**4.1. Discharge mode characteristics under upstream and downstream airflows**

With a pulse repetitive frequency of 1000 Hz, the applied voltage and current waveforms of the DBD volume discharge can be detected, which has been descripted in Section 3.1. By changing the air velocity, the discharge mode characteristics under different airflow velocities are shown in **Figure 9**.

For discharge in a static condition, as shown in **Figure 9(a)**, the upstream and downstream discharge exhibits a filament discharge mode. In the middle region of the electrode, the filaments exhibit a vertical distribution, and the path of the filaments close to the edge of the electrode shows a curved state because of the edge effect of the metal electrode.

For *t <sup>f</sup> > tp*  , the discharge filaments move along the direction of flow, the filament path changes from the original vertical state to the curved state along the direction of airflow, and some parts of the discharge area become uniform. The result is shown in **Figure 9(b)**.

For *t <sup>f</sup>* ≤ *tp*  , with increasing airflow velocity, most of the discharge area exhibits a uniform state, and the paths of the discharge filaments at the edge of the metal electrodes are attracted by the central discharge region. The discharge filaments shrink to the inside region of discharge, and the discharge region exhibits a pinched state, as shown in **Figure 9(c)** and **(d)**.
