[in Japanese]

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[in Japanese]

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[in Japanese]

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This article describes the relationship of the surface roughness with the surface-level winds and the vertical profile of winds in the atmospheric boundary layer. The roughness conditions in urban areas are specifically focused on. In urban areas, the spatial distribution and height of buildings largely vary, which impacts on the variability of winds. The results of quantitative analysis of strong winds in urban areas during typhoon conditions are demonstrated.

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A developed tropical cyclone (TC) can be regarded as a system like a Carnot cycle engine. When water vapor over warm ocean turns into liquid water and ice in the upper air, a part of released diabatic heating is converted to mechanical work accelerating the wind. The surface friction due to ocean surface waves and others play a vital role to cease such acceleration at some point because the frictional energy loss increases roughly proportional to the cube of wind speed. In other words, the surface friction limits the upper bound of maximum wind speed of TCs. In general, ocean surface waves develop under strong wind conditions, and thus the drag coefficient becomes larger with increasing wind speed. One of the critical remaining issues is the difficulty in actual measurements of the magnitude of the drag force. Wave breakings under high wind speed might alter the surface roughness through wave breaking and sea spray layer. The friction over land is also important for the decay of a TC after the landfalling. For a given wind speed, drag coefficients over land are several times larger than those over ocean. It significantly weakens the intensity of a TC after landfalling.

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Tribology plays an important role for understanding the dynamic earthquake slip process because the earthquake slip process is a frictional phenomenon that occurs on a crack plane embedded in underground rock. For instance, the rate-and-state friction law, which depends on slip velocity and a state variable, has been widely employed in seismology to understand the process. For seismology, it is important to emphasize that the unified system, which includes the crack plane and the surrounding rocks, is treated. Microcracks generation around the crack plane and changes in the pressure of fluid in the pores within the surrounding rocks are important features of the system. The interaction among these processes can provide seismologists with valuable insights and have implications for nonlinear dynamics.

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Several examples of tribological research in volcanic settings are presented in conjunction with an overview of volcanic phenomena. Friction plays a fundamental role in both the formation and erosion of volcanoes. During eruptions where lava ascends through conduits, however, abrasion predominates over friction. Many studies of pyroclastic flow descent incorporate the concepts of shallow and two-layer flow friction, with friction coefficients typically determined through simulation. The air content ratio during pyroclastic flow descent is cited as a key factor in controlling this friction coefficient. Many studies examine slope sediment collapse related to volcanic erosion processes from a geotechnical perspective. These studies use laboratory testing to determine the internal friction angle and cohesion of collected samples. When assessing the safety of slope failures, the Coulomb failure criterion is typically employed. Mitigation measures for volcanic hazards like pyroclastic flow or lahar should consider their runout distance and treat them as density flows. The balance between air or water content, friction, and turbulent dissipation associated with particle collisions (i.e., particle-fluid interaction) also influences the velocity, runout distance, and deposition morphology of friction flows. Taking these factors into account, it is crucial to evaluate the various physical properties related to 'friction' to prepare for volcanic hazards.

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In tsunami numerical simulations, the bottom shear stress has commonly been evaluated using the steady flow friction coefficient such as Manning’s roughness coefficient and Darcy-Weisbach equation. The bottom shear stress is an important quantity that causes tsunami height attenuation, sediment transport, and resultant morphology changes. Therefore, in order to make an accurate simulation for tsunami height variation and tsunami-induced morphological evolution, it is highly important to choose suitable friction coefficient under tsunami. This study investigates the boundary layer characteristics beneath tsunami by using k-ω turbulence model. In the present numerical simulation, a tsunami wave with the period of 15 min and the wave height of 1 m is applied at the offshore boundary of 4,000 m depth. The shoaling process of the tsunami is calculated using Greenʼs law, and the time-variation of the free stream velocity outside the boundary layer is evaluated from the Airy wave theory to impose the pressure gradient term in the boundary layer equation. It is revealed that the steady flow resistance law cannot always be applied in the cases considered in this study, even if the long wave condition is satisfied.

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In this county, the river has characteristics that length is short and steep slope. Then the riverbed of the upstream was dug and produces the gravel when a flood occurs. It becomes at great risk of the flooding by the gravel which began to flow depositing in a downstream river, and the river product sand becoming cross-sectional area of river small. Thus, the sand take an important role with water flowing downstream in the flow of the river channel in the river. The sand of various particle size was distributed over the riverbed widely spatially. The small sand particle starts movement when the speed exceed constant size. Therefore, the sand repeat erosion and sedimentation in a riverbed, flow of river and sand made a river shape and a section.

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In order to ensure the oil sealing performance of floating seals used in the axle seals of crawler tracks for construction machinery, it is important to understand the behavior of seal rings and O-rings. In this study, we measured the eccentricity between the seal rings and the position of O-rings during load application in the assembly of floating seals, clarifying their respective behaviors. Furthermore, we investigated the relationship between the circumferential distribution of O-ring compression reaction force and the occurrence of eccentricity between the seal rings. As a result, it was found that the O-ring did not compress uniformly in the circumferential direction when loaded, causing variations in the O-ring compressive reaction force around the circumference. And it was found that when the integral value of the radial component of this O-ring compression reaction force exceeds the maximum static frictional force on the sealing surface, the sealing rings slip relative to each other, causing eccentricity between the sealing rings. Furthermore, eccentricity between the seal rings is thought to generate a moment that causes declination of the seal rings, as it induces a shift in the pressing vector on the sealing surface between the axial movement and stationary sides.

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In artificial joints, adsorbed protein films from synovial fluid significantly influence tribological behavior. While film structure (lamellar for low friction, heterogeneous for high friction) and frictional properties are known, their formation process has been unclear. This study investigates normal load history effects on the formation of adsorbed bovine serum albumin (BSA) films on CoCrMo-UHMWPE interfaces and their frictional characteristics. Friction tests using a reciprocating tribometer with in-situ electrochemical analysis monitored both the coefficient of friction (COF) and net protein adsorption/desorption behavior. Load was varied incrementally, from low to high (L→H) or high to low (H→L). Results showed distinct, history-dependent behaviors. The L→H case exhibited an exponential COF increase, correlated with a shift from protein desorption to adsorption. Conversely, the H→L case demonstrated a stable low COF, where an initial adsorption event indicated film stabilization. This study proposes a mechanism where initiating friction at high loads denatures BSA, forming a uniform and low-friction film. Initiating at low loads forms BSA aggregates, creating a heterogeneous and high-friction film. These findings clarify that load history is a critical parameter governing protein film formation, providing a mechanical basis for previously observed phenomena.

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[in Japanese]

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