Helical Flow Structures Research Articles

Experimental and numerical study of a novel inbuilt helical ridge small-caliber graft for artery bypass surgery

Small-caliber (≤ 6 mm) vascular grafts are commonly used to treat vascular diseases, yet intimal hyperplasia at the distal end-to-side anastomosis remains a significant complication with long-term use. Helical flow has been shown to reduce the risk of cardiovascular complications effectively. Integrating helical flow through an inbuilt helical ridge in vascular grafts is intended to improve hemodynamic performance and mitigate issues such as flow-induced thrombosis and intimal hyperplasia. However, inadequate helical flow strength may limit its effectiveness. This study proposes a novel design for an inbuilt helical ridge in small-caliber grafts to achieve enhanced helical flow. Two novel small-caliber vascular grafts with variable-pitch helical ridges, incorporating both decreasing and increasing pitches, were designed. Their effectiveness was assessed through numerical simulations and in vitro experiments to evaluate their performance in arterial graft applications. The small-caliber vascular grafts incorporating variable-pitch helical ridges significantly improved helical flow structure and distribution, optimized wall shear stress distribution, and reduced platelet adhesion compared to conventional and constant-pitch helical ridge grafts. Among the various new designs, the small-caliber vascular graft featuring decreasing variable-pitch helical ridges has demonstrated better performance. This design is well-suited for small-caliber bypass surgeries, as it is effective in improving hemodynamic performance by improving helical flow into asymmetric structures and increasing helical flow density distribution.

Numerical investigation on the impact of different coronary aneurysms morphologies on thrombus formation and hemodynamics: a comparative study.

Coronary artery aneurysms (CAAs) are morphologically classified as saccular and fusiform. There is still a great deal of clinical controversy as to which types of CAA are more likely to cause thrombosis. Therefore, the main objective of this study was to evaluate the trend of thrombus growth in CAAs with different morphologies and to assess the risk of possible long-term complications based on hemodynamic parameters. Utilizing computed tomography angiography (CTA) data from eight healthy coronary arteries, two distinct morphologies of coronary artery aneurysms (CAAs) were reconstructed. Distribution of four wall shear stress (WSS)-based indicators and three helicity indicators was analyzed in this study. Meanwhile, a thrombus growth model was introduced to analyze the thrombus formation in CAAs with different morphologies. The research results showed the distribution of most WSS indicators between saccular and fusiform CAAs was not statistically significant. However, due to the presence of a more pronounced helical flow pattern, irregular helical flow structure and longer time of flow stagnation in saccular CAAs during the cardiac cycle, the mean and maximum relative residence time (RRT) were significantly higher in saccular CAAs than in fusiform CAAs (P < 0.05). This may increase the risk of saccular coronary arteries leading to aneurysmal dilatation or even rupture. Although the two CAAs had similar rates of thrombosis, fusiform CAAs may more early cause obstruction of the main coronary flow channel where the aneurysm is located due to thrombosis growth. Thus, the risk of thrombosis in fusiform coronary aneurysms may warrant greater clinical concern.

Numerical investigations of pipe flow downstream a flow conditioner with bundle of tubes

Flow conditioners are widely utilized in pipeline systems to improve the precision of flow rate measurement in the pipeline systems of the offshore and subsea oil and gas industry. There is a lack of knowledge about the influences of the conditioners on the flow inside a bend pipe due to the measurement inaccuracy caused by geometries complexity. In this study, numerical simulations are carried out solving the three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations with the κ–ω SST model to investigate the large-scale flow characteristics inside a double bend pipe and the performance of a conditioner with a bundle of 19 tubes. The obtained axial velocity inside a double bend pipe flow with no flow conditioner are compared with those of the previously published numerical simulations results and experimental data as the validation study. Helical flow structures are found behind the double bend and effectively removed by the flow conditioner. The performance of the flow conditioner is evaluated based on the axial flow velocity profiles, the swirl intensities and the deviation from the flow inside a straight pipe. The effects of Reynolds numbers and the lengths of the tube bundle on the flow downstream the flow conditioner are discussed.

