Understanding the isothermal flow features associated with different vortex breakdown (VB) structures is of great significance to the design of swirl-stabilized gas turbine combustors. A recent experimental study [A. Dolai et al., Phys. Fluids 37, 025212 (2025)] has identified three distinct VB structures (bluff-swirl, bubble, and conical) in the isothermal flow field of a twin-swirl combustor by varying the outer-to-inner jet momentum ratio. The present study employs stress-blended eddy simulations to investigate the recirculation zone structure, pressure field, mixing characteristics, and dynamic stability under these conditions using budget analysis, entrainment mass evaluation, and proper orthogonal decomposition (POD), after reproducing the experimentally measured velocity fields with high fidelity. The recirculation zone shape is governed by the formation of a radial pressure gradient, which is conventionally attributed to centripetal acceleration; here we demonstrate that advective and turbulent contributions play a significant role in its development. The entrainment velocity is another crucial parameter that governs the mixing between two coannular streams in the near field region. It is shown that the entrainment velocity is better approximated by the rms components rather than by the inner or outer mean velocities. Interestingly, the level of mixing is found to be independent of the momentum ratio and the swirling direction. The dynamic stability of the flow field is further assessed using POD analysis. A precessing vortex core (PVC) structure is identified rotating at a nearly constant frequency ($\sim 200\mathrm{Hz}$). The PVC frequency is again found to be independent of both the momentum ratio and the swirling direction. However, the rotation direction of the PVC follows the core swirler. Overall, the present study sheds light on the similarities (mixing, dynamic stability) and the differences (recirculation zone structure, pressure gradient formation mechanisms) between the isothermal co- and counterswirling flow fields under varying operating conditions.

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• Turbulence
• Large eddy simulations
• Navier-Stokes equation