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Fluid mechanic forces play a key role in the early development and progression of cardiovascular diseases, which predominantly occurs in areas of disturbed flow and low wall shear stress (WSS). In the present study, we perform particle image velocimetry (PIV) measurements in an anatomically realistic transparent flow phantom of a human carotid arte...
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... streamlines do not form closed loops in which fluid particles (and thus, nutrients and blood borne agonists) would become trapped. On the contrary, flow in these regions is progressively restored to streamwise flow. Downstream, sectional streamlines converge to a bifurcation line or asymptotic trajectory as it typically occurs in free shear flows. The branching and vessel curvature of the carotid artery causes a radial pressure gradient, which introduces secondary flow formation. This instability is known as Dean instability and the formation of secondary vortex pairs depends on the vessel curvature, radius and flow velocity. The secondary flow motion in both models was measured in the cross-sectional planes in the carotid sinus for mean and peak flow as indicated in Figure 3. The secondary velocity field in the physiological carotid bifurcation is shown in Figure 8 for mean flow at Re = 453. Consistent with previous findings, very strong secondary flows are observed in the carotid sinus. At the proximal location, fluid flows through the centre of the vessel towards the flow divider wall (left to right in Figure 8) and low momentum fluid near the wall flows towards the outer sinus wall setting up an asymmetric vortex pair. The vortex pair almost vanishes at the mid-sinus location (S2) before it changes its swirling direction at the distal sinus due to changes in vessel curvature. Unlike in the idealised model (Figure 9) with a symmetric and planar bifurcation, secondary flow is asymmetric in the in-vivo geometry with only a single predominant vortex occurring in the distal sinus. In both models, the strength and centre of the vortex changes with time as the flow moves downstream and similar behaviour is observed for steady peak flow. The streamwise vorticity distribution in the physiological model is shown in Figure 10 for Re = 704. A strong shear layer exists along the inner wall and streamwise vorticity increases towards the distal internal carotid. At S5, a second and possibly third vortex occur, giving rise to a highly unstructured flow ...
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Velocity measurements techniques are essential for a tremendous number of applications. The number of velocity measurement techniques for flows also reflects the need in fluid mechanics. Especially optical methods as Laser Doppler or particle image velocimetry are advantageous, because they can be used in-situ without disturbing the fluid flow. Ano...
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... Intriguingly, these intricate flow patterns elude detection in the PIV measurements. Buchmann and Jermy [28] similarly observed non-uniform velocity profiles in the bifurcation area based on MRI data. Region C focuses on the flow downstream of the bifurcation. ...
... However, the PIV contours suggest greater flow instability and the presence of vortices, while the CFD simulation indicates a relatively orderly re-establishment of organized flow. The curvature, radius, and flow velocity collectively contribute to the generation of secondary vortex pairs in this dynamic system [28]. Table 4 presents axial velocity profiles at two locations (A and B) within three different regions of the carotid artery, displaying a comparison between the PIV measurements and CFD predictions. ...
Cardiovascular disease remains the leading cause of morbidity and mortality globally, necessitating extensive research into the hemodynamics of blood flow under pathological conditions, such as atherosclerosis in carotid arteries. In vitro studies, particularly Computational Fluid Dynamics (CFD), are crucial for advancing our understanding of arterial blood flow and predicting pathological states. However, the accuracy of CFD simulations relies heavily on their validation against empirical data, such as those obtained from Particle Image Velocimetry (PIV). This study focuses on the comparative analysis of CFD predictions and PIV measurements of blood velocity vectors in a stented carotid artery bifurcation model under steady flow conditions derived from patient-specific data. The methodology involves simulating blood flow within a CFD framework and conducting PIV experiments using a blood-mimicking fluid seeded with particles in a carotid artery bifurcation phantom. The results indicate a reasonable agreement between the axial velocity vector profiles obtained via PIV and those predicted by CFD, with CFD predicting 10% higher than that recorded by PIV, especially in terms of recirculation areas and velocity values, despite some discrepancies in the velocity contours distribution, highlighting potential differences in how each method captures flow separation or recirculation areas. Despite some discrepancies in velocity contour distribution, which highlight potential differences in capturing flow separation or recirculation areas, the findings confirm that CFD simulations can effectively replicate the hemodynamics observed in carotid arteries and potentially other arterial segments. This study emphasizes the importance of integrating CFD simulations with experimental PIV data to validate and refine our understanding of arterial flow dynamics, significantly contributing to cardiovascular research and the development of interventions for arterial diseases.
