碎波帶紊流及剪應力之大尺度實驗觀測研究

Large-scale laboratory observations of turbulence and shear stress in the surf zone

陳羲(Hsi Chen)

指導教授 ： 黃志誠

國立中央大學/地球科學學院/水文與海洋科學研究所/碩士(2015年)

摘要

本研究以大尺度實驗觀測碎波帶紊流在時空上的演變。碎波帶底床為斜率1:100的混凝土鋪面，應用單頻(monochromatic)規則波以三種不同的造波條件進行試驗，每種造波條件重覆超過16次以上的試次，以觀測在碎波後流速的剖面與紊流動能k的演變。因受到亞重力波的影響，即使在控制優良的實驗水槽中，碎波帶的各個波浪週期與碎波位置仍會有些微的差距，因此本實驗利用較嚴謹的修正相位總體平均法(Phase-corrected ensemble averaging method)來分離紊流項之流速。利用傅立葉頻譜分析(FFT)紊流能量頻域上的分佈，並將此紊流頻譜與Kaimal et al. (1972)的紊流曲線(KCs)做比較，發現觀測資料與該曲線的趨勢相似，證明了碎波的紊流能量隨頻率的分布與邊界剪力層的紊流(boundary shear layer turbulence)有相似的特性。
由本研究的垂直測量剖面發現紊流動能的分布具有由上往下減少的趨勢，顯示水體的紊流來源為上方的碎波所產生。將紊流動能隨相位作圖。發現當波峰經過之後，水體之紊流強度會增強，並持續一段時間後才消散。此外，本實驗也對波剪應力(wave shear stress)與紊流剪應力作時序列分析，發現本研究量測波剪應力強度約大於紊流剪應力一個量級。
相對以往認為波浪中的波引流速呈現正交，近年學者則認為水平和垂直波引流流速 , (wave-induced velocity)在具有傾斜底床、底床摩擦與碎波消散作用的行進波中兩向量可能不為正交[Zou et al., (2006); Wilson et al., (2014)]，而產生對波剪應力的貢獻。本研究發現時間平均的波剪應力 剖面在此為由上往下向負值增強，而近底床趨近零，從時序列 發現是在波谷位置的能量貢獻主導這現象。

關鍵字

大尺度均向波浪實驗室研究 ； 碎波帶 ； 紊流 ； 頻譜 ； 波剪應力

並列摘要

Large-scale laboratory observation of the temporal and vertical evolution of turbulent properties in the surf zone is presented. The experiments were conducted in a wave flume with a slope of 1:100. Monochromatic regular waves of three different incident wave conditions were generated. More than 16 test runs with the same initial and boundary conditions were repeated for each wave condition.
Because of the effect from infra-gravity waves, the wave period and breaking location in a wave train is slightly different in the surf zone even the experiment was conducted using well-controlled facilities. A phase-corrected ensemble averaging method is used to separate the turbulent velocities. Fourier transform analysis is used to get the turbulent spectrum. The results are compared to the universal turbulent spectrum proposed by Kaimal et al.(1972) (KCs). The observed turbulent spectra under wave-breaking are compared to that of boundary shear layer turbulence. We found that are the trend of the two turbulence spectra are similar. This result confirms the energy cascade process of the wave-breaking turbulence is similar to that of boundary shear layer turbulence.
Vertical profile of the Turbulent Kinetic Energy (TKE) density are examined. We found that the TKE decreases with the depth in the surf zone. The temporal variation of turbulent intensity as a function of wave phase is also presented. We found that the turbulent intensity increases and lasts for a while after the broken wave crest passes. In addition, analysis for the time series of turbulent shear stress and wave shear stress were performed. It is found that the wave shear stress is one order of magnitude larger than the turbulent shear stress in the experiment.
The horizontal and vertical wave-induce velocities, and were considered to be orthogonal in most of previous studies and theories. However, they may not be orthogonal and the wave shear stress may become important due to the effects of bottom slope, bottom friction, and wave breaking.
The time-average wave shear stress is close to zero near the bottom; the value is negative and the intensity increases with increasing distance from the bed. We found that most of are under the wave trough phases.

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