Different from conventional streamwise-aligned riblets, converging and diverging (C-D) riblets are arranged obliquely along the main-flow direction, intending to induce a large-scale secondary flow that modulates or controls the existing large-scale vortical structures in wall turbulence. In this study, we perform direct numerical simulations of turbulent flow over the C-D riblets at a low Reynolds number and investigate the impacts of the C-D riblets on aerodynamic forces, turbulence statistics, and coherent structures. The results show that the C-D riblets can reduce the skin-friction drag, but at the expense of a significantly increased pressure drag in the diverging region, which leads to an increase in the total drag. We present detailed mean and statistical flow-fields to assess the flow characteristics in the presence of the C-D riblets, including modifications on the distributions of velocities, pressure, Reynolds stresses, and turbulence kinetic energy. Moreover, the re-arrangement of turbulence structures over the C-D riblets is examined in terms of instantaneous flow-fields and two-point correlations. The findings provide more insight into the physics of flow over C-D riblets and may contribute to the further development of this flow control strategy.

Vacuum carburizing and high-pressure gas quenching process is being widely used in the heat treatment industry. There is an urgent need for a mature technology to predict the influence of carbon content and quenching gas flow on the microstructure and mechanical performance of workpieces. In this paper, a coupling simulation system of the flow field, temperature field, phase field, and stress–strain field was established. And simulations and experiments were carried out for an 18CrNiMo cylindrical sample and a 20CrMnTi gear hub. The results prove that this system can realize the synchronous coupling simulation of the above physical fields, and the simulation results are accurate and applicable to complex shape parts, therefore can guide the design of the process control scheme of the vacuum carburizing and high-pressure gas quenching process.