Influences of plunge pool length and backward cavity on flow field in plunge pool of free overfall
Date Issued
2009
Date
2009
Author(s)
Chen, Guan-Ru
Abstract
This study mainly measures and analyzes the flow field of skimming, napped and periodic oscillatory flow of free overfall with a plunge pool in various lengths (Lp). It also discusses the influences of backward cavity in the plunge pool on flow conditions. Series of experiments are carried out in a rectangular flume made of acrylic material. Single approaching flow rate is considered and the length of the plunge pool is adjustable to create required flow conditions (Lp = 5 – 30 cm). Point gauge and the laser-sheet imaging technique are used to measure the steady and oscillating water surface. In addition, the PIV (Particle Image Velocimetry) is applied to visualize the flow field in the plunge pool. For periodic oscillatory flow, oscillation period increases with the increasing plunge pool length. On the other hand, the water stage amplitude decreases while the plunge pool length stretching. The backward cavity evidently influences the range of plunge pool for periodic oscillatory flow. However, for skimming and napped flow, the measured water stages are rather similar in spite of the plunge pool with/without the backward cavity. Obvious influences by backward cavity are investigated in the periodic oscillatory flow. The oscillation period and water stage amplitude become longer and smaller, respectively, comparing with the same plunge pool length. Regarding air pocket beneath the falling jet, it appears except Lp = 14.5 and 15 cm without backward cavity. To application of end depth method to flow rate estimation, the mean value of the maximum and minimum water depth at the brink of the drop used as the end depth is workable. However, the huge deviation of flow rate estimation is found as the air pocket disappearing. The visualized velocity field in the plunge pool shows unstable phenomena. In addition, the backward cavity significantly affects the flow field. For example, the major vortex in the plunge pool shows narrow shape and its centre position locates closing to the vertical wall.
Subjects
Free overfall
Skimming flow
Napped flow
Periodic oscillation flow
Laser-sheet imaging technique
PIV(Particle Image Velocimetry)
Type
thesis
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