| 1. |
- Fredriksson, Sam, 1966, et al.
(author)
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An evaluation of gas transfer velocity parameterizations during natural convection using DNS
- 2016
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In: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 121:2, s. 1400-1423
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Journal article (peer-reviewed)abstract
- Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air-water gas exchange at no-wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath the surface, the heat flux through the surface, or the wind speed above the surface. The gas transfer is modeled via a passive scalar. The Schmidt number dependence is studied for Schmidt numbers of 7, 150 and 600. The methods using divergence, dissipation and heat flux estimate the transfer velocity well for a range of varying surface heat flux values, and domain depths. The two evaluated empirical methods using wind (in the limit of no wind) give reasonable estimates of the transfer velocity, depending however on the surface heat flux and surfactant saturation. The transfer velocity is shown to be well represented by the expression, k(s) = A (Bv)(1/4) Sc2(n), where A is a constant, B is the buoyancy flux, m is the kinematic viscosity, Sc is the Schmidt number, and the exponent n depends on the water surface characteristics. The results suggest that A = 0.39 and n approximate to 1/2 and n approximate to 2/3 for slip and no-slip boundary conditions at the surface, respectively. It is further shown that slip and no-slip boundary conditions predict the heat transfer velocity corresponding to the limits of clean and highly surfactant contaminated surfaces, respectively.
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| 2. |
- Fredriksson, Sam, 1966
(author)
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Estimating the Air-Water Gas Transfer Velocity during Low Wind Conditions
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The abundances of atmospheric carbon dioxide, CO2, and methane, CH4, are increasing. These increases affect e.g., the global carbon cycle and the climate both regionally and globally. To better understand the present and future atmospheric CO2 and CH4 concentrations and their climate impact, the gas exchange between water and the atmosphere is important. This exchange can occur in two directions. Oceans take up approximately one third of the anthropogenic CO2 release (the ocean carbon sink). At the same time coastal waters and inland waters emit large amounts of CO2 and CH4, altogether corresponding to a similar amount as the ocean sink. The interfacial gas-flux for CO2 and CH4 is controlled by the water-side. The gas-flux, F_g, is for such gases typically estimated as F_g=k_g(C_wb-ϑC_as) where k_g is the gas transfer velocity, C_wb and C_as are the gas concentrations in the water bulk and in the air at the surface, and ϑ is the dimensionless Ostwald solubility coefficient. The subject of this thesis is to describe and estimate k_g for gases that have a water-side controlled gas-flux (e.g., CO2, and CH4). Besides being important for the geophysical sciences, k_g is also used to design and optimize many applications in e.g., chemical and environmental engineering. The transfer velocity is influenced by interfacial shear stress from wind, natural convection due to surface heat flux, microscale breaking waves at moderate wind speeds, breaking waves at high wind speeds, bubbles, surfactants, and rain. This thesis focuses on the low wind condition where the forcings due to shear stress, natural convection, and surfactants are important. The relative importance of buoyancy and shear forcing is characterized via a Richardson number Ri=Bν⁄(u_*^4 ). Here B, ν, and u_* are the buoyancy flux, kinematic viscosity, and friction velocity, respectively. The thesis summarizes three papers where k_g has been studied numerically with direct numerical simulations (DNS) and one paper where field observations have been used. The results from the field measurements show close relationships for the method using flux-chambers and the parameterization using the rate of turbulent kinetic energy dissipation, and the quantities surface rms velocity and the significant wave height. A parameterization of area-integrated values of k_g from wave measurements was proposed. The DNS comprise flow conditions ranging from convection-dominated to shear-dominated cases. The results are used to: (i) evaluate different parameterizations of the air-water gas-exchange, (ii) determine, for a given buoyancy flux, the wind speed at which gas transfer becomes primarily shear driven, (iii) find an expression for the gas-transfer velocity for flows driven by both convection and shear, and (iv) investigate the influence of surfactants on gas transfer velocity. Parameterizations using either the rate of turbulent kinetic energy dissipation or the horizontal surface flow-divergence show a larger disadvantageous dependence on the type of forcing than the parameterization using the surface-normal heat-flux. Two parametrizations using the wind-speed above the surface give reasonable estimates for the transfer-velocity, depending however on the surface heat-flux. The transition from convection- to shear-dominated gas-transfer-velocity is shown to be at Ri≈0.004. This means that buoyancy fluxes in natural conditions are not important for gas exchange at wind velocities U_10 above approximately 3 ms^(-1). Below this wind speed the buoyancy fluxes should be taken into account. The transfer velocity is shown to be well represented by two different approaches: (i) Additive forcing as k_(g,sum)=A_Shear u_* (Ri⁄Ri_c +1)^(1⁄4)Sc^(-n), where Ri_c=(A_Shear⁄A_Buoy)^4 is a critical Richardson number, and (ii) either buoyancy or shear-stress forcing that gives k_g=A_Buoy (Bν)^(1⁄4)Sc^(-n) for Ri>Ri_c and k_g=A_shear u_* Sc^(-n) for Ri
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| 3. |
- Fredriksson, Sam, 1966, et al.
(author)
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Near-surface physics during convection affecting air-water gas transfer
- 2016
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In: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 35:1, s. 012007-
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Conference paper (peer-reviewed)abstract
- The gas flux at the water surface is affected by physical processes including turbulence from wind shear, microscale wave breaking, large-scale breaking, and convection due to heat loss at the surface. The main route in the parameterizations of the gas flux has been to use the wind speed as a proxy for the gas flux velocity, indirectly taking into account the dependency of the wind shear and the wave processes. The interest in the contributions from convection processes has increased as the gas flux from inland waters (with typically lower wind and sheltered conditions) now is believed to play a substantial role in the air-water gas flux budget. The gas flux is enhanced by convection through the mixing of the mixed layer as well as by decreasing the diffusive boundary layer thickness. The direct numerical simulations performed in this study are shown to be a valuable tool to enhance the understanding of this flow configuration often present in nature.
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| 4. |
- Fredriksson, Sam, 1966, et al.
(author)
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Surface shear stress dependence of gas transfer velocity parameterizations using DNS
- 2016
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In: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291 .- 0148-0227 .- 2156-2202. ; 121:10, s. 7369-7389
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Journal article (peer-reviewed)abstract
- Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0Ric or kg=Ashearu*Sc-n, Ri
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