On flow topology changes accompanying aerodynamic bifurcation for aircraft with vented strakes

For the vast majority of cases, a symmetric body in a flow with symmetric boundary conditions will produce symmetric forces and moments. Counterexamples to this rule-of-thumb include flow over a circular cylinder at certain Reynolds numbers and flow over ogive-shaped bodies at high angles of attack. This research focuses on another example of asymmetric moments being produced over a symmetric body—an aircraft with vented shakes at moderate angles of attack. During developmental flight tests of the F/A-18E/F Super Hornet, abrupt uncommanded rolling motions were observed with the vents on the strake deflected in the open position in approach configuration. This behavior, consistent with an aerodynamic bifurcation, was not expected to occur since it had not been predicted analytically or empirically. Closing the vents on the strake eliminated the uncommanded lateral dynamics.; In order to understand the flow topology changes associated with the aerodynamic bifurcation, three wind tunnel experiments were conducted using two F/A-18E wind tunnel models in two low-speed wind tunnels. In addition to measuring the effect of vent deflection on the forces and moments, a wide range of flow visualization techniques were used to characterize the on-body and off-body flow topologies. The vents-open force and moment data show critical states indicative of a subcritical flow bifurcation, the abrupt replacement of an unstable flow topology with a stable one. The flow visualization data show that opening the vents has the effect of lifting the powerful strake vortices off the surface of the wing and inboard toward the fuselage. This creates a separated flow region on the midboard part of the wing that grows as angle of attack is increased. At the critical state, the separated region affects the flow on the outboard panel, causing an abrupt loss of lift. This flow topology change may occur asymmetrically due to the unsteady nature of the bifurcation and the interaction of the strake vortices across the plane of symmetry. Recommendations are presented regarding the detection of critical states in the wind tunnel, the suitability of the various flow visualization techniques for studying bifurcation phenomena, and design considerations for future aircraft.