Influence of aortic valve tilt angle on flow patterns in the ascending aorta

Transcatheter aortic valve implantation (TAVI) has become the alternative procedure to high-risk patients who are diagnosed with aortic valve stenosis. Differently from the traditional open-chest surgical procedure, a small variation on the prosthetic aortic valve deployment angle is expected with the TAVI procedure. The hemodynamic patterns of the blood flow in the ascending aorta are related to the development of many cardiovascular diseases. There are, however, few data available in the literature correlating the aortic valve tilt angle to hemodynamic effects. In this work, a 3D printed aorta model made of a transparent silicon resin was produced, based on the anatomy of a specific patient submitted to a TAVI procedure. The stereoscopic Particle Image Velocimetry technique was employed to measure three-component velocity fields at closely spaced cross-sectional planes, along the ascending aorta. The measurements were performed for a constant flow rate corresponding to the peak of the systolic phase of the cardiac cycle. Averaged velocity fields and turbulent quantities were determined for both, the base case, with no valve tilt, and for cases with an inclination of 4° and 8°, oriented at the four anatomical directions of the human body reference system, namely anterior, posterior, right and left. The results revealed the dominant flow patterns in the ascending aorta formed by a jet-like inlet flow impinging on the curved aorta right wall, inducing a significant eccentricity on the axial velocity profile. Regions of reverse flow were identified and linked to the abrupt area change associated with the typical reduced inlet diameter of TAVI implants. The impinging flow and wall curvature effects established circulation patterns defining a helical flow structure. The influence of the inlet flow orientation on the flow turbulent characteristics was assessed by the spatial evolution of the turbulent kinetic energy (TKE), Reynolds and viscous stresses. The maximum values of TKE were found around the inlet jet boundaries and concentrated in the neighborhood of the right aorta wall where the eccentric axial flow prevailed. Spatial distributions of the maximum Reynolds stresses were similar to the TKE distributions and presented maximum stresses typically one order of magnitude higher than the maximum average viscous shear stresses. Maximum average viscous stress distributions were revealed at the jet-like flow boundaries and in the vicinity of the right wall, displaying moderate stress levels that, according to the literature, can be sufficient to produce cell damage and platelet activation. The complex nature of the flow field was revealed by streamlines obtained from the measured flow fields, allowing the identification of the influence of the inlet flow orientation and tilt angle on the position of the stagnation point on the aorta right wall, as well as the angle of incidence of the jet-like flow on the wall. A simple model based on momentum balance was used to estimate the pressure increment on the wall due to flow impingement. The model captured the influence of the inlet flow orientation, indicating that pressure increases of the order of 40% in relation to the base case condition were obtained for the 8°, left inlet flow orientation.

Flow Physics During Surge and Recovery of a Multi-Stage High-Speed Compressor

Abstract Stall followed by surge in a high-speed compressor can lead to violent disruption of the flow, damage to the blade structures and eventually engine shutdown. Knowledge of unsteady blade loading during surge is crucial for compressor design such as axial gap optimization. The aim of this paper is to demonstrate the feasibility of using three-dimensional full assembly unsteady Reynolds-averaged Navier–Stokes (URANS) CFD for modeling surge cycles of an eight-stage high-speed compressor rig. Results from this work show stalling of the mid-stages is the surge trigger. During the flow reversal, a strong acoustic reflection occurs when the convected entropy perturbations reach the intake opening, which increase the blade loading significantly. During recovery, a hysteresis loop was recorded due to hot air re-ingestion, which led to a strong shear at mid-span of the inlet guide vane (IGV)/R1 domain and the formation of rotating helical flow structures. The final phase of recovery was accompanied by a four-cell multi-row tip rotating stall, which was cleared as the compressor recovered to the forward flow characteristic. It was also shown that the single passage model, despite its limitations and shortcomings in modeling recovery, can predict reasonably accurate transient flow features during surge and thus provide considerable insight to the flow behavior, which can be used to obtain a first approximation of casing and blade loading.

Modeling capabilities of unsteady RANS for the simulation of turbulent swirling flow in an annular bluff-body combustor geometry

