Several mechanistic models have been already developed for predicting the onset of liquid carryover in gas-liquid cylindrical cyclone (GLCC) separators. However, currently no model is available to predict gas carryunder. A bubble trajectory model has been developed that can be used to determine the initiation of gas carryunder in the GLCC and to design GLCC for field applications. The bubble trajectory model uses a predicted flow field in GLCC that is based on swirl intensity. This paper describes the development of a general correlation to predict the decay of the swirl intensity. The correlation accounts for the effects of fluid properties (Reynolds number) as well as inlet geometry. Available experimental data as well as computational fluid dynamics (CFD) simulations were used to validate the correlation. The swirl intensity is used to calculate the local axial and tangential velocities. The flow model and improved bubble trajectory results agree with experimental observation and CFD results. Examples are provided to show how the bubble trajectory model can be used to design GLCC.