Session FA: Biofluid Dynamics VII: Cardiovascular Flow

A Multi-Scale Model of the Circulatory System for the Study of Left Ventricular Assist Devices

A computer model of the cardiovascular system has been developed to study the hemodynamic effects of a non-pulsatile axial flow left ventricular assist device (LVAD). The model is multi-scale and consists of a distributed quasi-one-dimensional arterial tree, based on integrated Navier-Stokes with a pressure/area state equation representing the compliance of the arteries; and lumped parameter models for the systemic return, pulmonary circulation, coronary circulation, and heart. Physiologically consistent aortic pressure and flow histories have been obtained by including a dynamic aortic valve model that allows back-flow by representing leaflet motion. In addition, a three-dimensional finite element model of the aorta with nonlinear elastic arterial walls can be integrated with the quasi-one-dimensional and lumped parameter models, with the lower fidelity models providing boundary conditions for the detailed model. The three dimensional model allows investigation of the detailed flow characteristics induced by the LVAD. The effect of an LVAD and its implant configuration on the hemodynamics of the cardiovascular system and coronary perfusion are studied for various patient conditions and levels of assist.   

Numerical Analysis of Blood Flow in an Arteriole using Immersed Boundary Lattice Boltzmann Method

In an arteriole with an internal diameter of 10$\sim $100$\mu $m, blood flow has a various flow characteristics such as a decrease in hematocrit, a decrease in viscosity, and axial migration of red blood cells (RBCs). These phenomena are caused by the interaction between RBCs and plasma. The conventional method such as finite difference or finite element method is difficult to track a lot of RBCs which change their shapes depending on shear of flow. In this paper, an immersed boundary lattice Boltzmann method is used to predict the behavior of RBCs in the arteriole. The RBC is assumed as a 2D circular or biconcave capsule, which has a stress-free membrane with the same perimeter in the initial condition. Deformation of capsule is caused by stretching, bending and dilatational energies. The three types of energies are calculated for each capsule in the shear flow. As a result, the total energy of circular capsule is higher than that of biconcave one. Therefore, the membrane of biconcave capsule is stiffer than that of circular one. An analysis is conducted for a channel flow in order to estimate flow resistance varying parameters such as hematocrit, Reynolds number, and the effective radius of capsule.   

A 3D velocimetry study of the flow through prosthetic heart valves

Blood damage commonly appears in medical valve prothesis. It is a mayor concern for the designers and surgeons. It is well known that this damage and other complications result from the modified fluid dynamics through the replacement valve. To evaluate the performance of prosthetic heart valves, it is necessary to study the flow through them. To conduct this study , we have built a flow channel that emulates cardiac conditions and allows optical access such that a 3D-PIV velocimetry system could be used. The experiments are aimed to reconstruct the downstream structure of the flow through a mechanical and a bio-material tricuspid heart valve prothesis. Preliminary results show that the observed coherent structures can be related with haemolysis and trombosis, illnesses commonly found in valve prothesis recipients. The mean flow, the levels of strain rate and the turbulence intensity generated by the valves can also be directly related to blood damage. In general, bio-material made valves tend to reduce these complications.   

Blood flow and wall motion in an idealized left ventricle

During diastole of the heart, the left ventricle (LV) expands as a result of both incoming blood flow and wall material relaxation. In this work, we simulate both of these effects, along with the fluid-structure interaction between the blood and the heart wall. As a first step leading to more realistic studies, we approximate the LV by a prolate ellipsoid and the valves by cylindrical tubes. The mitral valve is open, allowing blood to enter the LV, whereas the aortic valve is closed. To account for the effects of muscle fibers in the heart wall, we model the wall as a multi-layered orthotropic linear elastic material with different material properties in the fiber, sheet, and sheet-normal directions within each layer. Results will be presented for this idealized configuration, while simulations of blood flow in realistic canine left and right ventricles are currently underway.   

Fluid structure interaction (FSI) simulation of a bileaflet mechanical heart valve (MHV)

MHVs are widely used as prosthetics for dysfunctional heart valves. All current MHV designs, however, are prone to thrombus formation, which is believed to be strongly associated with the non-physiological hemodynamics patterns and elevated shear stress level induced by the valve; it is, therefore, of enormous practical importance to study the hemodynamics through MHVs. Here we present an FSI solver modeling the physiological MHV hemodynamics. The solver uses a strong coupling scheme for the FSI problem and a recently developed curvilinear grid/immersed boundary method for flow simulation. The FSI solver is applied to model an in-vitro MHV hemodynamics measurement. The experimental pulsatile flow waveform with peak Reynolds number of 4000 is specified at the inlet and the flow is modeled by DNS. The results, including the dynamics of wake vortical structure, shear distribution and leaflet kinematics, are validated against the experimental data.   

