Search

This study examines the flow of the ternary nanofluid over a rotating stretchable cylinder with a torsional motion under the influence of a Cattaneo–Christov heat flux. In order to accomplish the goals of this simulation, a ternary hybrid nanofluid (THNF) that is based on water and incorporates nanoparticles with three distinct morphologies is considered. For assessing the colloidal mixture of spherical, cylindrical, and platelet shaped nanoparticles, the necessary models are used. By using appropriate similarity transformations, the modeled equations are reduced to a set of ordinary differential equations (ODEs). These ODEs are then solved using the finite element approach. The numerical integration's validity and reliability, as well as the newly obtained results, were thoroughly analyzed. Results reveal that the larger values of Reynolds number enhance the system's inertial force, which resists the liquid accelerating force and declines both velocities and heat transport. The rise in values of volume fractions and Reynolds number declines the skin friction coefficients along swirling and radial directions. The increase in values of both the thermal relaxation time parameter and Reynolds number improves the rate of heat transport. [ABSTRACT FROM AUTHOR]

The onset of flow-induced oscillations in valve-like configurations remains not completely understood, despite the wide relevance in fluid transport across human physiology and various industrial applications. The present work explores the excitation mechanisms of self-sustained oscillation with key operating parameters in a general-purpose configuration by means of high-fidelity simulations. The investigation is carried out with a partitioned framework that resolves the fluid field by a finite-difference fractional step scheme, discretizes the structural domain via an isogeometric method, and considers an immersed boundary forcing through the interpolation/spreading kernel built by moving-least squares. Our findings confirm the onset of flapping motion in valvular shells, jointly influenced by geometric parameters, structural properties, and flow conditions. Specifically, at a Reynolds number (Re) of 800 and shell aspect ratio of 1.0, a critical reduced velocity exists at around 6, bifurcating static and periodic oscillation modes. After this criterion, flexible shells flutter in the third-plate-mode natural frequency, with oscillation amplitudes approaching an asymptotic value, coupled with intensified vortex shedding, as the reduced velocity increases. Re mainly imparts a destabilizing effect on the fluid-shell system; a lower Re suppresses flow-induced vibrations through viscous dissipation, while a higher Re introduces three-dimensional complexities, asymmetrical oscillations, and quasi-periodicity in the flapping dynamics, especially within the critical regime of reduced velocity. The impact of shell aspect ratio is intricate; in contrast, the case with an aspect ratio of 1.3 displays more intensive flapping motion compared to the reference case of 1.0, whereas further increasing to 1.6 mainly shows stabilizing effects in the shell dynamics. [ABSTRACT FROM AUTHOR]

Mosquitoes exhibit a distinctive and remarkable flight pattern, flapping their wings at a high frequency with relatively small stroke amplitude. However, until recently, the underlying aerodynamic mechanisms have remained unclear. Furthermore, there is a lack of understanding about their flight behaviors after blood-feeding and the corresponding aerodynamic characteristics. This study aims to explore this uncharted area, conducts experiments to acquire kinematic and morphological data and numerical simulations to obtain three-dimensional flow characteristic. Further analysis uncovers several key findings. Both before and after blood-feeding hovering exhibit a similar flapping wing pattern, characterized by downstroke and upstroke with three stages of each half stroke. After blood-feeding, there are significant increases in stroke amplitude, mid-downstroke duration, velocity, and flip angles. Additionally, body pitch, stroke plane tilt, and Reynolds number experience increments. In hovering, mosquitoes balance vertical force with weight, with substantial peaks observed in each stage, particularly during the mid-stroke. After blood-feeding, the vertical force experiences a 3.3-fold increase, with the majority of the increase occurring during the mid-downstroke. The study identifies three unsteady mechanisms for aerodynamic force generation without blood-feeding hovering, namely, added-mass force, delayed stall, and fast-pitching-up rotation. These mechanisms persist after blood-feeding, with a greater reliance on delayed stall to support increased weight. [ABSTRACT FROM AUTHOR]

Scale-adaptive simulations (SAS) of three-dimensional flow around a circular cylinder fitted with a passive version of a novel flow control method (passive Venturi actuator, PVA) are performed at a diameter-based Reynolds number of Re = 28 000. The PVA consists of one or more narrow slits located at the top and/or bottom sides of the cylinder that connected to the throat of the axial Venturi slit in this cylinder. The main purpose of the study is to investigate the influence of divergence angle and narrow slit location relative to the axial Venturi slit on the aerodynamic performance of the cylinder. To this end, four models were designed with various PVAs. Additional models, a plain cylinder (unmodified model) and a cylinder fitted with an axial Venturi slit (model without a narrow slit), were also used for quantitative comparison. SAS predicts that an additional 5% reduction in the time-averaged drag coefficient, ⟨ C D ⟩ , was observed when two narrow slits located on the surface at an angle of ±80° from the front stagnation line were fitted to the cylinder with an axial Venturi slit. Reducing the divergence angle of the PVA leads to improvements in ⟨ C D ⟩ and root mean square of fluctuating force coefficients, C D − rms and C L − rms . It is found that a cylinder with a PVA that has two narrow slits and a divergence angle of 6° can produce a 28.6% reduction in ⟨ C D ⟩ , a 58.5% reduction in C D − rms , and an 81.2% reduction in C L − rms , when compared to the plain cylinder. [ABSTRACT FROM AUTHOR]

The stability analysis of laminar channel flow subject to spanwise thermal modulations is presented. Modulations create streamwise streaks and rolls, producing three-dimensional flow structures. It is shown that these structures induce a new type of instability which persists at low Reynolds numbers. Detailed characterization and quantification of this instability are given, including an explanation of its mechanism. It is shown that heating intensity and spatial distribution control this instability; its intensity increases with a reduction of the Prandtl number, and it can be induced by heating of either wall. [ABSTRACT FROM AUTHOR]

Downwind yacht sails, such as spinnakers, are low-aspect-ratio highly cambered wings with a sharp leading edge. They are characterised by substantial three-dimensional flow separation and are thus modelled with difficulty with numerical simulations. Furthermore, accurate full-scale validation data are not available. The first quantitative flow measurements have only recently been achieved in water tunnels. In this study, we aim to provide guidelines on this emerging sail testing methodology. We consider six model-scale rigid models at average-chord-based Reynolds numbers ranging from 5 870 to 61 870. A critical Reynolds number is identified, below which relaminarisation of the reattached boundary layer downstream of the leading edge separation bubble occurs. Both lift and drag increase monotonically at subcritical Reynolds numbers while remaining about constant at transcritical Reynolds numbers. The critical Reynolds number decreases with increasing incidence and is insensitive to the blockage ratio. Spinnakers are normally sailed in front of the mainsail, whose circulation is found to generate an approximately 3 ∘ upwash on the spinnaker and higher flow velocity on both sides of it. These findings provide guidelines for the experimental testing of spinnaker-like wings in water tunnels and provide new insights into the flow and experimental testing of highly cambered wings with massive flow separation at low Reynolds numbers. [ABSTRACT FROM AUTHOR]

