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Predicting rupture risk in intracranial aneurysms is among one of the most critical questions in vascular surgery. The processes that govern an aneurysm growth are multifaceted and complex, but may be summarized into three components: hemodynamics, biology, and mechanics. We review and connect the literature in the three disciplines, identifying considerable strides in recent history and current gaps in research. Taken together, the findings from each field elucidate how and why certain aneurysms rupture, whereas others remain stable. These parameters could eventually inform a translatable predictive model that optimizes risk evaluation and physician's decision-making in treatment options for aneurysms.

The COVID-19 pandemic pushed dental health officials around the world to reassess and adjust their existing healthcare practices. As studies on controlled COVID-19 transmission remain challenging, this review focuses on particles that can carry the virus and relevant approaches to mitigate the risk of pathogen transmission in dental offices. This review gives an overview of particles generated in clinical settings and how size influences their distribution, concentration, and generation route. A wide array of pertinent particle characterization and counting methods are reviewed, along with their working range, reliability, and limitations. This is followed by a focus on the effectiveness of personal protective equipment (PPE) and face shields in protecting patients and dentists from aerosols. Direct studies on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are still limited, but the literature supports the use of masks as an important and effective non-pharmaceutical preventive measure that could reduce the risk of contracting a respiratory infection by up to 20%. In addition to discussing about PPE used by most dental care professionals, this review describes other ways by which dental offices can protect patients and dental office personnel, which includes modification of the existing room design, dental equipment, and heating, ventilation, and air conditioning (HVAC) system. More affordable modifications include positioning a high-efficiency particulate air (HEPA) unit within proximity of the patient's chair or using ultraviolet germicidal irradiation in conjunction with ventilation. Additionally, portable fans could be used to direct airflow in one direction, first through the staff working areas and then through the patient treatment areas, which could decrease the number of airborne particles in dental offices. This review concludes that there is a need for greater awareness amongst dental practitioners about the relationship between particle dynamics and clinical dentistry, and additional research is needed to fill the broad gaps of knowledge in this field.

Embolization has tremendously evolved in recent years and has expanded to treatment of a variety of pathologic processes. There has been emerging evidence that the level of arterial occlusion and the distribution of embolic particles may play an important role in the clinical outcome. This is a comprehensive literature review to identify variables that play important role in determination of level of occlusion of blood vessels and distribution of embolic particles. The literature searches between 1996 to 2020 through PubMed and Ovid-MEDLINE yielded over 1030 articles of which 30 studies providing details on the level of occlusion are reviewed here. We divided the playing factors into characteristics of the particles, solution/injection and vascular bed. Accordingly, particle size, type and aggregation, compressibility/deformability, and biodegradability are categorized as the factors involving particles' behavioral nature. Infusion rate and concentration/dilution of the medium are related to the carrying solution. Hemodynamics and the arterial resistance are characteristics of the vascular bed that also play an important role in the distribution of embolic particles. Understanding and predicting the level of embolization is a complex multi-factor problem that requires more evidence, warranting further randomized controlled trials, and powered human and animal studies.

The process of an intracranial aneurysm development, growth, and rupture is multifaceted and complex. In addition, clinical observations have identified the potential of thrombus formation within such aneurysms. While the underlying mechanism is not fully understood, the thrombi represent a potential risk factor for ischemic stroke. Emerging studies indicate that blood residence time (RT) is a promising hemodynamic metric associated with the aneurysm rupture and formation of intra-aneurysmal thrombi. Here, we present a methodology to experimentally evaluate both trajectory-wise and local RT based on magnetic resonance imaging (MRI) velocimetry, and apply it to in vitro flow measurements in scaled-up replicas of 9 patient-specific intracranial aneurysms. Lagrangian tracks of massless tracers are integrated from the velocity fields and averaged to return the mean RT in the aneurysm sac. This is found to be closely approximated by a simple time scale based on the sac diameter and space-time average of the aneurysmal fluid velocity. The mean RT is also correlated with the inflow time scale at the parent artery. These results also provide a basis for the estimation of RT when high-resolution hemodynamic maps are not available. With the continuous increase in accuracy and resolution enabled by progress in MRI technology, the methodology described here may in the future be applicable to in vivo data.

