Intense Turbulent Mixing Research Articles

Tidal and river discharge forcing upon water and sediment circulation at a rock-bound estuary (Guadiana estuary, Portugal)

The relative impacts of tidal (neap, spring) and river discharge (including a flood event) forcing upon water and sediment circulation have been examined at the rock-bound Guadiana estuary. Near-bed and vertical profiles of current, salinity, turbidity, plus surface suspended sediment concentrations (SSC, at some stations only), were collected at the lower and central/upper estuary during tidal and fortnightly cycles. In addition, vertical salinity and turbidity profiles were collected around high and low water along the estuary. Tidal asymmetry produced faster currents on the ebb than on the flood, especially at the mouth. This pattern of seaward current dominance was enhanced with increasing river flow, due to horizontal advection that was confined within the narrow estuarine channel. The freshwater inputs and, at a degree less, the tidal range controlled the vertical mixing and stratification importance. Well-mixed (spring) and partially stratified (neap) conditions alternated during periods of low river flows, with significant intratidal variations induced by tidal straining (especially at the partially stratified estuary). Highly stratified conditions developed with increasing river discharge. Intratidal variability in the pycnocline depth and thickness resulted from current shear during the ebb. A salt wedge with tidal motion was observed at the lower estuary during the flood event. Depending on the intensity of turbulent mixing, the residual water circulation was dominantly controlled either by tidal asymmetry or gravitational circulation. The SSC was governed by cyclical local processes (resuspension, deposition, mixing, advection) driven by the neap-spring fluctuations in tidal current velocities. More, intratidal variability in stratification indicated the significance of tidal pumping at the partially and highly stratified estuary. The estuary turbidity maximum (ETM) was enhanced with increasing current velocities, and displaced downstream during periods of high river discharge. During the flood event, the ETM was expelled out of the estuary, and the SSC along the estuary was controlled by the sediment load from the drainage basin. Under these highly variable river flow conditions, our observations suggest that sand is exported to the nearshore over the long-term (>years).

Spatial structure of the flow at round jet outflow into a narrow channel

Results on visual studies of the flow structure in case of the round submerged jet in a narrow channel are presented. These studies were carried out for the laminar and turbulent flows. The typical large-scale structures and zones of intensive turbulent mixing were identified in the flow.

Natural factors controlling the temporal variability of the major-element chemical composition of mineral aerosols over the Northern Caspian

This paper considers characteristic features of the composition and distribution of chemical elements in aerosols over the Northern Caspian, which can be used for a more reliable prediction of possible negative consequences of atmospheric pollution related to the beginning of the large-scale exploitation of oil and gas deposits in the shelf zone of the Caspian Sea. It was shown that the contents of aerosols, their grain-size composition, and major-element composition change under the influence of (1) transboundary transport of terrigenous dust by air masses, (2) variations in the intensity of turbulent and convective mixing in the near-surface atmosphere, and (3) variations in air humidity.

Hydrodynamic effect on the heterogeneity of active sites of butadiene polymerization upon formation of titanium catalyst in situ

The effect of hydrodynamic conditions of the reaction mixture flow during formation of active sites sites of butadiene polymerization on their distribution by kinetic and stereospecific heterogeneity was studied. The enhanced intensity of the turbulent mixing at the stage of formation of active sites results in lowering of the number of types of active sites of chain propagation and the polydispersity coefficient with changes in stereoregular composition of polybutadiene.

Estimating daytime subcanopy respiration from conditional sampling methods applied to multi-scalar high frequency turbulence time series

