John R. Culp

Transient Separation Control Using Pulse-Combustion Actuation

The transitory response of the flow over a stalled, two-dimensional (NACA 4415) airfoil to pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale is investigated experimentally (Re = 570, 000). Actuation is effected by momentary [O(1 ms)] pulsed jets that are generated by a spanwise array of combustion-based actuators integrated into the center section of the airfoil. The flowfield in the cross-stream plane above the airfoil and in its near wake is computed from multiple high-resolution particle image velocity images that are obtained phase locked to the actuation waveform and allow for tracking of vorticity concentrations. The brief actuation pulse leads to a remarkably strong transitory change in the circulation about the entire airfoil that is manifested by a severing of the separated vorticity layer and the subsequent shedding of a large-scale clockwise vortex that forms the separated flow domain. The clockwise severed vorticity layer that follows behind this detached vortex has a distinct sharp streamwise edge that grows and rolls up as the layer is advected along the surface. It is shown that the shedding of the severed vortex and the accumulation of surface vorticity are accompanied by a transitory increase in the magnitude of the circulation about the airfoil that lasts 8—10 convective time scales. The attached vorticity layer ultimately lifts off the surface in a manner that is reminiscent of dynamic stall, and the flow separates again.

DYNAMIC FLIGHT MANEUVERING USING TRAPPED VORTICITY FLOW CONTROL

Closed-loop feedback control is used in a series of wind tunnel experiments to effect commanded 2-DOF maneuvers (pitch and plunge) of a free airfoil without moving control surfaces. Bi-directional changes in the pitching moment over a range of angles of attack are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is applied on both surfaces by hybrid actuators that are each comprised of a miniature [O(0.01c)] obstruction integrated with a synthetic jet actuator to manipulate and regulate the vorticity concentrations. In the present work, the model is trimmed using position and attitude feedback loops that are actuated by servo motors and a ball screw mechanism in the plunge axis. Once the model is trimmed, the position feedback loop in the plunge axis is opened and the plunge axis is controlled in force mode so to maintain the static trim force on the model, and alter its effective mass. Meanwhile the servomotor in the pitch axis is only used to alter the dynamic characteristics of the model in pitch, and to introduce disturbances. Attitude stabilization and position control of the model is achieved by closing the position loop through the flow control actuators using a model reference adaptive controller designed to maintain a specified level of tracking performance in the presence of disturbances, parametric uncertainties and unmodeled dynamics associated with the flow. The controller employs a neural network based adaptive element and adaptation laws derived by a Lyapunov-like stability analysis of the closed loop system.

A Closed-Loop Flight Control Experiment using Active Flow Control Actuators

Closed-loop pitch control on a moving 1-DOF wing model is investigated in wind tunnel experiments. The models attitude is controlled over a broad range of angles of attack when the baseline flow is fully attached using bi-directional pitching moment that is effected by flow-controlled trapped vorticity concentrations on the pressure and suction surfaces near the trailing edge. In the present work, the model is trimmed using a position feedback loop and a servomotor actuator. Once the model is trimmed, the position feedback loop is opened and the servomotor acts like an inner loop control to alter the dynamic characteristics and to introduce disturbances. Position control of the model is achieved by the flow control actuation using an arbitrary reference model based adaptive outer loop controller. The control architecture employs a neural network based adaptive element that permits adaptation to both parametric uncertainty and unmodeled dynamics.

Closed- Loop Aerodynamic Flow Control of a Free Airfoil

Transitory flow arising from the dynamic response of a free-moving airfoil model to commanded pitch and plunge maneuvers is investigated in wind tunnel experiments. The airfoil is mounted on a 2-DOF traverse and its trim and dynamic characteristics are controlled using position and attitude feedback loops that are actuated by servo motors. Commanded maneuvers are achieved without moving control surfaces using bi-directional changes in the pitching moment over a range of angles of attack that are effected by controllable, nominally-symmetric trapped vorticity concentrations on both the suction and pressure surfaces near the trailing edge. Actuation is applied on both surfaces by hybrid actuators that are each comprised of a miniature [O(0.01c)] obstruction integrated with a synthetic jet actuator to manipulate and regulate the vorticity concentrations. The present work focuses on the transitory response of the flow to step-modulated changes in the actuation input while the model’s position is maintained using the systems controller. Flow control effectiveness is demonstrated by the closed-loop response in plunge to a momentary force disturbance which is analogous to the free flight response to a sudden gust.