Plunging Low Aspect Ratio Wings in Low Reynolds Number Flows

Abstract

A growing desire exists to develop Micro Air Vehicles (MAVs) that fall within a 15cm span. Their small scale and low operating flight speeds encourage a low Reynolds Number (Re) regime, in the order of Re ~ 104 - 105. Wings under these conditions are highly susceptible to separated flows, posing a significant challenge for the MAV. Natural flyers are able to confront these issues through flapping flight, which has inspired an entire research field on the aerodynamics of oscillating wings. While the number of parameters that govern the problem is exhaustive, studies are required to explain the contribution of each and any phenomena that may ensue. This lends itself to a canonical approach. This thesis presents an experimental study on various wing geometries, undergoing a small amplitude oscillation in the form of a pure plunge. The focus lies on understanding the three-dimensional effects of oscillating a finite wing with a positive geometric angle of attack, to encourage greater lift than that achieved from an unforced wing. This expands on the current research which predominantly focuses on the thrust generating capabilities of a ‘flapping’ airfoil. Force measurements, hot-film measurements, Particle Image Velocimetry (PIV) and volumetric velocimetry, are used to examine the performance and flow topology that ensues from actuating the various wings.

The study presents time-averaged force measurements as a function of Strouhal number (non-dimensionalised plunge frequency) for the various low aspect ratio wings. It is shown that while the finite nature of these wings suppresses lift, significant improvements are nonetheless possible. For example, a semi Aspect Ratio = 2 NACA0012 rectangular wing, is able to achieve 180% more lift than the unforced wing. A phenomenon arises in which peaks are observed in the time-averaged lift curve, for various rectangular and delta wing planforms. This suggests optimal lift conditions at particular Strouhal numbers. In a similar manner to a 2D airfoil the oscillating wing stimulates the formation of both leading and trailing-edge vortices. The trajectory and timing of these vortices, in relation to the plunge cycle, appear to be significantly affected by Strouhal number. At particular frequencies, the vortices interact in such a way that their induced flow generates a significant region of low velocity, recirculating flow near the wing. The size of the recirculating region closely correlates with the shape of the time-averaged lift curve, agreeing well with points of troughs and peaks when this region is maximised and minimised, respectively. It is thought that these wing/vortex and vortex/vortex interactions contribute to the selection of optimal frequencies, and therefore determine optimal lift for the oscillating wing.

The effect of profile and planform shape is also considered. Thinner profiles, on rectangular wings, appear to influence the manner in which the vortices interact, by enhancing flow separation and therefore, earlier formation of the leading edge vortex. This alters its relative convection within the plunge cycle, reducing the optimal plunge frequency. Furthermore, greater improvements in lift are possible with the thinner profile, however, limited to low Strouhal numbers. The effects of different sweep angles on a delta wing are also investigated, showing higher optimal plunge frequencies for higher sweep angles. A moderate sweep angle of = 27°, is shown to improve the lift coefficient achieved by the oscillating wing.

The spanwise variation of the vortex structures is also studied, using a volumetric velocimetry technique which allows simultaneous interrogation of a finite volumetric flow field. A numerical simulation was also carried out by Dr. Miguel Visbal at that the US Air Force Research Laboratory at the Wright-Patterson Air Force Base for the sAR = 1 rectangular flat plate. This was compared to the experimental data obtained at the University of Bath. A good agreement between the two is provided in this thesis. Leading edge vortices are observed to form on the low aspect ratio wings, initially forming a loop with trailing edge vortex. While a wake vortex system evolves downstream, the leading edge vortex separates and anchors onto the surface in order to satisfy Helmholtz’s Second Theorem, evolving in a manner which is highly Strouhal number and planform dependent. In some cases, a strong spanwise undulation is observed along the leading edge vortex filament, coinciding with a peak in the time-averaged lift curve. In relation to the tip vortex, the study shows that at sufficiently high frequencies, a novel ‘tip vortex ring’ appears, consisting solely of tip vortices from different half cycles. Its self-induced velocity propels it away from the wing in the spanwise direction. To study the effect of planform, these three-dimensional structures are compared across different planforms for a range of Strouhal numbers.

Date of Award	1 Jul 2014
Original language	English
Awarding Institution	University of Bath
Supervisor	Ismet Gursul (Supervisor) & Zhijin Wang (Supervisor)