Control of Thick Airfoil, Deep Dynamic Stall Using Steady Blowing

Abstract

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 ∞

Control of Unsteady Aerodynamic Loads Using Adaptive Blowing

“…An earlier investigation of the passive effect of the slot indicated that the position of laminar separation is fixed at the lip of the jet and that the onset of static stall is slightly delayed. 45 Therefore, the baseline data presented here somewhat deviates from results obtained on NACA 0018 airfoil models with no surface discontinuities.…”

“…This destabilizing effect of low momentum blowing on the separation bubble has previously been described in detail. 45 At α = 11 • , a similar destabilizing effect is observed at momentum coefficients below C µ ≈ 2%. In both cases a reduction of the lift coefficient of approximately ∆c l ≈ 0.5 can be obtained irrespective of the Reynolds number.…”

“…Blowing was applied from the slot located at x/c = 5% since prior investigations had shown that its proximity to the leading-edge makes control at this location far more effective and versatile than control at mid-chord. 45,46 Aerodynamic coefficients recorded during a quasistatic pitching motion at Re = 3·10 5 are presented in figure 4. With steady blowing at a sufficiently high momentum coefficient, leading-edge stall is suppressed, yielding lift coefficients far exceeding the baseline c l,max .…”

Dynamic stall on a pitching and surging airfoil

“…Particle image velocimetry (PIV) analysis of Raffel (2012, 2013) on an airfoil pitching around a statically attached angle of attack identified a 'primary stall vortex' that pinched off at the point of dynamic stall. Müller-Vahl et al (2015) used PIV and pressure measurements on a NACA-0018 airfoil pitched about the quarter chord to investigate the effect of constant blowing near the leading edge and at half chord. They found that with sufficient leading edge blowing the dynamic stall vortex can be eliminated entirely and separation can be prevented even up to α = 25 • .…”

Unsteady Thick Airfoil Aerodynamics: Experiments, Computation, and Theory

“…However, we have chosen a NACA 0018 airfoil as the focus of our study. 51 Most of the static testing of NACA 0018 airfoils has been conducted at high Reynolds numbers appropriate to the aircraft industry and large turbines. 52,53 Less research has been conducted at Re < 10 6 .…”