Experimental study of aortic flow in the ascending aorta via Particle Tracking Velocimetry

A three-dimensional, pulsatile flow in a realistic phantom of a human ascending aorta with compliant walls is investigated in vitro. Three-Dimensional Particle Tracking Velocimetry (3D-PTV), an image-based, non-intrusive measuring method is used to analyze the aortic flow. The flow velocities and the turbulent fluctuations are determined. The velocity profile at the inlet of the ascending aorta is relatively flat with a skewed profile toward the inner aortic wall in the early systole. In the diastolic phase, a bidirectional flow is observed with a pronounced retrograde flow developing along the inner aortic wall, whereas the antegrade flow migrates toward the outer wall of the aorta. The spatial and temporal evolution of the vorticity field shows that the vortices begin developing along the inner wall during the deceleration phase and attenuate in the diastolic phase. The change in the cross-sectional area is more distinct distal to the inlet cross section. The mean kinetic energy is maximal in the peak systole, whereas the turbulent kinetic energy increases in the deceleration phase and reaches a maximum in the beginning of the diastolic phase. Finally, in a Lagrangian analysis, the temporal evolution of particle dispersion was studied. It shows that the dispersion is higher in the deceleration phase and in the beginning of the diastole, whereas in systole, it is smaller but non-negligibl

A comparative study on the analysis of hemodynamics in the athlete's heart

The pathophysiological mechanisms underlying the development of the athlete's heart are still poorly understood. To characterize the intracavitary blood flows in the right ventricle (RV) and right-ventricular outflow tract (RVOT) in 2 healthy probands, patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) and 2 endurance athletes, we performed 4D-MRI flow measurements to assess differences in kinetic energy and shear stresses. Time evolution of velocity magnitude, mean kinetic energy (MKE), turbulent kinetic energy (TKE) and viscous shear stress (VSS) were measured both along the whole RV and in the RVOT. RVOT regions had higher kinetic energy values and higher shear stresses levels compared to the global averaging over RV among all subjects. Endurance athletes had relatively lower kinetic energy and shear stresses in the RVOT regions compared to both healthy probands and ARVC patients. The athlete's heart is characterized by lower kinetic energy and shear stresses in the RVOT, which might be explained by a higher diastolic compliance of the R

NUMERICAL INVESTIGATION OF FLUID FLOW AND CELL PERFORMANCE AT CATHODE OF PROTON EXCHANGE MEMBRANE FUEL CELLS

Bu çalışmada bir PEM yakıt hücresinin katot tarafındaki akışkan akışı ve konsantrasyon dağılımı sayısal olarak incelenmiştir. İnceleme alanı, katot gaz akış kanalı, katot gaz difüzyon tabakası ve katot katalizör tabakasını içermektedir. Problemin temel denklemleri olan süreklilik, momentum ve konsantrasyon denklemleri kontrol hacim metoduyla ayrıklaştırılarak ve SIMPLE algoritması kullanılarak geliştirilen bir bilgisayar programı ile çözülmüştür. Geliştirilen programın doğruluğunu denemek için, düz kanal, tabanı gözenekli kanal ve PEM yakıt hücresinin katot tarafındaki akışlar simüle edilerek sonuçları, literatür sonuçlarıyla karşılaştırılmıştır. Gaz difüzyon tabakası gözeneklilik değeri, Reynolds sayısı, işletme sıcaklığının ve hücre geriliminin farklı değerleri için simülasyonlar yapılarak bu parametrelerin akışa, konsantrasyon ve akım yoğunluğu dağılımına etkileri incelenmiştir. Yapılan çalışma sonucunda, gaz difüzyon tabakasının gözeneklilik değerinin ve Reynolds sayısının artması ile akım yoğunluğunun, güç yoğunluğunun ve katalizör tabakasındaki oksijen konsantrasyonunun arttığı belirlenmiştir. Gerçekleştirilen simülasyonlar sonucu elde edilen verilere göre, hücre işletme sıcaklığının artması ile akım yoğunluğu ve güç yoğunluğunun arttığı ancak katalizör tabakasındaki oksijen konsantrasyonunun azaldığı belirlenmiştir.In this study, fluid flow and concentration distribution in cathode section of a PEM Fuel Cell were numerically investigated. The problem domain consists of cathode gas channel, cathode gas diffusion layer and cathode catalyst layer. The governing equations, continuity, momentum and concentration equations were discritized by the control volume method and solved using a computer program developed using SIMPLE algorithm. Comparing the results of numerical simulations and the result of analytical solution and results from literature, the mathematical model and program developed were tested. Simulations were made for different values of GDL porosity, Reynolds number and operation temperature. Using the results of these simulations, the effects of these parameters on flow, concentration and current density distribution were analyzed. It is determined that increasing the porosity of the gas diffusion layer increases current density of the PEM fuel cell, power density of the PEM fuel cell and oxygen concentration in catalyst layer. According to the simulations, it is also determined that increasing Reynolds number increases current density of the PEM fuel cell, power density of the PEM fuel cell and oxygen concentration in catalyst layer. Analyzing the data obtained from simulations shows that increasing operation temperature of PEM fuel cell increases current density of the PEM fuel cell and power density of the PEM fuel cell but decreases oxygen concentration in catalyst layer

