The origin and formation mechanism of a submerged vortex and an air-entraining vortex have been fully clarified by largeeddy simulation (LES) that used approximately 2 billion hexahedral elements with maximum resolution of 0.255 mm and was applied to the internal flows of a model pump sump. The model pump sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm and a water depth of 100 mm and a vertical suction pipe with a 100 mm diameter installed at its downstream end with an offset of 10 mm from the centerline of the rectangular channel. At the upstream end of the channel, a uniform velocity of 0.37 m/s is given. LES with different wall boundary conditions have revealed that the origin of a submerged vortex is the mean shear of the approaching boundary layers that develop on the bottom and side walls of the pump sump. From detailed investigations of LES computed for a long time period of 16 seconds have revealed that deviation of the mean flow that approaches the suction pipe triggers conversion of the axis of the vorticity that was originally aligned to the lateral direction in the approaching boundary layers to that aligned to the vertical direction. The local acceleration of the vertical flow stretches the afore-mentioned vertical vortex, which results in formation of a submerged vortex. The separated flows downstream of the suction pipe generate vertical vorticity, and forms an air-entraining vortex when such a vortex is sucked into the suction pipe. Computations with a different bellmouth height and a different water-surface height have supported the above mentioned origin and formation mechanism of these vortices.

This study highlights especially the mechanism of vortex cavitation occurrence from the end of the suction duct in a double-suction volute pump and pump oscillation which causes cavitation noise from the pump. In this study, full 3D numerical simulations have been performed using a commercial code inside the pump from the inlet of suction duct to the outlet of delivery duct. The numerical model is based on a combination of multiphase flow equations with the truncated version of the Rayleigh-Plesset model predicting the complicated growth and collapse process of cavity bubbles. The experimental investigations have also been performed on the cavitating flow with flow visualization to evaluate the numerical results.

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Unsteady numerical calculation of an entire multistage pump was performed by using a large eddy simulation (LES) at the design flow rate to investigate the flow field in the pump in detail and to evaluate the accuracy of LES by comparing the results with an experimental and a conventional CFD result based on a Reynolds-averaged Navier-Stokes (RANS) equation. We investigated a four-stage centrifugal pump consisting of a suction bend, impellers, vaned diffusers, return channels, and a discharge volute. The interaction between the impeller and the stator was taken into account by using a moving overset grid in LES calculations, and the flow field in the inlet portion of each hydraulic part was investigated using the calculated result. In the experimental investigation, velocity distributions and pressure fluctuation were measured at several points by using a scaled model pump. RANS calculation was performed with respect to a single-stage pump composed of the first-stage component of the four-stage pump. We found that the hydraulic design of the four-stage pump is satisfactory and that LES was a very effective design tool for investigating the flow field in detail including the unsteadiness in the hydraulic passageway of the multistage pump.

The origin, formation mechanism, and dynamics of suction vortices in a pump sump have been clarified by large eddy simulation (LES) applied to two different computational models. The first one is a pump-sump model with uniform flow entering a water channel of rectangular cross section and a vertical suction (outlet) pipe installed at its downstream end. LES with different wall boundary conditions have revealed that the origin of a submerged vortex is the mean shear of the approaching boundary layers that develop on the bottom and side walls of the sump. Detailed investigations have revealed that deviation of the mean flow triggers conversion of the vorticity axis to the vertical direction. The local acceleration of the vertical flow stretches the aforementioned vertical vortex, which results in the formation of a submerged vortex. The second one is a simplified computational model composed of a paraboloid of revolution and aims to accurately simulate the stretch of the viscous core of a submerged vortex that has appeared under the suction pipe of the pump-sump model. The differences between the models, especially predictions of the minimum pressure, imply that cavitation could have been initiated in the viscous core, if it had been taken into account, as is observed in the pump-sump experiment at the same condition. Parametric studies with different initial swirl numbers from 0.12 to 16.3 have clarified the behavior of the submerged vortex. It was found that a strong submerged vortex appears only at a relatively small range of the swirl numbers from 1.25 to 3.