Using Vanes to Reduce Flow Separation and Head Loss in Warped Transition

Abstract

Irrigation and water-supply systems commonly need channel transitions to provide changes of channel section. Warped transition (WT) is an option for transitioning from a narrow rectangular channel to a wide trapezoidal channel. The problem is that the flow separates from the sidewalls, disturbing the flow for a great distance downstream. This paper introduces vane(s) to WT as an alleviation measure. This paper aims to improve our understanding of the characteristics of subcritical flow past a WT with vane(s) or without. Numerical predictions of Reynolds-averaged flow were obtained under the same conditions as laboratory experiments. The predicted velocity structures and flow separation features compare well with laboratory measurements. The suitability of turbulence schemes and modeling strategies are discussed. The key findings are: in a pure WT, the flow separates from one sidewall, creating large energy-bearing turbulent eddies and hence causing significant head loss. The use of one vane effectively improves the efficiency of the transition. The use of three vanes further improves the efficiency, enhancing exit-flow uniformity and reducing head loss as well as maximum velocity. The mechanism at work is that the partition of the channel section by one or more vanes into smaller parts sets a limit on the maximum possible size of eddies. Thus, no large energy-bearing eddies appear and disturb the flow. This paper is of practical relevance to flow energy conservation, enhanced command of farmland served by irrigation networks, and channel erosion protection. The efficient numerical predictions are complementary to time-consuming measurements from expensive experiments.