Examination of the Maskell III Blockage Correction Technique for Full Scale Testing in the NRC 9-Meter Wind Tunnel

The 9-meter wind tunnel of the National Research Council (NRC) of Canada is commonly employed in full-scale testing of class 8 tractors. In this configuration the model blocks 10 - 15% of the test section cross-sectional area, which is greater than generally advocated blockage limits. The NRC utilizes the Maskell III method to correct data for wall interference but the effectiveness of this technique at such blockage levels remained to be seen. Corrected full-scale data was compared to data acquired with a half-scale model to determine how closely the corrected high-blockage data would agree with the low-blockage baseline. The half-scale model presented an opportunity to test at full-scale Reynolds numbers, with less than 4% blockage, which falls within most recommendations of maximum allowable blockage. It will be shown that after correcting the data using the Maskell III method, the wind-averaged drag coefficient of the full- and half-scale baseline cases were within 3 drag counts (ΔC_D = 0.003) of each other