Showing papers on "Vortex shedding published in 2016"
TL;DR: In this paper, a new vortex identification criterion called W -method is proposed based on the ideas that vorticity overtakes deformation in vortex and W = 0.52 is a quantity to approximately define the vortex boundary.
Abstract: A new vortex identification criterion called W -method is proposed based on the ideas that vorticity overtakes deformation in vortex. The comparison with other vortex identification methods like Q -criterion and l 2-method is conducted and the advantages of the new method can be summarized as follows: (1) the method is able to capture vortex well and very easy to perform; (2) the physical meaning of W is clear while the interpretations of iso-surface values of Q and l 2 chosen to visualize vortices are obscure; (3) being different from Q and l 2 iso-surface visualization which requires wildly various thresholds to capture the vortex structure properly, W is pretty universal and does not need much adjustment in different cases and the iso-surfaces of W =0.52 can always capture the vortices properly in all the cases at different time steps, which we investigated; (4) both strong and weak vortices can be captured well simultaneously while improper Q and l 2 threshold may lead to strong vortex capture while weak vortices are lost or weak vortices are captured but strong vortices are smeared; (5) W =0.52 is a quantity to approximately define the vortex boundary. Note that, to calculate W , the length and velocity must be used in the non-dimensional form. From our direct numerical simulation, it is found that the vorticity direction is very different from the vortex rotation direction in general 3-D vortical flow, the Helmholtz velocity decomposition is reviewed and vorticity is proposed to be further decomposed to vortical vorticity and non-vortical vorticity.
305 citations
TL;DR: In this article, a survey of existing studies on different methods of controlling the wake destructive behavior and suppression of vortex shedding behind bluff bodies are discussed, including the very recent developments, and the effectiveness of each technique on the drag reduction.
176 citations
TL;DR: In this paper, the authors present an experimental and numerical study that describes the motion of flexible blades, scaled to be dynamically similar to natural aquatic vegetation, forced by wave-induced oscillatory flows.
143 citations
27 Sep 2016
TL;DR: In this paper, a series of results provided by three-dimensional numerical simulations of nearly spheroidal bubbles freely rising and deforming in a still liquid in the regime close to the transition to path instability was reported.
Abstract: We report on a series of results provided by three-dimensional numerical simulations of nearly spheroidal bubbles freely rising and deforming in a still liquid in the regime close to the transition to path instability. These results improve upon those of recent computational studies [Cano-Lozano et al., Int. J. Multiphase Flow 51, 11 (2013); Phys. Fluids 28, 014102 (2016)] in which the neutral curve associated with this transition was obtained by considering realistic but frozen bubble shapes. Depending on the dimensionless parameters that characterize the system, various paths geometries are observed by letting an initially spherical bubble starting from rest rise under the effect of buoyancy and adjust its shape to the surrounding flow. These include the well-documented rectilinear axisymmetric, planar zigzagging, and spiraling (or helical) regimes. A flattened spiraling regime that most often eventually turns into either a planar zigzagging or a helical regime is also frequently observed. Finally, a chaotic regime in which the bubble experiences small horizontal displacements (typically one order of magnitude smaller than in the other regimes) is found to take place in a region of the parameter space where no standing eddy exists at the back of the bubble. The discovery of this regime provides evidence that path instability does not always result from a wake instability as previously believed. In each regime, we examine the characteristics of the path, bubble shape, and vortical structure in the wake, as well as their couplings. In particular, we observe that, depending on the fluctuations of the rise velocity, two different vortex shedding modes exist in the zigzagging regime, confirming earlier findings with falling spheres. The simulations also reveal that significant bubble deformations may take place along zigzagging or spiraling paths and that, under certain circumstances, they dramatically alter the wake structure. The instability thresholds that can be inferred from the computations compare favorably with experimental data provided by various sets of recent experiments guaranteeing that the bubble surface is free of surfactants.
97 citations
TL;DR: In this paper, a three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400, 000, with an angle of attack of A o A = 5 ∘.
95 citations
TL;DR: The regular-to-turbulent transition of the vortex cluster shedding reveals remarkable similarities between a superfluid and a classical viscous fluid.
Abstract: We report on the experimental observation of vortex cluster shedding from a moving obstacle in an oblate atomic Bose-Einstein condensate. At low obstacle velocities v above a critical value, vortex clusters consisting of two like-sign vortices are generated to form a regular configuration like a von Karman street, and as v is increased, the shedding pattern becomes irregular with many different kinds of vortex clusters. In particular, we observe that the Stouhal number associated with the shedding frequency exhibits saturation behavior with increasing v. The regular-to-turbulent transition of the vortex cluster shedding reveals remarkable similarities between a superfluid and a classical viscous fluid. Our work opens a new direction for experimental investigations of the superfluid Reynolds number characterizing universal superfluid hydrodynamics.
