Zoology of Unstable Modes in a Stratified Cylinder Wake

This paper delineates experimental and numerical results concerning the effect of the stratification and the tilt on the 3D instabilities of a cylinder wake led at moderate Reynolds numbers from the 2D von Karman vortex street. For a vertical cylinder, shadowgraph visualisations put forward an unstable mode similar to the mode A well known in homogeneous fluids. When the cylinder is tilted a new unstable mode is observed at moderate Froude numbers, characterized by undulated layers of strong density gradients and axial flow corresponding to Kelvin-Helmholtz billows created by the strong shear present in the critical layer of each tilted von Karman vortex. Eventually, for large tilt angles, two other modes have been observed. For a weak stratification, an instability appears due to the overturning of the isopycnals by the von Karman vortices whereas for a strong stratification, a short wavelength unstable mode can be observed, even in the absence of von Karman vortices.