Mixing Intensity Research Articles

Multi-time scale control of Southern Ocean diapycnal mixing over Atlantic tracer budgets

Oceanic cross-density (diapycnal) mixing helps sustain the ocean density stratification and its Meridional Overturning Circulation (MOC) and is key to global tracer distributions. The Southern Ocean (SO) is a key region where different overturning cells connect, allowing nutrient and carbon rich Indian and Pacific deep waters, and oxygen rich Atlantic deep waters to resurface. The SO is also rife with intense diapycnal mixing due to the interaction of energetic eddies and currents with rough topography. SO diapycnal mixing is believed to be of secondary importance for the MOC. Here we show that changes to SO mixing can cause significant alterations to biogeochemical tracer distributions over short and long time scales in an idealized model of the AMOC (Atlantic MOC). While such alterations are dominated by the direct impact of changes in diapycnal mixing on tracer fluxes on annual to decadal time scales, on centennial time scales they are dominated by the mixing-induced variations in the advective transport of the tracers by the AMOC. This work suggests that an accurate representation of spatio-temporally variable local and non-local mixing processes in the SO is essential for climate models’ ability to (i) simulate the global biogeochemical cycles and air sea carbon fluxes on decadal time scales, (ii) represent the indirect impact of mixing-induced changes to AMOC on biogeochemical cycles on longer time scales.

The importance of marketing mix planning and customer orientation for venture capital–financed startups: impacts on valuation, performance, and survival

PurposeExisting literature shows that marketing capabilities of new ventures are critical success factors affecting venture capital funding, startup performance and business failure. The purpose of this study is to investigate whether venture capitalists reward extensive marketing strategies in their startup valuation and whether the marketing mix planning and early strategies on customer orientation predict long-term development of startups.Design/methodology/approachTo address these gaps, this study investigate 107 business plans of new ventures which received venture capital based on these planning documents. The authors use computer-aided text analysis and regression analyses.FindingsThis study’s findings show that customer orientation has positive effects on new venture performance and intensive marketing mix planning increases the likelihood of survival. However, venture capitalists decrease their startup valuation when they read too much about customer orientation and operative marketing mix planning.Originality/valueThis study relies on unique internal documents and therefore provides valuable and new insights for research and practice. Further, this study investigate various short- and long-term effects from marketing and customer orientation for a startups’ development.

Measurements and modelling of the three-dimensional near-field dispersion of particulate matter emitted from passenger ships in a port environment

The maritime sector poses significant challenges in controlling the emission of harmful pollutants, such as particulate matter, and reducing their impact on coastal areas and the atmospheric environment. Efforts to regulate the sector necessitate new knowledge and methods to characterise the evolution and physicochemical transformation of maritime particle emissions dispersing in port areas. An experimental campaign at the Port of Rafina, Greece, was conducted with this in mind. In this paper, we report on the first multi-characteristic particle measurements, including particle number (PN), lung deposited surface area (LDSA), and black carbon (BC), performed at both land and sea using a novel instrument set-up mounted on an unmanned aerial vehicle (UAV). Land-based measurements showed that LDSA averages, which are influenced by particle number and size, increase up to 20 times above background levels as an emission plume progresses downwind, whereas BC concentrations, which are dominated by mass, are ∼12 times higher than the background concentration. Ground and UAV-based particle comparisons showed that PN and LDSA measurements exhibit greater differences than BC relative to the plume's location. Ground-based sensors had ∼50% lower LDSA and PN concentrations, whereas BC was about equal. The experimental observations are further substantiated by coupling a Gaussian plume dispersion model and a new computationally attractive approach, known as the Incompletely Stirred Reactor Network (ISRN) method, to predict the three-dimensional evolution of particle characteristics considering the effects of dilution, segregation, and physicochemical transformations, such as coagulation. Based on simplifying assumptions for the particle size distribution sources within the port area, the ISRN estimates suggest that the discrepancy between various metrics can be partly explained by coagulation, responsible for a non-linear increase in particle size, depending on the local level of dilution and mixing intensity, leading up to a ∼25% decrease in PN and LDSA compared to BC. Combined, measurements and modelling highlight the effect of the sampling location and the importance of monitoring more than one particle metric to characterise particle evolution.

