City Research Online

Abstract

Detailed analysis of the flow around a NACA0020 aerofoil at moderate low chord Reynolds number (Rec = 2×104) in completely stalled conditions has been carried out by means of Direct Numerical Simulations. The stalled condition is either a steady configuration at a fixed angle of attack (α = 20o) or it is reached via a ramp-up manoeuvre, increasing the angle of attack from 0o to 20o. Concerning this last case, new insights on the vorticity dynamics leading to the lift overshoot, lift crisis and the damped oscillatory cycle that gradually matches the steady condition, are discussed using a number of post-processing techniques. These include a detailed analysis of the flow ensemble average statistics and coherent structures identification that has been carried out using the Q-criterion and the Finite-Time Lyapunov Exponent technique.

Based on the fundamental knowledge achieved in studying the static and the dynamic stall, we introduced a biomimetic passive control technique to mitigate the aerodynamic performance degradation typical of such flow conditions. In particular, the envisaged control technique has been inspired by the dorsal feathers that 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 producing beneficial modifications of the unsteady vorticity field. The adoption of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to their aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material.

In this framework, we have used numerical simulation to study the effects of this passive control technique in both steady and dynamic stall. In particular, for the static case, we have defined an optimal condition as the one that delivers the highest lift coefficient CL, preserving or improving the aerodynamic efficiency E = CL/CD. To achieve a condition close to optimality we started by considering a simplified scenario, to determine the main characteristics of the flap (i.e., variations of its length, position and natural frequency). Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the pop-up of the optimal flaplet. It is found that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20o degrees. The analysis of direct numerical simulation data of the flow field around the aerofoil equipped with the optimal flap allowed to elucidate the main mechanism that promotes the aerodynamic improvements. In particular, it is found that the flaplet movement, induced by the transit of a large recirculation bubble on the aerofoil suction side, displaces the trailing edge vortices further downstream, away from the wing. The downstream displacement of the trailing edge generated vortices, limits the downforce generated by those vortices also regularising the shedding cycle that appears to be much more organised when the flaplet is activated.

A similar study has also been carried out for the dynamic case. We have analysed the effects produced by the presence of an elastically mounted flap on the transient behaviour of the flow fields. For a specific ramp-up manoeuvre characterised by a reduced frequency slower the shedding one, it is found that it is possible to design flaps that limit the severity of the dynamic stall breakdown. In particular, it is possible to increase the value of the lift overshoot and to smooth its abrupt decay in time. A detailed analysis on the modification of the unsteady vorticity field due to the flap-flow interaction during the ramp-up motion is also provided to explain the physical mechanism that lead to more benign aerodynamic response.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	School of Science & Technology > Engineering Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses

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