LASER INDUCED FLUORESCENCE MEASUREMENTS OF CARBON DIOXIDE DISSOLUTION IN WAVE-BREAKING TURBULENCE

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LASER INDUCED FLUORESCENCE MEASUREMENTS OF CARBON DIOXIDE DISSOLUTION IN WAVE-BREAKING TURBULENCE Yasunori Watanabe 1, Junichi Otsuka 2 and Ayumi Saruwatari 3 Spatial distributions of dissolved

1 LASER INDUCED FLUORESCENCE MEASUREMENTS OF CARBON DIOXIDE DISSOLUTION IN WAVE-BREAKING TURBULENCE Yasunori Watanabe 1, Junichi Otsuka 2 and Ayumi Saruwatari 3 Spatial distributions of dissolved carbon dioxide in aerated, strong turbulent flows due to wave breaking are experimentally studied by using image measurements on the basis of Laser Induced Fluorescence (LIF) in this paper. Physical mechanism of CO2 transport from air to surf zone water is explained in this study high concentration of C0 2 is dissolved into water via strong mixing of wave surfaces and entrainment of air bubbles after wave breaking, and dissolved CO2 rapidly spreads to wide area due to strong turbulent diffusion. While there is a high correlation of C0 2 concentration and image intensity of the entrained bubbles in a transition region where active jet splashing is undertaken, the mean CO2 concentration is much less than that in a bore region where fully developed turbulence significantly diffuses CO2 within a shallower water region. INTRODUCTION Air-sea gas transfer at equilibrium wave/wind state has been conventionally estimated using a simple model parameterized by only wind velocity over ocean (e.g. Liss and Merilvat 1986). However, local gas dissolution and transfer depend on other wave-breaking induced factors: turbulent intensity, breaker type, aeration effects etc, which are excluded in the conventional models. Toba and Koga (1986) presented an important role of wave breaking to affect air-sea gas and material exchanges in deep ocean. Komori et al. (1989) found that sub-surface turbulence disturbs and renews the air-water interface dissolved gas to entrain to depths. This surface renewal is a major process to induce downward gas flux under braking waves which significantly produce strong turbulence in the vicinity of the surface (Watanabe and Mori 2008). Another major factor enhancing the gas transfer is aeration in breaking waves. Numbers of air bubbles entrained through a wave splashing process significantly increase total area of air-sea interfaces per unit volume and enhance the gas dissolution into sea. For accurately estimating local C0 2 flux across the air-sea interface, dissolution and transport processes of CO2, which are also subjected to physical process of wave breaking, has to be understood. Mori (2004) presented capabilities of two-color laser induced fluorescence (LIF) image measurement that visualizes spatial distributions of dissolved C0 2 with dynamic correction of laser attenuation. In this study, the two-color LIF was applied to breaking waves for finding roles of the entrained air bubbles and 1 School of Engineering, Hokkaido University, North 13 West 8, Sapporo, , Japan 2 Cold Region Engineering Research Institute, Hiragishi 1-3-1, Sapporo, , Japan 3 School of Engineering, Hokkaido University, North 13 West 8, Sapporo, , Japan 51

52 COASTAL ENGINEERING 2008 8.0 m grabbers generator Figure 1. Experimental setup. wave-breaking induced turbulence to affect the dissolved CO2 concentration.

2 52 COASTAL ENGINEERING m grabbers generator Figure 1. Experimental setup. wave-breaking induced turbulence to affect the dissolved CO2 concentration. A final goal of the study is to develop an appropriate model describing quantitative dissolved C0 2 and the transfer velocity in near-shore breaking wave field. EXPERIMENTAL PROCEDURE Experimental Setup Experiments were performed in a wave flume of 8 m in length, 0.6 m in depth and 0.25 m in width, sloping at 1:20, with transparent acrylic walls, bottom and cover (see Fig. 1). Waves were generated by a piston-type wave generator set up at one end of the wave flume. A compressed CO2 cylinder was connected to the top cover of the flume for providing CO2 in gas phase over wave interfaces. Three 8-bit digital video cameras (Kodak ES1.0) each equipped with different optical filters were placed in front of the transparent sidewall to be focused on the same measurement area where illuminated by a YAG laser sheet emitting from the transparent flume bottom. Resolution of the camera was 1024 x 1024 pixels, and acquisition frequency was 29.5 Hz. The cameras were connected to image grabbers (Coreco PC-DIG) installed in a personal computer (Pentium 4,2.0 GHz), which were controlled to be synchronised. Image acquisition was triggered by a TTL signal generated when a wave gage placed in front of a wave paddle detected the first generated wave (see Fig. 1). The captured images were stored in a computer during the measurement, and they were then converted into 8-bit bitmap image files by using image converting software (CTC ViewFinder). Another wave gage was set at a measurement site to be triggered by the same signal with the image acquisition system. Simultaneous velocity and wave fields can be obtained at the same site by the synchronised images and wave records. The measurements were performed at three locations of water depth, h=\5.5 cm (Site 1), 14.0 cm (Site 2) and 10.0 cm (Site 3) (see Fig. 2). Experimental waves breaking at /i&=17.5 cm have plunged at Sitel, and typical splash-up jets

COASTAL ENGINEERING 2008 53 Figure 2. Measurement locations. are observed at S2, and then turbulent bores propagated in a bore region (Site 3). The breaking wave height and wave period were 13.

