Daegyoum Kim

Characteristics of vortex formation and thrust performance in drag-based paddling propulsion

SUMMARY Several characteristics of drag-based paddling propulsion are studied with a simple mechanical model and a measurement technique for mapping three-dimensional flow fields. In drag-based propulsion, the temporal change of the vortex strength is an important parameter in the relationship between vortex formation and thrust generation. Our results indicate that spanwise flow behind the paddling propulsor significantly affects tip vortex development and thrust generation. The distribution of spanwise flow is dependent on the propulsor shape and the Reynolds number. A delta-shaped propulsor generates strong spanwise flow compared with a rectangular propulsor. For the low Reynolds number case, spanwise flow is not as strong as that for the high Reynolds number case. Without sacrificing total impulse, the flexible propulsor can smooth out thrust peaks during sudden stroke motions, which is favorable for avoiding structural failures and stabilizing body motion. We also explored the role of stopping vortex shedding in efficient thrust generation by determining the relationship between stroke angles and total impulses generated by paddling propulsors.

Aerodynamics of a translating comb-like plate inspired by a fairyfly wing

Unlike the smooth wings of common insects or birds, micro-scale insects such as the fairyfly have a distinctive wing geometry, comprising a frame with several bristles. Motivated by this peculiar wing geometry, we experimentally investigated the flow structure of a translating comb-like wing for a wide range of gap size, angle of attack, and Reynolds number, Re = O(10) − O(103), and the correlation of these parameters with aerodynamic performance. The flow structures of a smooth plate without a gap and a comb-like plate are significantly different at high Reynolds number, while little difference was observed at the low Reynolds number of O(10). At low Reynolds number, shear layers that were generated at the edges of the tooth of the comb-like plate strongly diffuse and eventually block a gap. This gap blockage increases the effective surface area of the plate and alters the formation of leading-edge and trailing-edge vortices. As a result, the comb-like plate generates larger aerodynamic force per unit ar...

Free-fall dynamics of a pair of rigidly linked disks

We investigate experimentally the free-fall motion of a pair of identical disks rigidly connected to each other. The three-dimensional coordinates of the pair of falling disks were constructed to quantitatively describe its trajectory, and the flow structure formed by the disk pair was identified by using dye visualization. The rigidly linked disk pair exhibits a novel falling pattern that creates a helical path with a conical configuration in which the lower disk rotates in a wider radius than the upper disk with respect to a vertical axis. The helical motion occurs consistently for the range of disk separation examined in this study. The dye visualization reveals that a strong, noticeable helical vortex core is generated from the outer tip of the lower disk during the helical motion. With an increasing length ratio, which is the ratio of the disk separation to the diameter of the disks, the nutation angle and the rate of change in the precession angle that characterize the combined helical and conical k...

Aerodynamic characteristics of unsteady gap flow in a bristled wing

A micro-scale flying insect has a unique wing configuration consisting of a central frame and several bristles. For the low-Reynolds-number regime in which the insect lives, the bristled wing utilizes a virtual fluid barrier inside gaps produced by strong viscous diffusion of shear layers to overcome its morphological limitations. Considering the unsteady flapping motion of such a wing, the aerodynamic characteristics of gap flow formation are investigated numerically using a two-dimensional bristled wing model for a wide range of Reynolds numbers. Inside a gap between bristles, the development of a stopping vortex during the deceleration phase and its effect on the extinction of an existing vortex generated at the same edge are dependent on the Reynolds number, which leads to a significant change in vorticity distribution at stroke reversal even with just a small change in the Reynolds number. As the Reynolds number decreases, the gap flow responds more rapidly to wing motion, and its pattern does not deviate significantly from the kinematics of the wing. A noticeable difference is also observed in the behavior of the aerodynamic force acting on each bristle at low and high Reynolds numbers. With regard to aerodynamic force generation by the bristles, each bristle behaves independently and produces similar force because of strong gap flows relative to the wing at high Reynolds numbers. Meanwhile, at low Reynolds numbers, each bristle experiences a different force depending on its relative position, which indicates the existence of collective interaction of bristles through a virtual fluid barrier.A micro-scale flying insect has a unique wing configuration consisting of a central frame and several bristles. For the low-Reynolds-number regime in which the insect lives, the bristled wing utilizes a virtual fluid barrier inside gaps produced by strong viscous diffusion of shear layers to overcome its morphological limitations. Considering the unsteady flapping motion of such a wing, the aerodynamic characteristics of gap flow formation are investigated numerically using a two-dimensional bristled wing model for a wide range of Reynolds numbers. Inside a gap between bristles, the development of a stopping vortex during the deceleration phase and its effect on the extinction of an existing vortex generated at the same edge are dependent on the Reynolds number, which leads to a significant change in vorticity distribution at stroke reversal even with just a small change in the Reynolds number. As the Reynolds number decreases, the gap flow responds more rapidly to wing motion, and its pattern does not de...

Flow Visualization of a jet generated by a sweeping jet actuator

A sweeping jet actuator (SJA) is an instrument generating pulsing jets with no moving elements. Because of its simple design and high durability to shock and vibration, SJA has recently drawn increasing attention for the application to flow control such as aerodynamic control of a wing and thrust vectoring of a jet engine. However, experimental and numerical studies on SJA have been limited to internal flow structure of SJA. In this study, we investigated the flow structure and its variation in the outlet of SJA. We carried out the experiment to understand the flow structures using PIV (Particle Image Velocimetry). The flow structure varies with a degree of the outlet and volume flow rate. There is leaking process during half jetting cycle. The process of the main jet can occur because the jet moving time increased from one side to the other side.