Flow patterns in ascending aortic aneurysms: Determining the role of hypertension using phase contrast magnetic resonance and computational fluid dynamics.

Thoracic aortic aneurysm (TAA) is a local dilation of the thoracic aorta. Although universally used, aneurysm diameter alone is a poor predictor of major complications such as rupture. There is a need for better biomarkers for risk assessment that also reflect the aberrant flow patterns found in TAAs. Furthermore, hypertension is often present in TAA patients and may play a role in progression of aneurysm. The exact relation between TAAs and hypertension is poorly understood. This study aims to create a numerical model of hypertension in the aorta by using computational fluid dynamics. First, a normotensive state was simulated in which flow and resistance were kept unaltered. Second, a hypertensive state was modeled in which blood inflow was increased by 30%. Third, a hypertensive state was modeled in which the proximal and peripheral resistances and capacitance parameters from the three-element Windkessel boundary condition were adjusted to mimic an increase in resistance of the rest of the cardiovascular system. One patient with degenerative TAA and one healthy control were successfully simulated at hypertensive states and were extensively analyzed. Furthermore, three additional TAA patients and controls were simulated to validate our method. Hemodynamic variables such as wall shear stress, oscillatory shear index, endothelial cell activation potential (ECAP), vorticity and helicity were studied to gain more insight on the effects of hypertension on flow patterns in TAAs. By comparing a TAA patient and a control at normotensive state at peak-systole, helicity and vorticity were found to be lower in the TAA patient throughout the entire domain. No major changes in flow and flow derived quantities were observed for the TAA patient and control when resistance was increased. When flow rate was increased, regions with high ECAP values were found to reduce in TAA patients in the aneurysm region which could reduce the risk of thrombogenesis. Thus, it may be important to assess cardiac output in patients with TAA.

Endothelium resolving simulations of wall shear-stress dependent mass transfer of LDL in diseased coronary arteries.

In the present study, we investigate blood flow and mass transfer of the low-density lipoprotein (LDL) in a simplified axisymmetric geometry with a mathematically well-defined narrowing (stenosis), which mimics a diseased human coronary artery. The interior of the arterial wall is represented as a porous media containing multi-layered structures of different thickness. This multi-layered structure includes anatomically realistic sublayers: endothelium, intima, internal elastic layer (IEL), media and adventitia. The coupling between the blood flow and mass transfer of LDL in the lumen (interior of artery) and arterial wall is established through a multipore model at the lumen/endothelium interface. This multipore model takes into consideration three different contributions for transport of LDL: normal and leaky junctions of endothelial cells, as well as their vesicular pathway. A comprehensive mathematical model, which is based on solving the set of PDEs for conservation of mass, momentum, and concentration, is completed by introducing the wall shear-stress (WSS) dependent transport properties of the arterial wall. Several variants of the model are evaluated, including the constant and wall shear-stress dependent transport properties of the endothelium, as well as different representation of the arterial wall internal structure. The response of the model on changing the transmural pressure (to simulate hypertension effects) and geometrical shapes of the stenosis (to mimic the various stages of atherosclerosis development) is also presented. It is shown that the present model can predict the levels of LDL inside the arterial wall in good agreement with experimental studies in pressurized rabbit aorta under similar conditions. The model is recommended for future simulations of LDL accumulation in the patient-specific cardiovascular system conditions.

Assessment of human left ventricle flow using statistical shape modelling and computational fluid dynamics.

Blood flow patterns in the human left ventricle (LV) have shown relation to cardiac health. However, most studies in the literature are limited to a few patients and results are hard to generalize. This study aims to provide a new framework to generate more generalized insights into LV blood flow as a function of changes in anatomy and wall motion. In this framework, we studied the four-dimensional blood flow in LV via computational fluid dynamics (CFD) in conjunction with a statistical shape model (SSM), built from segmented LV shapes of 150 subjects. We validated results in an in-vitro dynamic phantom via time-resolved optical particle image velocimetry (PIV) measurements. This combination of CFD and the SSM may be useful for systematically assessing blood flow patterns in the LV as a function of varying anatomy and has the potential to provide valuable data for diagnosis of LV functionality.

Application of full field optical studies for pulsatile flow in a carotid artery phantom.

A preliminary comparative measurement between particle imaging velocimetry (PIV) and laser speckle contrast analysis (LASCA) to study pulsatile flow using ventricular assist device in a patient-specific carotid artery phantom is reported. These full-field optical techniques have both been used to study flow and extract complementary parameters. We use the high spatial resolution of PIV to generate a full velocity map of the flow field and the high temporal resolution of LASCA to extract the detailed frequency spectrum of the fluid pulses. Using this combination of techniques a complete study of complex pulsatile flow in an intricate flow network can be studied.

Oscillatory states in thermal convection of a paramagnetic fluid in a cubical enclosure subjected to a magnetic field gradient.

We report experimental and numerical studies of combined natural and magnetic convection of a paramagnetic fluid inside a cubical enclosure heated from below and cooled from above and subjected to a magnetic field gradient. Values of the magnetic field gradient are in the range 9≤|grad|b(0)|(2)|≤900 T(2)/m for imposed magnetic field strengths in the center of the superconducting magnet bore of 1≤|b(0)|(max)≤10 T. Very good agreement between experiments and simulation is obtained in predicting the integral heat transfer over the entire range of working parameters (i.e., thermal Rayleigh number 1.15×10(5)≤Ra(T)≤8×10(6), Prandtl number 5≤Pr≤700, and magnetization number 0≤γ≤58.5). We present a stability diagram containing three characteristic states: steady, oscillatory (periodic), and turbulent regimes. The oscillatory states are identified for intermediate values of Pr (40≤Pr≤70) and low magnetic field (|b(0)|(max)≤2 T). Turbulent states are generated from initially stable flow and heat transfer regimes in the range of 70≤Pr≤500 for sufficiently strong magnetic field (|b(0)|(max)≥4 T).