3 COASTAL ENGINEERING Figure 2. Measurement locations. are observed at S2, and then turbulent bores propagated in a bore region (Site 3). The breaking wave height and wave period were 13.6 cm and 1.43 s, respectively. Two-Color Laser Induced Fluorescence In this study, two-color LIF image measurements (Mori 2004) were performed in a regular breaking wave field. When C0 2 dissolves in water, there are three forms of dissolved inorganic carbon (DIC) in water: aqueous CO2, bicarbonate HCO^" and carbonate ion COg~. HCO^~ dominate DIC in a range of ph of ocean. Since fluorescent intensity of dye 'Uranine' used in this experiment is inversely proportional to amount of HCO^~, spatial distributions of DIC concentration can be quantitatively obtained through the image measurement of the fluorescent intensity of the dye excited by the YAG laser sheet, when a spatial distribution of the image intensity due to spatial attenuation of the laser sheet is eliminated from the measured results. In the two-color LIF, another florescent dye 'Rhodamine B', of which the fluorescence intensity is independent on CO2 concentration, is used to dynamically correct the laser attenuation. While a fluorescence distribution is affected by both of the CO2 concentration and the laser attenuation in the image for Uranine, the image for Rhodamine B indicates only the distribution of fluorescence that is proportional to the laser intensity. Therefore, using a rate of the image intensity of Uranine to Rhodamine B, the effect of the attenuation can be canceled out and the net intensity indicating the C0 2 concentration can be acquired. The fluorescence intensities of the both dyes were recorded by two cameras attached with optical band-pass filters that pass fluorescent wavelengths of 560nm for Uranine and 580nm for Rhodamine B, respectively (see Fig. 3). The Rate of the both fluorescent intensity images was preliminary calibrated with the CO2 concentrations measured by a glass electrode CO2 meter (see Fig. 4). An optical band pass filter passing wavelength 532nm for YAG laser light was equipped with another camera to record reflected laser lights from bubbles for

4 54 COASTAL ENGINEERING 2008 rag Lase' 3 v- CD a to 532 nm 560nm >580nm Figure 3. Spectra of fluorescent lights and band-pass filters used in the experiment. 600 _500 O& [ Intensity rate Figure 4. A calibration curve for the dissolved C0 2 concentration and the rate of two fluorescent intensity images. finding a correlation of bubble locations and C0 2 distribution (see Fig. 1). RESULTS Figure 5 shows the vertical distribution of dissolved C0 2 concentration in a still water after providing C0 2 over the surface. It is found that C0 2 slowly diffuses downward, and the mean concentration gradually increases. The approximate transfer velocity is roughly estimated to be less than O(10~ 4 cm/s). Both of the concentration and transfer velocity significantly increase under breaking waves (see Fig. 6). High concentration of C0 2 is found, from a comparison with a visible image of the laser reflection from bubbles, to appear within the aerated breaking wave front (see phases (a) and (b) in Fig. 6), while the concentration bellow the front rapidly diffuses due to strong turbulence. After the

COASTAL ENGINEERING 2008 55 (a) 5 min later (b) 10 min later (c) 15 min later (d) 30 min later 5 10 0 5 10 0 5 10 "0 5 ppm ppm ppm ppm Figure 5.

5 COASTAL ENGINEERING (a) 5 min later (b) 10 min later (c) 15 min later (d) 30 min later "0 5 ppm ppm ppm ppm Figure 5. Vertical distributions of dissolved C0 2 concentration in a still water. (a) Plunging phase (b) 0.20s later from (a) (c) 0.58s later from (a] (52 ISO ~?5«154»52 150!«1«^0 154 J52 tsq x (cm} x (cm) x (cm) (ppm) Figure 6. Visible images of laser reflection from the entrained bubbles (top) and distributions of C0 2 concentration (bottom) at Site 3. wave front has passed, high concentration of C0 2 locally distributes in a region

