THREE-DIMENSIONAL PHYSIOLOGICAL FLOW SIMULATION ON MULTI-BOX COMPUTATIONAL DOMAINS
The fluid flow phenomena in biological systems are typically complex. The complexity is originated from, for example, non-Newtonian behavior of body fluids, complicated geometry, as well as the interaction of muscle and fluid. With the advent of modern computational technology, both in hardware and software, gradually these problems can be resolved. The present research illustrates two such examples. Grid generation is a branch of applied mathematics that is essential for conducting numerical simulation of fluid flow. In this research, a new grid generation technique is developed and implemented in a flow solver. This technique enables one to perform grid generation for complex geometry using only a single computational zone. Fluid flow can then be analyzed without iteration between zones. The scheme is based on the composite transformation of an algebraic mapping and a mapping governed by the Laplace equation. The governing equations for the grid generation are derived first and then solved numerically. The scheme used for solving the grid generating equations is an extension of the traditional three-dimensional Douglass-Gunn scheme. Areas of extension include the inclusion of mixed derivative terms as well as first-order derivative terms. A unique feature of the proposed grid generation scheme is the concept of multi-box computational domains. In this scheme, the physical domain is mapped onto a geometry composed of many boxes in the computational space, rather than a single box as the traditional method does. The numerical solution routine is adjusted accordingly to accommodate this new feature. Grids were generated for two model geometries using the proposed grid generation software. The graft model features one inflow conduit and two outflow conduits, while the left atrium (LA) model has four inflow conduits and one outflow conduit. Flow simulation was performed using the research code INS3D, which employs the method of artificial compressibility. This method transforms the Navier-Stokes equations into a hyperbolic-parabolic set by adding to them pseudo-pressure gradient terms. The scheme is then marched along the pseudo-time axis, until the velocity field becomes divergence-free. For the flow simulation in side the graft, the effect of Reynolds number and flow-division ratio is examined. The Reynolds number effect is, as expected, demonstrated via the presence of a helical flow structure as well as the overall pressure drop. The flow-division ratio, on the other hand, alters the flow field in a way that moves the stagnation points. In particular, the flow pattern for the case with 50:50 flow-division ratio closely resembles that observed clinically, and the highlighted low wall stress area on the hood and toe of the reinforce strengthen the hypothesis about the formation of intimal hyperplasia. The complicated flow field demonstrated by the case with 100:0 division ratio, corresponding to an occluded distal artery, demonstrated that three-dimensional numerical simulation of the flow field can assist in interpreting data from a PIV (Particle Image Velocimetry) experimental session. The steady-state simulation of the flow field in the left atrium of the heart was another subject of interest. Although steady-state simulation is not as realistic as time accurate simulation, it nevertheless gives information on the long-term performance of the chamber. The simulation shows the existence of low wall shear regions. These low shear stress areas in the chamber are areas susceptible to blood clot formation. In fact, clinical evidence shows that of certain strokes are indeed caused by clots forming in the atrium and traveling through the arterial system and essentially lodging in the brain. Since this phenomenon is geometry-related and there is no practical way to alter it, common therapy for such conditions is to administer certain ‘blood thinners’ (Anticoagulation agenes) to reduce the possibility of blood clot formation. In summary, the present research demonstrates applications of computational fluid dynamics technique in the analysis of flow in biological system. A new grid generation technique is realized, and proved to be very useful in simulating these flows. Flow simulation results provide insights into the system and may be of use for clinic reference.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:cfd grid generation heart chamber graft left atrium
Date of Publication:01/01/2003