Incorporative mixing in microreactors: Influence on reactions and importance of inlet designation

Fast mixing is an essential feature of a microreactor. This study reveals the critical role of microscopic fluid incorporation on fast chemical reactions in a microreactor. The vortices in a microreactor produce tiny fluid segments, and the incorporation of these segments into the surrounding fluid triggers chemical reactions. The selectivity of chemical reactions highly depends on the type of the fluid segment incorporated into the fluid. Microreactor operations that consider incorporative mixing can achieve better reaction results, even with slow mixing. For example, a tee mixer showed better reaction performance with the vertical confluence configuration than with the symmetric configuration, although the symmetric 180-degree confluence provides more intense mixing. Computational fluid dynamics (CFD) simulations showed that vertical confluence enabled the division and incorporation of a stream from the horizontal inlet into another stream. The appropriate inlet designation is vital when two inlets of a microreactor differ in dimensions, directions, or positions because the different inlets provide different mixing profiles. The two inlets of the microreactor were distinguishable even though the fabrication error was insignificant. For instance, feed orientations changed the side-product yield seven times in a 3D-printed microreactor. Moreover, the impact of incorporative mixing on particle synthesis was confirmed. Switching the two inlets significantly influenced the morphology and size of the synthesized particles by changing the initial nucleation environment. In conclusion, considering incorporative mixing and optimal inlet orientation will enhance the performance and flexibility of microreaction technology.

Magnetic Resonance Imaging Scans of Cervical Spinal Cord Ependymoma with Changing Radiological Features over a Short Period of Time

In a 30-year-old woman with cervical ependymoma, magnetic resonance imaging (MRI) features changed over a short period of time. A T2-weighted MRI scan obtained 1 month before surgery showed an intramedullary tumor with mixed intensity suggesting solid and cystic components at the C4-7 level. Gadolinium-enhanced MRI showed ring enhancement, including a cystic component at the C4-7 level. However, fluorodeoxyglucose positron emission tomography revealed uptake at the C5 level, and another gadolinium-enhanced MRI scan performed 24 days after the previous scan showed only enhanced lesions corresponding to the solid component at the C5-6 level. These images changed over a short time because the spontaneous hematoma, which was enhanced on the first MRI scan, had disappeared later. Fluorodeoxyglucose positron emission tomography or repeated gadolinium-enhanced MRI may be useful if the initial MRI scan suggests the presence of a hematoma in spinal ependymoma.

Modelling and gas–solid mixing characterization in the jiggled bed reactor (JBR)

AbstractThe jiggled bed reactor (JBR) is a new multiphase laboratory‐scale microreactor consisting of a sealed container attached to a piston that is rapidly moved up and down by a pneumatically powered actuator. Particles and fluids in the container are mixed by this up and down motion instead of mechanical agitators or a fluidizing gas. This alternating motion provides intense mixing of all phases (gas, liquid, or solid) and intense contact between phases. Small rods inside the solids bed are heated by induction, allowing for excellent control of bed temperature and heating rate. The JBR is inexpensive and easy to operate, and it has been applied to catalytic gasification of bio‐oil, biomass pyrolysis, activated carbon production, high‐pressure oil hydrogenation, and hydrocarbons adsorption. Experiments demonstrated that solids mixing depends on the reactor platform maximum accelerations during both up and down strokes. A minimum acceleration, 55 m2/s for the tested JBR, was required to achieve good solids mixing. A physical model was developed to predict the reactor platform motion and its maximum acceleration. It requires a few preliminary experiments (around 10) to obtain its four empirical parameters. The model can then determine how to adjust the actuator compressed air pressure or modify the equipment to eliminate performance bottlenecks.

Net community production in the northwestern Mediterranean Sea from glider and buoy measurements

