###### Abstract  
The aerodynamics of airfoils can be seen in a wide range of applications. To obtain the aerodynamic loads, geometrically-scaled airfoil sections are tested in wind tunnels. However, due to the limited space of the wind tunnel, the mismatch of Reynolds numbers may lead to different aerodynamic loads. Previous works showed that decreased lifts and increased drag coefficients are associated with lower Reynolds numbers, which are accompanied by the changes in ambient flow, such as increased sizes of the separation bubbles and wake vortices. Although insightful, few direct connections between loads, pressures, and ambient flow were presented, leaving a critical knowledge gap for aerodynamic modifications to improve the aerodynamic performances at low Reynolds numbers. To bridge this gap, this work utilizes numerical simulations and wind tunnel experiments to study the aerodynamics of a thin airfoil (NACA64) and a thick airfoil (DU40), at two chord Reynolds numbers, i.e., 4000 and 60,000. The two-dimensional (2D) vortex particle method (VPM) with varying-sized particles is used to simulate the unsteady flow and compared to the steady-state simulations by XFOIL. As the Reynolds number increases, it reveals that the higher lift coefficients are associated with the increased upstream suction and positive pressures on the upper and lower surfaces of the airfoils, respectively. These changes are explained by the increased and decreased normalized wind speeds on the upper and lower surfaces of the airfoils, respectively. Stronger pressure recoveries observed downstream of the reattachment points are the main cause of drag reductions at higher Reynolds numbers. The smaller and more irregular vortices in the roll-up shear layers and wakes observed at the higher Reynolds number are similar to the previous experimental findings, which are shown in this work to make the force fluctuations more irregular at higher frequencies. Possibly due to missing 3D effects, the results obtained from the 2D VPM are observed to 'overestimate' the effects of increasing the Reynolds number at \(R_{\rm C}=60\),000. Furthermore, both VPM and XFOIL are found to work best in explaining the physics at low angles of attacks, i.e., \(-10^{\circ}\leq a\leq 10^{\circ}\), which are similar to the previous numerical works utilizing 2D methods. a 2023 15 November 20221 12 January 2023 15 November 20221 12 January  
circular cylinders, the aerodynamics of a curved-shaped body are known to be sensitive to Reynolds number variations (e.g., [1]). Due to the limitation of the wind tunnel space, the inevitable use of a geometrically-scaled model in wind tunnel tests implies that the measurements may not represent the full-scale wind loads. Therefore, the Reynolds number effects increase the complexities in designing a suitable wind turbine model for wind tunnel experiments. To simulate the performance of the Technical University of Denmark (DTU) 10 MW wind turbine [2] in wind tunnel experiments, for example, the authors of [3] developed a systematic iterative approach for designing a 1/75 geometrically-scaled model. The method first matches the full-scale blade aerodynamics using the 'low-Reynolds' number airfoils, and refines it to satisfy the characteristics of the full-scale structural dynamics. As a result, the shape, the chord lengths, and the pitch angles of the model blades are all different from the 1/75 downsized prototype. Although rigorous, this method requires the knowledge of the aerodynamic performances of another set of (low-Reynolds number) airfoils, implying a time-consuming task to only design a wind turbine model. Not only for wind turbine model design, but also the effects of low-Reynolds number flow play crucial roles in the blade design of small unmanned aerial vehicles [4]. Therefore, one of our intents in this work is to see if it is possible to simplify the model blade design to compensate for the Reynolds effects, e.g., using the same airfoils but rougher surfaces, given that only the tower base load is of interest.