Effects of free-stream turbulence and Reynolds number on the separated shear layer from a circular cylinder

Abstract

In this experimental investigation, hotwire measurements were used to study the development of a separating shear layer in the near wake of a circular cylinder. The main purpose was to investigate the effects of the free stream turbulence intensity, Ti, on the separating shear layer. The measurements were performed over a range of Re, based on the cylinder diameter and mean free-stream velocity, between 4.5×103 and 4.7×104, and free stream turbulence intensities of 0.25%, 2.2%, 3.4% and 6.2%.Mean and turbulent velocities in the separating shear layer plotted in self-similar coordinates showed that the shear layer evolved downstream approximately as a plane mixing layer when Ti≤2.2%, while for high Ti (Ti≥3.4%) a departure from similarity was observed for downstream locations x/D≥1.1. The shear layer transition point moved closer to the cylinder and the longitudinal distance at which the separating shear layer can no longer be approximated by a mixing layer decreased as Ti increased. Power spectra measurements showed peaks related to the shear layer frequency, fsl, and its harmonics. In agreement with the self-similarity study, these peaks became broader and indiscernible as Ti was increased, and at the highest Ti of 6.2%, peaks related to fsl and its harmonics could no longer be seen in the power spectra at all Re investigated in this work.The present measurements suggested that the appearance of peaks at fsl and its harmonics in the power spectra is well tied to the extent by which the separating shear layer behaves as a mixing layer. The free stream turbulence seemed to amplify the turbulence content of the separating shear layer thus promoting and accelerating the breakdown of the shear layer vortices. It also decreased the large vortex formation length which limited the downstream development region of the separating shear layer hence preventing the latter from developing in a manner similar to that of a mixing layer. The overall quantitative effect of the free stream turbulence on the development of the separating shear layer was measured by the change in the exponent of the functional relationship relating fsl and Re (i.e. fsh/fv~Ren where fv is the von Kármán vortex shedding frequency) with Ti. The present measurements have shown that the exponent n increased with increasing Ti namely, n was found to increase from 0.56 to 0.65 as Ti increased from 0.25% to 3.4%. Finally, a model has been developed showing that an increase in the free-stream turbulence intensity has, qualitatively, similar effects on the separated shear layer as an increase in Re.