In this paper, we investigate the capabilities of the unsteady Reynolds-averaged Navier–Stokes (URANS) equations for capturing the helical structures found in a three-dimensional, incompressible and isothermal annular swirling jet undergoing vortex breakdown. The flow topology is representative of a bluff-body combustor operating at a Reynolds number Re = 8500 and swirl number S = 0.39. As a turbulence model, the SSG Reynolds stress model (RSM) was chosen. The numerical simulation is validated by means of tomographic particle image velocimetry measurements of the same flow configuration. To detect the coherent structures in the flow field, the recently introduced spectral proper orthogonal decomposition technique is adopted, which gives both temporal and spatial information on the large scale coherent structures. The spectral analysis of the sampled velocity signals identified a precessing vortex core with a frequency of 24.3 Hz. In particular, two different large-scale helical flow structures are identified: a single and a double helix. Both of them are wound in the counter-swirl direction and wrapped around the central breakdown bubble. Those findings are quite similar to recent experimental tomographic particle image velocimetry results (Vanierschot et al., Phys. Rev. Fluids (2018) & J. Fluid Mech. (2020)). This study demonstrates that the unsteady RANS approach with RSM is able to predict the coherent structures found in an annular swirling jet undergoing vortex breakdown both temporally and spatially with reasonable accuracy and this approach can hence be used in the design of bluff-body combustors, where a multi-dimensional parameter space may require many simulations to find the optimal design.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

In this paper, we study the shape and dynamics of helical coherent structures found in the flow field of an annular swirling jet undergoing vortex breakdown. The flow field is studied by means of time-resolved tomographic particle image velocimetry measurements. The obtained flow fields are analysed using both classic and spectral proper orthogonal decomposition. Despite the simple geometrical set-up of the annular jet, the flow field is very complex. Two distinct large-scale helical flow structures are identified: a single and a double helix, both co-rotating with the swirl direction, and it is revealed that these structures are not higher harmonics of each other. The structures have a relatively low energy content which makes it hard to separate them from other dynamics of the flow field, notably turbulent motions. Because of this, classic proper orthogonal decomposition fails to identify both structures properly. Spectral proper orthogonal decomposition, on the other hand, allows them to be identified accurately when the filter size is set at around eight times the precession period. The precession frequencies of the single and double helices correspond to Strouhal numbers of 0.273 and , respectively. A global stability analysis of the mean flow field shows that these structures correspond to two separate global modes. The precessing frequencies obtained by the stability analysis and the related spatial structures match very well with the experimental observations. The current work extends our knowledge on turbulent vortex breakdown and on mean field global stability theory in general. It leads to the following conclusions. Firstly, single- and double-helix vortex breakdown are both manifestations of global modes. Previous studies have shown that both and modes can coexist in swirling jets. However, the mode has been identified as a second harmonic of the first mode, while this study identifies both as two independent global modes. Secondly, this work shows that the simultaneous occurrence of multiple helical global modes is possible within a turbulent flow and their shapes and frequencies are very well predicted by mean field stability analysis. The latter finding is of general interest as it applies to a wide class of fluid problems dominated by multiple oscillatory structures.

Computing patient-specific hemodynamics in stented femoral artery models obtained from computed tomography using a validated 3D reconstruction method

Patients with peripheral artery disease who undergo endovascular treatment are often inflicted by in-stent restenosis. The relation between restenosis and abnormal hemodynamics may be analyzed using patient-specific computational fluid dynamics (CFD) simulations. In this work, first a three-dimensional (3D) reconstruction method, based on an in-house semi-automatic segmentation algorithm of a patient's computed tomography (CT) images with calcification and metallic artifacts, and thrombus removal is described. The reconstruction method was validated using 3D printed rigid phantoms of stented femoral arteries by comparing the reconstructed geometries with the reference computer-aided design (CAD) geometries employed for 3D printing. The mean reconstruction error resulting from the validation of the reconstruction method was ~6% in both stented and non-stented regions. Secondly, a patient-specific model of the stented femoral artery was created and CFD analyses were performed with emphasis on the selection of the boundary conditions. CFD results were compared in scenarios with and without common femoral artery bifurcation, employing flat or parabolic inlet velocity profiles. Similar helical flow structures were visible in all scenarios. Negligible differences in wall shear stress (<0.5%) were found in the stented region. In conclusion, a robust method, applicable to patient-specific cases of stented diseased femoral arteries, was developed and validated.

