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被认为是海滩侵蚀、港口淤积以及港口水域和系泊船舶振荡的重要刺激因素。夹带的气泡引起水位上升，一些能量转化为势能，而部分波能可能作为气泡产生的波在离岸和岸上方向传播。水位上升的时间尺度似乎既与气泡上升的时间有关，又与高浪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种来源。此外，由于弗劳德相似模型对气泡卷吸过程的刻度不恰当，大多数实验室实验往往低估了它的影响。在一系列实验中，在一个产生非定常下沉射流的大型设施中，研究了空气卷吸引起的水位上升。记录了基本流型和水面高程。在第二组实验中，通过安装在狭窄水槽底部的气泡发生器将空气卷吸引起的水位上升和诱导波的传播理想化，并将气泡排放到静水中，然后测量了垂直圆形下沉射流和下沉破碎器中的平均含气率。根据平均含气率的实验数据，研究了夹带空气引起的能量耗散占总能量损失的比例，并对两种典型现象进行了比较。文中还研究了下沉射流和下沉破碎波在流体力学上的相似性。

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.