Pulsed blowing for amplifying unsteady lift on a cylinder

Energy harvesting from vortex-induced vibrations is a promising technology that relies on the vibrations of bluff bodies due to the vortex shedding phenomenon. Increasing the vibration amplitude at a given free stream kinetic energy is therefore equivalent to enhancing the efficiency of the harvesting device. In this study, we assess the potential of alternate slot blowing to amplify force fluctuations. Pressurized air is ejected alternately from the top and bottom parts of a cylinder. Through experimentation in a low-speed wind tunnel (Re = 8,000), we show that the magnitude of lift fluctuations can be enhanced by up to a factor of three compared to the unforced flow when the actuation is aligned with the natural vortex shedding frequency. Particle image velocimetry measurements indicate that this is caused by strong streamline bending, whereas at a higher forcing frequency, the vortex shedding is observed to be suppressed. This work elaborates on the physics of slot blowing for amplitude enhancement, and results suggest that a significant increase in the dynamic load acting on a cylinder can be achieved with carefully chosen active-flow control parameters, thereby promoting future energy harvesting applications.