... Such idealized models have been utilized for performing parametric investigation in previously published literature. [31][32][33] Blood is assumed as Newtonian in carotid arteries since the present CAB dimensions are large enough to suppress shear thinning effects (shear rate _ c ! 50 s À1 ). This assumption is also supported by a recent study, 34 where authors found negligible difference in CFD simulation on CAB with Newtonian and non-Newtonian models. ...
Investigation of sound-signal-based noninvasive diagnosis of arterial stenosis is an active area of research. This study focuses on computational investigation of hemodynamic and hemoacoustic parameters within the carotid bifurcation. The objective is to analyze the effect of 40 distinct geometric configurations on indicative sound signals, useful for understanding the feasibility of stethoscope-based diagnosis of stenosis. The study employs an in-house flow-solver based on the semi-implicit pressure-projection method on a curvilinear grid. Physiological condition-based pulsatile flow waveforms and three-element Windkessel model-based pressure are utilized at the inlet and outlets of the bifurcating carotid artery. The research involves assessment of parameters like wall shear stress (WSS) and integrated pressure force rate (IPFR) fast Fourier transform (FFT) spectrum. Geometric configurations are varied based on stenosis level S (0, 45%, 60%, and 70%), bifurcation angle BA (30°, 40°, 50°, and 65°), and length of stenosis L (1, 1.5, and 2). In the investigated geometries, WSS exhibits a distinct behavior, reaching a peak at stenosis and subsequently transitioning to a negative value. Furthermore, IPFR-spectrum analysis reveals distinguishable frequencies for S≥ 40%, hinting at the potential for stethoscope-based diagnosis. A novel correlation between the cutoff frequencies of IPFR FFT-spectrum and arterial geometry is established, which reflect the influence of artery geometry on sound signals. Computational fluid dynamics (CFD)-based flow-visualization approach is proposed to calculate characteristic frequencies, which are close to IPFR spectrum frequencies. Our study contributes to a framework for potential sound-based classification of plaque-induced constrictions.
... The three-dimensionality of the axial flow structures introduces in-plane fluid motion, which may contribute to the non-axial nature of the arterial stress and can lead to atherogenic degeneration. [58][59][60] To investigate the three-dimensionality, we compare the magnitude of secondary (in-plane) velocity at different phases of the pulsation cycle for healthy, 30% stenosis, and 50% stenosis carotid arteries (Fig. 7). In the healthy carotid artery, the magnitude of secondary velocity is maximum at the deceleration phase (P 2 ) [ Fig. 7(a)]. ...
Stenosis in the internal carotid arteries is a serious cardiovascular condition. It is well-reported that low and oscillatory wall shear stress enhances the risk of stenosis progression. However, the effects of increased heart rates in highly stenosed arteries are not well explored. A detailed understanding of the flow features and stress distribution in stenosed carotid arteries at different heart rates may help clinicians to prescribe better exercise schedules for patients. In this study, we probe the effects of elevated heart rates on the hemodynamics in healthy and stenosed carotid arterial geometries using an immersed boundary method-based computational framework. Our results reveal that a strong recirculation, secondary velocity, and oscillatory shear index (OSI) zone develop inside a severely stenosed carotid artery at normal heart rate. Higher heart rates may potentially improve arterial health by reducing OSI only for the healthy and mild stenosis carotid arteries. However, the increased heart rates worsen the arterial health of severely stenosed arteries by onsetting flow instabilities, enhancing the spread and severity of the recirculation zone and the magnitude of the secondary velocity, the pressure drops across the stenosis, and the spread of high OSI (≥0.2) zone downstream. Furthermore, in the case of severe stenosis, the wall shear stress at the stenosis throat rises significantly, which can contribute to plaque rupture and thrombus development. Here, we report in detail the behavior of stress levels and pressure fluctuations in the carotid artery model at different stenosis levels for normal and elevated heart rates.
... Mainly for its low refractive index, which can be matched using water-glycerol solution at an appropriate mixing ratio, the transparent silicone elastomer Dow Corning Sylgard® 184 was selected as the material for manufacturing the vessel models. Such procedure has been applied in several works already, for example in [5][6][7]. ...
... In the studies, e.g. [5,6,9], the refractive index was different hence it cannot be considered as a fixed value. ...
