# Partial Oscillation Flow Control on Airfoil at Low Reynolds Numbers
## 1 Introduction  
Near-space aircrafts (e.g., solar-powered aircrafts and stratospheric airships) have many advantages, such as high operational altitude and long endurance. This class of aircraft holds considerable potential for applications in intelligence gathering, surveillance, and reconnaissance activities, thereby constituting a pivotal developmental trajectory within the future realm of aerospace engineering [1, 2]. Nevertheless, the laminar separation bubble (LSB) emerges as a ubiquitous flow feature in near-space aircraft operating at low Reynolds numbers (\(\leq\)10\({}^{6}\)) [3, 4, 5], a phenomenon that severely compromises the lift-to-drag ratio and stands as a paramount challenge impeding further advancements in near-space aviation technology. Consequently, the efficacious manipulation of LSB phenomena occurring on airfoils under low Reynolds number conditions constitutes a pivotal factor in enhancing the overall aerodynamic efficiency of near-space aircraft, hence necessitating focused research efforts in this domain. Research findings indicate that the implementation of partial oscillation exerts a substantial influence on the aerodynamic attributes and flow patterns associated with the airfoil, thereby altering its performance characteristics profoundly. Accordingly, the implementation of partial oscillation has emerged as a burgeoning area of inquiry within active flow control strategies applied to airfoil design [6; 7]. To mitigate the detrimental impact of LSB on airfoils under low Reynolds number conditions and thereby enhance their aerodynamic properties, extensive research endeavors have been dedicated to exploring various active flow control methodologies [8; 9]. Typical active flow control methods prevent the formation of LSB by exciting the K-H (Kelvin Helmholtz) instability in the separated mixing boundary layer or the T-S (Tollmien Schlichting) wave in the laminar boundary layer upstream of the separation bubble [10; 11]. For the partial oscillation flow control of airfoil, the present achievements of research focus on the following two aspects:  
One aspect is the aerodynamic characteristics of flexible airfoil. Research indicates that the application of a flexible material coating on the wing's surface engenders an aeroelastic coupling interaction with the fluctuating flow field, thereby leading to vibrations. These vibrations, under suitable circumstances, have the capacity to influence the fluid dynamics structure and ultimately deliver an enhanced lift and reduced drag performance [12]. [PERSON] [13] conducted a wind tunnel experimentation to investigate the aerodynamic properties of a biomimetic bat wing, revealing that the synergistic interaction between the airflow field and the structural design significantly contributes to the postponement of stall occurrence on the membrane wing. [PERSON] [14] conducted an investigation into the interplay between the aerodynamic characteristics and structural behavior of membrane wings operating at low Reynolds numbers. The findings indicate that the temporal variation in membrane wing surface displacement exerts a substantial influence on the unsteady airflow around the wing, particularly at elevated angles of attack. This phenomenon not only retards the onset of stall but also enhances the wing's lift coefficient under such high-angle conditions. From previous studies, it has been ascertained that employing a membrane surface under suitable conditions can lead to the enhanced aerodynamic performance of airfoils. However, overall, the method of flow control utilizing self-excited vibrations in flexible airfoils is fraught with numerous unpredictable and uncontrollable factors, rendering its efficacy challenging to forecast and, consequently, making it difficult to implement practically. Another significant facet involves the application of active oscillatory flow control on the surface skin.