This study presents a new method to estimate daytime respiration from the subcanopy of forests directly from conventional eddy covariance (EC) measurements. The method primarily considers the respiration signal from root, litter and microbial respiration, which are known to be the main components of ecosystem respiration, R e, as well as decomposition of coarse and fine woody detritus, and respiration from low understory vegetation (forbs, herbs and grasses). The conceptual framework is based on the premise that upward moving air parcels carry a specific and unambiguous signal in their CO 2 and water vapour composition, which can be separated and distinguished into respiration and photosynthesis. The model employed a combination of conditional sampling methods, quadrant analysis and relaxed eddy accumulation with hyperbolic deadbands to identify respiration events and to quantify their flux contribution. Datasets from five sites, most of which had multiple sampling heights, were selected to test this technique among contrasting ecosystems and canopy structures. Respiration signals were successfully identified in daytime data of all sites. A hyperbolic deadband of size H = 0.25 applied to the plane constructed from perturbations of carbon dioxide c and water vapour q densities effectively separated the signals of respiration from photosynthesis. The time fraction of respiration events was ≤10% during daytime. The global correlation coefficient r c, q was found to be a universal predictor of this time fraction and was therefore used as a filter to identify periods of a meaningful and extractable respiration signal. Coherent structures, defined as ramp-shaped flow pattern observed in time series in and above forest canopies, are likely to be the underlying transport mechanism for these respiration events. Daytime subcanopy R e estimates derived from the new method agreed with those derived from: (i) the intercept of light-response curves and (ii) soil CO 2 efflux chambers for three of the five sites. Limitations were posed by the dense, multi-layered deciduous canopy and the intense vertical turbulent mixing at one coniferous site. In addition, refixation of respired CO 2 by the understorey (CO 2 recycling) may cause an underestimation of daytime R e or pose a limitation to the method proposed here. An indicator relating the canopy shear length scale, L s, to the adjustment length scale, L d, was proposed to predict the skill of the new method, and found to be useful in four of the five sites. Analysis of vertical coupling in the plant canopy using exchange regimes could explain the failure of the new method for the remaining site.

Thermal remote sensing of estuarine spatial dynamics: Effects of bottom-generated vertical mixing

In a recent paper, Hedger, R.D., Malthus, T.J., Folkard, A.M., Atkinson, P.M. [2007. Spatial dynamics of estuarine water surface temperature from airborne remote sensing. Estuarine, Coastal and Shelf Science 71, 608–615] demonstrate that airborne thermal remote sensing shows great potential for monitoring estuarine dynamics and surface currents. One aspect needing further attention is the impact of bottom-generated vertical mixing as this can create both stationary thermal features as well as thermal patterns that advect with the flow. This dual effect is illustrated using airborne infrared imagery of a mixing front having an embedded pattern of thermal boils. The boils are several meters in diameter (in water less than 4 m deep) and are ∼0.2 °C cooler than the ambient water surface. Time sequential imagery that captures the movement of individual boils as well as their growth rate can be used to deduce both the near-surface current and the intensity of turbulent mixing.

Visualisation of statistically periodic coherent structures in turbulent flows using a phase-locked averaging method

Coherent structures are the focus of many flow investigations, although their contribution to turbulent mixing is not well understood. When occurring together with high intense turbulent mixing, these structures require statistical methods for visualisation and analysis of simulation results obtained using high-resolution numerical schemes, such as LES or DES. The method of 'phase-locked' averaging separates transient processes with a given frequency. This paper reveals an online formulation for the phase-locked averaging technique at a certain frequency to be set by the user. The method is demonstrated for two realistic flow configurations, an isothermal double-concentric combustor flow and a jet-in-crossflow situation.

Role of nocturnal turbulence and advection in the formation of shallow cumulus over land

AbstractShallow cumuli over land are normally studied from a diurnal perspective. However, the thermodynamic vertical profiles of the morning transition may play an important role in setting up favourable conditions for the formation of shallow cumuli. In turn, these profiles are highly dependent on the evolution of the nocturnal boundary characteristics and of their layer aloft.By analysing thermodynamic profiles measured by radiosondes launched every three hours at four different stations, we are able to determine how horizontal advection and turbulent mixing modify the atmospheric stability and the differences in potential temperature and specific humidity at the interface between the atmospheric boundary layer and the layer above it. Two consecutive nights are studied. They show very similar boundary‐layer development; but variations in the layer aloft by a low‐level‐jet advection event during the second night, and intense turbulent mechanical mixing, lead to the development of two diurnal boundary layers with very different characteristics: the first one clear, the second cloudy.To complete the observational study, we perform a sensitivity analysis using a mixed‐layer model to examine the role of the morning initial conditions in the formation of shallow cumuli over land. The complexity and subtlety of the observed situation—namely, the interaction of a strongly‐mixed nocturnal boundary layer and horizontal advection—make this case suitable for testing the capacity of mesoscale models to reproduce cloudy boundary layers that are largely dependent on conditions during the previous night. Copyright © 2007 Royal Meteorological Society