On the accuracy of viscous and turbulent loss quantification in stenotic aortic flow using phase-contrast MRI

PURPOSE: To investigate the limits of phase contrast MRI (PC-MRI)-based measurements of viscous losses and turbulent kinetic energy (TKE) pertaining to spatial resolution, signal-to-noise ratio (SNR), and non-Gaussian intravoxel velocity distributions. THEORY AND METHODS: High-resolution particle tracking velocimetry data obtained in a realistic aortic phantom with stenotic flow were used to simulate PC-MRI measurements at different resolutions and noise levels. Laminar viscous losses were computed using the spatial gradients of the mean velocity vector field, and TKE levels were derived based on the intravoxel phase dispersion of flow-sensitized PC-MRI measurements. RESULTS: Increasing the voxel size from 0.625 to 2.5 mm resulted in an underestimation of viscous losses of up to 83%, whereas total TKE values showed errors of <15% and reduced sensitivity to voxel size. Relative errors in viscous loss quantification were found to be less dependent on noise levels when compared with TKE values. In general, a SNR of 20-30 is required for both methods. CONCLUSION: At spatial resolutions feasible in clinical three-dimensional PC-MRI measurements, viscous losses of stenotic flows are significantly underestimated, whereas TKE shows smaller errors and reduced sensitivity to spatial resolution. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc

Shear-scaling-based approach for irreversible energy loss estimation in stenotic aortic flow – An in vitro study

Today, the functional and risk assessment of stenosed arteries is mostly based on ultrasound Doppler blood flow velocity measurements or catheter pressure measurements, which rely on several assumptions. Alternatively, blood velocity including turbulent kinetic energy (TKE) may be measured using MRI. The aim of the present study is to validate a TKE-based approach that relies on the fact that turbulence production is dominated by the flow’s shear to determine the total irreversible energy loss from MRI scans. Three-dimensional particle tracking velocimetry (3D-PTV) and phase-contrast magnetic resonance imaging (PC-MRI) simulations were performed in an anatomically accurate, compliant, silicon aortic phantom. We found that measuring only the laminar viscous losses does not reflect the true losses of stenotic flows since the contribution of the turbulent losses to the total loss become more dominant for more severe stenosis types (for example, the laminar loss is 0.0094 ± 0.0015 W and the turbulent loss is 0.0361 ± 0.0015 W for the Remax = 13,800 case, where Remax is the Reynolds number based on the velocity in the vena-contracta). We show that the commonly used simplified and modified Bernoulli’s approaches overestimate the total loss, while the new TKE-based method proposed here, referred to as “shear scaling” approach, results in a good agreement between 3D-PTV and simulated PC-MRI (mean error is around 10%). In addition, we validated the shear scaling approach on a geometry with post-stenotic dilatation using numerical data by Casas et al. (2016). The shear scaling-based method may hence be an interesting alternative for irreversible energy loss estimation to replace traditional approaches for clinical use. We expect that our results will evoke further research, in particular patient studies for clinical implementation of the new method

Blood flow patterns and pressure loss in the ascending aorta: A comparative study on physiological and aneurysmal conditions

