Direct stability analysis based on the Floquet theory has been employed to clarify the effects
of planar shear on three-dimensional instabilities in the wake of two identical circular
cylinders of diameter D in tandem arrangement. The center-to-center separation (L x) in the
range of 1.2≤ L x/D≤ 2.5 was considered. The onset of the three-dimensional instabilities
was calculated, and the critical Reynolds number and corresponding spanwise
wavenumber varying with the separation were discussed for different shear rates …