Shallow and deep dynamic stall for flapping low Reynolds number airfoils

Ol, Michael V.; Bernal, Luis P.; Kang, Chang-Kwon; Shyy, Wei

doi:10.1007/978-3-642-11633-9_26

Cited by 45 publications

(65 citation statements)

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“…The experiments were performed at the US Air Force Research Laboratory's (AFRL) Horizontal Free-surface Water Tunnel, which is fitted with a three degree of freedom electric motion rig enabling independent control of pitch, plunge and surge (streamwise-aligned translation). More detail on the rig operation is given in Ol et al (2009a) and Granlund, Ol & Bernal (2011), while the facility is discussed in Ol et al (2005). The flow field is visualized by planar laser fluorescence.…”

Section: Validation and Resultsmentioning

confidence: 99%

“…The flow field is visualized by planar laser fluorescence. A high concentration of Rhodamine 6G in water is injected at the leading and trailing edges at 3/4-span locations by a positive-displacement pump at a prescribed volumetric infusion rate, via a 0.5 mm diameter internal rigid line, as documented by Ol et al (2009a). The dye is illuminated by an Nd:YLF 527 nm pulsed laser sheet of 1.5 mm thickness at 50 Hz and images are recorded with a PCO DiMax high-speed camera through a Nikon PC-E 45 mm Micro lens.…”

Section: Validation and Resultsmentioning

confidence: 99%

“…Advances in computational fluid dynamics (CFD) and experimental techniques have facilitated the detailed study and analysis of unsteady phenomena. Ol et al (2009a) and McGowan et al (2011) have analysed the forces and flow fields for unsteady motions over a broad parameter space using both experimental and computational methods. Garmann & Visbal (2011) and Granlund, Ol & Bernal (2013) have investigated pitching flat plates in detail through computational and experimental methods respectively.…”

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confidence: 99%

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Discrete-vortex method with novel shedding criterion for unsteady aerofoil flows with intermittent leading-edge vortex shedding

et al. 2014

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Unsteady aerofoil flows are often characterized by leading-edge vortex (LEV) shedding. While experiments and high-order computations have contributed to our understanding of these flows, fast low-order methods are needed for engineering tasks. Classical unsteady aerofoil theories are limited to small amplitudes and attached leading-edge flows. Discrete-vortex methods that model vortex shedding from leading edges assume continuous shedding, valid only for sharp leading edges, or shedding governed by ad-hoc criteria such as a critical angle of attack, valid only for a restricted set of kinematics. We present a criterion for intermittent vortex shedding from rounded leading edges that is governed by a maximum allowable leading-edge suction. We show that, when using unsteady thin aerofoil theory, this leading-edge suction parameter (LESP) is related to the A 0 term in the Fourier series representing the chordwise variation of bound vorticity. Furthermore, for any aerofoil and Reynolds number, there is a critical value of the LESP, which is independent of the motion kinematics. When the instantaneous LESP value exceeds the critical value, vortex shedding occurs at the leading edge. We have augmented a discrete-time, arbitrary-motion, unsteady thin aerofoil theory with discrete-vortex shedding from the leading edge governed by the instantaneous LESP. Thus, the use of a single empirical parameter, the critical-LESP value, allows us to determine the onset, growth, and termination of LEVs. We show, by comparison with experimental and computational results for several aerofoils, motions and Reynolds numbers, that this computationally inexpensive method is successful in predicting the complex flows and forces resulting from intermittent LEV shedding, thus validating the LESP concept.

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Section: Validation and Resultsmentioning

confidence: 99%

Section: Validation and Resultsmentioning

confidence: 99%

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confidence: 99%

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Discrete-vortex method with novel shedding criterion for unsteady aerofoil flows with intermittent leading-edge vortex shedding

et al. 2014

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“…[1][2][3] In recent years, interest in dynamic stall at moderate Reynolds numbers has re-emerged driven by its importance to small unmanned air vehicle system operation, as well as to the understanding of the remarkable performance found in natural flight. 4,5 Dynamic stall has been studied extensively [6][7][8][9][10][11][12][13] for effectively two-dimensional wing geometries represented by means of spanwise-periodic computations or wall-to-wall experimental configurations. These studies have provided a great deal of insight on the formation and transition of these dynamic stall vortices and on the highly unsteady loads elicited by their convection along the aerodynamic surface.…”

Section: Introductionmentioning

confidence: 99%

Flow Structure and Unsteady Loading over a Pitching and Perching Low-Aspect-Ratio Wing

Visbal

2012

42nd AIAA Fluid Dynamics Conference and Exhibit

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This study addresses the flow structure and unsteady loading arising over a low-aspect-ratio rectangular wing subjected to high-amplitude pitching and perching maneuvers under low-Reynolds-number conditions of interest in small unmanned aerial vehicle operation and gust interactions. Simulations are performed employing an extensively-validated high-fidelity computational approach capable of accurately capturing the complex unsteady transitional flows. Canonical pitching motions for a non-translating wing in the presence of a constant freestream are first investigated in order to elucidate the effects of pitch rate and Reynolds number on the flow structure and aerodynamic loading. The wing is pitched about its quarter-chord axis to a maximum incidence of 45 • with nominal non-dimensional angular rates 0.05 ≤ Ω + ≤ 0.2. The Reynolds number based on the wing chord varied from 10 3 to 4 × 10 4 . For the highest pitch rate, good agreement between the computed three-dimensional flow structure and recent experimental measurements is demonstrated. The 3D dynamic stall process is characterized by the formation of an initial spanwise-oriented leading-edge vortex which evolves into an arch-type structure with legs anchored on the wing surface. This dominant flow element of low-aspect-ratio wings exhibits a long residence time and establishes a well-defined low-pressure region and swirling pattern on the wing surface. Increasing pitch rate promotes an angular delay in the flow evolution and corresponding aerodynamic loads. Even for the lowest pitch rate, a significant increase in maximum lift is achieved relative to the static situation. The low-aspect-ratio of the wing also promotes further delay of dynamic stall and a much smoother behavior of the aerodynamic coefficients when compared to a pitching 2D wing section. The formation of the arch vortex and swirling pattern are found to be qualitatively similar over the range of Reynolds number studied. At fixed pitch rate, higher Reynolds number promotes a more compact arch vortex, a stronger suction on the wing and further stall delay. Canonical perching maneuvers involving simultaneous rotation and deceleration to rest are considered at two values of pitch rate. The reduction in translating velocity halts the development of the leading-edge vortex system which eventually moves upstream in front of the wing. Attempts to correlate the perching loads with the constant-freestream pitching cases indicate that agreement can only be achieved for the initial portion of the maneuver. Discrepancies arise at high angles of attack which depend on the value of pitch rate itself. Nomenclature c = wing chord C L , C D , C M = lift, drag and quarter-chord pitching moment coefficients C Lo , C Do , C M o = non-dimensional lift, drag and pitching moment for perching wing C Lw , C Dw , C M w = lift, drag and pitching moment for perching wing rescaled with translational velocityviscous vector fluxes J = Jacobian of the coordinate transformation LEV = leading-edge vortex M = Mach number p = stat...

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“…Sting effects were found to be minor and the junction flow was limited to regions within z ¼ c=8 of the wall. A recent investigation by Ol et al (2009) used a very similar strut configuration to drive the same pure-plunge motion as used in this study (reference case). Here three-dimensional effects associated with LEV breakdown were visualized using dye injection.…”

Section: Experimental Set-upmentioning

confidence: 94%