The unsteady lift of an oscillating airfoil encountering a sinusoidal streamwise gust

The unsteady lift of an oscillating airfoil encountering a sinusoidal streamwise gust is experimentally investigated. The sinusoidal streamwise gust is generated by a multiple-fan actively controlled wind tunnel. A two-dimensional airfoil with a NACA0015 profile oscillates in the wind tunnel with a pitch motion frequency of f_v = 1 Hz while the sinusoidal streamwise gust has a different oscillating frequency of f_β = 1.7 Hz. The non-dimensional unsteady lift coefficients determined from surface pressure show the same trends as Greenberg's prediction. Through spectral analysis, the sum frequency f_sum = f_β + f_v and the difference frequency f_diff = |f_β-f_v|, proposed by Greenberg, are firstly observed in the experiment. The experimental results are compared with Greenberg's theory in the case of a small amplitude of gust velocity variation σ = 0.2. The results of all cases indicate that the experimental results agree generally well with Greenberg's prediction, while there is a small deviation.