Naohisa Takagaki

Mass transfer velocity across the breaking air–water interface at extremely high wind speeds

Mass transfer velocity k L across the wind-driven air–water interface was estimated at extremely high wind speeds (up to U 10=70 m s−1) in a high-speed wind-wave tank by measuring changes in CO2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. k L increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of k L changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. k L is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present k L agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999) and Wanninkhof et al. (2009) for low and moderate free-stream wind speeds. However, for extremely high free-stream wind speeds, the present data deviate upward from the correlation curves of Wanninkhof (1992) and Wanninkhof and McGillis (1999) and approach to that of Wanninkhof et al. (2009) as the wind speed increases. This indicates that the correlation curve of Wanninkhof et al. (2009) is more appropriate for the correlation between k L and free-stream wind speed than those of Wanninkhof (1992) and Wanninkhof and McGillis (1999) in extremely high wind speed region.

Effects of turbulent eddies and Langmuir circulations on scalar transfer in a sheared wind-driven liquid flow

The effects of turbulent eddies and Langmuir circulations in liquid flow on scalar transfer across a sheared wind-driven gas-liquid interface are investigated by means of a direct numerical simulation of a gas–liquid two-phase turbulent flow with a wind-driven nonbreaking wavy interface. The wind-driven wavy gas-liquid interface is captured using an arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids. The results show that Langmuir circulations are generated on the liquid side below the sheared wind-driven gas-liquid interface. The marker particles on the gas-liquid interface, the turbulent eddies in the form of streamwise vortices on the liquid side (i.e., the typical horseshoe vortices associated with bursting motions), and the low scalar flux lines on the gas-liquid interface induced by the turbulent eddies on the liquid side tend to locally concentrate in the regions along the downward flows caused by the Langmuir circulations. It is suggested that the turbulent eddie...

Estimation of friction velocity from the wind-wave spectrum at extremely high wind speeds

The equilibrium range of wind-waves at normal and extremely high wind speeds was investigated experimentally using a high-speed wind-wave tank together with field measurements at normal wind speeds. Water level fluctuations at normal and extremely high wind speeds were measured with resistance-type wave gauges, and the wind-wave spectrum and significant phase velocity were calculated. The equilibrium range constant was estimated from the wind-wave spectrum and showed the strong relationship with inverse wave age at normal and extremely high wind speeds. Using the strong relation between the equilibrium range constant and inverse wave age, a new method for estimating the wind speed at 10-m height (U 10) and friction velocity (u*) was proposed. The results suggest that U 10 and u* can be estimated from wave measurements alone at extremely high wind speeds in oceans under tropical cyclones.

Mechanism of drag coefficient saturation at strong wind speeds

Previous studies [Powell et al., 2003; Donelan et al., 2004; Takagaki et al., 2012] have demonstrated the saturation of drag coefficients at strong wind speeds. But, the mechanism behind this saturation has not yet been fully clarified. In this study, at normal and strong wind speeds, we use a wind wave tank for investigating the peak enhancement factor of the wind-sea spectrum, which is an appropriate wave parameter for representing interfacial flatness. We measured the water-level fluctuation using wave gauges. At strong wind speeds, the result shows that the peak enhancement factor of the wind-sea spectrum decreases with decreasing inverse wave age and with increasing wind speed. This suggests that the distinctive wind-wave breaking occurs at strong wind speeds. It also suggests that this distinctive breaking of wind waves causes the saturation of drag coefficients at strong wind speeds.

The Effect of Raindrops on Interfacial Turbulence and Air-Water Gas Transfer

The effects of impinging raindrops on both turbulence below the airwater interface and CO2 transfer across the air-water interface are discussed using laboratory measurements by Takagaki and Komori [1]. The measurements of CO2 absorption rate and turbulence quantities in an open-channel flow show that impinging raindrops enhance both turbulent mixing near the free surface on the liquid side and CO2 transfer across the air-water interface, and that the mass transfer velocity due to impinging raindrops is well correlated with the mean vertical momentum flux of raindrops. The reason why the mass transfer velocity is well correlated by the mean vertical momentum flux is explained by showing the instantaneous velocity vectors induced by a falling single droplet. Further, in order to clarify the effects of rainfall on the global and local CO2 transfer across the air-sea interface, the mean annual net air-sea CO2 flux was estimated using both the daily precipitation data set and the empirical correlation [1] between the mass transfer velocity and mean vertical momentum flux. The rainfall effects are also compared with wind shear effects. The results show that rainfall effects are significant for the local CO2 budget between atmosphere and ocean in equatorial and mid-latitude regions, but are not so important for global budget, compared to the wind shear effect.