90 citations
TL;DR: In this paper, a combination of mathematical theory, scaling analysis and measurement is used to establish that this large-amplitude flapping motion is a vortex-induced vibration, verifying previous speculation based on a two-point measurement.
Abstract: The dynamics of a cantilevered elastic sheet, with a uniform steady flow impinging on its clamped end, have been studied widely and provide insight into the stability of flags and biological phenomena. Recent measurements by Kim et al. (J. Fluid Mech., vol. 736, 2013, R1) show that reversing the sheet’s orientation, with the flow impinging on its free edge, dramatically alters its dynamics. In contrast to the conventional flag, which exhibits (small-amplitude) flutter above a critical flow speed, the inverted flag displays large-amplitude flapping over a finite band of flow speeds. The physical mechanisms giving rise to this flapping phenomenon are currently unknown. In this article, we use a combination of mathematical theory, scaling analysis and measurement to establish that this large-amplitude flapping motion is a vortex-induced vibration. Onset of flapping is shown mathematically to be due to divergence instability, verifying previous speculation based on a two-point measurement. Reducing the sheet’s aspect ratio (height/length) increases the critical flow speed for divergence and ultimately eliminates flapping. The flapping motion is associated with a separated flow – detailed measurements and scaling analysis show that it exhibits the required features of a vortex-induced vibration. Flapping is found to be periodic predominantly, with a transition to chaos as flow speed increases. Cessation of flapping occurs at higher speeds – increased damping reduces the flow speed range where flapping is observed, as required. These findings have implications for leaf motion and other biological processes, such as the dynamics of hair follicles, because they also can present an inverted-flag configuration.
83 citations
TL;DR: In this paper, a three-dimensional two-phase model for sediment transport in the sheet flow condition, incorporating recently published rheological characteristics of sediments, was developed, which is applicable to a wide range of particle Reynolds number.
Abstract: Sediment transport is fundamentally a two-phase phenomenon involving fluid and sediments; however, many existing numerical models are one-phase approaches, which are unable to capture the complex fluid-particle and inter-particle interactions. In the last decade, two-phase models have gained traction; however, there are still many limitations in these models. For example, several existing two-phase models are confined to one-dimensional problems; in addition, the existing two-dimensional models simulate only the region outside the sand bed. This paper develops a new three-dimensional two-phase model for simulating sediment transport in the sheet flow condition, incorporating recently published rheological characteristics of sediments. The enduring-contact, inertial, and fluid viscosity effects are considered in determining sediment pressure and stresses, enabling the model to be applicable to a wide range of particle Reynolds number. A k − e turbulence model is adopted to compute the Reynolds stresses. In addition, a novel numerical scheme is proposed, thus avoiding numerical instability caused by high sediment concentration and allowing the sediment dynamics to be computed both within and outside the sand bed. The present model is applied to two classical problems, namely, sheet flow and scour under a pipeline with favorable results. For sheet flow, the computed velocity is consistent with measured data reported in the literature. For pipeline scour, the computed scour rate beneath the pipeline agrees with previous experimental observations. However, the present model is unable to capture vortex shedding; consequently, the sediment deposition behind the pipeline is overestimated. Sensitivity analyses reveal that model parameters associated with turbulence have strong influence on the computed results.
75 citations
TL;DR: This work challenges the status quo by presenting a numerical model able to deal with the numerical stability and accuracy challenges arising from the need to simulate the high liquid-gas density ratio across the sharp interfaces encountered in these flows and employing it in simulations of liquid jet in crossflow atomization.