This study is focused on measurement of velocity fields by the planar PIV experimental method. Velocity fields are measured within rigid blood vessel models with stenoses of different distances between each other. Stenoses of two different geometries, a symmetric and an asymmetric, though with the same 75% area reduction, are used. The vessel models are made of transparent silicone elastomer with low refractive index using external PMMA casting box with a 3D printed inner negative mould made of water soluble PVA material. Optical distortion on the curved inner wall of the vessel model is eliminated using a working fluid which has the refractive index of the same value as the model does. In order to match the refractive indices to each other, a water-glycerol solution mixed at an appropriate ratio was used as the working fluid. For simplicity of the experimental setup, a continuous laser diode was used as light source. Velocity fields were measured within five vessel models with stenoses: one model with a single symmetric stenosis, two models with double symmetric stenoses at different distances, and two models with a combination of symmetric and asymmetric stenosis at different distances. For each model, three steady flow regimes were measured with volume flow rates of 9, 13.5, and 18 ml/s, corresponding to Reynolds numbers of 83, 124, and 165 respectively.
... Effect of vessel wall compliance on flow structures is another factor that is of interest in physiological flow studies. Sylgard 184 (Dow Corning) is known as a suitable material for casting compliant and rigid test sections used for optical measurement techniques such as particle image velocimetry (PIV) [26][27][28][29][30]. One of the first studies that used a Sylgard test section with PIV was Hopkins et al. [31]. ...
Secondary flow structures in a 180 • curved pipe model of an artery are studied using particle image velocimetry. Both steady and pulsatile inflow conditions are investigated. In planar curved pipes with steady flow, multiple (two, four, six) vortices are detected. For pulsatile flow, various pairs of vortices, i.e., Dean, deformed-Dean, Lyne-type, and split-Dean, are present in the cross section of the pipe at 90 • into the bend. The effects of nonplanar curvature (torsion) and vessel dilatation on these vortical structures are studied. Torsion distorts the symmetric secondary flows (which exist in planar curvatures) and can result in formation of more complex vortical structures. For example, the split-Dean and Lyne-type vortices with same rotation direction originating from opposite sides of the cross section tend to merge together in pulsatile flow. The vortical structures in elastic vessels with dilatation (0.61%-3.23%) are also investigated and the results are compared with rigid model results. It was found that the secondary flow structures in rigid and elastic models are similar, and hence the local compliance of the vessel does not affect the morphology of secondary flow structures.
... Liepsch et al. (1998) and Liepsch (2002) present details of results that study anatomically realistic geometries using LDV. More recently, Bale-Glickman et al. (2003) and Buchmann et al. (2009) conducted planar PIV measurements in physiologically realistic and diseased models. ...
A method for the construction of both rigid and compliant (flexible) transparent flow phantoms of biological flow structures, suitable for PIV and other optical flow methods with refractive-index-matched working fluid is described in detail. Methods for matching the in vivo compliance and elastic wave propagation wavelength are presented. The manipulation of MRI and CT scan data through an investment casting mould is described. A method for the casting of bubble-free phantoms in silicone elastomer is given. The method is applied to fabricate flexible phantoms of the carotid artery (with and without stenosis), the carotid artery bifurcation (idealised and patient-specific) and the human upper airway (nasal cavity). The fidelity of the phantoms to the original scan data is measured, and it is shown that the cross-sectional error is less than 5% for phantoms of simple shape but up to 16% for complex cross-sectional shapes such as the nasal cavity. This error is mainly due to the application of a PVA coating to the inner mould and can be reduced by shrinking the digital model. Sixteen per cent variation in area is less than the natural patient to patient variation of the physiological geometries. The compliance of the phantom walls is controlled within physiologically realistic ranges, by choice of the wall thickness, transmural pressure and Young’s modulus of the elastomer. Data for the dependence of Young’s modulus on curing temperature are given for Sylgard 184. Data for the temperature dependence of density, viscosity and refractive index of the refractive-index-matched working liquid (i.e. water–glycerol mixtures) are also presented.
... Due to the point-wise nature of LDV measurements, the full threedimensional (and instantaneous) velocity and WSS maps are difficult to obtain and can only be inferred with considerable experimental effort. More recently, Bale-Glickman et al. (2003) and Buchmann et al. (2009) conducted planar Particle Image Velocimetry (PIV) measurements in physiologically realistic and diseased models. Furthermore, the derivation of arterial WSS from planar PIV data has been demonstrated for example by Buchmann et al. (2008); Poelma et al. (2008) and Zhang et al. (2008). ...
