## 1 Introduction  
Small wind turbine blades, which comprise a single airfoil, often experience relatively low Reynolds numbers in comparison with multi-MW turbine blades, which operate at high Reynolds numbers of above 10 million [1]. At the low Reynolds numbers, laminar flow is separated at the upper surface of the airfoil under various angles of attack (AoAs). The separated laminar flow often transitions to turbulent flow and may then reattach to the surface forming a laminar separation bubble (LSB) at a moderate AoA or remain separated at a large AoA. The bubble changes the boundary layer characteristics downstream and deteriorates the aerodynamic performance by loss of lift and an increase in drag [2,3]. The existence of the separation bubble is generally considered undesirable because it can have negative effects on the performance owing to the local alteration of the surface pressure distribution and lead to stall of the airfoil if the bubble bursts [4]. The leading-edge, stall which occurs at a high AoA, causes abrupt loss of lift and increase of drag [5,6]. At a low AoA, which causes the formation of an LSB, the strong amplification of disturbances within the laminar separated boundary layer gives rise to a transition to turbulence in the separated shear layer and may have a significant impact on airfoil performance [6]. The impact on aerodynamic performance is strongly dependent on the separation location and LSB size, which are affected by the transition process. Therefore, understanding the characteristics and effects of the LSBs is important in the aerodynamic design of airfoil shapes for maximizing wind turbine power and efficiency. Considering the importance of exploiting renewable energy resources in recent decades, the blade shape for a wind turbine can be optimized by finding the best combination of airfoil type, chord length, and twist angle with the highest local torque along the spanwise direction of the blade [7; 8]. Therefore, for prompt design evaluation using blade element momentum (BEM) theory, the aerodynamic lift and drag coefficients used in BEM theory-based calculations are required, depending primarily on 2D wind tunnel measurements of airfoils with a constant span. The general flow structure of an LSB and a separation-induced transition on an airfoil are illustrated in Figure 1. It shows that the LSB is composed of two regions: laminar (LS point to TO point) and turbulent (TO point to TR point) regions. The LSB is formed through the following processes; laminar boundary layer separation, transition to turbulence, and reattachment. When a laminar boundary layer separates at the LS point due to a strong adverse pressure gradient, the shear layer reattaches at the turbulent reattachment (TR) point, forming an LSB. The length between the LS point and the TR point is defined as the LSB that depends on the transition process in the shear layer. The length and location of the separation bubble strongly depend on the airfoil profile, free-stream turbulence intensity, free-stream Reynolds number, and the angle of attack [9; 10]. Separation bubbles can be either short or long. According to the [PERSON] [9], short bubbles reattach shortly after separation and therefore only have a local effect on the pressure distribution. In contrast, long bubbles completely change the pressure distribution creating a pressure plateau and hence produce large aerodynamic losses. In general, there are three types of transitions; natural, bypass, and separation-induced transition. Out of these, the most prevalent type of transition observed for airfoils at low Reynolds numbers is the separation-induced transition, where a laminar boundary layer separates under the influence of a pressure gradient on airfoil surface [11; 12]. As a result, a transition develops within the separated shear layer, which may reattach due to the enhanced mixing caused by the turbulent flow. This reattachment then forms an LSB at some distance downstream from the surface [13; 14].