Energetics and dynamics of flow through baffle drop shafts using physical and computational model studies
Date
2023
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Abstract
A drop shaft is one of the main hydraulic structures that is used to convey water from higher to lower elevations while dissipating potential energy in storm water management systems, water treatment plants, and hydropower stations. Drop shafts need to be adjusted for higher discharges because of the increased urban flooding due to climate change and rapid urbanization. Traditional baffle drop shafts have limited flow capacity and are unstable due to their asymmetric nature. The novel baffle drop shaft is proposed here for larger range of flow discharges. To the author's knowledge, there are no previous studies that have thoroughly investigated the energy dissipation potential of the novel baffle drop shaft. Hence, there is a need to establish a design relationship between key parameters such as the shaft diameter, baffle spacing, and discharge to inform best design practices. A 1:10 physical model study was carried out to investigate the energy dissipation of a novel baffle drop shaft using different discharges. Pressure and velocity were measured at two locations on the baffles using low range pressure sensors (100 mbar) and an electromagnetic velocity meter. Timed averaged pressure and velocity on the baffles increased with discharge. These averaged quantities were considered to calculate global and local energy dissipation through the shaft. The global energy dissipation efficiency was calculated based on the inlet and outlet channel flow data, and was found to range from 89.6% to 91.9%. The flow regime profiles were quite similar on each baffle section of the shaft; hence, we can consider the energy dissipation in each baffle to be equivalent. Under free-flow conditions, the energy dissipation efficiency decreases as the discharge increases. Physical models are costly and time-consuming for performing parametric studies of flowthrough such structures because each and every geometric configuration needs to be constructed in the lab. Computational Fluid Dynamics (CFD) is a more feasible option to conduct an in-depth investigation of the energetics and dynamics of flow in a baffle drop shaft since it is faster and more cost-effective than a physical model study. The CFD models have been built to simulate the hydraulic behavior of baffle drop shafts using OpenFOAM. This software is adaptable for modeling diverse flow issues due to the variety of models and numerical techniques that it incorporates. A suitable turbulence model that is commonly used in CFD for modeling turbulent flows such as in drop shafts is the RANS-based realizable k- ϵ model. Mesh sensitivity analysis was also performed to establish grid independences of the solution. Benchmark geometry CFD models were calibrated using four locations in the physical model, and velocity and pressure measurements at the edge of the baffle were used for validation with remarkable agreement. A parametric study was conducted using shaft diameters (D) of 0.8 m, 0.9 m, and 1 m, six baffle spacings (h) ranging from 0.23m to 0.48 m, and baffle rotating angles (θ) of 180◦, 250◦, and 270◦. Global energy dissipation efficiency (η) ranged from 92% to 97%. The η value decreased with discharge but was higher under free flow conditions in the baffle drop shaft. The geometric parameters D, h, and θ have little influence on energy dissipation. Considering structural integrity, available space, construction costs, and maintenance costs, the baffle drop shaft needs to be optimized to achieve the desired hydraulic performance. Maximum pressure was observed at the water jet impact location close to the outer shaft wall. Air entrainment is also a significant consideration in designing baffle drop shafts because its impact is critical in applications like hydro power generation. The bulking of the flow due to air entrainment needs to be considered to evaluate the maximum flow carrying capacity of baffle drop shafts. In summary, designing baffle drop shafts requires a multi-criteria approach that is mainly dependent on the design requirements on energy dissipation, structural integrity, construction costs, air entrainment, application, and location.
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Subject
CFD
energy dissipation
baffle drop shaft
physical model
energetics