STUDY OF FLOW AROUND NACA 0020 AEROFOIL WITH HAIRY FLAPS DURING RAMP-UP MOTION

The challenge of controlling flow separation on the suction side of wings at high angle of attack is the focus of many research activities. Among various designs and ideas, in the last decade biomimetic approaches, especially focusing on emulating the flying modalities of birds, have caught the attention of several researchers. In particular, the self-adaptive characteristics of the wings feathers have inspired passive control methodologies to delay and/or to palliate flow separation in critical flight conditions[2]. One of the observed features on the flight of certain types of birds is the feathers pop up induced by back-flow from the trailing edge towards the leading edge, which occurs when flow separates. It is reported that this mechanism counteract the back-flow, thus avoiding sudden drops in the lift force (e.g., stall condition) [1]. The self-adjusting reaction of feathers to incipient flow separation has inspired the design of hairy flaps located on the suction side of the aerofoils to mitigate the adverse effects of separation (i.e., enhanced lift) [6, 1]. Simple span-wise flaps distributed close to the trailing edge have shown to improve the lift by 10% at a chord Reynolds number of about one million [6]. A simple but appropriate study of stall performance of a given design is the ramp-up motion in which the angle of attack is increased from 0o to some angle after stall ' 20o. Brücker and Weinder [1] studied the span of the time in which it takes during the ramp-up motion for an aerofoil to stall; the authors show that the onset of stall could be delayed by a factor of 2 to 4 using a particular flaps configuration. The latter features 2 or 3 rows of flaps with the flap sizes and spacings in the order of the thickness of the separation bubble. They also observed interesting modulation of the wake in presence of flaps which may relate to the improvement of the aerodynamic performance in a stalled configuration. When the flaps pop up as a result of the interaction with the separating bubble, they interfere with instability of separated shear layer and inhibit merging of rollers which is believed to be a reason for stall. The separated rollers are shed downstream with a particular frequency despite the turbulent nature of the shear layer in the absence of flaps. Although a number of experimental measurements have put forward the potential for self adjusting hairy flaps in controlling separation bubbles in stalled and pre-stalled aerofoils conditions, to our knowledge no high fidelity numerical simulations have been carried out so far. Those simulations would pave the way towards a more detailed understanding of the physical mechanisms that determine their interaction with the separated flow field thus enabling the design of more efficient configurations. In this work, we aim to simulate for the very first time three-dimensional turbulent flow around an NACA0020 aerofoil with hairy flaps installed on the suction side and close to the trailing edge during a ramp-up motion. Through the analysis of the accumulated simulation data we will also shed some light onto the fluid interaction mechanisms that may introduce beneficial effects on the control of the separated regions.