Vortex breakdown over delta wings in an unsteady free stream.

摘要

Experiments were performed in a vertical unsteady water channel to investigate the phenomenon of vortex breakdown in an unsteady free stream. Varying both the frequency and amplitude of the free stream velocity allowed the study of the evolution of vortex breakdown in the spatial and temporal domain as it occurred over the wing.; Flow visualizations demonstrated that during a large amplitude unsteady free stream perturbation, variation of the location of the vortex breakdown position relative to the quasi-steady location is consistently observed, even at low angles of attack. Further, under flow acceleration, vortex breakdown suddenly appeared in an upstream position and then moved downstream during flow deceleration.; Quantitative measurements revealed that vortex shedding and breakdown strongly influenced the lift force. For delta wings with attached and stationary leading-edge vortices, the lift force is not a function of the reduced frequency. If, however, breakdown of the leading-edge vortices and vortex shedding occurs, then the lift force depends on the reduced frequency.; During vortex breakdown, velocity field measurements showed that there is a redistribution of vorticity in the vortex subcore, where the vorticity vector changes from a predominantly axial to a largely tangential direction. In addition, the helix angle for the velocity exceeds that of the vorticity globally in the vortex subcore only in the instant just before breakdown. Furthermore, it was found that through breakdown, global quantities such as circulation and flow rate vary in phase with the upstream velocity and vorticity as they change in time. This implied that an essentially inviscid mechanism underlies the breakdown process.; PIV measurements of the instantaneous flowfield indicated that while the crossflow streamline topologies can differ greatly depending on the flow conditions, the vorticity fields remain essentially unchanged through breakdown. In the pre-breakdown region, the flowfield is stable and streamline patterns contain stable foci, while unstable foci exist in the post-breakdown regime. In the transition to breakdown region, several types of instantaneous streamline topologies can exist in the crossflow plane, including combinations of limit cycles and foci.