Download alleviation by periodic excitation on a typical tilt-rotor configuration - Computation and experiment

Experiments and computations have been carried out that simulate a wing being immersed in the down-wash of a hovering rotor. The investigation was carried out for two-dimensional mean flow that was approximately normal to a flapped airfoil. After establishing the base drag, periodic excitation emanating from the flap shoulder was added and the drag was reduced. The excitation used was either generated by blowing and suction or by the oscillations of piezo-electric cantilever beams. Computations were performed with a seini-implicit finite element large eddy simulation method for three-dimensional, time-dependent turbulent flows. To reduce the computational requirements, only a slice of the wing was accounted for and the span-wise depth of this slice was about 30% of the chord. The calculations were first performed for the unforced case using about 10⁵ time steps, sufficient for the download caused by the flow field to reach a constant mean value. Thereafter, periodic excitation from the flap shoulder was introduced and the computation continued until the download again reached approximately a constant mean value. This value was in agreement with experimental results and predicted a decrease in the download of approximately 30% due to the periodic excitation. Flow visualization confirmed the computed flow fields.