Measurement of vortex shedding in the wake of a sphere at

Flow in the wake of a sphere has been studied for at least the last hundred years. The three-dimensional (3-D) flow structure has been observed many times using dye visualization and prior to the direct numerical simulations by Johnson & Patel (J. Fluid Mech., vol. 378, 1999, pp. 19–70), its structure at a Reynolds number of approximately 300, was believed to consist of a one-sided chain of hairpin-like vortices. However, the numerical simulations by Johnson & Patel (J. Fluid Mech., vol. 378, 1999, pp. 19–70) also showed that so-called ‘induced’ vortices were generated. The present results are the first spatially resolved measurements that elucidate the 3-D vortex shedding cycle in the wake of a sphere at a Reynolds number of 465. Tomographic particle image velocimetry (tomo-PIV) enabled snapshots of the vortical structure and by combining these results with temporally resolved planar PIV, the ensemble averaged shedding cycle in the wake of the sphere was reconstructed. The present results clearly indicate that besides the ‘primary’ vortex chain shed from the sphere, secondary (‘induced’) vortices are generated by transforming transverse vorticity into streamwise vorticity as a result of the interaction between the sphere’s separating shear layer and the counter-rotating longitudinal vortices extending downstream from the sphere.

Tricolor mechanochromic luminescence of an organic two-component dye: visualization of a crystalline state and two amorphous states

The tricolor mechanochromic luminescence (MCL) of a two-component mixture of a poor MCL dye and a non-MCL dye is described.

Controlling The Flow Structure of Circular Cylinder by Using Concentrically Installed Perforated Cylinder and Splitter Plate

The purpose of this work was to control undesired structure and vibrations caused from vortex shedding by controlling the flow downstream of the cylinder. The porosities of the outer cylinder and the splitter plate used for this purpose are available. The other parameter was plate angle based on the flow direction as a reference. The parameter expressed as porosities, ß was chosen to have a %70 openness for the outer cylinder (ß = 0.7) and was kept constant throughout all experiments. On the other hand, two different porosities were used for the splitter plate. For the first plate, there was no hole on the plate which we call ß = 0 or solid plate. For the second plate, it has ß = 0.7 porosity. ß = 0.7 porosity was also the value chosen for the outer cylinder. The range of the splitter plate angle was selected within 60o? ? ?180o with an increment of 30o. The results were extracted using two different methods. The first was the dye visualization experiment and the second is the PIV (Particle Image Velocimetry) measurement. As a result, it was observed that independently from the outer cylinder effect, the plate porosity and plate angle are effective on the level of turbulence and the flow structure in the annular region.

The kinematic genesis of vortex formation due to finite rotation of a plate in still fluid

We present a combined experimental and numerical study of an idealized model of the propulsive stroke of the turning manoeuvre in fish. Specifically, we use the framework of Lagrangian coherent structures (LCSs) to describe the kinematics of the flow that results from a thin plate performing a large angle rotation about its tip in still fluid. Temporally and spatially well-resolved velocity fields are obtained using a two-dimensional, incompressible finite-volume solver, and are validated by comparisons with experimentally measured velocity fields and alternate numerical simulations. We then implement the recently proposed variational theory of LCSs to extract the hyperbolic and elliptic LCSs in the numerically generated velocity fields. Detailed LCS analysis is performed for a plate motion profile described by $\dot{\unicode[STIX]{x1D703}}(t)=\unicode[STIX]{x1D6FA}_{max}\sin ^{2}(\unicode[STIX]{x1D714}t)$ during $0\leqslant t\leqslant t_{o}$ and zero otherwise. The stopping time $t_{o}$ is given by $t_{o}=\unicode[STIX]{x03C0}/\unicode[STIX]{x1D714}=10~\text{s}$, the value of $\unicode[STIX]{x1D6FA}_{max}$ chosen to give a stopping angle of $\unicode[STIX]{x1D703}_{max}=90^{\circ }$, resulting in a Reynolds number $Re=c^{2}\unicode[STIX]{x1D6FA}_{max}/\unicode[STIX]{x1D708}=785.4$, where $c$ is the plate chord length and $\unicode[STIX]{x1D708}=10^{-6}~\text{m}^{2}~\text{s}^{-1}$ the kinematic viscosity of water. The flow comprises a starting and a stopping vortex, resulting in a pair of oppositely signed vortices of unequal strengths that move away from the plate in a direction closely aligned with the final plate orientation at $t/t_{o}\approx 2$. The hyperbolic LCSs are shown to encompass the fluid material that is advected away from the plate for $t>t_{o}$, henceforth referred to as the advected bulk. The starting and stopping vortices, identified using elliptic LCSs and hence more objective than Eulerian vortex detection methods, constitute only around two thirds of the advected bulk area. The advected bulk is traced back to $t=0$ to identify five distinct lobes of fluid that eventually form the advected bulk, and hence map the long-term fate of various regions in the fluid at $t=0$. The five different lobes of fluid are then shown to be delineated by repelling LCS boundaries at $t=0$. The linear momentum of the advected bulk region is shown to account for approximately half of the total impulse experienced by the plate in the direction of its final orientation, thus establishing its dynamical significance. We provide direct experimental evidence for the kinematic relevance of hyperbolic and elliptic LCSs using novel dye visualization experiments, and also show that attracting hyperbolic LCSs provide objective characterization of the spiral structures often observed in vortical flows. We conclude by showing that qualitatively similar LCSs persist for several other plate motion profiles and stopping angles as well.

Flow Behavior in Side-View Plane of Pitching Delta Wing

In the present investigation, a delta wing which has 70° sweep angle, Λ was oscillated on its midcord according to the equation of α(t)=αm+α0sin(ωet). This study focused on understanding the effect of pitching and characterizing the interaction of vortex breakdown with oscillating leading edges under different yaw angles, β over a slender delta wing. The value of mean angle of attack, αm was taken as 25°. The yaw angle, β was varied with an interval of 4° over the range of 0°≤β≤ 16°. The delta wing was sinusoidally pitched within the range of period of time 5s≤Te≤60s and reduced frequency was set as K=0.16, 0.25, 0.49, 1.96 and lastly amplitude of pitching motion was arranged as α0=±5°.Formations and locations of vortex breakdown were investigated by using the dye visualization technique in side view plane.

Dye Visualization of In-Line Twin Synthetic Jets in Crossflows—A Parametric Study

This paper presents a parametric study on the interaction of twin circular synthetic jets (SJs) that are in line with a crossflow over a flat plate. The resulting vortex structures under different actuation, and flow conditions are investigated using two-plane dye visualization in a water tunnel. The influence of four independent nondimensional parameters, i.e., the Reynolds number (ReL), Strouhal number (St), velocity ratio (VR), and phase difference (Δϕ), on the behavior of the twin SJs is studied. It is found that the increase of Reynolds number causes the SJ-induced vortex structures more turbulent, making the twin SJ interaction less organized. The increase of velocity ratio pushes the occurrence of interaction further away from the wall and makes the resulting vortex structures more sustainable. The St has no obvious influence on the interaction. And three types of vortex structures are observed under different phase differences: one combined vortex, two completely separated vortices, and partially interacting vortex structures. Based on this parametric study, a simple model is proposed to predict the resulting vortex pattern for the twin SJ interaction.