Abstract: Recent advances in numerical methods coupled with the substantial enhancements in computing power and the advent of high performance computing have presented first principle, high fidelity simulation as a viable tool in the prediction and analysis of spray atomization processes. The credibility and potential impact of such simulations, however, has been hampered by the relative absence of detailed validation against experimental evidence. The numerical stability and accuracy challenges arising from the need to simulate the high liquid-gas density ratio across the sharp interfaces encountered in these flows are key reasons for this. In this work we challenge this status quo by presenting a numerical model able to deal with these challenges, employing it in simulations of liquid jet in crossflow atomization and performing extensive validation of its results against a carefully executed experiment with detailed measurements in the atomization region. We then proceed to the detailed analysis of the flow physics. The computational model employs the coupled level set and volume of fluid approach to directly capture the spatiotemporal evolution of the liquid-gas interface and the sharp-interface ghost fluid method to stably handle high liquid-air density ratio. Adaptive mesh refinement and Lagrangian droplet models are shown to be viable options for computational cost reduction. Moreover, high performance computing is leveraged to manage the computational cost. The experiment selected for validation eliminates the impact of inlet liquid and gas turbulence and focuses on the impact of the crossflow aerodynamic forces on the atomization physics. Validation is demonstrated by comparing column surface wavelengths, deformation, breakup locations, column trajectories and droplet sizes, velocities, and mass rates for a range of intermediate Weber numbers. Analysis of the physics is performed in terms of the instability and breakup characteristics and the features of downstream flow recirculation, and vortex shedding. Formation of “Λ” shape windward column waves is observed and explained by the combined upward and lateral surface motion. The existence of Rayleigh-Taylor instability as the primary mechanism for the windward column waves is verified for this case by comparing wavelengths from the simulations to those predicted by linear stability analyses. Physical arguments are employed to postulate that the type of instability manifested may be related to conditions such as the gas Weber number and the inlet turbulence level. The decreased column wavelength with increasing Weber number is found to cause enhanced surface stripping and early depletion of liquid core at higher Weber number. A peculiar “three-streak-two-membrane” liquid structure is identified at the lowest Weber number and explained as the consequence of the symmetric recirculation zones behind the jet column. It is found that the vortical flow downstream of the liquid column resembles a von Karman vortex street and that the coupling between the gas flow and droplet transport is weak for the conditions explored.
75 citations
TL;DR: In this paper, a numerical study on vortex-induced vibration (VIV) of a vertical riser subject to uniform and linearly sheared currents is presented. And the predicted numerical results are in good agreement with the experimental data.
74 citations
TL;DR: In this article, a bi-stable piezoelectric cantilever harvester is evaluated in a wind tunnel and a Von Karman vortex street is generated by placing a rectangular rod in the windward direction of the harvesters.
Abstract: Linear wake-galloping flow energy harvesters have a narrow frequency bandwidth restricted to the lock-in region, where the vortex shedding frequency is close to the natural frequency of the harvester. As a result, their performance is very sensitive to variations in the flow speed around the nominal design value. This letter demonstrates that the lock-in region of a wake-galloping flow energy harvester can be improved by exploiting a bi-stable restoring force. To demonstrate the enhanced performance, the response behavior of a bi-stable piezoelectric cantilever harvester is evaluated in a wind tunnel. A Von Karman vortex street is generated by placing a rectangular rod in the windward direction of the harvester and the voltage response of the harvester is evaluated as a function of the wind speed. It is shown that, compared to the linear design, bi-stability can be used to improve the steady-state bandwidth considerably.
TL;DR: In this paper, the authors investigated the lock-in in vortex-induced vibration of a circular cylinder in the laminar flow regime using direct time integration (DTI) and linear stability analysis.
Abstract: The phenomenon of lock-in in vortex-induced vibration of a circular cylinder is investigated in the laminar flow regime ( ). Direct time integration (DTI) and linear stability analysis (LSA) of the governing equations are carried out via a stabilized finite element method. Using the metrics that have been proposed in earlier studies, the lock-in regime is identified from the results of DTI. The LSA yields the eigenmodes of the coupled fluid–structure system, the associated frequencies ( ) and the stability of the steady state. A linearly unstable system, in the absence of nonlinear effects, achieves large oscillation amplitude at sufficiently large times. However, the nonlinear terms saturate the response of the system to a limit cycle. For subcritical , the occurrence of lock-in coincides with the linear instability of the fluid–structure system. The critical is the Reynolds number beyond which vortex shedding ensues for a stationary cylinder. For supercritical , even though the aeroelastic system is unstable for all reduced velocities ( ) lock-in occurs only for a finite range of . We present a method to estimate the time beyond which the nonlinear effects are expected to be significant. It is observed that much of the growth in the amplitude of cylinder oscillation takes place in the linear regime. The response of the cylinder at the end of the linear regime is found to depend on the energy ratio, , of the unstable eigenmode. is defined as the fraction of the total energy of the eigenmode that is associated with the kinetic and potential energy of the structure. DTI initiated from eigenmodes that are linearly unstable and whose energy ratio is above a certain threshold value lead to lock-in. Interestingly, during lock-in, the oscillation frequency of the fluid–structure system drifts from towards a value that is closer to the natural frequency of the oscillator in vacuum ( ). In the event of more than one eigenmode being linearly unstable, we investigate which one is responsible for lock-in. The concept of phase angle between the cylinder displacement and lift is extended for an eigenmode. The phase angle controls the direction of energy transfer between the fluid and the structure. For zero structural damping, if the phase angle of all unstable eigenmodes is less than 90°, the phase angle obtained via DTI evolves to a value that is close to 0°. If, on the other hand, the phase angle of any unstable eigenmode is more than 90°, it settles to 180°, approximately in the limit cycle. A new approach towards classification of modes is presented. The eigenvalues are tracked over a wide range of while keeping and mass ratio ( ) fixed. In general, for large values of , the eigenmodes corresponding to the two leading eigenvalues exhibit a decoupled behaviour with respect to . They are classified as the fluid and elastic modes. However, for relatively low such a classification is not possible. The two leading modes are coupled and are referred to as fluid–elastic modes. The regime of such occurrence is shown on the parameter space.
