Very-large-scale motions in the turbulent cylinder wake

Abstract

This work studied the very-large-scale motions (VLSMs), which were first observed in wall-bounded flows, in a turbulent cylinder near wake using two experimental datasets from Chen et al. [“Three-dimensional vorticity, momentum and heat transport in a turbulent cylinder wake,” J. Fluid Mech. 809, 135–167 (2016)] and Tang et al. [“Secondary vortex street in the intermediate wake of a circular cylinder,” Exp. Fluids 59, 119 (2018)]. It is found that the premultiplied spectra of both the streamwise velocity and temperature fluctuations measured with hot and cold wires by Chen et al. [“Three-dimensional vorticity, momentum and heat transport in a turbulent cylinder wake,” J. Fluid Mech. 809, 135–167 (2016)] display a peak at a low wavenumber corresponding to a frequency of about 1/4 of that of the Kármán vortices in the flow. A proper orthogonal decomposition method was utilized to decompose the velocity field measured with particle image velocimetry by Tang et al. [“Secondary vortex street in the intermediate wake of a circular cylinder,” Exp. Fluids 59, 119 (2018)] into modes with different frequencies. A prolonged coherent structure with a wavelength of about four times the Kármán vortices is observed in the visualization of the energetic velocity modes with the same frequency of the spectral peak, which is consistent with the spectral data and confirms the existence of VLSMs in the wake.