In-stent graft helical flow intensity reduces the risk of migration after endovascular aortic repair

During the last years endovascular aneurysm repair (EVAR) became the elective treatment for abdominal aortic aneurysms (AAAs) thanks to lower mortality and morbidity rates than open surgery. In face of these advantages, stent-graft performances are still clinically suboptimal. In particular, post-surgical complications derive from device migration as a consequence of the hemodynamic forces acting on the endograft. In this regard, while the importance of hemodynamic surface forces is well recognized, the role of the in-stent flow is still unclear. Here we hypothesize that in-stent helical blood flow patterns might influence the distribution of the displacement forces (DFs) acting on the stent-graft and, ultimately, the risk of stent migration. To test this hypothesis, the hemodynamics of 20 post-EVAR models of patients treated with two different commercial endografts was analyzed using computational hemodynamics.The main findings of the study indicate that: (1) helical flow intensity decreases the risk of endograft migration, as given by an inverse correlation between helicity intensity (h2) and time-averaged displacement forces (TADFs) (p < 0.05); (2) unbalanced counter-rotating helical structures in the legs of the device contribute, in particular along the systole, to significantly suppress TADFs (p < 0.01); (3) as expected, helical flow intensity is positively correlated with pressure drop and resistance to flow (p < 0.001). The findings of this study suggest that a design strategy promoting in-stent helical flow structures could contribute to minimize the risk of migration of implanted EVAR devices.

Helicoidal particles in turbulent flows with multi-scale helical injection

We present numerical and theoretical results concerning the properties of turbulent flows with strong multi-scale helical injection. We perform direct numerical simulations of the Navier–Stokes equations under a random helical stirring with power-law spectrum and with different intensities of energy and helicity injections. We show that there exists three different regimes where the forward energy and helicity inertial transfers are: (i) both leading with respect to the external injections, (ii) energy transfer is leading and helicity transfer is sub-leading and (iii) both are sub-leading and helicity is maximal at all scales. As a result, the cases (ii)–(iii) give flows with Kolmogorov-like inertial energy cascade and tuneable helicity transfers/contents. We further explore regime (iii) by studying its effect on the kinetics of point-like isotropic helicoids, particles whose dynamics is isotropic but breaks parity invariance. We investigate small-scale fractal clustering and preferential sampling of intense helical flow structures. Depending on their structural parameters, the isotropic helicoids either preferentially sample co-chiral or anti-chiral flow structures. We explain these findings in limiting cases in terms of what is known for spherical particles of different densities and degrees of inertia. Furthermore, we present theoretical and numerical results for a stochastic model where dynamical properties can be calculated using analytical perturbation theory. Our study shows that a suitable tuning of the stirring mechanism can strongly modify the small-scale turbulent helical properties and demonstrates that isotropic helicoids are the simplest particles able to preferentially sense helical properties in turbulence.

MRI-based modeling of spleno-mesenteric confluence flow

Characterization of hepatic blood flow magnitude and distribution can lead to a better understanding of the pathophysiology of liver disease. However, the underlying patterns and dynamics of hepatic flow, such as the helical flow structure that often develops following the spleno-mesenteric confluence (SMC) of the hepatic portal vein, have not yet been comprehensively studied. In this study, we used magnetic resonance image (MRI)-based computational models to study the effects of the helical flow structure and SMC geometry on portal blood flow distribution. Additionally, we examined these flow dynamics with four-dimensional (4D) flow MRI in a group of 12 cirrhotic patients and healthy subjects. A validation model was also created to compare computational data to particle image velocimetry (PIV) data. We found significant correlations between flow structure development, vessel geometry, and blood flow distribution in both virtually modified models and in healthy and cirrhotic subjects. However, the direction of these correlations varied among vessel configuration types. Nonetheless, validation model results displayed good qualitative agreement with computational model data.

Fluid dynamics and forces in the HH25 avian embryonic outflow tract.

The embryonic outflow tract (OFT) eventually undergoes aorticopulmonary septation to form the aorta and pulmonary artery, and it is hypothesized that blood flow mechanical forces guide this process. We performed detailed studies of the geometry, wall motions, and fluid dynamics of the HH25 chick embryonic OFT just before septation, using noninvasive 4D high-frequency ultrasound and computational flow simulations. The OFT exhibited expansion and contraction waves propagating from proximal to distal end, with periods of luminal collapse at locations of the two endocardial cushions. This, combined with periods of reversed flow, resulted in the OFT cushions experiencing wall shear stresses (WSS or flow drag forces) with elevated oscillatory characteristics, which could be important to signal for further development of cushions into valves and septum. Furthermore, the OFT exhibits interesting double-helical flow during systole, where a pair of helical flow structures twisted about each other from the proximal to distal end. This coincided with the location of the future aorticopulmonary septum, which also twisted from the proximal to distal end, suggesting that this flow pattern may be guiding OFT septation.

