The Titan IVB launch vehicle has experienced much larger than expected buffeting during transonic flight, according to flight telemetry. Computational-fluid-dynamics (CFD) simulations of the unsteady, full three-dimensional, transonic flow environment were conducted to identify the origin of the large vibration levels. Two simulation results are presented, at Mach 0.8 and 1.1, which identify a new, important fluid dynamic mechanism that produces a relatively strong buffet environment. This mechanism involves large vortex pairs, which are periodically shed along the core body. Comparisons between flight data and CFD simulation results demonstrate adequate emulation of both the frequency and magnitude of the unsteady aerodynamic loads. The vortex-pair shedding frequency is shown to be analogous to von Karman vortex shedding. However, a unique mechanism of instability is proposed. The implications of this work for wind-tunnel testing procedures are also addressed.
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