堰上游流場量測及分析

Abstract

本研究目的主要欲依照堰頂所量得之流速剖面迴歸出與表面流速及水深之關係，藉此將未知流量通過堰頂時之表面流速及水深反推求流速剖面，進而利用積分方式計算精確之流量。此外，亦針對堰上游之流速量測資料討論流體接近堰前之流速變化，其中包含縱向及水深方向之觀察。在量測儀器上，使用聲波都卜勒測速儀量測不同堰型及不同堰頂水深之堰頂與上游三維流速，本文針對堰高為20cm之銳緣堰和臥箕堰兩種形式進行量測。研究中量測之案例則設定6組不同堰頂水深，其範圍為12至17cm。 根據研究成果，可歸納出三項要點。第一，堰頂最大流速位置發生在接近堰頂處，有別於一般無水工結構物下之明渠水流的流速剖面，兩者之間不同的差異，將會導致流量估算上的誤差。第二，綜合堰頂流速剖面資料，藉由堰頂表面流速與流速剖面之關係，以堰頂表面流速和堰頂水深為特徵值，可得無因次化流速剖面圖，藉此可推得堰頂表面流速與水深之迴歸式( )，其迴歸式僅需要堰頂表面流速與水深兩個資訊，即可推求不同堰頂水深時之堰頂流速面與流量推估。第三，觀察綜合堰上游之縱向流場可發現，兩種堰形量測中皆有等流速交點之存在，可把交點視為一軸點( pivot )，其各水深方向u流速剖面以此交點為支點，在此支點上方為流速加速區，反之，下方則為流速減速區。

The main goal in this thesis is to measure the velocity profile at weir crest and then derive the relationship between water surface velocity and profile at crest. That relationship can be utilized to estimate velocity profile as flow over weir where only two kinds of information: water surface velocity and water dapth. Furthermore, the discharge will be carried out through the integration of velocity profile. In addition, the horizontal and vertical variations of velocity profile at upstream side will also be discussed. This thesis constructs three-dimensional flow field at crest and upstream side of sharp crest and Ogee weirs with height 20 cm using Acoustic Doppler Velocimetry at various water depth at weir crest. In this study, there are 6 different cases in water depth at crest and range of depths is from 12 to 17 cm. There are three topics according to results of measurements. First, the maximum longitudinal velocity of vertical velocity profiles at weir crest occurs near the crest which differs from general velocity profile in open channel flow without structure. The distinct pattern therefore causes the traditional estimation of velocity profile invalid. Hence the second topic holds an algorithm how to create the velocity profile at crest based on information of velocity at water surface and water depth. Surface velocity and water depth as characteristic values normalize the velocity profile and elevation. Summarizing all normalized measured data, similar non-dimensional profiles are presented. Afterward the regression is carried out between velocity profile and elevation ( ). Some examinations are performed and these show good agreement of estimating real velocity profile only according to surface velocity and water depth. Furthermore, the discharge calculations are discussed here and some comparisons are also exhibited with other methods. The last topic is an observation at upstream side of weir. Concluding the velocity profiles, there is a pivot of equilibrium longitudinal velocity that locates at approximate elevation of weir height. Zone above the pivot means velocity acceleration, and whereas the velocity deceleration appears below the pivot.