## 1 Introduction
According to [PERSON] [15], a free-stream turbulence level of 1% is a boundary between natural and bypass transition with and without downstream development region of instability waves and thus a higher turbulence intensity (TI) than 1% can cause a shorter LSB. Furthermore, [PERSON] et al. [16], numerically confirmed that the  
Figure 1: General characteristics of a laminar separation bubble (LSB) and separation-induced transition, reproduced from [PERSON] (1968). increase of TI at a NACA 0021 airfoil triggers a delay in LS, resulting in a larger laminar zone and decrease of separation bubble length (SBL). Understanding the characteristics of an LSB on an airfoil has been the subject of many investigations, both experimentally and numerically, over the last few decades [6; 10; 17], but the overall LSB dynamics are still not completely understood. The experimental approaches to LSBs have been discussed to understand the complex nature of this phenomenon. A combination of hot-wire anemometry, high-speed flow visualization, and velocity and surface pressure measurements was also used to study LSB dynamics on a NACA 0018 airfoil, with an emphasis on the formation of roll-up vortex, evolution, and detachment [18]. However, experimental approaches to the measurement of LS and transition onset (TO) points are both expensive and time consuming. Thus, as an alternative, computational fluid dynamics (CFD) approaches which use numerical modeling techniques, are more preferred for the accurate prediction of the aerodynamic performance of airfoils [19; 20; 21]. However, one of the major challenges in the accurate simulation of airfoil aerodynamics is to reproduce the transitional flow. In this regard, predicting the transition from laminar to turbulent flow is extremely crucial, especially when LSBs are present around the airfoil at high AoAs. To the authors' best knowledge, very few publications about characteristics and effects of the LSBs on the S809 airfoil are available in the literature, despite its importance regarding aerodynamic performance. A number of studies have used two equation turbulence models to acquire comparatively fast results, whereas direct methods of direct numerical simulation (DNS) and large eddy simulation (LES) are prohibitively expensive for industrial applications [22; 23]. To further avoid the difficulties of the methods, recently, approaches based on the Reynolds-averaged Navier-Stokes (RANS) equations coupled with linear stability theory have been suggested as an attractive alternative for accurate prediction of the separation-induced transition. [PERSON] et al. [24] extensively reviewed transition models for CFD and [PERSON] et al. [11; 12; 25] suggested a new transition model called \(\gamma\)-\(Re_{\theta}\), which can be readily implemented into an existing RANS solver. Therefore, the objective of this study is to achieve a better understanding of the LSB characteristics on pressure and suction airfoil surfaces to prevent the unintended degradation of airfoil performance and accurately predict the aerodynamic coefficients of an airfoil. Detailed analysis was performed to investigate the characteristics of the LSB and its subsequent effects on the aerodynamic performance of the NREL S809 airfoil as a function of AoA. A numerical approach utilizing the \(\gamma\)-\(Re_{\theta}\) transition model, first proposed by [PERSON] et al. [25], was adopted using commercial CFD software (ANSYS FLUENT 19.4 solver, Ansys, Canonsburg, PA, USA). The simulation results were compared and validated with the aerodynamic properties of the wind turbine airfoil experimentally obtained at the Delft University 1.8 m \(\times\) 1.25 m low-turbulence wind tunnel [26]. The following properties were studied as a function of AoA: the flow structure of the LSB, the TO point at the onset of turbulence, the reattachment point on the pressure and suction airfoil surfaces, and the relationship between the extent of the separation bubble and the lift-to-drag ratio.