Unsteady Aerodynamics of Wings in Turbulent Wakes

Abstract

From micro air vehicles flying in the wakes of buildings to aircraft operating in terrain and ship airwakes, wind gust generates significant variation in unsteady aerodynamic loadings, adversely affect controllability, or result in sustained oscillations of elastic structures. A physical understanding of wake-wing interactions is crucial not only for improving aerodynamic characteristics of wake interaction but also for the successful development and implementation of aeroelastic and flow control strategies. In this thesis, wings in unsteady turbulent vortex-dominant wakes are experimentally investigated to study vortex-wing/wake-wing interaction mechanism and a novel passive flow control method based on vortex-wing interaction is proposed. The flow control device provides significant lift enhancement with little drag penalty when deployed upstream of the leading edge.
The unsteady aerodynamics of a stationary wing in turbulent wakes are investigated to identify the underlying flow physics during wake-wing interaction. When the wing interacts with the von Kármán vortex street and experiences temporal variations of the effective angle of attack, the time-averaged lift increases substantially at post-stall angles of attack. As the wing traverse across the wake, despite of decreasing amplitude of oscillations in the effective angle of attack, an optimal offset distance from the wake centreline where the time-averaged lift becomes maximum is observed. Placing the wing at the optimal location in the wake, the stall angle is delayed by 9^o and the maximum lift coefficient can reach 64% higher than that in the freestream. The lift enhancement is found to be primarily dependent on the temporal change in the wake angle that excite the formation of high lift leading-edge vortex. In contrast to large velocity fluctuations at the wake centreline cause excursions into the fully attached and separated flows during the cycle, small-amplitude oscillations at the optimal location result in constantly separated flow at the leading edge and formation of LEV and separation bubble with reattachment further downstream. The maximum circulation of the LEVs for the optimal wing location can reach twice that of the wing at the wake centreline, which must be due to the larger vorticity flux even though the velocity fluctuations are smaller. This is attributed to continuous vorticity shedding from the leading edge for the optimal case, whereas the vorticity shedding from the leading edge is interrupted for the periods of attached flow when the wing is located at the wake centreline.
The effect of wing geometry on the strength of the leading-edge vortices, the ratio of the spanwise length scale of the incident vortex to the wing-span, and the degree of the two-dimensionality of the wake-wing interaction were investigated. The competition between the effects of spanwise length scale of the incident wake and the strength of the leading-edge vortices determined the optimal aspect ratio, which was found to be around 4. The effect of the wing sweep angle has also been found to be significant. Increasing sweep angle, the effective spanwise section of the wing that will be interacting with the wake vortices decreases, resulting in decreased two-dimensionality and thus decreased the mean lift. The leading-edge shape affects the maximum lift coefficient because the strength of the leading-edge vortices and their distance from the wing surface depend on the leading-edge geometry. Relative to the performance in freestream, the increases in the stall angle and maximum lift coefficient were not significantly affected by the leading-edge shape.
A novel post-stall flow control method utilising unsteady wakes of compliant flags to excite LEV formation thus enhancing lift characteristics of airfoil was shown to be possible at an airfoil chord Reynolds number of 100,000. The flag wakes substantially increase in the airfoil stall angle and the maximum lift coefficient. Oscillating flags could generate periodic wakes with better spanwise coherence than the stationary bluff body. This resulted in the excitation, formation and shedding of the leading-edge vortices periodically, providing mean lift enhancement. There is an optimal range of the flag mass ratio for which the flag frequency coincides with the natural frequency of the vortex shedding instability or its subharmonic of the baseline airfoil wake. The flag dimensionless frequency is a function of the mass ratio only, which can be predicted by a reduced order model in the limit of very large mass ratio and by using the modified free-streamline theory for the separated flow.

Date of Award	11 Oct 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Zhijin Wang (Supervisor) & Ismet Gursul (Supervisor)