Experiments are performed by using two circular cylinders made of porous filter material, that is, bronze sintered compact, with a different filter size, respectively, in a water tunnel. The effects of minute air bubbles injected from the surface of the circular cylinder into its boundary layer on its hydraulic force characteristics are investigated over the Reynolds number (1.2~4.2)×105, changing the airflow rate of bubble injection. The pressure distributions on the porous cylinders are also measured. Their cavitation characteristics are studied, too. Compared with a smooth-surface cylinder, the critical Reynolds numbers of the porous cylinders decrease owing to the increase in the surface roughness caused by the bronze particles which compose the sintered compact. The effects of the bubble injection on the hydraulic force characteristics are different according to the size of the bronze particles. Though a little difference is recognized, the hydraulic force characteristics show a similar tendency in both cases of bubble injection and cavitation occurrence.

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