Albert Medina

Lift Disturbance Cancellation with Fast-Flap Actuation

Experimental results examining a rapidly-deflecting simple flap of a wall-to-wall NACA 0006 wing in a water tunnel at Re = 40k are presented for a survey of flap deflections designed to negate the lift-transient from an imposed plunge motion of the entire wing. The plunge, over a period ranging from eight convective times to one convective time, is regarded as a vertical disturbance, or a “gust”. The flap deflection history is initially derived from Theodorsen’s formula for unsteady flap motions, from which one obtains phase and amplitude information. The theoretical derivation makes the standard assumptions of attached flow, planar wake, and no leading edge vortices. Experimental data measurements of lift on the fore-element and the flap of the wing are examined for pure-plunge, for pure flap deflection, and for the combined airfoil plunge and flap deflection motion. The latter shows up to 87% of lift cancellation, verifying the limited, but substantial applicability of Theodorsen’s formula. Improvements over the theoretical formulation of lift cancellation are sought by constructing empirical models for both airfoil plunge and flap deflection. The empirical models for airfoil plunge and flap deflection are constructed independent of one another and their superposition is employed to approximate the total lift in combined plunge and deflection motions. It is shown that although the empirical model approach performs similar to the inviscid theory of Theodorsen’s model, the empirical model proves more effective in suppressing the leading-edge vortex induced in plunge.

Separated flow response to rapid flap deflection

The transient response of a massively separated flow over an airfoil to rapid flap actuation is presented. A NACA 0006 airfoil is oriented at a fixed incidence of 20 deg for a Reynolds number of Re 4 × 104. The experiments are performed in awater tunnelwith awing spanning thewidthof the test section to produce anominally two-dimensional flowfield. The airfoil is bisected about the midchord position, resulting in a 50%-chord trailing-edge flap. The flap is rapidly deflected in a smoothed-ramp profile over a range of deflection speeds and amplitudes. The flap maneuver is completed in a fraction of a single convective time. Focus is given to a deflection amplitude of 2 deg to minimize geometric deviation from the nondeflected configuration. The desired response to such a flap motion is the evocation of vortical transients conducive to lift enhancement. Through this study, twodistinct transient responses are observed that are directionally dependent on flap actuation. In motions resulting in an increase in airfoil camber, the lift is increased instantaneously tomodest values before relaxation to a separated steady state. In effect, thismode expedites convergence to the steady-state value of the final airfoil configuration and is devoid of the “antilift” spike associated with the discrete actuation of conventional fluidic actuators. In motions resulting in a decrease in airfoil camber, the lift profile is characterized by an initial reduction before a surge in lift, culminating in a global peak and followed by relaxation. Both deflection modes prove disruptive to the leading-edge shear-layer dynamics through trailing-edge actuation and are cause for rollup of a leading-edge vortex.Ridges of the finite-timeLyapunov exponent field are used to determine that the net decrease in cambermotion induces significant entrainment near the trailing edge, leading to a smaller recirculation region and reattachment of the flow above the suction surface trailing-edge region. The net increase in cambermotion does not generate this entrainment, and therefore yields a significantly larger recirculation region.