CFD Analysis of Vortex Dropshaft Structure

Management of large sewer system flows often entails surface collection systems in conjunction with deep tunnels for storage and conveyance. Vertical dropshafts are used to transport water from the surface systems to the deep tunnels. Good dropshaft performance is a critical component of the overall system performance. In addition to simply conveying flows from surface systems to tunnels, dropshafts affect energy dissipation and entrained air release. Vortex dropshafts are frequently used because of their excellent energy dissipation and air removal characteristics. Because of the complex hydraulics associated with vortex dropshafts, physical model studies have often been used to help evaluate dropshaft performance and provide design guidance. However, CFD analysis is beginning to supplement or replace physical models, because CFD models can be assembled more quickly, provide greater visualization capabilities, are less costly, and are not subject to some of the physical model scale effects. For design of the Milwaukee dropshafts, CH2M HILL utilized physical models developed in the early 1980's at the University Of Iowa, Iowa Institute of Hydraulic Research (IIHR). CH2M HILL has designed several additional dropshafts since these original model studies were performed. Because of site considerations, dropshaft designs often deviate in some respects from the Milwaukee model tests. Hence, CFD simulations have been used to assist with the design of these dropshafts. To provide model verification, a CFD model of the physical model constructed at IIHR was developed. This paper compares CFD model simulations and IIHR physical model results.