應用同折射率材料與粒子追蹤測速技術探討非破碎駐波水流型態對淺瀨棲地之影響

Abstract

本文的研究目的在於探討非「破碎型駐波水流型態（unbroken standing wave）」對淺瀨棲地的影響。前輩學者研究河川底下礫石中的流場時，一般假設推動礫石間河川與地下水間交互作用的壓力差是來自水流通過不平整的河川底床地形或形貌所造成的壓力差。不過，因為無法直接測量底床內的流場，所以過去的研究主要是利用觀測液體濃度變化的方式，來推論自由液面流與孔隙介質間交換流量，間接驗證壓力差來源。 有別於過往的推論，筆者在可利用水流型態（flow type）進行河川棲地分類的脈絡下，提出另一種假設：淺瀨區域的表面波（非破碎型駐波）為震盪型水躍（undular hydraulic jump），此震盪型水躍會在底床表面產生壓力變化，可作為自由液面與孔隙間液體交換的驅力。為驗證此假設，以能夠直接測量液體在孔隙間的流場的「同折射率材料」與「粒子追蹤測速技術」作為實驗方法，並透過時間平均的Navier-Stokes方程計算近床區的壓力變化，最後再以線性管流網絡模式解釋壓力變化對孔隙介質流的影響。 透過實驗，可發現震盪型水躍底下存在穩定的液體交換結構，再以Navier-Stokes方程進行流場分析，顯示液體會在高壓區進入孔隙介質，並在低壓區回到自由液面流。管流網絡模式亦說明分析所得的壓力分布，足以產生液體交換結構，驗證了先前的假設。這個假設的成立，顯示以非破碎型駐波作為淺瀨的分類方法具有物理意義。

This study investigates the effect of unbroken standing waves on riffles. In the previous studies, the researchers believe that exchange of hyporheic zone caused by the pressure difference because of different fluvial morphological and bed form. Because they can’t measure the fluid velocity field inside sediment bed directly, they use concentration change to study the exchange flow rate between surface flow and porous medium flow. Newson & Newson (2000) believe the biotopes can be classified according to the flow types on the surface of river. We suppose unbroken standing wave on riffles are undular hydraulic jump and a pressure variation under the waves is drivers of fluid exchange between surface flow and porous medium flow. In order to prove this thought, we use Refractive Index Matching (RIM) materials and Particle Tracking Velocimetry (PTV) technique to set up a experiment to measure the velocity field in porous medium under undular hydraulic jump. According to our research, we find out there are a stable downwelling fluxes upwelling fluxes on porous medium surface. We calculate the pressure distribution on porous medium surface by using time-average Navier-Stokes eqns. and find out up and down welling fluxes are corresponding to low and high pressure zone. The velocity field in porous medium also can be simulated by our Pipe Flow Network Analysis model.