Abstract The shear layer on a supercavity wall is the gas entrainment channel to wake flow for ventilated supercavitating flows with various tail closure modes. To thoroughly understand the gas loss mechanism and the relationship between the gas loss mechanism and dynamic stability of the flows, the flow structures and associated characteristics in the shear layer must be studied. The multi-fluid model used in our previous work is further applied to consider the turbulence of each phase in the flows and then validated again via effectiveness analyses by simulating the ventilated supercavitating flows, including an obvious bubbly wake around a body and the shear-layer characteristic scale of the ventilated supercavity around a single disk cavitator at a low speed. Based on this model, the critical transition criterion is inspected from two vortex tubes to the re-entrant flow closure modes corresponding to different supercavity gas loss mechanisms. The gas volume fraction used to define the supercavity wall and the potential flow solution of the flow velocity on the wall are also examined via simulations. The shear flows are analyzed to reveal the distribution laws of gas velocity in the shear layer under different gravity effects. The shear-layer thickness is calculated and the influence of gravity and water viscous force acting on the outer wall of the supercavity is assessed by analyzing the gaseous velocity distributions in the supercavity cross sections. The results also show that the thickness is sensitive to the inner body shape.