Abstract. The Mediterranean Sea comprises just 0.8 % of the global oceanic surface, yet considering its size, it is regarded as a disproportionately large sink for anthropogenic carbon due to its physical and biogeochemical characteristics. An underwater glider mission was carried out in March–April 2016 close to the BOUSSOLE and DyFAMed time series moorings in the northwestern Mediterranean Sea. The glider deployment served as a test of a prototype ion-sensitive field-effect transistor pH sensor. Dissolved oxygen (O2) concentrations and optical backscatter were also observed by the glider and increased between 19 March and 1 April, along with pH. These changes indicated the start of a phytoplankton spring bloom, following a period of intense mixing. Concurrent measurements of CO2 fugacity and O2 concentrations at the BOUSSOLE mooring buoy showed fluctuations, in qualitative agreement with the pattern of glider measurements. Mean net community production rates (N) were estimated from glider and buoy measurements of dissolved O2 and inorganic carbon (DIC) concentrations, based on their mass budgets. Glider and buoy DIC concentrations were derived from a salinity-based total alkalinity parameterisation, glider pH and buoy CO2 fugacity. The spatial coverage of glider data allowed the calculation of advective O2 and DIC fluxes. Mean N estimates for the euphotic zone between 10 March and 3 April were (-17±36) for glider O2, (44±94) for glider DIC, (17±37) for buoy O2 and (49±86) mmolm-2d-1 for buoy DIC, all indicating net metabolic balance over these 25 d. However, these 25 d were actually split into a period of net DIC increase and O2 decrease between 10 and 19 March and a period of net DIC decrease and O2 increase between 19 March and 3 April. The latter period is interpreted as the onset of the spring bloom. The regression coefficients between O2 and DIC-based N estimates were 0.25 ± 0.08 for the glider data and 0.54 ± 0.06 for the buoy, significantly lower than the canonical metabolic quotient of 1.45±0.15. This study shows the added value of co-locating a profiling glider with moored time series buoys, but also demonstrates the difficulty in estimating N, and the limitations in achievable precision.

Colloidal properties of oil-contaminated sewage and its treatment

Oil refineries and other industrial enterprises generate a large amount of sewage contaminated with petroleum products. Sewage must be treated before it can be discharged into a water body. Sewage treatment methods often do not allow reducing the content of petroleum products to the maximum allowable values. This is because sewage is a microheterogenous system characterized by variable composition and high stability. It is thus important to study factors affecting the high aggregate stability of sewage as well as the development of theoretical foundations for optimizing sewage treatment. The paper presents the study of properties of multicomponent colloidal emulsion systems, factors affecting their high aggregative stability and develops theoretical foundations for optimizing sewage treatment. The systems simulating industrial effluents according to the basic chemical composition obtained by the dispersion method are studied herein. Research also includes the kinetics of precipitation of petroleum products from sewage, sewage treatment parameters during the intensive mixing of the emulsion with air. The express-method determination is used for the petroleum product content in emulsions. The regression analysis is used for the dependence of the optical density on the content of petroleum products. It is found that 10 -6 µm particle removal continues for 20 min, which corresponds to the rapid coagulation. The stability of clean-contamined emulsions decreases by about 10 times with increasing barbotage speed to 30 l/h. The latter being 10 l/h, is sufficient when using Ca(OH) 2

Removal of saxitoxin and anatoxin-a by PAC in the presence and absence of microcystin-LR and/or cyanobacterial cells

Cyanobacteria can produce cyanotoxins such as microcystin-LR (MC), saxitoxin (STX), and anatoxin-a (ANTX-a) which are harmful to humans and other animals. Individual removal efficiencies of STX and ANTX-a by powdered activated carbon (PAC) was investigated, as well as when MC-LR and cyanobacteria were present. Experiments were conducted with distilled water and then source water, using the PAC dosages, rapid mix/flocculation mixing intensities and contact times of two drinking water treatment plants in northeast Ohio. At pH 8 and 9, STX removal was 47%-81% in distilled water and 46%-79% in source water, whereas it was 0-28% for pH 6 in distilled water and 31%-52% in source water. When 1.6 µg/L or 20 µg/L MC-LR was present with STX, STX removal was increased with PAC simultaneously removing 45%-65% of the 1.6 µg/L MC-LR and 25%-95% of the 20 µg/L MC-LR depending on the pH. ANTX-a removal at pH 6 was 29%-37% for distilled water and 80% for source water, whereas it was 10%-26% for pH 8 in distilled water and 28% for pH 9 in source water. The presence of cyanobacteria cells decreased ANTX-a removal by at least 18%. When 20 µg/L MC-LR was present with ANTX-a in source water, 59%-73% ANTX-a and 48%-77% of MC-LR was removed at pH 9 depending on the PAC dose. In general, a higher PAC dose led to higher cyanotoxin removals. This study also documented that multiple cyanotoxins can be effectively removed by PAC for water at pH's between 6 and 9.

