Analysis of Spiral Vortex and Vertical Slot Vortex Drop Shafts

A theory is developed that is shown to give good prediction of the head‐discharge relationships for two designs of vortex drop structures. The first design is the widely used spiral guide wall design, and it is shown that control is shared between the critical section at the top of the shaft and the vortex inducing asymmetry of the guide wall inlet design. A second simplified and more space saving design, called the vertical slot vortex drop structure, is also analyzed. It is shown that, although the flow conditions and analysis are quite different, the resulting head‐discharge relationship is quite similar to the spiral guide wall design. For both designs the head‐discharge relationship is shown to be almost linear and the theoretical and measured discharges are in close agreement. The proposed vertical slot design produces a stable air core, similar to the spiral guide wall design, and consequently the flow is free from risk of surging to the pipe‐full flow condition. The compactness of the new design is especially suitable for underground installations.