We investigate the control of three-dimensional flow separation around a finite wall-mounted square cylinder by applying suction at the side leading edge. Direct numerical simulations are conducted at a Reynolds number of 250, with suction ratios Γ of 0–2 (where Γ is the absolute value of the suction velocity divided by the free stream velocity). The effect of Γ on the aerodynamic forces acting on the cylinder is studied. The results show that suction reduces the aerodynamic forces, with the best control effect for the fluctuating lift coefficient (corresponding to a reduction of over 70%) achieved at Γ = 0.375. As the suction ratio increases, the pressure drag experienced by the square cylinder decreases. Simultaneously, the mean frictional drag force exerted on the square cylinder increases. The optimal mean drag coefficient (corresponding to a reduction of nearly 20%) is achieved at Γ = 1. The effect of the suction ratio on the flow topology in the wake is also investigated. Suction significantly suppresses the flow separation. As the suction ratio increases, the spanwise counter-rotating vortices in the streamwise and transverse directions decreases in size, and the downwash vortex shrinks, and shifts toward the free end of the square cylinder. The far-wake streamwise base vortex disappears when active suction is applied to the side leading edge. However, a new pair of base vortices splits from the original base vortex and persists into the far wake flow field, forming a quadrupole vortex structure with the tip vortex. [ABSTRACT FROM AUTHOR]

Flow around a cylinder is an important research problem in fluid mechanics. However, studies on traveling wave wall (TWW) flow control for the wake generated by the flow around a 3D cylinder remain limited. This study performs large eddy simulation (LES) to investigate the wake characteristics of flow around a three-dimensional (3D) circular cylinder with the TWW. The influence of the main control parameters of TWW on the aerodynamic forces and wake of the 3D cylinder is analyzed. The wake characteristics of the 3D cylinder at Re = 4 × 104 are obtained, and the traveling wave propagating downstream is achieved on the rear surface of the 3D cylinder using dynamic mesh. The control effects of wave amplitude, number of waves, and wave velocity of the traveling wave on the aerodynamic forces of the cylinder are analyzed, and the optimal control parameter combination is determined. The results demonstrate that the TWW control method completely eliminates the spanwise 3D flow characteristics of the cylinder when the amplitude ratio, number of waves, and velocity ratio are 0.02, 4, and 1.5, respectively. Moreover, it effectively suppresses the flow separation on the cylinder surface, eliminates the wake vortex, and suppresses vortex-induced vibration (VIV). [ABSTRACT FROM AUTHOR]

Laminar hypersonic flows at Mach 7.10 with unit Reynolds numbers of 5.2 × 10 4 , 1.04 × 10 5 , and 4.14 × 10 5 m−1 over a 30°/55° double-wedge configuration were studied to investigate the spatial–temporal characteristics of the flow in a time-accurate manner. Close comparisons were made between previous kinetic and current continuum approaches to test the validity of the continuum assumption, especially considering the presence of large gradients associated with shock–shock and shock–boundary layer interactions, as well as spanwise instabilities. Previous results from direct simulation Monte Carlo, which inherently predicts rarefied effects such as velocity slip and temperature jumps, were found to be in very close agreement with the current work, even for the lowest Reynolds number. The impact of velocity slip and temperature jumps on flow and surface parameters was investigated, and comparisons were made with a no-slip and constant temperature wall model. The temporal and spatial variation of two- and three-dimensional flows were thoroughly investigated using two-dimensional (2D), three-dimensional (3D) periodic sidewall boundary conditions, and a full 3D configuration consistent with existing experimental data. Close comparisons among the 2D and 3D cases were made. The characteristics of 2D periodic oscillations were reported for the moderate Reynolds number case for the first time. The presence of spanwise instabilities, even at a relatively low free stream pressure of about 100 Pa, establishes that the flow field depends on spanwise effects and is fully 3D. High-fidelity numerical schlieren videos captured strong spanwise oscillations for 3D configurations. [ABSTRACT FROM AUTHOR]

The perforated shrouds have been proposed to control cylinder flows, while the effects and mechanisms at different Reynolds numbers (Res) remain unclear. Three-dimensional numerical simulations are conducted in this paper to compare the aerodynamic performance of the flow around a smooth cylinder and a shrouded cylinder at Re of 3900 and 1.4 × 10 5 . The results indicate that the drag of the perforated shrouded cylinder is reduced by 30.8% at the high Re, while increased by 26% at Re of 3900 compared with the smooth cylinder. Differently, the lift oscillations of the cylinder are greatly weakened by 83.3% at the Re of 3900 and 98.5% at the Re of 1.4 × 10 5 , which implies the wake oscillations are nearly eliminated at the Re of 1.4 × 10 5 . Further analysis exhibits the near wake region is elongated along the mainstream, with significantly recovered pressure. Especially, the greater pressure loss owing to outer shrouds even leads to the negative drag of the inner cylinder at the high Re. In addition, the incoming flow is broken up by outer shrouds, and different flow patterns appear in the gap. The discernible vortex pairs occur in the gap at the Re of 3900, while as Re increases to 1.4 × 10 5 , the quantities of small-scale vortex weaken the impact on the inner cylinder. The shear layer characteristics are elucidated by Lamb vector curl and Kelvin–Helmholtz instabilities. The vorticity stretching and tilting in the shear layer of the shrouded cylinder is much weaker at the high Re. Generally, the energy for shear layer instabilities at low frequencies is diminished with the presence of perforated shrouds. However, the energy at higher frequencies is strengthened at the low Re. [ABSTRACT FROM AUTHOR]

In this study, the three-dimensional flow fields in the steady state around the low-rise building models with gable roof where the roof pitch angle is 15°, 30° and 45°, and the eave pitch angle is 0°, 15°, 30°, 45° and 60° were computed for the 40000 value of the Reynolds number with Standard k-e, RNG k-e and Realizable k-e turbulence models. The effect of the change in roof and eave slope angles on the flow field was investigated as mean pressure coefficient distributions on the model surfaces. In numerical solutions, wind angle of incidence is considered as 90°. Geometric changes in building models create differences in surface pressure distributions. Differences in surface pressure distributions occur on the windward roof and eave surfaces. The most critical negative peak pressure coefficients are formed on the ridges of 15° inclined roofs. The difference between two peaks on the windward roof surfaces decreases as the angle of the eave's inclination increases. The numerical results calculated by three different turbulence models are close to each other. [ABSTRACT FROM AUTHOR]

The incompressible laminar isothermal flow of a Newtonian fluid at steady state around a surface-mounted rib is studied in a three-dimensional (3D) numerical experiment. The dimensionless Navier–Stokes equations are solved numerically using the Galerkin finite element method for Reynolds numbers 1 to 800. The expansion ratio of the problem is 1:9.6, while the aspect ratio is 1:20. The transition from the steady to the unsteady state and the identification of the critical Reynolds number are investigated in this paper. Numerical results of the skin-friction lines at the bottom and streamlines throughout the computational field are presented. A comparison between the 2D and 3D flow is made to show the effect of the walls on the flow, which reaches the plane of symmetry and affects the flow there; hence, also affecting the stability of the flow. It is concluded that the flow is three-dimensional even for a Reynolds number equal to 10. The critical Reynolds number is 600, and the steady-state equations can be used for any calculations up to this value. [ABSTRACT FROM AUTHOR]

Galloping cross-flow vibration responses of three in-line identical square cylinders are numerically studied for the mass ratio m * = 2 , streamwise gaps L x = 3 B and 5B, reduced velocity U * = 3 − 50 , and Reynolds numbers Re = 150 in two dimensions (2-D) and 2000, where the flow is three-dimensional (3-D). Here, B is the side of the cylinder. An isolated cylinder does not gallop since the mass ratio m * = 2 is less than the critical value in the Re = 150 flow, whereas for the three in-line bodies, galloping instability is triggered at the upstream cylinder due to the interference effect caused by the presence of downstream bodies. The interaction with the wake of galloping upstream cylinder promotes galloping instability for the two downstream cylinders almost immediately at Re = 150. In the three-dimensional wake at Re = 2000, downstream cylinders interact with less coherent Karman vortices shed by the galloping upstream cylinder, compared to the 2-D case. This phenomenon leads to the delayed on-set of galloping response for the first downstream cylinder, while the second one never gallops. [ABSTRACT FROM AUTHOR]

