The nozzle are currently presenting the most promising optimisation possibilities to supply the increasing needs of the aerospace industry. The dual bell nozzles allow the altitude adaptation of the flow. The inflection forces the flow to a stable and symmetrical separation, generating an optimised thrust for sea level operation. During the flight, when the ambient pressure decreases under a certain value, the flow suddenly attaches the nozzle wall down to the nozzle end, generating a higher expansion for higher performance. The study presented in this document was made at the DLR (German Aerospace Center) as part of a PhD. This fundamental study has for objective the understanding of the flow behaviour in a dual bell nozzle, in order to define the various parameters of influence. A cold flow test series has been conducted on various nozzle geometries. Pressure measurements have been made and the flow has been observed with schlieren optics in order to determine the influence of the geometry on the evolution of the flow transition from one operating mode to the other, its duration and stability toward pressure variations. Temperature and pressure measurements were also made during a test series under warm gas conditions to investigate the thermal load in the vicinity of the wall inflection.