Passive hovering of a flexible -flyer in a vertically oscillating airflow

We numerically investigate the passive flight of a flexible $\unicode[STIX]{x039B}$-flyer in a vertically oscillating airflow with zero mean stream. The flexibility of the flyer is introduced by a torsion spring installed at the hinged joint. We study the effects of spring stiffness, density, resting angle and actuation efforts on the hovering performance. The results suggest that the occurrence of resonance in flexible flyers can result in significantly different performances in flexible and rigid flyers. It is found that flexibility can have two opposing effects, reducing or increasing the actuation efforts for hovering, depending on the range of driving frequency. This result is explained by the modulation of relative motion between the flyer and the imposed background flow due to the involvement of passive angular oscillation. The angular oscillation patterns, the wake symmetry properties and the postural stability behaviours under different driving conditions are also explored. Based on the findings of the present study, the ideal parameter values for stable hovering are suggested. The results of this study offer novel insight into the mechanism by which the flexibility of the flyer affects the passive hovering performance.

van der Pol-Duffing Modeling for Torsional Galloping

This study is a continuation of the study presented in OMAE 2012 entitled: “Stability Analysis of a Yawing Flat Plate into the Water Current”[1]. The structure in the aforementioned study consists of a rectangular flat plate with an elastic axis in its mid-chord length. The elasticity is provided by torsion spring. The flat plate has only one degree of freedom which is angular oscillation about its axis. It is observed that as the current speed exceeds a critical velocity, the flat plate becomes unstable. The instability leads to torsional galloping occurrence, as a result of which the flat plate begins to oscillate angularly about the elastic axis. Through the present study, a phenomenological model is developed based on van der Pol-Duffing equation, in order to explain the instability leading to the torsional galloping.

Microstructure and Mechanical Properties of Squeeze Cast Semi-Solid Materials

Several rheocasting processes have been developed or applied in the world. One of the new rheocasting processes is the limited angular oscillation (LAO), in which the molten metal is rapidly cooled and slightly mixed during initial stages of solidification. Squeeze casting (SQC) using semi-solid slurry produced by LAO (Rheo-SQC) has been developed. Microstructure and mechanical properties of squeeze cast semi-solid slurries have been investigated. Complete parts with little defects have been produced. The ultimate tensile strength and elongation of semi-solid cast samples are higher than those of the liquid cast samples. It can be concluded that the rheo-SQC is a feasible process.

Development of an Angularly Oscillating Wave Energy Converter

The averaged power density of ocean waves is about 25 times as high as that of solar or winds. Yet, energy harvesting from ocean waves is far less competitive than that from solar and winds nowadays. The primary hurdle is the high installation/maintenance cost associated with wave energy harvesting devices. The present research focuses on the development of a wave energy converter (WEC) that is expected to have negligibly low cost on installation and maintenance. To achieve this goal, a new working mechanism is applied. The enabled WEC is a surface-floating device. It can be loosely anchored to the seabed through single-point slack mooring; that makes the installation as easy as anchoring a boat. The WEC uses wave-enabled angular oscillation to harvest energy. Such angular oscillation directly turns into nearly the same angular oscillation between the rotor and stator of a specially designed electric generator. The whole system is encapsulated in a rigid and watertight buoy — the hull of the WEC, thus the WEC is corrosion free. Furthermore, the only parts that subject to wear in the entire system are a couple of high-endurance bearings, which may make the WEC maintenance free in its designed lifespan (e.g., 5 years). In this paper, we present and discuss the design and testing of our first prototype WEC. Experimental exploration from hydrodynamic perspective was conducted in a wave tank to improve the shape design of the buoy, which plays a critical role on exciting large angular oscillation of the WEC in waves. Numerical simulation from electromagnetic perspective was carried out to guide the design of the electric generator; the resulted generator is capable of working efficiently in slow angular oscillation mode (e.g., at 1 Hz or lower).