Formation and Stability of a Ground Vortex in the Cross Flow over an Axisymmetric Inlet

The formation mechanisms of a ground vortex by cross flow along a plane surface and suction into an inlet of a cylindrical nacelle whose axis is normal to the wind direction and parallel to the ground plane is explored in wind tunnel experiments. The evolution of the columnar vortex and its ingestion into the inlet are investigated over a broad range of the primary formation parameters that include the speeds of the inlet and cross flow and the cylinder's elevation above the ground plane. The interactions between the inlet and cross flow and the formation of the ground vortex are measured using planar PIV in multiple planes normal to the nacelle's axis and parallel to the ground plane. The present investigations show that the vortex forms when the ratio of the inlet and cross flow velocities exceeds a critical value that varies linearly with the ratio of the elevation of the inlet's centerline and its diameter, as already pointed out in the past. It is also shown that when the critical value of the inlet/cross flow velocity ratio is exceeded for a given elevation ratio, the ground plane first begins to intersect the stream surface of the flow into the inlet, thereby enabling surface vorticity to roll into a "lambda" structure of two counter-rotating longitudinal vortices that begin to be advected towards the nacelle's inlet. Depending on the inlet/cross flow velocity ratio, one of the unstable longitudinal vortices is suppressed and the dominant vortex forms the ground vortex that is ingested into the inlet. Furthermore, it is shown that for given velocity ratio and ground plane elevation the ensuing flow field is self-similar.