The influence of shear layer thickness on the stability of confined two-dimensional wakes

The influence of confinement onto the inviscid and incompressible linear stability of the family of wakes introduced by Monkewitz [Phys. Fluids 31, 999 (1988)] is examined. The nondimensional parameters of the model, the velocity ratio Λ, defined as ratio of the velocity gap to the mean velocity, the profile shape parameter, which controls the shear layer thickness δw, and the confinement parameter, are varied and their effect onto temporal and spatiotemporal stability properties is considered. Particularly, the limit between absolute (A) and convective (C) instability is investigated as a function of the different parameters. For a given confinement, there exists an optimal value of the shear layer thickness for which the absolute instability is maximal. The absolute frequency and complex wavenumber of the mode at the C/A transition are discussed. Furthermore, the continuous profiles are approximated by means of piecewise broken-line profile, with similar spatiotemporal properties. As a typical application, a few nonparallel base flows, computed by direct numerical simulation at Re = 500, are analyzed on a weakly nonparallel basis by plotting the locus of the local velocity profile in the (δw,Λ) plane. The absolute frequency at the C/A transition point is seen to predict accurately the frequency prevailing in the nonlinear direct numerical simulations. These results further help interpreting the influence of confinement on the Strouhal number measured experimentally in the wake of confined cylinders.