Enhanced identification of coherent structures in the flow evolution of a pitching wing

The present work investigates the three-dimensional dynamics which affects the evolution of dynamic stall over a pitching wing. The phenomenon is traced over a finite section of a spanwise periodically-extended infinite blade at Reynolds chord number of Rec = 1.35· 105 and a reduced pitching frequency of k = 0.1. The geometry is obtained from NACA 0012 aerofoil unit chord extrusion. Modal analysis is conducted on the computed flow field through Spectral Proper Orthogonal Decomposition. SPOD inputs are gathered via Computational Fluid Dynamics results employing Delayed Detached Eddy Simulations with k− ω SST closure. The three-dimensional database is converted to a two-dimensional spatial distribution by exploiting the spanwise homogeneity of the system. In particular, the SPOD filter role is addressed by decomposing the velocity field. Thus, three different filter sizes are chosen to investigate the response of the modal set corresponding to the two dominant pairs. The results prove that stronger filtering actions are less effective over low-harmonic contents, already featuring high correlation levels around a single dominant frequency. Conversely, higher harmonics responses are more dependent on the filter dimension. The latter, in fact, enforces a temporal constraint that may introduce detrimental effects on the energetic optimality of the spatial modes, thus compromising the possibility to draw a direct connection with physical evolutions.