... The geometry is reproduced via rapid prototyping at 2.8 times scale with a layer thickness of 0.1 ± 0.05mm and subsequently cast into a clear silicone resin (Sylgard 184, Down Corning). The resulting transparent flow phantom is shown in Figure 3(a), while a more detailed description of the model manufacturing process can be found in Buchmann et al. (2009). ...
... Flow measurements are conducted in a closed-loop flow facility described in greater detail by Buchmann et al. (2009). The flow phantom is connected to a system of rigid and flexible pipes, which are designed to provide fully developed flow at the inlet of the flow phantom. ...
Hemodynamic forces within the human carotid artery are well known to play a key role in the initiation and progression of
vascular diseases such as atherosclerosis. The degree and extent of the disease largely depends on the prevailing three-dimensional
flow structure and wall shear stress (WSS) distribution. This work presents tomographic PIV (Tomo-PIV) measurements of the
flow structure and WSS in a physiologically accurate model of the human carotid artery bifurcation. The vascular geometry
is reconstructed from patient-specific data and reproduced in a transparent flow phantom to demonstrate the feasibility of
Tomo-PIV in a complex three-dimensional geometry. Tomographic reconstruction is performed with the multiplicative line-of-sight
(MLOS) estimation and simultaneous multiplicative algebraic reconstruction (SMART) technique. The implemented methodology
is validated by comparing the results with Stereo-PIV measurements in the same facility. Using a steady flow assumption, the
measurement error and RMS uncertainty are directly inferred from the measured velocity field. It is shown that the measurement
uncertainty increases for increasing light sheet thickness and increasing velocity gradients, which are largest near the vessel
walls. For a typical volume depth of 6mm (or 256pixel), the analysis indicates that the velocity derived from 3D cross-correlation
can be measured within±2% of the maximum velocity (or±0.2pixel) near the center of the vessel and within±5% (±0.6pixel)
near the vessel wall. The technique is then applied to acquire 3D-3C velocity field data at multiple axial locations within
the carotid artery model, which are combined to yield the flow field and WSS in a volume of approximately 26mm×27mm×60mm.
Shear stress is computed from the velocity gradient tensor and a method for inferring the WSS distribution on the vessel wall
is presented. The results indicate the presence of a complex and three-dimensional flow structure, with regions of flow separation
and strong velocity gradients. The WSS distribution is markedly asymmetric confirming a complex swirling flow structure within
the vessel. A comparison of the measured WSS with Stereo-PIV data returns an acceptable agreement with some differences in
stress magnitude.
... For the production of the experimental flow phantom, the process adapted by Buchmann et al. (15) was used. In brief, a physical prototype of the bifurcation model was created in a water soluble material by computer-controlled three-dimensional printing and subsequently cast into a clear silicone resin (Sylgard 184, Dow Corning). ...
... However, imaging and PIV processing errors can introduce considerable noise to the measured velocity field near stationary walls (32) , which is further amplified during the WSR estimation. One alternative to conventional velocity field differentiation and particularly well suited for complex flow geometries is iPIV as previously developed by the authors (15) (for detailed descriptions, see Ref. (15) and Ref. (35)). ...
... However, imaging and PIV processing errors can introduce considerable noise to the measured velocity field near stationary walls (32) , which is further amplified during the WSR estimation. One alternative to conventional velocity field differentiation and particularly well suited for complex flow geometries is iPIV as previously developed by the authors (15) (for detailed descriptions, see Ref. (15) and Ref. (35)). ...
Particle image velocimetry (PIV) and computational fluid dynamics (CFD) modelling of blood flow through a carotid artery bifurcation has been carried out in order to assess the role of haemodynamics in atherosclerosis and validate a novel wall shear stress (WSS) measurement method via detailed quantitative data analysis. Velocity and WSS data, obtained by PIV was compared against CFD-predicted data obtained for the same geometry and boundary conditions. The results demonstrate that both methods are capable of capturing essential flow characteristics, and are in good agreement. The axial velocity data clearly show the important flow features such as skewed velocity profiles and a low-momentum region near the sinus outer wall, although with slight errors between the two techniques in the order of 6.1-13.7%. The secondary velocity plots and three-dimensional particle traces reveal complex flow features within the sinus, such as Dean vortices and helicoidal flow. WSS data obtained by both techniques reveal the presence of a low-momentum and reversed-flow regions, and show agreement with errors in the order of 1% in the common carotid artery and 0.9-9.8% in the sinus. The error sources in both the experimental and numerical methodology are discussed and recommendations for future applications are made.
