This study aims to clarify particle adhesion characteristics on riblet surface using direct numerical simulations and experiments. Numerical simulations of particle-laden, fully developed turbulent channel flow with rectangular riblets were performed to investigate the influence of particle adhesion on drag reduction. A one-way coupling method was employed, in which the flow affects the particles but the particles do not affect the flow. The spatial distribution of particle adhesion on the riblet surfaces was evaluated. Most particles adhered to riblet crests and the upper half of the sidewalls owing to tumbling motion within the grooves. Outdoor and laboratory experiments also revealed nonuniform particle accumulation on the riblet groove floor. The riblet shape was then modified based on the particle adhesion distribution, and a series of numerical simulations with different shapes was performed. The results reveal a decrease in the skin-friction drag reduction effect compared with the initial riblet configuration.
This study proposes a novel Mass-Preserving Particle Level Set method with Kernel function correction (mpPLS/KC) method that achieves both mass conservation and interface shape reconstruction in multiphase flow simulations. While the conventional PLS/KC method exhibits high shape reconstruction accuracy, it suffers from mass conservation issues. Therefore, a global mass conservation process was added to address this limitation. The proposed method converts minute mass errors occurring throughout the computational domain into offset values for the level set function, performing mass correction while maintaining the distance function properties. Through this efficient algorithm, mass conservation errors were successfully reduced to below 1.0 & times;10-14 (machine epsilon error level) in all benchmark tests. This represents a significant improvement compared to the mass errors of the conventional PLS/KC method (10(-3) to 10(-6) order). Regarding interface shape reconstruction, the method demonstrated high-accuracy tracking of complex interface shapes involving large deformations in Zalesak's three-dimensional rigid body rotation test and LeVeque's three-dimensional deformation test. The computational time increase due to the addition of the mass conservation process was approximately 4-6%, which is within a practically acceptable range. For practical validation, the proposed method was applied to three engineering problems: dam-break flow, dam-break flow with obstacles, and tsunami flow through coastal forests. In all cases, the results showed good agreement with experimental data. This study demonstrates that simultaneous achievement of extremely high mass conservation and excellent shape reconstruction, which was previously difficult, is now possible. The method is expected to advance as a practical multiphase flow analysis tool for various engineering fields including ocean engineering and fluid machinery.
The effect of nozzle spacing on the flow characteristics of triple rectangular free jets was studied experimentally and numerically. The nozzle spacing ratio S/d(e), where d(e) is the equivalent diameter of a rectangular nozzle with an aspect ratio of 2, was varied from 2.06 to 4.13. The Reynolds number based on the nozzle exit velocity and the equivalent diameter was 1.5 & times; 10(4). The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer with an X-type probe. Three-dimensional numerical simulations of triple rectangular free jets were also performed using Open FOAM 5.0, and the numerical results depicted the vortical structures in the jets. Triple rectangular free jets with a small nozzle spacing ratio, specifically S/d(e) = 2.06, merge in the vicinity of the nozzle. The mean velocity and the turbulent kinetic energy near the jet centerline become higher than those with a large nozzle spacing ratio. An axis-switching phenomenon occurs after the jets merge, and the whole cross-sectional shape of the triple rectangular free jets changes to vertically long elliptical free jet. Farther downstream, the elliptical free jet becomes round. The flow characteristics of each of the triple rectangular free jets with a large nozzle spacing ratio, specifically S/d(e )= 4.13, are similar to those of a single rectangular free jet. Downstream, the streamwise mean velocities of triple rectangular free jets become identical for the dimensionless coordinates of x/S and y/S.
Surface tension modeling plays a significant role in diffuse-interface simulations of liquid-gas two-phase flows. However, widely used surface tension formulations in phase-field models often produce unphysical force even flat interfaces under equilibrium conditions, leading to spurious velocity fields and degraded numerical fidelity. The objective of this study is to identify an optimal combination of surface tension force formulation and discretization schemes that eliminates such unphysical force and velocity while maintaining computational locality. Several surface tension formulations are systematically examined using three-dimensional flat liquid films and spherical droplets under equilibrium conditions. Special attention is given to curvature-based formulations, which theoretically satisfy the requirement that the surface tension force vanishes at flat interfaces. Although these formulations can be written in mathematically equivalent forms, their numerical behavior differs significantly after discretization. The distributions of surface tension forces are analyzed to clarify how discretization errors arise and how they depend on the choice of formulation. Furthermore, the impact of finite difference schemes on computational locality and numerical accuracy is investigated. It is shown that evaluating the gradient of the order parameter using an isotropic difference scheme while computing the curvature term with a central difference scheme provides an effective balance between accuracy and locality, significantly reducing communication overhead in parallel computations. The proposed formulation and discretization strategy are further validated through phase-field lattice Boltzmann simulations of a three-dimensional static droplet. The results demonstrate a substantial reduction in spurious velocity around the droplet compared with conventional formulations. These findings provide practical and physically consistent guidelines for surface tension modeling in phase-field lattice Boltzmann simulations, particularly for large-scale and parallel computations of multiphase flows.
