Response of a Streamwise Vortex–Wall Interaction to Unsteady Forcing

Abstract

Streamwise vortices exist in a number of engineering flows in both external and internal aerodynamics, often with undesirable effects. An experimental investigation of a streamwise vortex–wall interaction is performed through the use of a vortex-generating wing section and a suspended splitter plate. A synthetic jet actuator is implemented below the vortex, and the response of the vortex to unsteady forcing is evaluated with a focus on spatial amplification through convective instabilities. At low forcing frequencies, a large displacement of the vortex is recorded near the actuator, but with almost no spatial amplification. This behavior agrees qualitatively with transient growth studies of columnar vortices. Within a specific frequency range, it is shown that spatial amplification of the vortex displacement occurs. The range of amplified forcing frequency agrees well with predictions for the Crow instability. The vortex–wall interaction generates a secondary vortex structure that rolls up from the boundary layer. When the actuator displaces the primary vortex structure, this causes a strong dynamic reaction in the secondary structure. This occurs due to a mutual induction mechanism between the two vortices as well as a spatial oscillation of the separation point from which the secondary structure originates.