# Control of Flow around an Oscillating Plate for Lift Enhancement by Plasma Actuators
## 1 Introduction  
Recently, to better understand the dynamics of flying insects, the development of micro air vehicles (MAV) has been progressed. The MAVs, with a maximal dimension of 150 mm, normally operate in the Reynolds number regimes of 10\({}^{4}\)-10\({}^{5}\) or lower, in which most flying animals fly [1] and the boundary layer tends to be laminar [2; 3]. A number of MAV designs including fixed, rotary and flapping wings have been proposed [4; 5; 6; 7]. The MAVs with fixed wings are capable of high-speed and efficient flight and the MAVs with a rotary wing are capable of hovering [7]. Compared with fixed-wing craft or rotorcraft, bio-inspired flapping-wing MAVs have more likely possibility to be scaled down in size [7]. The research on the bio-inspired MAVs have primarily focused on the wing shape of creatures [8] such as butterflies [9], beetles [10] and dragonflies [11; 12]. Particularly, the dragonfly has high flight performance such as hovering, accelerating in almost any direction and precise maneuver at a high speed [12; 13; 14]. It is well known that dragonflies fly with simultaneous feathering and flapping motions of their wings [15]. For the dragonflies in flight, the flow is separated on the top surface of the wing, when the wings are moved in down-stroke motion. At this time, lift force is generated by the negative pressure on the top surface due to the separation vortices. In addition, lift is generated during the up-stroke motion by the remaining vortices on the top surface, where the generation of vortices on the lower surface is suppressed by the feathering motion [16]. To develop a MAV that simulates the flight of dragonflies, a mechanism that combines both flapping and feathering motions is necessary. If the feathering motion of insects is able to be reproduced by combining simple vertical oscillations of wing and active flow control, it can provide new knowledge for MAVs. There are various active fluid control techniques including suction and blowing [17; 18; 19; 20], synthetic jets [21; 22; 23]. Recently, the research on the flow control by plasma actuators has been performed [24; 25; 26; 27; 28; 29; 30; 31]. The plasma actuator is a structure composed of upper and lower electrodes and a dielectric layer, as shown in Figure 1. By applying an alternating voltage with the high amplitude and frequency between the two electrodes, plasma is generated between the electrodes. As a result, induced flow occurs from the upper electrode toward the lower one. Compared with other flow control techniques, the plasma actuator has two key advantages; with its simple structure, installation is easy and disturbances in flow caused by installation are minimal owed to the actuator's thin structure and fast time response [24]. [PERSON] et al. [25] showed that the dynamic stall vortex can be suppressed with a sufficiently high excitation Strouhal number. [PERSON] et al. [26] clarified that the suppression control of tonal noise generation at an airfoil trailing edge was conducted by using a flush-mounted plasma actuator for a NACA0012 airfoil at an angle of attack of \(2^{\circ}\). [PERSON] et al. [27] found that the actuator controls the stall flow around a wing with pitching movement, where the time-average lift force increases. The ultimate goal of this investigation is to effectively enhance lift force with a simple flapping wing by using a plasma actuator to control flow and replace the feathering motion of insects. To work towards this goal, wind tunnel experiments and numerical simulations were performed to clarify the control effects of the plasma actuator on the flow around an oscillating plate in a uniform flow. In Section 2, the flow configuration, control parameters of the actuators, the experimental methods and the computational methodologies are explained. In Section 3, the computational validation on the control effects is discussed. In Section 4, the predicted control effects of the actuator on flow fields around the oscillating plate are presented. In Section 5, conclusions are summarized.