Abstract
The stall of wings and blades when the angle of attack exceeds the stall angle is a viscous flow phenomenon which limits the maximum lift force that can be achieved. This is highly undesirable for fixed wings, rotorcraft and wind turbines regardless of the size and Reynolds number range in which they operate. This study aims to develop a novel passive flow control method for aerofoils and wings at post-stall angles of attack which can be achieved by the fluid-structure actions between the leading-edge flag, a highly flexible compliant structure of length much less than the chord, and the surrounding flow, as flow separation and stall typically start suddenly near the leading edge for low Reynolds numbers flows. Two-dimensional aerofoils (symmetrical NACA0012, cambered NACA6409, flat plate with a sharp leading edge), finite wings (including a rectangular wing and a tapered wing with taper ratio of 0.33), and non-slender delta wings (leading edge sweep angles of 40° and 50°) were investigated. The experimental techniques involved in the present study include force measurements, flow visualisations by means of particle image velocimetry measurements, flag displacement measurements through digital image correlation method, and hot-wire measurements for flow velocity fluctuations. The freestream velocity was fixed at 15 ms-1, corresponding to a chord Reynolds number of 100,000 for aerofoils and finite wings, and 200,000 for non-slender delta wings. With regard to the possible applicability of this research, the testing conditions of experiments within this project are similar to those for the micro air vehicles (MAVs) in low Reynolds number flows. It is also believed that the concept can be extended to higher Reynolds numbers.In this study, substantial time-averaged lift enhancement as well as delay of the aerodynamic stall were found possible for all aerofoils and wings tested. The flow control mechanisms for aerofoils and finite wings are similar, where the flag oscillations are coupled with the natural wake instability of the clean aerofoils/wings. At the post-stall angles of attack, leading edge flags exhibit self-excited oscillations and form leading-edge vortices periodically. The leading-edge vortices then reattach to the aerofoil/wing surface, as indicated by the closed separation bubbles downstream of the flag visualised in flow field measurements. Two types of flag configuration are studied, including compliant flags made of soft latex membrane and nearly rigid flags fabricated based on compliant flags partially reinforced with plastic shims. The latter has larger bending stiffness in general and experienced better spatial and temporal coherence of the flag oscillations. Larger lift enhancements could be achieved by nearly rigid flags than compliant flags. The optimal flags have oscillation frequencies within a narrow band between the fundamental and subharmonic frequencies of the natural vortex shedding, revealing a wake resonance phenomenon.
The lift enhancement and stall delay effects observed for the rectangular wing and tapered wing are fundamentally the same as those on aerofoils, although the magnitude of lift enhancement is smaller due to the existence of the tip vortex. There is no noticeable difference in the flag oscillation frequency for flags on aerofoils or finite wings. Significant three-dimensionality is reported for flag oscillations on a linearly tapered wing, represented by a phase lag of the flag oscillations towards the wing tip. Oblique vortex shedding from the flag and the trailing edge is revealed by phase-averaged flow fields, assuming the flow has the same phase lag as that measured for the flag oscillations in different spanwise planes.
Unlike aerofoils and finite wings, the lift enhancement mechanism for flags on non-slender delta wings is the excitation of the shear layer. Flag oscillations are observed at pre-stall and post-stall angles of attack, although time-averaged lift enhancement only exists in the post-stall regime of the clean wings. Flags oscillate at their natural frequency, with the flag mass ratio being the dominant parameter. At post-stall angles of attack, shear layer reattachment onto the wing surface and re-formation of the leading-edge vortices are revealed by flow field measurements, whereas the clean wing has fully stalled flow at the same angle of attack. Lift enhancement can be obtained over a wide range of flag mass ratio and length, given the dimensionless frequency is in the optimal range and the flag tip velocity amplitude is sufficiently large.
| Date of Award | 27 Mar 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Zhijin Wang (Supervisor), Samuel Bull (Supervisor) & Ismet Gursul (Supervisor) |