Regions with reduced and increased values of total enthalpy are observed in a time-averaged flow behind a bluff body. This energy redistribution takes place both in the vortex formation region and in the developed vortex wake. The present paper focuses on studying the effect of the structure of a vortex street on the intensity of energy redistribution. Two approaches are used. The first one is direct numerical simulation of the flow behind a transversely oscillating cylinder, which is known for a variety of vortex patterns in the wake. The simulations are based on a finite element solution of the Navier-Stokes equations for a compressible perfect viscous gas. The second approach is based on simplified point vortex models for infinite periodic vortex streets, which contain a finite number of vortex chains in equilibrium. It turns out that these simple models make it possible to obtain satisfactory qualitative results, particularly if a more precise approximation of velocity fields in the vortex cores (Rankine vortices) is implemented. It is shown that the effect of energy redistribution significantly depends on the vortex structure, namely the mutual arrangement of the vortices and their intensities. The estimates of the energy separation efficiency in the time-averaged flow are obtained for the general case of an arbitrary number of chains. A more detailed analysis is performed for vortex streets with 2, 3, and 4 vortex chains.
The Karman Vortex Street has been investigated for over a century and offers a reference case for investigation of flow stability and control of high dimensionality, non-linear systems. Active flow control, while of considerable interest from a theoretical point of view and for industrial applications, has remained inaccessible due to the difficulty in finding successful control strategies. Here we show that Deep Reinforcement Learning can achieve a stable active control of the Karman vortex street behind a two-dimensional cylinder. Our results show that Deep Reinforcement Learning can be used…
The two-dimensional laminar vortex streets behind a triangle have two morphologically distinct structures depending on the Reynolds number and the aspect ratio of the triangle. These two structures are the conventional structure and the separated rows structure, where the latter is characterized by a thin layer of irrotational fluid between two vortex rows. In this paper, by means of numerical simulation, we find that the separated rows structure occurs when the thickness of boundary layers is less than 25% of their separation distance. We also show from the linear stability analysis that the …
We discuss two distinct spatial structures of vortex streets. The `conventional mushroom' structure is commonly discuss in many experimental studies, but the exotic `separated rows' structure is characterized by a thin irrotational fluid between two rows of vortices. In a two-dimensional soap film channel, we generated the exotic vortex arrangement by using triangular objects. This setup allows us to vary the thickness of boundary layers and their separation distance independently. We find that the separated rows structure appears only when the boundary layer thickness is less than 40% of the …
The splashing of a drop impacting onto a liquid pool produces a range of different sized micro-droplets. At high impact velocities, the most significant source of these droplets is a thin liquid jet emerging at the start of the impact from the neck that connects the drop to the pool. We use ultra-high-speed video imaging in combination with high-resolution numerical simulations to show how the ejecta gives way to irregular splashing. At higher Reynolds number, its base becomes unstable, shedding vortex rings into the liquid from the free surface in an axisymmetric von Kármán vortex street, thu…
The vortex streets produced by a flapping foil of span-to-chord aspect ratio of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the mechanisms giving rise to the symmetry breaking of the reverse Bénard-von Kármán vortex street that characterizes fish-like swimming and forward flapping flight are examined. Two-dimensional particle image velocimetry measurements in the mid-plane perpendicular to the span axis of the foil are used to characterize the different flow regimes. The deflection angle of the mean jet flow with respect to the horizontal observed in the average velocit…
Phase reduction theory has been applied to many systems with limit cycles; however, it has limited applications in incompressible fluid systems. This is because the calculation of the phase sensitivity function, one of the fundamental functions in phase reduction theory, has a high computational cost for systems with a large degree of freedom. Furthermore, incompressible fluid systems have an implicit expression of the Jacobian. To address these issues, we propose a new algorithm to numerically calculate the phase sensitivity function. This algorithm does not require the explicit form of the J…
A new Strouhal-Reynolds number relationship, $St=1/(A+B/Re)$, has been recently proposed based on observations of laminar vortex shedding from circular cylinders in a flowing soap film. Since the new $St$-$Re$ relation was derived from a general physical consideration, it raises the possibility that it may be applicable to vortex shedding from bodies other than circular ones. The work presented herein provides experimental evidence that this is the case. Our measurements also show that in the asymptotic limit ($Re\rightarrow\infty$), $St_{\infty}=1/A\simeq0.21$ is constant independent of rod s…
