Regulated Formation of a Ground Vortex in Suction Flow Over a Surface*

The formation of a columnar ground vortex by the interaction of uniform flow over a plane surface with suction into an adjacent cylindrical conduit and the ingestion of the vortex into conduit's inlet are investigated in wind tunnel experiments using planar/stereo PIV with specific emphasis on the role of the surface vorticity layer in the vortex initiation and sustainment. It is shown that when combinations of the inlet momentum flux, crosswind speed, and inlet elevation exceed critical threshold, wall-normal columnar vortices are spawned within a counter current shear layer that forms over the ground plane within a streamwise domain on the inlet's leeward side by the suction flow into the duct. At low suction speeds, these wall-normal vortices are advected downstream, but their celerity is reversed with increased suction and they are advected towards the inlet, gain circulation stretch and are ingested into the inlet. Reduction of the counter current shear within the wall vorticity layer by deliberate, partial bypass of the inlet face flow through the periphery of the cylindrical conduit can significantly delay the ingestion of the ground vortex to substantially higher formation threshold.