An ejector is a heat-driven device that compresses a low-pressure secondary flow by gasdynamics interactions with a co-flowing high-pressure primary flow. Ejectors have been employed in multiple engineering domains, including mechanical, aerospace, and energy conversion management systems. The performance of ejectors mainly depends on the mixing of the primary and secondary flows. Typical Reynolds numbers inside ejector are in the order of ~10⁶. Therefore, most previous studies have adopted RANS-based turbulence models for ejector analysis. The global properties such as entrainment and compression ratios have been estimated accurately with RANS-based models. However, local flow structures have not been well predicted. This work investigates the gasdynamic structures arising due to the mixing of the primary and secondary flows within an ejector for the critical flow regime using a large eddy simulations model. The deviations between CFD estimated and experimentally measured ER and non-mixed length are 17% and 4%, respectively. The flow instabilities associated with the mixing and re-compression shock structures are captured well with the LES model.