Ondřej Bublík

Numerical analysis of non-Newtonian blood flow and wall shear stress in realistic single, double and triple aorto-coronary bypasses.

Considering the fact that hemodynamics plays an important role in the patency and overall performance of implanted bypass grafts, this work presents a numerical investigation of pulsatile non-Newtonian blood flow in three different patient-specific aorto-coronary bypasses. The three bypass models are distinguished from each other by the number of distal side-to-side and end-to-side anastomoses and denoted as single, double and triple bypasses. The mathematical model in the form of time-dependent nonlinear system of incompressible Navier-Stokes equations is coupled with the Carreau-Yasuda model describing the shear-thinning property of human blood and numerically solved using the principle of the SIMPLE algorithm and cell-centred finite volume method formulated for hybrid unstructured tetrahedral grids. The numerical results computed for non-Newtonian and Newtonian blood flow in the three aorto-coronary bypasses are compared and analysed with emphasis placed on the distribution of cycle-averaged wall shear stress and oscillatory shear index. As shown in this study, the non-Newtonian blood flow in all of the considered bypass models does not significantly differ from the Newtonian one. Our observations further suggest that, especially in the case of sequential grafts, the resulting flow field and shear stimulation are strongly influenced by the diameter of the vessels involved in the bypassing.

ANALYSIS OF VIBRO-ACOUSTIC PROPAGATION IN A MOBILE SCREW COMPRESSOR FOR THE DETERMINATION OF EMITTED ACOUSTIC POWER RADIATED OUTSIDE ITS HOUSING

The method for a numerical solution of the vibro-acoustic problem in a mobile screw compressor is proposed and in-house 3D finite element (FE) solver is developed. In order to reduce the complexity of the problem, attention is paid to the numerical solution of the acoustic pressure field in the compressor cavity interacting with the linear elastic compressor housing. Propagation of acoustic pressure in the cavity is mathematically described by the Helmholtz equation in the amplitude form and is induced by periodically varying surface velocity of the engine and compressor assembly. In accordance with prescribed boundary conditions, numerical solution of the Helmholtz equation for the distribution of acoustic pressure amplitudes within the cavity is performed using the finite element method on tetrahedral meshes. For the FE discretisation of the elastic compressor housing, a new 6-noded thin flat shell triangular finite element with 21 DOF based on the Kirchhoff plate theory was developed and implemented. The resulting strong coupled system of linear algebraic equations describing the vibro-acoustic problem, i.e., the problem of interaction between the air inside the cavity and the screw compressor housing, is solved numerically by well-known algorithms implemented in MATLAB. By considering two different benchmark test cases, the developed 3D FE solver is successfully verified against the numerical results provided by the professional computational FE system Radioss. Finally, the vibro-acoustic problem is solved in a simplified model of a real mobile screw compressor by prescribing experimentally measured acoustic velocity on the surface of the engine and compressor assembly. The numerical solution is carried out only with the professional computational FE system Radioss as our developed solver is still unable to process large-sized problems without encountering memory limits. Thus, for the assessment of results computed by Radioss, we use results acquired during experimental measurements on a real mobile screw compressor under operating conditions.

PULSATILE NON-NEWTONIAN BLOOD FLOW MODELLING IN REALISTIC AORTO-CORONARY BYPASS GRAFTS

Nowadays it is generally accepted that hemodynamics has significantly influence on the performance and patency of implanted bypass grafts. In this regard blood flow modelling in realistic geometries can provide a valuable insight into the problem of graft failures associated with restenosis and/or occlusive intimal hyperplasia [Haruguchi, 2003]. Present study tries to contribute to this investigation by modelling pulsatile non-Newtonian blood flow in three realistic aorto-coronary bypass models and by analysing the distribution of hemodynamically significant factors such as wall shear stress (WSS) and oscillatory shear index (OSI).