Surprises and disagreements in wing performance at moderate Reynolds number

Due to the extreme sensitivity of the global flow state to small differences in geometry and/or environmental conditions, the state of the current technical literature in moderate Reynolds number aerodynamics can most charitably be described as in flux. Reference data from either experiment or computation are not readily available, and in many cases do not agree. Here we focus attention on a single case of the NACA 65(1)-412 airfoil at chord Reynolds number of 20 000. A combination of information from DNS, wind tunnel, and water-channel experiments will be described. Abrupt transitions between attracting flow states open the possibility of efficient control with large gain from small input disturbance amplitudes.