Parameter Variation and the Leading-Edge Vortex of a Rotating Flat Plate

Wojcik, Craig; Buchholz, James

doi:10.2514/1.j052381

Cited by 27 publications

(21 citation statements)

References 44 publications

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“…For φ 48 • , the leading-edge separated shear layer ahead of the LEV is deflected away from the plate, as also found by Ozen & Rockwell (2012) for A = 2 and larger φ. At φ = 60 • the negative vorticity is lifted farther from the plate by the LEV velocity (see also Harbig et al 2013;Garmann & Visbal 2014;Wojcik & Buchholz 2014a), and some of this vorticity has wrapped around the LEV itself (left portion). This overall behaviour is associated with the arch-like LEV observed in the iso-Q surfaces (figure 4).…”

Section: Lev Circulation and Flow Fieldsmentioning

confidence: 98%

“…Circulation normalization using U local was also employed by Carr, DeVoria & Ringuette (2013b) and Wojcik & Buchholz (2014a). For φ = 12 • , when the influence of the plate ends is small, the curves of Γ * total are nearly flat between the root and tip.…”

Section: Total Circulation and Vortex Structurementioning

confidence: 99%

“…They found that as Re decreases, the C L peak occurs earlier, the LEV circulation growth rate is higher and pinch-off is faster. Wojcik & Buchholz (2014a) used DPIV to investigate the LEV circulation and its scaling for A = 2 and 4 flat plates at various angles of attack for rotation angles of φ = 90-320 • . They examined spanwise positions of 25 % and 50 % span, and showed a concentration of opposite-sign vorticity between the LEV and the plate, which the LEV entrained.…”

Section: Introductionmentioning

confidence: 99%

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Aspect-ratio effects on rotating wings: circulation and forces

2015

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We employ experiments to study aspect ratio (A) effects on the vortex structure, circulation and lift force for flat-plate wings rotating from rest at 45 • angle of attack, which represents a simplified hovering-wing half-stroke. We use the time-varying, volumetric A = 2 data of Carr et al. (Exp. Fluids, vol. 54, 2013, pp. 1-26), reconstructed from phase-locked, phase-averaged stereoscopic digital particle image velocimetry (S-DPIV), and an A = 4 volumetric data set matching the span-based Reynolds number (Re) of A = 2. For A = 1-4 and Re span of O(10 3 -10 4 ), we directly measure the lift force. The total leading-edge-region circulation for A = 2 and 4 compares best overall using a span-based normalization and for matching rotation angles. The total circulation increases across the span to the tip region, and is larger for A = 2. After the startup, the total circulation for each A has a similar slope and a slow growth. The first leading-edge vortex (LEV) and the tip vortex (TV) for A = 4 move past the trailing edge, followed by substantial breakdown. For A = 2 the outboard, aft-tilted LEV merges with the TV and resides over the tip, although breakdown also occurs. Where the LEV is 'stable' inboard, its circulation saturates for A = 2 and the growth slows for A = 4. Aft LEV tilting reduces the spanwise LEV circulation for each A. Both positive and negative axial flow are found in the first LEV for A = 2 and 4, with the positive component being somewhat larger. This yields a generally positive (outboard) average vorticity flux. The average lift coefficient is essentially constant with A from 1 to 4 during the slow growth phase, although the large-time behaviour shows a slight decrease in lift coefficient with increasing A. The S-DPIV data are used to obtain the lift impulse and the spanwise and streamwise components contributing to the lift coefficient. The spanwise contribution is similar for A = 2 and 4, due to similar trailing-edge vortex interactions, LEV saturation behaviour and total circulation slopes. However, for A = 2 the streamwise contribution is much larger, because of the stronger, coherent TV and aft-tilted LEV, which will create a relatively lower-pressure region over the tip.

