In the present study, we consider the air entrainment into a suction pipe which is vertically inserted down into a suction sump across a mean free-water surface. This configuration is often referred to as the "vertical wet-pit pump", and has many practical advantages in construction, maintenance and operation. In particular, we focus our concern upon the critical submergence depth S c , which is one of the prime and conventional indicators for the air-entrainment occurrence. By a systematic

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... oach, we experimentally investigate the influences of kinetic and geometric parameters upon S c . As the kinetic parameters, we consider the Reynolds number Re and the Weber number We, in addition to the Froude number Fr, on such a basis as Fr is not much larger than unity in many actual cases. As the geometric parameters, we consider back clearance X, sump breadth B and bottom clearance Z. Here, all parameters are non-dimensionalised by the outside diameter D and the intake velocity V i of the suction pipe. As a result, we reveal the effects of such six parameters upon S c . The We effect, namely, the surface-tension effect can be ignored at We > 12. And, the Re effect, namely, the viscous effect becomes negligibly small at Re > 3×10 4 . Under such conditions for We and Re, we could consider only the Fr effect, namely, the gravitational effect. Concerning the X/D and B/D effects, S c /D attains the maximum at a certain X/D or B/D. On the other hand, the Z/D effect is monotonic, and becomes small at Z/D > 2.5. Some aspects of these geometric effects can be evaluated by a local-Froude-number effect on the basis of the global relation between S c /D and Fr. And, the other aspects is necessarily considered to be related with the flow structure in the suction sump.