An in vitro investigation of the influence of stenosis severity on the flow in the ascending aorta.

Cardiovascular diseases can lead to abnormal blood flows, some of which are linked to hemolysis and thrombus formation. Abnormal turbulent flows of blood in the vessels with stenosis create strong shear stresses on blood elements and may cause blood cell destruction or platelet activation. We implemented a Lagrangian (following the fluid elements) measurement technique of three dimensional particle tracking velocimetry that provides insight on the evolution of viscous and turbulent stresses along blood element trajectories. We apply this method to study a pulsatile flow in a compliant phantom of an aorta and compare the results in three cases: the reference case (called "healthy" case), and two cases of abnormal flows due to mild and severe stenosis, respectively. The chosen conditions can mimic a clinical application of an abnormal flow due to a calcific valve. We estimate the effect of aortic stenosis on the kinetic energy of the mean flow and the turbulent kinetic energy, which increases about two orders of magnitude as compared with the healthy flow case. Measuring the total flow stress acting on a moving fluid element that incorporates viscous stresses and the apparent turbulent-induced stresses (the so-called Reynolds stresses) we find out similar increase of the stresses with the increased severity of the stenosis. Furthermore, these unique Lagrangian measurements provide full acceleration and, consequently, the forces acting on the blood elements that are estimated to reach the level that can considerably deform red blood cells. These forces are strong and abrupt due to the contribution of the turbulent fluctuations which is much stronger than the typically measured phase-averaged values.

Experimental investigation of the influence of the aortic stiffness on hemodynamics in the ascending aorta.

A three-dimensional (3-D) pulsatile aortic flow in a human ascending aorta is studied to investigate the effect of the aortic stiffness on the flow field and turbulent fluctuating velocities in the ascending aorta. A nonintrusive optical measurement technique, 3-D particle tracking velocimetry (3D-PTV), has been applied to anatomically accurate phantoms under clinically realistic conditions. A compliant silicon phantom was used to mimic the healthy aorta, and a rigid model was used to imitate the pathological case that appears in aortas for example as a result of aging. The realistic models are transparent which allows optical access to the investigation domain, and the index of refraction was matched to avoid optical distortions. Our results revealed that the aortic stiffness leads to an increase in systolic velocity and a decrease in the Windkessel effect, which is associated with the diastolic blood pressure. Furthermore, we found that the turbulent kinetic energy is about an order of magnitude higher for the rigid aorta, that is, an increase in aortic stiffness increases the magnitude of turbulent fluctuating velocities. The spatial distribution of the flow velocity showed that the flow is more organized and coherent spiraling patterns develop for the compliant aorta which helps to dampen the influence of disturbed flow. Finally, we observed higher Lagrangian acceleration and hence higher instantaneous forces acting on blood particles in the stiff case which implies that aging and hence arterial stiffening provokes distinctive alterations in blood flow, and these alterations may cause pathological symptoms in the cardiovascular system.

Stretching and tilting of material lines in turbulence: the effect of strain and vorticity.

The Lagrangian evolution of infinitesimal material lines is investigated experimentally through three dimensional particle tracking velocimetry (3D-PTV) in quasihomogeneous turbulence with the Taylor microscale Reynolds number Re(lambda)=50. Through 3D-PTV we access the full tensor of velocity derivatives du(i)/dx(j) along particle trajectories, which is necessary to monitor the Lagrangian evolution of infinitesimal material lines l. By integrating the effect on l of (i) the tensor du(i)/dx(j), (ii) its symmetric part s(ij), (iii) its antisymmetric part r(ij), along particle trajectories, we study the evolution of three sets of material lines driven by a genuine turbulent flow, by "strain only," or by "vorticity only," respectively. We observe that, statistically, vorticity reduces the stretching rate l(i)l(j)s(ij)/l2, altering (by tilting material lines) the preferential orientation between l and the first (stretching) eigenvector lambda1 of the rate of strain tensor. In contrast, s(ij), in "absence" of vorticity, significantly contributes to both tilting and stretching, resulting in an enhanced stretching rate compared to the case of material lines driven by the full tensor du(i)/dx(j). The same trend is observed for the deformation of material volumes.