TR PIV Experimental Investigation on Bypass Transition Induced by a Cylinder Wake

The process of laminar to turbulent transition induced by a cylinder wake is studied by time-resolved (TR) particle image velocimetry (PIV) in a water channel. The combination of multi-scale local-averaged structure function analysis with criteria is used to identify the generation of secondary transverse vortex structure and to track its evolution along the streamwise. At the beginning of transition, with the decent of cylinder wake vortex, the secondary vortex structure is induced near the wall. As the secondary vortex moves downstream, it is induced to lift up by the wake vortex, meanwhile they are diffused and dissipated. According to the method of spatial conditional average, a low-speed hump is found in the near-wall region along the bypass transition zone, accompanied by a low-speed region in the free stream occupied by the wake vortex. With further downstream, the hump in the near-wall region becomes more and more obvious. At the later stage of transition zone, hairpin vortex can be seen by conditional-averaged low-pass filtered vorticity. The hairpin head is almost vertical to the wall with an inclination angle of about 90, which is attributed to the additional lift-up behavior induced by wake vortex. It can be concluded that in the process of bypass transition, the wake vortex would not only induce the secondary vortex but also leaven its growth and evolution, resulting in the robust and rapidly growing hairpin vortex.