靜水池長度與背向穴室對自由跌水池內流況影響之試驗研究

Abstract

本研究主要量測與分析自由跌水在不同靜水池長度時發生滑流、舌流及週期性振盪流之水理及觀察背向穴室存在時對水理之影響。本系列試驗於壓克力水槽中設置自由跌水模型，並於跌水垂直側壁後方建置矩形空間之背向穴室，下游處設置尾檻以形成靜水池。為造成不同流況，試驗之靜水池長度(Lp) 則為5至30 cm間。研究中僅針對單一入流量進行試驗。試驗中以水尺及雷射光頁影像技術量測穩定及振盪流況時之水位。於週期性振盪流況下，水位振盪週期隨靜水池長度增加而增加，然而水位振幅隨靜水池長度增加而減少。滑流與舌流時水位於有無背向穴室時之差異微小，然而明顯差異表現則於週期性振盪流況時，包括水位振盪週期及振幅。背向穴室之存在使造成週期性振盪流況之靜水池長度範圍改變，振盪週期變長振幅變小。此外，應用跌水水深(end-depth)於振盪流況時之流量推估亦被檢查。跌水處之最高與最低水位之平均水深仍可應用於流量推估。試驗中發現當靜水池長度為14.5及15 cm且無背向穴室時，水舌下方之氣室將逐漸消失，但此長度下且背向穴室存在時氣室則持續存在。當氣室消失時振盪流況之週期明顯增加且振幅減少，所推估流量之誤差值亦明顯增加。最後利用PIV (Particle Image Velocimetry)將流場可視化，主要說明有無背向穴室與不同靜水池長度下靜水池內部流況變化。量測結果顯示各流況時之流場皆非穩態。背向穴室之存在影響靜水池流場明顯，並使靜水池內主渦流之形狀變的較扁長，且渦心位置則較靠近底床。

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.