Different shapes and regimes can be observed in open channels depending on their velocity and fluid depth. This part presents a CFD model of a trapezoidal open channel with a deeper water depth, created using the Volume of Fluid (VOF) model. The results of this model are then compared with those obtained in the first part of the blog, which deals with a shallow open channel model.
Open Channels Shapes and Flow Regimes
Open channels are available in different shapes, including circular, trapezoidal, and rectangular, each with unique characteristics and applications. Circular channels offer flow stability and are commonly used in water distribution, irrigation, and stormwater drainage systems. Trapezoidal channels are versatile and efficient for varying flow rates and are often found in irrigation canals, wastewater treatment plants, and river systems. Rectangular channels with a constant flow rate are used in urban drainage, highway culverts, and industrial wastewater networks. The shape of an open channel is determined by factors such as space availability, flow rate, and hydraulic performance. These channels have numerous applications in hydraulic engineering, agriculture, transportation infrastructure, and environmental projects for restoring waterways, managing wetlands, and protecting aquatic ecosystems. Selecting the appropriate open channel shape is vital for successful hydraulic projects and ensuring efficient water flow management to benefit the environment.
Figure 1. (a) trapezoidal channel, (b) circular channel; (c) rectangular channel
Open channels have different flow regimes based on depth and fluid velocity: supercritical flow in shallow channels, characterized by fast-moving water and turbulence, and subcritical flow in deep channels, characterized by slower-moving water and smooth flow.
CFD Modelling for Open Trapezoidal Channel Flow
CFD modelling using the Volume-of-Fluid (VOF) model accurately predicts open channel flow behaviour, including flow patterns, pressure distribution, and velocity profiles. It is particularly useful for simulating free surface flows and complex phenomena like hydraulic jumps and sediment transport.
The below videos showcase the CFD simulation using VOF model and illustrate results such as flow pattern, free surface velocity, waves surfing over the water surface, etc. The first video is for an open trapezoidal channel with deeper water flow. The second video shows how water depth affects the flow that results from the interaction of the two streams coming from the two channels.
Video 1. Ansys Fluent simulation of open trapezoidal channel flow with deep water
Video 2. Open channel flow models with shallow and deep water flow
Summary
Open channel flow varies in shape and regime, with each having advantages and limitations. CFD modelling accurately predicts flow behaviour using the VOF model, enabling engineers to enhance performance and make informed decisions for efficient and reliable operation.