Investigation of the influence of low-frequency forcing on the 3-D turbulent wake of a cantilevered triangular prism

The influence of weak low-frequency actuation on the three-dimensional turbulent wake of a cantilevered triangular prism of aspect ratio 4 protruding a thin turbulent boundary layer is investigated experimentally at a Reynolds number of 12 000. Results are reported for surface pressure measurements on the leeward face and wake of the obstacle and particle image velocimetry for selected planes in the wake. Zero-net-mass-flux actuation (sinusoidally modulated synthetic jets) is used to excite the flow through two slits spanning the obstacle height along the edges of the leeward face. Vortex shedding lock-on is shown to occur over intervals corresponding to subharmonics of the actuation frequency. The synchronization mechanism is identified, where weak perturbations due to actuation at critical stages of the Karman vortex formation trigger shedding. A phenomenological model is presented, linking the concepts of vortex formation time and circulation transport, to describe lock-on phenomena for one-sided and symmetric two-sided actuation. The model further describes interactions with the synthetic jet leading to the splitting of shed vortices observed in earlier studies. Similarities to results observed for other geometric and actuation configurations suggest a broader relevance of the proposed model and highlight differences between weak and strong forcing.