Modeling and Experimental Verification of the Aeroelastic Behavior of a Typical Airfoil Section With Shape Memory Alloy Springs

This paper presents the modeling, simulation and wind tunnel experimental verification of the aeroelastic behavior of a two-degree-of-freedom (pitch and plunge) typical airfoil section with superelastic shape memory alloy helical springs in the pitch degree-of-freedom. A linearly elastic spring is considered in the plunge degree-of-freedom. Although viscous damping is considered in both degrees-of-freedom, hysteretic damping simultaneously takes place in the pitch degree-of-freedom due to the (stress-induced) pseudoelastic behavior of the shape memory alloy springs. The shape memory alloy phase transformation kinetics and constitutive modeling are based on Brinsons model and the shape memory alloy helical spring behavior is based on classical spring design. The nonlinear effects of shape memory alloy phase transformation are included in the shape memory alloy spring modeling for the representation of hysteretic force-displacement behavior. A two-state linear aerodynamic model is employed to determine the unsteady pitching moment and lift. The aeroelastic behavior of the typical section is numerically and experimentally investigated for different preload levels applied to the shape memory alloys. Numerical predictions and experimental results show that for large enough preload levels (such that shape memory alloy phase transformations take place at small pitch angles) unstable post-flutter regime is replaced by stable limit-cycle oscillations. Moreover, the amplitudes of aeroelastic oscillations decrease with increasing preload levels since more expressive phase transformations are achieved at small pitch angles. Although the amplitudes of the post-flutter limit-cycle oscillations increase with increasing airflow speed (since aerodynamic loads increase with the square of the airflow speed), they remain bounded within acceptable levels over a range of airflow speeds due to hysteretic damping. Moreover, the cutoff airflow speed increases with increasing preload. The experimentally verified results show that the pseudoelastic behavior of shape memory alloy elements can passively enhance the aeroelastic behavior of a typical section.