Periodic unsteady flow within a bypass dual throat nozzle with bypass closure

Bypass Dual Throat Nozzle (BDTN) is a novel Thrust Vectoring Control (TVC) nozzle featuring bypasses and the divergent–convergent cavity structure, while its complex geometry might induce unsteady flow. A BDTN model is developed and tested in a blowdown wind tunnel to investigate the evolution and characteristic frequency of unsteady flow within BDTN under bypass-closed state through synchronized high-speed schlieren imaging and dynamic pressure sensing. The distinct Primary/Recirculating Flow Interaction (PRFI) within the nozzle cavity has been observed from instantaneous snapshots, which leads the inflow–outflow process in recirculating flow and the evolution of shock waves and wave-shaped structures in mainstream. Then, the PRFI is identified as the dominant role for the unsteady flow within the cavity through statistical analysis of mean, Root-Mean-Square (RMS) and temporal partial derivative (pd). Synchronized schlieren–pressure pulsation analysis elucidates the bypass–cavity pressure balance mechanism, inducing separation bubbles and promoting inflow–outflow process. Moreover, the pressure signal spectra from both the cavity and bypass yield identical fundamental frequencies, identified as the characteristic frequency of unsteady flow. A preliminary frequency estimate derived from disturbance propagation demonstrates consistency with spectra of shock motions and pressure fluctuation, incorporating coupled bypass–cavity structural effects.