In this paper, we present our recent research activities on control of flow over a bluff body such as a circular cylinder, a two-dimensional bluff body with a blunt trailing edge and a sphere. First, we introduce a three-dimensional forcing applied to a two-dimensional bluff body and show that it significantly changes vortical structures in the wake and reduces mean drag and lift fluctuations. Second, by providing an appropriate active or passive control to a separating shear layer, one can destabilize the shear layer and reattach the flow on the surface before main separation, which delays main separation and decreases drag. Finally, we apply three-different active control methods based on control theories (i.e. linear proportional feedback control, suboptimal feedback control, and active open-loop control using surrogate management framework) to flow over a sphere and show that they successfully reduce the lift fluctuations.

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.

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 $\hbox{\it Re}\,{=}\,u_\infty h/\nu\,{=}\,20\,000$, 40 000 and 80 000, where $u_\infty$ is the free-stream velocity and $h$ 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 $\hbox{\it Re}\,{=}\,320$ 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.

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In this paper, the effect of free-stream turbulence (FST) on the flow over a sphere is experimentally investigated at the Reynolds numbers of 0.5×105–2.8×105. Three levels of FST are generated in a wind tunnel by installing three different types of grids upstream of the sphere. It is found that FST triggers the boundary layer instability above the sphere surface and delays the separation. Once laminar separation occurs, the disturbances both from the boundary-layer instability and FST trigger the shear-layer instability. Then, high momentum is entrained toward the sphere surface and the separated flow is reattached, forming a separation bubble on the sphere surface. Due to high momentum near the surface, the main separation is delayed and the drag is reduced. The critical Reynolds number, at which the drag coefficient decreases rapidly, decreases as the FST intensity increases. With increasing Reynolds number, the first separation point moves downstream, but the reattachment and main separation points are nearly fixed, resulting in a constant drag coefficient. As the Reynolds number further increases, the separation bubble finally disappears but the main separation point is still nearly unchanged, resulting in a constant drag coefficient within the Reynolds number range considered. Therefore, the formation, regression, and disappearance of a separation bubble on the sphere surface are the key mechanisms of the drag change by FST.