Rain erosion is a crucial topic of interest in the current development of offshore wind turbines because it significantly shortens the expected lifetime of wind turbine blades. Ground erosion tests can be conducted to estimate the lifetime of a full-scale wind turbine blade. The results are typically summarized in a V-N curve for erosion initiation, where V denotes the impact velocity of the droplet and N denotes the number of impacts. This study summarizes state-of-the-art ground rain erosion testers, such as whirling arm, pulsating jet, and rubber ball testers. First, the main features of the various ground erosion testers are reviewed. Second, the measurement uncertainties of impact velocity and number of impacts are examined using analytical and phenomenological approaches. Third, the influence of liquid film on the erosion initiation and future research prospects are discussed to understand the different erosion performances of ground testers.

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Aerostatic bearing spindles are very useful machining tools owing to their low friction, long life cycle, and high speed. Although various control methods have been developed for spindles, spindles with higher accuracies should be developed. This paper describes a preliminary study on the control method, which considers gyroscopic precession, for high-speed aerostatic bearing spindles. First, gyroscopic pre-cession in high-speed spindles and its effect on control are discussed. Next, a pneumatic control circuit for spindle control is proposed and a method for calculating the characteristic coefficients is described. Numerical experiments were conducted using the derived characteristic coefficients and the control using the mounted aerostatic journal bearing was simulated through numerical calculations. The results showed that the proposed method increased the stability of the spindle by adjusting feedback gain between input and output when response of the control system was sufficiently fast.

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