Evolution and rupture of vulnerable plaques: a review of mechanical effects

Atherosclerosis occurs as a result of the buildup and infiltration of lipid streaks in artery walls, leading to plaques. Understanding the development of atherosclerosis and plaque vulnerability is of critical importance, since plaque rupture can result in heart attack or stroke. Plaques can be divided into two distinct types: those that rupture (vulnerable) and those that are less likely to rupture (stable). In the last few decades, researchers have been interested in studying the influence of the mechanical effects (blood shear stress, pressure forces, and structural stress) on the plaque formation and rupture processes. In the literature, physiological experimental studies are limited by the complexity of in vivo experiments to study such effects, whereas the numerical approach often uses simplified models compared with realistic conditions, so that no general agreement of the mechanisms responsible for plaque formation has yet been reached. In addition, in a large number of cases, the presence of plaques in arteries is asymptomatic. The prediction of plaque rupture remains a complex question to elucidate, not only because of the interaction of numerous phenomena involved in this process (biological, chemical, and mechanical) but also because of the large time scale on which plaques develop. The purpose of the present article is to review the current mechanical models used to describe the blood flow in arteries in the presence of plaques, as well as reviewing the literature treating the influence of mechanical effects on plaque formation, development, and rupture. Finally, some directions of research, including those being undertaken by the authors, are described

Mixing in a vortex breakdown flow

International audienceThis paper presents experimental and theoretical results on the mixing inside a cylinder with a rotating lid. The helical flow that is created by the rotation of the disk is well known to exhibit a vortex breakdown bubble over a finite range of Reynolds numbers. The mixing properties of the flow are analyzed quantitatively by measuring the exponential decay of the variance as a function of time. This homogenization time is extremely sensitive to the asymmetries of the flow, which are introduced by tilting the rotating or the stationary disk and accurately measured by Particle Image Velocimetry (PIV). In the absence of vortex breakdown, the homogenization time is strongly decreased (by a factor 10) with only a moderate tilt angle of the rotating lid (of the order of 15 degrees). This phenomenon can be explained by the presence of small radial jets at the periphery which create a strong convective mixing. A simple model of exchange flow between the periphery and the bulk correctly predicts the scaling laws for the homogenization time. In the presence of vortex breakdown, the scalar is trapped inside the vortex breakdown bubble, and thus increases substantially the time needed for homogenization. Curiously, the tilt of the rotating lid has a weak effect on the mixing, but a small tilt of the stationary disk (of the order of 2 degrees) strongly decreases (by a factor 10) the homogenization time. Even more surprising is that the homogenization time diverges when the size of the bubble vanishes. All these features are recovered by applying the Melnikov theory to calculate the volume of the lobes that exit the bubble. It is the first time that this technique has been applied to a 3D stationary flow with a non-axisymmetric perturbation and compared with experimental results, although it has been applied often to 2D flows with a periodic perturbation

The evolution of a subharmonic mode in a vortex street

The development of a subharmonic three-dimensional instability mode in a vortex street is investigated both numerically and experimentally. The flow past a ring is considered as a test case, as a previous stability analysis has predicted that for a range of aspect ratios, the first-occurring instability of the vortex street is subharmonic. For the flow past a circular cylinder, the development of three-dimensional flow in the vortex street is known to lead to turbulent flow through the development of spatio-temporal chaos, whereas subharmonic instabilities have been shown to cause a route to chaos through the development of a period-doubling cascade. The three-dimensional vortex street in the flow past a ring is analysed to determine if a subharmonic instability can alter the route to turbulence for a vortex street. A linear stability analysis and non-axisymmetric computations are employed to compute the flow past a ring with an aspect ratio AR=5, and comparisons with experimental dye visualizations are included to verify the existence of a subharmonic mode in the wake. Computations at higher Reynolds numbers confirm that the subharmonic instability does not initiate a period-doubling cascade in the wake

Control of vortex breakdown by density effects

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Experimental investigation of in-line flow-induced vibration of a rotating circular cylinder

