| The next generation of rotorcraft will be required to operate with much higher performance levels and high maneuverability is a major objective of future designs. The maneuvering capability of rotorcraft can be improved by reducing or suppressing vibratory loads on the main rotor blades caused by unsteady separation and stall processes. Therefore, a firm understanding of the unsteady separation at the leading edge of the blades that lead to stall is important. Once this is accomplished, rational control measures can be introduced to inhibit the separation process.; This research is, in part, a computational study of possible control mechanisms for unsteady boundary-layer separation from the leading edge of a two-dimensional airfoil placed in a uniform flow at a constant angle-of-attack above a critical angle where separation normally occurs. Processes of suction and injection are considered. First, the effect of suction slot placed near the leading edge is addressed. Computations were carried out both in Eulerian and Lagrangian coordinates with various distributions and strengths of surface suction, and different angles-of-attack. The results showed that delays in separation of six to ten times are possible using a suction slot. The effect of wall motion was also studied. When a small portion of the wall near the nose is moved in the direction of the local mainstream velocity, separation can be suppressed. Finally, a passive suction/injection device was designed in which fluid is removed in a suction slot and reintroduced near the airfoil nose through an injection slot; the device relies on naturally occurring pressure differences to produce suction. The calculations showed that the device works reasonably well for small angles of attack.; Separation process on helicopter blades are generally three-dimensional and in the second part of this thesis computational methods for three-dimensional unsteady separation a viscous boundary layer were developed. A model problem is selected for simplicity and to develop the general capability to compute and assess the complex characteristics of three-dimensional boundary-layer flows. The calculated results for a three-dimensional vortex in motion above a wall are among the first produced for a three-dimensional boundary-layer in Lagrangian coordinates. |
