Two-dimensional Vortex Structures Research Articles

Seal vibrissa-based three-fourths open-jet wind tunnel nozzle optimization

This study investigated the effects of using bionics in reducing the low-frequency fluctuation phenomenon in three-fourths open-jet automobile wind tunnels. Low-frequency fluctuation in these wind tunnels can cause pulsations of velocity and pressure in the test section, leading to measurement accuracy reduction. The unique shape of the seal vibrissa had been demonstrated to effectively suppress vortex-induced vibrations. Here the wind tunnel nozzle was optimized with seal-vibrissa-shaped protrusions. The effects and corresponding mechanism of seal-vibrissa-shaped nozzle toward jet shear layer vortex structures and low-frequency fluctuation phenomenon were analyzed. A 1:15 scaled three-fourths open-jet wind tunnel was used in the study. The results indicated that after the implementation of the seal-vibrissa-shaped nozzle, the turbulence intensity was decreased at the bottom of the test section and the pressure and velocity fluctuations were effectively suppressed. The three-dimensional vortex structures separating from the surface of vibrissa destroyed the original two-dimensional vortex structures produced by the original nozzle. Many small vortex structures with dispersed frequencies and energies were formed, which decreased the low-frequency fluctuations. The study showed that the seal-vibrissa-shaped nozzle had the potential of inhibiting the large-scale vortex separation in the jet shear layer, which could effectively reduce the low-frequency fluctuations.

Experimental and numerical investigation of bow wave and wake flows around towed surface-piercing cylinders

An investigation was conducted to study the flow characteristics around towed surface-piercing cylinders and understand the mechanisms behind the bow wave and wake region formation. The experiments took place in a towing tank using three different cylinder models, covering a range of Froude numbers based on cylinder diameter from 0.28 to 1.73. Three-cylinder models are considered M1 is circular, M2 is semicircular ellipse and M3 is elliptical. Numerical simulations were performed using the OpenFOAM framework, considering turbulence effects through the unsteady Reynolds-Averaged Navier Stokes equations with the Shear Stress Transport k-ω turbulence model. The numerical results matched well with the experimental data. The bow wave's general features were similar across all cylinder models, with the height increasing as the Froude number rose. Froude numbers between 0.28 and 0.58 displayed a smooth surge surface, transitioning to a rough surface with roll-up initiation between 0.87 and 1.15. At Froude numbers 1.44 to 1.73, a fountain-like bow wave formed. Models M1 and M2 had the same average bow wave height, approximately 15% higher than that of Model M3. The depression depth or air ventilation at the trailing edge of the cylinders increased with the Froude number. Model M1 exhibited a greater depression depth than Models M2 and M3 at Froude number 1.15, and this difference further increased at Froude number 1.73 due to high vorticity and adverse pressure gradient generated by Model M1. Vortex coherent structures visualization revealed that at a low Froude number of 0.58, only the Karman vortex at the trailing edge was observed. At Froude number 1.15, the deep wake region maintained predominantly two-dimensional vortex structures, while the free surface displayed pronounced three-dimensional vortex structures. Model M1 had a drag coefficient approximately 40% higher than Model M3. These findings are valuable for engineering applications, particularly in the design of submarine masts.

Characteristics of geometry-and pressure-induced laminar separation bubbles at an enhanced level of free-stream turbulence

Responses of a geometry-induced separation bubble (GISB) and a pressure-induced separation bubble (PISB) at enhanced levels of free-stream turbulence (FST) have experimentally been investigated for a comparative study using the particle image velocimetry (PIV) technique. The outlines of separation bubbles based on the dividing streamlines are self-similar for different levels of FST and Reynolds numbers. The spectral analyses of the time-resolved PIV data show that the vortex shedding frequency of a separated shear layer remains unchanged for the GISB cases even with an enhanced level of FST. In contrast, it is different for the PISB cases. We propose a criterion that determines whether the frequency will remain the same even for the cases with FST. Linear stability analyses reveal that the inviscid-inflectional instability dominates the transition process, and the linear stages of transition are not completely bypassed even at an enhanced level of FST. The most amplified frequencies, while scaling with the displacement thickness and the boundary layer edge velocity, collapse in a single curve for all the cases. Furthermore, measurements in the spanwise plane show that the streamwise velocity streak/Klebanof mode at an enhanced level of FST is not a general flow feature for all types of separation bubbles. However, at an enhanced level of FST for the PISB case, the boundary layer streaks are found to distort the two-dimensional vortex structure associated with the Kelvin–Helmholtz instability, eventually leading to a three-dimensional $\varLambda$ -like structure in the spanwise plane.

