Blood Flow Simulation Based On Smoothed Dissipative Particle Dynamics

Blood,as the carrier of substances and oxygen,plays an important role in life survival.It also stores the health information of a human,and many diseases are closely related to blood,such as malaria,anemia,thrombosis,and so on.Therefore,blood test is the first step performed for the diagnosis and treatment of many diseases.The studies on blood are not only helpful to understand the mechanism of blood circulation,but also conducive to develop the diagnosis and treatment of blood-related diseases.In this thesis,we performed a numerical simulation on blood flow to investigate the plasma separation and the temporalspatial heterogeneity of hematocrit.Considering the complexity of vascular networks and the size scale of cells,a particle based mesoscopic numerical method,named smoothed dissipative particle dynamics,is used to model the behaviors of fluid in the network.The interaction between cells and fluid is modeled by immersion boundary method.The combination of these two methods is the numerical method(SDPD-IBM)adopted in the present work.It is convenient to solve the problems with fluid-structure interaction in complex simulation domains,and also able to describe the mesoscopic fluctuation.However,there are also several shortcomings,including the inflow/outflow boundary condition and the low computational efficiency.The main contributions of this thesis are: 1)development of high-resolution inflow/outflow boundary condition,2)construction of a high-efficient parallel framework,3)numerical design of a microfluidic chip for plasma separation,and 4)numerical investigation of the temporal-spatial heterogeneity of hematocrit.Different from the mesh based method,the particle based method is generally based on Lagrange description,such that particles are required to continuously flow from the inlet,and the particles moved out from the outlet are required to be settled down.Hence,the implementation of inflow/outflow boundary condition is one of the challenges for particle based methods.In this thesis,we developed such an inflow/outflow boundary condition,by adding an inflow domain and an outflow domain at both ends of the simulation domain,respectively.The inflow domain is a generation region that can continuously generate new particles and cells,where a periodic boundary condition is applied at the same time.When a particle or cell flows through its bounds,it is not only put into the domain due to the periodic boundary condition,but also duplicated into the simulation domain to guarantee particles and cells flowing into the simulation domain continuously.The outflow domain is a deletion region where the particles and cells are settled down.When a particle or cell flows out of this domain,the system mass and momentum may be not conserved any more,if it is directly deleted.To conserve the mass and momentum,we add conservative and dissipative forces on each particle in the domain to adaptively compensate for the losses of mass and momentum.The simulation results showed that the proposed inflow/outflow boundary condition can accurately simulate the blood flow in multiple-outlet vessels.To save the computational cost,particle based methods often scatter a large number of particles to discrete simulation regions,and thus the computational efficiency is another challenge.We constructed a parallel framework with masterslave structure by MPI,in which the main thread is responsible for dividing and assigning the computing tasks.The slave threads are divided into three types,including the cell,inflow and hybrid slave threads.Each cell thread is responsible for a single cell only,and used to calculate the mechanical behaviors of the cell,such as deformation and aggregation forces.In this way,a great deal of message communication can be reduced between cell threads.The inflow threads are divided by the domain decomposition,and responsible for updating the physical quantities such as the position and velocity,as well as the hybrid threads.For a typical simulation case,having 100000 particles and 30 cells,the parallel framework can roughly save CPU time eight times over the serial one.Plasma separation is one of the widely-used operations in experimental analysis and pharmaceutic process.We numerically design a more efficient plasma separation microfluidic chip,based on an experimental chip.First,we extract a single bifurcation from the experimental chip,to investigate the effects of flow rate and bifurcation angle on the separation efficiency.A set of optimal parameters are chosen,i.e.,the bifurcation angle is 2?/3,and the flow ratio between the cell outlet and plasma outlet is 2.5:1.Then,a new microfluidic chip is designed by these optimal parameters,and is compared with the experimental chip.The results showed that the plasma purity can reach 100% and the separation efficiency can reach 64% for the new designed,while 100% purity but 25% efficiency for the experimental chip.Finally,we determine the flow rate and dilution level of blood upon using the new designed chip.The result showed that the separation efficiency first increases then decrease as the hematocrit increases,and a maximum is achieved at 10.4%.In addition,when the flow rate is less than 13.3?l/h at the inlet,the separation purity can reach 100%.However,the purity will decrease if the flow rate continues to increase.Therefore,the optimal hematocrit is 10.4%,and the inlet flow rate is 13.3 ?l /h,the separation efficiency and separation purity are 64% and 100%.It can be seen that such a simulation is quite helpful in the design of biomedical devices.Hematocrit is one of the indicators of human health,and is of great significance in the diagnosis and treatment of diseases.We explore the temporal-spatial heterogeneity of hematocrit,by investigating the distribution of hematocrit in three vascular networks.With respect to time,there are two types of hematocrit behaviors observed.One is that the hematocrit varies steadily,but the other is periodic oscillation.Spatially,the hematocrit distributes quite differently in the vessels with different diameters,and different shapes such as straight,curved and bifurcated.Such a temporal-spatial heterogeneity is found to be attributed to the presence of vascular bifurcations.If the vessel is straight without bifurcations,the hematocrit will be more stable in time and space.Hence,we further analyze the effects of flow rate,vessel diameter and vessel curvature on the hematocrit in a single bifurcation.Our investigations provide more accurate information for the diagnosis and treatment of related diseases,compared with the popular hematocrit measurements,such as blood tests that only gives an average hematocrit,and MRI that provides two-dimensional measurement data.