This study aims to address the problem of fine particle entrainment in the underflow of a hydrocydone. To improve the separation performance of hydrocyclones, we investigated the effect of the boundary layer structure. Several completely cylindrical structures were developed based on the conventional cylindrical-conical structure. By modifying the bottom structure of the completely cylindrical hydrocyclones, the entrainment of fine particles can be reduced, and the separation sharpness can be improved. Ultimately, a W-shaped apex was proposed. A computational fluid dynamics (CFD)-based numerical simulation was conducted to compare the performance of different hydrocyclone structures. The variations of the pressure field, velocity field, air-core shape, density field, and separation performance were investigated, and the results were compared with classical experimental data to verify the accuracy of the numerical simulations. The results show that the total pressure drops and the radial pressure gradients in type B (with a cylindrical section), type C (with a 3 degrees gradient cylindrical section), and type E (with a W-shaped apex) hydrocyclones are lower than those in the type A (a conventional cylindrical-conical-shaped hydrocyclone), which is beneficial for reducing the energy loss and increasing the throughput. In the completely cylindrical hydrocyclones, the axial velocity is reduced, and the residence time of the particles is prolonged, which facilitates the sufficient separation of the particles. In addition, a high-density suspension layer forms near the apex in the completely cylindrical hydrocyclones. This suspension layer causes elutriation and hence reduces the entrainment of fine particles in the underflow. In the completely cylindrical hydrocyclones, the recovery rate and entrainment of fine particles in the underflow are reduced, and the separation sharpness is improved. The type E hydrocydone has the least entrainment of fine particles and has a separation sharpness that is between those of type A and types B, C, and D (with a 3 degrees gradient cylindrical section and a 150 degrees sloping bottom). (C) 2019 Elsevier B.V. All rights reserved.