On the development of low frequency structures in near and far laminar wakes

Abstract

Flow past any geometry contains structures of diverse sizes, albeit the shedding structure is the most prevalent. There are situations where the flow also has large-scale secondary dominant structures. This study attempts to provide the sources of such low frequency secondary structures in a laminar regime. To this end, we analyze the velocity field through fast Fourier transform in the near and far wake of an elliptic cylinder of axis ratio 0.4 whose major axis is kept perpendicular to the incoming flow. Numerical simulation is carried out for Reynolds number 130, where the flow is reported to be two-dimensional. This study reveals the presence of low-frequency structures besides the primary shedding structures in linear, transition, and saturation regions of temporal wake development. We show that the temporal source of the primary frequency is the saturated state of the wake development, while its physical source is the periodic nature of the saturation region, which inhibits the transmutation of the wavelength of flow structures. On the other hand, the secondary low frequency is embedded in the transitional developing stage of the wake and its physical source is the irregular behavior of the transition process, which aids transmutation of the wavelength of the structures.