An aortic aneurysm is defined as a balloon-shaped bulging of all three histologic components of the aortic vessel walls (intima, media and adventitia). This dilation results from vessel weakening owing to remodeling, i.e. due to cystic degeneration of the Tunica media (Marfan), progression of atherosclerosis or presence of a bicuspid aortic valve. The growth rate of the aortic diameter varies from patient to patient and may progress until the aneurysm ultimately ruptures. The role of hemodynamics, i.e. blood flow patterns, and shear stresses that are supposed to intensify during aneurysm growth are not yet fully understood, but thought to play a key role in the enlargement process. The aim of this study is to characterize the aortic blood flow in a silicone model of a pathological aorta with ascending aneurysm, to analyze the differences in the blood flow pattern compared to a healthy aortic model, and to single out possible blood flow characteristics measurable using phase contrast magnetic resonance imaging (MRI) that could serve as indicators for aneurysm severity. MRI simulations were performed under physiological, pulsatile flow conditions using data obtained from optical three dimensional particle tracking measurements. In comparison to the healthy geometry, elevated turbulence intensity and pressure loss are measured in the diseased aorta, which we propose as a complimentary indicator for assessing the aneurysmal severity. Our results shed a light on the interplay between the blood flow dynamics and their contribution to the pathophysiology and possible role for future risk assessment of ascending aortic aneurysms

A Novel Approach for 3D-Structural Identification through Video Recording: Magnified Tracking

Advancements in optical imaging devices and computer vision algorithms allow the exploration of novel diagnostic techniques for use within engineering systems. A recent field of application lies in the adoption of such devices for non-contact vibrational response recordings of structures, allowing high spatial density measurements without the burden of heavy cabling associated with conventional technologies. This, however, is not a straightforward task due to the typically low-amplitude displacement response of structures under ambient operational conditions. A novel framework, namely Magnified Tracking (MT), is proposed herein to overcome this limitation through the synergistic use of two computer vision techniques. The recently proposed phase-based motion magnification (PBMM) framework, for amplifying motion in a video within a defined frequency band, is coupled with motion tracking by means of particle tracking velocimetry (PTV). An experimental campaign was conducted to validate a proof-of-concept, where the dynamic response of a shear frame was measured both by conventional sensors as well as a video camera setup, and cross-compared to prove the feasibility of the proposed non-contact approach. The methodology was explored both in 2D and 3D configurations, with PTV revealing a powerful tool for the measurement of perceptible motion. When MT is utilized for tracking &ldquo;imperceptible&rdquo; structural responses (i.e., below PTV sensitivity), via the use of PBMM around the resonant frequencies of the structure, the amplified motion reveals the operational deflection shapes, which are otherwise intractable. The modal results extracted from the magnified videos, using PTV, demonstrate MT to be a viable non-contact alternative for 3D modal identification with the benefit of a spatially dense measurement grid

A Novel Estimation Approach of Pressure Gradient and Haemodynamic Stresses as Indicators of Pathological Aortic Flow Using Subvoxel Modelling

Objective: The flow downstream from aortic stenoses is characterised by the onset of shear-induced turbulence that leads to irreversible pressure losses. These extra losses represent an increased resistance that impacts cardiac efficiency. A novel approach is suggested in this study to accurately evaluate the pressure gradient profile along the aorta centreline using modelling of haemodynamic stress at scales that are smaller than the typical resolution achieved in experiments. Methods: We use benchmark data obtained from direct numerical simulation (DNS) along with results from in silico and in vitro three-dimensional particle tracking velocimetry (3D-PTV) at three voxel sizes, namely 750 μm, 1 mm and 1.5 mm. A differential equation is derived for the pressure gradient, and the subvoxel-scale (SVS) stresses are closed using the Smagorinsky and a new refined model. Model constants are optimised using DNS and in silico PTV data and validated based on pulsatile in vitro 3D-PTV data and pressure catheter measurements. Results: The Smagorinsky-based model was found to be more accurate for SVS stress estimation but also more sensitive to errors especially at lower resolution, whereas the new model was found to more accurately estimate the projected pressure gradient even for larger voxel size of 1.5 mm albeit at the cost of increased sensitivity at this voxel size. A comparison with other methods in the literature shows that the new approach applied to in vitro PTV measurements estimates the irreversible pressure drop by decreasing the errors by at least 20%. Conclusion: Our novel approach based on the modelling of subvoxel stress offers a validated and more accurate way to estimate pressure gradient, irreversible pressure loss and SVS stress. Significance: We anticipate that the approach may potentially be applied to image-based in vivo, in vitro 4D flow data or in silico data with limited spatial resolution to assess pressure loss and SVS stresses in disturbed aortic blood flow.ISSN:0018-9294ISSN:1558-253