Computational simulations of magnetic particle capture in arterial flows.

The aim of Magnetic Drug Targeting (MDT) is to concentrate drugs, attached to magnetic particles, in a specific part of the human body by applying a magnetic field. Computational simulations are performed of blood flow and magnetic particle motion in a left coronary artery and a carotid artery, using the properties of presently available magnetic carriers and strong superconducting magnets (up to B approximately 2 T). For simple tube geometries it is deduced theoretically that the particle capture efficiency scales as [see text], with Mn (p) the characteristic ratio of the particle magnetization force and the drag force. This relation is found to hold quite well for the carotid artery. For the coronary artery, the presence of side branches and domain curvature causes deviations from this scaling rule, viz. eta approximately Mn (p) (beta) , with beta > 1/2. The simulations demonstrate that approximately a quarter of the inserted 4 microm particles can be captured from the bloodstream of the left coronary artery, when the magnet is placed at a distance of 4.25 cm. When the same magnet is placed at a distance of 1 cm from a carotid artery, almost all of the inserted 4 microm particles are captured. The performed simulations, therefore, reveal significant potential for the application of MDT to the treatment of atherosclerosis.

Magnetic particle motion in a Poiseuille flow.

The manipulation of magnetic particles in a continuous flow with magnetic fields is central to several biomedical applications, including magnetic cell separation and magnetic drug targeting. A simplified two-dimensional (2D) equation describing the motion of particles in a planar Poiseuille flow is considered for various magnetic field configurations. Exact analytical solutions are derived for the particle motion under the influence of a constant magnetization force and a force decaying as a power of the source distance, e.g., due to a current carrying wire or a magnetized cylinder. For a source distance much larger than the transversal size of the flow, a general solution is derived and applied to the important case of a magnetic dipole. This solution is used to investigate the dependence of the particle capture efficiency on the dipole orientation. A correction factor to convert the obtained 2D results to a three-dimensional cylindrical geometry is derived and validated against computational simulations. Simulations are also used to investigate parameter ranges beyond the region of applicability of the analytical results and to investigate more complex magnetic field configurations.

Numerical simulation of a turbulent magnetic dynamo.

We present numerical simulations of a turbulent magnetic dynamo mimicking closely the Riga-dynamo experiment at Re approximately 3.5x10(6) and 15< or =Rem< or =20. The Reynolds-averaged Navier-Stokes equations for the fluid flow and turbulence field are solved simultaneously with the direct numerical solution of the magnetic field equations. The fully integrated two-way-coupled simulations reproduced all features of the magnetic self-excitation detected by the Riga experiment, with frequencies and amplitudes of the self-generated magnetic field in good agreement with the experimental records, and provided full insight into the unsteady magnetic and velocity fields and the mechanisms of the dynamo action.

Numerical insight into flow structure in ultraturbulent thermal convection.

Very large eddy simulations of high-aspect-ratio unbounded Rayleigh-Bénard convection for Pr=0.71 over a 10-decade range of Rayleigh numbers (Ra=10(5)-10(15)) reveal a consolidation and dramatic thinning of the wall boundary layer with an increase in the Ra number. The fingerlike plumes between planform structures become also thinner, more distant, but much more vigorous. The Ra exponent in the Nu proportional to Ra(n) correlation follows n approximately 0.31 scaling, but n begins to increase gradually above Ra=10(13). However, no trend towards "crossing" of the thermal and hydrodynamic boundary layers is observed.

Convective rolls and heat transfer in finite-length rayleigh-Benard convection: A two-dimensional numerical study

A two-dimensional (2D) numerical study using a single-point algebraic k-straight theta;(2)-varepsilon-varepsilon(straight theta) turbulence closure was performed to detect the existence, origin, creation and behavior of convective rolls and associated wall Nusselt (Nu) number variation in thermal convection in 2D horizontal slender enclosures heated from below. The study covered the Rayleigh (Ra) numbers from 10(5) to 10(12) and aspect ratios from 4:1 to 32:1. The time evolution of the convective rolls and the formation of the corner vortices were analyzed using numerical flow visualization, and the correlation between roll structures and heat transfer established. A major consequence of the imposed two dimensionality appeared in the persistence of regular roll structures at higher Ra numbers that approach a steady state for all configurations considered. This finding contradicts the full three-dimensional direct numerical simulations (DNS), large eddy simulations (LES), and three-dimensional transient Reynolds-averaged Navier-Stokes (TRANS) computations, which all show continuously changing unsteady patterns. However, the final-stage roll structures, long-term averaged mean temperature and turbulence moments, and the Nusselt number (both local and integral), are all reproduced in good agreement with the ensemble-averaged 3D DNS, TRANS, and several recent experimental results. These findings justified the 2D approach as an acceptable method for ensemble average analysis of fully 3D flows with at least one homogeneous direction. Based on our 2D computations and adopting the low and high Ra number asymptotic power laws of Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)], new prefactors in the Nu-Ra correlation for Pr=O(1) were proposed that fit better several sets of data over a wide range of Ra numbers and aspect ratios: Nu=0.1Ra(1/4)+0.05Ra(1/3). Even better agreement of our computations was achieved with the new correlation Nu=0.124 Ra0.309 proposed recently by Niemela et al. [Nature (London) 404, 837 (2000)] for 10(6)