Comparative analysis among the capabilities of the RANS, DES, and LES models to predict flow and turbulence distribution was conducted to come up with guidelines for hydraulic torque converter (TC) transient simulation. To ensure the accuracy of calculation, the complex geometry of hydrodynamic elements was accurately represented and the computational meshes of the structured hexahedron were appropriately distributed. Wall shear stress, pressure-streamline structure were analyzed. Compared with RANS, the transient vorticity features, including the birth, development, formation of a scroll; transportation along the blade surface; shedding and rupture at the trailing edge could be clearly captured by the LES and DES models. Rothalpy was used to quantitatively evaluate the hydraulic loss and a new computational formula was proposed to predict the efficiency of each element in TC. After the comparison of relative computing time, DES model was proved be a feasible method for efficiently and accurately simulating 3D unsteady turbulent flow of TC.

In recent years, computational fluid dynamics (CFD) has been widely used in calculating flow characteristics and heat-flow coupling. However, heat generated during fluid transportation is always ignored and the density and viscosity of the working medium are supposed to be constant. In this article, heat generation was taken into consideration. Accordingly, the thermophysical properties working of the medium are variable. A typical rotor-stator fluid machine, a hydraulic retarder, was chosen to investigate the relationship between the heat transfer characteristics of the fluid and the braking torque. The conclusions were obtained by analyzing the flow conditions which involved Re, vorticity, and rothalpy in the flow passage and viscous dissipation. When the viscous dissipation generated heat in the passage, temperature was increased and viscosity was decreased, thus accelerating the development of turbulence. Moreover, Re was increased at the same time. Furthermore, the dissipation of vorticity was reduced constantly and rothalpy reduced. The braking torque was decreased finally because the wall shear stress was decreased. The conclusions in this article are of practical significance for forecasting other turbomachine characteristics.

Computational fluid dynamics (CFD) has been widely applied as an effective tool for optimizing products and reducing production cycles in many industrial fields; consequently, engineers are constantly pursuing higher accuracy in the performance predictions of CFD methods. In this paper, an analysis for the flow field of a hydrodynamic torque converter (TC) is conducted to evaluate CFD applications in detail. In the past, Reynolds-averaged Navier-Stokes (RANS) simulations have always played a dominant role in the numerical modeling of TCs because of their efficient calculation speed. However, most RANS models are unable to capture the complicated transient flows whose performance estimation errors are generally greater than 10%. Therefore, large eddy simulation (LES) with various sub-grid scale (SGS) models are applied in order to explore feasible methods for improving numerical accuracy and capturing the detailed transient flow phenomena. The effectiveness of the LES method is verified by comparing the numerical results with experimental data. Although the grid resolution is not fine enough due to the limitations of the high-performance computer (HPC) used, LES with dynamic kinetic energy transport (KET) models were still able to obtain an excellent description of both the near-wall flow and the main-stream flow via quantitative and qualitative analyses. The maximum error in the capacity factor (CF) is remarkably reduced to 4.4%. It is therefore beyond doubt that applying LES methods using coarser grid resolutions can still guarantee higher prediction accuracy through the reasonable selection of SGS models, which can effectively reduce the computing capacity requirements and contribute to the design process of TCs.

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This paper presented a numerical simulation of drag reduction on a superhydrophobic surface with a groove structure. The computational fluid dynamics (CFD) method was used to analyze the effect of the groove microstructure when droplets impacted a superhydrophobic surface. The simulation results revealed three main characteristics: (1) The distance the droplet spread was larger along the direction parallel to the groove and smaller perpendicular to the groove; (2) Two protruding small spheres were formed at the edge of the droplet along the groove direction; (3) During retraction, the droplets presented a narrow, cross-shaped morphology. The effect of the groove structure in the two-dimensional microchannel on drag reduction near the wall was analyzed based on slip theory by the coupled level set/volume of fluid (CLSVOF) method. The air in the superhydrophobic pit formed a low-velocity vortex, which made the fluid roll on the air surface. The rolling on the surface produced a velocity slip at the gas-liquid interface. In addition, the superhydrophobic surface had an obvious drag reduction effect in the laminar flow state, but the drag reduction effect in the turbulent state was not ideal and even increased the flow resistance at the wall.