Rocket nozzles experience the problem of undesirable side-loads during start-up and tail-off phases of thrusting. The major reason for this is attributed to asymmetric flow separation and associated shock unsteadiness caused due to over-expanded flow in the nozzle. Control of flow separation in the nozzle is hence the approach adopted by several researchers to tackle this problem. This paper presents the computational results of a novel strategy to control flow separation in an over-expanded supersonic nozzle with a rectangular cross-section. 2D and 3D steady Reynolds-Averaged Navier-Stokes (RANS) simulations were carried out in ANSYS Fluent with air as the working fluid. The pressure-based coupled solver was used along with SST k- \(\omega \) turbulence model. The results indicated a significant delay in flow separation on the walls of the nozzle. The grid independence was established, and the results were validated with that of a similar case in the literature.