Vector Projectile Imaging: Time-Resolved Dynamic Visualization of Complex Flow Patterns

Evaluation of Turbulence Index and Flow Pattern for Atherosclerotic Carotid Stenosis: A High-Frame-Rate Vector Flow Imaging Study

We study the collapsible behaviour of a vessel conveying viscous flows subject to external pressure, a scenario that could occur in many physiological applications. The vessel is modelled as a three-dimensional cylindrical tube of nonlinear hyperelastic material. To solve the fully coupled fluid–structure interaction, we have developed a novel approach based on the Arbitrary Lagrangian–Eulerian (ALE) method and the frontal solver. The method of rotating spines is used to enable an automatic mesh adaptation. The numerical code is verified extensively with published results and those obtained using the commercial packages in simpler cases, e.g. ANSYS for the structure with the prescribed flow, and FLUENT for the fluid flow with prescribed structure deformation. We examine three different hyperelastic material models for the tube for the first time in this context and show that at the small strain, all three material models give similar results. However, for the large strain, results differ depending on the material model used. We further study the behaviour of the tube under a mode-3 buckling and reveal its complex flow patterns under various external pressures. To understand these flow patterns, we show how energy dissipation is associated with the boundary layers created at the narrowest collapsed section of the tube, and how the transverse flow forms a virtual sink to feed a strong axial jet. We found that the energy dissipation associated with the recirculation does not coincide with the flow separation zone itself, but overlaps with the streamlines that divide the three recirculation zones. Finally, we examine the bifurcation diagrams for both mode-3 and mode-2 collapses and reveal that multiple solutions exist for a range of the Reynolds number. Our work is a step towards modelling more realistic physiological flows in collapsible arteries and veins.

Swirl flame stabilization is widespread among burners’ manufacturers, but the complex flow patterns are not yet fully understood. The interaction of two confined swirling jets leads to the formation of two recirculation zones being the flame located on the shear layer between the both zones. In such conditions, the lean mixtures can be burned producing low emissions. In the present study, flow structure and turbulent mixing of two isothermal coaxial jets are investigated using Large Eddy Simulation (LES). This is a challenging tool to achieve accuracy but it requests demanding spatial resolution and special treatment of results. By contrasting time-averaged radial profiles with experimental data of a classical benchmark, the model is validated. Results show that LES is able to reproduce the basic features of the flow pattern. Besides, the spectra analysis of instantaneous flow fields provides not only the energy decay but also the most energetic flow structures.

When organisms move forward or backward through their environment, they produce particular patterns of optic flow on their retina, i.e., the images of objects move in characteristic ways away from or towards a focus of expansion or contraction (Gibson, 1950, 1954). For straight motion without eye or head movements, the flow pattern is radial with a speed gradient and complete symmetry (Gibson, 1950). It is then straightforward to compute the heading direction, calculate the time to collision with objects or, indeed, take action to avoid collisions (van den Berg, 2000; Warren, 1998; Warren et al., 1988, 1991). The situation becomes more difficult when eye movements (e.g., pursuit movements) are made simultaneously, which is the rule and not the exception. Fixating an object that moves from left to right would create a lateral flow on the retina, giving a rightward motion to the images of all other objects. We call this a parallel flow to express the fact that all motion vectors would be parallel. This motion would be added to the appropriate radial motion for each image point, resulting in a complex flow pattern. This complex flow still contains the original radial flow that is needed for the extraction of heading direction. But the complex pattern would have to be decomposed to yield this information.

Background and purposeHemodynamic factors are thought to play an important role in the initiation, growth and rupture of cerebral aneurysms. This report describes a study of the associations between qualitative intra-aneurysmal hemodynamics and the rupture of cerebral aneurysms.Methods210 consecutive aneurysms were analyzed using patient-specific CFD simulations under pulsatile flow conditions. The aneurysms were classified into categories depending on the complexity and stability of the flow pattern, size of the impingement region, and inflow concentration by two blinded observers. A statistical analysis was then performed with respect to history of previous rupture. Inter-observer variability analysis was performed.ResultsRuptured aneurysms were more likely to have complex flow patterns (83%, p<0.001), stable flow patterns (75%, p=0.0018), 66% concentrated inflow (66%, p=<0.0001), and small impingement regions (76%, p=0.0006) compared to unruptured aneurysms. Inter-observer variability analyses indicate that all the classifications performed are in very good agreement, i.e. well within the 95% confidence interval.ConclusionsA qualitative hemodynamic analysis of cerebral aneurysms using image based patient-specific geometries has shown that concentrated inflow jets, small impingement regions, complex flow patterns, and unstable flow patterns are correlated with a clinical history of prior aneurysm rupture. These qualitative measures provide a starting point for more sophisticated quantitative analysis aimed at assigning aneurysm risk of future rupture. These analyses highlight the potential for CFD to play an important role in the clinical determination of aneurysm risks.

