A CFD-Based Nonlinear Reduced-Order Aerodynamic Model for Comprehensive Simulation of Rotorcraft with Active Microflaps

A comprehensive study on unsteady effects of oscillating Gurney flaps, or microflaps, has been conducted. Two-dimensional unsteady airloads, lift, moment and drag, due to an oscillating microflap were computed using a compressible Reynolds-Averaged Navier-Stokes (RANS) flow solver. The CFD results are generated with an overset mesh approach that captures oscillatory microflap motion. Three microflap configurations are examined so as to determine the type most suitable in terms of actuation efficiency and practical implementation. Furthermore, a reduced order model (ROM) for the unsteady microflaps is developed based on CFD simulations, using the Rational Function Approximation (RFA) approach. This approach has been applied successfully in the past to generate an approximate aerodynamic model in a mathematical form compatible with structural equations of motion as well as active control. The resulting RFA model is a state-space, time-domain aerodynamic model that accounts for unsteadiness, compressibility and time-varying freestream effect, suitable for use with comprehensive rotorcraft simulations. The unsteady aerodynamic responses are carefully studied for microflaps subject to a wide range of flow parameters including airfoil angle of attack, microflap oscillatory amplitudes, reduced frequencies, and freestream Mach numbers. The agreement between the ROM and direct CFD calculations is excellent even in presence of strong nonlinear flow effects, which suggests that the approximate model is suitable for incorporation in a comprehensive code, from which the potential of microflaps for active control of vibrations in rotor can be determined.