Vortex Dynamics and Shedding of a Low Aspect Ratio, Flat Wing at Low Reynolds Numbers and High Angles of Attack

Morse, Daniel; Liburdy, James A.

doi:10.1115/1.3112385

Cited by 15 publications

(7 citation statements)

References 21 publications

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“…High-velocity turbulence is apparent for x∕c > 1. 25, and the flow in this region is generally structured around an incoherent clockwise vortex (downward-facing triangle). At t 2.5 ms, much of the turbulent flow previously visible has been convected downstream and out of the field of view.…”

Section: Deep Stall Wake Evolutionmentioning

confidence: 99%

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Vortex Shedding from Airfoils in Reverse Flow

Lind

Jones

2015

AIAA Journal

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The vortex shedding characteristics of three airfoils held at static angles of attack through 360 deg are presented with a focus on reverse flow (150 ≤ α ≤ 180 deg). Wind tunnel testing was performed on one airfoil with a sharp trailing edge (NACA 0012) and two airfoils featuring a blunt trailing edge (ellipse and DBLN-526). Time-resolved particle image velocimetry and smoke flow visualization were used to identify three reverse flow wake regimes: slender body vortex shedding, turbulent, and deep stall vortex shedding. The slender body regime is present for low angles of attack and low Reynolds numbers. In the turbulent regime, separation occurs in reverse flow at the sharp aerodynamic leading edge of a NACA 0012, whereas flow separation occurs further down the chord of airfoils with a blunt geometric trailing edge. The deep stall vortex shedding frequency was measured using unsteady force balance measurements. The Strouhal number St d (based on the projected diameter d of the airfoils) was found to be 0.145-0.161 for 45 ≤ α ≤ 135 deg, which is well below the value of St d 0.19 for a corresponding cylinder. The results of the work presented here provide fundamental insight for rotor applications where flow separation and vortex shedding due to reverse flow can lead to unsteady loading, vibrations, and fatigue. Nomenclature = aspect ratio c = airfoil chord, m D u = component of drag force (measured from upper force balance), N d = diameter or projected diameter, m f St = Strouhal frequency, Hz f s = sampling frequency, Hz f vs = vortex shedding frequency (unforced), Hz f = deviation from calculated vortex shedding frequency, % k = scalar used in fast Fourier transform analysis N = signal length n = number of points used in n-point fast Fourier transform analysis Q = peak ratio t∕c = airfoil thickness-to-chord ratio t max = duration of a sample size, s U ∞ = freestream velocity, m∕s α = angle of attack, deg α rev = reverse flow angle of attack α ds rev = minimum angle for which the deep stall vortex shedding regime is present, deg α sb rev = maximum angle for which the slender body vortex shedding regime is present, deg Γ 1;2 = scalar vortex identification algorithm Δf = frequency resolution, Hz ω = vorticity, 1∕ŝ ω x o ∕c = sum of vorticity at station x o ∕c, 1∕s

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Section: Deep Stall Wake Evolutionmentioning

confidence: 99%

“…Vortex shedding in the wake of airfoils in forward flow has been studied (e.g., [22][23][24][25]), though less so than classical bluff bodies. Huang and Lin studied a NACA 0012 wing at Reynolds numbers O10 4 − 10 5 using smoke flow visualization and spectral analysis of hot-wire measurements collected in the wake [26].…”

Section: Introductionmentioning

confidence: 99%

Vortex Shedding from Airfoils in Reverse Flow

Lind

Jones

2015

AIAA Journal

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“…When the stress level and its cycles reach a certain value, degradation of mechanical performance will be evident, of which the tear strength decreases, puncture points and micro-cracks appear on the surface, and fractures finally occur. The fatigue life of silicone rubber is exponentially related to the applied stress [12].…”

Section: Fatigue Mechanism Of Silicone Rubbermentioning

confidence: 99%

Fatigue fracture of composite insulator sheds utilized in strong wind areas

Zhu

Jia

et al. 2015

IEEE Trans. Dielect. Electr. Insul.

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This paper presents the fracture failure mechanism of composite insulators that have been deployed for only one year on 750 kV transmission lines in strong wind areas. Investigations indicate that the flexible shed structure was vulnerable to flow-induced oscillation, which further brought cracking to the shed root. The interactive process of fatigue and fracture was reviewed by inspecting the failed samples to gain insight into the cracking mechanism. The fatigue theory of silicone rubber was introduced to illustrate the fatigue resistance of the material. Simulations of wind load and stress distribution were performed to demonstrate the load condition and stress concentration at the root area on sheds. Wind tunnel tests show the oscillation of two types of 750 kV insulators under strong winds of up to 60 m/s. Four samples of silicone rubber from major insulator manufacturers were tested to investigate the fatigue resistance property, and withstand limit of high-and lowperformance silicone rubbers. Results indicate that, composite insulators without specific design and wind tunnel tests were risky to apply on transmission lines in strong wind areas, which is common type of climate in northwestern China.

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“…And the average height of insulators hung on tower is 46 meters on this line, the instantaneous wind speed could reach 53.5m/s by theoretical calculation. High wind speed applies great pressure on the shed surface which leads to deformation, and the flow also vibrates the sheds [4][5][6][7][8]. Long-term oscillation of sheds creates stress concentration on the root of sheds.…”

Section: Introductionmentioning

confidence: 99%

Mechanical properties of HTV materials used in composite insulators in strong wind area

Zhu

Wang

Jia

et al. 2014

2014 IEEE Electrical Insulation Conference (EIC)

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Composite insulators were extensively applied in outdoor insulation for the extraordinary performance of hydrophobicity and anti-aging property. Whereas HTV silicone rubber had low elastic modulus and the stiffness was not high enough to resist high speed wind. In northwest China there are many storm-wind areas where the designed wind velocity of transmission lines could reach to 42 m/s. The high speed wind induced sustained and intensive oscillation of insulator sheds and finally leaded to fracture failure at root of it. The oscillation problem of sheds and fatigue and fracture properties of HTV materials haven't been paid enough attention before, for the composite insulators utilized in strong wind area. This paper researched on oscillation performance of one type of composite insulator in wind tunnel with applied wind speed from 0 m/s to 60 m/s. The oscillation phenomenon was recorded with high speed camera to measure the oscillation magnitude and frequency. Four HTV silicone rubber samples were tested for its fatigue resistance performance. The developing process of crack along the groove of silicone rubber samples was observed and the flexing cycles and the cycles that the micro-crack developed into fracture were measured.

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Vortex Dynamics and Shedding of a Low Aspect Ratio, Flat Wing at Low Reynolds Numbers and High Angles of Attack

Cited by 15 publications

References 21 publications

Vortex Shedding from Airfoils in Reverse Flow

Vortex Shedding from Airfoils in Reverse Flow

Fatigue fracture of composite insulator sheds utilized in strong wind areas

Mechanical properties of HTV materials used in composite insulators in strong wind area

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