A novel multiblock immersed boundary method enabling high order large eddy simulation of pathological and medical device hemodynamics

Computational fluid dynamics (CFD) simulations are becoming a reliable tool in understanding disease progression, investigating blood flow patterns and evaluating medical device performance such as stent grafts and mechanical heart valves. Previous studies indicate the presence of highly disturbed, transitional and mildly turbulent flow in healthy and pathological arteries. Accurate simulation of the transitional flow requires high order numerics together with a scale resolving turbulence model such as large eddy simulation (LES). This in turn limits one to use a structured fluid flow solver on which complex, branching arterial domains that are typical in the human blood circulatory system could not be handled. To overcome this, a novel multiblock based immersed boundary method (IBM) is developed based on high order discretization schemes that can efficiently simulate blood flow in complex arterial geometries using structured Cartesian fluid flow solvers. The developed solver, WenoHemo, is systematically validated for each of the newly introduced numerics using a variety of numerical and experimental results available in the literature. Three dimensional laminar flow over a sphere, laminar flow in a backward facing step, laminar and transitional flow in an abdominal aortic aneurysm (AAA), transitional flow in a model stenosed artery, and turbulent flow in a mixing layer are used as benchmark cases for validating the solver thoroughly. WenoHemo is then applied to study blood flow patterns in a pathological thoracic aortic aneurysm (TAA) and in a resulting thoracic aorta with a stent graft (TASG) geometry after an endovascular repair (EVAR). Phase averaged velocity profiles, turbulence kinetic energy levels, viscous wall shear stresses and turbulence energy spectra are used to compare the similarities and differences between the blood flow patterns obtained. Presence of well developed turbulence is detected in the case of TAA whereas TASG showed periodic vortex shedding with lower turbulence levels and improved blood flow to the descending aorta. Application of the solver to simulate blood flow patterns obtained in a bi-leaflet mechanical heart valve (BMHV) placed in a model aorta with imposed kinematics of the leaflets is also carried out, which reveals complex blood flow patterns that need to be considered in the design of the same for reliability and to reduce post surgical complications

Characterization of oscillatory instability in lid driven cavity flows using lattice Boltzmann method

In the present work, lattice Boltzmann method (LBM) is applied for simulating flow in a three-dimensional lid driven cubic and deep cavities. The developed code is first validated by simulating flow in a cubic lid driven cavity at 1000 and 12000 Reynolds numbers following which we study the effect of cavity depth on the steady-oscillatory transition Reynolds number in cavities with depth aspect ratio equal to 1, 2 and 3. Turbulence modeling is performed through large eddy simulation (LES) using the classical Smagorinsky sub-grid scale model to arrive at an optimum mesh size for all the simulations. The simulation results indicate that the first Hopf bifurcation Reynolds number correlates negatively with the cavity depth which is consistent with the observations from two-dimensional deep cavity flow data available in the literature. Cubic cavity displays a steady flow field up to a Reynolds number of 2100, a delayed anti-symmetry breaking oscillatory field at a Reynolds number of 2300, which further gets restored to a symmetry preserving oscillatory flow field at 2350. Deep cavities on the other hand only attain an anti-symmetry breaking flow field from a steady flow field upon increase of the Reynolds number in the range explored. As the present work involved performing a set of time-dependent calculations for several Reynolds numbers and cavity depths, the parallel performance of the code is evaluated a priori by running the code on up to 4096 cores. The computational time required for these runs shows a close to linear speed up over a wide range of processor counts depending on the problem size, which establishes the feasibility of performing a thorough search process such as the one presently undertaken

A novel multiblock immersed boundary method for large eddy simulation of complex arterial hemodynamics

Computational fluid dynamics (CFD) simulations are becoming a reliable tool to understand hemodynamics, disease progression in pathological blood vessels and to predict medical device performance. Immersed boundary method (IBM) emerged as an attractive methodology because of its ability to efficiently handle complex moving and rotating geometries on structured grids. However, its application to study blood flow in complex, branching, patient-specific anatomies is scarce. This is because of the dominance of grid nodes in the exterior of the fluid domain over the useful grid nodes in the interior, rendering an inevitable memory and computational overhead. In order to alleviate this problem, we propose a novel multiblock based IBM that preserves the simplicity and effectiveness of the IBM on structured Cartesian meshes and enables handling of complex, anatomical geometries at a reduced memory overhead by minimizing the grid nodes in the exterior of the fluid domain. As pathological and medical device hemodynamics often involve complex, unsteady transitional or turbulent flow fields, a scale resolving turbulence model such as large eddy simulation (LES) is used in the present work. The proposed solver (here after referred as WenoHemo ), is developed by enhancing an existing in-house high-order incompressible flow solver that was previously validated for its numerics and several LES models by Shetty et al. (2010) [33]. In the present work, WenoHemoWenoHemo is systematically validated for additional numerics introduced, such as IBM and the multiblock approach, by simulating laminar flow over a sphere and laminar flow over a backward facing step respectively. Then, we validate the entire solver methodology by simulating laminar and transitional flow in abdominal aortic aneurysm (AAA). Finally, we perform blood flow simulations in the challenging clinically relevant thoracic aortic aneurysm (TAA), to gain insights into the type of fluid flow patterns that exist in pathological blood vessels. Results obtained from the TAA simulations reveal complex vortical and unsteady flow fields that need to be considered in designing and implanting medical devices such as stent graft