TL;DR: In this article, a lattice Boltzmann method based numerical framework is established, in which the multi-block scheme and the overlap-mesh approach with improved information exchange mechanisms are used to balance the computational accuracy and efficiency.
TL;DR: In this article, the authors performed direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles and found that the particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude.
Abstract: Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are , while the solid to fluid density ratio . The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14 % for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated with the intermittent fast sedimentation of particle pairs in drafting–kissing–tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment.
TL;DR: In this article, the phase lag was shown to be a nonlinear function of the spacing ratio, Strouhal number and convection velocity of vortices in the gap between the cylinders.
TL;DR: In this article, the three-dimensional dynamics of a pair of counter-rotating streamwise vortices that are present in the wake of an ICE3 high-speed train typical of modern, streamlined vehicles in operation, is investigated in a 1/10th-scale wind-tunnel experiment.
TL;DR: It is demonstrated that topology optimization can successfully account for unsteady effects such as vortex shedding and time-varying boundary conditions in moderate Reynolds number flows.
TL;DR: In this paper, a physics-based feedback control is applied to symmetrize the bimodal dynamics of a turbulent blunt body wake with two lateral slit jets and monitored with pressure sensors at the rear surface.
Abstract: Feedback control is applied to symmetrize the bimodal dynamics of a turbulent blunt body wake. The flow is actuated with two lateral slit jets and monitored with pressure sensors at the rear surface. The physics-based controller is inferred from preliminary open-loop tests and is capable of symmetrizing the wake. A slight pressure recovery is achieved due to the net balance between the favourable effect of wake symmetrization and adverse effect of shear-layer mixing and vortex shedding amplification.
TL;DR: In this article, the results obtained from the numerical analysis regarding prediction of unsteady vortex behavior at the draft tube and flow instabilities specific to Francis hydro turbines occurring at the part load operating regime, using ANSYS CFX.
TL;DR: In this article, a sinusoidal wavy cylinder with a large spanwise wavelength and a constant wave amplitude was investigated using three dimensional (3D) large eddy simulation (LES) at a subcritical Reynolds number Re=3×103, based on incoming free stream velocity (U∞) and Dm.
TL;DR: In this article, a homogeneous mixture model is used to study cavitation over a circular cylinder at two different Reynolds numbers ( and 3900) and four different cavitation numbers (, 1.0, 0.7 and 0.5).
Abstract: A homogeneous mixture model is used to study cavitation over a circular cylinder at two different Reynolds numbers ( and 3900) and four different cavitation numbers ( , 1.0, 0.7 and 0.5). It is observed that the simulated cases fall into two different cavitation regimes: cyclic and transitional. Cavitation is seen to significantly influence the evolution of pressure, boundary layer and loads on the cylinder surface. The cavitated shear layer rolls up into vortices, which are then shed from the cylinder, similar to a single-phase flow. However, the Strouhal number corresponding to vortex shedding decreases as the flow cavitates, and vorticity dilatation is found to play an important role in this reduction. At lower cavitation numbers, the entire vapour cavity detaches from the cylinder, leaving the wake cavitation-free for a small period of time. This low-frequency cavity detachment is found to occur due to a propagating condensation front and is discussed in detail. The effect of initial void fraction is assessed. The speed of sound in the free stream is altered as a result and the associated changes in the wake characteristics are discussed in detail. Finally, a large-eddy simulation of cavitating flow at and is studied and a higher mean cavity length is obtained when compared to the cavitating flow at and . The wake characteristics are compared to the single-phase results at the same Reynolds number and it is observed that cavitation suppresses turbulence in the near wake and delays three-dimensional breakdown of the vortices.
TL;DR: In this article, a small aeroacoustic wind tunnel on a set of circular cylinders with a soft porous cover was used to investigate the possible reduction of noise from struts and other protruding parts.
