臥箕溢洪道入流場特性之試驗研究 臥箕溢洪道入流場特性之試驗研究 臥箕溢洪道入流場特性之試驗研究

Abstract

近年來集集攔河堰在颱洪過後，其臥箕溢洪道之堰面上常有磨損，除可能因穴蝕現象產生之破壞外，另可能因溢洪道前之砂石經由水流渦漩帶起砸落至溢洪道堰面而產生磨損。溢洪道入口之流場為一紊亂之三維流場，流況複雜，過去鮮少探討其流場特性。因此，本研究將利用質點影像測速法沿水深方向及水流方向量測集集堰溢洪道入流流場，藉此探討其流場特性。 就模型試驗可將流場結構分成沿水流及水深方向兩部分作探討。在沿水流方向部分，發現上游階梯底床後存在一主要渦漩(recirculation vortex)，其成因是由於階梯結構造成負壓力梯度，驅使自由流向下偏折所致，另由於主要渦旋之產生，在其下方將伴隨著一較小之角落渦旋(corner eddy)，而兩渦漩之旋轉方向相反，此可視為一變形之背向階梯流場。於水深方向，因近邊壁及渠心之速度差，在邊壁與階梯頂之交接處產生由上而下之垂直渦旋(vortex tubes)，其中心位置由於背斜迎水面及上述渦旋之影響，會以固定角度向下延伸。

In recent years, the ogee spillway weir surface of Chi-Chi Weir often wears after the flood. It is suspected that occurs, in addition to the damage arising from cavitation, abrasion occurs due to the gravels in front of the spillway picked up by flow vortex and dropped on spillway weir surface. Flow field in spillway inlet is complicated and three-dimensional, and few studies can be found in the past. Therefore, this study measures the inflow field of ogee spillway by using the particle image velocimetry to along the horizontal and vertical directions. The experimental results can be analyzed in the flow direction and in the depth direction. Along the flow direction, we can find a main recirculation vortex appears behind the upstream stepped bed, which is due to sudden cross-sectional expansion behind the stepped structure, and the associated adverse pressure gradient, drivers the flow with downward deflection. In addition, due to the occurrence of the main recirculation vortex, it will be accompanied underneath by a small corner eddy, and the two vortex’s direction of rotation are opposite. It can be considered as a deformed backward-facing step flow field. Along the depth direction, because of the existence of velocity difference between the wall and channel center line, top-down vortex tubes will occur at the intersection of the wall and top of the step. Also, due to the upstream anticline face of the weir and recirculation vortex, the top-down vortex tubes will extend downwardly at a fixed angle.