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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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Journal ArticleDOI
TL;DR: In this paper, the authors used a strobed laser sheet to illuminate the flow, which was seeded with a mineral oil fog, and found that the general flowfield structure consists of a folded wake, with a relatively large starting vortex at the end of each half-stroke.
Abstract: Afl ow-visualization experiment was conducted on an insect-based flapping-wing mechanism. This enabled greater understanding and insight to be gained on the unsteady aerodynamic phenomena that are responsible for the enhanced lift of wings operating at low Reynolds numbers in hovering flapping flight. Flow-visualization images were acquired with a strobbed laser sheet to illuminate the flow, which was seeded with a mineral oil fog. The general flowfield structure was found to consist of a folded wake, with a relatively large starting vortex at the end of each half-stroke. A large flow recirculation region was generated in the plane of flapping, which was centered around the two extreme flapping displacements. These general flowfield features were enhanced by detailed observations of the local flowfield around the wing section. One observation was the presence of multiple vortices on the top surface of the wing as it underwent translation. Furthermore, the local flowfield images clearly showed the growth of the leading-edge vortex as a function of span and identified the presence of separated flow on the outboard regions of the wing. These experimental results were supported by a free vortex modeling of the wake developments. The model was found to predict similar wake flowfield dynamics to that found in the experiments. This research has contributed to a better understanding of the unsteady aerodynamic mechanisms that are responsible for the enhanced lift of insect-based flapping wings in hover.

76 citations

Proceedings ArticleDOI
08 Jan 2007
TL;DR: In this paper, the roles of the plunging and pitching amplitude and frequency, and Strouhal number were studied for a symmetric plunging airfoil NACA0012 at zero geometric angle of attack and chord Reynolds number of 2×10 4, at the same plunging frequency.
Abstract: It is known that plunging airfoil can produce both lift and thrust with certain combination of plunging amplitude and frequency. Motivated by our interest in micro air vehicles (MAVs), we utilize a NavierStokes equation solver to investigate the aerodynamics of a flapping airfoil. The roles of the plunging and pitching amplitude and frequency, and Strouhal number are studied. For a symmetric plunging airfoil NACA0012 at zero geometric angle of attack and chord Reynolds number of 2×10 4 , at the same plunging frequency, it can produce either drag or thrust depending on the plunging amplitude. At the considered plunging amplitude (from 0.0125c to 0.075c), the flow history has more influence than the kinematic angle of attack to determine the lift. When drag is produced, the viscous force dominates the total drag with decreasing influence as the plunging amplitude increases. For an airfoil experiencing combined plunge and pitch motion, both thrust and input power increase with the Strouhal number (within the range of 0.03 to 0.5). For the case studied, the thrust is induced by the lift, which approximately follows the curve of the kinematic angle of attack. Leading edge vortex moves downstream and interacts with the trailing edge vortex. We also study the impact of gust on stationary airfoil and flapping airfoil. Within the range of the parameters tested, for stationary airfoil the lift is in phase with the velocity but the drag is slightly out of phase. For flapping airfoil, neither lift nor drag is in phase with the velocity. Nomenclature CD =Drag coefficient per unit span CL =Lift coefficient per unit span CP =input power coefficient CP,mean =time-averaged input power coefficient CT =thrust coefficient CT,mean =time-averaged thrust coefficient c =Chord length

76 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the wake properties of a wind turbine rotor in a water flume using particle image velocimetry and laser Doppler anemometry.
Abstract: The flow behind a model of a wind turbine rotor is investigated experimentally in a water flume using particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The study performed involves a three-bladed wind turbine rotor designed using the optimization technique of Glauert (Aerodynamic Theory, vol. IV, 1935, pp. 169–360). The wake properties are studied for different tip speed ratios and free stream speeds. The data for the various rotor regimes show the existence of a regular Strouhal number associated with the development of an instability in the far wake of the rotor. From visualizations and a reconstruction of the flow field using LDA and PIV measurements it is found that the wake dynamics is associated with a precession (rotation) of the helical vortex core.

76 citations

Journal ArticleDOI
TL;DR: In this paper, a study of the three-dimensional flow in a linear compressor cascade with stationary endwall at design conditions is presented for tip clearance levels of 1.0, 2.0 and 3.3 percent of chord.
Abstract: Experimental results from a study of the three-dimensional flow in a linear compressor cascade with stationary endwall at design conditions are presented for tip clearance levels of 1.0, 2.0, and 3.3 percent of chord, compared with the no-clearance case. In addition to five-hole probe measurements, extensive surface flow visualizations are conducted. It is observed that for the smaller clearance cases a weak horseshoe vortex forms in the front of the blade leading edge. At all the tip gap cases, a multiple tip vortex structure with three discrete vortices around the midchord is found. The tip leakage vortex core is well defined after the midchord but does not cover a significant area in traverse planes. The presence of the tip leakage vortex results in the passage vortex moving close to the endwall and the suction side.

76 citations

Journal ArticleDOI
TL;DR: In this paper, the occurrence and structure of longitudinal vortices are examined by a flow visualization technique for various sinusoidal wavy channels, and it is found that an unstable mode, in which the vortexices formed at the upper and lower walls of the channel have some interaction, leads to vortex destruction and significantly promotes the transition to turbulence.
Abstract: The occurrence and structure of longitudinal vortices are examined by a flow visualization technique for various sinusoidal wavy channels Longitudinal vortices are confirmed to appear in a channel with a narrow spacing in the range of geometrical parameters considered here The geometrical conditions for the onset of such vortices are shown by a diagram The longitudinal vortex is a kind of Taylor–Goertler vortex observed in a curved rectangular-section channel However, the wavy channel is different from the curved rectangular-section channel in the processes of vortex development and destruction It is found that an unstable mode, in which the vortices formed at the upper and lower walls of the channel have some interaction, leads to vortex destruction and significantly promotes the transition to turbulence

75 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202336
202278
20217
20207
20196
201815