Periodic Structures within the Formation Region of Trailing Vortices

Water tow-tank tests were performed for the Ahmed model at a range of “high-drag” backlight angles at Reynolds numbers of up to 1.3 × 10⁵. Dye was injected just upstream of the c-pillars and visualizations were recorded with a submerged CCD camera moving with the model.

Discrete sub-vortices were found to be shed periodically along the length of the c-pillar at Strouhal numbers (based on square root of frontal area) between 8 and 12. These sub-vortices were observed to undergo vortex pairing and then to roll up into the familiar c-pillar vortices. These observations are consistent with previously published observations for delta wings.

Wind tunnel tests were performed in order to provide Reynolds numbers of up to 1.6 × 10⁶. These revealed some spectral features which could be due to the shedding and pairing of discrete vortices from the c-pillar but the evidence was much less conclusive than at low Reynolds number.