Complex physics and biochemistry, the wide range of temporal and spatial scales to be captured, and the significant inter- and intraindividual variations make the description of the blood flow in the human aorta a difficult task. Although computational tools are mature, said aspects challenge aortic biomechanical models, and the approach must always be tailored to the question at hand. Besides being highly pulsatile, the curvature and tortuosity of the aortic geometry strongly impacts the flow dynamics with aortic pathologies may even lead to turbulent flow. In the following chapter, the characteristics of blood and different modeling approaches for rheology, species transport and thrombus/stenosis development will be addressed.

Purpose: The purpose of this study is to assess the importance of non-Newtonian rheological models on blood flow in the human thoracic aorta. Methods: The pulsatile flow in the aorta is simulated using the models of Casson, Quemada and Walburn-Schneck in addition to a case of fixed (Newtonian) viscosity. The impact of the four rheological models (using constant hematocrit) was assessed with respect to (i) magnitude and deviation of the viscosity relative to a reference value (the Newtonian case); (ii) wall shear stress (WSS) and its time derivative; (iii) common WSS-related indicators, OSI, TAWSS and RRT; (iv) relative volume and surface-based retrograde flow; and (v) the impact of rheological models on the transport of small particles in the thoracic aorta. Results: The time-dependent flow in the thoracic aorta implies relatively large variations in the instantaneous WSS, due to variations in the instantaneous viscosity by as much as an order of magnitude. The largest effect was observed for low shear rates (tens s-1). The different viscosity models had a small impact in terms of time- and spaced-averaged quantities. The significance of the rheological models was clearly demonstrated in the instantaneous WSS, for the space-averaged WSS (about 10%) and the corresponding temporal derivative of WSS (up to 20%). The longer-term accumulated effect of the rheological model was observed for the transport of spherical particles of 2 mm and 2 mm in diameter (density of 1200 kg/m3). Large particles' total residence time in the brachiocephalic artery was 60% longer compared to the smaller particles. For the left common carotid artery, the opposite was observed: the smaller particles resided considerably longer than their larger counterparts. Conclusions: The dependence on the non-Newtonian properties of blood is mostly important at low shear regions (near walls, stagnation regions). Time- and space-averaging parameters of interest reduce the impact of the rheological model and may thereby lead to under-estimation of viscous effects. The rheological model affects the local WSS and its temporal derivative. In addition, the transport of small particles includes the accumulated effect of the blood rheological model as the several forces (e.g., drag, added mass and lift) acting on the particles are viscosity dependent. Mass transport is an essential factor for the development of pathologies in the arterial wall, implying that rheological models are important for assessing such risks.

This Special Issue comprises original research and reviews articles in the research area of computational biofluid dynamics, fluid–structure interaction, and acoustic phenomena of relevance to confined, unsteady, transitional, or turbulent fluid flow scenarios in the human body. The selected papers in this issue represent several trends in biofluid mechanics. The scope of the papers ranges from extending numerical methodology for handling the interaction between the flow and the vessel (fluid–structure interactions, FSI) to using computational tools to guide intervention in clinical situations.

Patient specific based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases. The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. This fact has not been recognized in the literature up to this point, making patient specific simulations potentially less reliable.

Blood clot formation can be initiated by local flow conditions where regions of high shear and long residence time regions, such as flow separation and stagnation, have been identified as risk factors. This study highlights coherent structures,some of which not yet considered in the literature that may contribute to blood clot formation in the ECMO (Extra Corporeal Membrane Oxygenator) circuit. The centrifugal ECMO pump investigated in this study is compact and delivers adequate volume of blood with relatively high pressure in order to compensate for the large pressure drop in the membrane oxygenator. These requirements lead to regions with high shear in several different parts of the pump. In the narrow gap between the pump house and the impeller body (the magnet) a Taylor-Couette-like flow is observed with azimuthally aligned wavy vortices, which are also pushed towards the bottom of the pump-house by the flow generated by the blades. At the bottom gap between the impeller house and the pump house one finds spiraling flow structures, due to the rotation of the former structure. Separation bubbles are found near the tongue of the pump and at the lee sides of the blades. Such vortical structures have in literature been identified as regions where platelets may be activated whereby clots may develop.