A new piloted premixed jet burner to study strong finite-rate chemistry effects

This paper describes the initial characterization of a piloted premixed jet burner (PPJB) designed to investigate finite-rate chemistry effects in highly turbulent lean premixed combustion. The PPJB consists of a high-velocity lean premixed central jet, piloted by a low-velocity stoichiometric premixed pilot, surrounded by a large-diameter coflow of lean premixed hydrogen–air combustion products. The configuration of a lean central jet supported by a stoichiometric pilot is similar to that of a lean premixed gas turbine combustor, but without additional complications such as swirl, recirculation, and complex boundary conditions. A significant feature of the PPJB is that under certain conditions the central jet combustion process appears to undergo an extinction–reignition process. It is considered likely that intense turbulent mixing after the nozzle drives an initial extinction process that reduces flame luminosity, with reignition occurring downstream where turbulent mixing has decreased, causing an increase in flame luminosity. Four flames are selected for further study, each with an equivalence ratio of 0.5 and with central jet velocities of 50, 100, 150, and 200 m/s. Simultaneous two-dimensional (2D) Rayleigh–OH planar laser-induced fluorescence (PLIF) imaging results are presented for the selected flames, showing that in the “extinction” region OH concentrations occur at reduced levels in isolated patchy regions, supporting the idea that extinction is predominantly occurring. Laser Doppler velocimetry (LDV) data are also reported for the flow field turbulence statistics, with the most significant result being that for the reacting cases the pilot delays the occurrence of peak turbulence intensity downstream to near the observed “extinction” region.

Modeling of Chemical Processes in a Diesel Engine With Alcohol Fuels

Methanol utilization in a compression ignition engine has held tentative promise for a number of years, and, in fact, the concept has seen large scale field trials intended to demonstrate this option as a precursor to commercial implementation. However, results from those tests have identified some of the practical problems encountered with this fuel, namely, (1) its difficulty of vaporization and (2) its high autoignition temperature. Luminosity promoting additives, which facilitate radiative transport as a component of flame spread (because pure alcohol burns with little luminosity, continuum radiation as a reaction transport mechanism is essentially absent), intake air heating, active and passive heat sources, etc., represent some of the attempts to overcome limitations of these two factors. Except for intake air preheat, these augmentation methods have been noted to result in poor off-load thermal cycle efficiency. Focusing on the case of intake air preheat (which can be achieved by elevated compression ratio), and to model the chemical reaction kinetics, the partially stirred reactor model in CHEMKIN was used. This approach provided examination of the chemistry and reaction rates associated with an actual trial in which methanol was the fuel under study. To initiate this simulation, literature available reaction mechanisms were obtained, and then the experimental cylinder pressure history was matched by control of heat release rate via the partially stirred reactor model. This is represented within the reactor model by changing the turbulent mixing intensity factor. The overall reaction sequence, which models cylinder pressure, and attendant extent of reaction were the major focus. The minor focus included production of emission gases, e.g., the aldehydes and unburned fuel. Not only are the model results consistent with actual findings, they also support a method for addressing causes of off-load inefficiency and engine failures due to engine oil dilution with fuel.

Diurnal variations in the spectrum of lower-ionosphere plasma irregularities

We examined the variations caused in the spectrum of lower-ionosphere plasma irregularities by diurnal changes of ionospheric parameters. We demonstrate that at a height of 95 km, where the irregularities arise due to the turbulence of the neutral atmosphere, the expected level of plasma irregularity fluctuations at a constant turbulent mixing intensity is smaller in the daytime than at night. The one-dimensional spectrum which can be measured in experiments also changes, namely, the spectrum slope is greater at night.

