Lan Yao

Bio: Lan Yao is an academic researcher from Hong Kong University of Science and Technology. The author has contributed to research in topics: Trailing edge & Fin. The author has an hindex of 1, co-authored 1 publications receiving 3 citations.

Effects of gradual flexibility and trailing edge shape on propulsive performance of pitching fins

Abstract: This paper addresses hydrodynamic performance of fins regarding their trailing edge convexity–concavity and flexibility distribution. The effects of trailing edge convexity–concavity on propulsive performance and vortex dynamics were investigated experimentally utilizing time-resolved particle image velocimetry and force sensors. It was found that the convex trailing edge shape always outperforms the concave shape. Wake contracting by the bent shape of the trailing edge vortex of a convex trapezoidal form resulted in higher thrust and efficiency. The results also showed that the rounded edges of fish fins did not provide additional hydrodynamic advantages. Furthermore, we found that a gradually flexible fin delivered better propulsive performance over a uniformly flexible fin. The hydrodynamic performance of the flexible fins depended on the strength and relative positions of the trailing edge vortexes shed by each fin, which were affected by the flexible deformations of the fins. In the lower Reynolds number operation (approaching, but below the first resonant mode), the fins with larger camber produced a stronger momentum footprint especially considering the far wake elements, while in the higher Reynolds number range due to resonant deformation the extent of trailing edge excursion became dominant in affecting the propulsive performance. The results showed that gradually flexible fins can improve the performance of future watercraft.

Interplay of chordwise stiffness and shape on performance of self-propelled flexible flapping plate

Abstract: The locomotion of a flapping flexible plate with different shapes and non-uniform chordwise stiffness distribution in a stationary fluid is studied numerically. The normalized effective bending stiffness K∗ for three-dimensional plates with arbitrary stiffness distribution and shape parameters is proposed, and the overall bending stiffness of non-uniform plates with different shapes is reasonably characterized. It is found that the propulsion performance in terms of cruising speed and efficiency of the self-propelled flapping plate mainly depends on the effective bending stiffness. Plates with moderate flexibility K∗ show better propulsion performance. Meanwhile, both a large area moment of the plate and a flexible anterior are favorable to significantly improve their propulsive performance. The evolution of vortical structures and the pressure distribution on the upper and lower surfaces of the plate are analyzed, and the inherent mechanism is revealed. These findings are of great significance to the optimal design of propulsion systems with different fins or wings.

Effect of non-uniform flexibility on hydrodynamic performance of pitching propulsors

Abstract: Many aquatic animals propel themselves efficiently through water by oscillating flexible fins. ese fins are, however, not homogeneously flexible, but instead their flexural stiffness varies along their chord and span. Here, we developed a low order model of these functionally-graded materials where the chordwise flexibility of the foil is modeled by one or two torsional springs along the chordline. The torsional spring structural model is then strongly coupled to a boundary element fluid model to simulate the fluid-structure interactions. We show that the effective flexibility of the combined fluid-structure system scales with the ratio of the added mass forces acting on the passive portion of the foil and the elastic forces defined by the torsional spring hinge. We further detail the dependency of the propulsive performance on the flexibility and location of the single torsional spring for a foil that is actively pitching about its leading edge. Our results show that increasing the flexion ratio by moving the spring away from the leading edge leads to enhanced propulsive efficiency, but compromises the thrust production. Proper combination of two flexible hinges, however, can result in a gain in both the thrust production and propulsive efficiency.

Hydrodynamic performance of a penguin wing: Effect of feathering and flapping

Abstract: The penguin is the fastest underwater swimmer among the wing-propelled diving birds. To figure out the mechanism for its excellent swimming, the hydrodynamic performance of a penguin wing is numerically investigated using an immersed boundary method with the incompressible flow solver. This study examines the effects of feathering, flapping, and Strouhal number (St) under preset motion. Results indicate that feathering is the primary contributor to thrust generation. The change in angle of attack (AoA) can qualitatively reflect the change in lift but not thrust. Therefore, a new variable, angle of thrust (AoT, αT), is introduced to effectively reflect the change of thrust across different kinematic parameters. Optimal feathering amplitude balances the decrease in AoA and the increase in feathering angle to achieve the highest AoT and thrust. Excessive feathering amplitude degrades the leading-edge vortex to shear layers, transforms the pressure side to the suction side, and ultimately causes negative thrust (drag). Spatial analysis of the thrust shows that the outer three-fifths of the wing are the primary source of thrust, contributing 85.4% of thrust generation at optimal feathering amplitude. Flapping amplitude has little impact on the optimal feathering amplitude. The optimal feathering amplitude increases linearly with the St number in the scope of examination, leading to larger thrust but lower swimming efficiency. Thus, a dimensionless number, Stm, is introduced to describe the optimal wing motion. This work provides new insights into the propulsion mechanism of aquatic swimmers with flapping–feathering wings and helps design novel bio-inspired aquatic vehicles.

Underwater oscillations of rigid plates with H-shaped cross sections: An experimental study to explore their flow physics

Abstract: In this work, we present a comprehensive experimental study on the problem of harmonic oscillations of rigid plates with H-shaped cross sections submerged in a quiescent, Newtonian, incompressible, viscous fluid environment. Motivated by recent results on the minimization of hydrodynamic damping for transversely oscillating flat plates, we conduct a detailed qualitative and quantitative experimental investigation of the flow physics created by the presence of the flanges, that is, the vertical segments in the plate cross section. Specifically, the main goal is to elucidate the effect of flange size on various aspects of fluid–structure interaction, by primarily investigating the dynamics of vortex shedding and convection. We perform particle image velocimetry experiments over a broad range of oscillation amplitudes, frequencies, and flange size-to-width ratios by leveraging the identification of pathlines, vortex shedding and dynamics, distinctive hydrodynamic regimes, and steady streaming. The fundamental contributions of this work include novel hydrodynamic regime phase diagrams demonstrating the effect of flange ratio on regime transitions, and in the investigation of their relation to qualitatively distinct patterns of vortex–vortex and vortex–structure interactions. Finally, we discuss steady streaming, identifying primary, and secondary structures as a function of the governing parameters.