Woo-Pyung Jeon

Mechanism of drag reduction by dimples on a sphere

In this Letter we present a detailed mechanism of drag reduction by dimples on a sphere such as golf-ball dimples by measuring the streamwise velocity above the dimpled surface. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in the generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with a high momentum near the wall and overcomes a strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay the main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e., a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder.

Drag reduction in flow over a two-dimensional bluff body with a blunt trailing edge using a new passive device

In this paper, we present a new passive control device for form-drag reduction in flow over a two-dimensional bluff body with a blunt trailing edge. The device consists of small tabs attached to the upper and lower trailing edges of a bluff body to effectively perturb a two-dimensional wake. Both a wind-tunnel experiment and large-eddy simulation are carried out to examine its drag-reduction performance. Extensive parametric studies are performed experimentally by varying the height and width of the tab and the spanwise spacing between the adjacent tabs at three Reynolds numbers of

Active control of flow over a sphere for drag reduction at a subcritical Reynolds number

\hbox{\it Re}\,{=}\,u_\infty h/\nu\,{=}\,20\,000

Active Control of Turbulent Flow over a Model Vehicle for Drag Reduction

, 40 000 and 80 000, where

Mixing enhancement behind a backward-facing step using tabs

u_\infty

Effect of free-stream turbulence on the flow over a sphere

is the free-stream velocity and

Mechanism of drag reduction by a surface trip wire on a sphere

h

Effect of a localized time-periodic wall motion on a turbulent boundary layer flow

is the body height. For a wide parameter range, the base pressure increases (i.e. drag reduces) at all three Reynolds numbers. Furthermore, a significant increase in the base pressure by more than 30% is obtained for the optimum tab configuration. Numerical simulations are performed at much lower Reynolds numbers of

Hydrodynamic characteristics of the sailfish (Istiophorus platypterus) and swordfish (Xiphias gladius) in gliding postures at their cruise speeds.

\hbox{\it Re}\,{=}\,320

Measurements of Boundary Layer on the Flat Plate in Non-uniform Incoming Velocity Profiles of Wake

and 4200 to investigate the mechanism responsible for the base-pressure increase by the tab. Results from the velocity measurement and numerical simulations show that the tab introduces the spanwise mismatch in the vortex-shedding process, resulting in a substantial reduction of the vortical strength in the wake and significant increases in the vortex formation length and wake width.