Scanning PIV measurements of a laminar separation bubble

Burgmann, Sebastian; Brücker, Christoph; Schröder, Wolfgang

doi:10.1007/s00348-006-0153-6

Cited by 125 publications

(59 citation statements)

References 8 publications

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“…Hain, Kähler & Radespiel (2009) showed that the K-H instability led to a spanwise vortex formation in the shear layer above the separation bubble over an SD7003 aerofoil at the Reynolds number of Re = 66 000. Burgmann, Brücker & Schröder (2006) investigated a similar flow by using scanning particle image velocimetry (PIV), where they observed the quasi-periodic development of large vortex rolls at the downstream end of the separation bubble. Using time-resolved PIV and volumetric PIV measurements, Burgmann, Dannemann & Schröder (2008) further discussed the effects of the K-H instability on a transitional separation bubble and the temporal and spatial evolution of vortical structures.…”

Section: Laminar Separation Bubblesmentioning

confidence: 99%

Flow control over an airfoil using virtual Gurney flaps

2015

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Flow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin-Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.

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Section: Laminar Separation Bubblesmentioning

confidence: 99%

Flow control over an airfoil using virtual Gurney flaps

2015

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“…Fortunately, technical developments in recent years are increasingly making the move to three dimensions feasible for experimentalists (Lauder, 2006;Lauder and Tytell, 2006), and increases in computing power and the sophistication of computational algorithms have made 3D computation of fluid flows more practical also (Mittal, 2004;Mittal et al, 2006;Mittal and Iaccarino, 2005). On the experimental side, the use of multiple high-speed cameras to provide a variety of views of complexly deforming fish fins is now practical Standen and Lauder, 2005), and the use of stereo particle image velocimetry, multiple simultaneous light sheets, transverse light sheet orientations and scanning PIV all make 3D reconstruction of fluid flows feasible (Burgmann et al, 2006;Nauen and Lauder, 2002b;Standen and Lauder, 2007;Tytell, 2006). The coming marriage of 3D experimental and computational approaches promises exciting progress toward understanding fish locomotor dynamics.…”

Section: Conclusion and Prospectusmentioning

confidence: 99%

Escaping Flatland: three-dimensional kinematics and hydrodynamics of median fins in fishes

Tytell

Standen

Lauder

2008

Journal of Experimental Biology

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SummaryFish swimming has often been simplified into the motions of a two-dimensional slice through the horizontal midline, as though fishes live in a flat world devoid of a third dimension. While fish bodies do undulate primarily horizontally, this motion has important three-dimensional components, and fish fins can move in a complex three-dimensional manner. Recent results suggest that an understanding of the three-dimensional body shape and fin motions is vital for explaining the mechanics of swimming, and that two-dimensional representations of fish locomotion are misleading. In this study, we first examine axial swimming from the twodimensional viewpoint, detailing the limitations of this view. Then we present data on the kinematics and hydrodynamics of the dorsal fin, the anal fin and the caudal fin during steady swimming and maneuvering in brook trout, Salvelinus fontinalis, bluegill sunfish, Lepomis macrochirus, and yellow perch, Perca flavescens. These fishes actively move the dorsal and anal fins during swimming, resulting in curvature along both anterio-posterior and dorso-ventral axes. The momentum imparted to the fluid by these fins comprises a substantial portion of total swimming force, adding to thrust and contributing to roll stability. While swimming, the caudal fin also actively curves dorso-ventrally, producing vortices separately from both its upper and lower lobes. This functional separation of the lobes may allow additional control of three-dimensional orientation, but probably reduces swimming efficiency. In contrast, fish may boost the caudal finʼs efficiency by taking advantage of the flow from the dorsal and anal fins as it interacts with the flow around the caudal fin itself. During maneuvering, fish readily use their fins outside of the normal planes of motion. For example, the dorsal fin can flick laterally, orienting its surface perpendicular to the body, to help in turning and braking. These data demonstrate that, while fish do move primarily in the horizontal plane, neither their bodies nor their motions can accurately be simplified in a two-dimensional representation. To begin to appreciate the functional consequences of the diversity of fish body shapes and locomotor strategies, one must escape Flatland to examine all three dimensions.

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“…As vortices in the wakes of animal systems are rarely axisymmetric and often convect into and out of the laser plane, true 3D flow visualization and quantification of vortex structures are needed for reliable force and efficiency calculations. Modifications to conventional DPIV, incorporating multiple simultaneous light sheets (Standen and Lauder, 2007), stereo-DPIV (Nauen and Lauder, 2002) and multi-plane scanning DPIV (Burgmann et al, 2006), do provide greater spatial resolution in planar cross-sections, but either the number of planes is too small or they are not sufficiently synchronized in time to provide a truly global picture of the flow.…”

Section: Introductionmentioning

confidence: 99%

Volumetric flow imaging reveals the importance of vortex ring formation in squid swimming tail-first and arms-first

Bartol

Krueger

Jastrebsky

et al. 2015

Journal of Experimental Biology

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Squids use a pulsed jet and fin movements to swim both arms-first (forward) and tail-first (backward). Given the complexity of the squid multi-propulsor system, 3D velocimetry techniques are required for the comprehensive study of wake dynamics. Defocusing digital particle tracking velocimetry, a volumetric velocimetry technique, and high-speed videography were used to study arms-first and tail-first swimming of brief squid Lolliguncula brevis over a broad range of speeds [0-10 dorsal mantle lengths (DML) s] in a swim tunnel. Although there was considerable complexity in the wakes of these multi-propulsor swimmers, 3D vortex rings and their derivatives were prominent reoccurring features during both tail-first and arms-first swimming, with the greatest jet and fin flow complexity occurring at intermediate speeds (1.5-3.0 DML s −1). The jet generally produced the majority of thrust during rectilinear swimming, increasing in relative importance with speed, and the fins provided no thrust at speeds >4.5 DML s ) to counteract negative buoyancy. Propulsive efficiency (η) increased with speed irrespective of swimming orientation, and η for swimming sequences with clear isolated jet vortex rings was significantly greater (η=78.6±7.6%, mean±s.d.) than that for swimming sequences with clear elongated regions of concentrated jet vorticity (η=67.9±19.2%). This study reveals the complexity of 3D vortex wake flows produced by nekton with hydrodynamically distinct propulsors.

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Scanning PIV measurements of a laminar separation bubble

Cited by 125 publications

References 8 publications

Flow control over an airfoil using virtual Gurney flaps

Flow control over an airfoil using virtual Gurney flaps

Escaping Flatland: three-dimensional kinematics and hydrodynamics of median fins in fishes

Volumetric flow imaging reveals the importance of vortex ring formation in squid swimming tail-first and arms-first

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