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

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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.