The von Kármán street behind a circular cylinder: flow control through synthetic jet placed at the rear stagnation point

• Published in: Journal of fluid mechanics Vol. 901
• Main Authors: Greco, Carlo Salvatore, Paolillo, Gerardo, Astarita, Tommaso, Cardone, Gennaro
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.10.2020
• Subjects: Asymmetry; Circuits; Circular cylinders; Configurations; Cylinders; Diameters; Digital imaging; Digital particle image velocimetry; Drag coefficient; Drag coefficients; Drag reduction; Flow control; Flow velocity; Fluid dynamics; Fluid flow; JFM Papers; Momentum; Particle image velocimetry; Reynolds number; Stagnation point; Statistical methods; Synthetic jets; Turbulent flow; Two dimensional analysis; Velocity; Velocity distribution; Vortices; Wind tunnels; wakes; vortex interactions; drag reduction
• DOI: 10.1017/jfm.2020.427
• ISSN: 0022-1120

The present paper aims at establishing the synthetic jet technology capabilities in controlling the von Kármán street behind a circular cylinder. The circular cylinder, placed in an open-circuit wind tunnel, presents a slot in its rear position, through which the synthetic jet is issued. The Reynolds number, based on the circular cylinder diameter and the free-stream velocity, is equal to 4600 and the von Kármán street is characterized, in the baseline configuration (i.e. without synthetic jet), by a shedding frequency of 16.2 Hz. Several synthetic jet operating conditions are tested. Therefore, the effects of the momentum coefficient ($C_{\mu } = 5.4$%, 10.8% and 21.6%) and the dimensionless frequency ($f^{+} = 0.49$, 0.98 and 1.96) on the von Kármán street behaviour can be analysed. Instantaneous two-dimensional in-plane velocity fields are measured in a plane containing the synthetic jet slot axis using multigrid/multipass cross-correlation digital particle image velocimetry. These measurements have been used to investigate the mean flow quantities and turbulent statistics of the phenomenon. In addition, the wake extent and behaviour (i.e. symmetric or asymmetric) are analysed as well as the drag coefficient, for each configuration. The extent of the wake region decreases as the momentum coefficient and/or the dimensionless frequency increase, while the symmetric/asymmetric wake behaviour is found to be governed by a different control parameter: the synthetic jet Reynolds number based on its impulse. As regards the drag coefficient, a maximum reduction, of approximately 35%, is found for the configuration at $C_{\mu }=10.8\,\%$ and $f^{+}=0.98$.