Flow Bifurcation Research Articles

Numerical study of the effects of bifurcation angle on hemodynamics during pulsatile flow in a carotid artery bifurcation

Numerical study of the effects of bifurcation angle on hemodynamics during pulsatile flow in a carotid artery bifurcation

Free stream turbulence intensity effects on the flow over a circular cylinder at [formula omitted]: bifurcation, attractors and Lyapunov metric

Varying the free-stream turbulence intensity (Ti= 0%, 5%, 10%) the flow over a circular cylinder for Re=3900 is examined using high fidelity eddy-resolving simulations. Results show that with growth of the turbulence intensity, the drag coefficient increases and the pressure base coefficient drops, as well as the flow bifurcation in the near wake occurs, when the length of the separation zone is reduced by about two, and the stream velocity profile is reshaped from U-shaped to V-shaped. The flow transformation is accompanied by the separation of the wall stress scales. At the same time, dynamics of the vortex shedding (Strouhal number) remains unchanged. The flow bifurcation, confirmed experimentally in practice, and the phase transition are examined using the Lyapunov metric (predictability time and phase-space diagrams). The predictability times, i.e. the phase transition between the two states Ti = 0% → 5% and Ti = 0% → 10% are estimated at one half and one of the characteristic convective time, after which the saturation occurs when both dynamical systems reach a statistical stationary state. Finally, phase-space states are visualized in 2D and 3D, and shown that related attractors can be bounded by simple geometrical ellipsoids.

Bifurcation of coherent vortex flow in a magnetic island through nonlinear parity instability

The topology of the vortex flow associated with the magnetic island plays a significant role in modulating the turbulent transport near the magnetic island. In this paper, self-consistent nonlinear simulations of multi-scale interactions among large scale tearing mode, vortex flow, and small scale ion temperature-gradient (ITG) mode are numerically investigated based on the five-field Landau-fluid model. We found that the coherent vortex flow in a magnetic island has different parities in the nonlinear quasi-steady state, and this can be described by a theoretical framework—nonlinear parity instability. In the ITG stable case, the structure of the vortex flow bifurcates from tearing parity to twisting parity, which is characterized by modulational parity instability, modeled by a four-wave nonlinear coupling process. In the ITG unstable case, the vortex flow stays in tearing parity without parity bifurcation, and the energy is transferred from the twisting parity modes to the tearing parity modes. The impact of the parity instability on the magnetic island width is discussed as well.

Effect of flow bifurcation transitions of shear-thinning fluids on hydrodynamic resistance of channels with sudden contraction and expansion

The aim of this work was to study the relationship between hydrodynamic resistance and flow instability of shear-thinning fluids in channels with sudden contraction and expansion. To this end, the critical conditions of bifurcation transitions from symmetric to asymmetric vortex flow depending on the channel geometry and rheological properties of Carreau model fluid were investigated. It was found that increasing the contraction ratio leads to decreasing the critical Reynold number as a power function, while variation in the length of the channel narrow part has almost no effect on the position of the bifurcation transition point, but changes the corner vortex lengths. It was found that the considered bifurcation transitions in shear-thinning fluids lead to a non-monotonic dependence of the hydrodynamic resistance of channels with sudden contraction and expansion on Reynolds number with a minimum located around the critical Reynolds number.

Numerical Simulation of Blood Flow in Aortoiliac Bifurcation with Increasing Degree of Stenosis

The worldwide lethal prevalence of atherosclerotic diseases has made it a crucial topic of research, the descending aorta is a major artery with complex geometry involving curvature, branches, and bifurcation leading to common iliac arteries. This paper aims to scrutinize the intricate blood flow patterns and the flow parameters with the increasing degree of stenosis in the infrarenal aorta, which has been accomplished through computational fluid dynamics modeling. A 3D CAD model of the healthy aortoiliac bifurcation was constructed from MR images, and three diseased models with 32%, 47%, and 71% occlusion in the infrarenal aorta region were constructed. At the inlet, pulsatile velocity and at the outlet pressure boundary conditions were applied, blood was considered Newtonian and turbulence was modeled using Large Eddy Simulation (LES). The numerical simulation was carried out using finite volume method on ANSYS. The predicted hemodynamic parameters like velocity, wall shear stress (WSS), oscillating shear index (OSI), Q-Criterion and turbulence intensity were post-processed for all the models, the analysis of which provides an insight into the myriad processes involved in the inception and evolution of atherosclerosis. The transition of blood flow from laminar to turbulent with increase in the degree of stenosis is a very eminent feature of this study, turbulence is identified in cases with 47%, and 71% occlusion and is dominant in the latter.

