Seonghyeon Hahn

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

Direct numerical simulation of turbulent channel flow with permeable walls

\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

Direct numerical simulation of jets in a cross turbulent boundary layer flow

u_\infty

Unsteady Simulation of Jets in a Cross Flow

is the free-stream velocity and

Wake disturbing apparatus and method for reducing drag

h

Turbulence Modification in a Spatially Evolving Pressure-Driven Three-Dimensional Turbulent Boundary Layer

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

On the possibility of skin-friction reduction using d-type roughness

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

ACTIVE CONTROL OF TURBULENT FLOW OVER A MODEL VEHICLE FOR DRAG REDUCTION

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.

Wake Disrupter as a New Passive Device for Drag Reduction in Flow behind a Model Vehicle

The main objectives of this study are to suggest a proper boundary condition at the interface between a permeable block and turbulent channel flow and to investigate the characteristics of turbulent channel flow with permeable walls. The boundary condition suggested is an extended version of that applied to laminar channel flow by Beavers & Joseph (1967) and describes the behaviour of slip velocities in the streamwise and spanwise directions at the interface between the permeable block and turbulent channel flow. With the proposed boundary condition, direct numerical simulations of turbulent channel flow that is bounded by the permeable wall are performed and significant skin-friction reductions at the permeable wall are obtained with modification of overall flow structures. The viscous sublayer thickness is decreased and the near-wall vortical structures are significantly weakened by the permeable wall. The permeable wall also reduces the turbulence intensities, Reynolds shear stress, and pressure and vorticity fluctuations throughout the channel except very near the wall. The increase of some turbulence quantities there is due to the slip-velocity fluctuations at the wall. The boundary condition proposed for the permeable wall is validated by comparing solutions with those obtained from a separate direct numerical simulation using both the Brinkman equation for the interior of a permeable block and the Navier–Stokes equation for the main channel bounded by a permeable block.