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of \(9.21\) mm to simulate the induction flow at crank-angle \(270^{\circ }\)bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., \(y \ge -20\) mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.

Assessment of normal flow patterns in the pulmonary circulation by using 4D magnetic resonance velocity mapping

Bicuspid aortic valve (BAV) causes complex flow patterns in the ascending aorta (AAo), which may compromise the accuracy of flow measurement by phase-contrast magnetic resonance (PC-MR). Therefore, we aimed to assess and compare the accuracy of forward flow measurement in the AAo, where complex flow is more dominant in BAV patients, with flow quantification in the left ventricular outflow tract (LVOT) and the aortic valve orifice (AV), where complex flow is less important, in BAV patients and controls. Flow was measured by PC-MR in 22 BAV patients and 20 controls at the following positions: (i) LVOT, (ii) AV, and (iii) AAo, and compared with the left ventricular stroke volume (LVSV). The correlation between the LVSV and the forward flow in the LVOT, the AV, and the AAo was good in BAV patients (r = 0.97/0.96/0.93; P < 0.01) and controls (r = 0.96/0.93/0.93; P < 0.01). However, in relation with the LVSV, the forward flow in the AAo was mildly underestimated in controls and much more in BAV patients [median (inter-quartile range): 9% (4%/15%) vs. 22% (8%/30%); P < 0.01]. This was not the case in the LVOT and the AV. The severity of flow underestimation in the AAo was associated with flow eccentricity. Flow measurement in the AAo leads to an underestimation of the forward flow in BAV patients. Measurement in the LVOT or the AV, where complex flow is less prominent, is an alternative means for quantifying the systolic forward flow in BAV patients.

A continuum description of granular flows would be of considerable help in predicting natural geophysical hazards or in designing industrial processes. However, the constitutive equations for dry granular flows, which govern how the material moves under shear, are still a matter of debate. One difficulty is that grains can behave like a solid (in a sand pile), a liquid (when poured from a silo) or a gas (when strongly agitated). For the two extreme regimes, constitutive equations have been proposed based on kinetic theory for collisional rapid flows, and soil mechanics for slow plastic flows. However, the intermediate dense regime, where the granular material flows like a liquid, still lacks a unified view and has motivated many studies over the past decade. The main characteristics of granular liquids are: a yield criterion (a critical shear stress below which flow is not possible) and a complex dependence on shear rate when flowing. In this sense, granular matter shares similarities with classical visco-plastic fluids such as Bingham fluids. Here we propose a new constitutive relation for dense granular flows, inspired by this analogy and recent numerical and experimental work. We then test our three-dimensional (3D) model through experiments on granular flows on a pile between rough sidewalls, in which a complex 3D flow pattern develops. We show that, without any fitting parameter, the model gives quantitative predictions for the flow shape and velocity profiles. Our results support the idea that a simple visco-plastic approach can quantitatively capture granular flow properties, and could serve as a basic tool for modelling more complex flows in geophysical or industrial applications.

BackgroundComparing four-dimensional flow against two-dimensional flow measurements in patients with complex flow pattern is still lacking. This study aimed to compare four-dimensional against the two-dimensional flow measurement in patients with bicuspid aortic valve and to test potentials of four-dimensional operator-dependent sources of error.ResultsThe two- and four-dimensional flow data sets of sixteen patients with bicuspid aortic valve and eighteen healthy subjects were studied. Flow analyses were performed by two observers blindly. Patients with bicuspid aortic valve mean differences between the two- and four-dimensional measurements in both observers were − 8 and − 4 ml, respectively. Four-dimensional measurements resulted in systematically higher flow values than the two-dimensional flow in bicuspid aortic valve patients. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 12/− 28 ml and + 14/− 21 ml, respectively. In the healthy volunteers, mean differences between the two- and four-dimensional measurements in both observers were ± 0 and + 1 ml, respectively. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 21/− 18 ml and + 12/− 13 ml, respectively. Inter-observer variability in four-dimensional flow measurement was 4% mean net forward flow in bicuspid aortic valve patients and 8% in healthy volunteers.ConclusionInter-observer variability in four-dimensional flow assessment is 8% or less which is acceptable for clinical cardiac MRI routine. There is close agreement of two- and four-dimensional flow tools in normal and complex flow pattern. In complex flow pattern, however, four-dimensional flow measurement picks up 4–9% higher flow values. It seems, therefore, that four-dimensional flow is closer to real flow values than two-dimensional flow, which is however to be proven by further studies.

