To simulate the change rate of the friction coefficient μ with respect to the sliding speed V, that is, the μ-V slope, a model combining macroscale and microscale phenomena is proposed. The macroscale model obtains distributions of the fluid pressure and fiber contact pressure over the whole engagement face, and the microscale model obtains the friction coefficient of each fiber contact through a detailed model for single-protuberance fiber contact. An experiment was conducted to obtain the μ-V slope by changing the wave height of separator faces, and the simulation and experimental results were compared. The combined model is advantageous for representing experimental μ-V relationships at small and large wave heights in comparison with models using only the macroscale behavior. Both experimental and simulation results showed the μ-V slope becoming more negative with increasing wave height. The simulation results revealed possible causes for the negative slope. In the wavy separator, the fluid friction that contributes to the positive slope is difficult to achieve due to the large film thickness, and the load-sharing ratio of the fiber contact tends to decrease due to wedge action of the fluid film. These phenomena shift the μ-V slope to the negative.

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