Computational investigations on the hemodynamic performance of a new swirl generator in bifurcated arteries

Hemodynamic behaviour of blood in the bifurcated arteries are closely related to the development of cardiovascular disease. The secondary flows generated at the bifurcation zone promotes the deposition of atherogenic particles on the outer walls. The present study aims at suppressing the development of atherosclerosis plaque by inducing helical flow structure in the arterial passage. To realize this objective a novel swirl generator (stent like structure with an internal groove) has been developed to induce helicity in the bifurcated passage. The functional requirement of the swirl generator is to minimize the relative residence time (RRT) of the fluid layer near the endothelial wall without generating any additional pressure drop. Different configurations of the swirl generator have been tested computationally using large eddy simulation (LES) model. It is observed that the induced helical flow redistributes the kinetic energy from the centre to the periphery. A single rib swirl flow generator proximal to the stent treated passage can generate sufficient helicity to bring down the RRT by 36% without generating any additional pressure drop. The swirl flow adds azimuthal instability which increase vortex formations in the passage. The induced helical flow in the domain provokes more linked vortices, which may act as self-cleaning mechanism to the arterial wall.

Discrete-Phase Modelling of an Asymmetric Stenosis Artery Under Different Womersley Numbers

Understanding the hemodynamics in the post-stenotic region of an asymmetric stenosis is of paramount importance in the study of atherosclerosis progression. Numerically, the analysis becomes more complex when a discrete phase is added to the continuous phase in order to understand the behaviour of atherogenic particles in a pulsatile flow environment. In the present study, discrete-phase modelling (DPM) of an asymmetric and symmetric stenosed artery models has been carried out at different Womersley numbers. The objective is to understand the correlation between the discrete-phase (atherogenic) particle behaviour with the characteristics of continuous phase (blood) under varying pulse frequencies. Continuous phase is modelled by time-averaged Navier–Stokes equations and solved by means of pressure implicit splitting of operators algorithm. DPM has been carried out with one-way coupling. The transport equations are solved in the Eulerian frame of reference, and the discrete phase is simulated in Lagrangian frame of reference. The study brings out the importance of helicity in the atherosclerosis progression. Result shows that the asymmetric stenosis model exhibits less helical flow structure and the vortical structures are not getting transported to the downstream. Consequently, the average particle residence time (PRT) of the atherogenic particles is one order higher than the symmetric stenosis model. Low PRT leads to enhanced mass transport in the arterial flow and triggers further occlusion/plaque build-up at the post-stenotic region. The extent of asymmetry in a diseased artery may be considered as a useful parameter in understanding the rate of progression of atherosclerosis.

Computational study of aortic hemodynamics for patients with an abnormal aortic valve: The importance of secondary flow at the ascending aorta inlet.

Blood flow in the aorta is helical, but most computational studies ignore the presence of secondary flow components at the ascending aorta (AAo) inlet. The aim of this study is to ascertain the importance of inlet boundary conditions (BCs) in computational analysis of flow patterns in the thoracic aorta based on patient-specific images, with a particular focus on patients with an abnormal aortic valve. Two cases were studied: one presenting a severe aortic valve stenosis and the other with a mechanical valve. For both aorta models, three inlet BCs were compared; these included the flat profile and 1D through-plane velocity and 3D phase-contrast magnetic resonance imaging derived velocity profiles, with the latter being used for benchmarking. Our results showed that peak and mean velocities at the proximal end of the ascending aorta were underestimated by up to 41% when the secondary flow components were neglected. The results for helical flow descriptors highlighted the strong influence of secondary velocities on the helical flow structure in the AAo. Differences in all wall shear stress (WSS)-derived indices were much more pronounced in the AAo and aortic arch (AA) than in the descending aorta (DAo). Overall, this study demonstrates that using 3D velocity profiles as inlet BC is essential for patient-specific analysis of hemodynamics and WSS in the AAo and AA in the presence of an abnormal aortic valve. However, predicted flow in the DAo is less sensitive to the secondary velocities imposed at the inlet; hence, the 1D through-plane profile could be a sufficient inlet BC for studies focusing on distal regions of the thoracic aorta.

