Blood flow dynamics at the pulmonary artery bifurcation

Knowledge of physiologic hemodynamics is a fundamental requirement to establish pathological findings. However, little is known about the normal flow fields in the pulmonary arteries, especially for children. The purpose of this study is to characterize flow patterns in the pulmonary artery bifurcation of healthy pediatric subjects using direct numerical simulations. A realistic geometry is obtained via statistical shape modeling, by averaging five subject-specific digital models extracted from cardiovascular magnetic resonance datasets of healthy volunteers. Boundary conditions are assigned to mimic physiological conditions at rest, corresponding to a peak Reynolds number equal to 3400 and a Womersley number equal to 15. Results show that the normal bifurcation is highly hemodynamically efficient, as measured by an energy dissipation index. The curvature of the pulmonary arteries is sufficiently small to prevent flow separation along the inner walls, and no signs of a turbulent-like state are found. In line with previous imaging studies, a helical structure protruding into the right pulmonary artery is detected, and its formation mechanism is elucidated in the paper. These findings might help to identify abnormal flow features in patients with altered anatomic and physiologic states, particularly those with repaired congenital heart disease.Peer ReviewedPostprint (published version

Computational study of pulmonary flow patterns after repair of transposition of great arteries

Patients that undergo the arterial switch operation (ASO) to repair transposition of great arteries (TGA) can develop abnormal pulmonary trunk morphology with significant long-term complications. In this study, cardiovascular magnetic resonance was combined with computational fluid dynamics to investigate the impact of the postoperative layout on the pulmonary flow patterns. Three ASO patients were analyzed and compared to a volunteer control. Results showed the presence of anomalous shear layer instabilities, vortical and helical structures, and turbulent-like states in all patients, particularly as a consequence of the unnatural curvature of the pulmonary bifurcation. Streamlined, mostly laminar flow was instead found in the healthy subject. These findings shed light on the correlation between the post-ASO anatomy and the presence of altered flow features, and may be useful to improve surgical planning as well as the long-term care of TGA patients.Postprint (author's final draft

Abnormal pulmonary artery bending correlates with increased right ventricular afterload following the arterial switch operation

Purpose: In transposition of great arteries, increased right ventricular (RV) afterload is observed following arterial switch operation (ASO), which is not always related to pulmonary artery (PA) stenosis. We hypothesize that abnormal PA bending from the Lecompte maneuver may affect RV afterload in the absence of stenosis. Thus, we sought to identify novel measurements of three-dimensional cardiac magnetic resonance (CMR) images of the pulmonary arteries and compare with conventional measurements in their ability to predict RV afterload. Methods: Conventional measurements and novel measurements of the pulmonary arteries were performed using CMR data from 42 ASO patients and 13 age-matched controls. Novel measurements included bending angle, normalized radius of curvature (Rc), and normalized weighted radius of curvature (Rc-w). Right ventricular systolic pressures (as the surrogate for RV afterload) were measured by either recent echocardiogram or cardiac catheterization. Results: Conventional measurements of proximal PA size correlated with differential pulmonary blood flow (r = 0.49, P = .001), but not with RV peak systolic pressures (r = -0.26, P = .18). In ASO patients, Rc-w correlated with higher RV systolic pressures (r = -0.57, P = .002). Larger neoaortic areas and rightward bending angles correlated with smaller right pulmonary artery Rc (r = -0.48, P = .001; r = 0.41, P = .01, respectively). Finally, both pulmonary arteries had significantly smaller Rc compared to normal controls. Conclusions: Pulmonary arteries exhibit abnormal bends following ASO that correlate with increased RV afterload, independent of PA stenosis. Future work should focus on clinical and hemodynamic contributions of these shape parameters.Postprint (author's final draft

Computational modeling of right ventricular motion and intracardiac flow in repaired tetralogy of fallot

Purpose Patients with repaired Tetralogy of Fallot (rTOF) will develop dilation of the right ventricle (RV) from chronic pulmonary insufficiency and require pulmonary valve replacement (PVR). Cardiac MRI (cMRI) is used to guide therapy but has limitations in studying novel intracardiac flow parameters. This pilot study aimed to demonstrate feasibility of reconstructing RV motion and simulating intracardiac flow in rTOF patients, exclusively using conventional cMRI and an immersed-boundary method computational fluid dynamic (CFD) solver. Methods Four rTOF patients and three normal controls underwent cMRI including 4D flow. 3D RV models were segmented from cMRI images. Feature-tracking software captured RV endocardial contours from cMRI long-axis and short-axis cine stacks. RV motion was reconstructed via diffeomorphic mapping (Deformetrica, deformetrica.org), serving as the domain boundary for CFD. Fully-resolved direct numerical simulations were performed over several cardiac cycles. Intracardiac vorticity, kinetic energy (KE) and turbulent kinetic energy (TKE) was measured. For validation, RV motion was compared to manual tracings, results of KE were compared between CFD and 4D flow. Results Diastolic vorticity and TKE in rTOF patients were 4.12¿±¿2.42 mJ/L and 115¿±¿27/s, compared to 2.96¿±¿2.16 mJ/L and 78¿±¿45/s in controls. There was good agreement between RV motion and manual tracings. The difference in diastolic KE between CFD and 4D flow by Bland-Altman analysis was - 0.89910 to 2 mJ/mL (95% limits of agreement: - 1.351¿×¿10-2 mJ/mL to 1.171¿×¿10-2 mJ/mL). Conclusion This CFD framework can produce intracardiac flow in rTOF patients. CFD has the potential for predicting the effects of PVR in rTOF patients and improve the clinical indications guided by cMRI.Peer ReviewedPostprint (author's final draft

