Effects of Human Postures on Flow Characteristics in Iliac Vein Compression Syndrome / 医用生物力学

Objective To investigate effects of human postures on flow characteristics of iliac vein compression syndrome. Methods The numerical model of iliac vein was reconstructed from CT images of a typical patient with pelvic-type iliac vein compression syndrome with collateral veins. Based on the computational fluid dynamics method, the non-Newtonian model and the porous media model were adopted to describe effects of abnormal structures on blood flow and acquire the wall shear stress and pressure of iliac vein. The discrete phase model was used to study the residence conditions of erythrocytes under three human postures. Results The pressure gradient at two ends of the compressive region was lowest under lying state, while the iliac vein showed a high pressure under sitting and walking states. The local maximum wall shear stresses under three postures were found at narrow segment of the collateral vein and convergence region of two flows of right iliac vein. The maximum shear stress was largest under lying state and smallest under sitting state. The blood residence time of 52.2 s in the left iliac vein was the longest under sitting state. The residence time of 14.8 s was shortest under lying state. The blood residence time was 23.8 s under walking state. Conclusions Porous media model used to simulate the effect of abnormal structures was highly consistent with the angiography data. The venous hypertension under sitting and walking states was consistent with the clinical results, and the lying state could relieve the hypertensive condition. In terms of wall shear stress and blood residence time in iliac vein, the continual change between three human postures would cause endothelial damage and blood flow stasis alternately, thus increase the risk of thrombosis.

Modelagem da vegetação aquática em estudos de dinâmica dos fluidos computacional / Modeling aquatic vegetation in computational fluid dynamics studies

RESUMO O objetivo deste trabalho foi apresentar, por meio da técnica dinâmica dos fluidos computacional (CFD), dois métodos utilizados nas representações conceitual e física da vegetação em meio aquático: meio poroso e elementos geométricos simplificados. Três estudos de caso, que incluem um wetland flutuante e manchas de vegetação, exemplificam a aplicação dos métodos, mostrando suas vantagens e desvantagens. Nas etapas da geometria e da malha, a representação da vegetação como meio poroso é mais simples, prática e rápida do que a da vegetação como elementos geométricos simplificados. Porém, na parte da modelagem das equações, o método do meio poroso não consegue capturar os processos de mistura no interior da vegetação, enquanto o método dos elementos geométricos simplificados consegue.

ABSTRACT The goal of this work was to present, through computation fluid dynamics (CFD), two methods used in the conceptual and physical representation of vegetation in aquatic environments: the porous media approach and the simplified geometric elements. Three case studies, including a floating wetland and patches of vegetation, exemplify how the methods are applied, showing their advantages and disadvantages. At the geometry and meshing stage, the porous media approach shows to be simpler, faster, and more practical than the simplified geometric elements. However, in the equation modeling, the porous media approach is not able to capture the mixing processes inside the vegetation, while the simplified geometric elements method can capture those processes.

Effects from Side Branch Diameter of Intracranial Aneurysm on Hemodynamic Parameters after Flow Diverter Implantation / 医用生物力学

Objective To comprehensively consider the effect of low diverter (FD) implantation on aneurysmal sac and its branches, so as to provide references for making a more reasonable surgical strategy for intracranial aneurysm embolization in clinical practice. Methods Based on computational fluid dynamics (CFD) method, the FD implantation procedure was simulated by using porous media model innovatively. Changes in hemodynamic parameters of aneurysmal sac and side branch with different diameters before and after FD implantation were compared and analyzed, such as blood flow field, velocity, wall pressure and wall shear stress (WSS). Results FD changed the hemodynamic characteristics of aneurysms. The blood flow velocity decreased significantly. The WSS on aneurysmal neck increased, while the difference of WSS between proximal and distal cervical area reduced conversely. Different side branch diameters of vessels had different effects on hemodynamic characteristic changes. The larger diameter would cause the greater blood flow reduction in side branch after FD implantation, but the decrease in velocity of aneurysmal sac and pressure on aneurysmal roof became smaller simultaneously. Meanwhile, the increase of WSS on aneurysmal neck was inversely proportional to the diameter of side branch. Conclusions The larger branch diameter of vessels would cause the worse effect of FD embolization therapy for intracranial aneurysm, worse atherosclerosis improvements and greater possibilities of branch occlusion or other ischemic complications. Doctors should pay more attention to such cases in FD interventional intravascular embolization in clinic.

Sensitivity Analysis of Flow Diverter Treatment of Intracranial Aneurysm Using Porous Media Model / 医用生物力学

Objective To simulate the hemodynamic effects of different flow diverters (FD) parameters by using computational fluid dynamics (CFD) technology, so as to develop a more reasonable FD embolization strategy before operation. Methods The porous media model was used to simulate the process of FD implantation into aneurysms, and the initial values of its own unique porous media momentum source parameters (permeability and inertial resistance) were calculated for a specific FD (Tubridge).The changes of hemodynamic parameters such as blood flow velocity, wall shear stress (WSS), volume flow and pressure of aneurysm-roof were compared under different situations (initial values of 80%, 90%, 100%, 110% and 120%), and the sensitivity analysis on hemodynamic parameters of patient-specific intracranial aneurysms (IA) for the porous media model was further conducted. Results The sensitivity of IA hemodynamic parameters to the permeability of porous media model was as follows: WSSparent-artery＞WSSaneurysm＞paneurysmal-roof, but the sensitivity to inertia resistance was relatively lower. Conclusions By using the porous media model, different metal coverages (MC) of FD could be simulated by choosing different permeability parameters, so it is necessary to adjust specific permeability settings during modeling of FD with different MC.