Computational Study of the Plasma Actuator Flow Control for an Airfoil at Pre-Stall Angles of Attack

Large-eddy simulations of the flows over an NACA0015 airfoil were conducted to investigate a flow control authority of a dielectric barrier discharge plasma actuator at pre-stall angles of attack. The Reynolds number was set to 63,000, and angles of attack were set to 4, 6, 8, and 10 degrees. The plasma actuator was installed at 5% chord length from the airfoil’s leading edge. Good flow control authority was confirmed in terms of lift-to-drag ratio increase and drag reduction. These improvements mainly result from the reduction of the pressure drug, which is due to the change in pressure distribution accompanying the movement and shrink of the laminar separation bubble on the airfoil surface. Additionally, although flow control using a burst drive with a nondimensional burst frequency of six improves the lift-to-drag ratio at all angles of attack, the phenomena leading to the improvement differ between near-stall angles (10 and 12 degrees) and the other lower angles. At near-stall angles, the turbulent transition is rapidly promoted by PA, and the flow is reattached. Whereas, at the lower angles, the transport of two-dimensional vortex structures, which maintain their structures up to downstream and suppress the turbulent transition, makes the flow reattachment.

Numerical simulation of bubbly flow around a cylinder by semi-Lagrangian–Lagrangian method

Purpose The purpose of this study is to design numerical simulations of bubbly flow around a cylinder to better understand the characteristics of flow around a rigid obstacle. Design/methodology/approach The bubbly flow around a circular cylinder was numerically simulated using a semi-Lagrangian–Lagrangian method composed of a vortex-in-cell method for the liquid phase and a Lagrangian description of the gas phase. Additionally, a penalization method was applied to account for the cylinder inside the flow. The slip condition of the bubbles on the cylinder’s surface was enforced, and the outflow conditions were applied to the liquid flow at the far field. Findings The simulation clarified the characteristics of a bubbly flow around a circular cylinder. The bubbles were shown to move around and separate from both sides of the cylinder, because of entrainment by the liquid shear layers. Once the bubbly flow fully developed, the bubbles distributed into groups and were dispersed downstream of the cylinder. A three-dimensional vortex structure of various scales was also shown to form downstream, whereas a quasi-stable two-dimensional vortex structure was observed upstream. Overall, the proposed method captured the characteristics of a bubbly flow around a cylinder well. Originality/value A semi-Lagrangian–Lagrangian approach was applied to simulate a bubbly flow around a circular cylinder. The simulations provided the detail features of these flow phenomena.

Two-dimensional vortex structures in the bottom boundary layer of progressive and solitary waves

AbstractThe two-dimensional vortices characterizing the bottom boundary layer of both progressive and solitary waves, recently discovered by experimental flow visualizations and referred to as vortex tubes, are studied by numerical solution of the governing equations. In the case of progressive waves, the Reynolds numbers investigated belong to the subcritical range, according to Floquet linear stability theory. In such a range the periodic generation of strictly two-dimensional vortex structures is not a self-sustaining phenomenon, being the presence of appropriate ambient disturbances necessary to excite certain modes through a receptivity mechanism. In a physical experiment such disturbances may arise from several coexisting sources, among which the most likely is roughness. Therefore, in the present numerical simulations, wall imperfections of small amplitude are introduced as a source of disturbances for both types of wave, but from a macroscopic point of view the wall can be regarded as flat. The simulations show that even wall imperfections of small amplitude may cause flow instability and lead to the appearance of vortex tubes. These vortices, in turn, interact with a vortex layer adjacent to the wall and characterized by vorticity opposite to that of the vortex tubes. In a first stage such interaction gives rise to corrugation of the vortex layer and this affects the spatial distribution of the wall shear stress. In a second stage the vortex layer rolls up and pairs of counter-rotating vortices are generated, which leave the bottom because of the self-induced velocity.

Dynamics of isolated vortices in shallow flows

Turbulent shallow flows are characterized by large-scale strongly two-dimensional vortex structures that are triggered and grow under the kinematic and dynamic conditions of shallowness. Shallow mixing layers, wakes and jets are prominent examples in hydro-environmental and geophysical flows, in which a series of such vortex structures, interacting with each other, are generated by continuous forcing. Isolated turbulent shallow vortices are key elements in the fluid mechanics of such shallow flows. Experimental investigations on two types of shallow vortices generated by instantaneous forcing have been conducted: the single forced vortex that evolves into a tri-polar vortex system, and the vortex couple (dipole). Scaling of the vorticity transport equation shows that the gradual spin-down of these vortices is governed by the bottom friction acting at the base of the vortex flow. A simple spin-down model is in good agreement with the observed data with different regimes for the initial turbulent stage and the final laminar stage. It is argued that the turbulent stage is of direct relevance for actual hydro-environmental and geophysical flows. Hence, laboratory experimentation must be performed at a sufficiently large scale to capture a Reynolds-invariant regime.

Bubble deformation and flow structure measured by double shadow images and PIV/LIF

An experiment on bubble motion in a simple shear layer was performed in order to obtain fundamental knowledge of the force on the bubble and its lateral motion induced by the surrounding flow field. We explored the flow structure in the vicinity of the bubble in one plane and its deformation in two planes by particle image velocimetry (PIV)–laser-induced fluorescence (LIF) and a projection technique for two perpendicular planes, respectively. For our experiment, we chose a single air bubble with an equivalent bubble diameter D eq of 2~6 mm in a vertical shear flow. Velocity measurements were made using a digital high-speed CCD camera for PIV with fluorescent tracer particles. The second and third CCD cameras were used to detect the bubble’s shape and motion via backlighting from an array of infrared LEDs. We quantitatively studied the three-dimensional wake structure from measurements of the two-dimensional vortex structure and approximated three-dimensional shape deformation arranged from two perpendicular bubble images.

