Covert-inspired flow control devices for lift enhancement and stall mitigation

Duan, Chengfang

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https://hdl.handle.net/2142/113201 Copy

Description

Title

Covert-inspired flow control devices for lift enhancement and stall mitigation

Author(s)

Duan, Chengfang

Issue Date

2021-07-15

Director of Research (if dissertation) or Advisor (if thesis)

Wissa, Aimy

Doctoral Committee Chair(s)

Wissa, Aimy

Committee Member(s)

• Chamorro, Leonardo
• Elliott, Gregory
• Goza, Andres

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Covert-inspired flap
• Lift enhancement
• Stall mitigation

Abstract

Unmanned aerial vehicles (UAVs) are taking on more expansive roles in military and commercial applications. However, their adaptability and agility are still inferior to that of their biological counterparts like birds, especially at low and moderate Reynolds numbers. A system of aeroelastic devices used by birds, known as the covert feathers, has been considered as a natural flow-control device for mitigating flow separation, enhancing lift, and delaying stall. In order to improve the mission adaptability of small-scale UAVs, this dissertation focuses on the design and evaluation of the covert-inspired flow control device consisting of single and multiple rows of flaps for lift enhancement and stall mitigation. The effects of a single static covert-inspired flap during post-stall were first characterized. Specifically, the single flap effects on the aerodynamic forces for various single flap designs, i.e., flap locations, flap deflection angles, and flap mobility, were identified, and the flap effects on various baseline airfoils were explored at Reynolds numbers in the order of 10^5,where small scale UAVs operate. Detailed experiments and simulations were used to investigate how the covert-inspired flap affects lift and drag on an airfoil that exhibits sharp or sudden stall (i.e., the NACA 2414 airfoil) and one that exhibits soft or gradual stall (i.e., an E387(A) airfoil). The effects of the flap chord-wise locations and deflection angles on lift and drag were investigated through wind tunnel experiments for two types of flaps, namely a freely-moving flap and a static flap. Results show that the single static covert-inspired flap can delay stall by up to 5 deg and improve post-stall lift by up to 23%. However, the post-stall lift improvement characteristics and sensitivities are highly affected by the airfoil choice. For the soft stall airfoil (i.e., E387(A)), the stall onset delay is insensitive to changing the flap deflection angle, and the flap becomes ineffective when the flap location is changed. In contrast, for the sharp stall airfoil (i.e., NACA 2414), the post-stall lift improvements can be tuned using the flap deflection angle, and the flap remains effective over a wide range of chord-wise locations. Numerical studies reveal that the lift improvements are attributed to a step in the pressure distribution over the airfoil, which allows for lower pressures on the suction side upstream of the flap. These lower pressure regions on the airfoil with the flap are a result of the leading-edge vortices being closer to the airfoil surface when compared to the baseline, which increases the velocity over the suction side of the airfoil. The distinctions between the flap-induced lift enhancements on the soft and sharp stall airfoils suggest that the flap can be used as a tunable flow control device for the sharp stall airfoil, while for the soft stall airfoil, it can solely be used as a stall mitigation device that is either on or off. In an effort to design a tunable flow control device, the remainder of the dissertation focused on the sharp stall airfoil. More specifically, the aerodynamic effects and dynamics of multiple freely-moving covert-inspired flaps were characterized on the NACA 2414 airfoil, which is the sharp stall airfoil. A simplified engineering analogy of the covert inspired flow control devices with multiple flaps was created, which helped to explore the feasibility of the multiple-flap designs and helped to formulate the first hypothesis about why birds deploy multiple rows of covert feathers during high-AoA flight. Detailed wind tunnel experiments were conducted, and the results showed that the multiple covert inspired flaps have additive, thus higher, post-stall lift improvement compared to the single flap systems. The additive effects of the flap-induced lift improvements on an engineering wing section were shown for the first time. Hot wire anemometry along with numerical simulations were carried out to study the effects of the multi-flap system on the flow field, including the turbulence wake profile, velocity fluctuation, pressure distribution, and flow visualization. Moreover, the flap structural dynamics were also studied for the first time. In the post-stall regime, a flap's deflection remains the same regardless of whether other flaps are deployed or not. Data-driven model of the multi-flap system was developed to create a framework that facilitates designing a system of covert-inspired flaps for various flight conditions. The detailed aerodynamic characterization and data-driven modeling efforts of this work enable the design of multiple spatially distributed flow control devices on engineering airfoils to achieve tunable post-stall lift improvement.

Graduation Semester

2021-08

Type of Resource

Thesis

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Copyright 2021 Chengfang Duan

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