Abstract: Rotor-Stator cavities are frequently encountered in engineering applications such as gas turbine engines. They are usually subject to an external hot mainstream crossflow which in general is highly swirled under the effect of the nozzle guide vanes. To avoid hot mainstream gas ingress, the cavity is usually purged by a stream of sealing flow. The interactions between the external crossflow, cavity flow, and sealing flow are complicated and involve all scales of turbulent unsteadiness and flow instability which are beyond the resolution of the Reynolds-Average approach. To cope with such a complex issue, a wall-modeled large eddy simulation (WMLES) approach is adopted in the current paper. In the simulation, a 20-degree sector model is used and subjected to a uniform pre-swirled external crossflow and a stream of radial sealing flow. It is triggered by a convergent RANS result in which the SST turbulent model is used. In the WMLES simulation, the Smagoringsky SGS model is applied. A scalar transportation equation is solved to simulate the blending and transportation process in the cavity. The overall flow field characteristics and deviation between RANS and WMLES results are discussed first. Both RANS and WMLES results show a Batchelor flow mode, while distinct deviation is also observed. Deviations in the lower radius region are caused by the insufficiency of the RANS approach in capturing the small-scale vortex structures in the boundary layer while deviations in the high radius region are caused by the insufficiency of the RANS approach in predicting the external crossflow ingestion. The boundary layer vortex and external ingestion are then discussed in detail, highlighting the related flow instabilities. Finally, the large-flow structures induced by external flow ingress are analyzed using unsteady pressure oscillation signals.