Unsteady Reynolds-Averaged Navier-Stokes modelling (URANS) is a valuable and cost-effective tool for Computational Fluid Dynamics (CFD), including the investigation of mainstream-cavity interaction in turbines. Despite the gap in accuracy with higher order CFD methodologies, URANS is among the few simulation strategies of industrial interest suitable for predicting ingress/egress over a wide range of conditions. This paper presents a numerical study of the flow-field in the upstream double-radial seal of a 1.5 stage turbine. Various configurations are tested, including non-purged and purged conditions. Rigour of the approach is ensured by a set of sensitivity analyses, allowing the delineation of a best practice on the use of URANS in rim seal simulations. Time-averaged and time-resolved flow predictions capture coherent structures in the rim gap. An association between the 3D morphology of these structures and different ingress/egress mechanisms is proposed. Regions of enhanced radial activity are identified to correspond with the blade leading edges. A frequency analysis of unsteady pressure signals probed in the rim gap leads to a calculation of the structure number and speed. The structures are synchronous with the disc rotation for non-purged cases but rotate at slower speed when purge is introduced. The relative number of blades and vanes directly influences the structure count and velocity. There is a reduction in radial activity and structure speed at lower flow coefficient, fundamentally related to the reduced pressure asymmetry and gradient of swirl across the rim seal.

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