Description
Flow over a NACA 65(1)-412 airfoil is studied for a moderate chord-based Reynolds number of 20,000 typically encountered in small-scale technologies such as micro-turbines and small unmanned air vehicles. Experiments are conducted at the San Diego State University (SDSU) Low-Speed Wind Tunnel with a turbulence intensity level of 0.27% in the test section. The SDSU data is compared to a similar experiment conducted at the University of Southern California Dryden Wind Tunnel (DWT) which has notably lower turbulence intensity of 0.035%. Finite span wing models are fabricated independently and vertically centered with endplates in their respective wind tunnel test sections to mimic quasi-2D flow. The SDSU’s model has an aspect ratio of 12.6 while the DWT model is 12.9. Experimental lift and drag polars are measured with force balances, compared and verified against X-Foil computations for varying turbulence intensities and Reynolds numbers. China Clay is used to visualize the separation location in the SDSU experiment. The lift and drag polars of both SDSU and DWT experiments show a sudden increase in the lift and decrease in the drag at a critical angle of attack, αcrit, at which the flow changes with increasing angle of attack from a laminar separated flow to a flow with a laminar separation bubble that transitions the laminar boundary layer to a turbulent boundary layer. However, the αcrit occurs at different values; in SDSU’s experiment αcrit occurs at 8◦ whereas for DWT it occurs at 9.5◦. Increasing the freestream turbulence appears to promote earlier onset of the critical angle. Beyond the critical angle, SDSU’s lift polar increases for two degrees before reaching the maximum lift and after which the lift gradually decreases. By comparison, DWT’s reaches its maximum lift at the αcrit and reduces beyond it. Comparison with X-Foil trends confirm that higher freestream turbulence intensities promote an earlier onset of boundary layer transition. Recommendations for future experimental work following this effort should consider emulating varying levels of freestream turbulence intensities in a single wind tunnel using a controlled turbulence generation instruments.