Columnar and surface urban aerosol in the Moscow megacity according to measurements and simulations with the COSMO-ART model

Abstract. Urban aerosol pollution was analyzed over the Moscow megacity region using the COSMO-ART (COSMO – COnsortium for Small-scale MOdelling, ART – Aerosols and Reactive Trace gases) online coupled mesoscale model system and intensive measurement campaigns at the Moscow State University Meteorological Observatory (MSU MO, 55.707∘ N, 37.522∘ E) during the April–May period in 2018 and 2019. We analyzed mass concentrations of particulate matter with diameters smaller than 10 µm (PM10), black carbon (BC) and aerosol gas precursors (NOx, SO2, CHx) as well as columnar aerosol parameters for fine and coarse modes together with different meteorological parameters, including an index characterizing the intensity of particle dispersion (IPD). Both model and experimental datasets have shown a statistically significant linear correlation of BC with NO2 and PM10 mass concentrations, which indicates mostly common sources of emissions of these substances. There was a pronounced increase in the BC/PM10 ratio from 0.7 % to 5.9 %, with the decrease in the IPD index related to the amplification of the atmospheric stratification. We also found an inverse dependence between the BC/PM10 ratio and columnar single-scattering albedo (SSA) for the intense air mixing conditions. This dependence together with the obtained negative correlation between wind speed and BC/PM10 may serve as an indicator of changes in the absorbing properties of the atmosphere due to meteorological factors. On average, the relatively low BC / PM10 ratio (for urban regions) of 4.7 % is the cause of the observed relatively high SSA = 0.94 in Moscow. Using long-term parallel aerosol optical depth (AOD) measurements over the 2006–2020 period at the MSU MO and under upwind clean background conditions at Zvenigorod Scientific Station (ZSS) of the IAP RAS (55.7∘ N, 36.8∘ E), we estimated the urban component of AOD (AODurb) and some other parameters as the differences at these sites. The annual mean AODurb at 550 nm was about 0.021 with more than 85 % of the fine aerosol mode. The comparisons between AODurb obtained from the model and measurements during this experiment have revealed a similar level of aerosol pollution of about AODurb=0.015–0.019, which comprised 15 %–19 % of the total AOD at 550 nm. The urban component of PM10 (PM10urb) was about 16 µg m−3 according to the measurements and 6 µg m−3 according to the COSMO-ART simulations. We obtained a pronounced diurnal cycle of PM10urb and urban BC (BCurb) as well as their strong correlation with the IPDs. With the IPD index change from 3 to 1 at night, there was about a 4 times increase in PM10urb (up to 30–40 µg m−3) and a 3 times increase in BCurb (up to 3–3.5 µg m−3). At the same time, no pronounced daily cycle was found for the columnar urban aerosol component (AODurb), although there was a slight increase in model AODurb at night.

Anisothermal flow dynamics in ventilated open space involved with indoor comfort

Energy cooling systems or thermal buildings are increasingly fertile subject for reducing the energy demand or for the indoor air quality coupled with energy demand. This work is devoted to studying cooling ability within an open domain. The space model is considered as a cavity heated on one external sidewall and cooled by three vertical boards which separate the room into four sub-domains. A complex flow structure is obtained in the present “cooperating” case, in which dynamic and buoyancy forces are added together on the active wall. It shows that the natural convection and the forced flow (injection) act directly on the resulting patterns, heat exchange, mixing ability, which control the thermal field homogeneity and the cooling necessity. The computation results show the influence of the parameters and the vertical boards on heat transfer in the enclosure. Under the wide range of Rayleigh (Ra) and Reynolds (Re) numbers, the features of dynamic and thermal fields permit to categorize the different convection phenomena. The intensity of mixing was quantified versus Ra and Re and we find the conditions and threshold of the different domains (diffusive, natural, forced and mixed convection).

Impact of Vegetable Oil Type on the Rheological and Tribological Behavior of Montmorillonite-Based Oleogels

We formulated and characterized oleogels based on montmorillonite clay and vegetable oils that could serve as eco-friendly semi-solid lubricants. In particular, we studied the influence of the physical-chemical properties of olive, castor, soybean, linseed, and sunflower oils on the rheological, chemical, thermal, and tribological properties of the semi-solid lubricants. We prepared the oleogels via the highly intensive mixing of vegetable oils with clay at a concentration of 30 wt.%. The oleogels exhibited shear-thinning, thixotropy, structural recovery, and gel-like behavior commonly related to that of a three-dimensional network. The results were corroborated via XRD measurements showing the presence of intercalated nanoclay structures well-dispersed in the vegetable oil. Empirical correlations between the content of saturated (SFAs), unsaturated (UFAs), mono-unsaturated (MUFAs) and poly-unsaturated (PUFAs) fatty acids and the plateau modulus of the aerogels were found. From these experimental results, we can conclude that the fatty acid profile of the vegetable oils exerts an important influence on the rheological and tribological properties of resulting clay and vegetable oil oleogels.