The role of the external boundary conditions of the nozzle surface on the azimuthal mode selection of impinging supersonic jets is demonstrated for the first time. Jets emanating from thin- and infinite-lipped nozzles at a nozzle pressure ratio of $3.4$ and plate spacing of $5.0D$, where $D$ is the nozzle exit diameter, are investigated using high resolution particle image velocimetry (PIV) and acoustic measurements. Proper orthogonal decomposition is applied to the PIV fields and a difference in dominant instability mode is found. To investigate possible explanations for the change in instability mode, additional nozzle external boundary conditions are investigated, including the addition of acoustic dampening foam. A difference in acoustic feedback path is suggested to be the cause for the change in dominant azimuthal modes between the flows. This is due to the thin-lip case containing a feedback path that is concluded to be closed exclusively by a reflection from the nozzle base surface, rather than directly to the nozzle lip. The ability of the flow to form a feedback path that maximises the impingement tone gain is discussed with consideration of the numerous acoustic feedback paths possible for the given nozzle external boundary conditions.

A double-walled stent-graft (DWSG) design with a compressible gas layer was conceived with the goal of treating hypertension in patients receiving an aortic stent-graft. Early prototypes were developed to evaluate the design concept through static measurements from a finite element (FE) model and quasi-static inflation experiments, and through dynamic measurements from an in vitro flow loop and the three-element Windkessel model. The amount of gas in the gas layer and the properties of the flexible inner wall were the primary variables evaluated in this study. Properties of the inner wall had minimal effect on DWSG behavior, but increased gas charge led to increased fluid capacitance and larger reduction in peak and pulse pressures. In the flow loop, placement of the DWSG decreased pulse pressure by over 20% compared to a rigid stent-graft. Capacitance measurements were consistent across all methods, with the maximum capacitance estimated at 0.07 mL/mmHg for the largest gas charge in the 15 cm long prototype. Windkessel model predictions for in vivo performance of a DWSG placed in the aorta of a hypertensive patient showed pulse pressure reduction of 14% compared to a rigid stent-graft case, but pressures never returned to unstented values. These results indicate that the DWSG design has potential to be developed into a new treatment for hypertensive patients requiring an aortic intervention.

Magnetic resonance velocimetry is used to investigate the steady inhalation, steady exhalation, and oscillatory flow in a 3D-printed realistic airway geometry for physiologically relevant regimes ranging from quiet breathing to respiration under high-frequency ventilation. Additionally, the pendulluft phenomenon is demonstrated experimentally for the first time, using both Eulerian velocity fields and Lagrangian path lines.

To reach a predictive understanding of how particles travel through bifurcating vessels is of paramount importance in many biomedical settings, including embolization, thromboembolism, and drug delivery. Here we utilize an in vitro model in which solid particles are injected through a rigid vessel that symmetrically bifurcates in successive branching generations. The geometric proportion and fluid dynamics parameters are relevant to the liver embolization. The volumetric flow field is reconstructed via phase-contrast magnetic resonance imaging, from which the particle trajectories are calculated for a range of size and density using the particle equation of motion. The method is validated by directly tracking the injected particles via optical imaging. The results indicate that, opposite to the common assumption, the particles distribution is fundamentally different from the volumetric flow partition. In fact, the amount of delivered particles vary substantially between adjacent branches even when the flow is uniformly distributed. This is not due to the inertia of the particles, nor to gravity. The particle distribution is rather rooted in their different pathways, which in turn are linked to their release origin along the main vessel cross-section. Therefore, the tree geometry and the associated flow streamlines are the prime determinant of the particle fate, while local changes of volumetric flow rate to selected branches do not generally produce proportional changes of particle delivery.

