Reynolds number effects on three-dimensional vorticity in a turbulent wake

When Reynolds number Re (is identical to U infinityd/nu, where U infinity is the free stream velocity, d is the cylinder diameter, and nu is the kinematic viscosity of fluid) varies from 103 to 104, there is a large change in the turbulent near-wake dynamics (e.g., the base pressure coefficient, fluctuating lift coefficient, and vortex formation length) of a circular cylinder, which has previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. This work aims to investigate how this Re variation affects the three components of the vorticity vector and to provide a relatively complete set of three-dimensional vorticity data. All three components of vorticity were simultaneously measured in the intermediate region of a turbulent circular-cylinder wake using a multiwire vorticity probe. It is observed that the root-mean-square values of the three vorticity components increase with Re, especially the strearnwise component, which shows a large jump from Re = 5 x 103 to Re = 104. At Re = 2.5 x 103, the maximum phase-averaged spanwise vorticity variance (omega2x)*, normalized by d and U infinity, is twice as large as its counterpart for the strearnwise component, (omega2x)*, or the lateral component, (omega2y)*. However, at Re = 104, the maximum (omega2z)* is only 55 % larger than the maximum (omega2x)* or 47% larger than the maximum (omega2y)*. The observation is consistent with the perception that the three-dimensionality of the How is enhanced at higher Re due to the occurrence of Kelvin-Helmholtz vortices. The effect of Re on vorticity signals, spectra, and coherent and incoherent vorticity fields is also examined. [PUBLICATION ABSTRACT]