| 1. |
- Andric, Jelena, 1979, et al.
(författare)
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A study of a flexible fiber model and its behavior in DNS of turbulent channel flow
- 2013
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Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 0001-5970 .- 1619-6937. ; 224:10, s. 2359-2374
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particlelevel fiber model has been integrated into a general-purpose, open source Computational Fluid Dynamics (CFD) code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct Numerical Simulation (DNS) of the incompressible Navier–Stokes equations is used to describe the fluid flow in a plane channel and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation time-scales of fibers suspended in a turbulent flow field.
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| 2. |
- Bastviken, David, et al.
(författare)
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Determination of the piston velocity for water-air interfaces using flux chambers, acoustic Doppler velocimetry, and IR imaging of the water surface
- 2013
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-8953. ; 118:2, s. 770-782
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Tidskriftsartikel (refereegranskat)abstract
- The transport of gasses dissolved in surface waters across the water-atmosphere interface is controlled by the piston velocity (k). This coefficient has large implications for, e.g., greenhouse gas fluxes but is challenging to quantify in situ. At present, empirical k-wind speed relationships from a small number of studies and systems are often extrapolated without knowledge of model performance. This study compares empirical k estimates from flux chamber and surface water gas concentration measurements (chamber method), eddy cell modeling and dissipation rates of turbulent kinetic energy (dissipation method), and a surface divergence method based on IR imaging, at a fetch limited coastal observation station. We highlight strengths and weaknesses of the methods, and relate measured k values to parameters such as wave height, and surface skin velocities. The chamber and dissipation methods yielded k values in the same order of magnitude over a 24h period with varying wind conditions (up to 10ms−1, closest weather station) and wave heights (0.01–0.30m). The surface divergence method most likely did not resolve the small turbulent eddies that cause the main divergence. Flux chamber estimates showed the largest temporal variability, with lower k values than the dissipation method during calm conditions, where the dissipation method failed as waves and instrument noise dominated over the turbulence signal. There was a strong correspondence between k from chambers, the RMS of surface velocities from IR imaging, and wave height. We propose a method to estimate area integrated values of k from wave measurements.
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| 3. |
- Fredriksson, Sam, 1966, et al.
(författare)
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An evaluation of gas transfer velocity parameterizations during natural convection using DNS
- 2016
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 121:2, s. 1400-1423
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Tidskriftsartikel (refereegranskat)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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| 4. |
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| 5. |
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| 6. |
- Fredriksson, Sam, 1966
(författare)
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Estimating the Air-Water Gas Transfer Velocity during Low Wind Conditions
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 7. |
- Fredriksson, Sam, 1966, et al.
(författare)
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Large eddy simulation of the tidal power plant Deep Green using the actuator line method
- 2017
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Ingår i: IOP Conference Series: Materials Science and Engineering. - 1757-8981 .- 1757-899X. ; 276:1
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Konferensbidrag (refereegranskat)abstract
- Tidal energy has the potential to provide asubstantial part of the sustainable electric power generation. Thetidal power plant developed by Minesto, called Deep Green, is anovel technology using a ‘flying’ kite with an attached turbine,moving at a speed several times higher than the mean flow.Multiple Deep Green power plants will eventually form arrays,which requires knowledge of both flow interactions betweenindividual devices and how the array influences the surroundingenvironment. The present study uses large eddy simulations(LES) and an actuator line model (ALM) to analyze theoscillating turbulent boundary layer flow in tidal currentswithout and with a Deep Green power plant. We present themodeling technique and preliminary results so far.
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| 8. |
- Fredriksson, Sam, 1966, et al.
(författare)
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MODEL TIDAL POWER IN A TURBULENT OCEAN
- 2018
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Ingår i: Oceanera-Net Final Conference, Edinburg, Scotland, 30-31 January 2018..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 9. |
- Fredriksson, Sam, 1966, et al.
(författare)
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Modelling Deep Green tidal power plant using large eddy simulations and the actuator line method
- 2021
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Ingår i: Renewable Energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 179, s. 1140-1155
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Tidskriftsartikel (refereegranskat)abstract
- The Deep Green technique for tidal power generation is suitable for moderate flows which is attractive since larger areas for tidal energy generation hereby can be used. It operates typically at mid-depth and can be seen as a "flying" kite with a turbine and generator attached underneath. It moves in a lying figure-eight path almost perpendicular to the tidal flow. Large eddy simulations and an adaption of the actuator line method (in order to describe arbitrary paths) are used to study the turbulent flow with and without Deep Green for a specific site. This methodology can in later studies be used for e.g. array analysis that include Deep Green interaction. It is seen that Deep Green creates a unique wake composed of two velocity deficit zones with increased velocity in each wake core. The flow has a tendency to be directed downwards which results in locally increased bottom shear. The persistence of flow disturbances of Deep Green can be scaled with its horizontal path width, D-y, with a velocity deficit of 5% at approximately 8-10D(y) downstream of the power plant. The turbulence intensity and power deficit are approximately two times the undisturbed value and 10%, respectively, at 10D(y). (C) 2021 The Authors. Published by Elsevier Ltd.
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| 10. |
- Fredriksson, Sam, 1966, et al.
(författare)
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Near-surface physics during convection affecting air-water gas transfer
- 2016
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Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 35:1, s. 012007-
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Konferensbidrag (refereegranskat)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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