Numerical Simulation of a Passive Control of the Flow Around an Aerofoil Using a Flexible, Self Adaptive Flaplet

Abstract

Self-activated feathers are used by almost all birds to adapt their wing characteristics to delay stall or to moderate its adverse effects (e.g., during landing or sudden increase in angle of attack due to gusts). Some of the feathers are believed to pop up as a consequence of flow separation and to interact with the flow and produce beneficial modifications of the unsteady vorticity field. The use of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to the mentioned aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material. In this framework, this numerical study is divided in two parts. Firstly, in a simplified scenario, we determine the main characteristics that render a flap mounted on an aerofoil at high angle of attack able to deliver increased lift and improved aerodynamic efficiency, by varying its length, position and its natural frequency. Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the flaplet movement induced by the interaction with the flow field. The parametric study that has been carried out, reveals that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20o degrees. The results obtained from the direct numerical simulation of the flow field around the aerofoil equipped with the optimal flap at a chord Reynolds number of 2 × 104 shows that the flaplet movement is mainly induced by a cyclic passage of a large recirculation bubble on the aerofoil suction side. In turns, when the flap is pushed downward, the induced plane jet displaces the trailing edge vortices further downstream, away from the wing, moderating the downforce generated by those vortices and regularising the shedding cycle that appears to be much more organised when the optimal flaplet configuration is selected.

Passive control of the flow around unsteady aerofoils using a self-activated deployable flap

“…A proper design must keep into account that the flap motion and its effects on the flow field are controlled by various parameters, such as its length, inertia, position, the torsional spring stiffness and its damping factor. In a previous study, focusing on stall at fixed angle of attack (at α = 20 o ), we have carried out a parametric study aimed to identify an optimal control condition defined as the one that delivers the highest lift coefficient C L while preserving or improving the aerodynamic efficiency E = C L /C D (see [32]). For the present study, where the incidence varies with time, we proceed with a similar parametric study but pursuing a different aerodynamic condition.…”

“…The chosen stiffness values correspond to flap natural frequencies ranging between the half and the double of the shedding frequency of the baseline foil at the maximum angle of attack achieved in the ramp-up motion, i.e., f 0 = 0.58U ∞ /c at α = 20 o . In previous studies (see [31] and [32]), we have also analysed both the effects of the flap hinge position, and of a different configuration consisting of two flaps positioned in tandem on the suction side of the aerofoil. The effect of these parameters has not been considered in this study, since it was found that the aerodynamic performances were not very sensitive to their introduction.…”

“…Moreover, they suggest that the onset of the shear layer non-linear growth is delayed via modelocking of the fundamental instability mode with the motion of the flaps. Rosti et al [31,32] performed a DNS of the flow around a NACA0020 aerofoil with a flap mounted via a torsional spring on its suction side at a fixed angle of attack 20 o . They found an increase in lift by around 20% when the spring stiffness, geometry and location are properly tuned.…”

Wake characteristics of a freely rotating bioinspired swept rotor blade

“…The wake of the straight blade exhibits a strong recirculation region whereas the swept-back blade wake does not. This phenomenon might be attributed to the straight blade profile which may have been subjected to a critical angle of attack at the blade root due to the high effective angle of attack [ 44 , 45 ]. On the other hand, the swept-back blade does not show evidence of strong flow separation, and its wake exhibits nearly constant velocity deficit at different streamwise positions.…”

Numerical Simulation of Flow over Bionic Airfoil

“…Ge et al [17] extracted the bionic airfoil from the wing of the long-eared owl and cut out a slat on the upper surface of the bionic airfoil leading edge to form a multielement airfoil and then carried out the numerical simulation. Rosti et al [18] focused on the use of passive, self-actuated flaps as lift enhancement devices in nominally stalled conditions. They firstly designed the optimal motion parameter configuration for flap in 2D case and then focused on the interaction between the self-actuated flap devices and the unsteady flow field generated by wing at high angle of attack and how to improve the aerodynamic efficiency of a stalled wing.…”