Experimental Study on Energy Evolution of Coherent Structure in Turbulent near Wake of Circular Cylinder ^†

The evolution of a coherent structure in a cylindrical wake was studied through observing its energy contribution to the flow field. Analysis using the proper orthogonal decomposition on the PIV data measured at two Reynolds numbers (Re) of 3840 and 9440 was performed. The coherent structure was identified by checking the Fourier power spectrum for each temporal mode coefficient and selecting those whose peak magnitudes were greater than the smallest magnitude of the identified harmonic frequency family as the large-scale organized motions. The energy contribution by the coherent structure is significantly dependent on Re. The evolution of the energy contribution by the coherent structure exhibits a monotonously decaying trend when moving downstream. The coherent structure primarily contains the Kármán vortices in the near wake. The contribution weight of the secondary vortices gradually increases, along with the streamwise distance, except in the very upstream subregions for the case of Re = 9440. The energy contribution by the secondary vortices immediately behind the cylinder (x/d = 0.5–5.5) was 30% for Re = 9440, in comparison with <1% for Re = 3840, but decayed rapidly to the value of <10% in the downstream subranges.