As almost all combustion processes of practical interest take place in the presence of turbulence, the development of the increasingly refined turbulence-chemistry interaction (TCI) models has led to highly sophisticated approaches. Nearly all of the studies comparing different models focus on stable premixed/non-premixed flame configurations. In this work, the focus is on well-documented, lean premixed bluff-body stabilized flames approaching blow-off and on the blow-off sequence itself. Large Eddy Simulations (LES) have been used to capture the time-dependent, three-dimensional flow-field using Transported Probability Density Function (TPDF), Partially Stirred Reactor Model (PaSR), and Implicit LES (ILES) models. Furthermore, the influence of finite-rate chemistry and different chemical mechanisms is evaluated to determine the limitation and capability of the different TCI approaches for modeling flames just prior to and during the transient blow-off process. While the average flow-fields do not reveal any significant differences between modeling approaches, detailed analysis of the flame reveals that there are differences in the predicted flame thickness and composition. The ability of the considered TCI models to predict local as well as full-flame extinction during the blow-off is investigated as well. It is demonstrated that such a blow-off sequence is not always governed by complex chemistry.

In this paper we consider the effects of acceleration and deceleration on the forcing of an intermittent jet. This experimental study specifically focuses on the effect of the acceleration and deceleration on the mixing of an intermittent jet with the ambient fluid and on the growth of disturbances that may lead to turbulence. The influence of different injection strategies has been evaluated. The results show that the deceleration phase may be able to contribute significantly to enhance the mixing of the jet with the ambient fluid. This effect is manifested primarily around the tail of the jet, towards the end of injection. The acceleration phase on the other hand has mainly impact at the leading part of the jet, where the leading part of the jet forms a mushroom shaped structure with minor mixing effect.

The mechanisms leading to blowoff were investigated numerically by analyzing bluff body stabilized flame at two conditions: a condition far from blowoff to a condition just prior to blowoff. Large eddy simulations have been used to capture the time dependent, three-dimensional evolution of the field. The results were first validated to available experimental data, showing very good agreement for the flow and overall good agreement for the flame. Changes in the large-scale structures are investigated by means of proper orthogonal decomposition and the wavelet method, elucidating the underlying dynamics of the complex flow-flame interaction of a flame approaching blowoff. Our results reveal that, when the flame approaches blowoff conditions, significant changes are found in the large-scale structures responsible for entrainment of species into the recirculation zone located downstream of the bluff body. Possible causes of this shift in large-scale structures are also discussed, which may be useful for extending the blowoff limits of bluff body stabilized burners.

A swirl stabilized gas turbine burner has been simulated in order to assess the effects of the fuel properties on spray dispersion and fuel-air mixing. The properties under consideration include fuel surface tension, viscosity and density. The turbulence of the gas phase is modeled applying the methodology of large eddy simulation whereas the dispersed liquid phase is described by Lagrangian particle tracking. The exchange of mass, momentum and energy between the two phases is accounted for by two-way coupling. Bag and stripping breakup regimes are considered for secondary droplet breakup, using the Reitz-Diwakar and the Taylor analogy breakup models. Moreover, a model for droplet evaporation is included. The results reveal a high sensitivity of the spray structure to variations of all investigated parameters. In particular, a decrease in the surface tension or the fuel viscosity, or an increase in the fuel density, lead to less stable liquid structures. As a consequence, smaller droplets are generated and the overall spray surface area increases, leading to faster evaporation and mixing. Furthermore, with the trajectories of the small droplets being strongly influenced by aerodynamic forces (and less by their own inertia), the spray is more affected by the turbulent structures of the gaseous phase and the spray dispersion is enhanced.