投潭水流作用下沖刷坑中流速分佈係數之探討

本研究之主要目的係利用均一粒徑爲渠床質所得局部冲刷試驗之結果，設計一系列模擬投潭水流作用下之瞬時動床冲刷坑型態，配合電磁流速計檢測堰壩下游的流速場，經由動量及能量的分析方法，探討投潭水作用下局部冲刷過程中，能量校正係數α、動量校正係數β之力學機制變化情形。經實驗數據及研究分析得知，於冲刷坑中能量、動量校正係數，二者皆由堰趾（X=0）開始逐漸增加，在冲刷延時t之相對最大坑長X/L_(it)≤0.3位置時，其值屬於同一數量級約10^2，接近最大刷深處X=L_(st)（=0.3L_(it)）時最大，亦卽X/L_(it)=0.3時其值增加1～3個數量級，然後又逐漸衰減，至離開冲刷坑後（X/L_(it)≥1）約為大於1，且有α＞β之定性關係。

關鍵字

投潭水；局部冲刷；能量校正係數；動量校正係數

並列摘要

Experimental results of local scouring channel beds made of uniform particles have been applied in this study to determine the instantaneous cross sectional profile of scour holes under free over-fall flow. An electromagnetic current meter was applied to measure the velocity field of the downstream of weirs. Energy coefficient (α) and momentum coefficient (β) were calculated for local scouring process under free over-fall flow in order to obtain their characteristics. Based on the analysis of the observed data, several conclusions can be drawn. The energy coefficient (α) and momentum coefficient (β) increase from the location of the weir (X=0) in the scour hole. They are at the same order (about 10^2) as the normalized downstream distance (X/L_(it), L_(it) being the length of scour hole) is less than 0.3, become maximum as X/L_(it) equals 0.3, then decrease as X/L_(it) is in the range of 0.3 to 1.0. Further downstream (X/L_(it)≥1), the relationship α>β≥1 holds.

並列關鍵字

Free over-fall flow ； Local scour ； Energy coefficient ； Momentum coefficient

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