Jooha Kim

The alula is a small structure located at the joint between the hand-wing and arm-wing of birds and is known to be used in slow flight with high angles of attack such as landing. It is assumed to function similarly to a leading-edge slat that increases lift and delays stall. However, in spite of its universal presence in flying birds and the wide acceptance of stall delay as its main function, how the alula delays the stall and aids the flight of birds remains unclear. Here, we investigated the

Abstract In some specific conditions, a flying spinning ball deflects in a direction opposite to that predicted by the Magnus effect, which is known as the inverse Magnus effect. To elucidate when and why this effect occurs, we measure the variations of the drag and lift forces on a rotating sphere and the corresponding flow field with the spin ratio (the ratio of the rotational velocity to the translational one). This counterintuitive phenomenon occurs because the boundary layer flow moving aga

A new adaptive-passive control device is introduced to optimally reduce the drag on a sphere over a wide range of Reynolds numbers, Re = 0.4 × 105–4.4 × 105. The device, called an adaptive moving ring (AMR), is designed to change its size (i.e., protrusion height) adaptively depending on the wind speed (i.e., the Reynolds number) without energy input. An empirical model is formulated to accurately predict the drag coefficient as a function of the size of AMR and the Reynolds number. Based on the

Leading-edge tubercles on a humpback whale flipper are known to enhance its hydrodynamic performance at post-stall angles of attack (Miklosovic et al 2004 Phys. Fluids 16 39-42). We investigate vortical structures above a three-dimensional wing with tubercles using surface-oil-flow visualization and particle image velocimetry measurement. Two wing models with and without tubercles, previously studied by Miklosovic et al (2004 Phys. Fluids 16 39-42), are considered at the Reynolds number of 180 0

Leatherback sea turtles (Dermochelys coriacea) are known to have a superior diving ability and be highly adapted to pelagic swimming. They have five longitudinal ridges on their carapace. Although it was conjectured that these ridges might be an adaptation for flow control, no rigorous study has been performed to understand their hydrodynamic roles. Here we show that these ridges are slightly misaligned to the streamlines around the body to generate streamwise vortices, and suppress or delay flo

In this paper, the effects of rotor-rotor interaction on the wake structure and thrust generation of a quadrotor unmanned aerial vehicle (UAV) are experimentally investigated in the rotor tip Reynolds number range of 34000 - 54000. The interaction strength is manipulated by varying the number of rotating rotors and the normalized rotor separation distance. A stronger rotor-rotor interaction places the inner tip vortices between rotors closer to each other, forming an upflow region through vortex

With particle image velocimetry (PIV), cross-correlation and optical flow methods have been mainly adopted to obtain the velocity field from particle images. In this study, a novel artificial intelligence (AI) architecture is proposed to predict an accurate flow field and drone rotor thrust from high-resolution particle images. As the ground truth, the flow fields past a high-speed drone rotor obtained from a fast Fourier transform-based cross-correlation algorithm were used along with the thrus

In this study, the effects of rotor–rotor interaction on wake characteristics were investigated experimentally for a twin-rotor configuration in axial descent. The wake velocities were measured at descent rates (descent speed/induced velocity at the rotor disk during hover) from 0.87 to 1.52, and the rotor–rotor interaction strength was controlled by adjusting the distance between the rotor tips. As the descent rate increased, the wake of the isolated rotor gradually entered the vortex ring stat

In this study, we investigate the aerodynamics of our newly designed golf ball that does not have dimples but grooves on its surface. The smooth part of its surface is approximately 1.7 times that of a golf ball with dimples. We directly measure the drag and lift forces on two versions of this golf ball in the ranges of real golf-ball velocity and rotational speed, and compare them with those of smooth and dimpled balls. At zero spin, the drag coefficient of our balls shows a rapid fall-off at a

Longitudinal strips are suggested as a new device that can significantly increase the aerodynamic performance of a low Reynolds number airfoil at post-stall angles of attack. Its mechanism relies on the generation of corner vortices that delay flow separation on the airfoil suction surface.

This study investigates the effects of rotor–rotor interaction on the wake and thrust characteristics of a small tandem rotor operating in a crosswind. Flow velocity and force measurements were conducted in a wind tunnel with two rotors arranged parallel to a crosswind. The results show that the rotor–rotor interaction significantly influences the wake characteristics and thrust generations of the tandem rotor and its effects vary depending on the crosswind speed and distance between rotor tips.

For a rotating sphere or cylinder, the lift coefficients become negative at some specific Reynolds numbers Re and spin ratios α (ratio of surface velocity to the free-stream velocity), called inverse Magnus effect. In the present study, the inverse Magnus effect on a rotating sphere is experimentally investigated at Re = 0.6 × 105 − 1.8 × 105, based on the free-stream velocity U0 and sphere diameter d. By varying the spin ratio from 0 (no spin) to 1.7, we measure the lift, drag, and velocity fie

When rod surface roughness is introduced in a turbulent Couette–Poiseuille flow (CP-flow), it is known that the Reynolds stresses near the centerline decrease due to weakened very-large-scale motions (VLSMs) and roll-cell motions [Lee, Y. M. et al., “Direct numerical simulation of a turbulent Couette–Poiseuille flow with a rod-roughened wall,” Phys. Fluids 30, 105101 (2018)]. In the present study, we examine the origin of the weakened turbulent structures near the centerline in a CP-flow with ro