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The present paper describes the vortex structure and the flow field of model experiments simulating the inlet flow of turbomachines at low flow coefficients. A new experimental apparatus was devised to freely set the axial velocity of the main flow and the axial and tangential components of the swirling backflow. The vortex structure was visualized by small air bubbles. It occurs in the shear layer between the main flow and the backflow. The number and the radial location of vortices are determined mainly by the axial and tangential velocities of the backflow normalized by the axial velocity of the main flow. These characteristics agree with those of the backflow vortex structure of real turbomachines. This shows that the vortex structures are caused by the roll-up of the shear layer between the axial main flow and swirling backflow, not associated with the flow interaction with individual blade of the impeller. It was shown that a two-dimensional linear stability analysis can reasonably predict the relation between the number of vortices and their radial location.

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