垂直式跌水消能池之週期性振盪流場特性探討

Abstract

本文旨在探討垂直式跌水消能池之振盪流場特性，藉以瞭解消能池構造物發生之各種流況與振盪機制。藉由高速攝影機以1200 Hz頻率擷取高畫素影像(1024×1024 pix)，配合波高計量測水面時序列資料，並透過快速富立葉轉換(FFT)將時序列資料轉換為頻域之頻譜圖，找到具高能量之相對應頻率，即第一調和頻率(the first harmonic frequency；f0)。 為正確找出週期性振盪流場發生的範圍，吾人先以固定尾檻高度(h)，逐漸增加來流流量(q)方式觀察消能池內流場變化。第二種方法為固定來流流量(q)，逐漸增加尾檻高度(h)進行實驗。經定性與定量資料交叉比對分析後得知，當(1)來流流量(q)逐漸遞增；(2)尾檻高度(h)逐漸增高時；消能池內產生之流況次序相同，其次序為：舌流(napped flow)→過渡流(transition flow)→週期性振盪流(periodic oscillation flow)→過渡流(transition flow)→滑流(skimming flow)。此外，週期性振盪流(periodic oscillation flow)發生的高度比(尾檻高除以跌水高；h/H)範圍為0.121 ~ 0.857，其中高度比(h/H)以0.429發生週期性振盪流(periodic oscillation flow)之流量(q)範圍最大(自154.6 ~ 223.5 cm2/s)。 研究中發現週期性振盪流(periodic oscillation flow)之第一調和頻率(f0)隨流量(q)增加，出現「遞增」、「先遞增後遞減」與「遞減」三種趨勢。 在上述實驗發現，週期性振盪流場中振盪現象有所差異，遂對週期性振盪流場之振盪模態作進一步研究。透過可視化影像與第一調和頻率(f0)特性作綜合分析，於週期性振盪流場歸納出三種振盪模態；其模態特性分述如下： 1. 模態一之定性特性之一為當水舌下擺時，尾檻上角隅(corner)附近會產生氣體捲入形成挾雜氣泡的漩渦；而定量特性為第一調和頻率(f0)隨流量(q)增加呈現「遞減」趨勢。高度比(h/H)範圍為0.121 ~ 0.429。 2. 模態二之定性特性之一為當水舌下擺時，尾檻上角隅(corner)不具有明顯漩渦，不同於模態一；定量特性為第一調和頻率(f0)隨流量(q)增加呈現「遞增」趨勢。高度比(h/H)範圍為0.329 ~ 0.571。 3. 模態三定性特性之一為水舌撞擊位置(impinging point；IP)範圍在消能池尾檻角隅(corner)和尾檻垂直壁面間來回擺盪；定量特性為第一調和頻率(f0)隨流量(q)遞增呈現「遞減」趨勢，與模態一趨勢相同。高度比(h/H)為0.714。

ABSTRACT A vertical drop pool is frequently used in an open channel to dissipate the energy of the approach flow. As the combination of pool length (L), end sill height (h), and discharge (q) meets neither the napped flow nor the skimming flow conditions, the flow oscillates periodically in the vertical drop pool. The purpose of this study is to investigate the characteristics of the periodic oscillatory flow over a vertical drop pool with a subcritical approach flow experimentally. Free surface oscillations of dropping flows were measured using a wave gauge located in the pool. The primary frequency (f0) of the periodic oscillation was then determined by applying spectral analysis to the time series of the gauge measurements. Two flow visualization techniques were employed to reveal the structures of dropping flows qualitatively. In order to understand the relationship between the periodic oscillatory flow pattern with the discharge of the approach flow and the influence of end sill height, series of experiments were conducted by varying discharge (q) and end sill height ratios (h/H, where H is the dropping depth). The flow behaves as the periodic oscillatory flow for h/H ranging from 0.121 to 0.857 with different discharges. When the h/H was equal to 0.429, the largest range of discharge (q) was obtained from 154.6 ~ 223.5 cm2/s for the periodic oscillatory flow patterns in the vertical drop pool. With analyzing the flow visualization results and the time series measurements of wave gauge, flow patterns of the periodic oscillation could be classified by three different modes as follows: (1) Mode1 shows the trend that the f0 decreases with the increase of q for h/H varying from 0.121 to 0.429. A large amount of air bubbles was entrained to the vortex upon the end sill corner. (2) Mode2 presents the trend that the f0 increases with the increase of q for h/H changing from 0.329 to 0.571. By comparing with Mode1, there is no air bubbles entrained into the vortex upon the end sill corner. (3) Mode3 shows that the f0 decreases with the increase of q as h/H is equal to 0.714. The flow features a falling jet impinging on the corner and vertical wall of the end sill without entraining air bubbles upon the end sill corner.