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In order to solve the problem of small and asymmetrical plasma area of the gliding arc (GA) in the chemical treatment process, a plasma actuator driven by alternating current (AC) with a frequency of 23.2kHz was designed to multichannel gliding arcs (MGAs) in a rotational air flow at atmospheric pressure. The spatiotemporally resolved discharge characteristics of the MGA, including long-length breakdown, long-term extinction and typical breakdown features, were investigated combining optical and electrical diagnostic methods simultaneously at a flow rate of 250 SLM (standard liter per minute). The response characteristics of MGA under different flow rates were analyzed. On average, the MGA exhibit 48.8% more discharge power compared to traditional single-channel GA under flow rates of 50–250 SLM, which is due to the GAs continual existence at long-length state, representing better stability of the MGA plasma actuators. It was observed that the frequency of typical breakdown and long-term extinction increased as the flow rate accelerated, and the maximum height the GA could reach decreased with the number of channels increasing from 1 to 5, which can be attributed to the power decline of separate channels of GAs at 50 SLM. It was also found that MGA showed a broader plasma distribution of low electron temperature than traditional single-channel GA.

This study investigates the vortex interaction, the spatial distributions in the flow and the streamwise evolution of the spectral amplitude along the shear layer behind side-by-side cylinders of diameter ratio two at different gap ratios via dye flow visualization and particle image velocimetry (PIV). The frequency responses are measured at Re = 1000, 2000, 5000 as the gap ratio changes. Velocity measurements are made and analyzed at Re = 1000 for gap ratios of 1.25, 0.75 and 0.25. It is found that, as the gap ratio increases, the Strouhal number of the narrow wake decreases monotonously but that of the wide wake increases also in the monotonous way. The gap flow is always stably deflected toward the small cylinder. For side-by-side cylinders of diameter ratio two and Re = 1000, two different vortex interaction scenarios are found leading to two different flow categories. The critical gap ratio for diameter ratio two is slightly smaller than that for equal diameter. The frequency ratio of the narrow and wide wakes depends strongly upon the gap ratio and the diameter ratio; but is independent of the Reynolds number studied. This frequency ratio is related to the ratio of the averaged streamwise distance of the local maxima of each spectral component. For side-by-side cylinders of diameter ratio two, two flow characteristics are modified relative to that of a single cylinder. First, the onset locations of the shear layer instability. Second, the spatial growing rates of the shear layers. For the side-by-side cylinders of diameter two (D/d = 2), the influence on the wide wake is more significant but is less pronounced on the narrow wake. Besides, the influences on both the narrow and the wide wakes are even pronounced for D/d = 1 than those for D/d = 2.

The flow characteristics, mutual interactions and downstream evolutions of the wakes behind the side-by-side cylinders of unequal diameter are illustrated by flow visualization and Laser Doppler Anemometry. All the experiments are conducted in a recirculating water channel at Reynolds number 600. The gap ratio is 0.75 and the diameter ratio is 2.0. Periodic excitations are applied to the large cylinder with constant excitation amplitude. Within the excitation frequency range studied, the wake behind the large cylinder experiences the primary and the one third sub-harmonics lock-on; whereas the wakes behind the small cylinder are not locked-on. For the wake behind the large cylinder experiencing the primary lock-on, the gap flow becomes unbiased and the gap vortices are observed to interact and shed alternately into the wake behind each cylinder in synchronization with the excitation frequency (the natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, only one wider wake dominated by the excitation frequency is detected. While the wake behind large cylinder is locked-on at the one third sub-harmonics of the excitation frequency, the gap vortices are observed to entrain alternately into the wake behind each cylinder in synchronization with the one third of the excitation frequency (e.g., natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, the wake is relatively weak, less organized and turbulent. For both lock-on cases, the near and the far wake structures change suddenly within a short distance.