Mitral regurgitation (MR) is the most common type of valvular heart disease in patients over the age of 75 in the US. Despite the prevalence of mitral regurgitation in the elderly population, ...however, almost half of patients identified with moderate-severe MR are turned down for traditional open heart surgery due to frailty and other existing co-morbidities. MitraClip (MC) is a recent percutaneous approach to treat mitral regurgitation by placement of MC in the center of the mitral valve to reduce MR. There are currently no computational simulations to elucidate the role of MC on both the fluid and solid mechanics of the mitral valve. Here, we use the Smoothed Particle Hydrodynamics (SPH) approach to study various positional placements of the MC in the mitral valve and its impact on reducing MR. SPH is a particle based (meshless) approach that handles flow through narrow regions quite efficiently. Fluid and surrounding anatomical structure interactions is handled via contact and hence can be used for studying fluid-structure interaction problems such as blood flow with surrounding tissues/structure. This method is available as part of the Abaqus/Explicit solver. Regurgitation was initiated by removing targeted chordae tendineae that are attached to specified leaflets of the mitral valve and, subsequently, MC implants are placed in various locations, starting from the region near where the chordae tendineae were removed and moving away from the location towards the center of the valve. The MC implant location closest to where the chordae tendineae were removed showed the least amount of residual MR post-clip implantation amongst all other locations of MC implant considered. These findings have important implications for strategic placement of the MC depending on the etiology of MR to optimize clinical outcome.
The interaction between a flexible structure and the surrounding fluid gives rise to a variety of phenomena with applications in many areas, such as, stability analysis of airplane wings, ...turbomachinery design, design of bridges, and the flow of blood through arteries. Studying these phenomena requires modeling of both fluid and structure. Many approaches in computational aeroelasticity seek to synthesize independent computational approaches for the aerodynamic and the structural dynamic subsystems. This strategy is known to be fraught with complications associated with the interaction between the two simulation modules. The task is to choosing the appropriate models for fluid and structure based on the application, and to develop an efficient interface to couple the two models. In the present article, we review the recent advancements in the field of fluid–structure interaction, with specific attention to aeroelastic applications. One of the key aspects to developing a robust coupled aeroelastic model is the presence of an efficient moving grid technique to account for structural deformations. Several such techniques are reviewed in this paper. Also, the time scales associated with fluid–structure interaction problems can be very different; hence, appropriate time stepping strategies and/or sub-cycling procedures within the individual field need to be devised. The flutter predictions performed on an AGARD 445.6 wing at different Mach numbers are selected to highlight the state-of-the-art computational and modeling issues.
Purpose
Image-based computational fluid dynamics (CFD) is widely used to predict intracranial aneurysm wall shear stress (WSS), particularly with the goal of improving rupture risk assessment. ...Nevertheless, concern has been expressed over the variability of predicted WSS and inconsistent associations with rupture. Previous challenges, and studies from individual groups, have focused on individual aspects of the image-based CFD pipeline. The aim of this Challenge was to quantify the total variability of the whole pipeline.
Methods
3D rotational angiography image volumes of five middle cerebral artery aneurysms were provided to participants, who were free to choose their segmentation methods, boundary conditions, and CFD solver and settings. Participants were asked to fill out a questionnaire about their solution strategies and experience with aneurysm CFD, and provide surface distributions of WSS magnitude, from which we objectively derived a variety of hemodynamic parameters.
Results
A total of 28 datasets were submitted, from 26 teams with varying levels of self-assessed experience. Wide variability of segmentations, CFD model extents, and inflow rates resulted in interquartile ranges of sac average WSS up to 56%, which reduced to < 30% after normalizing by parent artery WSS. Sac-maximum WSS and low shear area were more variable, while rank-ordering of cases by low or high shear showed only modest consensus among teams. Experience was not a significant predictor of variability.
Conclusions
Wide variability exists in the prediction of intracranial aneurysm WSS. While segmentation and CFD solver techniques may be difficult to standardize across groups, our findings suggest that some of the variability in image-based CFD could be reduced by establishing guidelines for model extents, inflow rates, and blood properties, and by encouraging the reporting of normalized hemodynamic parameters.
