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

周期为几分钟的长波已被认为是海滩侵蚀、港口沉积、水和港口系泊船舶振荡的重要刺激因素。携带的气泡引起水位上升，部分能量转化为势能，而部分波浪能量可能以气泡产生的波的形式在离岸和岸上方向传播。水位上升的时间尺度似乎与气泡的上升时间和高破波的重复周期有关，并提出气泡夹带可能是短周期波向长周期波传递能量的一个来源。此外，由于气泡夹带过程是不正确的尺度弗劳德相似，大多数实验室实验往往低估了它的影响。首先在水槽中进行了两个不同的实验。在一个大型实验装置中，研究了空气夹带引起的非定常俯冲射流的水位上升问题。记录了基本的水流模式和水面高度。在第二个系列的实验中，通过在狭窄水槽底部安装气泡发生器并将气泡排放到静水中，理想化地实现了由空气夹带引起的水位上升和诱导波的传播。然后测量了垂直圆形俯冲射流和俯冲破碎管内的平均空隙率。根据平均孔隙率的实验数据，研究了夹带空气的能量耗散与总能量损失的比值，并对两种典型现象进行了比较。研究了俯冲射流与俯冲破碎波的水动力学相似性。

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.