Tests of Acoustic Target Strength and Bubble Dissolution Models Using a Synthetic Bubble Generator

Micro-fabricated electrolytic micro-bubblers

Fässler, S. M. M., Brierley, A. S., and Fernandes, P. G. 2009. A Bayesian approach to estimating target strength. – ICES Journal of Marine Science, 66: 1197–1204. Currently, conventional models of target strength (TS) vs. fish length, based on empirical measurements, are used to estimate fish density from integrated acoustic data. These models estimate a mean TS, averaged over variables that modulate fish TS (tilt angle, physiology, and morphology); they do not include information about the uncertainty of the mean TS, which could be propagated through to estimates of fish abundance. We use Bayesian methods, together with theoretical TS models and in situ TS data, to determine the uncertainty in TS estimates of Atlantic herring (Clupea harengus). Priors for model parameters (surface swimbladder volume, tilt angle, and s.d. of the mean TS) were used to estimate posterior parameter distributions and subsequently build a probabilistic TS model. The sensitivity of herring abundance estimates to variation in the Bayesian TS model was also evaluated. The abundance of North Sea herring from the area covered by the Scottish acoustic survey component was estimated using both the conventional TS–length formula (5.34×109 fish) and the Bayesian TS model (mean = 3.17×109 fish): this difference was probably because of the particular scattering model employed and the data used in the Bayesian model. The study demonstrates the relative importance of potential bias and precision of TS estimation and how the latter can be so much less important than the former.

Hydrodynamic study of bubble characteristics and bubble rise velocities in batch electrocoagulation with vibration-induced electrode plates using the PIV technique

Tracking methane transport becomes more complicated in the deep ocean where seafloor release of methane gas bubbles occurs at the high pressures and low temperatures conducive to hydrate formation on bubble surfaces. Gas hydrate formation can make the bubble surface rigid, preventing the dynamic interplay between bubble size, shape and rise velocity that gas‐transport models commonly rely upon when using bubble size to predict bubble rise velocity. To better constrain gas‐transport model predictions, we conducted controlled laboratory measurements of rise velocity, , for hydrate‐free air, methane, and xenon bubbles and hydrate‐coated xenon bubbles. Experimental results for were compared to predicted values from several published parameterizations used to study dissolution of gas bubbles rising in the ocean. For both hydrate‐free and hydrate‐coated gas bubbles, the McGinnis et al. (2006), https://doi.org/10.1029/2005jc003183 parameterization provides the most accurate predictions.

Bubble size distribution and gas–liquid mass transfer in airlift contactors

In order to clarify the microscopic flow structure, the ultrasonic Doppler method was applied to the measurement of two-phase bubbly flow in vertical pipe (i.d.50mm). Liquid flow structure might strongly be influenced by the characteristic of the injected bubbles, i.e. bubbles’ size and void fraction. In this study, a bubble generator was newly designed with the purpose to control the bubble size and void fraction, independent of liquid main-flow rate. The experiment was performed at z/d = 66 from the bubble generator. Liquid flow rates were of the Reynolds numbers ranging from Rem = 3700 to 6200. The gas flow rate was constant at JG = 0.00348(m/s) at the measurement position. By analyzing the bubbles’ picture, it was confirmed that bubble size distribution and average bubble size were almost constant if the liquid flow rate were changed. The ultrasonic Doppler method has the capability of measuring the instantaneous velocity profiles of both phases at the same time. By processing the data based on pattern recognition, the recorded data can be classified to several groups. Using this method, the authors have tried to measure the bubbly flow in rectangular channel. In the present study, the application of this method to bubbly flow in circular pipe was satisfactory to obtain the liquid velocity distribution in bubbly flow and surrounding bubbles. From these results, it was clarified that velocity profile in bubbly flow in circular pipe has a maximum value near the pipe wall. Furthermore, velocity profiles around the bubble are influenced by leading bubbles.

Redlip mullet (Chelon haematocheilus) is distributed in coastal waters of the North-Western Pacific Ocean and is a cultured fish in Korea. A hydroacoustic technique constitutes a useful method to assess the biomass and spatial distribution of mullet in sea cages or in coastal waters, and acoustic target strength (TS) information of the target fish is an essential parameter in using this method. In this study, ex situ TS measurements of 16 live mullets were made in an aquaculture sea cage in Korea. The split-beam scientific echo-sounder used for measurements was comprised of 38, 120, 200, and 420 kHz frequencies. An underwater video camera was simultaneously used to observe the mullets’ behavior during the TS measurements. The mullet TS data was analyzed from a wide range of total fish length (FL: 14.3–40.3 cm). As results for all frequencies, the frequency dependence of the mean TS values were relatively low, and the difference in mean TS was within 2.5 dB. When the slope of the least-squares regression line was forced to 20 into the TS equation, the resulting value for the constant term (b20) at each frequency was −67.0 dB, −68.3 dB, −66.3 dB, and −68.5 dB, respectively. The data tended to be frequency dependent. Additionally, the maximum TS appeared between tilt angles of 0° and 10°. These results indicate that TS measurements can be applied to estimate the biomass of the mullet in sea cages or in coastal waters.

Pre- and post-spawning capelin from the Barents Sea stock were observed in a net pen with a calibrated 38 kHz scientific split-beam echosounder. The transducer was positioned vertically or tilted. The acoustic target strength (TS) of capelin depended on swimbladder length. In female capelin within 15.5–18.0 cm length the logarithm of weight was significantly negatively related to TS. The negative effect of weight on TS could be due to a higher condition factor (assumed higher fat content), which gave the fish additional buoyancy and less need for swimbladder volume. The effect of gonad weight on TS was not significant. Comparisons of measurements made with vertical and tilted transducers demonstrated a small but significant effect of tilt angle on TS.

Knowledge of the acoustic target strength (TS) of capelin, Mallotus villosus (Müller), is important to scale integrator outputs for use in acoustic surveys, but experimental measurements of capelin TS are scarce. This paper reports a new ex situ experiment on capelin TS. Small groups of the fish were kept in a large sea cage that allowed them to swim freely between 4.5 and 10 m deep. TS measurements were made vertically with a scientific split-beam echosounder. Differences in behaviour could induce at least a 5 db change in TS owing to changes in tilt angle. The observed mean TS for capelin from the local Balsfjord stock was >1 db higher than expected from the TS–length relationship for Barents Sea capelin used during abundance estimation.

Calculations of interactions of gas bubbles with glass liquids containing sulphates

A 1/4th scale low temperature electrolytic model of the Hall-Heroult cell was constructed to investigate the electrolytic bubble formation mechanism, coalescence and movement under the horizontal anode surface. Geometric and dynamic similarity between the model and real cell was maintained through using similarity criteria. A 0.28M CuSO4+20%H2SO4 solution was selected as an electrolyte where Cu was deposited at the cathode and O2 bubbles were generated underneath the anode, similar to the phenomena of real cell. The bubble generation mechanism, movement, coalescence and detachment under the electrolytic medium were observed using a high speed camera. It was found that electrolytic bubbles generate uniformly under the whole anode surface and grow through gas diffusion and coalescence. At higher current density and higher anode inclination angles, bubbles escape quickly from underneath the anode surface. The bubble layer thickness and bubble sizes were also found to decrease with an increase in anode inclination angle.

A coupling model for gas diffusion and seepage in SRV section of shale gas reservoirs

Influence of viscosity and surface tension on bubble dynamics and mass transfer in a model bubble column

The mechanism of bubble generation using a slit elastic tube and an acoustic pressure wave in the gas phase

Mechanism and modeling of Taylor bubble generation in viscous liquids via the vertical squeezing route