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Section: Lev Circulation and Flow Fieldsmentioning

confidence: 98%

Section: Total Circulation and Vortex Structurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Aspect-ratio effects on rotating wings: circulation and forces

2015

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“…The thickness was 4 % of the chord and the edges were square. A detailed investigation of leading-edge vortex evolution for this configuration with varying Reynolds number, aspect ratio, angle of attack, and spanwise and azimuthal positions is presented in Wojcik & Buchholz (2014). In the present work, the analysis is limited to the single blade aspect ratio at a geometric angle of attack α = 35 • and azimuthal positions of ψ = 75, 90, 120, 180, 235, 270 and 320 • .…”

Section: Experimental Set-upmentioning

confidence: 99%

Vorticity transport in the leading-edge vortex on a rotating blade

2014

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Vorticity transport is analysed within the leading-edge vortex generated on a rectangular flat plate of aspect ratio 4 undergoing a starting rotation motion in a quiescent fluid. Two analyses are conducted on the inboard half of the blade to better understand the vorticity transport mechanisms responsible for maintaining the quasi-equilibrium state of the leading-edge vortex. An initial global analysis between the 25 and 50 % spanwise positions suggests that, although spanwise velocity is significant, spanwise convection of vorticity is insufficient to balance the flux of vorticity from the leading-edge shear layer. Subsequent detailed analyses of vorticity transport in planar control volumes at the 25 and 50 % spanwise positions verify this conclusion and demonstrate that vorticity annihilation due to interaction between the leading-edge vortex and the opposite-sign layer on the plate surface is an important, often dominant, mechanism for regulation of leading-edge-vortex circulation. Thus, it provides an important condition for maintenance of an attached leading-edge vortex on the inboard portion of the blade.

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“…[17,74,75,76,77,78] Since the LEV circulation is related to the lift produced, calculating the LEV circulation can provide a uniquely defined value to understand how wing kinematics and geometry affect circulation. Recently, Baik et al [76] experimentally studied the effect of reduced frequency on the development of the LEV circulation and LEV size of a nominally 2D pitch and plunge wing.…”

Section: Chapter VII Vortex Circulation and Lift A Introductionmentioning

confidence: 99%

Numerical study of leading edge vortex circulation development on finite aspect ratio perching wings.

Hord¹

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ACKNOWLEDGEMENTSMy deepest gratitude goes to the following individuals for making this dissertation possible.Yongsheng Lian for providing the opportunity to pursue my Ph.D even when there was no funding available. Finding support was a perpetual struggle, but he always was able to find it. He also had constant patience with me investing time into other projects unrelated to my research. The perching maneuver of natural fliers is a complex motion involving fast change of angle of attack, complicated wing kinematics, large wing deformation and agile body motion control, but the prominent aerodynamic features can be revealed using a simple pitch-up wing motion coupled with a stream wise deceleration. In this dissertation the aerodynamic forces, the leading edge vorterx (LEV) development, and LEV circulation of pitch-up and perching wings are extensively studied at low Reynolds number conditions. 2D and 3D wings of different aspect ratios were linearly pitched up from 0 • to 90• at three reduced pitch rates. The numerical investigation was conducted at a Reynolds number of 500 and the flow field was described using the unsteady three-dimensional incompressible Navier-Stokes equations on a set of composite overlapping grids. The Q-criterion was used to identify and isolate the LEV structure from shear layer vorticity. Results have shown that the LEV circulation depends primarily on the wing aspect ratio: increasing wing aspect ratio increased the rate of LEV circulation generation during the pitch-up motion. The reduced pitch rate for the pitch-up motion was found to delay the LEV circulation development when the aspect ratio was greater than two. For perching, pitch rate only altered the LEV propagation away from the wing, and not the magnitude of the circulation.iv

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Parameter Variation and the Leading-Edge Vortex of a Rotating Flat Plate

Cited by 27 publications

References 44 publications

Aspect-ratio effects on rotating wings: circulation and forces

Aspect-ratio effects on rotating wings: circulation and forces

Vorticity transport in the leading-edge vortex on a rotating blade

Numerical study of leading edge vortex circulation development on finite aspect ratio perching wings.

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