# Vortex Breakdown Control by the Plasma Swirl Injector
## 1 Introduction  
As an active flow and combustion control device, the dielectric barrier discharge (DBD) actuator has drawn much attention for its fast response, low power consumption, and simple structure, as described by [PERSON] et al. [1]. The operation of the DBD actuator is purely electric without any moving part, which is attractive to many applications. Recent investigations on plasma actuators have been reviewed by [PERSON] [2], [PERSON] et al. [3], [PERSON] et al. [4], [PERSON] et al. [5], [PERSON] et al. [6] and [PERSON] [7], to name but a few. Flow separation control by the DBD actuator was widely researched. [PERSON] et al. [8] observed that the heat released by nanosecond pulse actuation can produce shock waves, where the associated secondary vortex flows disturbed the main flow and caused an efficient transversal momentum transfer into the boundary layer. Flow separation control was investigated experimentally by [PERSON] et al. [9] on an airfoil leading edge up to 62 m/s with nanosecond pulse plasma actuator. They pointed out that the plasma actuator was similar to an active trip, which can generate coherent spanwise vortices at post stall. [PERSON] [10] showed that actuation in burst mode was very effective for controlling flow separation at a Reynolds number of \(6.3\times 10^{4}\), where the three features of the flow structure, associated with flow separation control, were emphasized and guidelines for the effective use of DBD actuators were proposed. Meanwhile, the modelling approach of [PERSON] et al. [11] was adopted by [PERSON] et al. [12] to simulate the 3D separated flow over a hump model with the inlet velocity of 34.6 m/s. They demonstrated that the flow separation was completely suppressed with the actuator placed just downstream of the separation point at an applied frequency of 5 kHz. Through flowfield measurement by particle image velocimetry (PIV) and a pressure cap over a wing section, [PERSON] et al. [13] emphasized that the actuation frequency was critical to the control authority of a nanosecond-DBD actuator. [PERSON] et al. [14] used a micro plasma actuator to suppress flow separation inthe low-pressure turbine of small engines. They pointed out that the sinus waveforms outperformed other waveforms slightly. [PERSON] et al. (2015) tested the effective flow separation control over the rear end of a lorry (1:20 scale) by the plasma actuation with the free stream velocity at 30 m/s. They found that the shear layer was deflected more downwards due to the actuation. However, the flow characteristics in the wake region was not significantly modified by the actuation and they concluded that no flow control effect was observed in their study. All of these studies demonstrate the wide range of applications of plasma actuation, with further potentials to be explored. Using ionic wind (which is the airflow induced by electrostatic forces linked to plasma discharge) for local cooling is also an interesting research topic. [PERSON] et al. (2016) demonstrated that heat transfer was increased by ionic wind through distortion of the boundary layer. [PERSON] and [PERSON] (2017) developed the idea of film cooling enhancement by plasma actuator. Through PIV and infrared thermography measurements, [PERSON] et al. (2018) found that the actuation deflected the jet toward the wall and delayed its diffusion into the cross flow, and, as a result, the effectiveness of the film cooling was increased. Through numerical simulation, the mechanism of film cooling improvement by plasma actuator was analyzed by [PERSON] et al. (2019). They found that the counter rotating vortex pairs were weakened by the actuation, which led to less interaction and reduced mixing between the main flow and the jet flow. A pressure sensitive paint technique was adopted by [PERSON] et al. (2010) to investigate the effect of DBD actuation on the film cooling effectiveness of a 30\({}^{\circ}\) slot with the mainstream velocity at 10 m/s.