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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: Gharib et al. as discussed by the authors studied the effect of stroke ratio on entrainment and mixing of a vortex ring issuing from a nozzle into stationary fluid, and found that when stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective at mixing and entraining.
Abstract: Direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharib et al. J. Fluid Mech. vol. 360, 1998, p. 121), is found to be 3.6. Simulations are performed for a range of stroke ratios (ratio of stroke length to nozzle exit diameter) encompassing the formation number, and the effect of stroke ratio on entrainment and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar-containing fluid is found to depend linearly on stroke ratio until the formation number is reached, and falls below the linear curve for stroke ratios greater than the formation number. This behaviour is explained by considering the entrainment to be a combination of that due to the leading vortex ring and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result.
51 citations
TL;DR: In this article, the effect of Strouhal number on the shape of the leading edge vortex is investigated, showing that the resulting vortex structures are at least planform-dependent.
Abstract: Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity.
51 citations
TL;DR: In this paper, a NACA 0012 half-wing with rounded wing tip and the near-field wake of a wingtip vortex is modeled using two different turbulence models: linear and non-linear eddy-viscosity models.
51 citations
TL;DR: In this paper, the structure and mixing of impulsively started jets have been studied in a water tank utilizing an acid-base reaction, and it was shown that the fluid in the vicinity of the jet tip mixes with the ambient fluid faster than the rest of the jets.
Abstract: The structure and mixing of impulsively started jets have heen studied in a water tank utilizing an acid-base reaction. The flow consists of a starting vortex that separates from the rest of the jet in the near field. Penetration of the jet tip scales with the square root of time, normalized by the nozzle diameter and velocity. The celerity of the jet tip is approximately one-half of the centerline velocity of a steady jet, with the same nozzle exit velocity, at the same location. Results of chemically reactive experiments indicate that the fluid in the vicinity of the jet tip mixes with the ambient fluid faster than the rest of the jet. The extent of the region near the jet tip with improved mixing becomes larger as the jet travels further downstream. The more rapid mass mixing at the jet tip implies faster momentum diffusion, which corroborates the slowing down of the jet tip in comparison with the steady jet.
51 citations
TL;DR: In this paper, an extension of Darwin's (1953) method to the case of vortices propagating in a real fluid is described, and experiments are conducted to demonstrate the existence of an added-mass effect during uni-directional vortex motion, which is analogous to the effect of solid bodies in potential flow.
Abstract: Darwin (1953) introduced a simple heuristic that relates the Lagrangian fluid drift induced by a solid body propagating in irrotational flow to its virtual- or added-mass. The force required to accelerate the solid body must also overcome this added-mass. An extension of Darwin's (1953) method to the case of vortices propagating in a real fluid is described here. Experiments are conducted to demonstrate the existence of an added-mass effect during uni-directional vortex motion, which is analogous to the effect of solid bodies in potential flow. The definition of the vortex added-mass coefficient is modified from the solid body case to account for entrainment of ambient fluid by the vortex. This modified coefficient for propagating vortices is shown to be equal in magnitude to the classical coefficient for a solid body of equivalent boundary geometry. An implication of these results is that the vortex added-mass concept can be used as a surrogate for the velocity potential, in order to facilitate calculations of the pressure contribution to forces required to set fluid into unsteady vortical motion. Application of these results to unsteady wake analyses and fluid–structure interactions such as vortex-induced vibrations is suggested.
51 citations