P. Bowyer

A Study on the Temperature Variation of Rise Velocity for Large Clean Bubbles

Abstract A series of microphysical laboratory experiments studying the hydrodynamics of single bubbles were conducted to measure the variation of rise velocity, VB, with temperature, T, and radius, r. Bubbles with an equivalent spherical radius between 377 and 4500 μm were studied for T varying between 0° and 40°C. While for nonoscillating bubbles VB increases with T; due to the significance of oscillations, VB decreases with T for oscillating bubbles, in conjunction with an increase in trajectory oscillations with T. Using observations from this study and data from other researchers, a three-part parameterization of VB(r, T) is proposed with transitions at Re = 1 and the onset of oscillations, where Re is the Reynolds number. The T for the transition to oscillatory behavior was found to vary linearly with r. An empirical parameterization of VB(r, T) for oscillatory and nonoscillatory bubbles that correctly incorporates the effect of T is presented.

Influence of energetic wind and waves on gas transfer in a large wind–wave tunnel facility

Air–water gas exchange experiments were carried out in a large wind wave tunnel in Marseille, France, to investigate gas transfer processes under energetic wind and wave fields, where macroscale breaking waves create bubble plumes (white caps) and turbulence on the water surface. We measured the gas transfer velocities of N2O, DMS, He, SF6, CH3Br, and total air. Their diffusivity and solubility span a large range, allowing us to investigate gas transfer mechanisms under a variety of physical conditions. We observed that the gas transfer velocities varied with friction velocity in a linear manner. Gas transfer in the presence of pure wind waves is generally consistent with the surface renewal model, as the gas transfer velocity has a strong dependence on diffusivity with an exponent of 0.53(±0.02). Contrary to expectations, the bubble plumes generated by breaking waves contributed relatively little in our pure wind wave experiments. Superposition of mechanically generated waves onto the wind waves in the high wind regime attenuated DMS gas transfer (as a function of friction velocity) across the air–water interface by ~20% compared with gas transfer under pure wind waves, implying suppression of gas transfer directly across the sheared water surface. Greater transfer of less soluble gases may result from bubble-mediated gas transfer.

Some Factors Affecting the Size Distributions of Oceanic Bubbles

Abstract The effects of water temperature, dissolved gas saturation levels, and particulate concentrations on the size distribution of subsurface bubbles are investigated using numerical models. The input of bubbles, either at a constant rate in a “steady-state” model or in an initial injection where the development of a bubble “plume” is followed, is kept constant. So too are the model representations of Langmuir circulation and turbulence. An increase in temperature results in a reduction of bubble numbers, a halving at 4-m depth for a 10°C rise in temperature, while an increase in saturation level of 10% increases the bubble concentrations by factors of 3 to 4 at the same depth; the shape of the distribution curves are only slightly modified. The presence of particulates tends to increase the number of small bubbles by inhibiting dissolution.

Video measurements of near-surface bubble spectra

Large bubbles >300μm radius have been observed at depths of between 10 cm and 1.2 m using a small video camera attached to a surface following float. In salt water, bubble spectra are presented for both limited fetch and open sea conditions and in fresh water for limited fetch conditions. The void fractions and areas of the observed bubbles are calculated. Measurements were made in a limited range of wind conditions (greater than Beaufort force 5) and depths (between 10 cm and 1.2 m). Video records were analyzed by eye which introduced some uncertainty in the measured size and limited the number of sampled bubbles. Bubble populations are represented by power law spectra (dndr=ar-b). Near the surface, 2 < b < 3 on average; the distribution is very patchy: near breaking waves there are relatively more large bubbles: here 1 < b < 2. Away from the surface, 3.5 < b < 5.

