Annular cavities are found inside rotor shafts of turbomachines with an axial or radial throughflow of cooling air, which influences the thermal efficiency and system reliability of the gas turbines. The flow and heat transfer phenomena in those cavities should be investigated in order to minimize the thermal load and guarantee the system reliability. An experimental rig is set up in the Institute of Steam and Gas Turbines, RWTH Aachen University, to analyze the flow structure inside the rotating cavity with an axial throughflow of cooling air. The corresponding 3D numerical investigation is conducted with the in-house flow solver CHTflow, in which the Coriolis force and the buoyancy force are implemented in the time-dependent Navier-Stokes equations. Both the experimental and numerical results show that the whole flow structure rotating slower than the cavity rotating speed. The flow passing the observation windows in the experimental and numerical results indicates the quite similar trajectories. The computed sequences and periods of the vortex flow structure correspond closely with those observed in the experiment. Furthermore, the numerical analysis reveals a flow pattern changing between single pair, double pair, and triple pair vortices. It is suggested that the vortices inside the cavity are created by the gravitational buoyancy force in the investigated case, while the number and strength of the vortices are controlled mainly by the Coriolis force.