omplex flow separation in thrust optimized parabolic (TOP) nozzles in over-expanded condition is one of challenging issue of many numerical investigations. The correct estimation of a TOP nozzle performance extremely depends upon the accurate estimation of the onset of flow separation. Literature review indicates that conventional RANS turbulence models have a significant error in predicting the onset of flow separation in these types of nozzles due to the overestimating of turbulent kinetic energy production. Recently proposed GEKO (generalized model of kω) has made it possible to rectify numerical simulations based on governing physics and using limited experimental results. In the present study, the flow physics in the LEA_TOC nozzle has been investigated with the numerical simulation approach. At the first, the significant error of conventional RANS turbulence models is shown to simulate flow separation in this type of problem. Then, the GEKO parameters are modified based on limited experimental result of LEA_TOC nozzle, and the ability of this model has been evaluated to estimate the flow physics under different pressure ratios. Numerical investigations show that GEKO has a high capability for accurate estimating the onset of flow separation at wide range of nozzle pressure ratios. Applying the corrected GEKO has resulted in an improvement of about 30% in estimation of the onset of separation in over-expanded LEA_TOC nozzle compared to the k-ω-sst model

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