High temperature and pressure Gladstone–Dale coefficient measurements in air behind reflected shock waves

Understanding the optical properties of air is essential for the validation and characterization of plasmas and hypersonic flows. Beyond 6000 K, the dissociation of nitrogen and oxygen molecules, along with other reactions, alters the equilibrium composition of air, causing a temperature and pressure dependence in the Gladstone–Dale coefficient. Due to measurement complexities, there is currently very little experimental data to validate model predictions under these conditions. In this work, a unique quadrature fringe imaging interferometer technique is applied to high temperature and pressure measurements of air in the Sandia free-piston high enthalpy shock tube. The diagnostic method combines a narrowband and broadband source to capture large, nearly-discrete changes in the index of refraction by calibrating to interference pattern changes. For the experiments, the reflected shock front is used to generate temperatures between 6000 and 7800 K at pressures up to 300 psi (20 bars). Results behind the shock front exhibit complex flow bifurcation and tail shock feature before equilibrium conditions are reached. Measurements in these flows show close agreement with theoretical predictions of the nonconstant Gladstone–Dale coefficient at high temperatures and high pressures, providing new validation data for chemical equilibrium gas models.

Confinement width and inflow-to-sediment discharge ratio control the morphology and braiding intensity of submarine channels: insights from physical experiments and reduced-complexity models

Abstract. Submarine channels conveying sediment gravity flows are often topographically confined, but the effect of confinement width on channel morphodynamics is incompletely understood. We use physical experiments and a reduced-complexity model to investigate the effects of confinement width (B) and the inflow-to-sediment discharge ratio (Qin/Qs) on the evolution of submarine braided channels. The results show that a larger confinement width results in increased active braiding intensity (BIA) and that BIA takes longer to stabilize (i.e., a longer critical time; tc). At a fixed confinement width, a higher Qin/Qs slightly decreases the BIA. Digital elevation models of difference (DoD) of the experiments allow measurement of the morphological active width (Wa) of the submarine channels and the bulk morphological change (Vbulk) within an experiment, defined as the sum of total erosion and deposition. We find that Wa and Vbulk are proportional to B. We further confirm that BIA is proportional to both dimensionless sediment–stream power (ω**) and dimensionless stream power (ω*). These trends are consistent for submarine braided channels both with and without confinement width effects. Furthermore, we built a reduced-complexity model (RCM) that can simulate flow bifurcation and confluence of submarine braided channels. The simulated flow distribution provides reliable predictions of flow depth and sediment transport rate in the experiments. Using kernel density estimation (KDE) to forecast the probability and trends of cross-sectional flow distribution and corresponding BIA under extreme events, we find that skewness of the flow distribution decreases as discharge increases. The development of braided submarine channels, shown here to extend to conditions of topographic confinement, suggests that factors not modeled here (e.g., fine sediment) may be necessary to explain the abundance of single-thread submarine channels in nature.

Bifurcation control analysis based on continuum model with lateral offset compensation

The study of traffic flow bifurcation control analysis is of great significance for understanding the essential evolution law of traffic flow, controlling traffic flow with practical means and alleviating traffic congestion. This paper describes the influence of lateral distance compensation in the process of lane change, and adjusts the traffic flow from the perspective of bifurcation control, which can capture the characteristics of traffic flow better, so as to describe the practical significance of the actual traffic phenomenon better. For example, the green ratio of traffic lights in the road, the planning and design of traffic signs, and the guidance of traffic condition prediction in ITS. In addition, the traffic flow is introduced and applied to the traffic model from the perspective of random function, and the bifurcation control is carried out according to the bifurcation behavior in traffic. That is, the bifurcation characteristics of the system are modified by the designed feedback controller, and the appearance of the equilibrium point of the system is adjusted to make it move forward, backward or disappear, so as to prevent or alleviate traffic congestion. There are few researches on bifurcation control of traffic flow model with lateral distance compensation. This paper introduces stochastic function based on lateral compensation model to study Hopf bifurcation phenomenon and Hopf bifurcation control. Firstly, the existence condition of Hopf bifurcation at the equilibrium point in the model is proved theoretically. Then, a feedback controller is designed to control the amplitudes of the Hopf bifurcation and the limit cycles formed by the Hopf bifurcation. Finally, the theoretical results are verified by numerical simulation. The research shows that by adjusting the control parameters in the feedback controller, the influence of boundary conditions on the stability of the traffic system is fully described, the effect of unstable focus and saddle point on the system is inhibited, and the traffic flow is slowed down. In addition, the unstable bifurcation points can be eliminated, and the amplitude of the limit cycle formed by Hopf bifurcation can be adjusted, so as to achieve the stability of the control system.

