Fluid-Structure Interaction In Bio-bifurcating Flow And Artificial Heart

Fluid-structure interactions (FSI) in bio-bifurcating flow and artificial heart were numerically investigated. The fluid flow was resolved using CFD solver and the deformation of thin-walled structure was simulated using the 3D nonlinear FEM code developed in the thesis. A staggered method was used for the fluid-structure coupling.Firstly, the respiratory flow in bifurcating airways with COPD (Chronic obstructive pulmonary diseases), which is one of the weakest form of FSI problems, was simulated and more attention was paid to the effect of COPD on respiratory flow. COPD always results in inflammation with secreting mucus that leads to locally narrow and obstruction of the airways so that it has serious effect on respiratory flow. It was found that in numerical computations, the adopted inlet boundary condition has significant influence on the flow structures and it generally dominates the whole flow characteristics. On the other hand, COPD dramatically alters the bifurcating flow. In cases with high Re, in downstream of the throat, there occurs large recirculation which blocks fluid flowing into downstream branches and induces unbalance of mass distributions. The obstruction leads to large increasing of fluid resistance as well as the pressure drop. Generally, more upstream the COPD occurs, more pressure loss experiences. These results explain successfully why it is not benefit for oxygen exchange when COPD patients breathe violently. Moreover, it suggests that the patients should inspire the aerosol gently into the lung in COPD therapy.Secondly, based on the nonlinear shell theory, an FEM code, which can be used for both buckling problem in static analysis and response of structure in dynamic analysis for arbitrarily shaped thin-walled structures, was developed. Combining with the CFD solver Fluent, a numerical method, which can be used for both the steady FSI problems and unsteady FSI problems, was constructed using a staggered method. Then, it was used successfully for steady FSI studies in the bifurcating flow in human bodies and for unsteady FSI studies in artificial hearts.Tubes conveying fluid in human bodies are commonly flexible thin-walled bifurcating structures. Generally, the fluid and the flexible structure are strongly coupled. Based on the pulmonary arterial bifurcation and lung airways in forced expiration, the steady fluid-structure interaction in bifurcating structures conveying viscous fluid was studied. It was found that in three-generation models, the flexible tube, which is located on the second generation, collapses into mode of N = 3 . The deformation of the flexible tube is dominated by the external pressure and it collapses into regular configuration when the external pressure is much higher than the fluid pressure exerting on inner wall. When the fluid pressure is comparable with even larger than the external pressure, the Re plays an important role for the deformation of the tube, so that the flexible tube collapses into irregular configuration. In the expanding section of the collapsed tube, there exists inverse pressure gradient. In high Re case, the inverse pressure gradient is high enough so that the flow begins to separate. The secondary flow in the cross-sections nearly follows the shape of the collapsed tube, namely, the fluid near the wall of the depressions is moved into the tube center while that in the center is moved into the lobes.Finally, the unsteady fluid-structure interaction in centrally-actuated artificial heart was simulated. In centrally-actuated artificial heart, the blood with oxygen is sucked from atriums and pumped into aorta and pulmonary artery by deflation and inflation of the inner deformable diaphragm. As a result, the diaphragm and the blood flow are interacted strongly with each other. It was found that in the simplified centrally-actuated artificial heart, there forms a three-dimensional vortex ring due to the boundary layer separation in the conjunction of the inlet tube and the blood chamber in diastole. This vortex ring is dissipated by fluid viscosity when it moves downwards with the fluid, and a "belt-like" high wall shear stress region, which provides good washing on the blood chamber wall, generates. The "dead" zone forms in the left part of the blood chamber in systole can be avoided by optimizing the shape of the artificial heart and the flow dynamics can also be significantly improved. In the "Phoenix" artificial heart, the initially egg-shaped diaphragm folds into a scalene triangle-shaped configuration and three depressions form in diastolic duration. The motion of the diaphragm includes three stages: pre-buckling, buckling and post-buckling. In the conjunction of the inlet tube and the blood chamber, there also occurs flow separation and forms vortices. In the systolic duration, more vorticity is dissipated by viscosity of the blood but the dissipation is weak in the right part of the blood chamber so that a "dead" zone remains.