**2. Background for discharges under airflows**

Plasma flow control is a type of active flow control technology based on discharge plasma technologies, which is advantageous of little power, quick response, and perfect actuation. Russians, Americans, and other research groups [4–20] have done an in-depth study on plasma flow control, as well as the interaction between plasmas and airflows, to improve the aerodynamic characteristics and promote the scientific basis for efficiency. Discharge plasmas applied to flow control mainly include surface discharges [4–14] and volume discharges [15–20]. Surface discharges are used to flow separation control by DBD discharges with a momentum exchange to neutral airflows, which generate a complex pattern of quasi-planar and spherical compression wave [4–8], as well as which are related to a strong demand on stable discharges within the flow boundary layer [7–14]. Volume discharges are applied in MHD flow control to achieve the acceleration and deceleration of airflow, which require high-intensity volume discharges under airflows condition, as well as which essentially need discharge enhancement methods under MHD airflow environment [15–19].

Plasma material processing is promising for material modification and its industrial applications. Efficient discharges are important demands to realize the surface modification and functional structure construction [21–31]. The moderate power density and the uniform energy distribution are beneficial to material modification. However, under a gas flowing condition, discharges can easily transit from a stable state into an unstable state, which can cause a disaster to the industrial application of plasma material processing. In order to obtain a uniform and stable discharge, discharges under airflows are employed to excite plasmas. Work groups [29] illustrate the surface modification of polyimide films by the discharge under airflow, it is found that the plasma at a homogeneous DBD is evenly distributed than at a filamentary DBD, and by the more efficient introduction of atomic oxygen to the PP surface in the case of homogeneous DBD.

Plasma air purification attracts widespread attention in recent years, mainly related with corona discharges under an air supply channel. Plasma air purification has been developed in many applications, including electrostatic precipitation [32–40], industrial gas exhaust treatment [41–44], and indoor air purification [45–49]. Work groups [35] illustrate that the electric power and the energy loss of corona discharges highly depend on airflow velocities, and corona discharge modes are also related to airflow conditions.

In addition, discharges under airflows are of complicated technical challenges, and the mechanism and its characteristics need a deep and wide investigation.
