Experimental Study of Flow Characteristics in Wedge and Modified Wedge Transitions

Abstract

Water generally leaves a supply reservoir through a narrow rectangular concrete channel and enters a large trapezoidal earthen channel. The two channels are connected by a transition. Among the most common types of transitions, a wedge transition is relatively simple to construct. Even in well-designed transitions, the flow is often accompanied by a large zone of flow separation, causing considerable head loss. For the first time, this experimental study has explored modifications to a wedge transition to reduce the separation and head loss. The characteristics of three-dimensional flow in both the wedge and the modified wedge systems were investigated using non-intrusive measurements of the flow field. Transition flow parameters, including longitudinal flow profile, transition energy loss, maximum velocity at the transition outlet, and turbulence intensity in the downstream trapezoidal channel, were determined. Flow separation characteristics were recorded by tracing the separation zone and the points of reattachment based on velocity data, and were confirmed by visual observation involving dye tests. For both systems, separation occurred on one side and the flow was three dimensional. Compared with the wedge transition, the modified wedge transition effectively guides the flow and prevents abrupt changes in flow direction. The modified wedge transition results in a much earlier recovery of the flow in the downstream channel, lower levels of turbulence, and a less strong secondary flow. It also achieves a small gain in the energy at the flow recovery location and reduces the head loss coefficient as a result of the reduced separation zone.