This work experimentally investigates the pressure-driven flow of a pure Newtonian fluid through three-dimensional (3D) porous media models. The porous media model consists of square arrays of rods that also could be interpreted as a periodic tandem rod arrangement. We employed a time-resolved three-dimensional particle tracking velocimetry (3D Shake-the-Box) technique for a range of Reynolds numbers 111 ≤ R e ≤ 890 to observe flow structures and vortex formation between the rods in porous media structures with different porosities of ε = 0.7 , 0.8 , and 0.9 , which corresponds to the spacing ratio of L D = 1.75 , 2 , and 3 , where L is the distance between the centers of the rods, and D is the diameter of the rods. For all the examined cases, we further analyzed the effect of the Reynolds number and the spacing ratio on the instantaneous and averaged patterns of velocity, vorticity, and the other flow parameters after obtaining the two-dimensional velocity fields using the bin-averaging method. We observed both symmetrical and asymmetrical patterns of structure and recirculation regions between the rods depending on the Reynolds number and spacing ratio. Increasing the Reynolds number reduced the symmetrical patterns of flow structures with respect to the centerline of the gap region, while the spacing ratio was randomly affecting the symmetry degree. Vortex shedding was considerable for the two examined high Reynolds numbers of Re = 444 and Re = 890 behind the upstream rod as the porosity increased. The backward movement of the reattachment point has been observed by increasing the Reynolds number. [ABSTRACT FROM AUTHOR]

A numerical study estimated the mixed convection around a sinusoidal cylinder inside a rectangular cavity. The key parameters for sinusoidal circular cylinders were the wavelength and Reynolds number (Re = 100, 500, and 1000) with a fixed Prandtl number of 0.7. The flow regimes were greatly affected by these parameters. The unsteadiness of the flow pattern was also affected by λ and Re. The results were analyzed by the thermal structure, vortical structure, and Nusselt number. Furthermore, the orthogonal enstrophy was used to quantify the three-dimensional flow characteristics, and the frequency of the surface-averaged Nusselt number for the heated bottom wall was used to analyze the unsteady characteristics. The heat transfer performance was enhanced by 6.6% at Re = 100 for λ = 3 compared to the circular cylinder. In addition, the heat transfer performance was enhanced by 6.6% and 2.6% at Re = 500 for λ = 2 and Re = 1000 for λ = 1, respectively, compared to the circular cylinder. [ABSTRACT FROM AUTHOR]

The traditional multi-level grid multiple-relaxation-time lattice Boltzmann method (MRT-LBM) requires interpolation calculations in time and space. It is a complex and computationally intensive process. By using the buffer technique, this paper proposes a new multi-level grid MRT-LBM which requires only spatial interpolation calculations. The proposed method uses a center point format to store multi-level grid information. The grid type determination in the flow field calculation domain is done using the axis aligned bounding box (AABB) triangle overlap test. According to the calculation characteristics of MRT-LBM, the buffer grid is proposed for the first time at the interface of different levels of grids, which is used to remove the temporal interpolation calculation and simplify the spatial interpolation calculation. The corresponding multi-level grid MRT-LBM algorithm is also presented for two-dimensional and three-dimensional flow field calculation problems. For the two-dimensional problem of flow around a circular cylinder, the simulation results show that a four-level grid MRT-LBM proposed in this paper can accurately obtain the aerodynamic coefficients and Strouhal number at different Reynolds numbers, and it has about 1/9 of the total number of grids as a single-level grid MRT-LBM and is 6.76 times faster. For the three-dimensional flow calculation problem, the numerical experiments of flow past a sphere are simulated to verify the numerical precision of the presented method at Reynolds numbers = 100, 200, 250, 300, and 1000. With the streamlines and velocity contours, it is demonstrated that the multi-level grid MRT-LBM can be calculated accurately even at the interface of different size grids. [ABSTRACT FROM AUTHOR]

In recent years, with the improvement of computer performance, the application of large eddy simulation numerical calculations has also increased. The application research of sub-grid model is very important. In this study, based on the STAR-CCM + software, three different sub-grid model large eddy simulation methods and standard k-ε and k-ω methods were used to numerically simulate the three-dimensional flow near the cylinder under subcritical Reynolds number Re = 3900. Firstly, this research conducts a grid convergence study. Then it compares and analyzes the time-average pressure distribution on the cylindrical surface under several numerical methods, the vortex shedding characteristics, the time-average velocity profile distribution of the flow field downstream, and also compares with the experimental data. The results show that the simulation results of the DSGS model are relatively accurate. The dissipation of the other two sub-grid models is too large, and the result error of the flow parameters is also too large. The accuracy of conclusions obtained by the large eddy simulation method is better than that of the RANS method. The application of large-eddy simulation can better display the various data of the flow field, and the results of this article can also provide certain help for the selection of sub-grid model in the application of large eddy simulation. [ABSTRACT FROM AUTHOR]

Wall-resolved large-eddy simulations were carried out for the flow over a Gaussian bump configuration. The geometry and flow conditions were motivated by an experimental investigation, which was conducted in order to provide data for validating numerical modelling. The present computations were initiated as benchmark results that are accessible via wall-resolved large-eddy simulation. It was found that by increasing the bump height, the Reynolds number could be reduced and flow separation would occur. The modified bump then serves as a surrogate for the original Gaussian bump producing a smooth separated flow. Solutions to the unsteady three-dimensional compressible Navier-Stokes equations were obtained utilising a high-fidelity computational scheme and an implicit time-marching approach. Large-eddy simulations were performed and grid resolution studies were carried to ensure quality of computed results. Features of the flowfields are elucidated, and it was found that the time-mean surface streamline pattern had similar features to that of the experiment. [ABSTRACT FROM AUTHOR]

This article introduces a novel method to calculate the structural sensitivity in non-linear flows. The method is completely data-driven and only uses direct solutions, avoiding the need to calculate the adjoint solution. Moreover, the method can be directly applied to calculate the structural sensitivity in three-dimensional flows. Depending on the nature and the complexity of the data analysed, the methodology presented to calculate this so-called non-linear structural sensitivity, uses dynamic mode decomposition (DMD) or higher order DMD to identify the global modes leading the flow dynamics. The performance of this novel technique has been tested in the two-dimensional wake of a circular cylinder at Reynolds number (Re) 50, 60, 85, 100, 130 and 160, the three-dimensional wake of a circular cylinder at Re = 280 and spanwise wavelength L z = 1.67 and 6.99, in a Food and Drug and Administration nozzle at Re = 800 and in two-concentric jets with similar velocity ratio at Re = 1500. The results have been compared with the structural sensitivity predicted by the linear theory, finding some similarities and discrepancies between both methods. Nevertheless, the highest structural sensitivity regions identified by this novel algorithm are in good agreement with the literature, where the flow instabilities have been controlled using different mechanisms of passive flow control located in the regions predicted by the non-linear structural sensitivity calculated in this article. [ABSTRACT FROM AUTHOR]

The flow of emulsions under the action of an electric field has practical applications in microfluidic systems such as lab-on-a-chip devices. An emulsion of drops under uniform electric field is studied by numerical simulations. The familiar finite difference front tracking method is used to simulate the two-phase medium. The flow is studied at finite electric Reynolds numbers. In case of finite inertia, density appears as an important parameter that affects the behavior of the flow. Two- and three-dimensional systems have been studied in the present work. The full equation for conservation of charge has been applied in the numerical method. The drop deformation and shape agree with Taylor's leaky dielectric theory in the limit of small electric Reynolds number (Re2). However, the drop deformation deviates from the theory at relatively large electric Reynolds numbers, specifically the drop turns to a prolate shape instead of an oblate shape. It is found that emulsions of two-dimensional drops concentrate in the wall regions at small electric Reynolds numbers where drops have an oblate shape. The bond between drops become strong and it becomes difficult to break fibers when the Reynolds number based on the fluid density is raised (Re1). The increase in Reynolds number (Re1) reduces the actual flow rate inside the channel. At large Reynolds numbers (Re1) and relatively high capillary numbers, oblate drops form clusters at the center of the channel with size of the same order as the channel height in three-dimensional flow. At large electric Reynolds numbers (Re2), drops become prolate and form stable fibers across the channel. This reduces the net flow rate substantially. The net flow also decreases as the electric Reynolds number is raised (Re2). [ABSTRACT FROM AUTHOR]

