Research
Zigzag dynamics of a falling sphere with a filament
(Main investigator : Seungho Choi)
Organisms often utilize passive appendages to enhance their locomotion by effectively manipulating the surrounding flow. These passive appendages play a crucial role in influencing the fluid-dynamic forces around them, making them valuable for various engineering applications. In our experimental study, we investigate the impact of a fiber-shaped appendage on the free-fall dynamics of a sphere. We introduce a model where a filament is attached to the upper hemisphere of the sphere. Interestingly, while the sphere without the filament falls vertically, the filament induces a zigzag motion, causing the sphere to fall more slowly. This intriguing behavior holds the potential for controlling the falling motion of the sphere in practical applications.
Gravity-coupled flutter and contact of a flag near a wall
(Main investigator : Minseop Lee) doi.org/10.1017/jfm.2023.931
The stability and post-critical behaviour of a horizontal flag undergoing gravity-induced deformation and periodic contact with a nearby horizontal rigid wall are experimentally investigated. We propose a horizontal flag model in which the wall is above or below the flag to study the effects of gravity. In general, the behavior of a flag is determined by the relative ratio of the bending force of the flag and the fluid force induced by the uniform flow. However, in a flag model strongly affected by gravity, the mode and behavior of the flag are determined by the relative ratio of the gravitational force and fluid force, and the dynamics of the flag are classified into static, flutter, partial contact, and full contact modes. The results elucidate the combined effects of gravity and contact on flutter and reveal design principles for application to triboelectric energy harvesting.
Flow-induced snap-through dynamics of an elastic sheet
(Main investigator: Hyeonseong Kim)
Snap-through motion, which is a rapid transition from one equilibrium state to another state, can be utilized as a novel mechanism for the flow-induced vibration of an elastic sheet. A post-buckled sheet, which its both ends are clamped, maintains its equilibrium state at low free-stream velocity, and the sheet starts the periodic snap-through oscillation as the free-stream velocity reaches a certain critical velocity. We experimentally and theoretically investigate the critical conditions of the snap-through oscillation as well as the kinematics and kinetics of post-critical oscillation. Furthermore, we employ the principle of periodic snap-through dynamics under fluid flow to develop a novel fluid kinetic energy harvesting system.
Flow-induced vibration and energy harvesting of a cylinder between side walls
(Main investigator: Junyoung Kim)
The dynamics of a cylinder arranged between two side walls are experimentally investigated. The gap between the cylinder and the side walls is sufficiently small to allow the cylinder to impact the walls. In general, a circular cylinder undergoing vortex-induced vibration (VIV) can only oscillate in a limited range of the flow velocity. However, the periodic impact with the side walls allows the large-amplitude oscillations of the impacting cylinder to persist outside of the lock-in region. We devise a novel energy harvesting system based on an impacting cylinder model and a triboelectric nanogenerator (TENG). The impacting cylinder shows improved energy harvesting performance than the non-impacting cylinder.
Prediction of reed valve dynamics based on a deep learning model
(Main investigator: Janggon Yoo)
A reed valve is a type of passive flow controller which regulates flow in a single direction with the deflection of a flexible plate. Interaction between the flexible plate and fluid flow perpendicular to the valve generates a complicated flow structure, and the impact of the plate with a stopper induces nonlinear motion of the plate. We investigate a low-order numerical model to predict the dynamics of the reed valve under periodic pressure pulsation. Furthermore, a deep learning model is established to optimize the performance parameters of the reed value such as mass flux.
Stabilized falling motion of a bio-inspired disk
(Main investigator: Minhyeong Lee)
Some of the smallest insects such as thrips or fairyfly have novel porous wings that consist of many bristles connected to a central frame. Interestingly, these insects not only use their wings to fly actively, but also to parachute passively with no wing stroke. Because it is critical for the fliers to maintain their bodies stable during passive flight, we experimentally examine the effects of bio-inspired bristled disks on their lateral and angular motions during free fall for the Reynolds numbers ranging from O(1) to O(102). The characteristics of disk motions and flow structures are identified for the bristled disks having different numbers of bristles and a circular disk. The comparison between the bristled disks and the circular disk reveals aerodynamic principles underlying the effective stabilization of the bristled disks.