Effects of magnetic field on vortex dynamics in (Tl,Pb)(Sr,Ba)2Ca2Cu3Oy single crystal

Abstract The flux dynamics of a (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O y single crystal have been investigated by means of magnetic relaxation measurements, with the applied field parallel to the c -axis. A power law dependence of the activation energy on the current density for fields less than 2 T over the temperature range 15–45 K has been observed, consistent with three-dimensional large bundle vortex creep. For intermediate fields (2–3 T), the flux dynamics are dominated by plastic vortex creep, while for fields higher than 3 T over the temperature range 16–46 K, the vortex glass scaling approach implied that a two-dimensional vortex structure is unlikely for fields less than 5 T.

Structure of a two-dimensional vortex behind a highly heated cylinder

The two-dimensional vortex structure behind a highly heated cylinder has been studied experimentally. The air velocity was set at 1.0 m/s. The surface temperature of the heated cylinder was varied up to 600 °C. Particle image velocimetry (PIV) was used to measure the two-dimensional instantaneous and average velocities. Just downstream of the heated cylinder, the vortex was intensified by increasing the wall temperature of the heated cylinder. The vortex frequency decreased when the wall temperature increased. This indicates that the local kinematic viscosity plays a key role in the vortex structure. As a result, the vortex frequency of the heated cylinder can be expressed as a function of the Strouhal and Reynolds numbers, when the local kinematic viscosity is reasonably estimated by taking into account the effect of the wall heating.

Small angle neutron diffraction studies of vortex structures in high temperature superconductors

We have used neutron scattering to provide direct information about flux structures in the bulk of crystals of the superconductor Bi 2Sr 2CaCu 2O 8. Its extremely high effective mass anisotropy, makes the flux lattice susceptable to melting and also to decomposition into ‘pancake’ vortices, which would give a more two-dimensional vortex structure. At low temperatures and fields the scattered intensity is consistent with a three dimensional flux-line structure. At higher fields and temperatures, the scattering from the flux lattice dissapears well below T c. We can associate this dissappearance with the above changes in the vortex structure. We compare the neutron scattering results with macroscopic measurements of magnetisation.

On Chaplygin's investigations of two-dimensional vortex structures in an inviscid fluid

This paper describes exact solutions of two-dimensional vortex structures that were published by Chaplygin (1899, 1903) at the turn of the last century, which seem to have escaped the attention of later investigators in this field. Chaplygin's solutions include that of an elliptical patch of uniform vorticity in an exterior field of pure shear and that of a (symmetric or non-symmetric) dipolar vortex with a continuous distribution of vorticity translating steadily along a straight path. In addition, a solution is presented for a non-symmetric vortex dipole moving along a circular trajectory. A concise account of Chaplygin's solutions is given, complemented by a more detailed analysis of some of their relevant properties.

Interacting two-dimensional vortex structures: Point vortices, contour kinematics and stirring properties

This paper addresses the stirring properties of interacting two-dimensional vortex structures. It is well known from previous studies that essential features of dipole-dipole and dipole-monopole interactions, like partner exchange, are captured by simple point-vortex models of these structures. In order to investigate the mixing induced by such interactions, the method of contour kinematics is adopted here, which allows us to trace fluid particles that are initially confined within some closed contour. For example, by tracing the deformations of the separatrix enclosing the ‘atmosphere’ of a point-vortex dipole during the interaction with another single point vortex or vortex pair, the contour kinematics method provides information about the processes of entrainment and detrainment that take place during the interaction. Comparison of the modelling results with both full numerical simulations and laboratory experiments on dye-visualised vortex interactions in a stratified fluid reveals a surprisingly good qualitative agreement.

Two-dimensional vortex structures in an anisotropic plasma

Two-dimensional vortex structures in an anisotropic plasma

Response of plane shear layers and wakes to strong three-dimensional disturbances

Shear layers and wakes formed downstream of a splitter plate whose trailing edge has spanwise variations were briefly explored using the reacting flow visualization technique. A fundamental difference in behavior was observed between the two types of flows. The shear layer (which has only one sign of mean vorticity) rapidly forms its characteristic two-dimensional vortex structures, whereas, in contrast, the wake (which has both signs of mean vorticity) forms closed vortex loops. These loops do not grow appreciably in the spanwise direction over the test section length, even though the transverse growth is comparable to the two-dimensional wake behavior. The global structure of the shear layer quickly forgets the initial perturbations, while the wake remembers them. The results strongly suggest that the presence of only one or both signs of mean vorticity in these plane turbulent flows has a fundamental bearing on the spanwise structure and stability of the vorticity field.