The effect of unit size on the flow and heat transfer performance of the “Schwartz-D” heat exchanger

As a representative type of triple periodic minimal surface (TPMS) topology, the S-D structure exhibits distinctive heat transfer properties and is ideal for constructing high-performance heat exchangers (HXs). To explore the effect of unit size on the flow and heat transfer performance, three crossflow S-D HXs with different unit sizes and the same porosity are designed and produced by metal additive manufacturing in this study. Simulation results show that the reduction in unit size induces a larger scale of bifurcation flow and increases the frequency of flow area change in HX. Experimental results demonstrate that when the smallest unit size is adopted, the HX's effectiveness can be improved by at least 10%, but at the cost of a 42% increase in pressure drop. Meanwhile, the advantage of small unit size in heat transfer enhancement will be further expanded under large flow rate conditions. In addition, it is found that the effect of flow ratio on the heat transfer performance of the S-D HXs appears to be linear, and the unit-based Reynold number may be an appropriate dimensionless parameter for correlating their pressure drop and heat transfer data. All in all, this study is expected to provide a better understanding and guidance for the design and optimization of TPMS HXs.

Three-dimensional electro-convective flows of dielectric liquids between concentric cylinders

This study numerically investigates the flow structures and bifurcations of three-dimensional (3D) electro-convection (EC) between concentric cylinders. The flow motion is driven by the volumetric Coulomb force exerting on the free ions introduced by a strong unipolar injection. Three different sets of cases with periodic or symmetric boundary conditions are carried out. The finite volume method is used to numerically resolve the model problem. The co-effect of 2D roll mode and 3D polygon mode at the initial period of the EC flow is identified. The EC flows are made up of different types of regular or irregular centrally downflowing polygonal cells and are characterized by the central charge density cores surrounded by the charge void regions. The symmetry, periodicity, and staggered distribution characteristics of polygonal cells are identified. The cell patterns of EC flow are strongly influenced by the computational domain and the electric Rayleigh number (T). The subcritical bifurcation of linear instability together with a hysteresis loop is observed. In addition, the stability of the triangular flow pattern is analyzed at a large range of T and found that the annular EC flow is more likely to be unstable in three dimensions than in two dimensions.

Numerical study of viscoelastic upstream instability

In this work, we report numerical results on the flow instability and bifurcation of a viscoelastic fluid in the upstream region of a cylinder in a confined narrow channel. Two-dimensional direct numerical simulations based on the FENE-P model (the finite-extensible nonlinear elastic model with the Peterlin closure) are conducted with numerical stabilization techniques. Our results show that the macroscopic viscoelastic constitutive relation can capture the viscoelastic upstream instability reported in previous experiments for low-Reynolds-number flows. The numerical simulations reveal that the non-dimensional recirculation length (LD) is affected by the cylinder blockage ratio (BR), the Weissenberg number (Wi), the viscosity ratio (β) and the maximum polymer extension (L). Close to the onset of upstream recirculation, LD with Wi satisfy Landau-type quartic potential under certain parameter space. The bifurcation may exhibit subcritical behaviour depending on the values of L2 and β. The parameters β and L2 have nonlinear influence on the upstream recirculation length. This work contributes to our theoretical understanding of this new instability mechanism in viscoelastic wake flows.

Analysis of Blood Flow Bifurcation Phenomena in Mitral Valve: A Numerical Approach to Predict Cardiac Arrest

Employing FVM, we have investigated numerically the rheological behavior of bifurcation phenomena of blood flow at various Reynolds numbers (Re) and at various values of contraction ratio (h), defined as the ratio of the inlet to narrow sections width of a two-dimensional planner contraction-expansion channel. Blood flow bifurcation through a planar contraction-expansion channel is analogous to the case of regurgitation (i.e., abnormal leakage of blood) in the mitral valve. In this work, we have studied the blood flow bifurcation characteristics including the normalized axial velocity profile, velocity gradient, dimensionless pressure, dimensionless longitudinal pressure gradient, pressure and skin friction coefficients on both the channel walls and analyzed the pressure drop, excess pressure drop for different values of Re. Secondly, blockage in the mitral valve is studied for different values of h. Pressure drops for various values of h are also studied to measure blood pressure. Correlation analyses are presented for normalized vortex length in terms of critical values of Re and h. It is revealed that if Re goes on increasing to 14.4 or more, flow breaks the symmetry at h = 15, and for each , recirculation length increases linearly with the increase in Re but decreases valve flaps that reduce blood flow to the heart muscles.

