# Vortex Breakdown Control by the Plasma Swirl Injector
## 1 Introduction
They pointed out that the improvement was not significant and that the actuator configuration should be optimized. [PERSON] and [PERSON] (2018) developed a plasma actuator cooling device in millimeter scale channels with a height of 2.5 mm, 10 mm, and 50 mm, without an upper wall. They found that the heat transfer coefficient was inversely proportional to the channel height. However, when the height was 2.5 mm, the heat transfer coefficient was larger than that of the height of 5 mm. They attributed this to the suppression of backward flow which impeded the cooling performance. They were optimistic about the feasibility of the plasma actuator cooling for a confined space. The plasma actuator was also adopted to control noise and vibration. The experiments of [PERSON] et al. (2018) showed that plasma actuators exhibited the ability to suppress vortex-induced vibration around a rigid circular cylinder. During their experiments the flow velocity was varied between 3 to 4.25 m/s (6000 \(<\) Re \(<\) 8500). [PERSON] et al. (2019) demonstrated that the plasma actuation was effective in reducing the cavity tone with the acoustic resonances at freestream velocity of 30 m/s by introducing streamwise vortices with fine displacement. A new corner-type actuator configuration was designed by [PERSON] et al. (2019) to control flow induced noise in a cavity. They demonstrated that the aero-acoustic lock-on was suppressed for freestream velocities below 12.5 m/s by the inward-inducing actuator. PIV measurement showed that a secondary circulating flow region was created by the actuation which prevented lock-on. The experimental research of [PERSON] et al. (2019) demonstrated that the DBD actuators were able to reduce aerodynamic noise from slats with wind tunnel velocity of 27.4 m/s. They also pointed out that the control authority of the actuators tested was still poor for real flight conditions, and that further research is needed to produce more efficient plasma actuators. The control effects of the plasma actuation on the flow around an oscillating plate was investigated by [PERSON] et al. (2019). They pointed out that it was possible for the DBD actuator to control the lift with a mechanism similar to that of an insect. They found that the appropriate adjustment of the driving time played an important role in lift enhancement. [PERSON] et al. (2019) numerically assessed the effectiveness of plasma actuators for load alleviation on a compressor cascade. They demonstrated that it was effective to alter the blade loading by proper triggering of the pressure and suction side of the actuation. A number of studies have been carried out to enhance the performance of DBD actuators, such as the shape of the serpentine actuator ([PERSON] et al., 2019), wire type electrode ([PERSON] et al., 2019), microfabricated DBD actuator ([PERSON] et al., 2019), sawtooth electrode ([PERSON] et al., 2019), and DBD active grid ([PERSON] et al., 2019). Vortex breakdown (VB) observed in swirling flows is an interesting physical phenomenon relevant to a wide range of engineering applications such as aerodynamics (delta wing, and blade tip leakage flow) and combustion. [PERSON] and [PERSON] [33] first reported vortex breakdown on a delta wing at a high angle of attack. The vortex lines near the edges of the wing experience a sudden change in shape when the angle of attack is increased. This phenomenon is referred to as vortex breakdown. [PERSON] [34] observed three types of vortex breakdown: double helix, spiral, and axisymmetric (bubble). Bubble breakdown is characterized by a stagnation point on the swirl axis, followed by an abrupt expansion of the centerline to form an envelope of recirculating fluid [35]. [PERSON] [36] defined vortex breakdown as \"a disturbance characterized by the formation of an internal stagnation point on the vortex axis, followed by a reversed flow in a region of limited axial extension\".