In the flow around a circular cylinder, a sudden decrease in the drag coefficient occurs at a high Reynolds number, but the same phenomenon occurs at a lower Reynolds number in the case where there exist grooves or roughness on the cylinder surface. In this paper, in order to make clear the flow characteristics around a cylinder with 20, 26 and 32 triangular grooves, the drag coefficient, pressure, velocity and turbulence intensity distributions were measured. Moreover, the flow around the cylinder was analyzed by applying the RNG k-ε turbulent model, and the surface flow pattern was observed using the oil-film technique.

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A tandem arrangement of double rotating cascades with smaller outlet diameter and single diffuser cascade between impeller outlet and volute inlet has been proposed as a centrifugal pump with high performance in air-water two-phase flow condition. For further improvement, the influences of impeller diameter and diffuser blades on two-phase flow performance were investigated experimentally under the condition of the same volute casing. As results, the similarity on the diameter of impeller having the similar blade geometry and the rotational speed is confirmed even in two-phase flow condition. Pump performances between a large impeller without diffuser blades and a small impeller with diffuser blades were compared. A small impeller with diffuser blades yields high performance in the range of small water flow rates in air-water two-phase flow condition, while a large impeller of volute pump type gives high performance in the range of high water flow rate and small air flow rate.

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