Outer layer manipulation of smooth and rough wall turbulent boundary layers

The detailed effect of a large eddy break-up (LEBU) device on smooth and rough (rod-roughened) wall turbulent boundary layers is investigated and compared in this body of work. Skin friction was measured using the defect chart method for the smooth wall and the static pressure tube method for the rough wall turbulent boundary layer. LEBU drag was measured with a specially designed drag balance using two miniature straight-bar load cells. Data for turbulence statistical analysis was obtained via single-point hot-wire anemometry. In case of smooth wall the effectiveness of LEBU to reduce skin friction follows the order 0.8δ > 0.6δ > 0.4δ (where δ is the boundary layer thickness). However, for rough wall it follows the opposite trend: 0.4δ > 0.6δ > 0.8δ, due to the effect of surface roughness. In both cases, as suggested by existing literature, LEBU effectively reduces skin friction but fails to reduce net drag due to the additional drag introduced by the device itself. It is also noted that, while the rough wall exhibits a similar pattern to the smooth wall, its skin friction reduction is significantly lower. The wake deficit caused by LEBU placement in the boundary layer gradually shifts towards the near-wall region with increased downstream distance. This confirms the ‘downwash effect’ proposed by Chin et al. (Flow Turbulence and Combustion, vol. 99, 2017, pp. 823–835), in which the wake generated by LEBU propagates toward the wall, attenuating near-wall energy and reducing skin friction. Length scale analysis shows that broken structures directly impact the integral length scales and indirectly influence the Taylor microscale and Kolmogorov length scale through an energy cascade originating in the outer layer. Additionally, on a smooth wall, LEBU is more effective at higher Reynolds numbers, whereas for a rough wall, it is more effective at lower Reynolds numbers.