The proceedings of the second international symposium on gas transfer at water surfaces, ‘AIR-WATER MASS TRANSFER’, which contains two keynote lectures and fifty eight technical papers, is summarized to present the state of the art on the subject.

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The self-tuning control (STC) theory, which is a stochastic adaptive control theory, is applied for the real-time control of an estuary barrage gate to achieve a certain target water level in the reservoir dammed by the gate. The Kalman filter is used to estimate in real time the parameters of the system. Firstly, the optimal control of the estuary barrage system is generally formulated using the STC theory. Secondly, to demonstrate the effectiveness and to study the use of this method, it is applied to synthetically generated data and to an experimental open channel gate system. As a result the STC strategy has been found to be well suited for the real-time optimal control of an estuary barrage gate. Characteristics of the STC theory to control a barrage gate are presented and discussed.

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Analytical model on the two-dimensional density front with suspended solid material is presented. The fundamental equations for suspension fronts are derived from the mass conservation of fluids and solid materials, and the momentum equation. The model can express the simple density front caused by the dissolute materials in a special case. The numerical calculations are carried out to simulate the saline density fronts and compared with the experimental results. In the calculation, the drag coefficient c_D is chosen as a parameter. The agreement between the model and the experimental results is fairly well if we choose the likely value of c_D=0.5. Finally, the powder snow avalanche is simulated by the presented model. The characteristics of powder snow avalanches are depicted well.

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The behaviours of river plumes with the densimetric Froude numbers of 1 to 2 are examined by a three-dimensional, primitive numerical model with a free water surface. Both the stratification due to buoyancy and lateral spreading have effects on river plumes just at the river mouth to produce the flow acceleration and the divergence of river water. Their phenomena are peculiar to river plumes with small densimetric Froude numbers. Numerical experiments confirm the appearance of both phenomena and make clear that the small gradient of water surface plays an important role on such phenomena.

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The organized structures of turbulent flow at the air-plant interface have been investigated numerically using a newly derived Large Eddy Simulation formulation. The effect of leaf drag is expressed by a term added to the Navier-Stokes equation averaged over grid-scale. The effect of wake production and dissipation due to leaves are included in Sub Grid Scale turbu- lence energy equation.
The three dimensional organized structures composed of spanwise vortices (rolls) and streamwise vortices (ribs) have been simulated. These eddy structure can also account for some statistical features of turbulence within and above plant canopies observed in field.

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The predominant energy of swash oscillations has been observed to concentrate at frequencies lower than the frequency band of the incident waves. The reasons for the frequency shift are examined in laboratory experiments and numerical simulations based on one-dimensional, time-dependent continuity and momentum equations. The experimental and numerical results show that the interaction between uprush and backrush causes low-frequency swash oscillations in addition to low-frequency standing or edge waves. Bore-bore capture in the surf zone is observed to be rare and negligible. The low-frequency swash oscillations caused by the uprush and backrush interaction are determined by the relative magnitude between incident individual and long waves, and may need to be considered in interpreting low-frequency swash oscillations measured on beaches.

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The problem of second-order interaction of plane monochromatic incident water waves with two-dimensional structures of arbitrary form in finite water depth has been solved using the integral-equation method based on Green's theorem. The second-order double-frequency diffraction potential is obtained explicitly. It is shown that, to meet the nonhomogenous free surface condition, the contribution from the integration with respect to two imaginary vertical lines at infinity on both sides of a structure must be included in the integral equation. The present approach has been applied to a two-dimensional sumberged breakwater to investigate higher harmonic generation. Extensive convergence tests are made for the integration over the free surface. Numerical results are obtained for the second harmornic amplitude oscillation over the submerged breakwater.

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The effect of temperature on the kinetic characteristics of acidogenesis in anaerobic digestion was investigated using anaerobic chemostat-type reactor to which soluble starch was fed as a substrate. The digestion temperature ranged from 15°C to 30°C. It became clear that the relationship between the substrate concentration and specific substrate utilization rate at same temperature is suitably expressed by the Contois kinetic model. The maximum specific substrate utilization rate (ν_max) increased, as the temperature increased, while the saturation constant (κ_sν) and the growth yield (Y) decreased. The temperature characteristics of the kinetic constants are expressed using exponential equations. The extent of temperature dependence of the kinetic constants decrease in the order of ν_max>Y>κ_sν. The simulation model to express the temperature characteristics of the acidogenesis is obtained in the temperature range from 20°C to 30°C.

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