Passive separation control on a square cylinder at transonic speed

A method to control flow separation on a square cylinder has been investigated. The method incorporates the use of small tabs that were deployed in highly separated regions on the sides of the cylinder. The effectiveness of the tabs was investigated mainly at M = 0.56, with limited data also taken at M = 0.71. The tabs proved to be very effective in reducing drag and suppressing force fluctuations on the cylinder. The largest drag reduction, amounting to almost 35%, was achieved at M = 0.56. Pressure measurements on the cylinder surface revealed that large pressure increases were created across the tabs and on the cylinder's rear surface, leading to the observation of a large reduction in drag. Based on the experimental observations, a simplified flow model to explain these phenomena is proposed in which the tab effects are attributed mainly to the obstruction of the reverse flow in the separated region, creating pressure reduction upstream of the tab and pressure increase downstream of it.