Computational and experimental investigation of Brazilian Free-Tailed bat ear tubercles on an airfoil leading edge

Abstract

Preliminary experimental data has indicated that the tubercles on Brazilian Free-Tailed bat ears, when applied to an airfoil leading edge, reduce drag and delay aerodynamic stall. This study's objective was to investigate the potential drag reduction from the tubercles using a Computational Fluid Dynamics (CFD) model that was validated against experimental data. Initial CFD simulations at Reynolds numbers (Re) of 5,600 and 16,800 and angles of attack of 0 and +/-5 degrees showed slight drag reduction at the non-zero angles, but also indicated inconsistencies with previous experimental work. New experimental data was acquired using particle image velocimetry at Re of 5,600, 16,800, and 20,700. The angle of attack was varied between 0 and 6 degrees in 2 degree increments. The CFD simulations were updated to match these new experimental conditions. At 4 and 6 degrees for Re = 20,700, the experimental and CFD data both confirmed that the tubercles reduced drag, although they differed on the drag reduction magnitude. Since the clean models matched experimental data and the drag reduction trends followed experimental trends at Re = 20,700, these simulations were assumed to be capturing the drag reduction mechanism(s). Conversely, the simulations at low angles of attack (0 and 2 degrees) as well as all angles at Re = 5,600 showed disagreement with the experiments for both the drag reduction trends and magnitudes. The experimental data in these cases exhibited high uncertainties. Improvements are required to reduce experimental uncertainty at Re = 5,600, and further mesh refinements on the tubercled cases and possible changes in flow solvers at the low Reynolds numbers are required to investigate drag reduction magnitudes. Preliminary analysis of the CFD results showed that the drag reduction was possibly caused by increased boundary layer vorticity that streamlined the wake.