The present study deals with the flow between co-rotating disks, namely, the disks which rotate co-axially in the same direction at the same angular velocity, with a narrow gap enclosed by a stationary shroud at their circumferences. The flow often accompanies azimuthally-fluctuating instabilities; a non-axisymmetric secondary flow near the shroud. In the experiments, we use a high-speed video camera and a YAG laser to visualize the flow in the axial midplane and the meridional plane between the co-rotating disks and carry out the particle image velocimetry (PIV) analysis. The torus-vortex modes are defined. In numerical calculations, the authors confirm the experimental results. Based on such PIV results, the authors define experimentally and numerically the flow torus-vortex modes TVMs in addition to core-shape modes CSMs, and reveal the details of the flow structure in each mode.
