Effects of forebody deflection on separated vortex pairs over slender bodies

Morphable forebodies offer a promising flight control strategy for slender-body vehicles, yet the impact of forebody deflection on leeward vortex dynamics remains poorly understood. This study uses particle image velocimetry to investigate vorticity analysis, dynamic mode decomposition, proper orthogonal decomposition, and phase-averaged reconstruction, and we characterize the downstream evolution and unsteady behavior of the leeward vortex pair. Results indicate that the flow remains dominated by forebody vortices. Positive deflection strengthens these vortices, causing regular variations in the time-averaged intensity, position, and scale of the aftbody vortices. However, excessive deflection pushes the forebody vortices away from the aftbody surface, preventing entrainment by the aftbody shear layer and thereby weakening the overall vortex strength. Under most conditions, the dominant Strouhal number (St) remains between 0.09 and 0.12, consistent with the self-sustained oscillations observed in undeflected flows. Notably, at a high angle of attack (30°) and large deflections (20°), a novel dual-frequency oscillation emerges. A low-frequency component (St = 0.04–0.05) appears with higher energy, becoming the dominant oscillation mode. Vortex-core trajectories reveal that this oscillation maintains a periodic alternating pattern relative to the symmetry plane, but with the core positioned further from the body surface. These findings demonstrate that forebody deflection not only governs the time-averaged downstream development of the leeward vortex pair but also fundamentally alters its unsteady dynamics.