3-DIMENSIONAL FLOQUET STABILITY ANALYSIS OF THE WAKE OF A CIRCULAR-CYLINDER

Results are reported from a highly accurate, global numerical stabilit y analysis of the periodic wake of a circular cylinder for Reynolds nu mbers between 140 and 300. The analysis shows that the two-dimensional wake becomes (absolutely) linearly unstable to three-dimensional pert urbations at a critical Reynolds number of 188.5 +/- 1.0. The critical spanwise wavelength is 3.96 +/- 0.02 diameters and the critical Floqu et mode corresponds to a 'Mode A' instability. At Reynolds number 259 the two-dimensional wake becomes linearly unstable to a second branch of modes with wavelength 0.822 diameters at onset. Stability spectra a nd corresponding neutral stability curves are presented for Reynolds n umbers up to 300.