Coccolith dissolution versus productivity changes during the Plio-Pleistocene (3.14–1.80 MA) in the South Atlantic (ODP site 1090)

Calcareous nannofossil assemblages and fragmentation ratios of two coccolithophore species (Coccolithus pelagicus and Calcidiscus leptoporus) were analysed in samples from ODP Site 1090, South Atlantic (42° 54.8′S, 8° 53.9′E), to obtain valuable insights into the paleoceanographic evolution and carbonate preservation trends during the late Pliocene and Early Pleistocene (3.14–1.80 Ma). Coccolithophore species were identified in a total of 232 samples. Calcareous nannofossil counts revealed shifts in the assemblages that can be correlated with the climatic evolution of the Southern Ocean. Between 3.2 and 2.85 Ma, i.e. during the mid-Pliocene Warmth, the assemblage was dominated by small Gephyrocapsa, a group of species typically associated with upwelling conditions. The dominance of the opportunistic small Gephyrocapsa group can be linked to a more intense mixing of the upper water column most likely due to the more southward position of the Subtropical Front (STF) compared with its present position. After 2.85 Ma, a gradual increase in the cold water species C. pelagicus marked a progressive cooling of the South Atlantic coupled with a northward shift of the (STF). The changes in the assemblage composition co-varied with the δ18O record. Periods with isotopically heavier values (glacial) were dominated by C. pelagicus and periods with lighter values (interglacial) were dominated by very small reticulofenestrids (VSR; <3 μm). After 2.7 Ma, an intensification of the glacial-interglacial patterns of C. pelagicus and VSR was registered. The interval between 2.44 and 2.34 Ma was characterised by low abundance of C. pelagicus and higher biodiversity which are interpreted to be due to a temporal southward movement of the STF interrupting the previously registered cooling. After 2.0 Ma, the decreasing abundance of C. pelagicus suggests a slowdown of the relatively intense cooling registered since 2.7 Ma. The fragmentation ratios of C. pelagicus and C. leptoporus suggest moderate preservation along our interval, with a general trend toward better preservation after 2.2 Ma. This enhanced preservation has been related to a greater input of the less-corrosive northern component water mass into the Cape Basin after 2.2 Ma. The lack of significant glacial-interglacial variations in the fragmentation ratios suggests that the low amplitude of the glacial cycles did not have a substantial impact on the carbonate preservation throughout the entire interval. During MIS 99–98 (approximately 2.5 Ma), a remarkably high accumulation of VSR was found and identified as a short (9 ka) event of high productivity. The timing of this event coincides with that of the Eltanin impact (approximately 2.5 Ma), suggesting that, albeit speculatively, the two events may be related.

Numerical Experiments of Temperature Mixing and Post-Storm Re-Stratification over the Louisiana Shelf during Hurricane Katrina (2005)

Studying mixing and re-stratification during and after hurricanes have important implications for the simulation of circulation and bio-geochemical processes in oceanic and shelf waters. Numerical experiments using FVCOM on an unstructured computational mesh were implemented to study the direct effect of hurricane winds on the mixing and temperature redistribution of the stratified Louisiana shelf during Hurricane Katrina (2005), as well as the post-storm re-stratification timescale. The model was forced by Katrina’s wind stress obtained from a combination of H-Wind database and NCEP model. The climatological profiles of temperature and salinity for August (the month in which Katrina occurred) from the world ocean atlas (WOA, 2013) were used as the pre-storm conditions over the shelf. Model results for sea surface temperature (SST) and mixed layer depth (MLD) were validated versus SST data from an optimally interpolated satellite product, and the MLD was calculated from the heat budget equation of the mixed layer. Model results were used to examine the temporal and spatial responses of SST and MLD over the shelf to Katrina. Results showed that intense mixing occurred within 1–1.1 RMW (RMW is the radius of maximum wind for Katrina), with turbulent mixing as the dominant mixing force for regions far from the eye, although upwelling was an important contributor to modulating SST and MLD. During the peak of Katrina and for the shelf regions severely affected by the hurricane wind, three distinct temperature zones were formed across the water column: an upper mixed layer, a transition zone, and a lower upwelling zone. Shelf re-stratification started from 3 h to more than two weeks after the landfall, depending on the distance from the track. The mixing during Hurricane Katrina affected the seasonal summertime hypoxic zone over the Louisiana shelf and likely contributed to the water column re-oxygenation.

