Experimental investigation of the influence of weak low-frequency forcing on the 3D turbulent wake of a cantilevered triangular prism
Date
2021-09
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
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 12000. Results are reported for surface pressure measurement 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. The mean-field characteristics and vortex formation and shedding process for the unactuated and actuated flows are characterized. It is found that flow topology in the wakes of the cantilevered triangular prism and other cantilevered cylinders (circular and square) are similar. 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. The vortex-splitting phenomenon is then related to similar observations in earlier studies using different actuation methods. The existence of interharmonic frequencies in the wake are evidenced and attributed to the interactions between actuation pulses and Karman vortices for the unsynchronized actuation cases. 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.
Description
Keywords
Vortex formation, Vortex shedding, low-frequency forcing, Synthetic jet
Citation
Erfan, I. (2021). Experimental investigation of the influence of weak low-frequency forcing on the 3D turbulent wake of a cantilevered triangular prism (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.