Wake structure of a deformable Joukowski airfoil

We examine the vortical wake structure shed from a deformable Joukowski airfoil in an unbounded volume of inviscid and incompressible fluid. The deformable airfoil is considered to model a flapping fish. The vortex shedding is accounted for using an unsteady point vortex model commonly referred to as the Brown-Michael model. The airfoil’s deformations and rotations are prescribed in terms of a Jacobi elliptic function which exhibits, depending on a dimensionless parameter m, a range of periodic behaviors from sinusoidal to a more impulsive type flapping. Depending on the parameter m and the Strouhal number, one can identify five distinct wake structures, ranging from arrays of isolated point vortices to vortex dipoles and tripoles shed into the wake with every half-cycle of the airfoil flapping motion. We describe these regimes in the context of other published works which categorize wake topologies, and speculate on the importance of these wake structures in terms of periodic swimming and transient maneuvers of fish.     

Visualization study on suppression of vortex shedding from a cylinder

A narrow strip has been introduced as a control element to suppress vortex shedding from a cylinder. The strip is set parallel to the cylinder axis, and the key parameter of control in this study is the strip position, which is determined by the angle of attack of the strip and the distance between the strip and the cylinder axis. A circular cylinder and a square cylinder were tested respectively. Flow visualization and hot-wire measurement were performed in a low turbulence wind tunnel in the range of Reynolds numberRe=4.0×10³≈2.0×10⁴. Test results show that, vortex shedding from both sides of the cylinder can be effectively suppressed if the strip is located in a certain zone in the wake. The effective zones in circular cylinder wakes at different Reynolds numbers have been found out, and the mechanism of the suppression has been discussed.     

Control of a coupled map lattice model for vortex shedding in the wake of a cylinder

The flow behind a vibrating flexible cable at low Reynolds numbers can exhibit complex wake structures such as lace-like patterns, vortex dislocations and frequency cells. These structures have been observed in experiments and numerical simulations, and are predicted by a previously developed low-order coupled map lattice (CML). The discrete (in time and space) CML models consist of a series of diffusively coupled circle map oscillators along the cable span. Motivated by a desire to modify the complex wake patterns behind flexible vibrating cables we have studied the addition of control terms into the highly efficient CML models and explored the resulting dynamics. Proportional, adaptive proportional and discontinuous non-linear (DNL) control methods were used to derive the control laws. The first method employed occasional proportional feedback. The adaptive method used spatio-temporal feedback control. The DNL method used a discontinuous feedback linearization procedure, and the controller was designed for the resulting linearized system using eigenvalue assignment. These techniques were applied to a modeled vortex dislocation structure in the wake of a vibrating cable in uniform freestream flow. Parallel shedding patterns were achieved for a range of forcing frequency-forcing amplitude combinations studied to validate the control theory. The adaptive proportional and DNL methods were found to be more effective than the proportional control method due to the incorporation of a spatially varying feedback gain across the cylinder span. The DNL method was found to be the most efficient controller of the low-order CML model. The required control level across the cable span was correlated to the 1/1 lock-on behavior of the temporal circle map.     

On the velocity and dissipation signature of vortex tubes in isotropic turbulence

The properties of vortex tubes are extracted and analyzed from a DNS database at various Re_λ, with the objective to characterize the associated distributions of induced velocity and kinetic energy dissipation. The induced velocity exhibits an inverse power-law scaling in the far field, different from Burgers’ r⁻¹ scaling, supporting the interpretation that tubes are the remnants of vortex sheets after roll-up, and suggesting a possible link with the Kolmogorov k^−5/3 spectral scaling. The energy dissipation signature is characterized by a local maximum near the edge of the vortex core, and an absolute peak at its center, which can be tentatively explained appealing to the occurrence of a bi-axial configuration of the strain-rate tensor.     

