Computational Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios

Potsdam, Mark; Datta, Anubhav; Jayaraman, Buvana

doi:10.4050/jahs.61.022002

Cited by 24 publications

(13 citation statements)

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“…An elliptical airfoil (featuring a blunt geometric trailing edge) was also shown to delay deep stall to jα rev j 14 deg, but it produced a large downwardacting lift force and pitching moment about the geometric quarterchord. Recent experimental and computational studies on a full-scale UH-60A at high advance ratios have characterized some of the threedimensional unsteady flow features encountered in the reverse flow region that are responsible for rapid variations in blade airloads [1,33]. These studies focused on the influence of a dynamic stall vortex in reverse flow.…”

Section: Introductionmentioning

confidence: 98%

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: Introductionmentioning

confidence: 98%

Vortex Shedding from Airfoils in Reverse Flow

Lind

Jones

2015

AIAA Journal

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“…14-17) and coupled high-fidelity computational fluid dynamics with computational structural dynamics (CFD/CSD) analyses (Ref. 18), examine the performance, airloads, and structural loads of slowed rotor systems. It is quite clear from these investigations, a variation in rotor RPM is key to achieving high-speed forward flight.…”

Section: Introductionmentioning

confidence: 99%

Aeromechanics of a Slowed Rotor

Bowen-Davies¹,

Chopra²

2015

J Am Helicopter Soc

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This paper investigates the impact of decreasing rotor speed on the performance and loads of a utility helicopter similar to a UH-60A Black Hawk in steady level flight. The rotor is modeled with the comprehensive analysis University of Maryland Advanced Rotor Code (UMARC) incorporating blade flexibility and time marching free wake. Thrusts from 14,000 to 20,000 lb are evaluated at reduced rotor speeds between 74% and 100% of nominal revolutions per minute for airspeeds up to 180 kt (μ < 0.4). It is demonstrated how rotor power requirements can be reduced by slowing the rotor as a function of airspeed and thrust level. The largest decrease in power is possible at low thrusts across the flight envelope. At increasing thrust levels, there is less thrust margin for performance improvement. The minimum rotor power is defined by an optimum C T /σ as a function of advance ratio regardless of rotor thrust. The in-plane forces and moments for a slowed rotor increase significantly over the baseline rotor values at high speeds and are a strong function of advance ratio. On the other hand, the vertical vibratory shear is not affected for the slowed rotor.

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“…These aspects of the flow, combined with high angle of attack and reverse flow, contribute to unsteady airloads experienced by a rotor blade. Some studies have investigated the unsteady pressure distribution on rotor blades in the reverse flow region, but the magnitude of unsteady airloads in these flow regimes is still largely unknown [5,6,7,8].…”

Section: Introductionmentioning

confidence: 99%

Unsteady airloads on static airfoils through high angles of attack and in reverse flow

Lind

Jones

2016

Journal of Fluids and Structures

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The present work is aimed at understanding the sources, magnitude, and frequency of unsteady airloads acting on airfoils at high angles of attack and in reverse flow in order to improve the design of rotor blades for high-speed helicopters and wind turbines. Four rotor blade airfoils were tested at angles of attack through 180 • and at three Reynolds numbers, up to one million. The unsteady airloads acting on each airfoil were calculated by integrating time-resolved pressure measurements acquired along the midspan of the airfoil. Unsteady velocity fields for a NACA 0012 were calculated from time-resolved particle image velocimetry measurements. For all airfoils, the unsteady airloads were found to be large in magnitude near stall, when an unstable shear layer induces unsteady flow on the suction side of the airfoil. At post-stall angles of attack, the unsteady airloads generally decreased as a region of separated flow builds over the suction side. As the angle of attack was increased further, the airloads become periodic and the flow entered the deep stall vortex shedding regime. At high angles of attack (30 • ≤ α ≤ 150 • ), the unsteady airloads were greatest for airfoils with a blunt aerodynamic trailing edge due to unsteady induced flow from trailing edge vortices.Aerodynamic hysteresis was shown to cause large unsteady airloads as the static angle of attack is decreased near stall. The unsteady airloads of a NACA 0012 in reverse flow were observed to be insensitive to Reynolds number due to flow separation at the sharp leading edge of this relatively thin airfoil.

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Computational Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios

Cited by 24 publications

References 0 publications

Vortex Shedding from Airfoils in Reverse Flow

Vortex Shedding from Airfoils in Reverse Flow

Aeromechanics of a Slowed Rotor

Unsteady airloads on static airfoils through high angles of attack and in reverse flow

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