Effects of roughness elements on bypass transition induced by a circular cylinder wake

Abstract

The bypass transition of flat-plate boundary layer induced by a circular cylinder wake under the influence of roughness elements is experimentally investigated. The hydrogen-bubble visualization results show that the boundary layer separation occurs upstream of the roughness, and the separated shear layer is incised by roughness to different extent, resulting in different kinds of secondary vortices formed immediately downstream of the roughness. During the evolution of the secondary vortex, two types of instabilities are observed, which are denoted as large-and small-scale instabilities, respectively, according to different spatial scale of the hairpin vortices formed afterward. A merging process of hairpin vortices is also observed when the secondary vortices undergo the small-scale instability, and a potential new transition control strategy based on the present observation is proposed.

Initial growth of a disturbance in a boundary layer influenced by a circular cylinder wake

“…A circular cylinder with a diameter of D = 20 mm and a length of 600 mm was placed horizontally, spanning the whole width of the test section. Unlike the studies of Ovchinnikov et al (2006), Pan et al (2008Pan et al ( , 2012, Wang et al (2011) and Mandal & Dey (2011), where the boundary layer was triggered as soon as it was initialized, the present experiment placed the cylinder downstream of the leading edge with a streamwise distance of L/D = 15 to avoid the complicated problem of leading-edge receptivity (Schrader et al 2010), i.e. the boundary layer had been developed for a long distance before being exposed to the outside wake environment.…”