Abstract
For scale model tests to accurately predict the performance of real-world applications, similarity requirements must be met. Generally, there are three types of similarity requirements: geometric similarity, dynamic similarity and kinematic similarity. Geometric similarity requires the model to look identical to the full-sized body but scaled up or down. Dynamic similarity requires the ratio of relevant forces to be equal for the model test and real-world which is often accomplished by matching the Reynolds number and/or other relevant dimensionless variables. Finally, kinematic similarity requires that the streamlines around the body during the scale model testing match the streamlines around the real-world body. However, when scale model tests are conducted in a wind tunnel, or water tunnel, there are walls that can modify the streamlines during the model tests so as to inadequately match the corresponding streamlines that one would find under similar conditions without nearby walls. One consideration during wind tunnel testing that helps to account for this potential hurdle is blockage ratio, defined as the ratio of the model projected area to the cross-sectional area of the wind tunnel test section. The pinching of the flow between the model and walls both impacts the velocity experienced by the model and the streamlines. In this study, the impact of blockage ratio was experimentally studied in a subsonic wind tunnel. A series of NACA 0015 airfoils were used with chord lengths ranging from 42 mm to 151 mm which effectively varied the blockage ratio from 2.3% - 14.5% during testing. Tests were conducted at Reynolds numbers ranging from 50,000 to 200,000. To understand how blockage ratio affected flow characteristics, the velocity contours above the upper surface of the airfoil were measured at varying angles of attack. Additionally, the stall angle of the airfoils was found to be impacted by Reynolds number and blockage ratio.