Omid Amili

Extensive characterisation of a high Reynolds number decelerating boundary layer using advanced optical metrology

ABSTRACT Over the last years, the observation of large-scale structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations. Nevertheless, partly due to the lack of comprehensive experimental data at sufficiently high Reynolds number, our understanding of turbulence near walls, especially in decelerating situations, is still quite limited. The aim of the present contribution is to combine the equipment and skills of several teams to perform a detailed characterisation of a large-scale turbulent boundary layer under adverse pressure gradient. Extensive particle image velocimetry (PIV) measurements are performed, including a set-up with 16 sCMOS cameras allowing the characterisation of the boundary layer on 3.5 m, stereo PIV and high resolution near wall measurements. In this paper, detailed statistics are presented and discussed, boundary conditions are carefully characterised, making this experiment a challenging test case for numerical simulation.

Measuring dynamic phenomena at the sub-micron scale

Measuring the spatio-temporal evolution of dynamic phenomena at sub-micron level is non-trivial due to the diffraction limit of optical systems. This paper describes a technique which allows imaging of sub-micron features of fluid-based phenomena, specifically the determination of their velocity and trajectory.

Measuring shear layer growth rates in aeroacoustically forced axisymmetric supersonic jets

Underexpanded jets at a single pressure ratio and Reynolds number are studied in their free and impinging conditions. The influence of different kinds of aeroacoustic self-forcing is studied through Proper Orthogonal Decomposition and Dynamic Mode Decomposition analysis of high resolution Particle Image Velocimetry data. The free jet is primarily characterized by the presence of a strong helical instability, though a weaker axisymmetric instability is also present. The impinging jet shear layer is dominated by a fast-growing axisymmetric instability, with a weaker helical instability also evident. Ultra-high-speed schlieren imaging reveals a much more rapid instability growth close to the nozzle for the impinging jet case. Analysis of the streamwise growth rates via DMD suggests that the shear layer instability grows more than twice as fast in the near-nozzle region for the impinging jet. Further downstream the growth rate in the free jet case is higher.

Lipoprotein Transport in the Blood Stream to the Arterial Wall in a Carotid Artery Bifurcation

The carotid arteries are a common site of atherosclerotic plaque formation, which has been linked to the blood flow patterns and the mass transport phenomenon. The purpose of this research was to study the lipid transport in a human carotid artery model, focusing on the effects of local geometric and hemodynamic factors on mass transfer from blood flow to vessel wall and its concentration at the luminal surface of the artery. The Reynolds number, 250, and the Schmidt number, 6.66times10 5, were selected to model the mass transfer of LDL macro molecules, and in order to see the effect of Reynolds and Schmidt numbers to mass transport, the model was analyzed with different conditions. The steady state flow was used for two dimensional carotid geometry. At the inlet, the blood flow was assumed a steady fully developed laminar velocity profile with a uniform LDL concentration. The vessel wall was assumed permeable to water and semi-permeable to LDL macro molecules. The problem was analyzed with the finite element method. The results show 26% increase of LDL concentration from inlet value at the luminal surface of the artery located in the separated flow region. The maximum value of LDL concentration occurred at the separation point

Hemodynamics in a giant intracranial aneurysm characterized by in vitro 4D flow MRI

Experimental and computational data suggest that hemodynamics play a critical role in the development, growth, and rupture of cerebral aneurysms. The flow structure, especially in aneurysms with a large sac, is highly complex and three-dimensional. Therefore, volumetric and time-resolved measurements of the flow properties are crucial to fully characterize the hemodynamics. In this study, phase-contrast Magnetic Resonance Imaging is used to assess the fluid dynamics inside a 3D-printed replica of a giant intracranial aneurysm, whose hemodynamics was previously simulated by multiple research groups. The physiological inflow waveform is imposed in a flow circuit with realistic cardiovascular impedance. Measurements are acquired with sub-millimeter spatial resolution for 16 time steps over a cardiac cycle, allowing for the detailed reconstruction of the flow evolution. Moreover, the three-dimensional and time-resolved pressure distribution is calculated from the velocity field by integrating the fluid dynamics equations, and is validated against differential pressure measurements using precision transducers. The flow structure is characterized by vortical motions that persist within the aneurysm sac for most of the cardiac cycle. All the main flow statistics including velocity, vorticity, pressure, and wall shear stress suggest that the flow pattern is dictated by the aneurysm morphology and is largely independent of the pulsatility of the inflow, at least for the flow regimes investigated here. Comparisons are carried out with previous computational simulations that used the same geometry and inflow conditions, both in terms of cycle-averaged and systolic quantities.

