Search Results (28)

The Tesla valve is a fluidic diode that enables unidirectional flow while impeding the reverse flow without the assistance of any moving parts. Conventional Tesla valves share a distinctive feature of a bifurcated section that connects the inlet and outlet. This study uses computational fluid dynamic (CFD) simulations to analyze the importance of the bifurcated design to the efficiency of the Tesla valve, quantified by diodicity. Simulations over the range of the Reynolds number, Re = 50–2000, are performed for three designs: the T45-R, D-valve, and GMF valve, each with two versions with and without the bifurcated section. For the T45-R valve, removing the bifurcated section leads to a consistent increase in diodicity, particularly at high Re. In contrast, the diodicity of the GMF valve drops significantly when the bifurcated section is removed. The D-valve exhibits a mixed behavior. Without the bifurcated section, its diodicity is suppressed at low Re but begins to increase for Re > 1100, eventually matching the diodicity of the bifurcated version at Re = 2000. The results highlight the intricate relationship between valve geometry and efficiency of Tesla-type valves and the dependence of this relationship on the Reynolds number. Full article

Compared with T- and Y-shaped confluence pipelines, arc confluence pipelines have a smaller energy loss coefficient. In the study presented herein, numerical simulation analyses of the flow fields of T-shaped, Y-shaped, bifurcated, and arc confluence pipelines were carried out. An experimental system was designed to study the energy loss coefficient of the arc confluence pipeline by using five different pipe diameters and nine different radii with water as the working fluid and analyze the influence of pipe diameter, radius, and Reynolds number on the energy loss of the arc confluence pipeline. As the radius of the arc confluence pipeline increases, the energy loss coefficient of confluence first decreases and then increases, and the minimum energy loss corresponds to a radius of 65 mm; with an increase in Reynolds number, the energy loss coefficient of confluence first decreases gradually and tends to a certain value. With an increase in diameter, the energy loss coefficient of confluence decreases gradually. Full article

A finite element method is employed to examine the impact of a magnetic field on the development of plaque in an artery with stenotic bifurcation. Consistent with existing literature, blood flow is characterized as a Newtonian fluid that is stable, incompressible, biomagnetic, and laminar. Additionally, it is assumed that the arterial wall is linearly elastic throughout. The hemodynamic flow within a bifurcated artery, influenced by an asymmetric magnetic field, is described using the arbitrary Lagrangian–Eulerian (ALE) method. This technique incorporates the fluid–structure interaction coupling. The nonlinear system of partial differential equations is discretized using a stable P2P1 finite element pair. To solve the resulting nonlinear algebraic equation system, the Newton-Raphson method is employed. Magnetic fields are numerically modeled, and the resulting displacement, velocity magnitude, pressure, and wall shear stresses are analyzed across a range of Reynolds numbers (Re = 500, 1000, 1500, and 2000). The numerical analysis reveals that the presence of a magnetic field significantly impacts both the displacement magnitude and the flow velocity. In fact, introducing a magnetic field leads to reduced flow separation, an expanded recirculation area near the stenosis, as well as an increase in wall shear stress. Full article

Bubble flow in confined geometries is a problem of fundamental and technological significance. Among all the forms, bubble breakup in bifurcated microchannels is one of the most commonly encountered scenarios, where an in-depth understanding is necessary for better leveraging the process. This study numerically investigates the non-uniform breakup of a bubble slug in Y-shaped microchannels under different flow ratios, Reynolds numbers, and initial bubble volumes. Overall, the bubble can either breakup or non-breakup when passing through the bifurcation and shows different forms depending on flow regimes. The flow ratio-Reynolds number phase diagrams indicate a power–law transition line of breakup and non-breakup. The bubble takes longer to break up with rising flow ratios yet breaks earlier with higher Reynolds numbers and volumes. Non-breakup takes less time than the breakup patterns. Flow ratio is the origin of non-uniform breakup. Both the Reynolds number and initial volume influence the bubble states when reaching the bifurcation and thus affect subsequent processes. Bubble neck dynamics are analyzed to describe the breakup further. The volume distribution after breaking up is found to have a quadratic relation with the flow ratio. Our study is hoped to provide insights for practical applications related to non-uniform bubble breakups. Full article