Investigation of the hydrothermal phenomena in a wavy microchannel with secondary flow passages through mid-wall inflection points

Advancement in manufacturing processeshas fuelled research in microchannel heat sinks to meet the demands of modern microelectronics. The heat sink design in the present study considers longitudinally wavy microchannels incorporating secondary flow passages. The secondary flow has been investigated for abrupt flow bifurcation (at 60° angle) and smooth flow bifurcation (at 30° angle). Further, these geometries were tested with different inlet–outlet configurations. The main objective of the paper is to study how a secondary flow would affect the flow profile and thermal performance of a wavy microchannel heat sink. The flow profile, pressure drop, temperature profiles, and average Nusselt number for a wide range of flow rates have been obtained and compared with those for the conventional design, and associated trends have been explored. Furthermore, the overall performance of the heat sink designs has been quantified by plotting the thermal enhancement factor (TEF) against a wide range of Reynolds numbers. The heat sink with flow bifurcation of 30° attains the highest value of Nuselt number below the flow rate of 1.8 × 10-6 m3/s, whereas above this flow rate, the Nusselt number value of the conventional wavy heat sink dominates the proposed heat sink designs. The study corresponds to the demands of modern microchips such as the 13th Gen Intel® Core™ processor and serves as a guideline for manufacturing heat sinks with wavy microchannels incorporating secondary flow passages.

An experimental study of the merging flow of polymer solutions in a T-shaped microchannel.

The merging flow through a T-junction is relevant to sample mixing and particle manipulation in microfluidic devices. It has been extensively studied for Newtonian fluids, particularly in the high inertial regime where flow bifurcation takes place for enhanced mixing. However, the effects of fluid rheological properties on the merging flow have remained largely unexplored. We investigate here the flow of five types of polymer solutions along with water in a planar T-shaped microchannel over a wide range of flow rates for a systematic understanding of the effects of fluid shear thinning and elasticity. It is found that the merging flow near the stagnation point of the T-junction can either be vortex dominated or have unsteady streamlines, depending on the strength of elasticity and shear thinning present in the fluid. Moreover, the shear thinning effect is found to induce a symmetric unsteady flow in comparison to the asymmetric unsteady flow in the viscoelastic fluids, the latter of which exhibits greater interfacial fluctuations.

Characterizing pulsatile blood flow in a specific carotid bifurcation: insights into hemodynamics and rheology models

<abstract> <p>This study uses laminar and turbulent flow models to investigate the blood flow dynamics in a specific carotid bifurcation. Pulsatile boundary conditions and the rigid carotid artery wall are considered. Three viscosity models describe the non-Newtonian blood behavior. The Fluent solver and the finite volume method solve the equations. Results show a Poiseuille-like flow in the common carotid artery (CCA), unaffected by the flow regime, viscosity model, or boundary conditions. The branching zone exhibits a C-shaped stagnation zone with low velocity and wall shear stress due to the CCA widening and ICA/ECA curvature. Strong secondary flow is observed in the carotid sinus; the flow is directed towards the inner wall with higher velocity in the internal carotid artery. Discrepancies between viscosity models are pronounced in laminar flow, particularly with the natural boundary conditions. The non-Newtonian blood behavior is more apparent in the laminar flow of the external carotid artery, especially with the second set of boundary conditions.</p> </abstract>

Design, flow characteristics and performance evaluation of bioinspired heat exchangers based on triply periodic minimal surfaces

Combining additive manufacturing (AM) and bioinspired structures, this study reports the design, fabrication, flow characteristics and heat transfer performance of three novel cross-flow metal heat exchangers (HXs) based on triply periodic minimal surface (TPMS) topologies. Numerical simulations of the TPMS HXs are also validated using associated experimental results for the first time. Results show that, the helically curved channels in the Schwarz-D (S-D) and Gyroid HXs promote flow mixing and induce large-scale bifurcation flow, while the Primitive HX provides basically a through-flow structure. Owing to the complex three-dimensional flow pattern and significantly increased heat transfer surface, the S-D HX provides 41–52% and 4–12% higher heat transfer rates relative to the Primitive HX and Gyroid HX. In addition, the presence of the baffles at the inlet results in similar pressure drops among the three HXs. Compared with the reference HXs reported in the literature, the current Gyroid and S-D HXs show comparable heat transfer performance with significant advantages in terms of pressure drop penalties. Since the surface-to-volume ratio of the TPMS units increases exponentially with decreasing unit size, it is believed that TPMS HXs employing smaller unit size will greatly enhance their compactness and heat transfer performance.