Blind tees are widely used in subsea pipelines to enhance the mixing conditions of oil and gas products, but their structural design still relies on experience. In this paper, a series of numerical investigations have been carried out on blind-tee pipes in order to develop an in-depth understanding of their mixing mechanism and clarify the effects of blind-tee structures on the pipe flow. Firstly, the three-dimensional flow conditions in a typical blind tee have been simulated under different Reynolds numbers to investigate the mixing mechanism. Two critical Reynolds numbers for the vortex generations in blind tees are determined in the laminar flow regime, and the fitting curves of blind-tee vorticity dissipations are obtained. Then, the geometrical parameters, including the radial size ϕ, axial length (BSL) and position of the blind section, are varied systematically to study their effects on the flow characteristics and mixing conditions. The results indicate that increasing ϕ and BSL in an appropriate range can strengthen the flow circulation and promote the fluid exchange. Finally, an optimal configuration is obtained, which can improve the mixing capacity of blind tee by 53% in terms of the volume average vorticity as compared to the typical structure. [ABSTRACT FROM AUTHOR]

Cleaning is an important process step in the food industry, especially to avoid contamination during increasingly frequent product changes. The equipment is cleaned almost daily causing high ecological and economic expenses. Predicting the cleaning time required to remove a thin soil layer for given operating conditions can prevent the use of unneeded resources. The purpose of the present contribution is to apply a recently published model for an adhesively detaching soil at the next level of geometrical complexity. To this end, a square duct with a stepwise expansion of each wall is considered. The Reynolds numbers of the flows investigated are between 13,000 and 39,000, based on the hydraulic diameter of the upstream duet. In a square duct, the flow is three-dimensional due to secondary flows of the second kind, and the existing model for two-dimensional flow needs to be extended in the first step. In course of that, a simple method is presented for the consideration of turbulent fluctuations in the calculation of the hydrodynamic loads. The model is validated using cleaning experiments in a fully developed channel flow without step. After that, it is applied to the channel flow with sudden expansion. Various cleaning parameters, such as the soil mass coverage and the flow velocity, are varied and the quality of the model predictions is investigated. The results are compared with experiments in a channel flow with sudden expansion. Good agreement between experiments and simulation is achieved, proofing that the model can be used to investigate the cleaning in complex, three-dimensional flow scenarios. [ABSTRACT FROM AUTHOR]

This work focuses on magnetohydrodynamic (MHD) flow around either single or two identical tandem circular cylinders confined in a duct under external transverse magnetic field. The influences of transverse magnetic field, inlet velocity of conducting fluid, and the distance between two cylinders arranged along the flow direction on the flow modes, vortex shedding frequency, separation angle, and drag coefficient are investigated. The simulations are conducted with the Hartmann number Ha and Reynolds number Re and are varied from 0 to 1.35 and from 100 to 200, respectively. For the MHD flow around single cylinder, the map of flow instability is set up and the relation between critical Hartmann number Hacr and Re is concluded as Ha cr ∼ (Re − 100) 2 / 3 . The coupling effect of geometry confinement and external transverse magnetic field can increase the separation angles and drag coefficients and decrease vortex shedding frequencies and lengths of reverse flow. For the tandem double cylinder cases, the spacing ratio between two cylinders L/d changes from 2 to 8. The boundary between steady regime and unsteady regime is described as, (Re / Ha 0.5) cr ∼ (L / d). The three wake patterns are found under various magnetic fields. The nonlinear and nonmonotonous characteristics of coupling effects of Ha, Re, and L/d are summarized. The physical mechanisms of influences of MHD flow field in gap region between tandem cylinders and the existence of upstream cylinder on flow instability and wake pattern are revealed. Finally, the critical conditions corresponding to the transition from steady flow to unsteady flow are achieved with formulas. [ABSTRACT FROM AUTHOR]

This aerodynamic study is an attempt to build up efficient flow control computations on a detached high Reynolds number flow behind a 25°inclined ramp enabling to reduce the turbulence energy of recirculating zones. Flow control is performed with a synthetic jet row placed at the top edge of this inclined ramp. Baseline simulations are validated with experiments and a key target is to yield numerically a deeper knowledge of flow control impact on flow separation and vortex dynamics. Average results of large eddy simulations, performed with and without flow control, show a vortex size reduction in the recirculation zone associated to a small pressure increase on the ramp with the selected synthetic jet conditions. A dynamic modal decomposition (DMD) involving a singular value decomposition (SVD) is explained and realized in the wake with and without control. Periodic DMD modes, which are below the integral scale of the turbulence are split in three frequency ranges : frequencies of the shear layer close to the frequency of the integral scale, frequency of the recirculation vortex corresponding to the lowest frequency range and an intermediate frequency range, corresponding to vortex structures situated behind the shear layer. Flow control activated in the shear layer at a frequency equal to the integral scale enables to damp energy of the intermediate frequency range situated in the most energetic POD mode. Vortex structures behind the shear layer are then significantly reduced. [Display omitted] • High Reynolds number three-dimensional flow over a ramp is analyzed and studied. • Active flow control with synthetic jets is carried out. • A significant decrease of the recirculation area and wall pressure as well as shear effects' change is observed. • Dynamic Mode Decomposition (DMD) is implemented to deepen understanding of the control spectral behavior. • A crucial shift and reduction of some dominant frequencies is observed. [ABSTRACT FROM AUTHOR]

The present article assesses the capability of the partially averaged Navier-Stokes (PANS) method to reproduce accurately the self-sustained shock oscillations, also known as transonic buffet, occurring on airfoils and wings at transonic regime under certain conditions of Mach number and angle of attack. The test case under analysis is an OAT15A unswept wing at Mach number M ∞ = 0.73 and Reynolds number Re c = 3 × 10 6. The three-dimensional flow is studied by accounting for the wind tunnel walls adopted in the experiments of Jacquin et al. [1] in the simulations. The computations on a large-span, confined configuration reveal a strong three-dimensionality of the flow both before and after the buffet onset. Attention is paid to the comparison with unsteady Reynolds-averaged Navier Stokes (URANS) results, to show the benefits of PANS in resolving flow unsteadiness at different flow resolutions, especially on affordable CFD grids, at limited additional cost. In this context, the role of the mesh metrics and the local turbulence level in the formulation of the model is described, as well as the relation of this latter with the spatiotemporal discretization used for the numerical simulations. The aim is to extend the use of PANS and obtain accurate predictions of flow cases involving shock-wave boundary layer interactions without expensive approaches. [ABSTRACT FROM AUTHOR]