Abstract: The use of porous materials is one of several approaches to passively control or minimize the generation of flow noise. In order to investigate the possible reduction of noise from struts and other protruding parts (for example components of the landing gear or pantographs), acoustic measurements were taken in a small aeroacoustic wind tunnel on a set of circular cylinders with a soft porous cover. The aim of this study was to identify those materials that result in the best noise reduction, which refers to both tonal noise and broadband noise. The porous covers were characterized by their air flow resistivity, a parameter describing the permeability of an open-porous material. The results show that materials with low air flow resistivities lead to a noticeable flow noise reduction. Thereby, the main effect of the porous cylinder covers is that the spectral peak of the aeolian tone due to vortex shedding appears much narrower, but is not suppressed completely. Based on the measurement results, a basic model for the estimation of the total peak level of the aeolian tone was derived. In addition to the minimization of the vortex shedding noise, a reduction of broadband noise can be observed, especially at higher Reynolds numbers. The noise reduction increases with decreasing air flow resistivity of the porous covers, which means that materials that are highly permeable to air result in the best noise reduction.
TL;DR: In this paper, the wall proximity effects on the vortex-induced vibrations (VIV) of an elastically mounted circular cylinder with two degree-of-freedom (2-DoF) are systematically studied in two-dimensional (2D) laminar flow at Reynolds number, Re = 200 based on the diameter of cylinder.
TL;DR: In this article, the authors investigated the wake of a high-speed train in a wind-tunnel experiment and found that the wake region is of considerable importance as it is where slipstream velocities induced by the vehicles movement through air are largest.
TL;DR: In this article, the dynamics of the formation and growth of the leading edge vortex and the corresponding unsteady aerodynamic torque induced by large-scale flow-induced oscillations of an elastically mounted flat plate were investigated.
Abstract: We report on the dynamics of the formation and growth of the leading-edge vortex and the corresponding unsteady aerodynamic torque induced by large-scale flow-induced oscillations of an elastically mounted flat plate. All experiments are performed using a high-bandwidth cyber-physical system, which enables the user to access a wide range of structural dynamics using a feedback control system. A series of two-dimensional particle image velocimetry measurements are carried out to characterize the behaviour of the separated flow structures and its relation to the plate kinematics and unsteady aerodynamic torque generation. By modulating the structural properties of the cyber-physical system, we systematically analyse the formation, strength and separation of the leading-edge vortex, and the dependence on kinematic parameters. We demonstrate that the leading-edge vortex growth and strength scale with the characteristic feeding shear-layer velocity and that a potential flow model using the measured vortex circulation and position can, when coupled with the steady moment of the flat plate, accurately predict the net aerodynamic torque on the plate. Connections to previous results on optimal vortex formation time are also discussed.
TL;DR: In this article, a flexible cylinder of circular cross-section towed behind a stationary circular cylinder at various distances is described and the dynamics of the model immersed in the wake of the leading one are fully characterised after data processing and computation of amplitude and frequency responses, as well as modal content as a function of reduced velocity.
TL;DR: In this article, numerical simulation of the interaction between two counter-rotating longitudinal vortices was carried out for the plate-fin heat exchanger using two rows of delta winglet type vortex generators.
TL;DR: In this article, a curved flexible pipe was studied experimentally in the concave configuration and the results reveal that different vibration frequencies exist at different positions of the pipe with the presentation of multi-mode-response, and the excited modes vary with the incoming speed.
TL;DR: In this article, the authors studied the resonances of a forced turbulent wake past a flat-based bluff body using symmetric and antisymmetric actuation modes and showed that the robustness of the subharmonic resonance is confirmed at different Reynolds numbers.
Abstract: We study the resonances of a forced turbulent wake past a flat-based bluff body using symmetric and antisymmetric actuation modes. The natural, unforced wake flow exhibits broadband dynamics superimposed on oscillatory motions linked to the reminiscent laminar Benard-von Karman instability in the turbulent flow. Harmonic and subharmonic resonances can be controlled by the phase relationship of periodic forcing and are linked to the symmetry properties of vortex shedding. Symmetric forcing leads to a strong subharmonic amplification of vortex shedding in the wake, but no harmonic excitation. The robustness of the subharmonic resonance is confirmed at different Reynolds numbers. Antisymmetric actuation, however, promotes a harmonic resonance with very similar wake and drag features.
TL;DR: In this paper, a model for the transition from room acoustics to duct acoustic systems is presented. But the model is limited to the case of micro-perforated plates (MPPs).