Coherence and Reynolds stresses in the turbulent wake behind a curved circular cylinder

The turbulent wake behind a curved circular cylinder is investigated based on data obtained from a direct numerical simulation. Here, emphasis is placed in the assessment of two approaches for simplified modelling: reduced-order modelling (ROM) and Reynolds-averaged Navier–Stokes equations. To this end, the instantaneous vortical structures, the proper orthogonal decomposition (POD) of the flow, and relevant Reynolds stress components have been analysed. The results show that despite the complexity of the instantaneous vortical structures, the wake dynamics are governed by the quasi-periodic shedding of primary vortices. Between 24% and 50% of the kinetic energy in the POD is captured by the two most energetic modes, and about 200 modes are needed to capture 90% of the kinetic energy. These findings suggest that, as long as the large-scale structures of the von Kármán vortex shedding are concerned, the present case can be approached by ROM; but a detailed representation of the flow dynamics without an eddy viscosity model that accounts for the unresolved turbulent fluctuations would require a large amount of degrees of freedom. Concerning the Reynolds stresses, their magnitude varies considerably depending on the depth at which they have been sampled. This dependence is related to the strength of the vortex shedding, and the intensity of the secondary flows induced by the curvature of the cylinder. As a consequence of the combination of these two effects, the correlation between streamwise and vertical velocity fluctuations is highest in the wake behind the midspan of the curved cylinder, and the correlation between cross-flow and vertical velocity fluctuations reaches large values in the lower wake.     

Vortex shedding and vorticity fluxes in the wake of cylinders within a random array

This work investigates the impact of the background turbulence generated by randomly placed cylinders on the vortex shedding regime and the mechanisms associated to vorticity fluxes. The goals are achieved by exploring velocity databases acquired with a two-dimensional particle image velocimetry system in two types of turbulent flow experiments: flow around a single infinite cylinder and flow within random array of infinite cylinders. Formation lengths, power spectral density functions and vortex distributions are employed to discuss the vortex shedding regime. The effects of background turbulence and vorticity cancellation, due to opposite sign vorticity, on the vorticity fluxes are discussed. The results show that the background turbulence reduces the formation length and consequently increase the shedding frequency. The stronger decay of longitudinal vorticity flux in denser arrays is not accompanied by an increase of the lateral flux of vorticity. Furthermore, it was concluded that the decay of longitudinal vorticity flux is mainly caused by the vorticity cancellation due to the vorticity of opposite sign of close downstream cylinders.     

The vortex physics which comes from the vortex core

A theoretical view of vortex core states and of their effects on physics of vortices in clean s- and d-wave-type II superconductors is presented based on a semi-classical picture of a vortex core as an Andreev potential well containing many quasiparticle states. We discuss the density of states, the vortex dissipation, Hall effect, and the vortex mass. The dynamic characteristics are determined by relaxation of core excitations driven by a moving vortex. In a d-wave superconductor, gap nodes make the core states more extended and introduce novel features into thermodynamics and kinetics of vortices.     

Lattice Boltzmann simulations of flow past a cylindrical obstacle

We present lattice Boltzmann simulations of flow past a cylindrical obstacle. Our study is based on the Lévy walk model of turbulence in a lattice Boltzmann model. We discuss pressure around the cylinder with laminar and turbulent incident flows, as well as the dependence of the von Karman street on the analog of integral scale in our model.     

Luminescence spectrum from hexagonal YMnO3

Luminescence spectrum and its temperature dependence have been investigated for hexagonal YMnO₃ crystal which is ferroelectric below T_C=914 K and becomes simultaneously antiferromagnetic below T_N=74 K. The luminescence spectrum is as broad as two-magnon Raman scattering spectrum. The luminescence intensity is comparable to the latter at room temperature and it increases rapidly as the temperature decreases. These characteristics are discussed based upon other optical responses and theoretical results.     

Interaction of a submerged horseshoe vortex with a free surface

In this paper a submerged horseshoe vortex under a free surface is discussed and the algebraic expression of the wave elevation is obtained. From this expression, some characteristics of the ship wave are described. There exists a smooth region near $\theta = 0^\circ$, but when the uniform stream passes the other singularities (source, sink, doublet, etc.) there is no smooth region. The mechanism of synthetic aperture radar (SAR) images of the narrow ship wakes is also explained.     

