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Swirl-induced phenomena are used in gas turbine burners as a mechanism to stabilize the flame. The formation of coherent structures under turbulent swirling conditions plays a fundamental role in the stabilization and needs to be completely understood also in the absence of combustion. In this work, numerical calculations of the unsteady, Reynolds-averaged Navier-Stokes (URANS) equations for isothermal flow in an unconfined annular low swirl burner (50kW) are reported. The standard k-epsilon and Reynolds stress models are used to run computational cases at a Reynolds number of 12,000 and two swirl numbers (S-L = 0.57 and S-H = 0.64). The numerical method is validated with the experiments reported by Legrand et al. . Numerical results agree well with experiments for mean flow, temporal pressure measurements, and transient coherent structures. 2-D proper orthogonal decomposition (POD), 3-D iso-surfaces and advanced, vortex-related visualization methods are used to document the latter.