Evolution of an Initially Columnar Vortex Terminating Normal to a No-slip Wall

The early evolution of an initially columnar vortex normal to a solid wall was examined. The vortex was generated by a pair of ¯aps in a water tank. Detrimental effects from the wall during the vortex generation were avoided by producing the vortex normal to a free surface and subsequently bringing a horizontal plate into contact with the surface. Digital particle image velocimetry (DPIV) measurements of the velocity and vorticity, together with laser induced ¯uorescence (LIF) visualizations, in a meridional plane revealed a toroidal structure with the appearance of an axisymmetric vortex breakdown bubble. Agreement was found between the measurements and numerical simulations of the axisymmetric Navier±Stokes equations. The results show that the ¯ow in the effusive corner region is dominated by a Bo Èdewadt-type spatially oscillatory boundary layer within the core region and a potential-like vortex boundary layer at large radii. The toroidal structure results from the interaction between these two boundary layers, leading to the roll up of a dominant shear layer within the Bo Èdewadt structure, and does not develop from the columnar vortex itself. 1 Introduction The interaction of a vortex with a rigid wall is of interest for its prevalence in many practical situations, such as vortex chambers, and in geophysical ¯ows, such as tor-nado/ground interactions. Vortex/endwall ¯ows are also of fundamental interest and the subject has a long history of theoretical and experimental investigation which is ongoing. The ®rst study to examine the production of secondary ¯ow induced by the interaction of a vortex with a rigid boundary was that of Taylor (1950). Taylor's theoretical study only applied to the boundary layer formed away from the core, as have most other such studies, e.g., see Burggraf et al. (1971) and Belcher et al. (1972). These have relied on boundary layer approximations to the governing equations, and the question largely remains as to how the ¯ow evolves in the effusive corner region, where the boundary layer ¯ow is turned into the axial direction. For a recent review of rotating ¯ows with axis normal to a no-slip wall, see Lugt (1996). The nature of the ¯ow in the effusive corner is far more complicated than that in just a single boundary layer, or a vortex core. This is a region where the usual boundary layer approximation is not valid as the interaction involves two boundary layer type ¯ows that are perpendicular to each other so gradients …