This study explores the optimal left ventricular assist device (LVAD) cannula outflow configuration in a patient-specific replica of the aorta. The volumetric velocity field is measured using phase-contrast magnetic resonance imaging (PC-MRI) under a physiologically relevant steady flow. The effect of the LVAD outflow graft insertion site and anastomosis angle on the transport of embolic particles to cranial vessels is studied by solving the particle equation of motion for spheres in the range of 0.1–1.0 mm using the measured three-dimensional (3D) velocity field. Results show that for a given aorta anatomy, it is possible to design the cannula graft location and terminal curvature so that the probability of embolic transport to the cranial vessels is significantly minimized. This is particularly important since the complex flow pattern in each cannula case affects the embolic trajectories differently, and hence the common assumption that particles distribute by the volumetric flow division does not hold.

We present experimental observations of the velocity and spatial distribution of inertial particles dispersed in turbulent downward flow through a vertical channel at friction Reynolds numbers $\mathit{Re}_{\unicode[STIX]{x1D70F}}=235$ and 335. The working fluid is air laden with size-selected glass microspheres, having Stokes numbers $St=\mathit{O}(10)$ and $\mathit{O}(100)$ when based on the Kolmogorov and viscous time scales, respectively. Cases at solid volume fractions $\unicode[STIX]{x1D719}_{v}=3\times 10^{-6}$ and $5\times 10^{-5}$ are considered. In the more dilute regime, the particle concentration profile shows near-wall and centreline maxima compatible with a turbophoretic drift down the gradient of turbulence intensity; the particles travel at speed similar to that of the unladen flow except in the near-wall region; and their velocity fluctuations generally follow the unladen flow level over the channel core, exceeding it in the near-wall region. The denser regime presents substantial differences in all measured statistics: the near-wall concentration peak is much more pronounced, while the centreline maximum is absent; the mean particle velocity decreases over the logarithmic and buffer layers; and particle velocity fluctuations and deposition velocities are enhanced. An analysis of the spatial distributions of particle positions and velocities reveals different behaviours in the core and near-wall regions. In the channel core, dense clusters form which are somewhat elongated, tend to be preferentially aligned with the vertical/streamwise direction and travel faster than the less concentrated particles. In the near-wall region, the particles arrange in highly elongated streaks associated with negative streamwise velocity fluctuations, several channel heights in length and spaced by $\mathit{O}(100)$ wall units, supporting the view that these are coupled to fluid low-speed streaks typical of wall turbulence. The particle velocity fields contain a significant component of random uncorrelated motion, more prominent for higher $St$ and in the near-wall region.

Proper orthogonal decomposition can be used to determine the dominant coherent structures present within a turbulent flow. In many flows, these structures are well represented by only a few high-energy modes. However, additional modes with clear spatial structure, but low-energy contribution can often be present in the proper orthogonal decomposition analysis, even for flows with a high degree of periodicity. One such mode has been observed in both free and impinging jets determined from particle image velocimetry. Both experimental and synthetic data are used to investigate the role of this particular mode, linking its existence to the unsteadiness of shear-layer large-scale coherent structures.

Mean wall shear stress in a turbulent channel flow has been measured using a film-based shear stress sensor with a working principle based on the deformation of a thin elastic film. A direct comparison was made between the elastic film measurements and those using oil-film interferometry in the same experimental facility to evaluate the capability of the novel technique to measure the mean wall shear stress. The results indicate that the film-based sensor measures the wall shear stress within the uncertainty of the oil film measurement for the range of Reynolds numbers considered here, i.e. 2100–2900 based on the friction velocity and the half channel height.