High-order-accuracy finite-difference approximations are developed for problems involving arbitrary variable coefficients in the second-order derivatives, e.g., the heat equation or turbulence ...modeling. The methods investigated are discretely conservative, use narrow stencils, and provide stable approximations for these problems. It is known that high-order finite-difference approximations for these types of equations using the chain rule approach may be inadequate for approximating partial differential equations with certain types of variable coefficients. The new approximations are constructed to alleviate this problem by requiring that the operators are stable when the variable coefficients are positive. Examples in heat-transfer problems with variable coefficients are shown to retain the designed order of accuracy and stability with lower error norms than the usual alternative discretizations. Finally, an application of the new stencils is presented for the large-eddy simulation of compressible turbulent flows.
The geometric conservation law (GCL) is an important concept for moving grid techniques because it directly regulates the treatments of the fluid flow and grid movement. With the grid movement at ...every time instant, the Jacobian, associated with the volume of each element in curvilinear co-ordinates, needs to be updated in a conservative manner. In this study, alternative GCL schemes for evaluating the Jacobian have been investigated in the context of a pressure-based Navier-Stokes solver, utilizing moving grid and the first-order implicit time stepping procedure as well as the PISO scheme. GCL-based on first and second-order, implicit as well as time-averaged, time integration schemes were considered. Accuracy and conservative properties were tested on steady-state, laminar flow inside a 2D channel and time dependent, turbulent flow around a 3D elastic wing; both treated with moving grid techniques. It seems that the formal order of accuracy is not a decisive indicator. Instead, the speed of grid movement and the interplay between the flow solver and the GCL treatments make a more noticeable impact.
The geometric conservation law GCL is an important concept for moving grid techniques because it directly regulates the treatments of the fluid flow and grid movement. With the grid movement at every ...time instant, the Jacobian, associated with the volume of each element in curvilinear coordinates, needs to be updated in a conservative manner. In this study, alternative GCL schemes for evaluating the Jacobian have been investigated in the context of a pressurebased NavierStokes solver, utilizing moving grid and the firstorder implicit time stepping procedure as well as the PISO scheme. GCLbased on first and secondorder, implicit as well as timeaveraged, time integration schemes were considered. Accuracy and conservative properties were tested on steadystate, laminar flow inside a 2D channel and time dependent, turbulent flow around a 3D elastic wing both treated with moving grid techniques. It seems that the formal order of accuracy is not a decisive indicator. Instead, the speed of grid movement and the interplay between the flow solver and the GCL treatments make a more noticeable impact.
Thesis (Ph. D.)--University of Florida, 2004.
Title from title page of source document. Document formatted into pages; contains 158 pages. Includes vita. Includes bibliographical references.
A computational methodology for performing fluid-structure interaction computations for three-dimensional elastic wing geometries is presented. The flow solver used is based on an unsteady ...Reynolds-Averaged Navier-Stokes (RANS) model. A well validated k- turbulence model with wall function treatment for near wall region was used to perform turbulent flow calculations. Relative merits of alternative flow solvers were investigated. The predictor-corrector-based Pressure Implicit Splitting of Operators (PISO) algorithm was found to be computationally economic for unsteady flow computations. Wing structure was modeled using Bernoulli-Euler beam theory. A fully implicit time-marching scheme (using the Newmark integration method) was used to integrate the equations of motion for structure. Bilinear interpolation and linear extrapolation techniques were used to transfer necessary information between fluid and structure solvers. Geometry deformation was accounted for by using a moving boundary module. The moving grid capability was based on a master/slave concept and transfinite interpolation techniques. Since computations were performed on a moving mesh system, the geometric conservation law must be preserved. This is achieved by appropriately evaluating the Jacobian values associated with each cell. Accurate computation of contravariant velocities for unsteady flows using the momentum interpolation method on collocated, curvilinear grids was also addressed. Flutter computations were performed for the AGARD 445.6 wing at subsonic, transonic and supersonic Mach numbers. Unsteady computations were performed at various dynamic pressures to predict the flutter boundary. Results showed favorable agreement of experiment and previous numerical results. The computational methodology exhibited capabilities to predict both qualitative and quantitative features of aeroelasticity.
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