Sound generated by a wing with a flap interacting with a passing vortex

We study the acoustic signature of a rigid wing, equipped with a movable downstream flap and interacting with a line vortex, in a two-dimensional low-Mach number flow. The flap is attached to the airfoil via a torsion spring, and the coupled nonlinear fluid–structure interaction problem is analyzed using thinairfoil methodology and the Brown and Michael equation. Passage of the incident vortex above the airfoil initiates flap oscillations at the system natural frequency, amplified over all other frequencies excited by the vortex. Far-field sound radiation is analyzed, yielding the leading order dipole-type signature of the system. The acoustic radiation is dominated by vortex sound, consisting of relatively strong leading and trailing edge interactions of the airfoil with the incident vortex, together with late-time wake sound resulting from induced flap oscillations. In comparison with counterpart rigid (non-flapped) configuration, we find that the flap may act as sound amplifier or absorber, depending on the value of flap-fluid natural frequency.