Computational Studies on Hemodynamics in Striaght and Bifurcated Arteries

Abstract

Hemodynamic behaviour of blood in the complexed arteries are closely related to the development of cardiovascular disease. Atherosclerosis is the major cause for the cardiovascular disease and is a chronic inflammatory process characterized by thickening of arterial wall cuase for the plaque development. The secondary flows generated at the complexed zone promotes the deposition of atherogenic particles on the outer walls. The formation and subsequent rupture of the plaque depends on wall shear stress (WSS) and oscillatory shear index (OSI). The focus of this present study is to understand the hemodynamics in the complexed region. Numerically, the analysis become more complex when a discrete phase is added to the continuous phase in order to understand the behaviour of atherogenic particles in a pulsatile flow environment. To understand the correlation between the discrete phase (atherogenic) particle behaviour with the characteristics of continuous phase (blood) under varying pulse frequencies in the post stenosis region of complexed geometries are difficult. Hence in simplified way study has been carried out with straight idealized models with different stenosis nature to analyse hemodynamics in the post stenosis region. Continuous phase is modelled by time averaged Navier-Stokes equations and solved by means of Pressure Implicit Splitting of Operators (PISO) algorithm. DPM has been carried out with one way coupling. The transport equations are solved in the Eulerian frame of reference and the discrete phase is simulated in Lagrangian frame of reference. The study brings out the importance of helicity in the atherosclerosis progression. Result shows that the asymmetric nature of stenosis exhibits less helical flow structure and the vortical structures are not getting transported to the downstream. Consequently the average particle residence time (PRT) of the atherogenic particles are one order higher than the symmetric stenosis model. Low PRT leads to enhanced mass transport in the arterial flow and triggers further occlusion/plaque build-up at the post-stenotic region. The extent of asymmetry in a diseased artery may be considered as a useful parameter in understanding the rate of progression of atherosclerosis.To understand the effect of pulse frequency on the hemodynamics of a stenosed arterial wall nature. For which the study has used a flexible wall nature. stenosis parts are assumed as isotropic, elastic homogeneous and incompressible. Different material properties can be assigned to each volume (part) to reflect the complexity (Healthy and diseased). For a healthy arterial wall young’s modulus (E) of 0.4MPa and Poissons ratio of 0.499 are selected. Stiffness was presented in the stenosed region (diseased) hence diseased part chosen to be four times of the healthy part with a young’s modulus of 1.6MPa. At each time step fluid and solid models solved individually using updated solution provided by the other part (Two way coupling). Due to the presence of larger curvature (thick wall) on one side, flow is pushed to one side of the arterial wall leading to higher stress and wall displacement in the asymmetric stenosis. Study has implemented a new device which aims at suppressing the development of atherosclerosis plaque by inducing helical flow structure in the carotid arterial passage (bifurcated). An idealized carotid artery model, chosen for the computational study. It comprises of three bilateral arteries namely Common Carotid Artery (CCA), Internal Carotid Artery (ICA) and External Carotid Artery (ECA). The CCA empties into a smaller ECA and a large ICA. The functional requirement of the swirl generator is to minimize the relative residence time (RRT) of the fluid layer near the endothelial wall without generating any additional pressure drop. With these criteria, the grooves are provided with triangular ribs at the inner wall of the swirl generator. The parameters taken for analysis are height of the rib, the helical angle and the number of ribs. Each of these parameters are varied systematically to understand their influence on the hemodynamics and the atherogenises. A total number of 11 different cases are analysed computationally using large eddy simulation (LES) model. It is observed that the induced helical flow redistributes the kinetic energy from the centre to the periphery. A single rib swirl flow generator proximal to the stent treated passage can generate sufficient helicity to bring down the RRT 36% without creating any additional pressure drop. The swirl flow adds azimuthal instability which increase vortex formations in the passage. The induced helical flow in the domain provokesmore linked vortices (coherent vortices), which may act as self-cleaning mechanism to the arterial wall and increase the stent life from restenosis. Study has investigated influence of pulsatile flow on the mechanical wall parameter after swirl flow induced in the bifurcated arterial passage. Comparsions are made between bifurcated fluid wall with enhanced swirl flow generator (single rib case) and the base case simulation. Arterial wall can be considerd as healthy (free from disease), isotropic, elastic homogeneous and incompressible and the Swirl generator device part can be assumed as rigid. Result shows that Vonmises stress on the interaction surface was generally found in the swirl flow inducer case behaving similar fashion way of Base case stress, which is acting on the wall. Additionally at the site of bifurcation, Vonmises stress closer to the base case. This is an evidence there is no such extra additional stress acting at the flow divergence by enhanced swirl flow in the bifurcated passage. uniformly distributed turbulent kinetic energy around the arterial periphery by induced swirl flow in the complexed geometry does not affect on the vascular injury. Study is focused on understand the effect of hemodynamics on the spatial and temporal variation of WSS and OSI using realistic models with varying degree of carotid artery stenosis. A series of scanned Computed Tomography (CT) images of four patients covering the carotid bifurcation artery with significant carotid artery stenosis, lumen and wall surface of the carotid bifurcation were obtained. The lumen portion was traced out from the CT image using an in-house software (ImgTracerTM, courtesy of AtheroPoint, Roseville, CA, USA). The output of the ImgTracerTM is the x-y coordinates in terms of pixels. This was further converted into millimetre (based on the resolution) and then feed into a commercially available 3D geometry modeller ICEM CFD to prepare the required polylines upon which the surface will be created. Smoothening of the surface was done using 3D modelling software CATIA. A finite volume based CFD method was utilized to understand the hemodynamics in pulsatile flow conditions. It was observed that, high stenosis models occupied a large value of normalized WSS in the ICA whereas they had smaller values of normalized WSS in the CCA. Forclinical use, present study recommend using the spatial average value of oscillatory shear rather than the maximum value for an accurate knowledge about the severity of stenosis. The bulk flow hemodynamics is represented with the direction of resultant vorticity. It reveals that, after the bifurcation zone a change of spin happens with the resultant vorticity due to the secondary flows originated from the inner wall of the bifurcation zone. Additionally, we propose the use of limiting streamlines as a novel and convenient method to identify the disturbed flow region that are prone to atherogenesis. Study has been numerically tests the optimized conceptual design of a device (swirl generator) that generate helical flow structure in the patient specific carotid artery models. The length of the helicity generator is three times the CCA diameter and it is placed five diameter distal (-5D) to the bifurcation zone in the CCA passage. At the inlet and outlets sufficient length have been provided to enhance the fully developed flow. As a result of the helical flow movement inside the arterial passage, the kinetic energy has been redistributed from the centre of the arterial passage to the periphery. This helps in washes out the recirculation regions near the bifurcation passage. Hence fluid residence time decreases with induced helical flow in the arterial passage.