Bubbles emerging from a submerged granular bed

This paper explores the phenomena associated with the emergence of gas bubbles from a submerged granular bed. While there are many natural and industrial applications, we focus on the particular circumstances and consequences associated with the emergence of methane bubbles from the beds of lakes and reservoirs since there are significant implications for the dynamics of lakes and reservoirs and for global warming. This paper describes an experimental study of the processes of bubble emergence from a granular bed. Two distinct emergence modes are identified, mode 1 being simply the percolation of small bubbles through the interstices of the bed, while mode 2 involves the cumulative growth of a larger bubble until its buoyancy overcomes the surface tension effects. We demonstrate the conditions dividing the two modes (primarily the grain size) and show that this accords with simple analytical evaluations. These observations are consistent with previous studies of the dynamics of bubbles within porous beds. The two emergence modes also induce quite different particle fluidization levels. The latter are measured and correlated with a diffusion model similar to that originally employed in river sedimentation models by Vanoni and others. Both the particle diffusivity and the particle flux at the surface of the granular bed are measured and compared with a simple analytical model. These mixing processes can be consider applicable not only to the grains themselves, but also to the nutrients and/or contaminants within the bed. In this respect they are shown to be much more powerful than other mixing processes (such as the turbulence in the benthic boundary layer) and could, therefore, play a dominant role in the dynamics of lakes and reservoirs

Experimental investigation of the initial regime in fingering electrodeposition: dispersion relation and velocity measurements

Recently a fingering morphology, resembling the hydrodynamic Saffman-Taylor instability, was identified in the quasi-two-dimensional electrodeposition of copper. We present here measurements of the dispersion relation of the growing front. The instability is accompanied by gravity-driven convection rolls at the electrodes, which are examined using particle image velocimetry. While at the anode the theory presented by Chazalviel et al. describes the convection roll, the flow field at the cathode is more complicated because of the growing deposit. In particular, the analysis of the orientation of the velocity vectors reveals some lag of the development of the convection roll compared to the finger envelope.Comment: 11 pages, 15 figures, REVTEX 4; reference adde

The role of intratidal oscillations in sediment resuspension in a diurnal, partially mixed estuary

Using detailed observations of the mean and turbulent properties of flow, salinity and turbidity that spanned 2001/02, we examined the physical mechanisms underpinning sediment resuspension in the low-energy Swan River estuary, Western Australia. In this diurnal tidally-dominated estuary, the presence of intratidal oscillations, a tidal inequality lasting 2 to 3 hours on the flood tide, generated by interactions of the four main diurnal and semidiurnal astronomical constituents, K₁, O₁, M₂, and S₂, played a major role in modifying vertical stratification and mixing. These intratidal oscillations are controlled by phase differences between the tropic and synodic months rather than being temporally-fixed by bed friction, as occurs in semidiurnal estuaries. Intratidal oscillations are largest, at around 0.1 m, near to the Austral solstice when the lunar and solar declination are in-phase. Despite the seemingly small change in water level, shear-induced interfacial mixing caused destratification of the water column with the top-to-bottom salinity (ΔS) difference of 3.5 present early in the flood tide eroded to less than 0.3 by the end of the intratidal oscillation. High turbidity peaks, of 250 nephelometric turbidity units, coincided with these intratidal oscillations and could not be explained by bed friction since shear stress from mean flow did not exceed threshold criteria. High Reynolds stresses of ∼1 Nm⁻² did, however, exceed τcr and together with negative Reynolds fluxes indicate a net downward transport of material. Destratification of the water column induced by shear instabilities resulted in large overturns capable of moving in situ material towards the bed during intratidal oscillations and these turbidities were ∼10 times greater than those from bed-generated resuspension observed later during the flood tide

Does ohmic heating influence the flow field in thin-layer electrodeposition?

In thin-layer electrodeposition the dissipated electrical energy leads to a substantial heating of the ion solution. We measured the resulting temperature field by means of an infrared camera. The properties of the temperature field correspond closely with the development of the concentration field. In particular we find, that the thermal gradients at the electrodes act like a weak additional driving force to the convection rolls driven by concentration gradients.Comment: minor changes: correct estimation of concentration at the anode, added Journal-re

Lake-size dependency of wind shear and convection as controls on gas exchange

High-frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u*) and convection (w*) to turbulence in the surface mixed layer. Seasonal patterns of u* and w* were dissimilar; u* was often highest in the spring, while w * increased throughout the summer to a maximum in early fall. Convection was a larger mixed-layer turbulence source than wind shear (u */w*-1 for lakes* and w* differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake-air gas exchange, especially for small lakes. © 2012 by the American Geophysical Union.Jordan S. Read, David P. Hamilton, Ankur R. Desai, Kevin C. Rose, Sally MacIntyre, John D. Lenters, Robyn L. Smyth, Paul C. Hanson, Jonathan J. Cole, Peter A. Staehr, James A. Rusak, Donald C. Pierson, Justin D. Brookes, Alo Laas, and Chin H. W

Microstructure measurements along a quasi-meridional transect in the northeast Atlantic.

