Unsteady Aerodynamics of Flapped Airfoils and Rotors Using CFD and Approximate Methods

The ability to accurately and efficiently model the unsteady aerodynamic effects of actively controlled trailingedge flaps (ACFs) is crucial for practical application of such systems for vibration and noise reduction as well as performance enhancement. Two-dimensional unsteady airloads due to oscillating flap motion are calculated and compared using various CFD codes and a CFD based reduced order model (ROM). This ROM is based on the Rational Function Approximation (RFA) approach, which results in a state-space, timedomain aerodynamic model suitable for incorporation into comprehensive rotorcraft simulation codes. The accuracy of this model was demonstrated across the practical range of unsteady flow conditions encountered by the active flaps. Two Reynolds-Averaged Navier-Stokes (RANS) solvers (CFD++ and OVERFLOW) were employed in conjunction with various turbulence models, including large eddy simulation based models. Unsteady sectional airloads obtained using the different CFD methods were compared so as to determine code independence. Furthermore, detailed flow physics associated with three-dimensional effects, flap hinge gap, as well as compressibility effects were analyzed using CFD.