Roughness Effects on the laminar separation bubble

Laminar boundary layer separation represents a source of losses that sensibly affects the aerodynamic performances. The roughness surface has been widely used to control the separation, while the entropy generation and the turbulence production mechanisms due to the roughness element are not clear. To explore those mechanisms, this paper investigates the boundary layer development on the flat plate using high-resolution particle image velocimetry (PIV) in the water tunnel. The flow past the flat plate under an adverse pressure gradient and subject to the roughness surface. The entropy generation rate is analyzed by proper orthogonal decomposition (POD) applied to the measurements. The observations from this work show that, the roughness accelerates the transition process and can decrease the size of the separation bubble. The mean viscous dissipation contributes significant portions of entropy generation at low order modes and the high order modes contain most the turbulent contributions.