Unsteady flow phenomena associated with leading-edge vortices
TL;DR: In this article, the results from extensive experimental investigations on turbulent flow fields and unsteady surface pressures caused by leading-edge vortices, in particular, for vortex breakdown flow, were presented.
About: This article is published in Progress in Aerospace Sciences.The article was published on 2008-01-01. It has received 77 citations till now. The article focuses on the topics: Vortex & Angle of attack.
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TL;DR: The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 08 as mentioned in this paper.
Abstract: The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 08 Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude, while greater turbulent kinetic energy penetrates the vortex core An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core
53 citations
Cites background or result from "Unsteady flow phenomena associated ..."
...(2009b) confirmed this behavior for the present fin geometry using PIV measurements on the fin surface and additionally showed that the vortex remains stable despite predictions of vortex breakdown on the fin surface (Breitsamter 2008)....
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...Devenport et al. (1996) made the same assumption and Heyes et al....
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TL;DR: In this article, an overview of experimental investigations on a 65 deg swept delta wing as part of the International Vortex Flow Experiment 2 (VFE-2) is presented, with details of the delta wing vortex structure and breakdown phenomenon are discussed and analyzed.
48 citations
TL;DR: In this article, the effects of active flow control by oscillatory blowing at the leading edge of a nonslender delta wing with a Λ=50° sweep angle have been investigated.
Abstract: The effects of active flow control by oscillatory blowing at the leading edge of a nonslender delta wing with a Λ=50° sweep angle have been investigated. Pressure measurements and Particle Image Velocimetry measurements were conducted on a half wing to investigate the formation of leading edge vortices for oscillatory blowing, compared to the stalled flow for the no blowing case. Stall has been delayed by up to 8, and significant increases in the upper surface suction force have been observed. Velocity measurements show that shear layer reattachment is promoted with forcing, and a vortex flow pattern develops. The time averaged location of the centre of the vortical region moves outboard with increased excitation. The near-surface flow pattern obtained from the PIV measurements shows reattachment in the forward part of the wing. There is no measurable jet-like axial flow in the vortex core, which seems to break down at or very near the apex. This highlights that unlike slender delta wings, vortex breakdown is not a limiting factor in the generation of lift for nonslender delta wings. Phase averaged measurements reveal the perturbation due to the pulsed blowing, its interaction with the shear layer and vortex, apparent displacement of the vortex core, and relaxation of the reattachment region. The flow in a phase averaged sense is highly three dimensional. Experiments indicate that unsteady blowing at Strouhal numbers in the region of St=0.5 to St=0.75, and in the region of St=1.25 to St=1.5 can be a highly effective. Reattached flow can develop from stalled flow after pulsing has been initiated with a time constant of tU/c=5 for unsteady blowing at St=0.75, and tU/c=7 for St=1.5. Experiments with excitation from finite span slots located in the forward half of the wing show that partial blowing can be more effective at low momentum coefficients. Force measurements of a full delta wing confirmed that the effectiveness of this method of flow control was not only confined to half delta wings.
26 citations
TL;DR: In this paper, the effect of a 65° sweep reverse half-delta wing (RHDW) mounted at the squared tip of a rectangular NACA 0012 wing, on the tip vortex was investigated experimentally.
Abstract: The effect of a 65° sweep reverse half-delta wing (RHDW), mounted at the squared tip of a rectangular NACA 0012 wing, on the tip vortex was investigated experimentally at Re = 2.45 × 105. The RHDW was found to produce a weaker tip vortex with a lower vorticity level and, more importantly, a reduced lift-induced drag compared to the baseline wing. In addition to the lift increment, the RHDW also produced a large separated wake flow and subsequently an increased profile drag. The reduction in lift-induced drag, however, outperformed the increase in profile drag and resulted in a virtually unchanged total drag in comparison with the baseline wing. Physical mechanisms responsible for the RHDW-induced appealing aerodynamics and vortex flow modifications were discussed.
24 citations
TL;DR: In this article, the effect of the dielectric barrier discharge plasma actuators on the leading-edge vortex flow control over a delta wing with a 75-degree swept angle was investigated.
Abstract: This paper presents an experimental investigation of the effect of the dielectric barrier discharge plasma actuators on the leading-edge vortex flow control over a delta wing with a 75 deg swept angle. The Reynolds number based on the delta wing chord is 50,000. The smoke flow visualization and particle image velocimetry experiments in the wind tunnel indicate that the asymmetric control caused by the dielectric barrier discharge plasma actuator can significantly affect the global flow structure over the delta wing: the leading-edge vortex breakdown position on the same side of the actuator is advanced, whereas vortex breakdown on the other side is delayed. This phenomenon is observed for the first time and is only observed in the current full-span delta wing experiment, significantly different from the result of the semispan experiment. The force measurement result shows that the asymmetric control has a limited contribution to lift generation but can induce a relatively large roll moment.
24 citations
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TL;DR: In this article, a full-scale, production F/A-18 fighter aircraft in the 80 by 120 ft Wind Tunnel at NASA Ames Research Center was tested over an angle of attack range of 18-50 deg, and at wind speeds of up to 168 ft/s, corresponding to a Reynolds number of 12.3x10(exp 6) based on mean aerodynamic chord and a Mach number of 0.15.
Abstract: Tail buffet studies were conducted on a full-scale, production F/A-18 fighter aircraft in the 80 by 120 ft Wind Tunnel at NASA Ames Research Center. The F/A-18 was tested over an angle-of-attack range of 18-50 deg, and at wind speeds of up to 168 ft/s, corresponding to a Reynolds number of 12.3x10(exp 6) based on mean aerodynamic chord and a Mach number of 0.15. The port, vertical tail fin was instrumented and the aircraft was equipped with a removable leading-edge extension (LEX) fence. Time-averaged, power-spectral analysis results are presented for the tail fin bending moment derived from the integrated pressure field, for the zero side-slip condition, both with and without the LEX fence. The LEX fence significantly reduces the magnitude of the rms pressures and bending moments. Scaling issues are addressed by comparing full-scale results for pressures at the 60%-span and 45%-chord location with small-scale, F/A-18 tail-buffet data. The comparison shows that the tail buffet frequency scales very well with length and velocity. Root-mean-square pressures and power spectra do not scale as well. The LEX fence is shown to reduce tail buffet loads at all model scales.
96 citations
TL;DR: In this paper, the authors investigate the transition between the helical mode instability and the vortex shedding and find that this transition is abrupt, as indicated by a jump in the frequency parameter, and that it occurs at the angle of attack at which breakdown reached the apex.
Abstract: Experimental evidence suggests that vortex breakdown is not the only source of buffeting of delta wings and fins. Other unsteady flow phenomena that contribute to buffeting at high angles of attack are fluctuations of vortex breakdown location and vortex shedding. Flow visualization and velocity measurements were carried out over a delta wing, over a wide range of angles of attack, to understand the transition between the helical mode instability and the vortex shedding. It was found that this transition is abrupt, as indicated by a jump in the frequency parameter, and that it occurs at the angle of attack at which breakdown reached the apex. The unsteady nature of vortex breakdown location was investigated by flow visualization for the interaction of vortex breakdown with a rigid flat plate. Although there are indications of a feedback effect on vortex breakdown, the amplitude of the fluctuations of breakdown location is smaller for impinging flows
58 citations