... In fact, the stereo configuration simulated here would present practical difficulty because the oblique cameras must view through the bed in order to image the illuminated wall over an entire wavelength; this would require perfect matching of refractive indices of the fluid and bed material. For 2C IPIV, this has been done in experiments of a model carotid artery by Buchmann et al. (2009). Because of the shallower incidence of rays passing through the solid-liquid boundary, the present configuration would be more challenging. ...
... The principal conclusion was that the PIV/IG+ results were rather sensitive to errors in wall position (cf. similar analysis in Buchmann et al. (2009)). ...
In investigations of laminar or turbulent flows, wall shear is often important. Nevertheless, conventional particle image velocimetry (PIV) is difficult in near-wall regions. A near-wall measurement technique, named interfacial PIV (IPIV) [Nguyen, C., Nguyen, T., Wells, J., Nakayama, A., 2008. Proposals for PIV of near-wall flow over curved boundaries. In: Proceedings of 14th International Symposium on Applications of Laser Technique to Fluid Mechanics], handles curved boundaries by means of conformal transformation, directly measures the wall gradient, and yields the near-wall tangential velocity profile at one-pixel resolution. In this paper, we show the feasibility of extending IPIV to measure wall gradients by stereo reconstruction. First, we perform a test on synthetic images generated from a direct numerical simulation (DNS) snapshot of turbulent flow over sinusoidal bed. Comparative assessment of wall gradients derived by IPIV, stereo-IPIV and particle image distortion (PID) [Huang, H.T., Fiedler, H.E., Wang, J.J., 1993. Limitation and improvement of PIV. Experiments in Fluids 15(4), 263–273] is evaluated with DNS data. Also, the sensitivity of IPIV and stereo-IPIV results to the uncertainty of identified wall position is examined. As a practical application of IPIV and stereo-IPIV to experimental images, results from turbulent open channel flow over a backward-facing step are discussed in detail.
... We will refer to this generalized technique as ''Interfacial PIV'' or ''IPIV''. In addition to standard channel flow like that examined herein, IPIV has already proven itself suitable for in-vitro measurement of the wall-shear gradient of a flow in a modeled carotid artery, as carried out by Buchmann et al. (2008 Buchmann et al. ( , 2009). ...
... One pixel is equivalent to 1 wall unit wall units from the wall if interframe delay is shortened sufficiently. The method should be valuable in applications to complex, separating boundary flows, such as the flow over a sinusoidal wall presented in this paper or the flow in a modeled carotid artery (Buchmann et al. 2008Buchmann et al. , 2009). ...
PIV measurements near a wall are generally difficult due to low seeding density, low velocity, high velocity gradient, and strong reflections. Such problems are often compounded by curved boundaries, which are commonly found in many industrial and
medical applications. To systematically solve these problems, this paper presents two novel techniques for near-wall measurement, together named Interfacial PIV, which extracts both wall-shear gradient and near-wall tangential velocity profiles at one-pixel
resolution. To deal with curved walls, image strips at a curved wall are stretched into rectangles by means of conformal transformation. To extract the maximal spatial information on the near-wall tangential velocity field, a novel 1D correlation function is
performed on each horizontal pixel line of the transformed image template to form a “correlation stack”. This 1D correlation function requires that the wall-normal displacement component of the particles be smaller than the particle image diameter in order to produce a correlation signal. Within the image regions satisfying this condition, the correlation function yields peaks that form a tangential velocity profile. To determine this profile robustly, we propose to integrate gradients of tangential velocity outward from the wall, wherein the gradient at each wall-normal position is measured by fitting a straight line to
the correlation peaks. The capability of Interfacial PIV was validated against Particle Image Distortion using synthetic image pairs generated from a DNS velocity field over a sinusoidal bed. Different velocity measurement schemes performed on the same
correlation stacks were also demonstrated. The results suggest that Interfacial PIV using line fitting and gradient integration provides the best accuracy of all cases in the measurements of velocity gradient and velocity profile near wall surfaces.