6 56 COASTAL ENGINEERING 2008 B -8 N L 1 : "? > > V Jt } ^ V > "^ 5 ^^^r 4^" Gc w> ' f < Site 1 Site 2 Site 3 1, correlation coefficient Figure 7. Correlation between the image intensity of reflection lights from bubbles and the C0 2 concentration under a wave trough level. where air bubbles entrained underneath a rear part of the wave, indicating C0 2 is dissolved from the air bubbles (see phase (c) in Fig. 6). Since the bubbles involved in the vortices are trapped within them over long time (Watanabe et al. 2005), the bubble entrainment contributes to enhance the C0 2 concentration in a deeper water region. Figure 7 shows the correlation between the image intensity of reflection lights from bubbles and the CO2 concentration under a wave trough level at each site. High correlations were deserved in an aerated region (z >-9 cm) in a transition region (Site 2) and bore region (Site 3), which quantitatively indicates the bubble entrainment has a role to transfer C0 2 to depths. On the other hand, there is no correlation in a plunging region (Site 1) where entrained bubbles are much fewer than the other region. Figure 8 shows the temporal variations of the vertical distributions of C0 2 concentration. It has been found there are two mechanisms for transferring C0 2 in breaking waves. The fluid with high concentration in the highly aerated wave front is rapidly diffused downward in relatively short duration (less than 0.2s) by strong turbulence produced bellow the front, which typically appears at an early stage of wave breaking process (Site 1 and 2, see a solid circle in Fig. 8). It is known that breaking waves produce three-dimensional vortex structures organized by horizontal rollers and obliquely descending vortices in the splashing process (Watanabe et al. 2005). The later vortices are longitudinally developed under a rear part of the waves. The entrained bubbles trapped within these vortices are restrained to buoyant over long time. Therefore, high C0 2 concentration appearing under a rear part of waves over relatively long time (0.6s or more)

COASTAL ENGINEERING 2008 57 0 2 4 6 8 Ms) 0 10 20 30 40 50 ppm Figure 8. Temporal variations of the vertical distributions of C0 2 concentration at Site 1 (top), Site 2 (middle) and Site 3 (bottom).

7 COASTAL ENGINEERING Ms) ppm Figure 8. Temporal variations of the vertical distributions of C0 2 concentration at Site 1 (top), Site 2 (middle) and Site 3 (bottom). at Site 2 and 3 is caused by C0 2 dissolution from the bubbles involved in the obliquely descending vortices (see a broken circle in Fig. 8). The transfer velocity in a transition region was roughly estimated to be in O(10 cm/s) which is over 10 5 times faster than molecular diffusion velocity measured in the still water test shown in Fig. 5. The time records of surface elevation, spatial mean C0 2 concentration and image intensity reflected from the entrained bubbles are shown in Fig. 9. Intermittent increase of the mean concentration is observed at the arrival phase of successive breaking waves in the transition region, while there is no major temporal change in mean concentration. The mean concentration in the bore region gradually increases in time over every onsets of breaking waves, since water volume at shallow water depth is smaller than that at the other deeper sites, and therefore the concentration can easily increase if same downward gas flux is provided into water. CONCLUSIONS Concentrations of dissolved carbon dioxide under breaking waves have been

8 58 COASTAL ENGINEERING 2008 Time(s) Figure 9. Time records of surface elevation (top), spatial mean C0 2 concentration (middle) and image intensity of laser reflection from the entrained bubbles (bottom). measured using two-color laser induced fluorescence. Advection, diffusion and dissolution of CO2, which rapidly changes in time due to turbulence and aeration caused by wave breaking, can be visualized in this image measurement. Distributions of air bubbles entrained under breaking waves highly correlate with the concentrations of the dissolved CO2, which indicates amount of the entrained bubbles is a major factor to determine CO2 transfer into sea. The gas transfer velocity in a transition region, where breaking jets consecutively produce strong turbulence causing significant convection and diffusion, is over 10 5 times faster than that in a still water. The mean CO2 concentration rapidly increases in a bore region where CO2 entrained by aerated turbulent bore is dissolved in small volume of water at shallow water depth. ACKNOWLEDGMENTS Financial support for this study is provided by JSPS Grants-in-Aid Scientific Research ( ). REFERENCES Komori, S., Murakami, Y. and Ueda, H The relationship between surface-renewal and bursting motions in an open-channel flow, Journal of Fluid Mechanics, 203, Liss, P. S. and Merilvat, L The role of air-sea exchange in geochemical cycling, Air-sea gas exchange: introduction and synthesis,,dordrechkreidel,

COASTAL ENGINEERING 2008 59 Mori, N. 2004. Two-color LIF-PIV for gas exchange at air-water interface, PIV and Water Waves,, World Scientific, 291-293. Toba, Y. and Koga, M. 1986.

9 COASTAL ENGINEERING Mori, N Two-color LIF-PIV for gas exchange at air-water interface, PIV and Water Waves,, World Scientific, Toba, Y. and Koga, M Oceanic Whitecaps,Dordrecht:Reidel, Watanabe, Y. and Mori, N Infrared measurements of surface renewal and sub-surface vortices in near-shore breaking waves, Journal of Geophysical Research, 113, C07015, doi: /2006jc Watanabe, Y, H. Saeki, and R. J. Hosking Three-dimensional vortex structures under breaking waves, Journal of Fluid Mechanics, 545,

MEASUREMENT OF DISSOLVED CARBON DIOXIDE CONCENTRATION IN A SURF ZONE

MEASUREMENT OF DISSOLVED CARBON DIOXIDE CONCENTRATION IN A SURF ZONE Junichi Otsuka 1, Yasunori Watanabe 2 and Aumi Saruwatari 3 In this stud, we measured dissolved carbon dioide (D-CO 2) concentration