Powder fuel transport process and mixing characteristics in cavity-based supersonic combustor with different injection schemes

The transport process and mixing characteristics of powder fuel in the supersonic airflow are essential for the working performance of a powder fueled scramjet. In the current investigation, the effect of different injection schemes on the powder fuel jet flow field and mixing characteristics are numerically studied in the cavity-based supersonic combustion chamber. The simulation is employed by the Eulerian-Lagrangian scheme coupling with the gas-solid two-phase Reynolds-average Navier-Stokes (RANS) method. Verification of numerical method and grid-independent is conducted firstly, which demonstrated the correctness and credibility of the method in this research. The results show that the powder fuel injection into the combustor induces typical gas-solid two-phase jet flow characteristics. With respect to the fuel injection schemes upstream of the cavity, the powder fuel moves across the cavity section and is continuously mixed with the mainstream along the flow direction. The mixing intensity in the combustion chamber is associated with the motion and distribution patterns of the particles. For the cavity internal fuel injection scheme, the particles are mainly distributed inside the cavity. Some of the particles can be coiled and sucked into the supersonic mainstream. The cavity shear layer and expansion section near the bottom wall is the main region with better mixing performance.

Early taphonomic processes in a microbial-based sedimentary system from a temperate salt-pan site (Cervia salterns, Italy)

AbstractIn the only salt evaporation pond retaining its natural setting of the historic Salina di Cervia (Italy), the northernmost salterns of the Mediterranean area, a number of potentially preservable textures derive from the interactions between photosynthetic mat producers and the sedimentary substrate. These morphologies occur at the beginning of the taphonomic processes when repeated emerged-submerged conditions take place. In these conditions the cohesive nature of the diatom- and cyanobacterial-derived mucilage favours the stabilization of otherwise ephemeral structures. Surface micromorphologies for which diatoms and cyanobacteria have played some active role when still living in the soft microlayer and down to the sediment-water interface, such as during the gliding motility, can overcome the surface layer of most intense mixing (i.e., the taphonomically active zone) and keep traces of them in the fossil record either as body fossils or as texture contributors. Tiny microbial-derived remnants, such as filaments and biofilm strands of halotolerant microorganisms, while fragile upon their formation, can therefore stabilize as biosignatures when combined with salt precipitation. Halophilic and halotolerant ecosystems are models for life in extreme environments (analogue sites) with similarity to those strongly suspected to occur and/or have occurred on Mars and on other planetary bodies. The study of hypersaline systems such as Salina di Cervia which harbour diverse and abundant microbial life, can be relevant for astrobiology since it allows the investigation of potential biosignatures and their preservation, and of further understand the range of conditions and the planetary processes sustaining potentially habitable systems.

In situ flux estimates reveal large variations in methane flux across the bottom boundary layer of a eutrophic lake

AbstractMethane (CH4) produced in anoxic sediments plays a significant role in the carbon economy of many lakes and reservoirs. CH4 released from sediments first crosses the bottom boundary layer (BBL), the layer of water overlying the lakebed where currents are slowed by friction with the sediments below. Physical and biogeochemical conditions in the BBL, which can fluctuate hourly to daily with basin‐wide internal waves (seiches), likely influence CH4 transport from sediments into the hypolimnion. In this study, we estimated CH4 fluxes across the BBL of a eutrophic lake using a novel in situ flux gradient approach adapted from marine applications. For 2–6 h periods throughout the spring and summer, we estimated CH4 fluxes across the BBL using simultaneous measurements of CH4 concentrations, turbulent mixing, and thermal stratification. Sub‐daily variation in CH4 fluxes was high, and CH4 fluxes sometimes changed several‐fold within hours. These rapid shifts in BBL fluxes were likely influenced by fluctuations in seiche‐driven variations in the intensity of BBL turbulent mixing. Fluxes increased from spring to summer, concurrent with the development of lake stratification, and fueled an accumulation of CH4 below the thermocline. Throughout the summer, CH4 flux across the BBL exceeded CH4 accumulation below the thermocline, suggesting significant methanotrophy in the hypolimnion, consistent with incubation‐based oxidation rates. Our results are the first to demonstrate sub‐daily and seasonal variability in the timing and magnitude of CH4 fluxes within a lake BBL, and highlight a need to quantify such variability in other lentic systems.