Flow-induced vibrations of two circular cylinders in tandem arrangement which are mounted in a three-dimensional flow field are investigated at a high Reynolds number of 20,000. A third-order upwind finite element scheme and the arbitrary Lagrangian–Eulerian (ALE) formulation are used to solve the three-dimensional incompressible Navier–Stokes equations. Two circular cylinders which are treated as rigid bodies are free to vibrate in both in-line and cross-flow directions. Accordingly, both cylinders are assumed as a two-degree of freedom system. The upstream cylinder is set in a smooth flow and the downstream cylinder lies in the wakes which are shed from the upstream one. Spacing S between center-to-center of the cylinders is 2 D and 3 D (D is the diameter of the cylinders) less than the critical spacing. The Scruton number S c with respect to the flow-induced vibration is given as 0.99. The excitation vibrations to the in-line direction occur in the low range of the reduced velocity because of the low Scruton number. In addition, large cross-flow amplitudes of both cylinders are observed in the high range of the reduced velocity. [ABSTRACT FROM AUTHOR]

This paper aims to improve existing models for drag, lift, and pitching torque coefficients of cylindrical disc particles with a wider range of aspect ratios and particle Reynolds numbers. The full body-fitted direct numerical simulations (DNS) are carried out for three-dimensional flow past cylindrical discs in order to quantify the coefficients at aspect ratios from 0.2 to 0.6, incidence angles from 0°to 90°, and Reynolds numbers from 0.1 to 300. The CFD results are validated against the aerodynamic coefficients of both spherical and non-spherical particles presented in the literature. A full set of new correlations for the coefficients, dependent on aspect ratio, incidence angle, and Reynolds number, is further derived according to the CFD predicted data. The results show that the new correlations have a promising performance for the disc particles considered in this study. [Display omitted] • CFD simulations are performed for drag, lift, and pitching torque coefficients. • A complete set of correlations for C D , C L , and C T is proposed. • Proposed correlations work well outside the Stokes regime. • The correlation predictions agree with the CFD simulations. [ABSTRACT FROM AUTHOR]

The three-dimensional flow of a circular synthetic jet interacting with a turbulent crossflow is investigated with unsteady Reynold-Averaged Navier–Stokes simulations. The effects of jet momentum are examined using three blowing ratios ( C B = 0.32 , 0.67 , and 1.10) at constant actuation frequency (f = 300 Hz), approach crossflow Reynolds number, R e θ = 900 , and boundary layer thickness, δ / d = 7.25 , where d is the jet diameter. The results showed that the expelled jet is accompanied by a reverse flow region on the downstream side which undergoes alternating expansion and contraction during the jet cycle. The size of the reverse flow region and depth of penetration increased with increasing jet momentum. For the low momentum jet, the expelled flow structure evolved into a hairpin vortex which significantly enhanced the wall shear stress in the spanwise direction and near the jet exit. The higher momentum jets, on the other hand, exhibited strong vortex loops around the expelled jet column that transitioned into trailing vortex pairs and a tilted vortex ring further downstream. Along the wall, both horseshoe vortex upstream of the jet exit and tertiary vortices downstream were found attached to the wall. The strong tertiary vortices promoted downwash of fluids which significantly enhanced the wall shear stress along the symmetry plane for the medium and high momentum jets. The impact of the tertiary vortices increased with the jet momentum, offering greater potential for flow separation control. [ABSTRACT FROM AUTHOR]

This study carried out a three-dimensional, unsteady, numerical simulation to investigate the effects of the cylinder radius, Reynolds number, three-dimensionality and unsteadiness on the mixed convection in a rectangular enclosure containing a cold circular cylinder. The variations in the cylinder radius in the range of 0.1 to 0.3 times the length of the enclosure and the Reynolds number in the range of 100 to 1000 were considered. The flow and thermal structures reached three different regimes depending on the radius of cold cylinder and Reynolds number. The patterns in the flow and thermal fields were categorized into three regimes: steady two-dimensional convection, steady three-dimensional convection, and unsteady three-dimensional convection. The temperature distribution, vortical structure, and Nusselt number for a heated bottom wall were extensively analyzed. The transition of flow regime, flow instability and the heat transfer performances were influenced by the presence of cylinder. In addition, the three-dimensional flow characteristics were quantified using orthogonal enstrophy, and the unsteady characteristics were investigated by carrying out the frequency analysis of Nusselt number. As the three-dimensionality in flow and thermal fields increases and the flow regime changes to the three-dimensional unsteady convection regime, the difference in the results of two and three-dimensional simulations also increases. [ABSTRACT FROM AUTHOR]

The three-dimensional flow around a custom wing performing pitching oscillation is analyzed numerically using large-eddy simulation with the wall adapting local eddy-viscosity subgrid-scale model. The wake characteristics behind the pitching wing with three different aspect ratios, AR = 2 / 3 , 1 , 4 / 3 , is investigated in an upstream flow with Reynolds number, Re = 2640. The Q − function is employed to identify the swirling structures in the flow field, and four primary vortices, namely, primary leading edge vortex (PLEV), secondary leading edge vortex, wing tip vortices, and trailing edge vortex, are identified. The evolution of vortices over the wing surface is presented in detail, and the arrangement of vortex rings in the wake is compared for three different aspect ratios. The PLEV develops into an arch vortex using the self-induction phenomenon, and the interlinking between the subsequent vortex rings depends on the arch vortex leaving the trailing edge. The force coefficient computed over the wing shows a positive thrust for AR = 4 / 3. The mean velocity field contours indicate a spanwise loading on the wing, which is unaccountable in two-dimensional studies. The Reynolds stresses generated due to large-scale periodic velocity fluctuations in the wake are presented. The vorticity field in the midplane of the wing appears to be diffusive at higher aspect ratios. The comparison of vorticity from the three-dimensional and two-dimensional models suggests that a two-dimensional model will over predict the vorticity distribution and force coefficients in the wake. [ABSTRACT FROM AUTHOR]

Experimental and numerical investigations are conducted in order to understand the flow around identical tandem circular cylinders confined in a duct (blockage ratio b = 0.1 and aspect ratio a = 5). In this work, the Reynolds number Re ranges from 40 to 200 and the spacing ratio (distance between two centers of tandem circular cylinders to diameter, L/d) from 0 to 8. When fluid flows around a single cylinder placed symmetrically in the duct (L / d = 0), it is found that the Strouhal numbers St and mean drag coefficients C d ¯ increase with the increase in Re, although the separation angle θs decreases. For the flows around two tandem circular cylinders, the research is focused on examining the coupling effect of Re and L/d on St, flow structures, θs, C d ¯ and reverse region length Lr. Based on numerical results, the mathematical descriptions are established, C d ¯ ∼ R e − 1 , θ s ∼ R e − 1 / 2 , and S t ∼ R e − 1 . The dependence of flow structures, drag forces, θs and Lr, on L/d is described in detail with the help of approaching velocity profiles, pressure coefficient Cp, and vorticity ω z * distribution on a cylinder's surface. Through numerical simulations, the detailed mechanisms about influences of these factors on the flow properties are revealed. The experimental results evidence the occurrence of three flow modes, i.e., no vortex shedding mode, single body mode, and reattachment mode, which are determined practically by the Reynolds number and space ratio. Both the measured St based on the dominate frequencies of vortex shedding and the visual flow field match well with numerical simulations. [ABSTRACT FROM AUTHOR]

This paper computes the laminar flow in a three-dimensional (2D) two-sided lid-driven cubical cavity induced by a centrally located circular cylinder by Finite Volume Method and multigrid acceleration. The cavity has left and right parallel lid-driven walls and all the other walls completing the domain are motionless. Furthermore, different wake parameters are considered throughout this study; velocity ratios [ α = 0 , 0. 2 5 , 0. 5 and 0.75] and radius size of the inner cylinder [ R = 0. 1 2 5 , 0. 1 5 , 0. 1 7 5 and 0.2]. Thereafter, the study has been extended to the unsteady state by further increasing Reynolds number till reaching up to 2000. It is noticed that the highest critical Reynolds number corresponds to the lowest radius size and conversely. By further increasing the velocity ratio till reaching 0.25, 0.5 and 0.75, where the left moving lid began to have an effect, the opposite phenomenon occurs and we note that the highest critical Reynolds number corresponds to the highest radius. This can be explained by the fact that when the cavity is two-sided lid-driven, the velocity of the fluid particles accelerates. Besides, it has been proved that both parameters α = 0. 7 5 and a radius size R = 0. 1 2 5 promote faster transition to the unsteadiness rather than the remaining velocity ratios and radii sizes. [ABSTRACT FROM AUTHOR]