Three types of streamline topology in a Karman vortex street flow are shown under the variation of spatial parameters. For the motion of dilute particles in the Kármán vortex street flow, there exist a route of bifurcation to a chaotic orbit and more attractors in a bifurcation diagram for the proportion of particle density to fluid density. Along with the increase of spatial parameters in the flow filed, the bifurcation process is suspended, as well as more and more attractors emerge. In the motion of dilute particles, a drag term and gravity term dominate and result in the bifurcation phenom…
The unsteady hydrodynamics of two in-phase pitching foils arranged in side-by-side (parallel) configurations is examined for a range of Strouhal number and separation distance. Three distinct vortex patterns are identified in the Strohual number-separation distance phase maps, which include separated wake, merged wake, and transitional-merged wake. Furthermore, a novel model is introduced based on fundamental flow variables including velocity, location, and circulation of dipole structures to quantitatively distinguish vortex patterns in the wake. The physical mechanism of wake merging process…
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 Kármán 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 transiti…
The knifefish species propels itself by generating a reverse Karman street from an anal fin and without significantly moving its body. This unique feature makes this species' propulsion method highly efficient (Blake, 1983). It has been suggested that there is an optimal swimming range for fish based on the amplitude and frequency of the reverse Kármán street. Experiments have been performed to measure the ratio between the amplitude and wavelength of vortices in the wake of a knifefish. It is suggested that by optimizing the thrust created by the reverse Karman street the wave efficiency can …
Although the stability properties of the wake past impervious bluff bodies have been widely examined in the literature, similar analyses regarding the flow around and through porous ones are still lacking. In this work, the effect of the porosity and permeability on the wake patterns of porous rectangular cylinders is numerically investigated at low to moderate Reynolds numbers in the framework of direct numerical simulation combined with local and global stability analyses. A modified Darcy-Brinkman formulation is employed here so as to describe the flow behavior inside the porous media, wher…
The effects of cavitation on the flow around a circular-cylinder array are studied by using a molecular dynamics simulation. Cavitation significantly affects on vortex shedding characteristics. As the cavitation develops,the vibration acting on the cylinders decreases and eventually disappears. The further cavitation development generates a longer vapor region next to the cylinders, and the vortex streets are formed at further positions from the cylinders. The neighboring Karman vortexes are synchronized in the antiphase in the absence of the cavitation. This synchronization is weakened by the…
We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using 2D Particle Image Velocimetry (PIV). An analysis in terms of a flapping frequency-amplitude phase space allows to identify: 1) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and 2) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significanc…
The two-dimensional flows around a cylinder between two parallel walls at Re=40 and Re=100 are simulated with computational fluid dynamics (CFD). The governing equations are Navier-Stokes equations. They are discretized with finite volume method (FVM) and the solution is iterated with PISO Algorithm. Then, the calculating results are compared with the numerical results in literature, and good agreements are obtained. After that, the mechanism of the formation of Karman vortex street is investigated and the instability of the entire flow field is analyzed with the energy gradient theory. It is …
The effects of a stably-stratified boundary layer on flow and dispersion in a bi-dimensional street canyon have been investigated experimentally in a wind tunnel in combination with differential wall heating. Laser-Doppler anemometry together with a fast flame ionisation detector and cold-wire anemometry were employed to sample velocities, concentration, temperatures and fluxes. A single-vortex pattern was observed in the isothermal case, preserved also in the leeward case, but with an increment of the vortex speed. Heating the windward wall was found to generate a counter-rotating vortex, res…
Room temperature compressed air was pumped into a short (H/D = 0.18) vortex chamber. Experiments revealed temperature separation. The highest temperature of the periphery was 465 degC, and the lowest temperature of the central zone was -45 degC. During the process, heat was transferred to periphery in the opposite direction of the powerful vortex flow. This heat transfer cannot be explained by conventional heat transfer processes. To explain this phenomenon, the concept of Pressure Gradient Waves (PGW) is proposed in the paper. PGW are elastic waves which operate in compressible fluids with pr…
