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

When Reynolds number Re (≡ U∞d/v, where U∞is the free stream velocity, d is the cylinder diameter, and v is the kinematic viscosity of fluid) varies from 103to 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 streamwise component, which shows a large jump from Re - 5×103to Re = 104. At Re = 2.5×103, the maximum phase-averaged spanwise vorticity variance 〈ωz2〉*, normalized by d and U∞, is twice as large as its counterpart for the streamwise component, 〈ωx2〉*, or the lateral component, 〈ωy2〉*. However, at Re = 104, the maximum 〈ωz2〉* is only 55% larger than the maximum 〈ωz2〉 * or 47% larger than the maximum 〈ωy2〉*. The observation is consistent with the perception that the three-dimensionality of the flow 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.