Formation of a symmetric vortex configuration in a pure electron plasma trapped with a penning trap

In the formation of vortical crystal structures of electrons, the distribution of background electrons works as a “cooler” of randomly moving, intense vortices. We examine the supporting effect of background electrons to forming crystal structures from the perspective of a reduction of the vortices’ random motion.     

Direct visualization of the vortex distributions in a superconducting film with a Penrose array of magnetic pinning centers: Symmetry induced giant vortex state

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8H₁, H₁ and 1.6H₁, where H₁ corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H₁ corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6H₁ the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6H₁ interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.     

Vortex splitting in the interference effect for a quantized vortex released from an optical lattice

For a quantized vortex in a harmonic trap and an optical lattice, we study the interference effect after the combined potentials are switched off. Both numerical and analytical calculations show that there is a vortex splitting in the interference effect of this freely expanding quantum gas, i.e. every interference peak is also a quantized vortex. The experimental scheme to verify this interference effect is discussed.     

Possible vortex production by a vibrating wire in superfluid He–B

We observe spectacular features on the force–velocity characteristic of a vibrating wire resonator in superfluid ³He–B at ultralow temperatures. There are both plateaux and discontinuities in the characteristic. The plateaux seem to have two separate critical velocities where firstly some ‘event’ occurs, which causes the wire to lose energy and slow down, followed by a second critical velocity where the ‘event’ decouples. The shape of the pulses so created depends on the precise position on the particular plateau. At the low-velocity end of the plateau, the pulses show a sudden decrease in velocity followed by a gradual growth, whereas at the high-velocity end the pulses consist of a sudden increase in velocity followed by a gradual decrease. Eventually, the pulsing ceases indicating a saturation effect. We speculate that these events are due to vortex creation and annihilation/separation.     

Vortex generation in the cross-flow around a cylinder attached to an end-wall

A cylinder attached to an end-wall normal to its axis is a common feature of many practical flow systems, e.g. in turbo-machinery or when a bridge is supported by a pillar from the bed of a river. In this situation, the nominally two-dimensional boundary layer flow incident upon the cylinder develops strong three-dimensional features and a very pronounced vortex structure may arise in the upstream flow close to the wall. For the appropriate Reynolds number range, the upstream vortical structure is nominally steady and is commonly referred to as the “horseshoe vortex system”. In contrast, the flow downstream is unsteady and periodic over a wide range of Reynolds numbers and vortices aligned with the cylinder axis are shed at a regular frequency into the wake. The generation of both these vortex systems requires energy to be extracted from the incident flow with the result that the drag force on the cylinder is increased.This paper concentrates on the upstream region of the cylinder and discusses an investigation in which two-component Particle Image Velocimetry (PIV) has been used to visualise the flow behaviour for a circular cylinder on a plane end-wall. The use of PIV has enabled two orthogonal velocity components to be measured in planes defined by the upstream flow direction and the axis of the cylinder. The third (out-of-plane) velocity component was then calculated by integrating the continuity equation. Subsequently, the velocity field information has been manipulated and converted into time-averaged information.Discussion of the measured results confirms that colour displays are an invaluable aid to understanding this complex fluid flow situation since they reveal substantially more information than grey-scale plots of the same data. In particular, the source of the horseshoe vortex system can be identified when colour plots of the time-averaged velocity and vorticity distributions are obtained. A limited amount of information on the unsteady vortex structures appearing in the end-wall region upstream of the cylinder is also presented. Finally, the experimental findings are discussed in relation to the results of previous workers.     

Irregular mixing due to a vortex pair interacting with a fixed vortex

The paper examines scalar advection caused by a point–vortex pair encountering a fixed point vortex in a uniform flow. The interaction produces two types of vortex motion. First is unbounded as the pair moves unrestrictedly after encountering the fixed vortex. The scalar exchanging between the pair's bubble and fixed vortex's neighbourhood is numerically estimated. Second is bounded as the pair's vortices periodically oscillate about the fixed vortex. The pair's periodic motion perturbs scalar motion causing a portion of scalar trajectories to manifest chaotic behaviour. We analyse scalar transport using Poincaré sections, which reveal regular and chaotic transport regions.