Effect of Schmidt number on mass transfer across a sheared gas-liquid interface in a wind-driven turbulence

The mass transfer across a sheared gas-liquid interface strongly depends on the Schmidt number. Here we investigate the relationship between mass transfer coefficient on the liquid side, kL, and Schmidt number, Sc, in the wide range of 0.7 ≤ Sc ≤ 1000. We apply a three-dimensional semi direct numerical simulation (SEMI-DNS), in which the mass transfer is solved based on an approximated deconvolution model (ADM) scheme, to wind-driven turbulence with mass transfer across a sheared wind-driven wavy gas-liquid interface. In order to capture the deforming gas-liquid interface, an arbitrary Lagrangian-Eulerian (ALE) method is employed. Our results show that similar to the case for flat gas-liquid interfaces, kL for the wind-driven wavy gas-liquid interface is generally proportional to Sc−0.5, and can be roughly estimated by the surface divergence model. This trend is endorsed by the fact that the mass transfer across the gas-liquid interface is controlled mainly by streamwise vortices on the liquid side even for the wind-driven turbulence under the conditions of low wind velocities without wave breaking.

Laboratory Measurements of Heat Transfer and Drag Coefficients at Extremely High Wind Speeds

AbstractHeat and momentum transfer across the wind-driven breaking air–water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank. An original m...

Loop-Type Wave-Generation Method for Generating Wind Waves under Long-Fetch Conditions

AbstractIt is important to develop a wave-generation method for extending the fetch in laboratory experiments, because previous laboratory studies were limited to the fetch shorter than several dozen meters. A new wave-generation method is proposed for generating wind waves under long-fetch conditions in a wind-wave tank, using a programmable irregular-wave generator. This new method is named a loop-type wave-generation method (LTWGM), because the waves with wave characteristics close to the wind waves measured at the end of the tank are reproduced at the entrance of the tank by the programmable irregular-wave generator and the mechanical wave generation is repeated at the entrance in order to increase the fetch. Water-level fluctuation is measured at both normal and extremely high wind speeds using resistance-type wave gauges. The results show that, at both wind speeds, LTWGM can produce wind waves with long fetches exceeding the length of the wind-wave tank. It is observed that the spectrum of wind wave...

Estimation of the global air–sea CO 2 gas flux considering wave breaking

We propose a new model with microbreaking and linear dependence on wind speed in the whitecap-free area and linear dependence on the fractional area of whitecaps, based on the whitecap model proposed by Monahan and Spillane (The role of oceanic whitecap in air–sea gas exchange. In: Brutsaert W, Jirka GH (eds) Gas transfer at water surfaces. Reidel, Dordrecht, 495–503, 1984). The relationship between whitecap coverage and wind–sea Reynolds number proposed by Zhao and Toba (J Oceanogr 57:603–616, 2001) provides the link among waves, wind, and whitecap coverage. This model yields a mass transfer velocity that is linear in wind speed at light winds and smoothly transitions with increasing wind to approximately cubic in wind speed and linear in wave age.

The effects of rain on wind-driven turbulent flow

It is of great interest to investigate the effects of rain on mass transfer across the wavey-sheared air-water interface in precisely estimating the mass transfer rate across the air-sea interface. The mass transfer rate across the air-water interface changes depending on the conditions of the free surface and water flow near the free surface. The purpose of this study is, therefore, to investigate the effects of rain on wind-driven turbulence through laboratory experiments in a wind-wave tank with a rain simulator. The values of the wave height and the velocity in the liquid side were measured for both no rain case and rain case (R=133mm/h), and we compared the values in no rain case with that in the rain case. Figure 1 shows the significant wave height (H s ) against the free stream wind speed (U ~). It is found that Hs decreases due to rain at low wind speeds, because of the damping effect noted by Tsimplis and Thorpe[1]. However there is no rain effect on Hs at high wind speeds. Figure 2 shows the vertical distributions of the streamwize and vertical mean velocities (U w , V w ), the streamwize and vertical turbulence intensities (\( u_w^\prime ,v_w^\prime \)) and the Reynolds stress (\( \overline {u_w v_w } \)) in the liquid side. These values are increased by rain near the free surface. This suggests that the turbulence near the free surface in the liquid side is promoted by the impingement of raindrops.