Computational simulations of three-dimensional flow around a NACA 0018 wing with an aspect ratio of $AR=5$ were carried out by using the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations with the Shear-Stress Transport turbulence model closure. Simulations were performed to capture aerodynamic stall hysteresis by using the developed pseudo-transient continuation (PTC) method based on a dual-time step approach in CFD OpenFOAM code. The flow was characterized by incompressible Mach number $M=0.12$ and moderate Reynolds number $Re=0.67\times {10}^6$. The results obtained indicate the presence of noticeable aerodynamic hysteresis in the static dependencies of the force and moment coefficients, as well as the manifestation of bi-stable flow separation patterns, accompanied by the development of asymmetry in the stall zone. The URANS simulation results are in good agreement with the experimental data obtained for the NACA 0018 finite-aspect-ratio wing in the low-speed wind tunnel under the same test conditions. A new phenomenological bifurcation model of aerodynamic stall hysteresis under static and dynamic conditions is formulated and is proven to be able to closely match the experimental data. Full article

Characterization of the physiological hemodynamic environment in normal pulmonary arteries is a key factor in understanding pathological conditions. This study aimed to analyze the morphology and hemodynamics in the healthy adult pulmonary bifurcation in comparison to age-matched repaired Tetralogy of Fallot (rTOF) geometries. The pulmonary trunk of five healthy volunteers was reconstructed from 4D Flow-MRI data and was compared to rTOF results. Subject-specific boundary conditions were assigned in both the inlet and outlets of the models, and flow characteristics were analyzed computationally. The morphological and flow features were consistent among the healthy geometries, highlighting the ability of an averaged geometry derived from this small cohort to capture the main flow characteristics. A slightly higher mean time-averaged wall shear stress (TAWSS) was found in the right pulmonary artery, which was also the branch with a higher mean curvature and local Reynolds number. Compared to rTOF results, the averaged healthy geometry demonstrated more than an 8-fold lower value in TAWSS, with the individual patient-specific healthy volunteers showing further reduced TAWSS than the rTOF patients. These observations could be useful in clinical assessment and decision making based on hemodynamic indices. Full article

Inspired by the airway for phonation, fluid flow in an idealized model within a sudden expansion and contraction channel with a geometrically symmetric structure is investigated, and the nonlinear behaviors of the flow therein are explored via numerical simulations. Numerical simulation results show that, as the Reynolds number (Re = U₀H/ν) increases, the numerical solution undergoes a pitchfork bifurcation, an inverse pitchfork bifurcation and a Hopf bifurcation. There are symmetric solutions, asymmetric solutions and oscillatory solutions for flows. When the sudden expansion ratio (Er) = 6.00, aspect ratio (Ar) = 1.78 and Re ≤ Rec1 (≈185), the numerical solution is unique, symmetric and stable. When Rec1 < Re ≤ Rec2 (≈213), two stable asymmetric solutions and one symmetric unstable solution are reached. When Rec2 < Re ≤ Rec3 (≈355), the number of numerical solution returns one, which is stable and symmetric. When Re > Rec3, the numerical solution is oscillatory. With increasing Re, the numerical solution develops from periodic and multiple periodic solutions to chaos. The critical Reynolds numbers (Rec1, Rec2 and Rec3) and the maximum return velocity, at which reflux occurs in the channel, change significantly under conditions with different geometry. In this paper, the variation rules of Rec1, Rec2 and Rec3 are investigated, as well as the maximum return velocity with the sudden expansion ratio Er and the aspect ratio Ar. Full article

