The present paper proposes a new approach to control flow separation around a body. Flow separation is controlled by inserting simple tabs inside the separated region to suppress the reverse flow action. This is expected to increase the pressure in the base region of the body, thus reducing drag. Moreover, flow instability is also expected to decrease because of change in the wake profile. The cases considered in the present investigation are flows around a circular cylinder at M = 0.6 and 0.73. Tabs having lengths of 10% and 20% of the cylinder diameter were used. The results show that the base pressure of the cylinder can be increased when these tabs are inserted inside the separated region. The smallest drag on the cylinder/tab body was achieved when the two pairs of tabs were installed on both sides of the cylinder at angles of ±120• and ±140 • , measured from the front stagnation point of the cylinder. Compared to the plain cylinder, drag was reduced by 32% at M = 0.6 and by 18% at M = 0.73. Schlieren photography reveals that the vortex formation length is increased when the tabs are installed. Moreover, the tabs greatly suppress the level of pressure fluctuations on the cylinder surface. This can be attributed to change in the wake profile that is associated with drag reduction. Furthermore, the frequency of the Karman vortex street is also increased. The analysis of the results was assisted by numerical calculations based on Large Eddy Simulation (LES). From these results, five significant effects of the tabs were identified: restriction of the reverse flow action, trapping of vorticity in the region upstream of the tabs, suppression of the shear layers' movement, more rapid vortex roll-up downstream of the body, and reduced strength of the downstream vortices.

A second throat has been used to stabilize the ow in the test section of an induction-driven transonic wind tunnel. Different second throat con gurations were attempted by employing simple wall-xed aps and a centerline-xed strut, and their ef ciency based on the duration of stable test section ow was examined. Generally, a longer duration of stable ow could be achieved when the tunnel was operated with a second throat than without one. Moreover, a longer run time was achieved with the wall-xed aps than with the center strut. The performance of the center strut was so poor that it raised concerns about the installation of the sting support, which is usually installed in the same manner as the center strut. It was also found that when the aps were installed vertically on the sidewalls the low-momentum reentry ow from the plenum chamber reduced the effectiveness of the aps in choking the ow. The performance was greatly improved by repositioning the aps horizontally and directly behind the reentry region. However, there was an upper limit to the effectiveness of the aps in choking the ow. Beyond a certain injector total pressure the effective cross-sectional area of the diffuser at the injector's location was reduced by the jet, turning it into the effective throat area. The cross-sectional area at the second throat is rendered ineffective, causing the Mach number in the test section to temporarily drop.

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.

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