This paper illustrates the mechanism of U-shaped vortex formation which is found both by experiment and DNS. The main goal of this paper is to explain how the U-shaped vortex is formed and further develops. According to the results obtained by our direct numerical simulation with high order accuracy, the U-shaped vortex is part of the coherent vortex structure and is actually the tertiary streamwise vortices induced by the secondary vortices. The new finding is quite different from existing theories which describe that the U-shaped vortex is newly formed as the head of young turbulence spot and finally break down to small pieces. In addition, we find that the U-shaped vortex has the same vorticity sign as the original λ-shaped vortex tube legs and serves as a second neck to supply vorticity to the ringlike vortex when the original vortex tube is stretched and multiple rings are generated.
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…
This paper describes a study of the generation of a plughole vortex and its consequences in a drainpipe during drainage of water from a stationary rectangular tank. The critical and minimum depths of water above the inlet of the drainpipe, where a surface dip starts to develop for drainpipes of various diameters, were examined parametrically. This study explored the following naturally occurring phenomena arising from a plughole vortex. (i) A plughole vortex initially causes a surface dip to develop towards the inlet of the drainpipe and as the surface dip approaches the inlet of the drainpipe…
Fluidic transport in inverted T-shaped cavities with the flow entering through the top and exiting from the two bottom outlets experiences an interesting phenomenon that causes particles having density lower than that of the fluid medium to get trapped at the junction, in a horizontal formation. However, this only occurs across a small range of Reynolds numbers and that too in the laminar regime. The unexpected phenomenon is conjectured to be modulated by formation of vortex tubes in the flow. Interestingly enough, such T-shaped (or, more generically, Y-shaped) cavities are also seen quite wid…
Vortex-induced vibration (VIV) manifests its destructiveness on marine risers in deep-water oil and gas development. Many types of riser fairings are employed in the practical applications of ocean engineering due to their effectiveness in suppressing VIV. However, few reports have been found on the optimal design of the fairing profile. In this study, we conduct shape optimization by integrating computational fluid dynamics and genetic algorithm at a low Reynolds number. The fairing profile is parameterized by Bézier curves with control points adopted as design variables, and lift coefficient…
In this study, the new concept of vortex core line based on Liutex definition is applied to demonstrate that vortex ring is not part of the Lambda-vortex and the generation of ring like vortex is formed separately from the Lambda-vortex. The proper orthogonal decomposition is also applied to analyze the Kelvin Helmholtz instability happening in hairpin ring areas of flow transition on the flat plate to understand the ring like vortex formation. The new identification method named modified Omega-Liutex method is efficiently used to visualize and observe the shapes of vortex structures in 3D. Th…
This fluid dynamics video shows three-dimensional vortex formation process for plates in impulsive motion which is investigated experimentally by using defocusing digital particle image velocimetry (DDPIV). Rigid and flexible plate cases are compared in order to study the effect of flexibility on 3D vortex formation and associated hydrodynamic forces. This study was motivated by the general question of how the flexibility of flapping propulsors in flying and swimming animals affects vortex formation and propulsive force. For translating and rotating motion, the flexible plate generates a vorte…
We study the formation of the mitral vortex ring during early diastolic filling in a patient-specific left ventricle (LV) using direct numerical simulation. The geometry of the left ventricle is reconstructed from Magnetic Resonance Imaging (MRI) data of a healthy human subject. The left ventricular kinematics is modeled via a cell-based activation methodology, which is inspired by cardiac electro-physiology and yields physiologic LV wall motion. In the fluid dynamics videos, we describe in detail the three-dimensional structure of the mitral vortex ring, which is formed during early diastolic…
Playing a role in the locomotion of some animals such as the water strider, the formation and dynamics of a semi vortex ring connected to a free surface are experimentally investigated. This semi vortex ring is generated by the circular motion of a flat circular disk in water. Digital Particle Image Velocimetry provides velocity fields and vortex properties. We show how in a broad range of Reynolds numbers, the properties of the semi vortex rings are related to the disk characteristics. In particular, our results highlight a formation process more complex than for a complete vortex ring produc…
Pulsatile jet propulsion is a highly energy-efficient swimming mode used by aquatic animals that continues to inspire engineers of underwater vehicles. Here, we present a bio-inspired jet propulsor that combines the flexible hull of a jellyfish with the bivalve compression of a scallop to create individual vortex rings for thrust generation. Similar to biological jetters, our propulsor generates a non-linear temporal exit velocity profile and has a finite volume capacity. The formation process of the vortices generated by this jet profile is analysed using time-resolved velocity field measurem…
On the vortex ring formation and mixing in thin films upon droplet impact
Institute for Fluid Mechanics and Aerodynamics, TU Darmstadt（达姆施塔特工业大学流体力学与空气动力学研究所）；Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology（韩国科学技术院机械工程系）
Unsteady inviscid flow models of wings and airfoils have been developed to study the aerodynamics of natural and man-made flyers. Vortex methods have been extensively applied to reduce the dimensionality of these aerodynamic models, based on the proper estimation of the strength and distribution of the vortices in the wake. In such modeling approaches, one of the most fundamental questions is how the vortex sheets are generated and released from sharp edges. To determine the formation of the trailing-edge vortex sheet, the classical Kutta condition can be extended to unsteady situations by rea…
The formation of a vortex ring from a piston/nozzle apparatus depends on the nozzle exit's shape. Here we study the formation mechanism and the evolution of asymmetrical rings from notched nozzles using direct numerical simulations. Three types of nozzle are used: i) V-notched; ii) A-notched and iii)W-notched nozzles. The results show that a distinct main vortex ring is formed following the shape of nozzle exit at initial formation time. However entrainment from the surrounding ambient fluid through the trough locations creates pairs of streamwise structure after the piston stops. This structu…
This study employs an improved volume of fluid method and adaptive mesh refinement algorithm to numerically investigate the internal jet-like mixing upon the coalescence of two initially stationary droplets of unequal sizes. The emergence of the internal jet is attributed to the formation of a main vortex ring, as the jet-like structure shows a strong correlation with the main vortex ring inside the merged droplet. By tracking the evolution of the main vortex ring together with its circulation, we identified two mechanisms that are essential to the internal-jet formation: the vortex-ring growt…
In this paper, the flow around each ring-like vortex is investigated by high order DNS including first sweep, first ejection, second sweep, second ejection, positive spike, momentum deficit, vortex shape, vortex location, strength of sweeps, etc. Meanwhile, the mechanism about formation of momentum deficit is deeply studied. A new mechanism on how the multiple rings are formed one by one found both by experiment and by DNS in late boundary layer transition is presented. It also reveals that the relation between streamwise vortex and spanvise vertex rings, and how the vorticity is transferred b…
We study numerically the formation of a vortex lattice inside a rotating bucket containing superfluid helium, paying attention to an important feature which is practically unavoidable in all experiments: the microscopic roughness of the bucket's surface. We model this using the Gross-Pitaevskii for a weakly-interacting Bose gas, a model which is idealised when applied to superfluid helium but captures the key physics of the vortex dynamics which we are interested in. We find that the vortex lattice arises from the interaction and reconnections of nucleated U-shaped vortex lines, which merge an…