Anatomical airways parameters, such as length, diameter and angles, have a strong effect on the flow dynamics. Aiming to explore the effect of variations of the bifurcation angle (BA) and carina rounding radius (CRR) of lower human airways on respiratory processes, numerical simulations of airflow during inhalation and exhalation were performed using synthetic bifurcation models. Geometries for the airways models were parameterized based on a set of different BA’s and several CRR’s. A range of Reynolds numbers (Re), relevant to the human breathing process, were selected to analyze airflow behavior. The numerical results showed a significant influence of BA and the CRR on the development of the airflow within the airways, and, therefore, affecting the following relevant features of the flow: the deformation of velocity profiles, alterations of pressure drop, flow patterns, and, finally, enhancement or attenuation of wall shear stresses (WSS) appearing during the regular respiratory process. The numerical results showed that increases in the bifurcation angle value were accompanied by pressure increases of about 20%, especially in the regions close to the bifurcation. Similarly, increases in the BA value led to a reduction in peak shear stresses of up to 70%. For the ranges of angles and radii explored, an increase in pressure of about 20% and a reduction in wall shear stress of more than 400% were obtained by increasing the carina rounding radius. Analysis of the coherent structures and secondary flow patterns also revealed a direct relationship between the location of the vortical structures, the local maxima of the velocity profiles and the local vorticity minima. This relationship was observed for all branches analyzed, for both the inhalation and exhalation processes of the respiratory cycle. Full article

A numerical study of two dimensional lid-driven triangular and trapezoidal cavity flow is performed via using the lattice Boltzmann method (LBM) for steady solutions. The equilateral and right-angled isosceles triangular cavity flow at Reynolds numbers, respectively, 500 and 100 is employed as the benchmark case for code validation. The isosceles right-angled triangular cavity flow is studied for Reynolds numbers sweeping from 100 to 8100. Flow topologies are captured and analyzed. The critical Reynolds number of Hopf bifurcation is predicted by calculating the perturbation decay rate. Two different geometries of right-angled isosceles trapezoidal cavities, bowl-shaped and pyramid-shaped trapezoids, are studied at Reynolds numbers 1000 and 7000. For each type of the trapezoidal cavity, a geometric parameter $\lambda$ (top-line/base-line ratio) is presented to distinguish different geometries of trapezoidal cavities. The flow patterns regarding the streamlines, vortical structures, and velocity profiles are discussed. The impact of parameter $\lambda$ on the fluid characteristics are investigated. Full article

In this paper, bifurcations in the solution of the Navier–Stokes equations are studied and multiple solutions of the driven semi-elliptical cavity flow are presented. The two-dimensional steady incompressible driven viscous flow in a semi-elliptical cavity is solved numerically. To this end, the problem is formulated using an elliptic coordinate system that transforms the geometry conformally and provides a body fitted coordinate system. The presented results show that above a bifurcation Reynolds number the solution of the governing flow equations bifurcates and there exist multiple solutions for a particular Reynolds number when the aspect ratio of the semi-elliptical cavity geometry is 0.26 ⩽D⩽ 0.8. The bifurcation Reynolds numbers for different aspect ratios and also multiple solutions at different Reynolds numbers are presented in detail. Full article

An algorithm for the calculation of steady-state flowing under uncertain conditions is introduced in this work in order to obtain a probabilistic distribution of uncertain problem parameters. This is particularly important for problems with increased uncertainty, as typical deterministic methods are not able to fully describe all possible flow states of the problem. Standard methods, such as polynomial expansions and Monte Carlo simulations, are used for the formation of the generalized problem described by the incompressible Navier-Stokes equations. Since every realization of the uncertainty parameter space is coupled with non-linear terms, an incremental iterative procedure was adopted for the calculation. This algorithm adopts a Jacobi-like iteration methodology to decouple the equations and solve them one by one until there is overall convergence. The algorithm was tested in a typical artery geometry, including a bifurcation with an aneurysm, which consists of a well-documented biological flow test case. Additionally, its dependence on the uncertainty parameter space, i.e., the inlet velocity distribution, the Reynolds number variation, and parameters of the procedure, i.e., the number of polynomial expansions, was studied. Symmetry exists in probabilistic theories, similar to the one adopted by the present work. The results of the simulations conducted with the present algorithm are compared against the same but unsteady flow with a time-dependent inlet velocity profile, which represents a typical cardiac cycle. It was found that the present algorithm is able to correctly describe the flow field, as well as capture the upper and lower limits of the velocity field, which was made periodic. The comparison between the present algorithm and the typical unsteady one presented a maximum error of ≈2% in the common carotid area, while the error increased significantly inside the bifurcation area. Moreover, “sensitive” areas of the geometry with increased parameter uncertainty were identified, a result that is not possible to be obtained while using deterministic algorithms. Finally, the ability of the algorithm to tune the parameter limits was successfully tested. Full article

