De Palma, 2; de Tullio, 2; Pascazio, 1. • Brevetti: • Curatele: • Altra tipologia: De Palma, 1. PUBBLICAZIONI CON CO–AUTORI STRANIERI: De Palma, 4; de Tullio, 3; Pascazio, 3. MOBILITÀ INTERNAZIONALE: Coclite (Germania) PROGETTI COMPETITIVI: Multiscale Modelling of Cardiovascular Hemodynamics and Blood Damage Marco D. de Tullio and Giuseppe Pascazio Politecnico di Bari, Dipartimento di Meccanica, Matematica e Management Via Re David 200, 70125 Bari, Italy {marcodonato.detullio,giuseppe.pascazio}@poliba.it http://www.poliba.it Abstract. The main objective of this research project is to develop a computational model to study cardiovascular hemodynamics in realistic configurations, evaluating blood damage induced by non-physiological conditions. The model would serve as an inexpensive tool for scientific and medical research, providing unlimited access to blood flow data as well as fundamental information for the improvement in patient care. The problem is very challenging, with a wide disparity in length scales, from the large vessels to the cellular blood components, requiring a multiscale approach. The analysis includes very complex phenomena: at the blood flow level, one has complex moving geometries, intrinsic flow unsteadiness and very intense velocity gradients both in space and time; at the cellular level (e.g. red blood cells), a high-fidelity representation of the cell membrane is necessary in order to evaluate the response of the cells to the imposed hydrodynamic loading and then evaluate the blood damage (e.g. hemolysis). Keywords: Computational fluid dynamics; fluid-structure-interaction; hemolysis 1 Introduction Cardiovascular disorders are nowadays the leading cause of death in developed countries and the related health and economic issues have lead to a number of devices and surgical techniques for their treatment. The replacement of heart valves, for example, is necessary whenever the natural valve cannot be restored to its normal function. Despite their widespread clinical use and continuous improvements, however, prosthetic heart valves (PHVs), and in particular the mechanical devices, still carry significant risks, related to the non-physiological blood flow through them. Hemolysis is the breakdown of red blood cells (RBCs) leading to loss of hemoglobin, which is the oxygen-carrier protein. Among various pathological conditions, the dominant mechanism of hemolysis observed in patients with mechanical prostheses is mechanical hemolysis [1], caused by the higher hydrodynamic loading as RBCs move through the devices. 2 M.D. de Tullio and G. Pascazio In this work, a numerical approach is presented to accurately investigate the flowfield through aortic prostheses, evaluate the response of the cells to the imposed hydrodynamic loading and then evaluate the blood damage. 2 Method Modeling of mechanical hemolysis is very complex because it involves phenomena taking place over a wide range of spatio/temporal scales, from the large structures with a spatial scale comparable to the valve orifice (order of centimeters) down to the Kolmogorov scale, and even further down to the single RBC scale (about 8µm), and with a broad range of temporal scales associated. For this reason, the work requires a multiscale approach. Considering the blood flow level, the aortic flow is pulsatile, neither laminar nor turbulent, but rather transitional. In this intermediate range, turbulence models do not perform properly, therefore direct numerical simulation is utilized to correctly capture the physics of the transitional flow without introducing artificial dissipation mechanisms. Moreover, the aortic valve has a complex geometry: rigid semi–circular leaflets can rotate about their pivots for mechanical valves, while highly deformable leaflets open and close for the native or biological ones (see figure 1). In order to efficiently describe the motion of the structures, Fig. 1: Left: aortic root and mechanical/biological valve geometries. Right: sketch of the multi-scale approach to evaluate blood damage. a suitable version of the immersed boundary method [2] is employed, able of handling arbitrary moving and/or deforming bodies on a fixed structured grid, avoiding the time-consuming regeneration or deformation of the grid and the successive interpolation of the flow field. The problem is further complicated by the interconnected evolution of fluid and structure; in fact the latter moves under the loads exerted by the flow while the former, in turn, evolves in a domain whose instantaneous geometrical configuration depends on the structure Modelling of Cardiovascular Hemodynamics 3 dynamics. This fluid/structure interaction problem requires the coupled solution of the structure and flow problems [3]. A finite-element solver for the deforming structures is coupled with the fluid solver in a segregated approach, in order to reduce the computational cost and to use optimized solvers for both the fluid and structural problems. On the other hand, in order to model blood damage, a high-fidelity model of the RBC, which is based on coarse-grained molecular models of the erythrocyte membrane, spectrin cytoskeleton [4] is adopted. A large number of Lagrangian tracer particles are released at the inlet of the computational domain (upstream of the valve). The instantaneous shape distortion is then evaluated in time for each single tracer and then related to the blood damage [5], as shown in figure 1. 3 Results The results obtained for two different configurations are summarized, reproducing a biological and a bileaflet mechanical valve, mounted inside a realistic geometry of the aortic root and initial tract of ascending aorta with three sinuses of Valsalva, under physiological conditions. Simulations provide unlimited access to blood flow data that can be used to obtain fundamental information for the improvement in patient care. As an example, the backward Finite Time Lya- Fig. 2: Backward FTLE in the symmetry plane for biological and mechanical valves, at peak of flowrate (left)and during the deceleration phase (right). punov Exponent (FTLE) fields in the symmetry plane are presented in figure 2, at peak of flowrate and during the deceleration phase, enlightening Lagrangian coherent structures for such complicated flows. Results show that during the forward phase, a three-jet configuration is distinctive of bileaflet mechanical valves, with high turbulent shear stresses immediately distal to the valve leaflets, while a jet-like flow emerges from the central orifice of bio-prosthetic valves, with high turbulent shear stresses occurring at the edge of the jet. Moreover, mechanical stresses are evaluated for a large set of tracers flowing through the devices during the cardiac cycle, obtaining the Hemolysis Index (HI) as an indication of propensity of the prostheses to induce blood damage. The results indicate that the flow features and hemolysis level depend strongly on the valve type. Higher 4 M.D. de Tullio and G. Pascazio level of blood damage is observed in the case of mechanical valve: the three highspeed jets due to the leaflets configuration, the vortex shedding downstream of the leaflets, the dynamics of the occluder are all factors that affect hemolysis, giving levels of shear stress higher than that obtained in the biological valve case (see figure 3). Fig. 3: Left: probability density function of mechanical stress acting on the cells flowing through the valve. Right: averaged HI in a cardiac cycle (the dashed flowrate curve is reported for reference). 4 Conclusions A computational model to study cardiovascular hemodynamics in realistic configurations is presented, with the aim of evaluating blood damage induced by prosthetic devices. This accurate and inexpensive tool for scientific and medical research provides unlimited access to blood flow data that can be used to obtain fundamental information for the improvement in patient care. References 1. 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