As an extension of the immersed boundary method with unified interpolation stencil (Kor, Badri Ghomizad, and Fukagata, J. Fluid Sci. Technol., Vol. 12, 2017, JFST0011), we propose an immersed boundary method that can handle moving boundary problems with a lower level of spurious force oscillation. The key modification to the previously proposed method, which was validated for fixed boundary problems, is to adopt the reconstruction method, in which the velocities outside the body are reconstructed, instead of the ghost-cell method, in which the velocities inside the body are set to satisfy the boundary conditions. From the comparison between the ghost-cell and reconstruction methods, both methods work equally well for a fixed boundary problem, but the reconstruction method is found to be effective in suppressing the spurious force oscillations that appear in moving boundary problems. The capability of the proposed method is demonstrated by numerical investigations of some typical problems. Both predefined motions, such an oscillating cylinder and a hovering flat plate, and interacting motions of rigid bodies are simulated to validate the method. For the latter, sedimentation of a single cylinder as well as a group of interacting cylinders under the gravitational force is examined to demonstrate the capability of the present method for fluid-structure interaction problems. The results show that the proposed method can properly handle the moving boundary problems, while preserving the simplicity of the unified interpolation stencil.
We had examined the two type of the yield-like behavior in the α-gels, which are observed by the stress-ramp test in the stress-controlled rheometer. We called them the first yield and the second yield from a lower stress, respectively. The influence of the surface roughness of the flow cell in the yield-like behavior is examined in this study. The parallel plate made from the glass, the stainless steel and the glass with the sand-paper of the particles size #100 (Ra125) and #2000 (Ra20) are used in the experiments. The glass plates have the surface roughness in the order of a nanometer and the stainless plates are in the order of submicron scale. The first yield point, which is observed in the low shear stress, is affected by the surface roughness and the gap between the plates. The first yield stress measured by the glass plates is small and the first yield point is unclear compared with the results by the stainless plates. The stress difference between the upper plate and the under plate occurs clearly in the stainless plate in the different stress-ramp rate. It is considered that the uniform velocity distribution in the glass plate is formed and the shear-layer in the stainless plate gives the big influence to the flow field because it is formed stably at long intervals. On the other hand, the second yield point is influenced by the gap but not the surface roughness.
This paper reports our simulations of the metal powder fusion additive manufacturing process based on a two-fluid model. In simulations of metal behavior in which heat is applied by high-density energy sources (e.g., laser or electron beam), the aspects that need to be correctly modeled include boiling and evaporation, as well as melting and solidification. The potential of the two-fluid model to clarify numerous physical phenomena—deep penetration, plume generation, spatter generation due to interfacial pressure differences between gas and liquid phases, and the transport of spatter with high-speed-plume flow—are shown from a perspective that departs from conventional interpretations. This paper compares and discusses the results of simulating the penetration and width of single beads and experimental results.