A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.
The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25 · 10 5 to 3.75 · 10 5 . Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δc l ≈ 0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic stall vortex were fully suppressed. This led to a significant reduction in lift hysteresis and form drag while simultaneously mitigating moment coefficient excursions.airfoil pitching frequency, Hz h = control slot height (1.2 mm) k = reduced pitching frequency; πfc∕U ∞ M = Mach number N = number of samples Re = Reynolds number; U ∞ c∕ν s = airfoil span (610 mm) u = streamwise velocity component (wind-tunnel frame of reference), m∕s U e = local boundary-layer edge velocity, m∕s U j = blowing jet velocity, m∕s U ∞ = wind-tunnel speed, m∕s _ V = volumetric blowing flow rate, m 3 ∕s x, y = chordwise and chord-normal positions (airfoil frame of reference), m x 0 , y 0 = streamwise and normal positions (wind-tunnel frame of reference), m y m = distance of the point of maximum jet velocity from the wall, m α = angle of attack, deg α s = static stall angle, deg Δc l = relative change in lift coefficient produced by control η = angle of the control slots relative to the airfoil surface (20 deg) ν = kinematic viscosity, m 2 ∕s σ c l = standard deviation of the lift coefficient σ c m = standard deviation of the moment coefficient ϕ= phase angle of the sinusoidal pitching motion, deg ω = airfoil pitching angular frequency; 2πf ω z = nondimensional vorticity; ∂v∕∂x − ∂u∕∂y · c∕U ∞
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