A General Model for the Helical Structure of Geophysical Flows in Channel Bends

AbstractMeandering channels formed by geophysical flows (e.g., rivers and seafloor turbidity currents) include the most extensive sediment transport systems on Earth. Previous measurements from rivers show how helical flow at meander bends plays a key role in sediment transport and deposition. Turbidity currents differ from rivers in both density and velocity profiles. These differences, and the lack of field measurements from turbidity currents, have led to multiple models for their helical flow around bends. Here we present the first measurements of helical flow in submarine turbidity currents. These 10 flows lasted for 1–10 days, were up to ~80 m thick, and displayed a consistent helical structure. This structure comprised two vertically stacked cells, with the bottom cell rotating in the opposite direction to helical flow in rivers. Furthermore, we propose a general model that predicts the range of helical flow structures observed in rivers, estuaries, and turbidity currents based on their density stratification.

Healthy and diseased coronary bifurcation geometries influence near-wall and intravascular flow: A computational exploration of the hemodynamic risk

Local hemodynamics has been identified as one main determinant in the onset and progression of atherosclerotic lesions at coronary bifurcations. Starting from the observation that atherosensitive hemodynamic conditions in arterial bifurcation are majorly determined by the underlying anatomy, the aim of the present study is to investigate how peculiar coronary bifurcation anatomical features influence near-wall and intravascular flow patterns. Different bifurcation angles and cardiac curvatures were varied in population-based, idealized models of both stenosed and unstenosed bifurcations, representing the left anterior descending (LAD) coronary artery with its diagonal branch. Local hemodynamics was analyzed in terms of helical flow and exposure to low/oscillatory shear stress by performing computational fluid dynamics simulations.Results show that bifurcation angle impacts lowly hemodynamics in both stenosed and unstenosed cases. Instead, curvature radius influences the generation and transport of helical flow structures, with smaller cardiac curvature radius associated to higher helicity intensity. Stenosed bifurcation models exhibit helicity intensity values one order of magnitude higher than the corresponding unstenosed cases. Cardiac curvature radius moderately affects near-wall hemodynamics of the stenosed cases, with smaller curvature radius leading to higher exposure to low shear stress and lower exposure to oscillatory shear stress. In conclusion, the proposed controlled benchmark allows investigating the effect of various geometrical features on local hemodynamics at the LAD/diagonal bifurcation, highlighting that cardiac curvature influences near wall and intravascular hemodynamics, while bifurcation angle has a minor effect.

The effects of ice cover on flow characteristics in a subarctic meandering river

AbstractThe effects of ice cover on flow characteristics in meandering rivers are still not completely understood. Here, we quantify the effects of ice cover on flow velocity, the vertical and spatial flow distribution, and helical flow structure. Comparison with open‐channel low flow conditions is performed. An acoustic doppler current profiler (ADCP) is used to measure flow from up to three meander bends, depending on the year, in a small sandy meandering subarctic river (Pulmanki River) during two consecutive ice‐covered winters (2014 and 2015).Under ice, flow velocities and discharges were predominantly slower than during the preceding autumn open‐channel conditions. Velocity distribution was almost opposite to theoretical expectations. Under ice, velocities reduced when entering deeper water downstream of the apex in each meander bend. When entering the next bend, velocities increased again together with the shallower depths. The surface velocities were predominantly greater than bottom/riverbed velocities during open‐channel flow. The situation was the opposite in ice‐covered conditions, and the maximum velocities occurred in the middle layers of the water columns. High‐velocity core (HVC) locations varied under ice between consecutive cross‐sections. Whereas in ice‐free conditions the HVC was located next to the inner bank at the upstream cross‐sections, the HVC moved towards the outer bank around the apex and again followed the thalweg in the downstream cross‐sections. Two stacked counter‐rotating helical flow cells occurred under ice around the apex of symmetric and asymmetric bends: next to the outer bank, top‐ and bottom‐layer flows were towards the opposite direction to the middle layer flow. In the following winter, no clear counter‐rotating helical flow cells occurred due to the shallower depths and frictional disturbance by the ice cover. Most probably the flow depth was a limiting factor for the ice‐covered helical flow circulation, similarly, the shallow depths hinder secondary flow in open‐channel conditions. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Coronary fractional flow reserve measurements of a stenosed side branch: a computational study investigating the influence of the bifurcation angle.

