Shingo Obara

Fluid-rigid body interaction is a significant topic in research on particle methods. This study developed a fluid-rigid body coupling method based on a physically consistent particle method, i.e., the moving particle hydrodynamics (MPH) method, incorporating the passively moving solid (PMS) model. When the discrete particle system satisfies the fundamental laws of physics, i.e., mass conservation, linear and angular momentum conservation, and the second law of thermodynamics, the method is asserted physically consistent, and this feature is important for robust dynamic calculations. The PMS model is a pioneering approach that is practical for particle methods in which fluid and rigid-body particles are initially calculated as a fluid. Then, only rigid-body particles are modified to restore the initial shape by applying rigid-body constraints. Thus, combining the MPH method and the PMS model realizes a fluid-rigid body coupling method that satisfies fundamental physical laws. The proposed method was first verified via the fundamental rigid body and fluid-rigid body coupling problems: the Dzhanibekov effect on a T-shaped rigid body, a floating rectangular solid, a floating cylinder, and water entry of a two-dimensional cylinder. Second, the proposed method was validated via calculating a cylinder rolling on a liquid film as a fluid-rigid body coupling problem with rotation. By using a potential-based surface tension model, the computed results showed reasonable agreement with the experimental data obtained in this study. Overall, it was confirmed that the proposed method is a promising fluid-rigid body coupling approach, in which the surface tension and wettability can be considered as well.

New bearing units having lubricant supply structure were devised for realizing longer lifetime and better performance of rotation mechanisms of space machineries. The spacer sandwiched by two bearings was filled with grease and porous body was established between grease and the bearing for both preventing thickener transferred into bearings and supplying only oil for bearings. The oil supply function was verified by friction tests and bearing tests using fluorescent agent. The contribution for long lifetime of the oil supply system was confirmed by the lifetime tests under a vacuum which showed stable rotation for 2×10⁸ revolutions at a room temperature and 1×10⁹ revolutions at a temperature of 60℃. Moreover, large size bearing units with the oil supply structure were evaluated under high rotation speed for utilized in larger spacecraft requiring larger attitude control torque and observation systems requiring agile attitude control. Using larger size bearings and rotating with higher speed would cause to promote oil scattering by centrifugal force and oil evaporation by temperature rise, whereas the oil supply structure was conducted the feasibility to realize both larger size bearings, higher rotation speed, longer lifetime and low torque.

The Bingham fluid simulation model was constructed and validated using a physically consistent particle method, i.e., the Moving Particle Hydrodynamics (MPH) method. When a discrete particle system satisfies the fundamental laws of physics, the method is asserted as physically consistent. Since Bingham fluids sometimes show solid-like behaviors, linear and angular momentum conservation is especially important. These features are naturally satisfied in the MPH method. To model the Bingham feature, the viscosity of the fluid was varied to express the stress-strain rate relation. Since the solid-like part, where the stress does not exceed the yield stress, was modeled with very large viscosity, the implicit velocity calculation was introduced so as to avoid the restriction of the time step width with respect to the diffusion number. As a result, the present model could express the stopping and solid-like behaviors, which are characteristics of Bingham fluids. The proposed method was verified and validated, and its capability was demonstrated through calculations of the two-dimensional Poiseuille flow of a Bingham plastic fluid and the three-dimensional dam-break flow of a Bingham pseudoplastic fluid by comparing those computed results to theory and experiment.

In this study, we improve a multiresolution method to reduce the computation time of fluid lubrication simulation based on a particle method by applying an implicit algorithm for viscosity calculation. The present method is based on the moving particle simulation (MPS) method and the overlapping particle technique (OPT), which is a multiresolution method for particle methods under unsteady states. The MPS method is used to solve the Navier–Stokes equation. The OPT is used to reduce the number of required particles and reduce the computation time. We improve the OPT by applying an implicit method for viscosity calculation to eliminate restrictions regarding time increment due to the diffusion number. In addition, we enable the particle size to be changed significantly between subdomains in the OPT. To validate the proposed method, we simulate the fluid lubrication of line contact in two dimensions until the flow reached a steady state. Consequently, it is shown that the pressure obtained using the proposed method agrees well with that obtained using the Reynolds equation. The computation time of a particle simulation using the improved multiresolution method is significantly shorter than that of a single-resolution simulation. In addition, the effects of particle size and subdomain size are validated in the fluid lubrication simulation using the OPT.

Impulsive vibration during rocket launch has the risk of causing indentations inside the bearings of reaction wheel mounted on a satellite. The indentations induce the vibration disturbance during the bearing rotation, which is harmful to precision equipment. In order to investigate the mechanism of the indentation formation, we carried out the experiments and finite element simulation for simplified reaction wheel. Based on the results of simulation, we estimated the depth of plastic deformation and wear using Palmgren’s formula and PV value, respectively. The indentation depth of simulation shows good agreements with that of experiment. The results also show that the wear between balls and rings is dominant factor of indentation formation, as compared with the plastic deformation. The amount of wear is greatly affected by the location of ball. In the location where the contact direction between ring and ball is parallel to the vibration direction, contact pressure contributes the wear as compared with slip velocity. Inversely, in the location where the contact direction is perpendicular to the vibration direction, the slip velocity contributes the wear and induces the larger wear region.