The flow development and force generation of a two-dimensional flat plate undergoing a pitch ramp-hold-return motion are reported. Direct force measurements and PIV flow field measurements are described. The experiments were conducted in water at a Reynolds number based on free stream speed and chord of approximately 5,000 and reduced frequency k = 0.2, where the reduced frequency is based on the rate of change of the pitch angle normalized with chord and free stream speed. The maximum pitch angle and the duration of the hold part of the motion were varied and their impact on the force generation and flow evolution were measured. As expected the force and flow evolution during the pitch ramp part of the motion is independent of the hold and return parts and is in agreement with previous work reported in the literature. It is found that the evolution of aerodynamic parameters during the hold part is a function of time and maximum pitch angle but independent of hold duration. However the force and flow evolution during the return part depends on the hold duration and maximum pitch angle. PIV measurements are used to document the evolution of the LEV and TEV vortices.

No abstract

No abstract

The unsteady aerodynamics of pitching wings at high reduced pitch rate is investigated experimentally and theoretically. Simple potential flow analysis is used to compute lift, drag and pitching moment, and compared to experimental measurements. The wing motion is a linear pitch ramp between 0 and 45 degrees with smoothing at the start and end of the motion. Recent experimental results are reported for several reduced pitch rates in the range K = 0.06 and 0.39 which corresponds to pitch times of 1 and 6 convective times, respectively, and for several wing planform geometries, pivot locations and Reynolds numbers. It is shown that the lift during the motion is in agreement with linear potential flow theory including rotation rate and finite span effects. The theoretical predictions significantly underestimates drag coefficients in the measurement. At high rotation rates the wing planform shape significantly impacts aerodynamic force for leading edge pivot with a triangular wing producing 25% more transient lift than trapezoidal and rectangular wings. The effect of Reynolds number and smoothing kinematics are investigated experimentally. At high reduced pitch rates a longer smoothing transient produces larger transient lift coefficients.

We consider aerodynamic forces and flow development for several wing planforms with the same aspect ratio (AR = 4) and mean chord, undergoing a constant-rate pitch change between 0 and 45º. The wing planforms studied are rectangular, trapezoidal and triangular, which correspond to taper ratios 1, 0.5 and 0, respectively. Three pivot axes are considered: leading edge, mid chord and trailing edge. The reduced pitch rate based on chord and free stream speed is K = 0.39 and the Reynolds number is Re = 8.9k. We consider also the case K = ∞ with the same rotation rate as the other cases and zero free stream velocity. Noncirculatory effects are found for leading edge and trailing edge pivot axes, and absent for mid-chord pivot for all wing planforms. The lift and drag coefficients during the constant pitch rate part of the motion increase as the taper ratio decreases due to rotation rate effects, which is contrary to steady flow behavior in which lift and drag coefficients are only functions of aspect ratio, independent of wing planform geometry. Lifting-line theory including rotation rate effects gives reasonable estimation of lift. Flow visualization show primarily 2D flow during the pitch-up motion. The three-dimensional swirl flow in the wake due to tip vortices is observed after the end of the pitch motion. It develops faster for lower tapper ratio wings. OH 45433-7542, Michael.Ol@wpafb.af.mil, AIAA Associate Fellow Downloaded by KUNGLIGA TEKNISKA HOGSKOLEN KTH on July 27, 2015 | http://arc.aiaa.org | Nomenclature AR = physical aspect ratio (=2), 2 / bS , m AR eff = effective aspect ratio, 2*AR , m b = wing wetted span, 2*c, m C D = drag coefficient, 2*D/U  S, 1 C L = lift coefficient, 2*L/U ∞ S, 1 c = wing mean chord (=5.08cm), m c t = wing tip chord, m c r = wing root chord, m D = drag force, N e = relaxation coefficient, 1 F A = axial force, N F N = normal force, N Fx = X component of force in sensor frame of reference, N Fy = Y component of force in sensor frame of reference, N h = hold parameter, 1 K = reduced pitch rate, /2 m K c U     , 1 L = lift force, N  = kinematic viscosity, m 2 /s Re = Reynolds number, / cU   , 1 S = wing wetted area, * bc , m 2 St = Stoke's number, 2 / m c   , 1 s = start parameter, 1 t 1 = point in time when a wing starts to pitch-up in an unsmooth motion trace, s t 2 = point in time when a wing starts to hold in an unsmooth motion trace, s t 4 = point in time when a wing starts to return in an unsmooth motion trace, s t 3 = point in time when a wing returns back to initial position in an unsmooth motion trace, s t c = convective time, c/U ∞ , s t p = pitch time, / mm   , s U ∞ = free-stream velocity, m/s () t  = angle of attack in time, deg m  = maximum angle of attack, deg () t  = pitch rate in time, deg/s m   = maximum pitch rate, deg/s m   = maximum pitch acceleration, deg/s 2