利用ADCP估算地區藻礁潮間帶紊流特性;Estimation of the turbulent characteristics using ADCP in the intertidal reef region

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Title:	利用ADCP估算地區藻礁潮間帶紊流特性;Estimation of the turbulent characteristics using ADCP in the intertidal reef region
Authors:	阮氏素雲;Nguyen, Thi To Van
Contributors:	水文與海洋科學研究所
Keywords:	紊流;ADCP;ADV;方差方法;藻礁;Turbulence;ADCP;ADV;variance method;TKE dissipation rate;Coastal area;Algal reefs
Date:	2022-11-07
Issue Date:	2024-09-19 15:07:49 (UTC+8)
Publisher:	國立中央大學
Abstract:	紊流是影響流體中能量轉換時的重要現象，特別是沿海地區，紊流對營養鹽及沉積物的傳輸都扮演著重要的腳色。因此，了解紊流在藻礁沿海地區的特性是重要的課題。本研究以測量速度的統計值來觀察潮間帶礁區的湍流，研究地點位位於桃園大潭藻礁G1區，利用Nortek ADCP signature 1000流速儀的觀測值，以變異數分析法(variance method)推估藻礁地區的雷諾應力，並將其結果與其他三台ADV做比較。研究結果顯示，由ADCP所推估的雷諾應力高於ADV實際觀測值，其原因是為wave orbital velocities所導致的，當波浪的方向與海岸線垂直(u′w′) ̅時，其分量會高估於與海岸線平行 (v′w′) ̅。因此，同理可得知，與海岸線平行的((v′w′) ̅)分量與ADV3的相關性性也更優於垂直((u′w′) ̅)的分量。本研究運用兩種方法計算ADCP的摩擦速度，第一種方法由雷諾應力計算得知，其結果高於ADV3的2倍，第二種方法為利用log-fitting，運用ADV和ADCP兩組資料進行log-fitting計算，與ADV3的結果呈現良好的相關性。另外，本研究亦根據ADCP的雷諾應力計算TKE production，得以觀察TKE budget的變化情形，以高解析(high-resolution beam)的ADCP垂直速度的空間剖面為資料，運用結構函數方法(structure function)計算，用於估計TKE dissipation rate ε。TKE production 和TKE dissipation rateε呈現平衡狀態，但由於TKE production是根據尚未校正的雷諾應力計算得知，且本研究所計算的TKE production大於前人研究，仍需重新考慮TKE production的結果。然而，TKE dissipation rateε與相近於前人的研究結果，另外，根據P-ε 呈現平衡的假設，本研究以符合log-fitting的部分小時資料，並用ε和 u_^3/κz檢查其關係性，其結果顯示存在相關性。;Turbulence is a random phenomenon and necessary for energy exchanging and momentum transport in fluid flows. In particularly, in the coastal reef region, turbulent flows plays an important role to uptake nutrients for the ecosystem and control the sediment transports. Thus, study of turbulent characteristics is important in coastal ecosystem area as algal reefs. In this research, we try to observe the turbulent flows in the intertidal reef region through statistical quantities using velocity measurements. The field experiment was deployed at site G1-2 of Datan reef located in the northwest coast of Taiwan. Specifically, we applied the variance method to estimate the Reynolds stress using the Nortek ADCP signature 1000. The results were then compared to the stress computed from the data of three ADVs. The results of the Reynolds stresses estimated from ADCP are overestimated compared to the ADVs. These overestimations are caused by the motions of wave orbital velocities induced bias for the covariance of the beams velocity. Indeed, the results showed that the cross-shore (u′w′) ̅ component coincided with the wave direction gives larger overestimations compared to the along-shore (v′w′) ̅ component that has less wave affection. The shear velocity estimated from the Reynolds stress of the variance method is also overestimated (double larger) compare to the results of ADV3. However, the shear velocity estimated using another method, which is the log-fitting method applied for combined measurements of ADVs and ADCP, gives very good correlation with the ADV3’s estimation. Besides, to observe the balance of TKE budget, we computed the profile of TKE production using the Reynolds stress from ADCP. Then, we applied the structure function for spatial profile of vertical velocity measured by the high-resolution beam of the ADCP to estimate the TKE dissipation rate ε. Correlation of TKE production rate and TKE dissipation rate ε showed the balance of two quantities. Even so, the TKE production need to be re-consider because it has larger order of magnitude compared to previous studies deployed in similar environment. In contrast, ε not only gives approximated values compare to previous studies, but also gives good correlation to the ratio of u_^3/κz in bursts that qualified the log-fitting followed assumption of P-ε balance.
Appears in Collections:	[Graduate Institute of Hydrological and Oceanic Sciences] Electronic Thesis & Dissertation

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