Microstructure measurements in the mid-depth waters of the North Pacific

[1] Microstructure measurements from the surface down to maximum depths of ∼2000 m were carried out at 28 locations in the interior of the North Pacific. Special attention was directed to the diapycnal diffusivities Kρ in the significant generation regions of the semidiurnal internal tide such as the Aleutian Ridge, the Hawaiian Ridge, and the Izu-Ogasawara Ridge. In the profiles obtained near the Aleutian Ridge, only a few turbulent patches with ɛ ≥ 10−9 Wkg−1 were detected and Kρ was generally less than 10−5 m2s−1. In the profiles obtained near the Hawaiian Ridge, many turbulent patches with ɛ ≥ 10−9 Wkg−1 were observed, although Kρ was generally of the order of 10−5 m2s−1. Along the Izu-Ogasawara Trough running in parallel with the Izu-Ogasawara Ridge, in contrast, remarkably intense turbulent mixing (ɛ ∼ 10−8 Wkg−1, Kρ = 10−4 ∼ 10−3 m2s−1) was common throughout the observed water depth.

Experimental and numerical investigation of premixed acetylene flame

In this paper, the mean velocity, turbulence intensity and temperature profiles in different cross-sections of premixed acetylene flame are given. A mathematical model for prediction of velocity, temperature and concentration fields of axisymmetric free premixed turbulent flame is presented in this paper. A second-order closure for turbulent reacting flows is used. Special attentions is paid to model behavior with the respect to the prediction correlation coefficients of turbulent diffusion of the scalar components. Conditional and unconditional statistics of the LDA signals were performed using jet and/or air seed. Compared to commonly used unconditional statistics, conditional statistics of velocity fluctuations can give us more data about intensity of turbulent mixing in the flame.

Turbulent diapycnal mixing in the Nordic seas

The distribution of turbulent diapycnal mixing in the Nordic seas is mapped from observations of internal wave density and velocity fine structure. The uppermost 500–1500 m host two distinct mixing regimes. In the eastern basins, the diapycnal diffusivity (Kρ) straddles 10−5 m2 s−1, whereas in the weakly stratified Greenland and Boreas basins it is raised by an order of magnitude. Below ∼2000 m, low stratification is associated with intense turbulent mixing across the Nordic seas, with diffusivities in the range 3 × 10−4–10−2 m2 s−1. These mixing rates agree within uncertainties with three tracer‐based diffusivity estimates in the region and are associated with turbulent dissipation rates (ɛ) that are at most moderately enhanced above typical open ocean values. A minimum in both ɛ and Kρ is commonly found at ∼1500 m, a depth level that is most efficiently sheltered from shallow and bottom energy sources for the mixing. Available evidence points to wind work on upper ocean inertial motions as a shallow source, with semidiurnal internal tides generated at different levels of the topography contributing to both shallow and deep turbulence. While the closure of the North Atlantic meridional overturning circulation in the Nordic seas appears to be primarily driven by air‐sea interaction, turbulent mixing has the potential to play a critical role in shaping the stratification and ventilation of the region via a range of complex interactions with convection.

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.

Widespread intense turbulent mixing in the Southern Ocean.

Observations of internal wave velocity fluctuations show that enhanced turbulent mixing over rough topography in the Southern Ocean is remarkably intense and widespread. Mixing rates exceeding background values by a factor of 10 to 1000 are common above complex bathymetry over a distance of 2000 to 3000 kilometers at depths greater than 500 to 1000 meters. This suggests that turbulent mixing in the Southern Ocean may contribute crucially to driving the upward transport of water closing the ocean's meridional overturning circulation, and thus needs to be represented in numerical simulations of the global ocean circulation and the spreading of biogeochemical tracers.

Role of physical processes controlling oxycline and suboxic layer structures in the Black Sea

A one‐dimensional coupled physical‐biogeochemical model, together with reanalysis of the available data, are used to provide a new perspective for identification and interpretation of the suboxic layer (SOL) of the Black Sea. While the lower boundary appears to be stable at σt ≈ 16.15 ± 0.10 kg m−3, the upper boundary is found not to be isopycnally uniform as asserted previously. It is found to vary depending on the intensity of vertical diffusive and advective oxygen fluxes across the oxycline. Its position, therefore, does not always correspond to a distinct density surface irrespective of the circulation characteristics. Instead, it changes from σt ≈ 15.55 ± 0.1 in cyclonic to σt ≈ 15.9 ± 0.1 kg m−3 in anticyclonic regions, whereas its position in the peripheral Rim Current transition zone occurs at intermediate density values. These findings imply that vertical oxygen variations cannot be expressed in terms of density without taking into account physical characteristics of the water column. The presence of very strong density stratification is found to prevent ventilation of the SOL. Even intense turbulent mixing generated by a large buoyancy loss during an exceptionally cold winter season is unable to generate sufficiently dense water to overturn and temporarily supply oxygen into the suboxic zone.