Introduction Blood velocity and quantitative parameters such as wall shear stress (WSS) can be investigated non-invasively with 4D Flow MRI, and a comparison of these parameters between patients with ruptured and un-ruptured intracranial aneurysms (IAs) has been suggested to be useful in the differentiation between aneurysms at risk for rupture and stable aneurysms. In this context, although ruptured IAs cannot be easily scanned with 4 D flow MRI in-vivo, these, like un-ruptured IAs, can be modeled and 3D printed. The upscaled 3 D printed flow phantoms can then be scanned with 4D Flow MR at unsurpassed conspicuity. Herein, we investigate the feasibility of measuring detailed blood flow information in small IAs in-vivo (less than 7 mm in size) and comparing the data with data obtained from scanning in-vitro scaled-up and 3D printed models of the same IAs. Methods Four patients presenting with one, and one patient presenting two incidentally discovered small aneurysm(s) were scanned with 4D flow sequences at 7Twith a resolution of 0.76 × 0.76 × 0.8 mm3. Based on non-invasive imaging studies, the same aneurysms were numerically modelled, upscaled, 3D printed and assembled in an MR-compatible, dynamic flow circuit filled with a fluid chosen for its Reynolds and Womersley numbers. 4D Flow MR data were then acquired at 3T with this setting. Results In the patient with two IAs, one was missed (positioning error) while the other proved to be too small ( Conclusion We successfully obtained sub-millimetric in-vivo 4D Flow scans in small IAs at 7T. 4D Flow at 3T of aneurysm-specific 3D printed replicas yielded flow patterns comparable to those obtained in-vivo. These findings suggest that a larger comparative study of upscaled 3D printed of ruptured and unruptured small IAs could provide useful insight into the role of hemodynamics in rupture of small IAs. Disclosures B. Jagadeesan: 1; C; Microvention, Medtronic. 2; C; Microvention, CvRx. M. Toloui: None. O. Amili: None. S. Schmitter: None. S. Moen: None. K. Ugurbil: None. A. Grande: None. F. Coletti: None. P. Van de Moortele: None.

In 2017, the American Heart Association reported that one third of deaths in the United States, and 31% of deaths worldwide, are attributed to cardiovascular disease (CVD) [1]. A risk factor pervasive across most types of CVD is chronic high blood pressure, or hypertension [2].

This study reports on experimentally observed rare near-wall reverse flow events in a fully developed turbulent flat plate boundary layer at zero pressure gradient with Reynolds numbers between Re θ ≈ 2500 and Re θ ≈ 8000 ( Re τ ≈ 800 – 2400 ). The reverse flow events are captured using high magnification particle image velocimetry sequences with record lengths varying from 50 000 to 126 000 samples. Time resolved particle image sequences allow singular reverse flow events to be followed over several time steps whereas long records of nearly statistically independent samples provide a variety of single snapshots at a higher spatial resolution. The probability of occurrence lies in the order of 0.012–0.018% which matches predictions from direct numerical simulations (DNS). The typical size of the reverse flow bubble is about 30 wall units in length and 5 wall units in height which agrees well with similar observations made in existing DNS data.

This paper reports on near-wall two-component–two-dimensional (2C–2D) particle image velocimetry (PIV) measurements of a turbulent pipe flow at shear Reynolds numbers up to $Re_{\unicode[STIX]{x1D70F}}=40\,000$ acquired in the CICLoPE facility of the University of Bologna. The 111.5 m long pipe of 900 mm diameter offers a well-established turbulent flow with viscous length scales ranging from $85~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=5000$ down to $11~\unicode[STIX]{x03BC}\text{m}$ at $Re_{\unicode[STIX]{x1D70F}}=40\,000$. These length scales can be resolved with a high-speed PIV camera at image magnification near unity. Statistically converged velocity profiles were determined using multiple sequences of up to 70 000 PIV recordings acquired at sampling rates of 100 Hz up to 10 kHz. Analysis of the velocity statistics shows a well-resolved inner peak of the streamwise velocity fluctuations that grows with increasing Reynolds number and an outer peak that develops and moves away from the inner peak with increasing Reynolds number.

For the first time, a physical mechanism is identified to explain the phase lag term in Powell’s impinging feedback loop equation (Powell, J. Acoust. Soc. Am., vol. 83 (2), 1988, pp. 515–533). Ultra-high-speed schlieren reveals a previously unseen periodic transient shock in the wall jet region of underexpanded impinging flows. The motion of this shock appears to be responsible for the production of the acoustic waves corresponding to the impingement tone. It is suggested that the delay between the inception of the shock and the formation of the acoustic wave explains the phase lag in the aeroacoustic feedback process. This suggestion is quantitatively supported through an assessment of Powell’s feedback equation, using high-resolution particle image velocimetry and acoustic measurements.

