Formation of Ground Vortices in the Cross Flow over an Axisymmetric Inlet Above a Plane

The formation and sustainment of a ground vortex at the inlet of a cylindrical nacelle operating in suction near a ground plane in the presence of crosswind is investigated in wind tunnel experiments. It is shown that the wall-normal columnar vortex originates at the ground plane and is drawn into the nacelle when its global dimensionless formation parameters (momentum flux and ground plane distance) exceed critical levels. The vortex is precipitated by the development of countercurrent shear flow that is induced over the ground plane on the leeward side of the inlet by interactions between the opposing cross flow and the suction flow into the nacelle. This countercurrent flow engenders rollup of wall-normal vortices by transport and turning of boundary layer vorticity concentrations that, depending on the direction of the instantaneous local velocity, are advected either downstream from or upstream towards the nacelle. When the formation parameters exceed the critical levels, an upstream advected vortex can become anchored to and drawn into the inlet. The anchored ground vortex is sustained by funneling, turning, and stretching vorticity concentrations off the ground plane, while its circulation above ground plane remains invariant as it is drawn into the inlet.