Numerical Study of Owls\u27 Leading-edge Serrations

Owls\u27 silent flight is commonly attributed to their special wing morphology combined with wingbeat kinematics. One of these special morphological features is known as the leading-edge serrations: rigid miniature hook-like patterns found at the primaries of the wings\u27 leading-edge. It has been hypothesized that leading-edge serrations function as a passive flow control mechanism, impacting the aerodynamic performance. To elucidate the flow physics associated with owls\u27 leading-edge serrations, we investigate the flow-field characteristic around a barn owl wing with serrated leading-edge geometry positioned at 20° angle of attack for a Reynolds number of 40 000. We use direct numerical simulations, where the incompressible Navier–Stokes equations are solved on a Cartesian grid with sufficient resolution to resolve all the relevant flow scales, while the wing is represented using an immersed boundary method. We have simulated two wing planforms: with serrations and without. Our findings suggest that the serrations improve suction surface flow by promoting sustained flow reattachment via streamwise vorticity generation at the shear layer, prompting weaker reverse flow, thus augmenting stall resistance. Aerodynamic performance is negatively impacted due to the shear layer passing through the serration array, which results in altered surface pressure distribution over the upper surface. In addition, we found that serrations increase turbulence level in the downstream flow. Turbulent momentum transfer near the trailing edge increased due to the presence of serrations upstream the flow, which also influences the mechanisms associated with separation vortex formation and its subsequent development over the upper surface of the wing. This article was published as Open Access through the CCU Libraries Open Access Publishing Fund. The article was first published in Physics of Fluids: https://doi.org/10.1063/5.017414

Direct numerical simulations of a great horn owl in flapping flight

The fluid dynamics of owls in flapping flight is studied by coordinated experiments and computations. The great horned owl was selected, which is nocturnal, stealthy, and relatively large sized raptor. On the experimental side, perch-to-perch flight was considered in an open wind tunnel. The owl kinematics was captured with multiple cameras from different view angles. The kinematic extraction was central in driving the computations, which were designed to resolve all significant spatio-temporal scales in the flow with an unprecedented level of resolution. The wing geometry was extracted from the planform image of the owl wing and a three-dimensional model, the reference configuration, was reconstructed. This configuration was then deformed in time to best match the kinematics recorded during flights utilizing an image-registration technique based on the large deformation diffeomorphic metric mapping framework. All simulations were conducted using an eddy-resolving, high-fidelity, solver, where the large displacements/deformations of the flapping owl model were introduced with an immersed boundary formulation. We report detailed information on the spatio-temporal flow dynamics in the near wake including variables that are challenging to measure with sufficient accuracy, such as aerodynamic forces. At the same time, our results indicate that high-fidelity computations over smooth wings may have limitations in capturing the full range of flow phenomena in owl flight. The growth and subsequent separation of the laminar boundary layers developing over the wings in this Reynolds number regime is sensitive to the surface micro-features that are unique to each species.Peer ReviewedPostprint (published version

Computational modeling of the right ventricle in repaired tetralogy of Fallot, before and after pulmonary valve replacement

Postprint (published version

Fluid dynamics of right ventricular filling in the presence of pulmonary regurgitation: assessment using DNS and 4D Flow MRI

Postprint (published version

Cost vs. accuracy: second-order vs. high-order methods for eddy-resolving simulations of turbulent separated flows

We report a comparative study of three numerical solvers for the direct numerical simulation of the flow over a sphere at Re = 3700. A high-order spectral-element code (Nek5000), a general purpose, unstructured finite-volume solver (OpenFOAM) and an in-house Cartesian solver using the immersed-boundary method (IBM) are employed for the analysis; results are compared against previous numerical and experimental data. Numerical results show that Nek5000 and the IBM code operate within a similar computational performance range, in terms of cost-vs-accuracy analysis; on the other hand, OpenFOAM needed a significantly higher number of degrees of freedom (and,overall, a higher cost) to match some of the basic features of the flow. Overall, our results suggest that high-order methods and second-order, energy-conserving approaches based on the IBM may be both viable options for high-fidelity scale-resolving simulations of turbulent flows with separation.Postprint (published version