Entrainment of Cold Gas Into Turbulent Premixed Acetylene Flame

Entrainment of surrounding air into premixed acetylene flame leads to a change in velocity and temperature fields. In this paper the mixing process between the premixed acetylene flame and surrounding air, in different regions of the flame, has been analyzed. Conditional and unconditional statistics of the LDA signals were performed using jet and/or air seed. Compared to commonly used unconditional statistics, conditional statistics of velocity fluctuations can give us more data about intensity of turbulent mixing in the flame. Mean velocity, turbulence intensity and probability density function of instantaneous velocities have been measured at different cross-sections of the flame, for premixed acetylene/air mixture with stoichiometric ratio. The velocity field measurements were performed with the laser-Doppler anemometer, while for the temperature field measurements Pt-PtRh10% thermocouple was used. Relations between mean velocities and probability density functions have been established in cases of conditional and unconditional seeding. Mixing factor has been defined as a measure of mixing intensity.

Electrostatic charge generation during impeller mixing of used transformer oil

The purpose of this study was to examine the effects of varying conductivity and turbulent impeller mixing intensity on charge generation of used Shell Diala ® transformer oil. The oil was mechanically agitated by a pitched six-bladed impeller in a 1.5 quart fully baffled cylindrical vessel at impeller Reynolds numbers between 5 000 and 10 000. The electrical current generated from the mixing operation was measured on the wall of the mixing tank and also in the internal vapor space on a 1 cm 2 target disk positioned approximately 1.5″ above the free liquid surface. Fluid electrical conductivity was varied using different concentrations of two types of commercially available antistatic additives (Shell ASA-3 and DuPont Stadis 450). A generalized dimensional analysis performed by using the Buckingham Pi theorem showed that a dependent dimensionless group, π 1, which contains the measured wall current and other fluid dynamic and electrical variables was found to be dependent on six independent groups. Four of these groups were related to modified Reynolds numbers based on impeller diameter and width, and tank diameter and fluid depth or tank height. The two remaining independent groups were related to the mass transfer Schmidt number and the impeller rotational speed. The mean particle diameter from a Malvern ZetaSizer and π 1 were plotted versus fluid conductivity to investigate the functional dependence of these parameters. The dimensionless group π 1 was also plotted for different impeller Reynolds numbers. These different plots show results that appear to illustrate inverse dependencies. The current versus conductivity data for different impeller Reynolds numbers also show similar behavior to published data for both laminar and turbulent flow through capillaries (i.e., current first increases with increasing conductivity, hits a maximum and then decreases with further increases in conductivity). This apparently demonstrates that a single charge transport mechanism is responsible for the behavior in two different charge-generation situations for different geometrical conditions.

Agglomeration of yttrium oxalate particles produced by reaction precipitation in semi-batch reactor

The yttrium oxalate reaction, with precipitation and agglomeration in a semi-batch reactor, was investigated. It was assumed that aggregates were formed by particle adhesion following collision and then converted to the agglomerate by the molecular growth. Since the agglomerate was strong enough to resist against the breakage of turbulent shear and collision. Thus, the size-reduction of the agglomerate was mainly caused by the breakage of the aggregate. Both the aggregation and breakage depended simultaneously on the turbulent mixing. When the particle aggregation was dominantly promoted by the turbulent mixing, the mean particle (agglomerate) size in the product suspension increased with increasing the turbulent mixing intensity. When, however, the breakage became more sensitive to the turbulent mixing than the aggregation, the mean particle (agglomerate) size in the product suspension was reduced by increasing the turbulent mixing intensity. The trend of the total particle population with the turbulent mixing intensity was opposite to that of the mean particle size in the product suspension. This mechanism of the particle agglomeration was tested at various feed concentrations and with gelatin additive and was approximated by a simple model of aggregation and breakage equations depending on the turbulent mixing intensity.