This study attempts to characterize the variation of pressure loss and the evolution of vortex structures in the steady three-dimensional flow around a 180° sharp bend under a transverse magnetic field. This study is conducted with the Reynolds number, 100 ≤ R e ≤ 400 , and Hartmann number, 0 ≤ H a ≤ 2000. This range of Re and Ha captures both complex three-dimensional structures and the inception of quasi-two-dimensional flows. Numerical simulations display how the pressure loss across the bend region (Δ p 0 ) and the vortex structures undergo four regimes by increasing Ha for fixed Re. These regimes are referred to as regimes I–IV. N 1 c , N 2 c , and N 3 c , the critical values of interaction number Nc for the first appearance of regimes II–IV, are recognized at values 0.8, 3.2, and 40, respectively. In regime I (N ≤ 0.8), Δ p 0 increases slightly and the magnetohydrodynamic flow promotes the recirculating bubble. In regime II (0.8 < N ≤ 3.2), as Δ p 0 decreases, the scope of the recirculating bubble reaches its peak. In regime III (3.2 < N ≤ 40), Δ p 0 grows, while the shrinkage of the recirculating bubble is triggered. Simultaneously, the flow tends to be two-dimensional [J. Sommeria and R. Moreau, "Why, how, and when, MHD turbulence becomes two-dimensional," J. Fluid Mech. 118, 507 (1982)]. In regime IV (40 < N), Δ p 0 is linearly dependent on Ha/Re. The non-monotonic behavior of the recirculating bubble length is caused by the redistribution of momentum at low Ha and by the predominant effect of the Lorentz force at large Ha. The law of how to distinguish the three-dimensional (3D) flow and quasi-two-dimensional (Q2D) flow is discovered by assessing the recirculating bubble length in the center plane. [ABSTRACT FROM AUTHOR]

The study of the exchange of momentum and energy between wave components of the turbulent velocity field, the so-called triad interactions, offers a unique way of visualizing and describing turbulence. Most often, this study has been carried out by direct numerical simulations or by power spectral measurements. Due to the complexity of the problem and the great range of velocity scales in high Reynolds number developed turbulence, direct measurements of the interaction between the individual wave components have been rare. In the present work, we present measurements and related computations of triad interactions between controlled wave components injected into an approximately laminar and uniform flow from an open wind tunnel by vortex shedding from two rods suspended into the flow. This results in two-dimensional interactions of three-dimensional turbulence, which makes the analysis of the triadic interactions considerably less complex to analyze than in a fully developed three-dimensional flow. With the information obtained from the computations, we are able to isolate the individual triad interactions contributing to the generated frequency components as the flow develops downstream as well as understanding, mapping out and predicting the strengths of these interactions. The analysis also provides the time constants governing the development of higher order frequency components. We are thus able to see the pattern of frequency combinations, the strengths of the individual mode combinations and the time sequence in which they occur. Any of the higher order combinations is not just the result of a single term in the Navier–Stokes Equation, but a combination of various previous combinations occurring with different strengths and in a varied pattern of generation. The combination of these experiments and computations thus provide unique insight into the inner workings of turbulence and shows how the nonlinear term in the Navier–Stokes equation on average forces the energy towards higher frequencies, which is the reason for the so-called energy cascade. • Downstream spectral development of two fundamental spatial Fourier modes is measured. • Experiments confirm the expected behavior of the non-linear interactions. • Downstream development results from repeated action of the Navier–Stokes equation. • Excellent agreement with numerical simulations. • Simulations reveals the details of the nonlinear triadic energy exchanges. [ABSTRACT FROM AUTHOR]

• The seal-vibrissa-shaped cylinder can weaken the regular vortex shedding in its wake. • The seal-vibrissa-shaped cylinder can mitigate the spanwise coherence of wake vortices. • The seal-vibrissa-shaped cylinder can suppresses the fluctuations on cylinder surfaces. • The seal-vibrissa-shaped cylinder can reduce the aerodynamic noise by inhibiting its tonal peak. • The configurations of seal-vibrissa-shaped cylinder can be applied for aerodynamic noise control of cylindrical-like bars. Inspired by the special structure of seal vibrissae and their ability to effectively suppress vortex-induced vibrations, this paper studies the characteristics of flow and aerodynamic noise generated around a seal-vibrissa-shaped cylinder at a Reynolds number (based on the hydrodynamic diameter and freestream velocity) of 6 × 104 using the delayed detached-eddy simulation model combined with the acoustic analogy approach. The far-field aerodynamic noise has also been measured at Reynolds numbers of 6 × 104∼1.2 × 105 in an anechoic wind tunnel to compare the seal-vibrissa-shaped cylinder with cylindrical and elliptical bars of the same characteristic dimensions. The calculated spectra of aerodynamic noise agree well with the wind tunnel measurements, verifying the accuracy of the numerical simulations. It is found that the three-dimensional flow separations introduced by the alternative saddle and nodal planes of the seal-vibrissa-shaped cylinder inhibit shear layer interactions and reduce the spanwise coherence, resulting in a wake with no dominant coherent structures and thus suppressing pressure fluctuations on the solid surfaces. The configuration of seal-vibrissa-shaped cylinder eliminates the regular Kármán vortex street which generally occurs in a cylinder wake, and consequently inhibits the tonal peak in the aerodynamic noise. The sound pressure level is also reduced in most frequencies. The consistency of the normalized spectra of the far-field aerodynamic noise and the vortex shedding frequency at different inflow speeds demonstrates the high accuracy of the anechoic wind tunnel measurements. Compared with cylinders with circular or elliptical cross sections, the seal-vibrissa-shaped cylinder effectively reduces the flow-induced noise by, respectively, 13 dB or 11 dB for a far-field receiver of 90° at Reynolds number of 6 × 104, showing great potential in reducing aerodynamic noise. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) and three-dimensional (3D) numerical simulations are carried out for uniform flow past a normal flat plate with two semicircular ends to investigate the three-dimensional effects of the wake flow behind the plate. The Reynolds numbers under investigation based on the inflow velocity and the width of the plate (H) are R e = 125, 200, 250, 500, and 1000. The instantaneous vortical structures show that the three-dimensionality characterized by streamwise vortical ribs becomes obvious at R e = 200. Their spatial densities increase with the increasing R e. The time-averaged drag coefficient remains almost the same from R e = 500 to 1000. The relative difference of the predicted drag coefficients between 2D and 3D simulations can be up to 77%. The amplitude of the lift force obtained using 2D simulations can be up to 30 times that obtained using 3D simulations. A drag force element decomposition analysis is conducted for the results obtained using 2D and 3D simulations. It is found that although the time-averaged wake flows obtained using the 2D and 3D simulations are different, the relative difference of their contributions to the time-averaged drag coefficient is only 10% while the difference comes mainly from the contribution of the Reynolds shear stress. The three-dimensionality of the wake flow is further investigated using quadrant and the Lumley triangle analysis of the Reynolds stress. The spanwise spatial scales of the streamwise vortices are quantified using Hilbert transform along the spanwise direction. The width of the streamwise vortices is approximately 0.5 – 1 H in the near wake region. The dependence of the spatial scales on R e is discussed. [ABSTRACT FROM AUTHOR]