This study presents vertical profiles of turbulence parameters obtained in the upper 100 m of the northeastern Atlantic Ocean along a transect from tropical permanently stratified waters to subpolar seasonally stratified waters in July-August 2009. The focus is to fully characterize the vertical mixing along this transect for further studies related to phytoplankton and nutrient distributions. Derived values of temperature eddy diffusivity

Relative influence of shredders and fungi on leaf litter decomposition along a river altitudinal gradient

We compared autumn decomposition rates of European alder leaves at four sites along the Lasset–Hers River system, southern France, to test whether changes in litter decomposition rates from upstream (1,300 m elevation) to downstream (690 m) could be attributed to temperature-driven differences in microbial growth, shredder activity, or composition of the shredder community. Alder leaves lost 75–87% of original mass in 57 days, of which 46–67% could be attributed to microbial metabolism and 8–29% to shredder activity, with no trend along the river. Mass loss rates in both fine-mesh (excluding shredders) and coarse-mesh (including shredders) bags were faster at warm, downstream sites (mean daily temperature 7–8°C) than upstream (mean 1–2°C), but the differ- ence disappeared when rates were expressed in heat units to remove the temperature effect. Mycelial biomass did not correlate with mass loss rates. Faster mass loss rates upstream, after temperature correction, evidently arise from more efficient shredding by Nemourid stoneflies than by the Leuctra-dominated assemblage downstream. The influence of water temperature on decomposition rate is therefore expressed both directly, through microbial metabolism, and indirectly, through the structure of shredder commu- nities. These influences are evident even in cold water where temperature variation is small

Mixing and flushing in the Arabian/Persian Gulf

The assimilative capacities of estuaries and coastal seas for effluent discharges are predominantly determined by the rates at which pollutant-bearing effluents are first dispersed and then flushed from the coastal region into the open ocean. The dispersion coefficients and flushing, as measured by the water residence time in the Persian Gulf (Arabian Gulf), were investigated using the three-dimensional numerical model Estuary, Lake and Coastal Ocean Model (ELCOM). The model was first validated using the R/V Mt. Mitchell expedition profile data, collected from 27 January to 26 February 1992 and from 13 May to 12 June 1992. The validated model was then used to compute the geographic variability of the horizontal dispersion coefficients Kx throughout the gulf. Model results revealed that dispersion was principally driven by the shear associated with the tides, but along the Arabian coast, wind was an additional significant energy source for dispersion. The water residence time was found to be more than 3 years along the Arabian coast, but shorter along the Iranian coast

Evaluating the impact of kinetic energy removal by wind turbines on the technical wind energy potential of the German Bight

Scenarios of energy transition in Germany project large wind capacity deployment in the German Bight by 2050. They use models that estimate technical potential or annual generation by fixing energy loss from atmosphere-turbine interactions to 10% to manage computational cost. This approach, which we call Fixed, underestimates losses as it discounts impacts of wind resource depletion that manifest as reduced wind speeds and lowered turbine yields. We explore the influence of kinetic energy (KE) removal by wind turbines and stability conditions on wind resource depletion and turbine yield. Using wind speeds, turbine yields and capacity factor estimates from three approaches that include the Fixed approach, a simplified representation of atmospheric KE budgets and their depletion (KEBA), and mesoscale simulations from the Weather Research and Forecasting (WRF) model with a wind farm parameterisation we investigate the predominant influence on the Bight’s potential that is relevant for energy scenarios. WRF, the most physically comprehensive model among the three, reveals that reductions in these estimates are highest during stable conditions. KEBA, which incorporates only KE removal effects, aligns closely with WRF. Under highly unstable conditions KEBA’s wind speed and capacity factors estimates are within 10% and 20% of WRF, respectively. Under stable conditions they are within 20 and 45%. As unstable conditions dominate the German Bight, KEBA estimates of technical potential are within 35% of WRF, suggesting that KE removal primarily shapes depletion effects and technical potential. Disregarding it leads to an overestimation of almost 90%. Despite depletion effects, the Bight’s potential remains substantial, generating about 200–250 TWh yr−1 or 3000–3400 full load hours yr−1 from a 72 GW deployment. We conclude that using a simplified yet physical model of KE budgets provides more representative technical potential estimates for energy scenarios compared to the Fixed approach