| Plasma actuators were used to control the phase relationship between vortex shedding from two side-by-side circular cylinders from Red = 16,700 to 76,500 in a low-speed flow. The motivation was to create, in a compressible cascade, a 'clean' and predictable upstream-propagating potential disturbance using synchronized shedding from a row of multiple cylinders so that the interaction of the disturbance with the trailing edge of a cascade vane could be investigated. In the low-speed flow, two cylinders of L/d = 8 were installed perpendicular to the flow, with the pitch-to-diameter ratio of 4. The 'natural' vortex shedding for this spacing is, predominantly, 180° out-of-phase. The performance of the plasma actuators was best, when they were installed on the sides (approximately +/-90° from the forward stagnation point) of each cylinder, along the full span. These actuators, by the formation of plasma, produce an air jet in downstream direction. The phase relationship of vortex shedding was determined using cross correlation of streamwise velocities, measured by hot-wire probes located downstream of each cylinder at symmetric positions with respect to the cylinder axes. The cross correlation of the unsteady velocities showed the effectiveness of the plasma actuators in phase synchronizing the vortex shedding. The coefficients of +0.4 to +0.6 (or -0.6 to -0.4) were obtained when the plasma actuators were activated, showing the in-phase (or out-of-phase) synchronization of the vortex shedding. The effectiveness of the plasma actuators was further demonstrated by the arbitrary-phase synchronization of vortex shedding. The phase synchronization up to the Reynolds number of 76,500 was obtained. No other method has been able to achieve the vortex-shedding control up to this high a Reynolds number. The power input to the actuators, before reaching a certain asymptotic value, increased with the increase in Reynolds number. The phase synchronization was sensitive to the excitation frequency, input power, and Reynolds number. Flow visualization was used to obtain a global view of the phase-synchronized shedding. The spanwise coherence of vortex shedding increased when the flow was excited. The upstream effect of the phase-synchronized shedding of cylinders in the low-speed flow (Re = 16,700) was investigated using PIV. The controlled in-phase shedding also produced a synchronized unsteady velocity field upstream of the cylinders. Through the scaling of the body force produced by the plasma actuator, it is demonstrated that it is possible to phase synchronize vortex shedding in a higher-speed, weakly-compressible, geometrically-similar flow. |