In this paper, based on two lattice models (D3Q19 and D3Q27), two three-dimensional weighted-orthogonal multiple-relaxation-time pseudopotential lattice Boltzmann (WMRT-PLB) models with tunable thermodynamic consistency and surface tension are developed in which the high-order terms of the equilibrium density distribution function and discrete forcing term in moment space are eliminated, and thus, the implementation of the collision process is simplified. The Chapman–Enskog analysis shows that the WMRT-PLB models can correctly recover the macroscopic Navier–Stokes equations in the low Mach number limit. Then, six classical multiphase flows benchmark cases are performed to validate the performance of the proposed model. The numerical results of the first three cases indicate that the developed WMRT-PLB models effectively weaken the non-physical coupling between kinetic viscosity and density, enhance the numerical stability because of the low spurious velocity, improve the computational efficiency by about 25% because of the simplification of the collision process, and increase the numerical accuracy in the dynamic problems. Meanwhile, the numerical results of the last three cases with the density ratio of 857.7 and the kinetic viscosity ratio of 1/15 agree well with the analytical solutions and experimental results reported in the literature. Note that it is also found that the simulation of droplet bouncing is still stable even when the Reynolds number is more than 3000, which shows the good numerical stability of the proposed model. It has the potential to be applied to the simulation of the complex multiphase flows with large density ratio and large Reynolds number. [ABSTRACT FROM AUTHOR]

In the present study, a numerical simulation was conducted to investigate the influence of surface roughness on the aerodynamic performance of a 1.5-stage highly loaded axial compressor at low Reynolds number. It was especially considered how the roughness Reynolds number ( k + ) affected the change of the inlet and outlet conditions, the growth of the separation bubble (LSB), the status of the limiting streamline, and the patterns of the wake. Regarding the roughness settings, five roughness magnitudes and four roughness locations were mainly studied. The results showed that at low Re, surface roughness mainly improved the stage performance by reducing the length and width of the LSB as well as the rotor tip vorticity, delaying the occurrence of three-dimensional flow separation and increasing the turbulence level near the wall. However, it also aggravated the incoordination between the subsequent stages to a certain extent, which limited further improvement of the overall aerodynamic performance. Generally, with k + increasing, the compressor aerodynamic performance improved and achieved the best at k + = 137.8. The maximum increases in the total pressure ratio, peak efficiency, and chocked mass flow were approximately 4.01%, 5.34%, and 2.24%, respectively. In addition, for all the four roughness locations, the roughness covering from the leading edge to 50% of the axial chord length on the suction surface had a relatively evident advantage in improving the compressor peak efficiency because of the better control of the LSB and wall shear stress. [ABSTRACT FROM AUTHOR]

The Schmidt number, defined as the ratio of scalar to momentum diffusivity, varies by multiple orders of magnitude in real-world flows, with large differences in scalar diffusivity between temperature, solute, and sediment driven flows. This is especially crucial in gravity currents, where the flow dynamics may be driven by differences in temperature, solute, or sediment, and yet the effect of Schmidt number on the structure and dynamics of gravity currents is poorly understood. Existing numerical work has typically assumed a Schmidt number near unity, despite the impact of Schmidt number on the development of fine-scale flow structure. The few numerical investigations considering high Schmidt number gravity currents have relied heavily on two-dimensional simulations when discussing Schmidt number effects, leaving the effect of high Schmidt number on three-dimensional flow features unknown. In this paper, three-dimensional direct numerical simulations of constant-influx solute-based gravity currents with Reynolds numbers 100 ≤ R e ≤ 3000 and Schmidt number 1 are presented, with the effect of Schmidt number considered in cases with (R e , S c) = (100 , 10) , (100 , 100) , and (500, 10). These data are used to establish the effect of Schmidt number on different properties of gravity currents, such as density distribution and interface stability. It is shown that increasing Schmidt number from 1 leads to substantial structural changes not seen with increased Reynolds number in the range considered here. Recommendations are made regarding lower Schmidt number assumptions, usually made to reduce computational cost. [ABSTRACT FROM AUTHOR]

To investigate the flow field evolution and resultant aerodynamic characteristics of a square cylinder during flow acceleration, large-eddy simulations were undertaken to simulate flow with a dimensionless acceleration rate ap of 0.0048. The adopted ap resembles that is likely to be experienced during a severe downburst wind storm. The tested incidence angle α ranges from 0° to 45° and the time-dependent Reynolds number Re [Re = U(t)D/v, where U(t) is the time-dependent inflow velocity and D and v are the cylinder length and the kinematic viscosity, respectively] varies from 0 to 5 × 104 throughout the simulation. Results revealed that the flow around a square cylinder in accelerated flow evolves through three distinct temporal flow phases: namely, Phase I (Ia and Ib) where no vortex shedding (the instantaneous lift force through this period is zero) is observed, Phase II, where periodic vortex shedding and associated lift force oscillations are initiated and enhanced, and Phase III where stable vortex shedding occurs. Phase I can be further broken down into Phase Ia, which is characterized by laminar shear layer separation and the existence of steady recirculation vortices in the near wake region, and Phase Ib (only present at α = 0° and 45°) which is distinguished by the presence of quasi-steady recirculation vortices behind the cylinder. Aside from the variation of flow patterns, the transient lift and drag coefficients and the wind pressure distributions, as well as the spatial evolution of three-dimensional flow structures and pressure distributions, are also elucidated in detail. [ABSTRACT FROM AUTHOR]

• A 3D IB-LBM model is proposed to study particle interaction in Poiseuille flow. • A binding focusing phenomenon between hard and soft particles is reported. • Deformability, Reynolds number and initial positions affect the binding focusing. • Hydrodynamics balance in binding focusing analyzed by streamlines and pressure fields. In this paper, the inertial migration of particles in a periodic straight channel is studied numerically based on the immersed boundary-lattice Boltzmann method in two-dimensional and three-dimensional Poiseuille flow. In the model, on considering the inertial migration of particles, the interaction of a soft particle and a hard particle that are similar in size is studied. We observe an interesting particle interaction phenomenon named the binding focusing state (BFS). That is, when the focused soft particle gets close to the hard particle, both particles will switch their original runways, and then run together on a new equilibrium position. To investigate the characteristics of the BFS, the effects of the soft particle deformability, the Reynolds numbers and the initial particle positions on the BFS are studied in detail. The soft particle deformability and Reynolds numbers can affect the formation of the BFS. Moreover, the streamline and pressure around the particles are analyzed to show the formation laws of the BFS; this is more intuitive and aids in understanding the BFS. [ABSTRACT FROM AUTHOR]

We present an experimental and numerical study of the flow downstream of honeycomb flow straighteners for a range of Reynolds numbers, covering both laminar and turbulent flow inside the honeycomb cells. We carried out experiments with planar particle image velocimetry (PIV) in a wind tunnel and performed numerical simulations to perform an in-depth investigation of the three-dimensional flow field. The individual channel profiles downstream of the honeycomb gradually develop into one uniform velocity profile. This development corresponds with an increase in the velocity fluctuations which reach a maximum and then start to decay. The position and magnitude of the turbulence intensity peak depend on the Reynolds number. By means of the turbulence kinetic energy (TKE) budget it is shown that the production of TKE is dominated by the shear layers corresponding to the honeycomb walls. The near-field and far-field decay of the turbulence intensity can be described by power laws where we used the position where the production term of the TKE reaches its maximum as the virtual origin. [ABSTRACT FROM AUTHOR]