An experiment conducted in the framework of the EUHIT project and designed to characterize large scale structures in an adverse pressure gradient boundary layer flow is presented. Up to 16 sCMOS cameras were used in order to perform large scale turbulent boundary layer PIV measurements with a large field of view and appropriate spatial resolution. To access the span-wise / wall-normal signature of the structures as well, stereoscopic PIV measurements in span-wise/wall-normal planes were performed at specific stream-wise locations. To complement these large field of view measurements, long-range micro-PIV, time resolved near wall velocity profiles and film-based measurements were performed in order to determine the wall-shear stress and its fluctuations at some specific locations along the model., 50 pages

The use of random jet arrays, already employed in water tank facilities to generate zero-mean-flow homogeneous turbulence, is extended to air as a working fluid. A novel facility is introduced that uses two facing arrays of individually controlled jets (256 in total) to force steady homogeneous turbulence with negligible mean flow, shear, and strain. Quasi-synthetic jet pumps are created by expanding pressurized air through small straight nozzles and are actuated by fast-response low-voltage solenoid valves. Velocity fields, two-point correlations, energy spectra, and second-order structure functions are obtained from 2D PIV and are used to characterize the turbulence from the integral-to-the Kolmogorov scales. Several metrics are defined to quantify how well zero-mean-flow homogeneous turbulence is approximated for a wide range of forcing and geometric parameters. With increasing jet firing time duration, both the velocity fluctuations and the integral length scales are augmented and therefore the Reynolds number is increased. We reach a Taylor-microscale Reynolds number of 470, a large-scale Reynolds number of 74,000, and an integral-to-Kolmogorov length scale ratio of 680. The volume of the present homogeneous turbulence, the largest reported to date in a zero-mean-flow facility, is much larger than the integral length scale, allowing for the natural development of the energy cascade. The turbulence is found to be anisotropic irrespective of the distance between the jet arrays. Fine grids placed in front of the jets are effective at modulating the turbulence, reducing both velocity fluctuations and integral scales. Varying the jet-to-jet spacing within each array has no effect on the integral length scale, suggesting that this is dictated by the length scale of the jets.

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.

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., PLoS ONE, 13 (1), ISSN:1932-6203

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.

The three-dimensional spatial filtering and measurement noise associated with experimental planar and three-dimensional (3D) particle image velocimetry (PIV) measurements is investigated using a combination of direct numerical simulations (DNS) and experimental databases. Spatial filtering velocity fields from a DNS of a zero-pressure-gradient turbulent boundary layer (TBL) at resolutions typical of PIV experiments are shown to underestimate Reynolds stresses by as much as 50 %. Comparison of experimental PIV measurement of a turbulent channel flow and 3D tomographic PIV measurements of a TBL with higher-resolution simulations and hot-wire anemometry measurements show that in real experiments, measurement noise acts to offset this effect. This is shown to produce measurements that appear to provide a good estimate of the turbulent fluctuations in the flow, when in reality the flow is spatially under-resolved and partially contaminated by noise. Means of identifying this noise are demonstrated using the one-dimensional (1D) velocity power spectra and the 1D transfer function between the power spectra of the unfiltered velocity field and the power spectra calculated from the filtered experimental measurement. This 1D transfer function differs from the commonly used sinc transfer function of PIV owing to the integrated effect of filtering in multiple directions. Failure to incorporate this difference is shown to overestimate the maximum resolved wave number in the 3D spectra of the planar PIV by close to 10 %, while conversely underestimating the maximum resolved wave number in the 3D PIV by 50 %. Appropriate spatial filtering of the experimental data is shown to remove the noise-dominated small-scale fluctuations and bring the data inline with that which should be obtained for a noiseless PIV measurement at the corresponding spatial resolution.

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.

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.

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, 6.66x10sup5/sup, 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.