While the geomorphic consequences of Pleistocene megafloods have been known for some time, it has been only in the past 2 decades that hydrogeologists and glaciologists alike have begun to appreciate the important impact that ice sheet–aquifer interactions have had in controlling subsurface flow patterns, recharge rates, and the distribution of fresh water in confined aquifer systems across North America. In this paper, we document the numerous lines of geochemical, isotopic, and geomechanical evidence of ice sheet hydrogeology across North America. We also review the mechanical, thermal, and hydrologic processes that control subsurface fluid migration beneath ice sheets. Finite element models of subsurface fluid flow, permafrost formation, and ice sheet loading are presented to investigate the coupled nature of transport processes during glaciation/deglaciation. These indicate that recharge rates as high as 10 times modern values occurred as the Laurentide Ice Sheet overran the margins of sedimentary basins. The effects of ice sheet loading and permafrost formation result in complex transient flow patterns within aquifers and confining units alike. Using geochemical and environmental isotopic data, we estimate that the volume of glacial meltwater emplaced at the margins of sedimentary basins overrun by the Laurentide Ice Sheet totals about 3.7 × 104 km3, which is about 0.2% of the volume of the Laurentide Ice Sheet. Subglacial infiltration estimates based on continental‐scale hydrologic models are even higher (5–10% of meltwater generated). These studies in sum call into question the widely held notion that groundwater flow patterns within confined aquifer systems are controlled primarily by the water table configuration during the Pleistocene. Rather, groundwater flow patterns were likely much more complex and transient in nature than has previously been thought. Because Pleistocene recharge rates are believed to be highly variable, these studies have profound implications for water resource managers charged with determining sustainable pumping rates from confined aquifers that host ice sheet meltwater.

This paper describes an experimental investigation carried out on a cyclone combustor and discusses their potential applications in a variety areas, including low calorific value gases and low grade fuels. Liquid fuel of 42 MJ/kg calorific value and chemical composition of 86.3 &#37; C, 12.8 &#37; H, 0.9 &#37; S is used. The effect of equivalence ratio on the flame structure is studied involving the measurements of temperature, and species concentration. The study shows that the swirling action of the flow causes droplets to be retained within the main chamber of the cyclone until they are consumed. Highly complex turbulent flow patterns and long residence times within a region of intense reacting flow patterns developed within the chamber. This phenomena rise to a highly stable flame with wide blow-off limits whose characteristics varied in response to gave a change in equivalence ratio. The flow pattern obtained with the liquid fuel shows similar features to the gas- fired cyclone. There are several differences which can be attributed to the change in fuel and the characteristics of the atomizer and liquid fuel.

&amp;lt;p&amp;gt;When modeling subduction processes, the results are usually constrained by looking at the geological surface expressions, geochemistry and geophysical observations such as tomography and seismic anisotropy. Of these observations, seismic anisotropy is the only type of observation that can potentially be directly linked to the spatial flow pattern in the mantle. Seismic anisotropy in the mantle is due to lattice-preferred orientation (LPO) of olivine minerals. In subduction environments, which can have complex and changing flow patterns, it is not expected that the LPO necessarily aligns with the flow pattern. This is partly due to the fact that it takes time to realign the LPO and partly because the olivine fast axis alignment depends on the water content and the magnitude of stress. To overcome this problem, the LPO must be computed for realistic and end member subduction zones in order to be able to relate seismic anisotropy to mantle flow and thereby slab dynamics.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;There are many ways to compute LPO. For this study we have used DREX (Kaminski et al., 2004), because the underlying method is accurate and fast enough for use in geodynamic models. To achieve a good and native integration with ASPECT (Kronbichler et al., 2012; Heister et al., 2017; Bangerth et al,. 2019), we have rewritten DREX in CPP as a plugin for ASPECT. In this presentation we will show how it was implemented and what the limitations and possibilities are. Furthermore, we will show initial results from 3D subduction models to study the link between seismic anisotropy and mantle flow.&amp;lt;/p&amp;gt;

BackgroundComplex and unstable flow patterns are reported to be associated with the rupture status of cerebral aneurysms, while their evaluation depends on qualitative analysis of streamlines of bloodflow. Oscillatory velocity...

Using Computational Fluid Dynamics Analysis to Characterize Local Hemodynamic Features of Middle Cerebral Artery Aneurysm Rupture Points