Reconnection plays a significant role in the dynamics of plasmas, polymers and macromolecules, as well as in numerous laminar and turbulent flow phenomena in both classical and quantum fluids. Extensive studies in quantum vortex reconnection show that the minimum separation distance δ between interacting vortices follows a 1/2 scaling. Due to the complex nature of the dynamics (e.g., the formation of bridges and threads as well as successive reconnections and avalanche), such scaling has never been reported for (classical) viscous vortex reconnection. Using the direct numerical simulation of t…
Motivated by recent experimental and computational results concerning the three-dimensional structure of vortices behind a vortex shedding flow meter [M. Reik et al., Forsch. Ingenieurwes. 74, 77 (2010)], we study the Strouhal-Reynolds number dependence in the vortex street in two dimensions behind a trapezoid-shaped object by employing two types of Frisch-Hasslacher-Pomeau (FHP) models. Our geometry is intended to reproduce the operation of the vortex shedding flow meter in a two-dimensional setting, thus preventing the formation of three-dimensional vortex structure. In particular, we check …
Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient in reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures formed immediately downstr…
For problems with multiple bodies, the current integral approach needs the use of auxiliary potential functions in order to have an individual force formula for each body. While the singularity approach, based on an extension of the unsteady Lagally theorem, is restricted to multibody and multivortex flows without bound vortex and vortex production. In this paper, we consider multibody and multivortex flow and derive force formulas, in both forms of singularity approach and integral approach but without auxiliary function, that give individual forces of each body for unsteady two dimensional p…
Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters,…
In this paper a model for viscous boundary and shear layers in three-dimensions is introduced and termed a vortex-entrainment sheet. The vorticity in the layer is accounted for by a conventional vortex sheet. The mass and momentum in the layer are represented by a two dimensional surface having its own internal tangential flow. Namely, the sheet has a mass density per-unit-area making it dynamically distinct from the surrounding outer fluid. The mechanism of entrainment is represented by a discontinuity in the normal component of velocity across the sheet. The sheet mass is able to support a p…
Hysteresis has been observed in a study of the transition between laminar flow and vortex shedding in a quasi-two dimensional system. The system is a vertical, rapidly flowing soap film which is penetrated by a rod oriented perpendicular to the film plane. Our experiments show that the transition from laminar flow to a periodic Kármán vortex street can be hysteretic, i.e. vortices can survive at velocities lower than the velocity needed to generate them.
In this paper, the implicitly implemented LES method and fifth order bandwidth optimized WENO scheme are used to make comprehensive studies on the separation topology of the MVG controlled flow at M=2.5 and Reθ=5760. Experiments are also made to verify the prediction of the computation. Analyses are conducted on three categories of the topology: the surface separation, cross-section separation and the three dimensional structure of the vortices. A complete description about the separation topology and a series of new findings are obtained. Among them, a pair of spiral point is first predicted …
This article presents the results of simulation for a special type of vortex tubes - self-vacuuming vortex tube (SVVT), for which extreme values of temperature separation and vacuum are realized. The main results of this study are the flow structure in the SVVT and energy loss estimations on oblique shock waves, gas friction, instant expansion and organization of vortex bundles in SVVT.
We study the behavior of an elastic loop embedded in a flowing soap film. This deformable loop is wetted into the film and is held fixed at a single point against the oncoming flow. We interpret this system as a two-dimensional flexible body interacting in a two-dimensional flow. This coupled fluid-structure system shows bistability, with both stationary and oscillatory states. In its stationary state, the loop remains essentially motionless and its wake is a von Kármán vortex street. In its oscillatory state, the loop sheds two vortex dipoles, or more complicated vortical structures, within e…
A two-dimensional computational fluid dynamics model is used to predict the oscillatory flow through a tapered cylindrical tube section (jet pump) placed in a larger outer tube. Due to the shape of the jet pump, there will exist an asymmetry in the hydrodynamic end effects which will cause a time-averaged pressure drop to occur that can be used to cancel Gedeon streaming in a closed-loop thermoacoustic device. The performance of two jet pump geometries with different taper angles is investigated. A specific time-domain impedance boundary condition is implemented in order to simulate traveling …