High-Order Large Eddy Simulations of a Wind Turbine in Ducted and Open-Rotor Configurations

Abstract High-order large eddy simulations are performed to study the performance and flow fields of a ducted wind turbine (DWT) operating at different tip speed ratios. To evaluate the effects of the duct, simulations with the same tip speed ratios are also performed on the corresponding open-rotor turbine. It is found that the ducted turbine consistently obtains higher power outputs than the open-rotor counterpart, and the duct itself enhances flow turbulence and blade trailing-edge vortices but weakens tip and hub vortices. Flow bifurcation is observed at the largest tip speed ratio and is identified to be caused by blade blockage effects. Comparative simulations are also performed on both turbines under different yaw angles. It is noticed that the ducted configuration is insensitive to small yaw angles and maintains higher power outputs than the open-rotor configuration at all yaw angles. Moreover, it is observed that the wakes of both configurations recover more quickly as the yaw angle increases.

THERMAL ANALYSIS OF LAMINAR WATER FLOW OVER A BACKWARD FACING CHANNEL WITH CARBON NANOPARTICLES

In the last few years, the thermo-hydraulic simulation of nanofluid flow bifurcation phenomena has become of great interest to researchers and a useful tool in many engineering applications. FVM has been employed in this article to numerically explore the laminar water flow over a backward facing channel with or without carbon nanoparticles (CN). The problem formulated in this paper has been solved by considering the effects of nanoparticle weight percentages (𝑤%), such as 0.00, 0.12, and 0.25 for different Reynolds number (𝑅𝑒). Nusselt number distribution (𝑁𝑢(𝑥)), coefficient of skin friction (𝐶𝑓), characteristics of pressure drop (Δ𝑝), velocity contours, static temperature, pumping power (𝑃𝑝) and thermal resistance factor (𝑅) have been investigated to know the behavior of thermo-hydraulic flow bifurcation phenomena. The present study shows that the surface temperature and coefficient of heat transfer can be reduced due to the effect of 𝑅𝑒 or w%. For different w%, it has been found that in the rise in the values of 𝑅𝑒 causes the increase of vortex length and as a result velocity gradient and Δ𝑝 arises. Furthermore, it has also been studied that the enhancement of 𝑅𝑒 causes the increaseof 𝑃𝑝 and Δ𝑝.

Effect of Axial and Radial Flow on the Hydrodynamics in a Taylor Reactor

This paper investigates the impact of combined axial through flow and radial mass flux on Taylor–Couette flow in a counter-rotating configuration, in which different branches of nontrivial solutions appear via Hopf bifurcations. Using direct numerical simulation, we elucidate flow structures, dynamics, and bifurcation behavior in qualitative and quantitative detail as a function of axial Reynolds numbers (Re) and radial mass flux (α) spanning a parameter space with a very rich variety of solutions. We have determined nonlinear properties such as anharmonicity, asymmetry, flow rates (axial and radial) and torque for toroidally closed Taylor vortices and helical spiral vortices. Small to moderate radial flow α initially decreases the symmetry of the different flows, before for larger values, α, the symmetry eventually increases, which appears to be congruent with the degree of anharmonicity. Enhancement in the total torque with α are elucidated whereby the strength varies for different flow structures, which allows for potential better selection and control. Further, depending on control parameters, heteroclinic connections (and cycles) of oscillatory type in between unstable and topological different flow structures are detected. The research results provide a theoretical basis for simple modification the conventional Taylor flow reactor with a combination of additional mass flux to enhance the mass transfer mechanism.

Full bifurcation scenarios and pattern formation of laminar electroconvection in a cavity

This study numerically investigates the flow structures and bifurcation scenarios of three-dimensional (3D) laminar electroconvection (EC). An efficient parallel lattice Boltzmann model is undertaken to numerically solve the model problem. The results present three steady flow patterns and three pitchfork bifurcations. These three patterns each have one, two, or four charge void cells. The three critical values of electric Rayleigh number Tc are 242, 545, and 665, respectively. There are also two hysteresis loops whose nonlinear criteria Tf are 157 and 435, respectively. An unexpected flow pattern, which has two prism-shaped primary vortex structures, demonstrates the significance of 3D analysis. In addition, we find that the 3D flow in the cavity is more stable by studying the correlation between the 3D and two-dimensional laminar EC. Using dynamic mode decomposition for the flow structures, we reveal that the novel feature is the result of competition between the EC flow structure and the limitation of geometry.