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

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.