DHMPIV and Tomo-PIV measurements of three-dimensional structures in a turbulent boundary layer

In turbulent boundary layers, a large portion of total turbulence production happens in the near wall region, y/δ < 0.2. The aim of the present work is to measure three-dimensional velocity field in a turbulent boundary layer at a moderately high Reynolds number. Tomographic particle image velocitmery (Tomo-PIV) was used to extract the 3C-3D velocity field using a rapid and less memory intensive reconstruction algorithm. It is based on a multiplicative line-of-sight (MLOS) estimation that determines possible particle locations in the volume, followed by simultaneous iterative correction. Application of MLOS-SART and MART to a turbulent boundary layer at Refθ=2200 using a 4 camera Tomo-PIV system with a volume of 1000×1000×160 voxels is discussed. In addition, near wall velocity measurement attempt made by digital holographic microscopic particle image velocimetry (DHMPIV). The technique provides a solution to overcome the poor axial accuracy and the low spatial resolution which are common problems in digital holography [5]. By reducing the depth of focus by at least one order of magnitude as well as increasing the lateral spatial resolution, DHMPIV provides the opportunity to resolve the small-scale structures existing in near wall layers.

PC028 The Effect of Rigid Stent Grafts on the Propagation of Pressures in Aortic Dissection: A Lumped-Parameter Mathematical Model of Flow Through the Descending Thoracic Aorta

case of distal type I endoleak and device migration (>10 mm) of a right iliac leg component was noted during longer-term follow-up. No other limb-related endoleak, migration, component separation, or stent fracture was reported during a mean follow-up of 10.8 6 5.6 months. Conclusions: Results from this postmarket registry under routine clinical care demonstrate infrequent limb occlusions and limb-related reintervention, supporting the excellent performance of the Spiral-Z leg graft.

Interaction of a Supersonic Underexpanded Jet with a Flat Plate

High-spatial resolution measurements of the velocity field of a supersonic jet impinging on a flat surface were conducted. To study the effect of the boundary condition, two nozzle outer geometries were investigated. The experimental results presented here are for a nozzle pressure ratio of NPR = 3.0 and a nozzle to plate spacing of \(Z/D = 3.5\). The flow fields of the corresponding cases show that although the general features of the flow are similar, the ambient air entrainment into the shear layer is different. This change in entrainment influences the rest of the jet structure.

Measurements of Near Wall Velocity and Wall Stress in a Wall-Bounded Turbulent Flow Using Digital Holographic Microscopic PIV and Shear Stress Sensitive Film

In wall-bounded turbulent flows, a large portion of total turbulence production happens in the near wall region. Although the viscous and buffer layers are relatively easy for numerical simulations due to local low Reynolds numbers of the important structures, they are very difficult to study experimentally because of their physically small geometry. The aim of this paper is to measure the wall shear stress in a fully developed turbulent channel flow at moderately high Reynolds numbers using a novel stress meter which is capable of measuring surface forces over an extended region of the model. In addition, near wall region in a wall-bounded flow by means of digital holographic microscopic particle image velocimetry will be investigated. DHMPIV provides a solution to overcome the poor axial accuracy and the low spatial resolution. The paper addresses the experimental techniques, articulate calibration procedure, data acquisition, and analysis procedure.