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Starting vortex
About: Starting vortex is a research topic. Over the lifetime, 4785 publications have been published within this topic receiving 100419 citations.
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TL;DR: In this paper, a Lagrangian Coherent Structures (LCS) analysis was used to study the role of the TPM in vortex ring formation and identification of the vortex ring before separation.
Abstract: An investigation is made into the mechanism of pinch-off for turbulent vortex rings formed by a synthetic jet using time resolved particle image velocimetry measurements in air. During formation, measurements of the material acceleration field show a trailing pressure maximum (TPM) forms behind the vortex core. The adverse pressure gradient behind this TPM inhibits vorticity transport into the ring and the TPM is spatially coincident with the termination of vorticity flux into a control volume moving with the ring. A Lagrangian Coherent Structures (LCS) analysis is shown to be in agreement with the role of the TPM in pinch-off and in identifying the vortex ring before separation. The LCS analysis provides physical insights which form the basis of a revised model of pinch-off, based on kinematics, which predicts the time of formation (formation number) well for the present dataset. The delivery of impulse to the vortex ring is also considered. Two equally important mechanisms are shown to play a role: a material flux and a vortex force. In the case of long maximum stroke ratio, it is demonstrated that a vortex force continues to deliver impulse to the ring after the material flux is terminated at pinch-off and that this contribution may be substantial. This shows that the pinch-off and separation process cannot be considered impulse invariant, which has important implications for unsteady propulsion, present models of vortex ring formation, and existing explanations for vortex ring pinch-off.
39 citations
TL;DR: In this article, the isothermal flow in a model channel of a plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated, and velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000).
Abstract: The isothermal flow in a model channel of plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated. Velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000). Strong longitudinal vortices are observed downstream of each generator. The vortex roll-up process is highlighted by the evolution of the velocity vector field in the cross section of the flow. The modifications of the vortex characteristics after successively encountered generators are investigated. This work shows most of the flow features which are known to produce heat transfer enhancement, and shows that these smooth shaped vortex generators are very promising for enhanced heat exchangers.
39 citations
TL;DR: In this article, the state of the art in describing the eigenoscillations of a vortex ring in an ideal incompressible fluid is reviewed, and it is shown that allowing for even a very weak degree of core smoothing causes many oscillation modes to lose their stability.
Abstract: The state of the art in describing the eigen-oscillations of a vortex ring in an ideal incompressible fluid is reviewed. To describe eigen-oscillations, the displacement field is taken as the basic dynamic variable. A vortex ring with the simplest vorticity distribution in the core and with a potential flow in the vortex ring envelope is the commonest approximation used in treating the eigen-oscillations of vortex rings of a more general form. It turns out that allowing for even a very weak degree of core smoothing causes many oscillation modes to lose their stability. It is shown that the instability effect is determined by the sign of the vibration energy. The energies of the ring eigen-oscillations are calculated and two kinds of eigen-oscillations, those with a negative energy and those with a positive energy, are identified, of which it is the former which become unstable when the core vorticity is smoothed. The multiple instabilities of vortex ring oscillations together with the details of the spatial structure of its eigen-oscillations suggest that it is the nonlinear evolution of precisely these processes which might be the origin of vortex ring turbulence. A new method for the study of unsteady processes in turbulent vortex rings, which utilizes the experimental diagnostics of the ring's sound field, is presented. The structure of the sound field strongly supports the proposed model of the turbulent vortex ring.
39 citations
TL;DR: In this paper, a biology-inspired dynamic flight simulator for flapping wing-based micro-air vehicles is presented, where a horseshoe-shaped primary vortex is observed to wrap around each wing during early down-and upstroke, forming a doughnut-shaped vortex ring.
Abstract: Numerical investigation of vortex dynamics in near wake of a hovering hawkmoth and hovering aerodynamics is conducted to support the development of a biology-inspired dynamic flight simulator for flapping wing-based micro air vehicles. Realistic wing-body morphologies and kinematics are adopted in the numerical simulations. The computed results show 3D mechanisms of vortical flow structures in hawkmoth-like hovering. A horseshoe-shaped primary vortex is observed to wrap around each wing during the early down- and upstroke; the horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring with an intense jet-flow present in its core, forming a downwash. The doughnut-shaped vortex rings of the wing pair eventually break up into two circular vortex rings as they propagate downstream in the wake. The aerodynamic yawing and rolling torques are canceled out due to the symmetric wing kinematics even though the aerodynamic pitching torque shows significant variation with time. On the other hand, the time-varying the aerodynamics pitching torque could make the body a longitudinal oscillation over one flapping cycle.
39 citations
TL;DR: In this article, the interaction of a barotropic cyclonic vortex on a b plane with a strong topographic slope is studied by means of laboratory experiments and numerical simulations, and agreement between laboratory results and numerical runs suggests that conservation of potential vorticity is indeed the basic mechanism involved in the southward motion of the vortex and the northeastward meandering current.
Abstract: The interaction of a barotropic cyclonic vortex on a b plane with a strong topographic slope is studied by means of laboratory experiments and numerical simulations. In the laboratory, the vortex is produced in a rectangular rotating tank with a weak uniformly sloping bottom (slope angle 3.58) in order to simulate the b effect. The cyclonic vortex moves in the northwestward direction and interacts with an additional, pronounced linear topography (slope angle 358) along the western boundary of the tank. The laboratory experiments revealed that the original northwestward trajectory changes to the south until the vortex is dissipated by viscous effects and bottom friction. As it moves upslope, the exterior ring of the vortex forms a strong current to the northeast. From this current a new cyclonic vortex is created, repeating approximately the behavior of the original one. Later, two more vortices are formed in the same way. A finite difference numerical model is used to solve the nondivergent barotropic equation. Despite the presence of the strong slope, agreement between laboratory results and numerical runs suggests that conservation of potential vorticity is indeed the basic mechanism involved in the southward motion of the vortex and the northeastward meandering current.
39 citations