Fluid mixing by swimming organisms in the low-Reynolds-number limit

Recent publications in the fluid physics literature have suggested that low-Reynolds-number swimming organisms might contribute significantly to ocean mixing. These papers have focussed on the mass transport due to fluid capture and disturbance by settling or swimming particles based on classical fluid mechanics flows but have neglected the role of molecular property diffusion. Scale-analysis of the property conservation equation finds that, while properties with low molecular diffusivities can have enhanced mixing for typical volume fractions in aggregations of migrating zooplankton, this mixing is still well below that due to internal-wave breaking so unlikely to be important in the ocean

The relation between unstable shear layer thicknesses and turbulence lengthscales

This note explores the connection between the (i) lengthscales of unstable finescale shear layers which are responsible for turbulence production, (ii) turbulence patch thicknesses and (iii) the outer scales of turbulence, that is, the density overturn (Thorpe) and Ozmidov lengthscales, in the stratified ocean interior. Explanations are offered both for why (i) turbulence patches are often observed to be much thicker than outer turbulence scales and (ii) there is a spectral gap between finescale internal waves and the outer scales of turbulence. A finescale parameterization based on unstable shear predicts Ozmidov lengthscales smaller than unstable-shear-layer thicknesses for moderately unstable gradient Froude numbers |Vz|/N \u3c 5.5 [or equivalently, gradient Richardson numbers Ri = N2/V2z \u3e 0.03 where |Vz|is the instantaneous finescale vertical shear magnitude and N the instantaneous buoyancy frequency] for a critical gradient Froude number δc = 2; this is little changed for critical gradient Froude numbers as low as 1. Thus, assuming that patch thicknesses correspond to unstable shear-layer thicknesses, outer turbulence lengthscales will be smaller than patch thicknesses for moderately unstable shear but not strongly unstable shear. A spectral gap between internal wave and turbulent shear arises for similar reasons

The evolution of salt fingers in inertial wave shear

Shadowgraph profiles collected in the thermohaline staircase east of Barbados reveal nearly-horizontal banding—unlike the vertical banding that has been observed in other fingering-favorable parts of the ocean. A plausible interpretation of this optical microstructure is that vertical shear is tilting over fingers. This paper presents a model for shear-tilting of salt fingers. The Ri = 6 inertial wave shears observed in C-SALT would tilt over and damp out square planform (kx = ky) fingers so rapidly that they could not produce significant fluxes. Vertical sheets aligned with the shear (ky = 0) would behave like unsheared fingers if the shear was steady but oceanic shear is predominantly near-inertial so turns with time. Therefore, an across-sheet shear component will develop and initially-aligned sheets too will ultimately be tilted over. This happens slowly enough that sheets can grow to produce significant fluxes. When the growth of tilting sheets is limited by a critical inverse finger Richardson number, (∇ × V)2/N2 ∼ 3–16, the model produces microstructure and fluxes similar to those reported from C-SALT. However, this constraint does not explain the density ratio dependence in laboratory studies and numerical simulations. What constrains finger growth needs to be better understood

A proposed flux constraint for salt fingers in shear

Towed ocean microstructure observations reveal that the temperature Cox number depends inversely on interface temperature-gradient Tz in the fingering-favorable thermohaline staircase east of Barbados. This implies fluxes independent of interface gradients. These conclusions are contrary to theoretical expectations from a finger Froude number stability criterion, |Δw|/N \u3c 1.0: (i) Cox numbers independent of temperature gradient, and (ii) fluxes varying linearly with temperature gradient. We propose a hybrid wave/finger Froude number stability criterion, |(uz − wx)(wy − vz)|N2 ≤ 1.0 which reduces to Uz|Vw|/N2 ≤ 1.0 for salt sheets (δ/δx = 0, | Δw| = wy) aligned with background (internal-wave) shear Uz. The sheet criterion reproduces the observed behavior. For weak but nonzero background shears, 0.1 \u3c Uz/N \u3c 1.0, this constraint implies higher fluxes than unsheared square-planform fingers constrained by a finger Froude number | wxwy|/N2 \u3c 1.0

Spontaneous generation of near-inertial waves by the Kuroshio Front

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2381–2406, doi:10.1175/JPO-D-14-0086.1.While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10–100-km-scale frontal meanders, accompanied by O(10) mW m−2 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.“The Study of Kuroshio Ecosystem Dynamics for Sustainable Fisheries (SKED)” supported by MEXT, MIT-Hayashi Seed Fund, ONR (Awards N000140910196 and N000141210101), NSF (Award OCE 0928617, 0928138) for support.2016-03-0

Eddy stirring and horizontal diffusivity from Argo float observations : geographic and depth variability