Turbulent Mixing Variability in an Energetic Standing Meander of the Southern Ocean

Abstract This study presents novel observational estimates of turbulent dissipation and mixing in a standing meander between the Southeast Indian Ridge and the Macquarie Ridge in the Southern Ocean. By applying a finescale parameterization on the temperature, salinity, and velocity profiles collected from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper 1600 m, we estimated the intensity and spatial distribution of dissipation rate and diapycnal mixing along the float tracks and investigated the sources. The indirect estimates indicate strong spatial and temporal variability of turbulent mixing varying from O(10−6) to O(10−3) m2 s−1 in the upper 1600 m. Elevated turbulent mixing is mostly associated with the Subantarctic Front (SAF) and mesoscale eddies. In the upper 500 m, enhanced mixing is associated with downward-propagating wind-generated near-inertial waves as well as the interaction between cyclonic eddies and upward-propagating internal waves. In the study region, the local topography does not play a role in turbulent mixing in the upper part of the water column, which has similar values in profiles over rough and smooth topography. However, both remotely generated internal tides and lee waves could contribute to the upward-propagating energy. Our results point strongly to the generation of turbulent mixing through the interaction of internal waves and the intense mesoscale eddy field.

Observation of strong turbulent mixing in the Australian North West Shelf

The North West Shelf (NWS) of Australia is a region with strong barotropic tides up to 10 m, which provide a source for intense turbulent mixing. In the study, we used turbulent mixing measurements obtained on the NWS from October to November 2017 to investigate the mixing characteristics during the austral spring season. In the surface mixed layer, the averaged dissipation rate reached 1 × 10−5 W/kg. The dissipation rate decreased to 1 × 10−8 W/kg in the thermocline and increased to 7 × 10−8 W/kg near the bottom. The averaged diffusivity in the surface mixed layer reached 7 × 10−1 m2/s, decreased to 1 × 10−4 m2/s in the thermocline, and increased to 7 × 10−3 m2/s near the seafloor. These values were significantly larger than those in the open ocean. The study proposed a parameterization method that scaled the diffusivity in a function of the Richardson number, κρ=κ0+κm(Ri)−1.5, where κ0 and κm were derived from the observation data, to take into account the strong shear instability conditions in our observation. Applications of this method for other shelf regions need further validation.

Electrochemical corrosion of AA6061-AA7075 double sided FSW joints prepared with and without secondary heating

The present study aims to evaluate corrosion behavior of different zones in the double-sided dissimilar friction stir weld between precipitation hardening aluminum alloys AA6061-T6 and AA7075-T651 of 12.7 mm thickness obtained without and with employing secondary/additional heating. Each specific zone of the welds is tested for corrosion potential, corrosion current and impedance spectroscopy. The stir zone formed is more corrosion resistive than AA7075 but less in comparison to AA6061. The corrosion resistivity of AA7075 dominated region increases significantly on the application of secondary heating. The increase is attributed to the disintegration of large corrosion susceptible precipitates due to intense mixing and stirring facilitated at elevated temperature with the application of additional heating. Although, AA6061 experienced trivial decrease in corrosion resistance as high working temperature resulted in increased grain boundary along with high misorientation angle. The scanning electron microscopic images and elemental line scan at corroded stir zone identified AA7075 as the sacrificed alloy due to galvanic coupling with AA6061 at macroscopic scale and dissolution of Mg/Si at microscopic scale. The regions with higher percentage of high angle grain boundaries observed to undergo faster corrosion rates. The heat produced in second pass left annealing effect on prior welded region wherein lower microhardness is noted. • Corrosion resistance of SZ in AA6061-AA7075 DS-FSW joints is enhanced compared to AA7075 while reduced compared to AA6061. • HAZ and TMAZ on RS (AA7075) showed significantly increased corrosion resistance employing secondary heating. • Breakdown of corrosion sensitive intermetallic helped in increasing corrosion resistance. • Slight drop in corrosion resistance of AA6061 observed after secondary heating as increased fraction of HAGBs promoted IGC.