Altering the wake dynamics of a circular cylinder with harmonic forcing

Abstract

The time-averaged velocity profile in the wake of a circular cylinder resembles a “U” type or a “V” type profile due to a velocity defect. Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020)] showed experimentally that the wake dynamics could be altered with harmonic forcing in such a way that instead of a velocity defect, a “jet-like” profile emerged with a characteristic “W” profile. The harmonic forcing was created by modulating the waveform of a dielectric barrier discharge plasma actuators' supply signal with a frequency twice that of the shedding frequency. However, the reason for the appearance of the “W” profile was not clear in S. Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020).] In this paper, we use numerical simulation to recreate the test conditions implemented by Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020).] We apply large-eddy simulation to study the impact of the pulsed, harmonic forcing on the wake of a circular cylinder at a subcritical Reynolds number of 4700. The plasma actuators are modeled with a body-force approach. The frequency of the driving signal of the plasma actuator is modulated at twice the shedding frequency. The amplitude of the signal is set at 6 kV peak to peak to create a blowing ratio of 0.8. The goal is to understand how the wake changes in three dimensions and the impact on separation on the cylinder surface due to the harmonic forcing. Results show that pulsed forcing causes vortices from one side of the wake to cross the centerline. This crossing creates an effective jet-like velocity along the centerline, resulting in a W velocity profile. Such a W profile is observed at least up to a streamwise distance of five cylinder diameters. Additionally, the pulsed actuation significantly increases the magnitude of primary and secondary frequencies throughout the wake. Forcing caused a 50% increase in the transverse velocity fluctuations at the centerline of the wake at the streamwise location of x/d=5. There was a similar increase in 33% at the centerline in the streamwise velocity fluctuations at the same location.