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 3989–3997, doi:10.1002/2015GL063827.Stirring along isopycnals is a significant factor in determining the distribution of tracers within the ocean. Salinity anomalies on density surfaces from Argo float profiles are used to investigate horizontal stirring and estimate eddy mixing lengths. Eddy mixing length and velocity fluctuations from the ECCO2 global state estimate are used to estimate horizontal diffusivity at a 300 km scale in the upper 2000 m with near-global coverage. Diffusivity varies by over two orders of magnitude with latitude, longitude, and depth. In all basins, diffusivity is elevated in zonal bands corresponding to strong current regions, including western boundary current extension regions, the Antarctic Circumpolar Current, and equatorial current systems. The estimated mixing lengths and diffusivities provide an observationally based data set that can be used to test and constrain predictions and parameterizations of eddy stirring.This work was supported by the National Science Foundation under grants OCE-13-55668 and OCE-95-21468 and the Office of Naval Research under grants N00014-12-1-0336 and N00014-13-1-0484.2015-11-2

Comparison of methods for sampling particulate emissions from tires under different test environments

Traffic-related emissions are strongly criticised by the public because they contribute to climate change and are classified as hazardous to health. Combustion engine emissions have been regulated by limit values for almost three decades. There is currently no legal limit for non-exhaust emissions, which include tire wear particle emissions and resuspension. As a result, the percentage of total vehicle emissions has risen continuously. Some of the particles emitted can be assigned to the size classes of particulate matter (≤10 µm) and are therefore of particular relevance to human health. The literature describes a wide range of concepts for sampling and measuring tire wear particle emissions. Because of the limited number of studies, the mechanisms involved in on-road tests and their influence on the particle formation process, particle transport and the measuring ability can only be described incompletely. The aim of this study is to compare test bench and on-road tests and to assess the influence of selected parameters. The first part describes the processes of particle injection and particle distribution. Based on this, novel concepts for sampling and measurement in the laboratory and in the field are presented. The functionality and the mechanisms acting in each test environment are evaluated on the basis of selected test scenarios. For example, emissions from external sources, the condition of the road surface and the influence of the driver are identified as influencing factors. These analyzes are used to illustrate the complexity and limited reproducibility of on-road measurements, which must be taken into account for future regulations

Evaluating salt-fingering theories

Author Posting. © Sears Foundation for Marine Research, 2008. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 66 (2008): 413-440, doi:10.1357/002224008787157467.The NATRE fine- and microstructure data set is revisited to test salt-finger amplitude theories. Dependences of the mixing efficiency Γ, microscale buoyancy Reynolds number Re and thermal Cox number CxT on 5-m density ratio Rρ and gradient Richardson number Ri are examined. The observed mixing efficiency is too high to be explained by linear fastest-growing fingers but can be reproduced by wavenumbers 0.5-0.9 times lower than the fastest-growing wavenumber. Constraining these fingers with a hybrid wave/finger Froude number or a finger Reynolds number cannot reproduce the observed trends with Rρ or Ri, respectively. This suggests that background shear has no influence on finger amplitudes. Constraining average amplitudes of these lower-wavenumber fingers with finger Richardson number Rif ~ 0.2 reproduces the observed dependence of Re and CxT on density ratio Rρ and Ri at all but the lowest observed density ratio (Rρ = 1.3). Separately relaxing the assumptions of viscous control, dominance of a single mode and tall narrow fingers does not explain the difference between theory and data at low Rρ for a critical Rif ~ 0.2.We gratefully acknowledge the support of the Office of Naval Research (grant N00014-04-1-0212) and Natural Sciences and Engineering Research Council of Canada

Turbulence in a Sheared, Salt-Fingering-Favorable Environment: Anisotropy and Effective Diffusivities

Direct numerical simulations (DNS) of a shear layer with salt-fingering-favorable stratification have been performed for different Richardson numbers Ri and density ratios R[subscript]p. In the absence of shear (Ri = oo), the primary instability is square planform salt fingering, alternating cells of rising and sinking fluid. In the presence of shear, salt fingering takes the form of salt sheets, planar regions of rising and sinking fluid, aligned parallel to the sheared flow. After the onset of secondary instability, the flow becomes turbulent. The continued influence of the primary instability distorts the late-stage structure and hence biases isotropic estimates of the turbulent kinetic energy dissipation rate . In contrast, thermal and saline gradients evolve to become more isotropic than velocity gradients at their dissipation scales. Thus, the standard observational methodology of estimating the turbulent kinetic energy dissipation rate from vertical profiles of microscale gradients and assuming isotropy can underestimate its true value by a factor of 2–3, whereas estimates of thermal and saline dissipation rates using this approach are relatively accurate. Likewise, estimates of G from vertical profiles overestimate the true G by roughly a factor of 2. Salt sheets are ineffective at transporting momentum. Thermal and saline effective diffusivities decrease with decreasing Ri, despite the added energy source provided by background shear. After the transition to turbulence, the thermal to saline flux ratio and the effective Schmidt number remain close to the values predicted by linear theory