Associate Laboratory of Energy, Transports and Aeronautics

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Publications

Propulsive Enhancement and Dynamic Stall Mitigation in Flapping Airfoils

Publication . Camacho, Emanuel António Rodrigues; Silva, André Resende Rodrigues da; Marques, Flávio Donizetti

The study of oscillating airfoils is the centerpiece for the biomimetization of flight, exploring newer energy-extracting devices, and improving rotor blade design. Typically, these are affected by highly nonlinear aerodynamic effects, which in most cases, still require substantial research regarding the aerodynamics and parameters that govern these systems. The present investigation looks into some of these topics by scrutinizing two primary fronts: the propulsive enhancement and the dynamic stall mitigation of flapping airfoils. After a comprehensive review of past investigations, the current work proposes a newer airfoil, the NACA0012-IK30 airfoil, that introduces the concept of dynamic curvature. This bioinspired concept is applied to the conventional NACA0012, producing an innovative geometry by dividing the airfoil into two parts, where we can deflect the leading edge independently from the rest of the airfoil. This newer design went through parametrical and optimal analysis by employing a diverse set of experimental and numerical methodologies, divided into two domains: lower Reynolds number, where the propulsive capabilities are studied, and higher Reynolds number, where dynamic stall mitigation is explored. At the lower spectrum of the Reynolds number, results show that the activation of the leading edge offered by the IK30 mechanism demonstrated the ability to improve considerably the propulsive power and efficiency. When comparing the proposed geometry with standard flapping, it is clear that the suggested design can provide optimal thrust with near-optimal propulsive efficiency, something unachievable in traditional flapping. Results obtained at the higher range of the Reynolds number, where the IK30 mechanism functions with dynamic stall mitigation purposes, show the adequacy of the proposed geometry under different plunging and pitching conditions. When correctly deflecting the leading edge, the aerodynamic stall experienced both in static and dynamic conditions can be mitigated or even eradicated, leading to significant drag reductions and modest lift enhancements. While the present study yielded encouraging results, the IK30 mechanism and the broad concept of dynamic curvature need to be the focus of further research. Using newer technologies, for instance, continuous camber morphing, coupled with ingenious kinematics, will allow us to explore newer pathways to extend our knowledge and exploit the aerodynamics of unsteady airfoils.

2024-11-08Doctoral thesis

Open access

Insights on bubble encapsulation after drop impact on thin liquid films

Publication . Ribeiro, Daniela; Panão, Miguel; Barata, Jorge M M; Silva, A. R. R.

The accurate understanding of the phenomenology of drop impact onto dry/ wetted and cold/heated surfaces is increasingly relevant to implement biofuels in civil aviation. The outcome of drop impact depends on the pre-impact conditions and a seldom researched event is the encapsulation of a bubble when this impact occurs on thin liquid films. Therefore, the goal of the experimental work reported is to investigate the mechanism of this bubble encapsulation. Results show that the mechanism leading to a bubble formation has two stages. In the first stage, after the drop impacts a steady liquid film, a prompt splash occurs followed by a crown splash. The uprising sheet propagates in an almost normal direction relative to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown-bounding rim encapsulating air inside the dome. In the second stage, three different phenomenologies of bubble encapsulation can occur. At the top of the closed crown, one jet (phenomenology 1) or two jets are formed (phenomenologies 2 and 3). For phenomenology 2, the upward jet eventually collapses due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin sheet, forming a bubble. In phenomenology 3, the upward jet is high enough to allow its breakup and ejection of one large droplet before the collapse of the upward jet. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. Additionally, the first perturbation imposed on the liquid film by the droplet impact is studied and an empirical correlation is proposed for its propagation velocity. Finally, bubble geometry is investigated.

2023-03-30Journal article

Open access

The impact of temperature on heated liquid films: Crater and jetting impact dynamics

Publication . Vasconcelos, Daniel; Silva, A. R. R.; Barata, Jorge M M

The droplet impact phenomena onto liquid films are a field extensively researched for over a century, which are driven by many practical applications such as heat exchangers, internal combustion engines and spray cooling. Despite the extensive work on wetted surfaces, the influence of temperature on droplet outcome, local evaporation/boiling effects, and liquid film stability has been overlooked in the literature. Therefore, the main objective of this work is to evaluate the influence of the liquid film temperature on the crater and jet dynamics. The experimental setup was designed for this purpose, in which a borosilicate glass surface that contains the liquid film is placed over an aluminium block with embedded cartridge heaters, heating it by conduction. Water, n-decane and n-heptane are the fluids adopted for the experiments due to their differences in thermophysical properties and saturation temperature. Different conditions are considered, which include two dimensionless thicknesses, h*= 1.0 and h*=1.5, and a range of dimensionless temperatures, theta = 0, theta = 0.2, theta = 0.4 and theta = 0.6. Qualitative and quantitative analyses are performed regarding crater evolution, and central jet height and breakup measurements, respectively. Evaporation rate measurements are required due to the influence on the liquid film thickness variation. Qualitative results show that temperature differences promote the formation of recirculation zones near the impact surface and the crater boundaries, as well as the influence on the crater shape and curvature. In terms of the quantitative analysis, the central jet height measurements for the n-heptane and n-decane reveal that higher values of the dimensionless temperature lead to an increase in the jet height, as well as promoting and increasing the occurrence and number of secondary droplets, respectively. Water follows a similar trend with the exception of theta = 0.2, which can be explained by a time scale analysis.

2023-05-12Journal article

Open access

Optimal Operation of the NACA0012-IK30 Airfoil

Publication . Camacho, Emanuel A. R.; Silva, A. R. R.; Marques, Flávio D.

The kinematics of oscillating airfoils are crucial to understanding subjects such as rotor dynamics and bio-inspired flows. Unsteady airfoils have been studied extensively, but there is an overall lack of knowledge regarding newer and more complex kinematics. The present paper builds upon previous studies of the NACA0012-IK30 airfoil by implementing a gradient-based method that searches for a leading-edge pitching amplitude that maximizes propulsive power. All of this is done numerically by solving the Reynolds-Averaged Navier-Stokes equations coupled with the Intermittency Transition model. Results indicate that for higher reduced frequencies, higher leading-edge pitching amplitudes are required to maximize the mean propulsive power. Additionally, propulsive power is achieved with near-optimal propulsive efficiency, which is a common limitation of traditional flapping airfoils.

2023-06-08Conference object

Embargoed access

Influence of a Deflectable Leading-Edge on a Flapping Airfoil

Publication . Camacho, Emanuel A. R.; Marques, Flávio D.; Silva, A. R. R.

Flapping wing dynamics are of great interest in many research areas, such as bioinspired systems and aircraft aeroelasticity. The findings of the present study provide significant insight into the importance of the leading-edge dynamic incidence on the propulsive performance of flapping airfoils. The main objective is to improve the propulsive characteristics by adding a pitching leading-edge to a conventional NACA0012 airfoil at the lower spectrum of the Reynolds number. The problem is solved numerically at a Reynolds number of 104 under various flapping conditions. The results show that the leading-edge pitching amplitude has a great impact on the propulsive power and efficiency, providing meaningful improvements. The required power coefficient is reduced overall, although not as significantly as the propulsive power. The influence of the movable leading-edge on the pressure distribution is analyzed, showing that the enlargement of the frontal area is the root cause of propulsive augmentation. The proposed geometry provides an innovative way of flapping an airfoil with propulsive purposes, offering remarkable improvements that can defy conventional flapping.

2023-07-05Journal article

Open access