The endplate for a small H-Darrieus vertical axis wind turbine is optimized using a global optimization method with three-dimensional computational fluid dynamics (CFD) simulations. This study focuses on the design of a three-bladed wind turbine characterized by a diameter of 0.2 m, a height of 0.2 m, and an operational tip-speed ratio of 0.7. The design process is conducted in the following two phases. First, CFD simulations are performed on various endplate topologies to identify a potential one. Second, an aerodynamic shape optimization is performed on the potential topology using the framework of the surrogate model-based global optimization approach. The performance of wind turbines is evaluated using unsteady Reynolds-averaged Navier–Stokes simulations. The potential design obtained in the first phase (investigation of various topologies) is a three-pronged endplate whose outlines are defined by quadratic curves. The optimized design obtained in the second phase (shape optimization) covers more around the leading edges and less around the trailing edges of the blade airfoils than the baseline design obtained in the first phase. The optimal endplate generates low-pressure regions around the leading edge when the position of the blade is at a rotational angle of 0°. This enables a larger thrust force to be generated, thereby achieving an additional 4% improvement in performance compared with the baseline design. This performance improvement is also reproduced in wind tunnel experimental evaluations.

In this study, we investigated the mechanism of drag reduction in turbulent flow in an annular pipe subjected to large-scale buoyancy-driven control through direct numerical simulations. The control was achieved by imposing alternating heating and cooling along the outer wall, which induces a buoyancy force that generates large-scale vortical motion. Two radius ratios (ξ = 0.3 and 0.5) were examined at a bulk Reynolds number of 5600. A maximum reduction in skin friction of 20.1% was obtained at ξ = 0.3 with a Richardson number Ri = 1.25 × 10⁻² and azimuthal wavenumber Ω = 3. The results of an analysis of the flow fields revealed that the drag was reduced in the formation of two pairs of large-scale counter-rotating vortices and the establishment of stable thermal stratification. A three-component decomposition of the Reynolds shear stress and an annular Fukagata-Iwamoto-Kasagi (FIK) identity analysis indicated that the reduction primarily resulted from the suppression of the turbulent component near the outer wall. These findings demonstrate that buoyancy-induced large-scale control is an effective and environmentally sustainable strategy for reducing frictional drag in wall-bounded turbulent flows.

We developed a numerical method for simulating a stationary droplet with a high density ratio on a solid surface using an improved lattice Boltzmann method (LBM) for incompressible two-phase flows. This method uses the phase-field model and wettability of a solid surface is determined from the wetting boundary condition based on a solid surface free energy density. We first reviewed existing solid surface free energy densities from both physical and numerical viewpoints. We then proposed a new one as a cosine power form, which was derived by improving drawbacks in the existing forms. We next implemented a new wetting boundary condition in the LBM based on the proposed density function form. Using the developed method, we conducted three-dimensional simulations of a stationary droplet on a flat solid surface and investigated the range of static contact angles θ that can be computed with the numerical stability. As a result, we successfully demonstrated stable simulations over a wide range of θ, including the superhydrophobic (θ > 150◦) and superhydrophilic (θ < 30◦) surfaces. Furthermore, the simulated contact angles showed good agreement with the theoretical values predicted by Young’s equation within the range of 30◦ ≲ θ ≲ 150◦, confirming the validity of the proposed approach.