In this chapter we attempt to present a brief introduction to the subject of air-sea gas exchange First the basic equations governing such exchange are given, then a review of some models proposed to describe the gas transfer process Following this, experimental approaches through both laboratory (principally using wind/water tunnels) and field measurements are summarised Finally, we present what seems to us to be the best current synthesis of the wind tunnel and field results for the prediction of gas exchange rates across the sea surface

Air-Sea Gas Exchange Rates: Introduction and Synthesis

The Structure of Turbulent Shear Flow By Dr. A. A. Townsend. Pp. xii + 315. 8¾ in. × 5½ in. (Cambridge: At the University Press.) 40s.

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The Structure of Turbulent Shear Flow

The state of desalination and brine production: A global outlook.

Many engineering applications are characterized by implicit response functions that are expensive to evaluate and sometimes nonlinear in their behavior, making reliability analysis difficult. This paper develops an efficient reliability analysis method that accurately characterizes the limit state throughout the random variable space. The method begins with a Gaussian process model built from a very small number of samples, and then adaptively chooses where to generate subsequent samples to ensure that the model is accurate in the vicinity of the limit state. The resulting Gaussian process model is then sampled using multimodal adaptive importance sampling to calculate the probability of exceeding (or failing to exceed) the response level of interest. By locating multiple points on or near the limit state, more complex and nonlinear limit states can be modeled, leading to more accurate probability integration. By concentrating the samples in the area where accuracy is important (i.e., in the vicinity of the limit state), only a small number of true function evaluations are required to build a quality surrogate model. The resulting method is both accurate for any arbitrarily shaped limit state and computationally efficient even for expensive response functions. This new method is applied to a collection of example problems including one that analyzes the reliability of a microelectromechanical system device that current available methods have difficulty solving either accurately or efficiently.

Efficient Global Reliability Analysis for Nonlinear Implicit Performance Functions

Emissions of sulfur gases from both natural and anthropogenic sources strongly influence the chemistry of the atmosphere. To assess the relative importance of these sources we have combined the measurements of sulfur gases and fluxes during the past decade to create a global emission inventory. The inventory, which is divided into 12 latitude belts, takes into account the seasonal dependence of sulfur emissions from biogenic sources. The total emissions of sulfur gases from natural sources are approximately 0.79 Tmol S/a. These emissions are 16% of the total sulfur emissions in the Northern Hemisphere and 58% in the Southern Hemisphere. The inventory clearly shows the impact of anthropogenic sulfur emissions in the region between 35° and 50°N.

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Sulfur Emissions to the Atmosphere from Natural Sources

The mechanics of buoyant jet flows issuing with a general three-dimensional geometry into an unbounded ambient environment with uniform density or stable density stratification and under stagnant or steady sheared current conditions is investigated. An integral model is formulated for the conservation of mass, momentum, buoyancy and scalar quantities in the turbulent jet flow. The model employs an entrainment closure approach that distinguishes between the separate contributions of transverse shear (leading to jet, plume, or wake internal flow dynamics) and of azimuthal shear mechanisms (leading to advected momentum puff or thermal flow dynamics), respectively. Furthermore, it contains a quadratic law turbulent drag force mechanism as suggested by a number of recent detailed experimental investigations on the dynamics of transverse jets into crossflow. The model is validated in several stages: First, comparison with basic experimental data for the five asymptotic, self-similar stages of buoyant jet flows, i.e., the pure jet, the pure plume, the pure wake, the advected line puff, and the advected line thermal, support the choice and magnitude of the turbulent closure coefficients contained in the entrainment formulation. Second, comparison with many types of non-equilibrium flows support the proposed transition function within the entrainment relationship, and also the role of the drag force in the jet deflection dynamics. Third, a number of spatial limits of applicability have been proposed beyond which the integral model necessarily becomes invalid due to its parabolic formulation. These conditions, often related to the breakdown of the boundary layer nature of the flow, describe features such as terminal layer formation in stratification, upstream penetration in jets opposing a current, or transition to passive diffusion in a turbulent ambient shear flow. Based on all these comparisons, that include parameters such as trajectories, centerline velocities, concentrations and dilutions, the model appears to provide an accurate and reliable representation of buoyant jet physics under highly general flow conditions.

Integral Model for Turbulent Buoyant Jets in Unbounded Stratified Flows. Part I: Single Round Jet

