Kazuhiko Suga

Accurate numerical reproduction of contact line dynamics on three-dimensional curved solid surfaces remains a challenging issue in multiphase flow simulations. In this study, a wetting boundary condition applicable to curved surfaces is developed within a three-dimensional phase-field lattice Boltzmann framework. The proposed method extends an existing curved-surface wetting model and focuses on improving the evaluation of interface normals and order-parameter gradients on Cartesian lattices, while preserving the compact computational stencils and efficiency inherent to the lattice Boltzmann method. Three-dimensional simulations of liquid spreading on a concave spherical surface and droplet spreading on a convex solid sphere are performed over a wide range of prescribed contact angles. The results show that the proposed method eliminates nonphysical behaviors observed with conventional staircase-based boundary conditions, such as droplet sliding along the solid surface and droplet detachment into the surrounding gas phase. In the convex spherical surface cases, the droplet height converges stably to equilibrium through damped oscillations, and the equilibrium droplet shapes show good agreement with theoretical predictions derived from geometric considerations under zero-gravity conditions over a broad range of contact angles. These results demonstrate that the proposed wetting boundary condition can accurately reproduce wetting phenomena on three-dimensional curved solid surfaces.

To investigate the flow characteristics over a streamwise-preferential porous substrate, we design a layered porous medium that satisfies the drag-reducing conditions suggested by the DNS of Gomez-de Segura and Garcia-Mayoral (2019). Planar PIV measurements are carried out for fully developed turbulent flows over substrates made of the layered porous medium. Two (square duct and two-dimensional channel) flow cases are considered. Streamwise-wall-normal plane measurements are performed at the bulk Reynolds numbers Reb=5000-15000 for the square duct flows. The measurement data indicate that with the drag-reducing conditions, which are suggested by the DNS, turbulence over the porous substrate is suppressed to a similar level to that near a solid smooth wall. For further discussion, we then carry out another PIV campaign for channel flows. With streamwise-wall-normal and streamwise-spanwise plane measurements of channel flows at Reb=3000-15000, it is observed that the turbulence level near the porous medium is more significant than that near the solid wall. To investigate why turbulence over the layered porous medium behaves unlike in the DNS, we analyse the data comparing with our previously studied porous medium turbulence. With the spanwise streak distributions and quadrant analyses, their similarity and dissimilarity of turbulence structures are discussed.

Stator coils of automobiles in operation generate heat and are cooled by coolant poured from above. The flow characteristic of the coolant depends on the coil structure, flow condition, solid–fluid interaction, and fluid property, which has not been clarified due to its complexities. Since straight coils are aligned and layered with an angle at the coolant-touchdown region, the coil structure is simplified to a horizontal square rod array referring to an actual coil size. To obtain the flow and wetting characteristics, two-phase fluid flow simulations are conducted by using the phase-field lattice Boltzmann method. First, the flow onto the single-layered rod array is discussed. The wetting area is affected both by the rod gap and the wettability, which is normalized by the gap and the averaged boundary layer thickness. Then, the flow onto the multi-layered rod arrays is investigated with different rod gaps. The top layer wetting becomes longitudinal due to the reduction of the flow advection by the second layer. The wetting area jumps up at the second layer and increases proportionally to the below layers. These become remarkable at the narrow rod gap case, and finally, the dimensionless wetting area is discussed at each layer.