Gas Exchange and Bubble-Induced Supersaturation in a Wind-Wave Tank

Gas exchange and bubble-induced supersaturation were measured in a wind-wave tank using total gas saturation meters. The water in the tank was subjected to bubbling using a large number of frits at a depth of 0.6 m. A simple linear model of bubble-mediated gas exchange implies that this should force an equilibrium supersaturation of 3%. This is confirmed by experiment, but a small additional steady-state supersaturation is also forced by warming. The total steady-state supersaturation is approached asymptotically. When the bubblers were switched off, the total gas pressure approached a new steady state at much lower supersaturation, at a rate that depended on the state of the wind and waves in the tank. The rates of approach on the various equilibria enabled the gas flux across the surface of the bubbles or across the air–water interface to be calculated. In addition a series of experiments was conducted where the water was subjected to bubbling in the presence of wind or wind and paddle waves: in this case gas invasion from the bubbles was balanced by gas evasion near or at the surface resulting in an equilibrium at <3% and enabling the relative strength of the invasion and evasion to be estimated. Gas concentrations could be measured in a rapid, automated manner using simple apparatus. To derive gas fluxes, corrections for changes in water temperature and fluctuations in air pressure are necessary, and these are quantified. In addition, transient fluctuations in gas concentration at the start of bubbling periods allowed mixing within the tank to be observed.

Topographically controlled circulation and mixing in a lake

It is well known that the wind-driven circulation in a wide lake may be controlled by its topography. Wind stress acting on the smaller mass of the water column in the shallows results in downwind flow there and return flow in the deeper water. In lakes with large areas of shallow water this circulation may cause rapid mixing in weakly stratified conditions. In more strongly stratified conditions, topographically induced horizontal wind driven circulation may influence the vertical stratification of the lake. Lough Mask is about 10 km long and has a maximum width of about 8 km. The lake may be divided into shallow eastern and deeper western portions with mean depths of 10 and 35 m respectively. In late Autumn, water at depth in the deep part of the lake moves upwind at speeds of up to 50 cm s−1 in response to axial winds of up to 15 m s−1. At the same location a weaker upwind flow is observed in the near surface water. This return flow is topographically driven. Temperature differences between near surface and near bottom meters were small ( 20 m) surface mixed layer and a cooler layer at depth whose temperature fluctuates in response to wind stress along the axis of the lake. Modelling results, supported by drogue measurements, indicate that the return flow associated with axial winds in these summer conditions is confined to the surface mixed layer. If this layer is thin, shear between the surface layer and the underlying water can be sufficient to cause mixing and a thickening of the surface layer until the Richardson number is ∼1. Thus the vertical temperature structure of the deep part of the lake can be influenced by flows driven by the lake topography. In the shallow part of the lake, there is evidence of thermally driven motion in periods of calm weather in summer.

The Physics and Hydrodynamic Setting of Marine Renewable Energy

Increasing interest is apparent in marine energy resources, particularly tidal and wave. Some TeraWatts of energy propagate from the world’s oceans to its marginal seas in the form of surface waves (≈ 2 TW) and tides (≅ 2.6 TW) where that energy is naturally dissipated. The seas and coastlines around the UK and its neighbours are notable for dissipating a significant fraction of the global energy of waves (≈ 50 MW km−1 on the Atlantic coast) and especially tides (> 250 GW north of Brittany). Displacing a significant fraction of the natural dissipation by energy capture is a tempting and reasonable proposition, but it does raise technical and environmental issues. Sustainable exploitation of the energy needs to consider diverse effects on the environment, waves and tides having a role in maintaining the shelf sea, coastal, estuarine and shoreline environment through associated advection, stirring and other processes. Tides are particularly significant in controlling the stratification of shelf seas and their flow characteristics. Surface waves are more important in determining conditions nearshore and in the intertidal zone. Also, the exploitation of wave and tidal resources is only practical economically and technologically at a limited number of energetic and accessible sites, and societal and ecological considerations inevitably narrow the choice.

An Attempt to Determine the Space Charge Produced by a Single Whitecap Under Laboratory Conditions

The Whitecap Simulation Facility located in UCG has been used to generate repeatable whitecaps under controlled conditions over the last 4–5 years. Amongst other things, the electric charge transferred into the atmosphere has been measured using an Obolensky filter. The charge was found to vary with (1) time after splash of measurement; (2) intensity of splash (controlled by the height of water behind the tank gates before a splash), and (3) water temperature. These results can be combined with global results relating whitecaps and windspeed to produce a new estimate of the contribution of the ocean to the global electrical circuit. This ranges from 160A in January to 220A in June.