BackgroundCoronary hemodynamics and physiology specific for bifurcation lesions was not well understood. To investigate the influence of the bifurcation angle on the intracoronary hemodynamics of side branch (SB) lesions computational fluid dynamics simulations were performed.MethodsA parametric model representing a left anterior descending—first diagonal coronary bifurcation lesion was created according to the literature. Diameters obeyed fractal branching laws. Proximal and distal main branch (DMB) stenoses were both set at 60 %. We varied the distal bifurcation angles (40°, 55°, and 70°), the flow splits to the DMB and SB (55 %:45 %, 65 %:35 %, and 75 %:25 %), and the SB stenoses (40, 60, and 80 %), resulting in 27 simulations. Fractional flow reserve, defined as the ratio between the mean distal stenosis and mean aortic pressure during maximal hyperemia, was calculated for the DMB and SB (FFRSB) for all simulations.ResultsThe largest differences in FFRSB comparing the largest and smallest bifurcation angles were 0.02 (in cases with 40 % SB stenosis, irrespective of the assumed flow split) and 0.05 (in cases with 60 % SB stenosis, flow split 55 %:45 %). When the SB stenosis was 80 %, the difference in FFRSB between the largest and smallest bifurcation angle was 0.33 (flow split 55 %:45 %). By describing the ΔPSB−QSB relationship using a quadratic curve for cases with 80 % SB stenosis, we found that the curve was steeper (i.e. higher flow resistance) when bifurcation angle increases (ΔP = 0.451*Q + 0.010*Q2 and ΔP = 0.687*Q + 0.017*Q2 for 40° and 70° bifurcation angle, respectively). Our analyses revealed complex hemodynamics in all cases with evident counter-rotating helical flow structures. Larger bifurcation angles resulted in more pronounced helical flow structures (i.e. higher helicity intensity), when 60 or 80 % SB stenoses were present. A good correlation (R2 = 0.80) between the SB pressure drop and helicity intensity was also found.ConclusionsOur analyses showed that, in bifurcation lesions with 60 % MB stenosis and 80 % SB stenosis, SB pressure drop is higher for larger bifurcation angles suggesting higher flow resistance (i.e. curves describing the ΔPSB−QSB relationship being steeper). When the SB stenosis is mild (40 %) or moderate (60 %), SB resistance is minimally influenced by the bifurcation angle, with differences not being clinically meaningful. Our findings also highlighted the complex interplay between anatomy, pressure drops, and blood flow helicity in bifurcations.

Kinematics of the nucleus of the radio galaxy M 87

The superfine structure of the active region of the radio galaxy M 87 has been investigated at millimeter and centimeter wavelengths. A disk, a core, a jet, and a counterjet have been identified. The disk is inclined to the plane of the sky at an angle of 60◦ toward the jet. We show that the surrounding thermal plasma inflows onto the disk, is transferred in a spiral to the center, and is ejected, carrying away an excess angular momentum as it is accumulated. The remainder falls to the forming central massive body. The temperature of the plasma as it is transferred increases to relativistic values; the ejection velocity increases to 0.02c. The nozzle diameter decreases as the axis is approached. The central high-velocity bipolar outflow is surrounded by low-velocity components, tubes. The interaction of the rotating flow with the ambient medium collimates and accelerates the flows. Ring currents whose tangential directions are observed as parallel chains of components are generated in the flows. The ring currents of the disk and jets produce aligned magnetic fields. The ejection of the jet and counterjet plasma flows is equiprobable; the motion is along the field in one case and opposite to the field in the other case, causing an acceleration or deceleration. The velocity of the high-velocity jet is a factor of 1.7 higher than the counterjet velocity. The acceleration compensates for the losses of relativistic electrons observed at distances exceeding those corresponding to their radiative cooling time. The hydrodynamic instability of the flow ejection causes precession, the formation of a conical helical flow structure with an increasing pitch. The reaction of the flows curves the disk and the helix axes. The spectra of compact fragments in the high-velocity bipolar outflow have lowfrequency cutoffs determined by reabsorption and absorption in the thermal plasma of the screen. The cutoff frequencies in the spectra increase as the center is approached and pass into the millimeter wavelength range. The screen thickness reaches 0.05 pc; the electron density is 104–105 cm−3. The magnetic fields of the bipolar outflow fragments are 50 mG ≤ B ≤ 500 mG at the nozzle exit and B ≈ 200 μG in the remote part at ρ ≥ 0.25 mas. The rotation measure toward the core center is RM ≈ 104 rad m−2. The sign of the rotation measure in the remote part of the bipolar outflow determined by the helical shape is reversed. The longitudinal magnetic field of the screen toward the central part of the core is BII ≈ 10 μG and falls at the edges of the active region to BII ≈ 2 μG.