Flow Separation Response to Unsteady External Disturbances in Dual Bell Nozzles

The dual bell nozzle belongs to the family of Altitude Compensating Nozzles (ACN), which 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). Therefore the dual bell nozzle operates with a separated flow at take off and 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 unsteady ambient pressure on the separation point and on the shock system inside the dual bell nozzle, operating in the first mode. The external computational domain is characterized by an acoustically open downstream boundary. The numerical simulations show that the oscillation of the separation point is very sensitive to frequencies near the acoustic wave characteristic frequencies.