911 マイクロバブル抵抗低減における境界層気泡分布の超音波計測(2)(OS9-3 超音波を利用した流動場の計測(マイクロバブル・混相流),OS9超音波を利用した流動場の計測,オーガナイズドセッション)

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  • 911 Ultrasonic measurement of boundary layer bubble distribution for microbubble drag reduction(2)

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Friction drag accounts for 80 % of the total drag for large ships, and hence microbubble drag reduction is expected as one of the most feasible ways to reduce it because of less environmental impact and easy installation. The basic experiment to confirm the drag reduction effect was performed from 1970's, and it comes to the stage of a practical examination for moving ships. As a method to assess the bubble-originated alternation of the boundary layer structure beneath a moving ship, we have designed a ship-mounting type of ultrasound probe to apply ultrasound velocity profiler (UVP) and examined the measurement performance for a long towing-ship in NMRI 400m-run facility. The probe emits 2MHz-pulsed ultrasound at 75 degree to the main flow direction to acquire the turbulent velocity profile up to 7.2m/s in the fastest case. This report mainly describes the measurement method of bubble distribution in the boundary layer. The method is constructed by the probability theory of ultrasound reflection on individual bubbles, and examined by numerical simulation. This application to a 50m-model ship reveals that the water flow boundary layer changes to steep profile where the bubbles are concentrated.

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