**4. Conclusions**

The discharge energy of μs-DBD is to demonstrate that different length actuators mounted on the small flying wing and the large flying wing have similar energy density. The single energy intensity is positively correlated with input voltage but has nothing to do with the pulse repetition frequency. So, other interference factors were eliminated when the dimensionless frequency is contrasted and analyzed for two symmetric flying wings based on two kinds of scale models. Force and flow field characteristics of the two symmetric flying wings are the topic of the article.

The large flying wing is 2.5 times the size of the small flying wing. The purpose of this paper is to study whether the optimal dimensionless frequency of two flying wing scale models at different incoming flow velocities is consistent by measuring the lift, drag, moment, and surface flow field variation of the two scaling flying wing model before and after μs-DBD plasma flow control actuation. Through the corresponding experiments, the following conclusions can be drawn. The discharge

energy results show that the longer the length of the μs-DBD actuator is, the greater the discharge output energy is, but the energy density is basically unchanged and is independent of the length of the actuator. With the increase of actuation voltage, the output energy and the energy density increase. The single pulse energy of the microsecond pulse at di fferent actuation frequencies is basically unchanged and has nothing to do with actuation frequency.

The force measurement and flow field PIV of the small and large flying wing models are compared by experiments. The results show that the stall angles of attack at the corresponding Reynolds numbers of 4.6 × 10<sup>5</sup> and 2.9 × 10<sup>6</sup> are both 15◦. When the actuation frequency of plasma flow control is 150 Hz, the unsteady actuation e ffect is the best, and the corresponding dimensionless frequency *F*<sup>+</sup> is 1.07, which is appropriate for the Strouhal similarity criterion. After actuation of the small flying wing, the stall angle of attack is delayed by 4◦, the maximum lift coe fficient is increased by 30.9% and the drag coe fficient can be reduced by 17.3%. The e ffect of lift increase and drag reduction is good. For a large flying wing, the stall angle of attack is delayed by 4◦, the maximum lift coe fficient is increased by 15.1%, and the drag coe fficient is increased. Similarly, the inflection point of the pitch moment is delayed at the optimal dimensionless frequency. In addition, the e ffect of low frequency actuation is better than that at a high frequency. The inertia force of the incoming flow at a low Reynolds number is small, and the plasma actuation e ffect is obvious, but at a high Reynolds number, the ability to promote the complete reattachment of the separation area is not enough due to the limitation of actuation intensity. The PIV test results of the flow field at di fferent cross-sections show that the stall separation on the surface of the symmetric flying wing begins from the outer side, and then with the increase of the angle of attack the separation area begins to appear on the inside. The plasma flow control can not only delay the separation of the longitudinal boundary layer but also slow down the movement of the lateral vortex and inject momentum and energy into the flow field, thus e ffectively increasing the lift and reducing the drag.

**Author Contributions:** Conceptualization, H.L. and L.X.; Methodology, M.H.; Formal Analysis, B.W.; Investigation, Z.S.; Data Curation, Z.N.; Writing-Review & Editing, B.T.

**Funding:** This research was founded by the *National Natural Science Foundation of China* (Grant Nos. 11802341 and 11472306), *Open Project of State Key Laboratory of Aerodynamics* in 2018 (Grant No. SKLA20180207).

**Acknowledgments:** Thanks for the experimental platform and technical support provided by *AVIC Aerodynaiviics Research Institute.*

**Conflicts of Interest:** The authors declare no conflict of interest.