Partial filling of porous medium insert in a channel alleviates the tremendous pressure drop associated with a porous medium saturated channel, and enhances heat transfer at an optimum fraction of porous medium filling. This study pioneered an investigation into the viscous dissipative forced convective heat transfer in a parallel-plate channel, partially occupied with a porous medium at the core, under local thermal non-equilibrium condition. Solving the thermal energy equation along the Darcy–Brinkman equation, new exact temperature fields and Nusselt number are presented under symmetrical isoflux thermal boundary condition. Noteworthy is the heat flux bifurcation at the interface between the clear fluid and porous medium driven by viscous dissipation, in cases where the combined hydrodynamic resistance to fluid flow and thermal resistance to fluid conduction is considerable in low Darcy number porous medium insert. However, viscous dissipation does not affect the qualitative variation of the Nusselt number with the fraction of porous medium filling. By using Al₂O₃-Water nanofluid as the working fluid in a uniformly heated microchannel, partially filled with an optimum volume fraction of porous medium, the heat transfer coefficient improves as compared to utilizing water. The accompanied viscous dissipation however has a more adverse impact on the heat transfer coefficient of nanofluids with an increasing Reynolds number. Full article

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 ($\alpha$) 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 $\alpha$ initially decreases the symmetry of the different flows, before for larger values, $\alpha$, the symmetry eventually increases, which appears to be congruent with the degree of anharmonicity. Enhancement in the total torque with $\alpha$ 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 article

On the basis of the Prandtl boundary layer theory and an improved perturbation method, the process of laminar flow bifurcating into the Southwest China vortex (SWV) in the Hengduan Mountains is studied. The results show that the formation of SWV is mainly determined by the speed of incoming airflow in the direction of the main axis of the Hengduan Mountains. The vortex is generated in the leeward area of the Hengduan Mountains when the speed of incoming airflow is greater than the critical velocity. Moreover, it means that the laminar flow bifurcates into a vortex. The formation position of the SWV is mainly determined by the relative position of the incoming airflow in the windward area of the Hengduan Mountains and the main axis of the Hengduan Mountains. The seasonal distribution of SWVs is determined by both the velocity of the incoming airflow and the relative position of the incoming airflow to the main axis of the Hengduan Mountains. These findings are consistent with the SWV observation facts, which not only adequately explain the physical formation mechanisms and processes of SWVs, but also present the formation location and seasonal distribution of SWVs. Meanwhile, a solution from laminar to vortex in circumflow motion is also presented. Full article

Hydraulic vibration is a critical factor affecting the stability and health of the hydro-turbine generator unit (HGU). This paper investigates the nonlinear dynamic characteristics of the HGU occurring from the hydraulic vibration. First, using the turbulence flow model, the lift and drag coefficients of the asymmetric airfoil blades are calculated at different Reynolds numbers and angles of attack. Based on the lift and drag coefficients of the asymmetric airfoil blade, a novel nonlinear mathematical model of asymmetric hydraulic forces is proposed for the asymmetric airfoil blade. Then, combining mechanic and electric asymmetric excitations, a coupled nonlinear mathematical model of the HGU is built using Lagrange equations. Moreover, the correctness of this model is verified. Finally, dynamic behaviors of the HGU changing with attack angle, deviation of the outlet blade angle, outlet guide vane angle, and Reynolds number are analyzed by bifurcation diagrams. It is found that increasing the Reynolds number can enhance the hydraulic vibration, which can lead to more complex nonlinear bifurcation and chaotic motions. In addition, some interesting physical phenomena of HGU under critical ranges and values are obtained. More importantly, these results could give theoretical guidance in the designing and stability research of hydropower units. Full article