Separation Shock Cutoff Frequency in Dual Bell Nozzles

The dual bell nozzle constitutes an option of great interest to improve launcher first stage performance. The internal flow of this nozzle adapts to the external pressure by separating at the wall inflection at low altitude (first operating mode) and by full flowing at high altitude (second operating mode). In such a way the dual bell nozzle operates with a separated flow in the initial portion of the flight trajectory. This characteristic is of important concern since the external flow is neither steady nor axisymmetric, and its coupling with the internal flow separation can cause dangerous side loads. This work presents the results of a time accurate numerical analysis of the effect of an unsteady forcing (imposed by varying the ambient static pressure) on the separation point and on the shock system inside a sub-scale cold-gas dual bell nozzle, operating in the first mode. The spectral analysis of the separation point response to the forcing reveals that the system dynamics is characterized by low-pass filter behavior, with a cutoff frequency around 600 Hz. These results are compared with analogous findings present in literature.