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The unsteady force response of an accelerating flat plate, subjected to controlled spanwise bending, is investigated experimentally. The flat plate was held normal to the flow (at an angle of attack of $90^{\circ }$ ), and it was dynamically bent along the spanwise direction with the help of internal actuation. Two bending directions were tested. In one case, part of the plate (denoted by flexion ratio) was bent into the incoming flow (the bend-down configuration). In another case, the plate was bent away from the flow (the bend-up configuration). We used two different aspect ratio ( $AR$ ) plates, namely $AR = 2$ and 3. Three acceleration numbers, namely $A_c = 0.57$ , 1.6 and 3.2 (corresponding to dimensional acceleration of 0.036, 0.1 and 0.2 m s $^{-2}$ , respectively) were tested with a fixed terminal Reynolds number (Re) of 18 000. For each acceleration number, three bending durations, namely 1.2, 2.4 and 3.6 s were implemented. The results indicate that the highest impulse was imparted by the highest bending rate (duration 1.2 s) during all three accelerations tested. We show that controlled spanwise bending can significantly change the unsteady force response by manipulating the inertial forces during a start-up manoeuvre. The unsteady forces depend on the vector sum of the forward acceleration and the bending acceleration of the plate. The unsteady drag was augmented when the plate was bent towards the incoming flow. The initial force peaks were significantly reduced when the bending direction was reversed. The development of the edge vortices from the flat plate was measured with the help of particle image velocimetry (PIV) at the 70 % and the 90 % span locations. The PIV measurements were also carried out at the midchord plane closer to the tip region to capture the growth of the tip vortex. The vorticity field calculated from these PIV measurements revealed that controlled bending contributed to a variation in the circulation growth of the edge vortices. During the bend-down case, the circulation growth was faster and the tip vortices stayed closer to the plate. This resulted in increased interaction with the edge vortex at the 90 % span. This interaction was more severe for $AR = 2$ . During the bend-up case, the growth of the edge vortex was delayed, but the vortex grew for a longer time compared with the bend-down case. Finally, a mathematical model is presented which correctly captured the trend of the force histories measured experimentally during both the bend-up and bend-down cases.

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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.