Effect of vortex generators on the closing transient flow of bileaflet mechanical heart valves

The time-periodic closing of bileaflet mechanical heart valves is accompanied by a strong flow transient that is associated with the formation of a counter-rotating vortex pair near the b-datum line of leaflet edges. The strong transitory shear that is generated by these vortices may be damaging to blood elements and may result in platelet activation. In the present work, these flow transients are mitigated using miniature vortex generator arrays that are embedded on the surface of the leaflets. Two vortex generator designs were investigated: one design comprised staggered rectangular fins and the other one staggered hemispheres. The closing transients in the absence and presence of the passive vortex generators are characterized using phase locked PIV measurements. The study utilizes a 25 mm St. Jude Medical valve placed in the aortic position of the Georgia Tech left heart simulator. Measurements of the velocity field in the center plane of the leaflets demonstrate that the dynamics of the transient vortices that precede the formation of the leakage jets can be significantly altered and controlled by relatively simple passive modifications of existing valve designs. Human blood experiments validated the effectiveness of miniature vortex generators in reducing thrombus formation by over 42 percent.   

Three Dimensional Energetics of Left Ventricle Flows Using Time-Resolved DPIV

Left ventricular (LV) flows in the human heart are very complex and in the presence of unhealthy or prosthetic heart valves (HV), the complexity of the flow is further increased. Yet to date, no study has documented the complex 3D hemodynamic characteristics and energetics of LV flows. We present high sampling frequency Time Resolved DPIV results obtained in a flexible, transparent LV documenting the evolution of eddies and turbulence. The purpose is to characterize the energetics of the LV flow field in the presence of four orientations of the most commonly implanted mechanical bileaflet HV and a porcine valve. By decomposing the energy scales of the flow field, the ultimate goal is to quantify the total energy losses associated with vortex ring formation and turbulence dissipation. The energies associated to vortex ring formation give a measure of the energy trapped within the structure while estimations of the turbulence dissipation rate (TDR) give a measure of the energy dissipated at the smaller scales. For the first time in cardiovascular applications, an LES-based PIV method, which overcomes the limitations of conventional TDR estimation methods that assume homogeneous isotropic turbulence, was employed. We observed that energy lost at the larger scales (vortex ring) is much higher than the energy lost at the smaller scales due to turbulence dissipation.   

Vorticity dynamics of Bi- and Trileaflet Prosthetic Heart Valves

Fluid flow through prosthetic heart valves is associated with spatio-temporal complexity that may relate to clinical performance. We present PIV results of the instantaneous vorticity field in the wake of a bileaflet mechanical valve and a trileaflet polymeric valve in an idealized aorta model. For the bileaflet valve, the shear layer formed at the valve housing was observed to roll up into the sinus expansion during the acceleration phase while inducing vorticity of opposite sign near the sinus boundary. The fine structure of the shear layer roll up was influenced by a constellation of karman vortices that formed in the wake of the leaflets. During peak flow secondary instabilities created a more turbulent vorticity field downstream of the valve. The vorticity field during the deceleration phase was in general disorganized. In contrast, the large scale vorticity structure for the trileaflet valve differed drastically. The stability of the shear layer between the central jet and the surrounding fluid was found to govern the vorticity dynamics in the sinus and the downstream regions. Ensemble averaging revealed strikingly symmetric and smooth vorticity fields void of the rich instantaneous dynamics. Comparison with state of the art CFD simulations produced a complete thee-dimensional picture of the vorticity fields.   

Flow Through Deformable Orifice Diaphragms Used as Heart Valve Analogues

Both hemodynamic and structural performance are important considerations in designing replacement heart valves. In this study, compression-molded silicone diaphragms of varying orifice and modulus are used as simplified heart valve analogues. Structural quantities such as diaphragm orifice area and deformation are quantified simultaneously with hemodynamic quantities (flow characteristics). Diaphragms are positioned downstream of a steady fully-developed pipe flow, and velocity fields are quantified both upstream and downstream of each diaphragm using particle image velocimetry (PIV). Diaphragm deformation is obtained from each image, while pressure drop across the diaphragm and volumetric flow rate are measured independently. The combined flow and structural data can be used to validate fluid-structure interaction codes suitable for biomedical applications. The bulk flow results will be compared against the existing hydraulic performance formula for rigid orifice diaphragms, and details of instantaneous flow fields will be presented.   

Experimental Investigation of Flow trough a Mechanical Heart Valve

Turbulent flow trough a model of a mechanical heart valve is investigated using digital particle image velocimetry. The valve leaflets are represented by flat plates mounted in a duct. The emphasis is on the effect of the valve design on the platelet activation state associated with the resulting flow field. Global quantitative images corresponding to multiple planes of data acquisition provide insight into the three-dimensional nature of the flow. Turbulent flow structures including jet-like regions and shed vortices are characterized in terms of patterns of instantaneous and time-averaged velocity, vorticity, and streamline topology. Potential of bileaflet heart valves for being thrombogenic is assessed by quantitative comparison of the associated flow fields in terms of maximum values of turbulent stresses and platelet activation states.