http://iopscience.iop.org/article/10.1016/0169-5983(95)00053-G/pdf

Experimental study of plane turbulent wakes in a shallow water layer

1. Physico-Chemical Fundamentals.- Physical-chemical phenomena and molecular properties.- The determination of Henry's Constant for volatile organics by equilibrium partitioning in closed systems.- Volatilization of chlorinated hydrocarbons from water.- The ratio of gas-phase to liquid-phase mass transfer coefficients in gas-liquid contacting processes.- Absorption of gases at condensing and evaporating water surfaces.- The effects of evaporation and condensation on an absorption process.- 2. Turbulence near Gas-Liquid Interfaces.- Turbulence structure and turbulent diffusion near gas-liquid interfaces.- Turbulence measurements near the free surface in stirred grid experiments.- A laboratory technique for investigating the relationship between gas transfer and fluid turbulence.- Carbon dioxide transfer at the gas/water interface as a function of system turbulence.- Turbulence structure of wavy stratified air-water flow.- Current fluctuations in the surface waters of small lakes.- Moisture and heat transport in a stably stratified boundary layer over a water surface.- 3. Interfacial Motions and Instabilities.- Wind-wave effects on gas transfer.- Interfacial motion observed during experiments on air-water gas transfer.- A laboratory study of the velocity field below surface gravity waves.- The roles of Langmuir circulations in gas transfer across natural water surfaces.- Measurements of the fluctuating pressure in the turbulent boundary layer over progressive, mechanically generated water waves.- Experimental studies of surface wave breaking and air entrainment.- Bubbles produced by breaking wind waves.- The role of bubbles for gas transfer from water to air at higher windspeeds. Experiments in the wind-wave facility in Hamburg.- A new optical bubble measuring device A simple model for bubble contribution to gas exchange.- Contribution of bubbles to gas transfer across an air-water interface.- Entrapment and transport of bubbles by plunging water.- 4. Conceptual Models and Parameterizations of Gas Transfer.- Conceptual models of gas exchange.- A comparison of turbulent mass transfer at gas-liquid and solid-liquid interfaces.- The variation of the gas transfer coefficient with molecular diffusity.- Wind/wave-tunnel experiment on the Schmidt number - and wave field dependence of air/water exchange.- A model of interfacial gas transfer for a well-roughened sea.- Turbulent transfer across smooth and rough surfaces.- Reaeration of open channel flow.- Dependence of oxygen transfer rate on energy dissipation during surface aeration and in stream flow.- Wind effects on air-water oxygen transfer in a lake.- Reaeration and wind induced turbulence shear in a contained water body.- 5. Field and Laboratory Experimental Techniques.- Field techniques for reaeration measurements in rivers.- Gas transfer rate coefficient measurement of wastewater aeration equipment by a stable isotope krypton/lithium technique.- Reaeration studies on some New Zealand rivers using methyl chloride as a gas tracer.- Dichlorodifluoromethane (Freon-12) as a tracer for nitrous oxide release from a nitrogen-enriched river.- An assessment of the radiotracer technique for measuring reaeration rates in large river systems.- Measurements of wind effects on water-side controlled gas exchange in riverine systems.- Optimum application of the radon deficit method to obtain air-sea gas exchange rates.- Parametrization of air/lake gas exchange.- Simultaneous in situ determination of dissolved gases by gas chromatography.- 6. Climate and Oceanographic Applications.- Gas exchange measurements in natural systems.- The role of oceanic whitecaps in air-sea gas exchange.- On the exchange of oxygen and carbon dioxide between ocean and atmosphere in an eastern boundary current.- Invasion of fossil fuel CO2 into the ocean.- In situ pH measurements as an indicator of CO2 gas transfer in glacial meltwaters.- The transfer of mercury at the air/water interface.- Exchange rates of dimethyl sulfide between ocean and atmosphere.- Laboratory observations on transfer of atmospheric oxygen into stratified seawater.- 7. Water Quality and Engineering Applications.- The significance of gas exchange in water quality assessment and planning.- The sensitivity of the dissolved oxygen balance to predictive reaeration equations.- Estimation of volatilization of toxics for multimedia modeling.- Reaeration measurement in swamp streams: Radiotracer case studies.- Prediction of dissolved gas transfer in spillway and outlet works stilling basin flows.- Carbon dioxide desorption from the activated sludge at the waste water treatment plants.- Volatilization of fission products in nuclear reactor buildings.- Self-propelled weir aerators.

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Gas transfer at water surfaces

In order to elucidate the turbulent structure below a shear-free gas-liquid interface, turbulence measurements were made in a 50 cm square by 40 cm deep tank stirred by a vertically oscillating grid well below the surface, using a split-film anemometer probe rotating in a horizontal circle. This instrument is able to measure both vertical and horizontal velocity fluctuations to within 0.4 mm of the surface, from which spatial spectra and profiles of r.m.s. velocity fluctuations and integral lengthscales can be calculated. The turbulent structure is affected by the presence of the surface within a ‘surface-influenced layer’ roughly one integral scale, or ten per cent of the distance from the surface to the centre of the grid stroke, in thickness. The shapes of the spectra and profiles within the surface-influenced layer are predicted to a good first approximation by the source theory of Hunt & Graham (1978), which treats the turbulent structure as the superposition of homogeneous turbulence with an irrotational velocity field driven by a source distribution at the surface which cancels the vertical velocity fluctuations there. The magnitudes (as opposed to the shapes) of the profiles scale according to the values that would otherwise occur in the vicinity of the surface-influenced layer were the surface not present. These magnitudes are adequately predicted by the bulk relations determined by Hopfinger & Toly (1976) and Thompson & Turner (1975), with no apparent dependence on turbulent Reynolds number. There are some minor discrepancies between the measured profiles and those of Hunt & Graham. A thin layer of reduced velocity fluctuations below what would be expected from the theory was observed near the surface. Also, anisotropy in the velocity spectra at depths within the surface-influenced layer extended well into the inertial subrange, whereas the Hunt & Graham theory predicts no anisotropy at high wavenumbers.

Near-surface turbulence in a grid-stirred tank

Shallow turbulent flows occurring in wide rivers, estuaries, lakes or coastal regions, as well as the atmosphere, are readily susceptible to transverse disturbances that lead to two-dimensional coherent structures, the shallow jet, the shallow wake and the shallow mixing layer are examples of such flow patterns, three types of generation mechanisms are proposed for these flows; namely, topographical forcing, internal transverse shear instabilities, and secondary instabilities of the base flow, the large-scale coherent structures greatly influence the mixing and transport of pollutants that are released into such flows, they may also play an important role in transverse momentum exchange that controls friction in wide channel flows.

Gerhard H. Jirka

Papers

Integral Model for Turbulent Buoyant Jets in Unbounded Stratified Flows. Part I: Single Round Jet

Experimental study of plane turbulent wakes in a shallow water layer

Gas transfer at water surfaces

Near-surface turbulence in a grid-stirred tank

Large scale flow structures and mixing processes in shallow flows