Open-Loop and Closed-Loop Trailing-Edge Separation Control on a Natural Laminar Flow Airfoil

Abstract

Active Unsteady Flow Control experiments were performed on a Natural Laminar Flow, NLF 0414 airfoil at Rec = 1.0 × 10 6 in a 3-ft × 4-ft low-speed, low-turbulence wind tunnel. The NLF 0414 was designed with a region of favorable pressure gradient extending almost 70% of the chord on the upper surface of the airfoil. Aggressive pressure recovery in the aft 30% of the chord near the trailing edge results in the separation of the flow from the airfoil surface at the off-design conditions. The goal of this study was to control boundary-layer separation across the trailing-edge region of the airfoil in an effort to improve the performance beyond the designed angle-of-attack range. Active control of separation was achieved using a series of fast-switching solenoid valves connected to blowing slots at x/c = 0.75 on the upper surface of the airfoil. The airfoil in its baseline configuration was first evaluated to identify the dominant modes in the spectral content of unsteady Cp. Airfoil performance data were then acquired across a parametric range of blowing amplitudes, actuation frequencies and duty cycles in order to understand the effects of variations in the major forcing parameters on the performance of the model. Phase-averaged and phase-locked planar PIV measurements were also acquired across a horizontal plane near the trailing-edge region of the airfoil model in order to examine the spatio-temporal evolution of the flowfield and understand the mechanism responsible for the alleviation of separation as a result of actuation at the different flow control settings. A closed loop controller was developed to vary the actuation parameters in-situ using sensory feedback from the unsteady surface pressure measurements. Adaptive modal decomposition methods were used to identify the frequencies of natural instabilities in the flowfield in real-time. A proportional controller was designed to automatically control the blowing amplitude by estimating the state of the flow and the extent of boundary-layer separation. The closed-loop system was able to simultaneously control the blowing amplitude and the actuation frequency such that a desired value of Cl was obtained.

Flow-separation-control system operating in feedback closed loop

“…Investigations of CLC systems controlling the flow separation through the fluidic devices can concern a wide spectrum of lifting surfaces such as: airfoils (Gupta and Ansell, 2016; Pinier et al , 2007), compressor blades (Bright et al , 2005) and high-lift systems (Chabert et al , 2013, 2014). Within this area, the studies of Seifert et al (1996), Seifert and Pack (1999) and the Darabi and Wygnanski (2004) showed that periodic blowing or suction is more efficient than steady blowing or steady suction.…”