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タイトル: Laboratory Study on the Role of Steady Streaming in Oscillatory Sheetflow Transport
その他のタイトル: シートフロー漂砂における定常流れの役割に関する実験的研究
著者: Lwin, KyiKyi
著者(別言語): ルウィン, キー キー
発行日: 2012年9月27日
抄録: Many coastal activities are concerned with the interaction of coastal sedimentary processes and coastal works, such as the construction of structures for shore protection and stabilization, and beach nourishment. It is important to measure sand properties, sediment moving processes and transport rates, as well as the resulted nearshore morphology to understand the sediment transport mechanism under various wave and current conditions. In this study, we are interested in understanding the sediment transport mechanism, especially the influence of wave-induced boundary layer streaming on sediment transport under combined wave and current conditions in the sheetflow regime. Recently, sediment net transport rate measured through the large wave flume (LWF) experiments presents a more onshore tendency, i.e., a larger onshore net transport, than the result from the small oscillatory flow tunnel (OFT) experiments. Various researchers argue that the wave-induce onshore streaming could be the reason to cause such difference. The objective of this research is to understand the physical features of this phenomenon and answer the question: Does onshore streaming really enhance the onshore sheetflow net sand transport? If so, then, how and how much does it affect the onshore transport? If not, what is the real reason behind? The second is to obtain new insights into the importance of the boundary layer onshore streaming and to understand the transport processes under wave and current conditions. To achieve the objectives and to measure the sediment net transport rate under sheet flow conditions, laboratory experiments were conducted under the combined asymmetric wave-current conditions to quantitatively evaluate the influence from the onshore streaming. The second order Stokes'wave theory with a velocity asymmetric index of 0.57 was applied for wave generation with three well-sorted sands with medium sand size of D50 =0.3 mm (coarse), 0.16 mm (fine) and 0.13 mm (very fine). For fine sand without onshore current, the net transport increases with increasing velocity, and it is directed to the onshore. However, for larger velocity case, the net transport rate decreases and the direction also changes to the offshore. As for the very fine sand, the net transport rate decreases and is directed to the offshore even for a small velocity case. Considering the coarse sand, the net transport is in the onshore direction. To understand the effect of onshore streaming, a small current Uc of 10 cm/s and 20 cm/s was generated in the onshore direction. Experiment results for small current 10 cm/s indicate the magnitude offshore net transport rate reduces and the direction is to the offshore for the very fine sand and fine sand with large velocity case. When increasing the small current value to 20 cm/s, the net transport rate of fine and very fine sand increases and changes to onshore direction with small velocity case. But for large velocity case, even though the magnitude of offshore net rate reduces, the direction is still directed to offshore with fine sand case. Taking into account the net transport rate measured under the combined wave and current cases, the onshore net transport for coarse sand continuously increases. It is noted that the tendency of increasing of net transport rate is not observed in fine grains when the velocity becomes increases without the contribution of current. In case of the contribution of small onshore streaming, although the magnitude of offshore net transport rate of fine and very fine sand reduces, it still directs to offshore under large velocity case. It indicates that the onshore streaming, indeed, enhances offshore net transport rate for fine sand and very fine sand with large velocity. On the other hand, the small onshore streaming may be partly important for the case of fine sand under small velocity condition. Sediment particle velocity within the sand-laden sheetflow layer was measured by means of a PIV technique. By averaging the sediment particle velocity over one wave period, the mean flow velocity was also evaluated. From the mean velocity profile under pure wave conditions, it is found that, in case of the coarse sand, an onshore streaming is detected in the pick-up layer and leads to offshore in the upper sheet-flow layer. Nevertheless, in case of fine sand, the profiles show a negative streaming due to the strong phase-lag effect. The positive near-bed streaming is not observed. The large phase-lag can induce a negative (offshore) net transport. Thus, the phase-lag effect seems to play an important role for the sediment sheetflow transport in the OFT test. For coarse sand under combined wave and current conditions, a very small onshore current exists in the pick-up layer (z< 0 mm) and the mean flow velocity leads to onshore direction in the sheet flow layer. In the suspension layer, when the elevation is higher than 15 mm, the mean flow changes its direction from onshore to offshore. In the case of fine sand, the time-averaged velocity indicates the streaming is positive in the pick-up layer as well as in the sheet flow layer. After that, the velocity decreases for increasing the depth (z) mm. Clearly, the additional onshore current in the tunnel does contribute to more onshore sediment transport. Besides that, it is also confirmed that the phase-lag effect plays an important role in the sediment transport under the sheetflow conditions, especially for the fine sand case with large velocity case as it produces offshore net transport rate. Here also, the streaming profiles are very sensitive to sand size. Furthermore, the measured net transport rates are compared with the results from surface wave under same flow conditions. For very fine and fine sand with onshore streaming, the sediment rate under oscillatory flow tunnel can predict about 75 % of net rates under surface wave with small velocity case. Now, the new experiments indicated the difference of sediment rate between these was about 1.5 times for fine sand with onshore streaming. In addition, the results of coarse sand with streaming produce larger onshore net sediment rate compared to surface wave. It means the contribution of streaming is quite large enough to enhance the more onshore net transport rate for the coarse sand. As a result, the streaming effect is very dependent on sand size. The maximum erosion depth was estimated from the temporal change of the measured erosion depth. A linear relationship was found between the relative maximum erosion depth δem/D and the maximum Shields parameter, θm. The erosion depth under crest is larger than under trough for fine sand and coarse sand under combined wave and current conditions. The influence of wave period and velocity on erosion depth was also measured for two types of sand. And then, in order to know how much the distribution of small onshore streaming enhanced the larger net rate, the results of net transport rate with onshore streaming are compared with SANTOSS model which include surface wave effects. In this study, SANTOSS model was also considered as streaming-related model including the streaming effect by analytically to represent the surface wave phenomenon. The comparison results showed that although the results of fine and coarse sand with onshore streaming overestimated compared to the results of streaming-related model, it lies with a factor of two differences.
内容記述: 報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 課程博士 ; 学位の種類: 博士(工学) ; 学位記番号: ; 研究科・専攻: 工学系研究科社会基盤学専攻
URI: http://hdl.handle.net/2261/52641
出現カテゴリ:021 博士論文
1130220 博士論文(社会基盤学専攻)

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