# Control of Flow around an Oscillating Plate for Lift Enhancement by Plasma Actuators
## 5 Conclusions  
To investigate the control effects by the plasma actuator on the flow around an oscillating plate in a uniform flow, wind tunnel experiments and numerical simulations were performed. In the experiments, the flow with and without control was visualized. In addition, the lift fluctuations and flow fields around the plate were investigated with the numerical simulations based on the volume penalization method. The effects of the installation surface and driving conditions on lift enhancement were investigated.  
Figure 19: Lift with each time offset for the control only on the top surface (\(R=50\%\)). (a) Time-averaged lift coefficient; (b) Time fluctuations of lift coefficient.  
Figure 20: Iso-surfaces of second invariant of velocity gradient tensor and contours of pressure coefficient around oscillating plate under the condition of \(t_{0}/T=0.5\). (a) \(t/T=0.18\); (b) \(t/T=0.75\); (c) \(t/T=0.94\).  
In the baseline flow, the flow separation occurs from the leading edge on the top surface throughout most of the period. By continuously driving the plasma actuator installed near the leading edge on the top surface, the flow separation was almost suppressed while the flow was separated around the center of the plate in the up-stroke motion. These control effects were confirmed in both the experimental and predicted results.  
In all the present control conditions of the installation surface, driving ratio and time offset, the driving of the actuator only on the top surface during the up-stroke motion shows most significantly enhances lift, whereas the time-averaged lift coefficient was increased by 11.6% compared to the baseline flow. This is because the vortex shedding from the bottom surface is suppressed in the up-stroke motion due to the stabilization of the wake, along with the suppression of the flow separation on the top surface by the control. In other words, it is possible for the plasma actuator to increase lift with a mechanism similar to the feathering motion of insect flight.  
The control in the down-stroke motion successfully minimized decrease of lift in the up-stroke motion; however, this control also suppressed the flow separation of the vortices on the top surface in the down-stroke motion and the increase of the time-averaged lift was limited.  
Although there is a possibility that increased lift could be achieved by more detailed adjustments to the driving conditions for the control by the actuators installed on both surfaces, the present results indicate that appropriate adjustment of the time for driving the actuator plays an important role in lift enhancement.  
[PERSON] analyzed the data, performed the experiments and simulation, wrote the paper; [PERSON] conceived the experiments and established the basis of the computations; [PERSON] designed the experimental setup and supervised this research.  
This work was supported by JSPS KAKENHI grant number 26820044 and 15K05793 and through the application development for Post K computer (FLAGSHIP 2020) by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).  
This research is partially supported by Initiative on Promotion of Supercomputing for Young or Women Researchers, Information Technology Center, The University of Tokyo.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript and in the decision to publish the results.