The task of dynamic flow estimation is to construct an approximation of an evolving flow---and particularly, its response to disturbances---using measurements from available sensors. Building from previous work by Darakananda et al.~(Phys Rev Fluids 2018), we further develop an ensemble Kalman filter (EnKF) framework for aerodynamic flows based on an ensemble of randomly-perturbed inviscid vortex models of flow about an infinitely-thin plate. In the forecast step, vortex elements in each ensemble member are advected by the flow and new elements are released from each edge of the plate; the ele…
Ring vortices are efficient at transporting fluid. They are often produced by ejecting a volume of fluid through a circular orifice. The impulse given to the vortex rings results in a propulsive force on the generator. Propulsive vortex rings have been widely studied and characterised. After four convective times, the vortex moves faster than the shear layer it originates from, and separates. This separation corresponds to a maximum in circulation and a maximal spread of the vorticity, quantified by the non-dimensional energy. The simultaneity of these three events obfuscates the causality bet…
This study presents a numerical investigation of how fluid viscoelasticity influences the flow dynamics past a transversely forced oscillating cylinder in the laminar vortex shedding regime at a fixed Reynolds number of 100. In particular, we examine how fluid viscoelasticity influences the boundary between the lock-in and no lock-in zones and the associated wake topology compared to that seen in a simple Newtonian fluid. All in all, we find that the fluid viscoelasticity facilitates the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than that re…
We compute the solutions of Prandtl's and Navier-Stokes equations for the two dimensional flow induced by a rectilinear vortex interacting with a boundary in the half plane. For this initial datum Prandtl's equation develops, in a finite time, a separation singularity. We investigate the different stages of unsteady separation for Navier-Stokes solution at different Reynolds numbers $Re=10^3-10^5$, and we show the presence of a large-scale interaction between the viscous boundary layer and the inviscid outer flow. We also see a subsequent stage, characterized by the presence of a small-scale i…
Two-dimensional numerical simulations have been performed to understand the effect of wall proximity on the vortex-induced vibrations (VIV) of an elastically mounted circular cylinder having two degrees of freedom for gap ratios g/D = {0.1,0.2,0.3,0.4,0.5,0.6}, where D is the cylinder diameter, and g is the gap between the cylinder's bottom surface and the wall. Parametric simulations have been performed using a quasi-monolithic coupled fluid-structure interaction solver with exact interface tracking for Re = 100 and m*=10 over a range of U* to present the effect of the wall's proximity on the…
We use holography to investigate the dynamics of a vortex-anti-vortex dipole in a strongly coupled superfluid in 2+1 dimensions. The system is evaluated in numerical real-time simulations in order to study the evolution of the vortices as they approach and eventually annihilate each other. A tracking algorithm with sub-plaquette resolution is introduced which permits a high-precision determination of the vortex trajectories. With the increased precision of the trajectories it becomes possible to directly compute the vortex velocities and accelerations. We find that in the holographic superflui…
Flow past a circular cylinder is investigated in the subcritical regime, below the onset of Benard-von Karman vortex shedding at Re_c ~ 47. The transient response of infinitesimal perturbations is computed. The domain requirements for obtaining converged results is discussed at length. It is shown that energy amplification occurs as low as Re=2.2. Throughout much of the subcritical regime the maximum energy amplification increases approximately exponentially in the square of Re reaching 6800 at Re_c$. The spatiotemporal structure of the optimal transient dynamics is shown to be transitory Bena…
There are different ways for fluid flow to become turbulent, but usually many instabilities take place before that. Among these instabilities the transition from laminar flow to vortex shedding carries significant practical importance. Here we study a flow, where at high enough flow rates, vortices are generated by a fixed obstacle (cylinder), which penetrates a flowing, quasi-two dimensional soap film. We present experimental results that demonstrate that the transition from laminar flow to a periodic Kármán vortex street can be hysteretic, i.e. changing the mean flow rate $\bar V$ vortices c…
Large eddy simulations (LES) are employed to investigate the role of time-varying currents on the form drag and vortex dynamics of submerged 3D topography in a stratified rotating environment. The current is of the form $U_c+U_t \sin(2πf_t t)$, where $U_c$ is the mean, $U_t$ is the tidal component and $f_t$ is its frequency. A conical obstacle is considered in the regime of low Froude number. When tides are absent, eddies are shed at the natural shedding frequency $f_{s,c}$. The relative frequency $f^*=f_{s,c}/f_t$ is varied in a parametric study which reveals states of high time-averaged form…
Bioinspired Tapered-Spring Turbulence Sensor for Underwater Flow Detection
Dyson School of Design Engineering, Imperial College London（帝国理工学院伦敦校区设计工程学院）；Imperial College London（帝国理工学院）；Department of Computer Science, University of Aberdeen（阿伯丁大学计算机科学系）
