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Air entrainment and bubble dispersion were studied systematically at vertical circular plunging jets in freshwater, saltwater, and seawater. Although the inception conditions for air entrainment are identical for all water solutions, experimental results show lesser air entrainment in seawater plunging jets for identical inflow conditions. Comparison between seawater and saltwater results suggest that organic matter and biochemicals harden the induction trumpet at plunge point and diminish air entrapment at impingement in seawater. Froude-similar experiments with different geometric scales show lesser air entrainment for inflow conditions satisfying . The results show that classical dimensional analysis of air entrainment is incomplete, while they would imply scales effects in terms of air entrainment for small laboratory studies with wave height at breaking less than 0.3 m.

Void fractions under breaking waves have been measured by a conductivity probe. Empirical coefficients k0 and C0 have been determined through the experimental results and found to be function of the local wave height and relative distance, respectively. The results indicate that the distributions of void fraction follow closely analytical solution of the diffusion equation.

This study investigates energy dissipation due to air bubble entrainment for three typical phenomena; a hydraulic jump, a 2-D vertical plunging jet and a vertical circular plunging jet into water. A simple model is presented here which enables to estimate the energy transformation and dissipation achieved by air bubbles quantitatively for three above phenomena. The average rate of energy dissipation by air bubbles obtained from the experimental data are 25%, 1.4%, and 2.15% with respect to total energy loss for the hydraulic jump, 2-D vertical plunging jet and vertical circular plunging jet, respectively.

When a plunging jet impinges into a pool of liquid, air bubble entrainment takes place if the inflow velocity exceeds a threshold velocity. This study investigates air entrainment and bubble dispersion in the developing flow region of vertical circular plunging jets. Three scale models were used and detailed air-water measurements (void fraction, bubble count rate, bubble sizes) were performed systematically for identical inflow Froude numbers. The results highlight that the modelling of plunging jet based upon a Froude similitude is affected by significant scale effects when the approach flow conditions satisfied We1 1E+3. Bubble chord time measurements showed pseudo-chord sizes of entrained bubbles ranging from less than 0.5 mm to more than 10 mm with an average pseudo-chord size were between 4 and 9 mm. However bubble size data could not be scaled properly.

At plunging breakers, air bubbles are entrained at the impingement of the water jet, formed at the top of the wave, with the water free surface in front. During the present study, air bubble entrainment at a pseudo-plunging breaker was investigated at near full-scale and further experimental work studied the bubble detrainment process. Experimental observations included the generation and propagation of waves downstream of the plunge point. Experimental results highlighted a number of unsteady air-water flow patterns and emphasise high levels of aeration: i.e., depth-averaged void fraction of more than 10% next to jet impact in shallow waters. Unsteady bubble injection experiments showed a strong vortical motion induced by the rising bubbles. Altogether, the results suggest that a dominant time scale is the bubble rise time d1/ur, which cannot be scaled properly with an undistorted Froude model. The study contributes to a better understanding of unsteady bubble entrainment at a pseudo-plunging breaker and the associated vortical circulation.

Hamanako is an economically and culturally important tidal lake on the Pacific coast of Honshu, Japan. Between 1954 and 1973, entrance works were constructed to regulate the channel to the lake. In...

In this study, a simple air-bubble model was developed for unidirectional, homogeneous, plunging-wave propagation over a sloping beach to calculate wave height and wave setup in the surf zone. Two parameters used in the model, the reference void fraction and the decay factor, were determined by comparing model results with experimental results, and they are functions of the local wave height and the relative distance from the breaking point, respectively. Moreover, a free parameter of the model was calculated using the best fit of a curve based on experimental data from different flow regimes of plunging wave breaking. The results of the air-bubble model were compared with laboratory data and the computed results showed good agreement, except for some discrepancies in the vicinity of the breaking point and near the shoreline.

Field measurements of chlorophyll concentration and other water quality indexes are carried out for several times in Mikawa Bay, one of eutrophyic inner bays. The data are compared with LANDSAT TM data and the possibility of remote sensing from satellite images is discussed. An algorithm is proposed for the prediction of chlorophyll concentration, where bands 1 and 3 are used and atmospheric correction is made by considering effects of aerosol concentration.

A series of model test is carried out to investigate the critical condition of the occurence of impulsive tension in offshore oil booms moored slack in waves and curre nt. The critical condition, at which the maximum tension suddenly increases with con trol parameters changing, is discussed in terms of representative dimensionless parame ters such as gap ratio or Froude number.A numerical model is proposed for predicting the critical conditoin and compared with the experiments.

A numerical model is proposed for the estimation of wave transformation characteristics and motions of a pile-supported floating breakwater. The model takes account of friction force between rolllers and piles, where nonlinear Coulomb friction is linearized by using equivalent damping coefficients. Experimental results are compared with the calculation results, which shows significant wave energy dissipation and attenuation of floating body motion because of the friction effect.

Two kinds of resonant oscillation due to transient ship-generated waves are studied from the viewpoint of dynamic response of the mooring system, where ship-generated waves are idealized as a wave packet. Short periodic resonance with the same period as ship-generated waves is discussed compared with steady-state response due to monochromatic waves. Long periodic resonance due to slowly-varying wave drift force associated with grouped waves is also investigated including transient free oscillation of the system.

A new numerical model of regional sediment transport and barrier elongation, supplied by sediment coming from longshore sediment transport (LST), was developed. The spit growth and barrier elongation model was based on the model proposed by Kraus (1999). The LST rate used as input for the spit growth and barrier elongation model was simulated by the one-line model of shoreline change (Hanson, 1987) combined with the inlet reservoir model (Kraus, 2000; Larson et al., 2006). The model was applied to field data from the south coast of Long Island, United States. The simulation results were compared with measurements of the annual net LST rate and the barrier elongation. Overall, the model simulations were in good agreement with the measured data. © 2011 ASCE. (Less)

A new procedure for estimating and improving harbour tranquillity is first presented where the tranquillity is evaluated through the analysis of ship motion in a harbour. A numerical model for the ...

本論文は, 港内静穏度の評価法として, 従来の波高のみによる評価法でなく, 係留船舶の動揺量から評価する新しい手法を論じたものである. そして港内係留船の動揺算定法を示し, その手法の妥当性を水理実験で検証するとともに, その計算法を用いて, 船舶動揺の低減化対策のための港側の対策構造物の効果について考察している.

Air bubble entrainment at plunging jet takes place when the jet impact velocity exceeds a critical velocity function of the inflow conditions. This study investigates scale effects affecting air entrainment and bubble dispersion at vertical circular plunging jets. Three scale models were used and detailed air-water measurements were performed systematically for identical Froude numbers. The results highlight significant scale effects when We1 < 1E+3 or V1/ur < 10. Bubble chord times were also measured and presented in terms of pseudo-bubble chord length which was found to overestimate real bubble chords by 10 to 30%. The data show pseudo-bubble chord sizes ranging from less than 0.5 mm to more than 10 mm. The average pseudo-chord sizes were between 5 and 7 mm.

Three events of storm surge which yielded different tidal departures in the head of Mikawa Bay, Japan are compared and discussed from the viewpoint of response to the wind forcing. The mechanism of sudden sea level rise is investigated based on the data of wind, sea level and velocity obtained in the period of three different Typhoons which passed near the bay. The magnitude of preceding east wind which gives rise to the sea level in Ise Bay and quick inversion in wind direction from east to west are key factors to the development of large tidal departure in the head of Mikawa Bay. The propagation of surge waves as a free long wave is confirmed through the velocity profile data and also tested using a 2D simple numerical model.