A flow control technique for noise reduction of a rod-airfoil configuration

Adaptive and flow control techniques have been investigated as possible noise reduction approaches in modern commercial aircraft and small unmanned air vehicles. In the present paper, a rotating cylinder is examined as a noise reduction device in a simplified airframe component. The rod-airfoil canonical benchmark is used as a test case and noise prediction is realized by a 3D hybrid computational aeroacoustics approach. The rod is rotated at frequencies ranging from 0.5 to 2 times the natural shedding frequency of the nonrotating case. Evaluation of the directivity of generated noise around the rod and airfoil demonstrated that cylinder rotation leads to reduction of noise emissions, which became more pronounced for the highest rotational frequencies. Rod rotation also proved to be beneficial from an aerodynamic perspective, generally increasing the lift forces and reducing the drag forces acting on the rod and airfoil. Evaluation of the shedding Strouhal number displayed an ascending trend with increasing rotational frequency, which in turn resulted in a shift of the dominant acoustic tonal components towards higher frequencies. Moreover, rod rotation led to gradual suppression and deflection of the vortex street away from the symmetry plane. Since periodic vortex shedding is the dominant noise mechanism, vortex shedding suppression and minimization of the airfoil interaction with the vortex street is the cause of reduced acoustic emissions. The present study thus shows the potential of the rotating cylinder as a noise reduction device in aeronautical applications, while underlining its capabilities of enhancing aerodynamic performance.