Aerodynamic Characteristics of Finite Pitching Wings with Leading-Edge Tubercles

Abstract

Blades in rotor systems such as helicopters and wind turbines experience highly dynamic inflow conditions such as rapid pitching that can lead to dynamic stall. In turn, that causes large force fluctuations which translate into fatigue for the blade structure and controllability issues for the vehicles. Biomimetics brings a potential solution in the shape of leading edge tubercles inspired from the flippers of humpback whales. These possess impressive manoeuvrability for the size and their flippers pitch up and down rapidly. While the effect of this bio-inspired leading edge shape is well-documented in steady conditions, being known to delay stall and make its character more benign, there are very few studies investigating the effect in unsteady conditions. Therefore, the scope of this study is to analyse the effect of leading edge tubercles on the flow structure evolution throughout sinusoidal oscillations of finite wings and on their tip vortex strength.

This thesis combined two methodologies: one experimental and another computational. The experimental study focused on the wind tunnel analysis of two different rectangular finite wings with leading-edge tubercles against a straight leading edge finite wing subjected to sinusoidal pitching oscillations. The study uses 3D Particle Tracking Velocimetry (PTV) technique, with the state-of-the-art Shake-The-Box (STB) algorithm which uses Helium-filled soap bubbles as flow tracer particles in order to obtain the complex instantaneous flow field around the wings throughout the oscillation cycles. After obtaining the particle tracks, phase-averaging was used to improve the quality of experimental results by making use of the multiple oscillation cycles recorded. The computational study replicates the wind tunnel conditions. U-RANS simulations were run using the Ansys CFX solver and using a deforming mesh to model the pitching motion of the airfoil. Both the CFD and the experiment were conducted at a Reynolds number of Re = 3.3 × 10⁴.

The results of both the experimental and the CFD correlate well in terms of the flow structure, allowing for an interesting comparison between the two methods. These show that the wings with leading edge tubercles do show a more benign stall behaviour and quicker reattachment on the downstroke of the oscillations thanks to the compartmentalisation effect of the streamwise vortices shed by the tubercles. However, surprisingly the angle of attack at which the maximum lift coefficient is produced is not increased or delayed compared to the straight leading edge (SLE) wings. Furthermore, the tubercle wings exhibit reduced tip vortex strength throughout the oscillation cycle, thanks in part to the destructive effect of the streamwise vortices near the wing tip on the tip vortex. Finally, in close correlation with the tip vortex circulation, the tubercle wings have a lower induced drag coefficient compared to SLE wings, with a better span efficiency thanks to the compartmentalisation effect of the spanwise flow.