Experimental Evaluation of Wind Tunnel Test Apparatus for Active Flow Control

Abstract

Active control of flow dynamics is an ever-increasing field of study. This paper presents details on a new experimental apparatus for the testing of dynamic fluid-structure interaction and the control thereof. Specifically, the experimental apparatus involves an open return wind tunnel with an actuated rigid airfoil. The airfoil is replaceable, but this initial evaluation uses a flat plate as a basic geometry airfoil. The pitch of the airfoil is actively controlled in real-time via an angle sensor, microcontroller, and brushed DC electric motor. For a given PID controller, sine sweeps are performed to identify the closed-loop frequency response at varying wind speeds. The wind creates a destabilizing feedback torque on the airfoil which increases with wind speed and results in a change in the closed-loop frequency response. A model of the system is augmented with a mathematical model for the wind dynamics. The assumed form wind model is manually tuned to match the closed-loop frequency response for all wind speeds. Finally, steady-state tests are conducted in which the setpoint for angle of attack is incrementally changed and the frequency spectrum is found for the angle sensor, in the control loop, and a hotwire behind the airfoil which is not in the control loop. The results from the angle sensor show the system holds steady, with slight variation at low frequencies for high angles of attack. The results from the hotwire show a distinct sheading frequency at low angles of attack, which is replaced by chaotic low frequency flow at high angles of attack characteristic of flow separation.