Water tunnel experiments on flow separation and vortex control by applying suction

In this experimental study, the effectiveness and methods of applying suction as a tool for flow control, specifically to prevent flow separation and to curtail vortices that promote separation, were studied utilizing a water tunnel facility. All tests were conducted at moderately low Reynolds numbers ranging from 1,550 to 21,180. Experiments were conducted with four different configurations: (1) Cylinder model of diameter 4.83 cm, (2) Cylinder - flat plate junction model with a cylinder diameter of 11.43 cm, (3) Streamlined body - flat plate junction model, where the streamlined body is a wing with NACA 0012-64 airfoil and chord length 18.1 cm, and (4) 30° swept-forward wing with NACA 0012-64 airfoil and of chord length 16.2 cm at angles of attack of 30° and 40°. Instead of random, large-scale, brute-force applications of suction, in this study suction was applied discretely only at certain selected locations (determined based on guiding principles that were drawn fi-om some fundamental study of the physics of the flow), and at moderately low suction rates. Tests were conducted at suction rates ranging from 0 to 21.0 cc/sec. The suction coefficients, C_Q = Q/AV_∞ ranged from 0.0028 to 0.0760 at various Reynolds numbers. With suction applied at low suction rates and only at selected locations, very effective flow control was achieved around the models tested. In a severe case of flow over the cylinder, separation was delayed up to a maximum of 19°. In the cases of the cylinder and streamlined body - flat plate junctions, the horse-shoe vortex was almost totally eliminated. In the case of the swept-forward wing, a ‘horn' vortex which forms as a consequence of reversed flow was moved towards the root of the wing. Row reversal was avoided, and nearly full-chord attached flow was established up to 45% span.