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Study on influences of entrained air bubbles on long period oscillation of mean water level in the surf zone

夹带气泡对海浪区平均水位长周期振荡的影响研究

基本信息

批准号：
12650511
负责人：
AOKI Shin-ichi
金额：
$ 2.24万
依托单位：
Toyohashi University of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

项目摘要

Long waves with periods of several minutes have been recognized as an important exciting component to beach erosion, sedimentation in harbors, and oscillations of water and moored ships in harbors. The entrained air bubbles induce a rise in water level associated with some energy transfer into potential energy while part of the wave energy may propagate in off- and on-shore directions as bubble-generated waves. The time scale of the water level rise seems to be related to both the rising time of the bubbles and the repetitive period of high wave breaking, and it is proposed that the air bubble entrainment may be one source of energy transfer from short to long period waves. Furthermore, since the air bubble entrainment process is improperly scaled by the Froude similitude, most laboratory experiments tend to underestimate its effects.Two different experiments were first carried out in a flume. In one series of experiments, the water level rise caused by air entrainment was investigated in a large-size facility generating an unsteady plunging jet. The basic flow patterns and water surface elevations were recorded. In the second series of experiments, the water level rise caused by air entrainment and the propagation of the induced waves were idealized by an air bubble generator installed at the bottom of a narrow flume and discharging air bubbles into still water.The averaged void fractions in vertical circular plunging jet and in plunging breakers were next measured. The ratios of energy dissipation due to entrained air with respect to total energy loss were investigated from the experimental data of averaged void fraction and compared between the two typical phenomena. The hydrodynamicai similarities betweeh the plunging jet flows and plunging breaker waves were also investigated.

周期为几分钟的长波已被公认为是海滩侵蚀、港内沉积、港内水和系泊船舶振荡的重要激振分量。夹带的气泡引起水位上升，这与一些能量转换成势能相关，而部分波能可以作为气泡生成的波在离岸和岸上方向上传播。水位上升的时间尺度似乎与气泡的上升时间和高波破碎的重复周期有关，并提出气泡夹带可能是短周期波向长周期波能量传递的一个来源。此外，由于气泡夹带过程是不正确的弗劳德相似尺度，大多数实验室实验往往低估其影响。在一系列的实验中，空气卷吸引起的水位上升，在一个大尺寸的设备产生的非定常暴跌射流进行了研究。记录了基本流态和水面高程。在第二组实验中，通过在水槽底部安装气泡发生器，将气泡排放到静水中，使空气卷吸引起的水位上升和诱导波的传播理想化，然后测量了垂直圆形倾翻射流和倾翻破碎器中的平均空隙率。根据平均含气率的实验数据，研究了掺气能耗占总能耗的比例，并对两种典型现象进行了比较。研究了倾翻急流与倾翻破碎波的相似性。