Analysis of Cerebral Hemorrhage Based on Microcirculation / 医用生物力学

Objective To study the effect and mechanism of capillary microcirculation disturbance on intracerebral hemorrhage. Methods The loading effect of capillaries was replaced by the introduction of porous media. A microcirculation model from the capillaries to the veins was established. The appropriate mechanical boundary conditions were set up for the model by referring to various physiological conditions of human body, and the changes in blood pressure and stress of vascular wall under various conditions were simulated. Results Under normal circumstances, the whole blood pressure of the LSA was relatively low, and the pressure difference between the beginning and the end of the LSA was more obvious, and the stress of all parts of the vascular wall was at the same level. In the case of microcirculation disorder, the whole blood pressure of the LSA increased and the pressure difference between the beginning and the end of the LSA significantly decreased. The stress for each part of the vessel increased and the stress at the end of the LSA increased most significantly. Conclusions The influence of microcirculation disturbance on hemodynamics of the LSA was particularly significant. It was an important factor leading to hemorrhage of the LSA rupture. The research findings are of important theoretical and practical significance for understanding the mechanism of cerebral vascular rupture and preventing the occurrence of cerebral hemorrhage in the case of microcirculation disturbance.

Articular cartilage simulation using depth-dependent mechanical parameters / 医用生物力学

Objective To evaluate the difference in articular cartilage simulation due to the application of either homogeneous or depth-dependent mechanical parameters. Method The nonlinear， biphasic， porous model of articular cartilage was built using poroelasticity module in COMSOL. Under the static load, the model was computed using homogeneous and depth dependent parameter, respectively. The difference between the results was analyzed. Results For the total stress of cartilage， there was no remarkable difference between two parameter configurations. However, for the analysis such as solid phase stress, fluid pressure and flow velocity， the difference between two parameter configurations must be considered. Conclusions Different parameter configuration has negligible effect on the total stress of cartilage, but it is influential to the flow velocity. Therefore， homogeneous mechanical parameters should be used in order to simplify the total stress computation problem. The other more detailed analysis should be based on the depth dependent parameters. These conclusions could be referred to for future cartilage modeling and numerical computation and thus laying a foundation for the design and computation of artificial joint.

Two-dimensional numerical analysis of impact response of the human tibia in Car-pedestrian accidents / 国际生物医学工程杂志

Objective The purpose of this paper was to use a new biphasic poroelastic tibia model to develop a two-dimensional numerical method for simulating impact responses of human tibia in car-pedestrian accidents. Methods The geometry of tibia model was reconstructed from CT scans of the left tibia of a living human volunteer. A poroelastic approach was utilized to establish the governing equations of the model and the finite element method was applied to solve these governing equations. Both cortical and cancellous components of tibia were represented using a poroelastic material model consisting of solid phase (matrix) and fluid phase (marrow). A lateral-medial impact direction was selected in the simulation analysis and the impact responses of the pedestrian tibia during 0-200 ms were analyzed. Results The bending deformation of the tibia predicted by the computer simulation was primarily concentrated on the impact zones. The displacement response of Node 107 in the impact zone indicated a peak displacement of -6 mm at around 75 ms, and the significant time delay between the impact force and the displacement response of the skeleton. The axial stress response at the center of element E77 in the impact zone indicated a peak stress of 140 MPa at around 30 ms,and the significant time delay was observed between the impact force and the axial stress response of the skeleton, too. Conclusion This research developed a two-dimensional numerical method for simulating impact responses of human tibia in car-pedestrian accidents. It was able to approximately simulate the bending deformation, lateral displacement response and axial stress response of pedestrian tibia in the impact zones,and the effects of the fluid phase on the solid phase. More in-depth investigation is helpful to further the biofidelity of tibia dynamics model.

Análise teórico-experimental da dispersão de poluentes líquidos em solos / Theoretical-experimental analysis of liquid pollutant dispersion in soils

Acidentes envolvendo o contato de substâncias químicas tóxicas com o meio ambiente são muito freqüentes. Exemplos são derramamentos acidentais de combustíveis e solventes industriais. Seria interessante predizer a distribuição da substância no solo em função do tempo de contato. Isso possibilitaria prever a extensão do impacto ambiental causado por algum acidente e auxiliar na aplicação de técnicas de remediação. Sendo assim, desenvolveu-se um código computacional para simular a dispersão de poluentes líquidos em solos, a partir de um modelo matemático baseado nos princípios de conservação de massa e transporte de poluentes em meios porosos. Resultados obtidos em simulações foram comparados com experimentos realizados em pequena escala, apresentando uma boa concordância.

Accidents which result in the contact of hazardous chemical substances with the environment are frequent. Typical examples are solvents accidental spills and inadequate waste disposal. Therefore, it would be interesting to predict the distribution of the substance throughout the soil as a function of contact time, in order to foresee the environmental impact and help in the application of remediation techniques. It has been developed a computational code to simulate the dispersion of pollutants in soils, by solving a mathematical model based on the mass conservation and on the transport rate of pollutants in porous media. Numerical results were compared with experimental data, showing a good agreement.