Pulsed Blowing on a Laminar Airfoil at Low Reynolds Number

Abstract

The range of effectiveness of pulsed actuation with varying frequency, duty cycle and blowing ratio is explored on a NACA 64 3 -618 airfoil at a Reynolds number of 6.4x10 4 . Pulsed blowing from a row of discrete holes located at five percent chord on the upper surface of the airfoil successfully reduces separation over a wide range of reduced frequency (0.1-1), blowing ratio (0.5-2), and duty cycle (0.6-50%). A phase-locked investigation, by way of particle image velocimetry, at ten degrees angle of attack illuminates physical mechanisms responsible for separation control of pulsed actuation at a low frequency and duty cycle. Temporal resolution of large structure formation and wake shedding is obtained, revealing a key mechanism for separation control. The Kelvin-Helmholtz instability is identified as responsible for the formation of smaller structures in the separation region which produce favorable momentum transfer, assisting in further thinning the separation region and then fully attaching the boundary layer. The proximity to the wall dampens the Kelvin-Helmholtz instability, leaving no further evidence until the proceeding jet perturbation arrives. Nomenclaturelocal -P s, ∞ ) / (P T, ∞ -P s, ∞ )] C µ = jet momentum coefficient [see Eq. (1)] c = chord d = diameter of actuator and pressure tap holes DC = actuation duty cycle f = actuation frequency f s = shear layer shedding frequency f+ = reduced frequency (fc/U ∞ ) n = wall normal distance from airfoil surface N = number of actuation holes P = pressure Re = Reynolds number based on airfoil chord (U ∞ c/v) S = planform area Sr θs = Strouhal number based on conditions at the point of separation (f s θ s /U es ) T = actuation period t = time elapsed from beginning of actuation waveform U ∞ = freestream velocity U es = boundary layer edge velocity at the separation location x = chordwise coordinate y = wall normal distance α = airfoil angle of attack θ s = momentum thickness at the separation location ν = kinematic viscosity

Effect of Spanwise Jet Spacing on Separation Control for Swept and Unswept Airfoils

“…The NACA 64 3 -618 laminar airfoil has been studied exstensively 3,4,6,17 . It is designed to maintain laminar flow over a large portion of its surface.…”