Malik Fesal, Siti Natasha (2023) Flow visualization on downstream of rectangular sharp-crested weir with free fall flow. Doctoral thesis, Universiti Tun Hussein Onn Malaysia.

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Abstract

The characteristic of counter-current phenomenon on downstream channel for sharp crested weirs, and vertical drop hydraulic structures has been a challenge in fluid dynamics for many years. This study focuses on the interaction between two distinct flow regimes, namely free fall, and modular transition flow, in an open channel. The experiments were conducted with several upstream conditions of head over weir crest, hc /P for 0.3, 0.4, 0.5 and 0.6 with tailwater depth of ht of 0.7 P, 0.8 P and 1.0 P, with P is the height of the sharp-crested weir. The experiments were performed in two-dimensional (2D) plane the flow visualization method with Froude number, Fr ranging from 1.0 to 5.4. The flow characteristics were analyzed, including velocity distributions, hydraulic Froude number, Fr, and excitation frequency from downstream of rectangular sharp-crested weirs. The study revealed the occurrence of two types of Von Karman vortex streets; regular and reverse, in the vicinity of the downstream channel, with significant vortex frequency shedding based on each tailwater depth. The excitation frequency, fst range, was discovered to vary with tailwater depth. The maximum value for excitation frequency, fst, with a tailwater depth of ht = 0.7 P, 0.8 P and 1.0 P, were 40 Hz, 50 Hz, and 85 Hz respectively, while the minimum excitation frequency, fst, for all cases was zero. The excitation frequency played a significant role in the oscillatory period within the counter-current region until it diminished into the recovery zone. Overall, our research extends the knowledge of nappe flow in open channels by providing new insights into the velocity distribution and the relationship between vortices and the frequency of Strouhal. This information can be used to optimize the design and operation of open channel systems, improving their safety, sustainability, and efficiency

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