An incompressible Navier–Stokes solver has been developed to analyze the three-dimensional viscous flow around a pair of square cylinders at a low Reynolds number (Re = 100) at various arrangements. A semi-implicit finite difference scheme was employed on a staggered grid, which facilitated the easier application of boundary conditions while achieving satisfactory accuracy and stability. A third-order upwind (Kawamura–Kuwahara) scheme was applied to discretize the convective terms and a second-order central difference scheme to discretize the diffusive terms. Time integration was performed using a second-order accurate Adams–Bashforth scheme. An iterative pressure–velocity correction was used to obtain a divergence-free velocity field. Aspects of the flow such as vorticity profiles, the time evolution of lift and drag coefficients, and three-dimensionality appearance were studied. Three-dimensional characteristics ceased to appear in a tandem arrangement, irrespective of the gap ratio. In contrast, the side by side arrangement showed a clear three-dimensional transition in the flow when analyzing the vortex structures. Further investigations carried out for staggered arrangement keeping the center to center distance constant and varying the stagger angle (α) reveal that the three-dimensional behavior starts at α = 30° and keeps progressing with the angle increase. [ABSTRACT FROM AUTHOR]

Three-dimensional flow past surface mounted elliptic cylinder has been investigated numerically using OpenFOAM. Axis ratio (AR) of the elliptic cylinder is varied from 0.5 to 2. Effects of shear intensity (K) of the nonuniform incoming flow and Reynolds number (Re) varying from 150 to 250 on wake dynamics have been examined using Hilbert Huang transformation and Recurrence analysis. Qualitative and quantitative analyses of wake unsteadiness are illustrated using Hilbert spectra and degree of stationarity. Various recurrence quantification estimates have been reported to reveal insight about unsteady wake transition with the change in Re, AR and K. Four types of distinguishable wake flows, viz., steady, periodic, quasi-periodic and chaotic have been characterized with the help of vorticity contours, frequency spectra and bifurcation diagram. In aperiodic flow, wake oscillation contains multiple laminar states or slow changing states which has been disclosed using laminarity and trapping time. Nonstationarity and synchronous wake behavior along the height of the cylinder have been examined using cross-recurrence analysis and quantified in terms of recurrence rate, determinism and average diagonal length. In addition, the effect of change in K for each value of Re and AR has been studied on the mean drag coefficient of the cylinder. [ABSTRACT FROM AUTHOR]

In the present study, numerical investigations have been performed to study the flow transition mechanism in wavy channels using finite volume-based open source field operation and manipulation. Two different wavy channel configurations are chosen, which represent two different flow destabilization mechanisms, viz., Kelvin–Helmholtz and centrifugal instabilities. Sinusoidal walls with out-of-phase and in-phase channel configurations have been considered in the present study. Steady to chaotic flow transition in two different channel configurations are investigated by varying Reynolds number. A detailed flow regime map is presented for the two different wavy channel configurations. Unsteady flow features have been illustrated with the help of instantaneous streamlines, velocity contours, vorticity contours, and iso-Q surfaces. For the out-of-phase configuration, the flow changes from two-dimensional steady to two-dimensional unsteady in the Re range of 175–185, and then three-dimensional unsteady flow is observed for the Re varying from 250 to 260. On the contrary, for the in-phase configuration, the transition happens directly from steady two-dimensional flow (Re < 101) to unsteady three-dimensional (Re > 102) in a very narrow range of Re. Transitions in the two different wavy channels have been examined in detail using Hilbert–Huang transformation, phase-space reconstruction, Poincaré section, recurrence plot, and dynamic mode decomposition. Frequency, growth rate, and vortex structures of the dominant modes are illustrated corresponding to each value of Re for the considered channel configurations. [ABSTRACT FROM AUTHOR]

Kassem, H.; Sutherland, T.F., and Amos, C.L., 2021. Hydrodynamic controls on the particle size of resuspended sediment from sandy and muddy substrates in British Columbia, Canada. Journal of Coastal Research, 37(4), 691–707. Coconut Creek (Florida), ISSN 0749-0208. A benthic annular flume, Sea Carousel, was deployed at both sand-dominated (Baynes Sound) and mud-dominated (Carrie Bay) stations in British Columbia, Canada, to examine the character of near-bed flow over these contrasting bottom types and its control on particle size of resuspended sediment. An assessment has also been made of the turbidity-induced drag reduction due to suspension of bottom sediments. The median sizes of suspended material from the sandy sites have been compared with the well-known Rouse theory, whereas the aggregates resuspended from muddy stations were scaled with the energy dissipation rate (ε) determined from high-frequency three-dimensional flow measures in the flume. There was no evidence in the turbulence spectra in the Sea Carousel of energy inputs in the paddle and lid rotational frequencies, and a f–5/3 slope for f > 2 Hz in turbulent transitional flows was evident. The bed roughness length of sandy sites was Reynolds-number dependent but was asymptotic to a constant value of 2 mm at high flows. This equated to a dimensionless drag coefficient at 1 m above bed of a constant, 3 × 10–3 (also at high Reynolds numbers), which agrees well with values reported in the literature. The median size of suspended sand (from the sandy sites) and equivalent still water settling rate (ws) scaled with the friction velocity (u*) in the form ws/u* = D*/8. The median size of resuspended aggregates (df) scaled inversely with dissipation (ε) in the form df = 5 × 10–6ε–0.24m, which is close to the relationship found in the literature. [ABSTRACT FROM AUTHOR]

The response and recovery of turbulent pipe flow to three-dimensional perturbed wall changes were examined numerically in a wide range of Reynolds numbers between R e = 5 × 10 3 and 1.58 × 10 5 . The perturbations were based on distinct azimuthal Fourier modes corresponding to m = 3, 15, and 3 + 15. The long-lasting response of the flow was examined by characterizing both the mean and turbulent field in the wake of pipe inserts for each Re. The variation of the recovery with increasing Reynolds number revealed an asymptotic behavior for R e ≥ 7.5 × 10 4 , which scaled with Re4 for both mean velocity and turbulence kinetic energy. Two peaks were observed for the mean velocity along the wake centerline, where the location of peaks followed a power-law trend in the form of L p / D ∝ R e 4 / 3 , where D is the pipe diameter. A fast decay of turbulence past the wall change further suggested that maximum Reynolds shear stress in the downstream wake decays as (x / D) − 1 / 3 for all Re. The flow also exhibited long-lasting responses that obstructed its relaxation at 20D downstream of the perturbation, even for low Re of 5 × 10 3 . Overall, the recovery exhibited a second-order response. [ABSTRACT FROM AUTHOR]

Hill's vortex is a three-dimensional vortex structure form-preserving solution of the Euler equations (Hill in Philos Trans R Soc Lond A 185:213–245, 1894). For small amplitude axisymmetric disturbances on the external surface, the linear stability analysis by Moffat and Moore (J Fluid Mech 87:749–760, 1978) predicted the formation of a tail. Successive linear and nonlinear investigations confirmed this fact and in addition observed that the shape of the tail was linked to number of small amplitude azimuthal disturbances of the surface. In this paper, the Navier–Stokes equations are solved, at high Reynolds number, by imposing large amplitude axisymmetric and three-dimensional disturbances on the surface of the vortex. The axisymmetric disturbances are convected in the rear side, are dumped and form an axisymmetric wave increasing at the same rate as that in the linear stability analysis. The azimuthal disturbances produce a hierarchy of structures inside the vortex, and in a short-time evolution, the shape of the vortex is maintained. For a long-time evolution, direct numerical simulations show that Hill's vortex for azimuthal disturbances loses its original form for the formation of a wide range of energy containing scales characteristic of three-dimensional flows. Although a true turbulent state has not been reached, the DNS of this simple vortex structure shows the passage from a vortex dominated to a turbulent state. [ABSTRACT FROM AUTHOR]