This study experimentally investigates the in-line flow-induced vibration (FIV) of an elastically mounted circular cylinder under forced axial rotation in a free stream. The present experiments characterise the structural vibration, fluid forces and wake structure of the fluid–structure system at a low mass ratio (the ratio of the total mass to the displaced fluid mass) over a wide parameter space spanning the reduced velocity range

Simulation of the Control of Vortex Breakdown in a Closed Cylinder Using a Small Rotating Disk

The enhancement or suppression of vortex breakdown in a closed cylinder caused by a small rotating disk embedded in the nonrotating endwall is simulated in this study. This paper shows that corotation or counter-rotation of the control disk with respect to the driving lid is able to promote or suppress the “bubble-type” vortex breakdown. This is achieved using only a small fraction of the power required to drive the main lid. The simulations show that the vortex breakdown induced or suppressed by flow control displays similar characteristics near the breakdown region as produced by varying the flow Reynolds number. These include near-axis swirl, centerline axial velocity, and centerline pressure. The influence of the size of the control disk is also quantified

Vortex-induced vibration of a transversely rotating sphere

Vortex-induced vibration (VIV) of a sphere is one of the most basic fluid-structure interaction problems. Since such vibrations can lead to fatal structural failures, numerous studies have focused on suppressing such flow-induced vibrations. In this study, for the first time, the effect of an imposed transverse rotation on the dynamics of the VIV of an elastically mounted sphere has been investigated. It was observed that the non-dimensional vibration amplitude for a rotating sphere (A∗ = √2yrms/D, where yrms is the root mean square of the displacement in the transverse direction and D = sphere diameter) exhibits a bell-shaped evolution as a function of reduced velocity, similar to the classic VIV response of a non-rotating sphere. The sphere is found to oscillate freely up to a rotation ratio α (ratio of the equatorial velocity of the sphere to the free-stream velocity) close to 0.5. For lower rotation ratios (α ≤ 0.3), the response looks similar to the non-rotating case but with slightly smaller vibration amplitude. For higher α values, the amplitude was found to decrease significantly with the rotation up to α = 0.5. The amplitude dropped drastically after it reached the peak amplitude. This is unlike the VIV response of a rotating circular cylinder where the vibration amplitude increases up to three times the maximum vibration amplitude in the non- rotating case due to a novel asymmetric wake pattern (see [1]

Effect of Central Body Size on the Leading Edge Vortex of a Rotating Insect Wing

The stable attachment of a leading-edge vortex (LEV) is responsible for the high lift observed from insect wings. In experiments, we study the flow structure over a model wing mounted on a central body. The diameter of the central body and the change in Rossby number (Ro) due to placement of the wing root away from the centre can affect the flow structure. Normally, the LEV splits to form dual LEVs in a rotating wing, with the spanwise split location changing with Reynolds number. The results presented here show that the LEV structure is minimally affected by changes in the central body size for a wide range of body sizes

REPORTAJE FOTOGRÁFICO DEL TABOR DE AGÜIMES [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201

Numerical and in vitro experimental study of arterial deformation and buckling under hypertension and atherosclerotic conditions

Cardiovascular diseases remain the major cause of mortality worldwide. Pathologies of the vasculature such as atherosclerosis are often related to biochemical and genetic factors as well as mechanical effects that strongly change the function and shape of arteries. The present work is part of a general research project which aims to better understand the mechanical mechanisms responsible for atherosclerotic plaque formation and rupture. The chosen approach is to use numerical fluidstructure interaction (FSI) methods to study the relative influence of hemodynamic parameters on the structural stresses generated on plaques. To this aim, a numerical study of a simplified straight vessel exposed to lumen pressure was investigated under quiescent and steady flow conditions. As the internal pressure or the steady velocity increases, the vessel buckles lead-ing to a non-linear large deformation behaviour. The results have been validated using theoretical predictions for the buckling thresholds. Further studies on idealised cardiovascular conditions such as stenosis (i.e., lumen constriction) or aneurysm like (i.e., arterial wall expansion) formation have also been performed